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UNIVERSIDAD PABLO DE OLAVIDE DEPARTAMENTO DE GEOGRAFÍA, HISTORIA Y FILOSOFÍA LABORATORIO DE HISTORIA DE LOS AGROECOSISTEMAS
OLD WHEAT VARIETIES. AN OPPORTUNITY TO IMPROVE THE SUSTAINABILITY OF MEDITERRANEAN DRYLANDS AND ORGANIC FARMING
VARIEDADES TRADICIONALES DE TRIGO.
UNA OPORTUNIDAD PARA MEJORAR LA SUSTENTABILIDAD DE LOS SECANOS MEDITERRÁNEOS Y LA AGRICULTURA ECOLÓGICA
Doctoral thesis Guiomar Carranza Gallego
Supervisors Gloria Isabel Guzmán Casado
Manuel González de Molina Navarro
Sevilla, Febrero 2019
Porque el tiempo es el ingrediente esencial, pero en el mundo moderno no hay tiempo. Rachel Carson
Dedicado a todas las mujeres que cultivan las semillas para un mundo justo y solidario. Quisieron enterrarnos, pero no sabían que éramos semilla.
A mi gente.
INDEX
ABSTRACT 1
RESUMEN 2
1. INTRODUCTION 4
1.1. AGRICULTURE. ACTUAL PROSPECTS AND CHALLENGES 4
1.1.1 The climate change challenge 6
1.1.2 Climate change and cereals 7
1.1.3 Climate change and drylands 8
1.2. THE MEDITERRANEAN CLIMATE. VULNERABILITIES AND CLIMATE CHANGE
PROJECTIONS IN THE MEDITERRANEAN REGION 9
1.2.1. Mediterranean vulnerabilities 9
1.2.2. Climate change and the Mediterranean climate 12
1.2.3. The abandonment of Mediterranean drylands. Threats and possibilities
12
1.3. WHEAT LANDRACES 14
1.3.1. Definition of “landraces” 14
1.3.2. Genetic erosion 15
1.3.3. Genetic material conservation 17
1.3.4. The Green Revolution high‐yielding varieties. Two sides of the same
coin 19
1.3.5. The performance of genetically improved varieties under unfavourable
conditions 21
1.3.6. Adaptations of old cereal varieties 23
1.3.6.1. Root biomass 23
1.3.6.2. Aerial biomass production 24
1.3.6.3. Nitrogen uptake and grain nitrogen content 25
1.3.6.4. Grain micro‐ and macronutrient content 26
1.3.6.5. Drought resistance 27
1.3.6.6. Weed competition 28
1.3.6.6.1. Alellopathy 28
1.4. ORGANIC FARMING 29
1.4.1. Benefits, challenges and limits of Organic Agriculture 29
1.4.2. Climate change mitigation and adaptation 33
1.4.3. Organic cereal under Mediterranean rainfed conditions 34
1.4.4. Organic farming needs of appropriate varieties 37
1.5. CLIMATE CHANGE, SOIL QUALITY AND CARBON SEQUESTRATION 39
1.5.1. Carbon sequestration 40
1.5.2. Strategies to improve soil organic carbon content 41
2. OBJECTIVES 43
3. THEORETICAL FRAMEWORK 44
3.1. AGROECOLOGY 44
3.1.1. Agroecology as a new agriculture and food production approach 44
3.1.2. Agroecology and traditional varieties 45
3.1.3. Traditional knowledge and Agroecology 48
3.1.4. Traditional varieties and knowledge need each other 49
3.2. ECOSYSTEM SERVICES 51
3.2.1. Ecosystem services and sustainability 53
3.2.2. Ecosystem services and agriculture 55
3.2.3. Soil and ecosystem services 58
3.2.4. Biodiversity and ecosystem services 61
Biodiversity of weeds in agroecosystems 63
Soil biota diversity 64
Biodiversity and traditional cultivars 65
3.3. SOCIAL AND AGRARIAN METABOLISMS. THE NECESSARY APPROACH TO
SUSTAINABILITY 66
3.3.1. Humans and Nature relationship. Social metabolism 66
3.3.2. Agrarian metabolism and a biophysical approach to sustainability 68
3.3.3. The role of biomass in agroecosystems and societies 69
3.3.3.1. Biomass, internal cycles and fund elements 70
Thermodynamics and sustainability 70
Biomass beyond its economic utility 73
3.3.3.2. The land cost of sustainability 75
3.3.4. Experimental history 75
4. STUDIES 76
4.1. FIRST STUDY 76
4.2. SECOND STUDY 95
4.3. THIRD STUDY 126
5. DISCUSSION AND GENERAL CONCLUSIONS 156
5.1. THE AGRONOMIC PERFORMANCE OF OLD AND MODERN WHEAT VARIETIES UNDER
MEDITERRANEAN RAINFED CONDITIONS 156
5.1.1. Grain, straw and total biomass 156
5.1.2. Weed competition 157
5.2. SOCIO‐ENVIRONMENTAL IMPLICATIONS 158
5.2.1. Genetic breeding under Mediterranean rainfed conditions 158
5.2.2. Seeds and farmers’ right to their autonomy 159
5.2.3. A profound change in other links of the food chain 160
5.3. OLD VARIETIES AND ECOSYSTEM SERVICES BEYOND PRODUCTIVITY 162
5.3.1. Straw biomass and ecosystem services 162
5.3.2. Higher belowground inputs to agroecosystems 164
5.3.3. Reduction in herbicide application 165
5.3.4. The effect of varietal replacement 165
5.4. AN OPPORTUNITY FOR CLIMATE CHANGE MITIGATION/ADAPTATION STRATEGIES
166
5.5. OLD WHEAT VARIETIES AND ORGANIC CEREAL DRYLANDS 167
5.6. LIMITATIONS AND PROJECTIONS OF THE STUDY 168
5.7. GENERAL CONCLUSIONS 169
6. AKNOWLEDGEMENTS 170
7. REFERENCES 173
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ABSTRACT Wheat is grown in more than 220 million ha in the world, being the most widely cultivated crop on Earth. Additionally, it is considered the most important crop for global food security, accounting for 20% of total protein intake worldwide, while in countries of the Mediterranean basin this share amounts to 31%. Global wheat production has increased 3.5‐fold since the 1960s and it has been a key element in the development of the so‐called Green Revolution. Modern varieties were designed to respond to the increase of industrial inputs associated with the intensification of agriculture (e.g. chemical fertilizers, pesticides, irrigation), which involved the abandonment of old wheat varieties, or their displacement to marginal agricultural lands. However, under less optimal conditions for agriculture such as rainfed Mediterranean conditions, or under management schemes that exclude the use of chemical inputs (such as organic agriculture), modern cereal varieties may not constitute an advantage over those grown before modern genetic breeding. Investigations comparing modern and traditional varieties are usually conducted under conditions that clearly favor modern ones, introducing an important bias in the widely extended idea that old varieties are less productive. In addition, most studies tend to focus on grain yield when comparing traditional and modern varieties. Yet, in the face of global change, the goal of crop cultivation should not only be grain production but also the provision of ecosystem services and the generation of sustainable and resilient agroecosystems. This is especially important in Mediterranean agriculture, which is particularly vulnerable to the expected impacts of climate change and global change. In this sense, it is necessary to investigate the role played by old varieties in these agroecosystems, and the consequences of their replacement by modern ones. Against this background, the main goal of this PhD is to compare the production and supply of ecosystem services of modern and traditional wheat varieties under rainfed Mediterranean conditions, both in the present and in the past. To this purpose, a field experiment comparing both types of cultivars under organic, traditional and conventional managements, along with a broad bibliographic research from historical sources, has been carried out with the objective of identifying traits of traditional varieties that should be considered for the development of sustainable rainfed agroecosystems. In this PhD dissertation, some of the main ecosystem services derived from the cultivation of old wheat cultivars are analyzed. In Study 1, we evaluated the effects of soil incorporation of the straw of old and modern wheat cultivars under laboratory‐controlled conditions. Results show that ecosystems services such as soil nutrient conservation and carbon accumulation can be enhanced after the incorporation of the straw of old varieties. In Study 2, a life cycle assessment is carried out to compare the carbon footprint of old and modern varieties grown under rainfed organic and conventional farming systems, in order to elucidate the climate change mitigation potential of old varieties related to their carbon sequestration potential. Finally, the main consequences of wheat varietal replacement in the last century in Spanish cereal drylands are investigated in Study 3, considering the impacts on ecosystem services found in Studies 1 and 2. The results show that the cultivation of old wheat varieties under Mediterranean rainfed conditions is advisable in many aspects. Results of Study 1 show that the soil incorporation of the straw residues of old varieties is advantageous for soil carbon accumulation and the reduction of nitrogen losses from the soil. Accordingly, Study 2 shows that the higher straw and root biomass production of old cultivars can result in higher carbon sequestration rates, which is responsible for a lower carbon footprint of these varieties with respect to modern ones. The central role of carbon sequestration in the reduction of the environmental impact of cereal cultivation highlights the necessity to include it in carbon footprint accountings. Additionally, synthetic fertilizer in conventional farming systems and the use of machinery in organic ones were the main hotspots of the GHG emissions profiles, indicating that climate change mitigation
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efforts in Mediterranean rainfed systems should focus on these steps. Last, Study 3 shows that the reduction in crop residues due to varietal replacement entailed the degradation of the fund elements soil and biodiversity during the last century, threatening the agroecosystem sustainability. In conclusion, the cultivation of old wheat varieties in Mediterranean drylands can involve environmental advantages and climate change adaptation and mitigation synergies, without diminishing grain yields, especially under organic farming conditions. RESUMEN El trigo es sembrado en más de 220 millones de hectáreas en todo el mundo, siendo el cultivo que más extensión ocupa en la Tierra. Además, es considerado el cultivo más importante para la seguridad alimentaria a nivel global, ya que representa el 20% de la ingesta total de proteínas en todo el mundo, mientras que en los países de la cuenca mediterránea esta proporción asciende al 31%. La producción global de trigo se ha multiplicado por 3.5 desde la década de 1960 y ha sido un elemento clave en el desarrollo de la llamada Revolución Verde. Las variedades modernas fueron diseñadas para responder al aumento en el uso de insumos industriales asociado a la intensificación de la agricultura (por ejemplo, fertilizantes químicos, pesticidas, irrigación), lo que implicó el abandono de las variedades de trigo tradicionales o su desplazamiento a tierras agrícolas marginales. Sin embargo, bajo condiciones menos óptimas para la agricultura, como las condiciones del secano mediterráneo, o bajo esquemas de manejo que excluyen el uso de insumos químicos (como la agricultura ecológica), las variedades modernas de cereal pueden no constituir una ventaja sobre aquellas que se cultivaban antes de la mejora genética moderna. Las investigaciones que comparan variedades modernas y tradicionales se realizan generalmente en condiciones que favorecen claramente a las modernas, lo que introduce un importante sesgo en la idea ampliamente extendida de que las variedades tradicionales son menos productivas. Además, la mayoría de estos estudios tienden a centrarse en el rendimiento del grano. Sin embargo, frente al cambio global, el objetivo de la actividad agrícola no solo debe ser la producción de grano, sino también la provisión de servicios ecosistémicos y la generación de agroecosistemas sustentables y resilientes. Esto es especialmente importante para la agricultura mediterránea, particularmente vulnerable a los impactos esperados del cambio climático y el cambio global. En este sentido, es necesario investigar el papel que juegan las variedades antiguas en estos agroecosistemas, así como las consecuencias de su reemplazo por las modernas. Dentro de este contexto, el objetivo principal de la presente tesis es comparar la producción y el suministro de servicios ecosistémicos de variedades de trigo modernas y tradicionales bajo las condiciones del secano mediterráneo, tanto en el presente como en el pasado. Para dicho propósito, se llevó a cabo un experimento de campo que compara ambos tipos de cultivares bajo los manejos ecológico, tradicional y convencional, y una amplia investigación bibliográfica de fuentes históricas, con el objetivo de identificar los rasgos de las variedades tradicionales que deben ser tenidos en cuenta para el desarrollo de agroecosistemas de secano sostenibles. En esta tesis doctoral se analizan algunos de los principales servicios ecosistémicos derivados del cultivo de variedades tradicionales de trigo. En el Estudio 1, evaluamos los efectos de la incorporación al suelo de la paja de cultivares de trigo antiguos y modernos bajo condiciones controladas en laboratorio. Los resultados muestran que determinados servicios ecosistémicos, como la conservación de nutrientes y la acumulación de carbono en el suelo, pueden potenciarse tras la incorporación de paja de variedades antiguas. En el Estudio 2 se lleva a cabo un análisis de ciclo de vida para comparar la huella de carbono de variedades antiguas y modernas cultivadas en sistemas de producción ecológica y convencional bajo condiciones de secano, a fin de dilucidar el potencial de mitigación del cambio climático de las variedades antiguas a partir de su potencial para el secuestro de carbono. Finalmente, las principales
3
consecuencias de la sustitución varietal del trigo en los sistemas cerealísiticos del secano español durante el pasado siglo se investigan en el Estudio 3, a partir de los impactos en los servicios ecosistémicos encontrados en los Estudios 1 y 2. Los resultados muestran que el cultivo de variedades tradicionales de trigo, en condiciones de secano mediterráneas, es recomendable en muchos aspectos. Los resultados del Estudio 1 indican que la incorporación al suelo de los residuos de paja de las variedades antiguas es ventajosa para la acumulación de carbono en el suelo y la reducción de las pérdidas de nitrógeno del suelo. En consecuencia, el Estudio 2 muestra que la mayor producción de paja y de biomasa de raíz de los cultivares antiguos puede conducir a mayores tasas de secuestro de carbono, lo que conlleva una menor huella de carbono de dichas variedades con respecto a las modernas. El papel central del secuestro de carbono en la reducción del impacto ambiental del cultivo de cereales destaca la necesidad de incluirlo en los cálculos de la huella de carbono. Asimismo, el empleo de fertilizantes sintéticos en los sistemas agrícolas convencionales, y el uso de maquinaria en los ecológicos, fueron los principales hotspots en los perfiles de emisiones de GEI, lo que indica que los esfuerzos de mitigación del cambio climático en los sistemas mediterráneos de secano deberían centrarse en dichos factores. Por último, el Estudio 3 muestra que la reducción de los residuos de cosecha tras la sustitución varietal conllevó la degradación de los elementos fondo, suelo y biodiversidad, durante el último siglo, lo que amenaza la sostenibilidad de los agroecosistemas. En conclusión, el cultivo de variedades tradicionales de trigo en las tierras de secano mediterráneas puede implicar ventajas ambientales y sinergias para la adaptación y mitigación del cambio climático, sin disminuir los rendimientos de grano, especialmente en condiciones de agricultura ecológica.
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1. INTRODUCTION 1.1. AGRICULTURE. ACTUAL PROSPECTS AND CHALLENGES Agriculture is considered one of the main human activities impacting the global environment (Matson et al. 1997), extending over about 38% of terrestrial area, the largest use of land of the Earth (FAO, 2018a). Vitousek et al. (1986) estimated that approx. 30% of Earth`s terrestrial net primary productivity (NPP) was appropriated by humans but this value can rise up to the staggering amount of 60%‐100% in some regions under a more detailed regional analysis (Haberl et al. 2007). Population growth, together with the technological development proper of the History of human beings, are the main factor influencing the biosphere functioning. Due to concern about these trends, Crutzen and Stroermer (2000) coined the term Anthropocene to allude the new geological epoch Earth had entered in the late XVIII century, coinciding with the Industrial Revolution and the beginning of growing global concentrations of carbon dioxide (CO2) and methane in polar ice, driven by the impact of human activities as a new force able to control fundamental processes of biosphere. Agriculture is considered one of the main drivers of environmental change in recent decades and centuries (Leff et al. 2004). The Green Revolution arrival, in mid‐20th century, involved a huge change in agricultural practices that clearly entailed great impacts on the biosphere. This new productive approach provoked an unprecedented agricultural productivity increment (understood as the increment of harvested product by land and work unit) due to the increased fertilization, pesticide application and mechanization, improved irrigation and soil management regimes, high‐yielding crop varieties as well as massive land conversions (Matson et al. 1997; Tilman et al., 2002). Likewise, it replaced the need of the traditional fallow and N fixing crops rotations in arable lands and left space to monoculture cropping (Bradshaw, 2017). The intense use of chemical inputs allowed the increase in yields, but also brought, and still does, dramatic environmental problems and negative effects on human health (Foley et al. 2005; Pimentel et al. 1996; Tilman et al. 2001). During the last four decades of the last century, the consumption of nitrogen (N) fertilizer increased 6.6‐fold, phosphate fertilizers increased 3.0‐fold, irrigated land and total cultivated land increased 85% and 10%, respectively, and total agriculture emissions increased 70% (FAOSTAT, 2018a). Along with the increase in chemical inputs and the change in agricultural practices, breeding programs are also responsible for this yield increase in last decades. The proportion of yield increase attributed to the genetic improvement of crops has been estimated to be about 50%‐60% (McLaren, 2000). Nevertheless, the annual gains in yields derived from these programs are declining, while the research cost associated are escalating (Tilman, 1999). Accordingly, the pesticides sales increased 15‐20 fold during the same period (Oerke, 2006). Despite the increment in pesticides and herbicides use, productive losses did not diminish significantly in last decades of the 20th century: for wheat, yield losses due to pests and weeds were calculated in 29% by the end of the century (Oerke, 2006). The ecological impacts of agricultural intensification affect both terrestrial and aquatic ecosystems, including farmlands and also landscapes outside production (Stoate et al. 2009), and local, regional and global scales (Matson et al. 1997). In addition, environmental impacts derived from agriculture are “double sided coins”, as they expressed through the over‐exploitation of natural resources, as well as by using them as a sink for wastes and pollution (Pretty et al. 2001). Harmful effects derived from agricultural intensification are related to increased rates of soil erosion (Cerdá et al. 2010; Vanwalleghem et al. 2017), water pollution (Berka et al. 2001; Scanlon et al. 2007), the loss of biodiversity (CBD, 2010; Geiger et al. 2010), the reduction of crop pollinators (Deguines et al. 2014; Kremen et al. 2002), the increase in greenhouse gas (GHG) emissions (Matson et al. 1997; Snyder et al. 2009), the impact on the main biogeochemical cycles (Galloway et al. 2008; Lal, 2004a; Smil, 1999a), etc. Some of these
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alterations are shared with many other human activities, but some of them are almost exclusively derived from agricultural activity, such as the alteration of the global N cycle (Smil, 1997). Briefly, we can see that the current agricultural food system faces a tradeoff between the long‐term maintenance of ecosystem services and the short‐term agricultural production (Foley et al. 2005). This tradeoff can be a significant risk, as the environmental harmful impacts due to agricultural intensification have in turn negative impacts on agricultural production (Tilman et al. 2001; Deguines et al. 2014). For instance, the biodiversity loss due to agricultural intensification can, in turn, reduces agricultural productivity (Lanz et al. 2018), as well as erosion rates (Den Biggelaar et al. 2004). Since 1961, global cereal production increased by 193% while population did by 144% (FAO, 2018b,c). That is, on average, agrarian production has increased at a higher rate than population (Hazel and Wood, 2008; Tilman, 1999). However, despite there is enough food produced to feed every person of the world, roughly 11% of the global population is affected by hunger. As recently reported (FAO, 2017:8), “world hunger is on the rise: the estimated number of undernourished people increased from 777 million in 2015 to 815 million in 2016”. So, environmental damages are increasing, while the challenge of producing food for everyone is far from being reached: contrarily to what is usually believed, the application of Green Revolution techniques did not lead to a decrease in the number of hungry people in the world (Jordan, 2002). There are important socio‐political implications of these trends, as there are many social inequalities underlying, to some extent, this challenge. Many authors have defended that the point might not be to produce higher amounts of food, but to focus in inequality and the lack of access to resource and land of many people in the world (Altieri and Rosset, 1996). As Alexandratos (1999) stated, “the undernourished and the food‐insecure persons are in these conditions because they are poor in terms of income to purchase food, or in terms of access to agricultural resources, education, technology, infrastructure, and credit to produce their own food”. In wealthy nations, protein intakes are over the necessity threshold, while in many poorer regions, millions of people do not take the protein they need (Smil, 2002). On average, caloric demand of wealthy countries is 250% higher than that of impoverished countries (Tilman et al. 2011), and this is linked to the shift in diet patterns towards an increment in animal products consumption (Keyzer et al. 2005; Lassaletta et al. 2014a; Tilman and Clark, 2014). Indeed, animals are fed with cereals, soybeans and forages instead of grazing pastures, thus needing more farm area than “an equivalent plant‐based diet” (Bradshaw, 2017). This inequality is also expressed in the elevated GHG emissions derived from livestock (Goodlang and Anhang, 2009). In this regard, the search for sustainable agro‐food systems should focus on diet changes towards a lower animal product consumption and on methods to reduce food waste (Niles et al. 2018). Additionally, the rise in grain demand is also related to the rising biofuel production (Lotze‐Campen et al. 2010) and this non‐nutritional use can affect food security in many regions of the world (Nonhebel, 2012), strengthening the need to search for alternative biomass to produce biofuels (Tilman et al. 2009). Giving all of these circumstances, it could be said that one of the main challenges of this century is linked to supplying enough food for every people in the world, while reducing environmental harm derived from agriculture activity (Foley et al. 2011), that is, to build a sustainable food production system. Contrastingly, there are studies supporting the need for the spearing of land through the intensification of already existing croplands (Balmford et al. 2018; Searchinger et al. 2018). In the same way, ecosystem reaction to land use changes is not immediate, but it takes long term until we can realize consequences, especially in semiarid regions (Scanlon et al. 2007).
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Therefore, we should be addressing long term strategies to build sustainable systems for the future, and be concerned about the time that it takes to reverse the negative impact of agricultural practices, as restauration and recuperation of harm agroecosystems could require centuries (Gliessman, 2000). Indeed, these agriculture challenges should be tackled with care, as the implementation of measures will depend on the socio‐economic and geopolitical contexts (Hazel and Wood, 2008). As Foley et al. (2011) stated, “the challenges facing agriculture today are unlike anything we have experienced before, and they require revolutionary approaches to solving food production and sustainability problems”. 1.1.1. The climate change challenge
All these challenges that agriculture will be facing the next decades will be under a climate change context. Climate change is defined as “a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods (UNFCCC).” Climate change is easily resumed in the temperature and CO2 concentration increase since 19th century (IPCC, 1996) due to the rise in fossil fuel combustion and the emissions of GHG. It has great impacts on nature and human beings, although evidence of climate change related impacts is more robust and comprehensive for the former. The alterations of hydrological cycles and the quantity and quality of water resources, the changes in species ecology and migration patterns and the acidification of oceans, the alteration of the intensity of extreme climate events such as droughts, heavy precipitations events, windstorms, fires and pest outbreaks are some of the negative impacts of climate change on nature (IPCC, 2014). Human influence on the climate system is undoubted (IPCC, 2014). The main human factor contributing to climate change is GHG emissions, which have constantly increased since there are estimates, with only a slight reduction due to global economic crisis in 2009 (Boden et al. 2017). Such has been this increase that GHG emissions in 2014 were more than 18 times those in 1900 (Boden et al. 2017). NASA’s monthly measurements of atmospheric CO2 levels agree with that trend, and CO2 levels of January 2019 were 410 ppm (NASA, 2019). In turn, agriculture contribution to these gasses has been said to be relatively low, about 11%, however, this share can increase up to 21% when accounting for other activities as land use change (Tubiello et al. 2015). In the other way around, many impacts of climate change on agriculture have been drawn and predicted. Negative effects such as the reduced water availability for irrigated lands (Elliot et al. 2014) and rainfed crops (Anwar et al. 2015), the increase in soil erosion rates (Mondal et al. 2016) and in plant diseases affectations (Chakraborti et al. 2000), etc. are some of the examples of the harmful effects. Likewise, the increase in temperature has been said to have an adverse effect on interactions of crops with soil biotic community (Tian et al. 2018) and crop growth and phenology, although the advancement of phenology depends on crops and regions (Anwar et al. 2015; Olesen et al. 2012) and its effects are conditioned by water availability (Olesen et al. 2012). Regarding the increment in atmospheric CO2, its interactive effects with water and temperature entails some discrepancies about its effect (O’Learly et al. 2015). Fitzgerald et al. (2016) found that heavy water stress limited yield responses to elevated CO2 under semiarid rainfed conditions, and that timing of temperatures and water inputs, along with the crop’s ability to translocate carbohydrates to the grain at post‐anthesis were determinant for the response to elevated CO2. At this regard, the positive effect on yield due to higher CO2 concentration can be counteracted by the effect of lower rainfall and higher temperatures in semiarid environments under climate change scenario (Anwar et al. 2007; Ferrise et al. 2011). Therefore, the C‐fertilizer effect on specific environments, such as drylands, should consider the impact of high temperature, water and CO2 simultaneously (Fitzgeralds et al. 2016).
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Furthermore, effects beyond grain yield should be also considered, since, for instance, the eventual increase in grain yield due to elevated CO2 could come with a decrease in wheat grain protein concentration and lower end product quality (Fernando et al. 2015). All of these impacts have a final common output, the reduction of crops yields. Albeit the adverse impacts are regionally specific and depend on climatic, soils and farming conditions (Olesen and Bindi, 2002), agriculture will have to cope with them. Indeed, it is worrisome that adverse consequences will be increasingly detrimental with time (Anwar et al. 2015). Regarding climate change, agriculture has a two‐fold challenge in the very next future: mitigation and adaptation to climate change. That is, to reduce GHG emissions while adapting to a changing and more variable climate (Smith and Olesen, 2010). Strategies in one or the other sense, even the synergies that could be derived from strategies initially designed for one purpose, should be carefully design and planned. In addition to the aforementioned, facing climate change effects has also socio‐political implications, such as those linked to human health impacts. Global food availability and fruit and vegetables consumption have been modelled to be reduced by 2050, and this prediction would have a different impact on population deaths depending on the global region implied (Springmann et al. 2016), with higher affectation in global Southern regions. Therefore, coping with climate change impacts is not only a question of environmental concern, but it should also be a question of ethical concern. 1.1.2. Climate change and cereals
Cereals constitute the most prevalent crop family cultivated worldwide, exceeding 20% of global land area and 61% of the total cultivated land (Leff et al. 2004). However, cereal yield growth rates have slowed down during last decades due to different factors such as natural resource degradation, pest resistance or market prices slowdown (Wood et al. 2000). Cereals constitute the major staple crop for world population and wheat, rice and corn entail about 44% of human daily calorie intake (Shiferaw et al. 2013). Within these proportions, wheat occupies 22% of the total cultivated area in the world (Figure 1, Leff et al. 2004). It is grown in 220 million ha in the world, being the most widely cultivated crop (FAO, 2018b). Additionally, it is considered the most important crop for global food security (Shiferaw et al. 2013), accounting for 20% of total protein intake worldwide, while in countries from Mediterranean basin the average percentage amounts to 31% (FAO, 2018d). Wheat harvest has increased 310% since 1961 to 2017, however, the correlation with cereal land area increase disappeared since the 1980s (Figure 2, FAO, 2018b). Therefore, the increase in wheat production, following the global situation of agriculture, was due to a great intensification of its cultivation.
Figure 1. Global distribution of wheat. Source: Leff et al. 2004.
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Figure 2. Global wheat production and area cultivated in the period between 1961‐2017. Source: FAOSTAT.
Given the relevance of cereals and wheat to human nutrition, many studies have evaluated the negative effect of climate change on global wheat yield (Asseng et al. 2015; Challinor et al. 2014; Deryng et al. 2011; Lobell and Field, 2007), as well as under Mediterranean conditions (Farina et al. 2011; Ludwig et al. 2006), predicting a probable yield decrease during next decades (Asseng et al. 2015; Deryng et al. 2011; Ferrise et al. 2011; Saadi et al. 2015; Yang et al. 2014). In many European countries, climate change has been found to be a factor for the wheat yield stagnation (Brisson et al. 2010), as its harmful effects can offset a proportion of technological‐based increases in cereal yields (Lobell et al. 2011). Recently, Tian et al. (2018) found that the increase in temperature shortened the growing period of wheat, and decreased grain yield and quality in Asia. Under rainfed Mediterranean conditions, the predicitons of wheat yield decrease are about 29% (Anwar et al. 2007). Accordingly, Olesen et al. (2007) forecasted an average yield decrease of winter wheat of 21% by the end of the present century in Spain, with Southern regions being more affected than Northern ones. In addition, wheat croplands of the North of Spain will be affected by warming temperatures, because of the requirements for low temperatures for flower induction (vernalisation) (Olesen et al. 2007). Moreover, under Mediterranean rainfed conditions, the expected decrease in rainfall can imply that many current and traditional wheat cropping areas would become unviable in a climate change future scenario (Luo et al. 2005; Maracchi et al. 2005). Adaptation and mitigation of climate change strategies in cereal fields are manifold. To enumerate some of them, the optimization of sowing date (Bassu et al. 2009; Olesen et al. 2012), the use of earlier heading varieties (Gouache et al. 2012), the use of straw residues to increase soil quality, cereal yields and C secuestration (Lal et al. 2011; Mandal et al. 2007; Triberti et al. 2008), and the cultivation of varieties with a vigorous root biomass production (Sprigg et al. 2014) and more tolerant to heat or drought stress (Semenov et al. 2014) are some of the strategies suggested to cope with the changing climate situation. 1.1.3. Climate change and drylands
Drylands can be defined as land areas where mean annual precipitation is below 600 mm and the ratio of mean precipitation to potential evapotranspiration is below 0.65 (Feng and Fu, 2013). Global climate change is likely to affect rainfall distribution (Wood et al. 2000), which will make these areas specially vulnerable to climate change, to desertification and land degradation (Safriel 2009) and water shortages (Huang et al. 2017). Drylands have suffered from a higher temperature rise due to climate change than humid areas, especially because of their lower soil
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225
230
235
240
245
1961
1963
1965
1967
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
2011
2013
2015
2017
Mg
ha
World area harvested (ha)
World production (Mg)
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moisture and vegetation cover, that are responsible of a lower specific heat than that of wetter surfacces and have a lower capacity to buffer heating process. However, they are a lower contributor to GHG emissions than the latters (Huang et al. 2017). This assimetry between geographic distribution of the CO2 emissions and the warming rates has not received much attention, and it is predicted to continue growing in future decades (Huang t al. 2017). Besides these trends, a fifth of agricultural land is in arid or dry semiarid land (Wood et al. 2000). In this regard, it would be easily assumed that both adaptation and mitigation climate change strategies should be specifically designed for these regions. We should be seriously addressing this issue, especially if we take into acount that drylands area will spread over the global land surface and become drier under a climate change scenario during the next decades (Sherwood and Fu, 2014). 1.2. THE MEDITERRANEAN CLIMATE. VULNERABILITIES AND CLIMATE CHANGE PROJECTIONS IN THE MEDITERRANEAN REGION
Mediterranean climate is characterized by dry and hot summers and humid and temperate winters. Averaged annual rainfall values oscillates around 500 mm (Acevedo et al. 1999), ranging from 275 mm to 900 mm and at least 65% of the annual rainfall is concentrated in winters and falls (Aschmann, 1973). As a result, Mediterranean regions are characterized by an important drought stress and strong soil water deficit in summer (Bolle, 2003a). The average temperature is below 15°C, and total hours per year with temperatures below freezing (0º) does not exceed 3% of the total (Aschmann, 1973). Mediterranean climate is also characterized by a high spatial variability, even at small scales (Bolle, 2003a). This climate is found on the west coast of continents between latitudes 30° and 40° (Figure 3). Globally, five regions of the world show this climate: The Mediterranean basin, the center and southern coasts of California and Northern coast of Mexico, central Chile, the Southern tip of South Africa and Southwest Australia (Acevedo et al. 1999).
Figure 3. Adaptation of the Mediterranean ecoregion (in red) in Ecoregions 2017 ©Resolve, from
data by Dinerstein et al. (2017). An interactive map is available at ecoregions2017.appspot.com. 1.2.1. Mediterranean vulnerabilities
Water scarcity. Water resources are limited in the Mediterranean region, and there is a growing competition for water sources among irrigated agriculture, domestic uses and tourism development. In this context, water availability has become a key factor of agriculture in these regions (Verheye and De la Rosa, 2005). Indeed, climate change will impact negatively in future
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water availability in Mediterranean areas (Iglesias et al. 2009) by increasing potential evapotranspiration, decreasing rainfall and increasing the intensity and frequency of droughts (IPCC, 2014). Predictions of several climate change scenarios rise the percentage of Mediterranean population that would be living under water stress up to 14%‐38% (Schröter et al. 2005). However, water demand is increasing due to demographic shifts, economic development, irrigation and changes in our lifestyle, largely exceeding the available water in many countries of the region (Iglesias et al. 2011). Groundwater extraction has increased during last decades to meet this growing demand in most semiarid or arid countries (Fornes et al. 2005), and the main fate of this groundwater is irrigation (Fornes et al. 2005). Along with the rising use of groundwater, the degradation of Mediterranean aquifers is increasingly affecting water quality, due to several factors: pumping is exceeding natural recharge, contamination of water with agrochemicals and leakage from urban areas, among others (Barraque, 1998). The reduction in water availability affect humans direct and indirectly through the impacts on related ecosystem services (Schröter et al. 2005). Soil degradation and desertification. Natural and anthropogenic causes make Mediterranean soils a fragile component of ecosystems (Salvati and Bajocco, 2011). Together with climatic characteristics and erodible parental material (Verheye and De la Rosa, 2005), Mediterranean soils fragility is related to low structural stability of surface soils (Usón and Poch, 2000), water and wind erosion, soil salinization and SOM depletion (Cerdá et al. 2009, 2017; García‐Orenés et al. 2012). The increase in urbanization processes (Al‐Adamat et al. 2007) and touristic pressure (Salvati and Bajocco, 2011) have also adverse effect on Mediterranean soils. Crop production, deforestation and grazing are major practices related to human impact on Mediterranean soils’ structure, biological activity and organic matter loss (Verheye and De la Rosa, 2005). Agricultural production of Mediterranean drylands strongly relies on soil quality, along with winter rainfall volume and summer drought stress (Verheye and De la Rosa, 2005). In this sense, soils from the Mediterranean regions typically have low SOC concentrations due to low rainfall, high temperatures and low organic input (Aguilera et al. 2013a). This condition distances Mediterranean cropland soils from their potential for SOC storage (Aguilera et al., 2013a), making them even more vulnerable to land degradation (Al‐Adamat et al. 2007). Moreover, climate change impacts could also pose new threats for Mediterranean soils (Anaya‐Romero et al. 2015), since climatic change predictions make the Mediterranean biome extremely vulnerable to desertification (Lavee et al. 1998; Underwood et al. 2009). In other words, SOC depletion is a major vulnerability factor for the sustainability of Mediterranean agriculture in a climate change context (Iglesias et al., 2011). The low organic C content of Mediterranean soils is responsible of low fertility and low agriculture productivity (Cerdá et al. 2010), thus management practices enlarging this pool are relevant for the sustainability of agriculture. Biodiversity erosion. Mediterranean climate areas are recognized as hot‐spots of biodiversity (Myers et al. 2000). More than a half of the plant species of the Mediterranean basin are endemic, which constitute 4.3% of total plant diversity worldwide (Myers et al. 2000). Biomes such as tropical and southern temperate forest are largely affected by one single driver of biodiversity loss, in this case, land‐use change (Sala et al. 2000). Regarding biodiversity of artic and alpine biomes, the main threat is global warming (Chapin and Körner, 2013:320; Sala et al. 2000). Contrastingly, Mediterranean ecosystems are sensitive to all biodiversity changes drivers (Sala et al. 2000). Climate change, land degradation and land use change, N deposition and air pollution of Mediterranean areas are factors affecting biodiversity in this region (Sala et al. 2000; Schröter et al. 2005). Other drivers underlying species decline in this region are the infrastructures’, urbanization and tourism development, chemical pollutants like PCBs (polychlorinated biphenyls), overfishing and by‐catch (Vié et al. 2008).
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Agricultural dependence on external inputs. During the past 50 years, global ecosystems have suffered from a more intensive and extensive alteration never seen before in history (Steffen et al. 2011), especially in the Mediterranean region (Serra et al., 2008), where soils have been cultivated for millennia. In those decades, Mediterranean agriculture suffered from a previously unknown industrialization processes, which involved the increase in the use of external inputs such as fertilizers, crop protection and irrigation (Guzmán et al. 2017). In Spanish agroecosystems, the use of external inputs increased by 5‐fold, within which crop protection, irrigation energy and mineral fertilizers did by 35.2, 7.7. and 2‐fold, respectively (Guzmán et al. 2018). Agroecological indicators show that even with an increase in energy and water inputs and agroecosystems NPP, Spanish farms have lowered their efficiency (Guzmán et al. 2017). The increase in the use of external energy and materials in Spanish agriculture has not only led to the inefficiency of the agroecosystems, but also to the increase of the pressure on them (Soto et al. 2016) and, thus, their unsustainability (Guzmán et al. 2018). Soil quality has been affected by erosion, desertification, salinization and pollution, while the detrimental effects of agricultural management on water quality are related to nutrient and agrochemical leaching (Zalidis et al. 2002). The increase in N fertilizer applications due to agricultural intensification is responsible for the increase in N deposition and its associated negative effects (Ochoa‐Hueso et al. 2011). For example, N deposition can affect plant biodiversity and species endemisms of the Mediterranean basin due to the proliferation of nitrophilous species and abiotic stresses (Phoenix et al. 2006). In addition, the excess of N fertilizer use, along with an intensive irrigation, is responsible of N pollution of streams and aquifers (Lassaletta et al. 2012). Irrigation of Mediterranean croplands is clearly linked to agricultural intensification. In Spain, irrigated lands have greatly increased in last decades. From 1950 to 2008, irrigated area increased by 207% (Aguilera et al. 2018), while from 2008 to 2017 it showed an increase of 11% (MAPA, 2018). In 2017, irrigated lands constituted 7.4% of total land area in Spain, 22% if considering agricultural lands (MAPA, 2018). Cereals, olive groves and vineyards are the crops with the highest irrigated areas, and their irrigated lands represent 25%, 21% and 10% of the Spanish cropland area (MAPA, 2018). Indeed, climate change will increase the irrigation requirements of crops grown in these regions by 4‐18%, and by 22‐74% when taking into account the predicted population growth (Fader et al. 2016). Some of the negative impacts of this practice is the increase in energy consumption and soil GHG emissions (Sanz‐Cobena et al. 2017), soil salinization, waterlogging and leaching of nitrate and other pollutants (Fernández‐Cirelli et al. 2009). Particularly in the Andalusian region, the transformation from traditional to industrialized agriculture and the decline of traditional crops is also inextricably linked to the spread of intensive greenhouse cropping destined to European market, the intensive urbanization and the development of an industrial and tourism‐based economy during the last 60 years (Bermejo et al. 2011). Furthermore, the panorama of resource depletion challenges the maintenance of the dependence on external inputs for food production, adding a huge vulnerability to Mediterranean farming systems. The mining of phosphorus for fertilizer production is faster than geological cycles can replenish it (Vaccari, 2009), and its scarcity is forecasted by 2100 (Van Vuuren et al. 2010). In Mediterranean areas, the scarcity of water is of special relevance due to limited water availability during summer. Predictions under the climate change scenario amplify the adverse effect on water depletion in Mediterranean areas (García‐Ruíz et al. 2011), making the dependence on irrigation also a factor of vulnerability for this region (Fader et al. 2016). Likewise, regarding oil production, Wang et al. (2017) concluded that it is likely that the global production of conventional fossil fuels will reach its peak by this century, even when accounting for non‐conventional oil and gas in their projection. This peak will be “abrupt and revolutionary”, comparing to “previous energy transitions (wood to coal, coal to oil) (Hirsch et al. 2005). In a context of global change, the high use of external energy by Mediterranean agriculture
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challenges the need for the reduction in energy consumption and GHG emissions (Guzmán et al. 2017), the resource depletion and climate change (Alexander and Gleeson, 2019). Certainty about the unsustainability of our misuse of natural resources and the reliance on external energy should warn us about the future of food security. There is much at stake, also for societal concern, since economy depends on energy and not on money to run (Hall and Klitgaard, 2018). 1.2.2. Climate change and the Mediterranean climate
The Mediterranean area is especially vulnerable to global change (Giorgi and Lionello, 2008). In this context, the effects of global warming on the Mediterranean climate patterns are a growing concern in last decades (Ulbrich et al. 2006) and the study of Mediterranean climate characteristics have received attention, since they are relevant to understand how Mediterranean ecosystems will react to global climate change (Bolle, 2003b). The Mediterranean region has suffered from large climate shifts in the past (Luterbarcher et al. 2006). In this sense, the land use change carried out in the Mediterranean area has a long history, and is responsible for high levels of deforestation, over‐exploitation and overpopulated landscapes (Bolle, 2003a). In 2009, a study reported that 30.8% of the Mediterranean biome had been converted into urban or high‐intensity agriculture land (Underwood et al. 2009), features that cannot offer much room to mitigate and adapt to the effect of extreme climate events. Particularly, the Mediterranean region has been identified as a hotspot in future climate change projections (Giorgi, 2006; IPCC, 2007), and so the Iberian Peninsula (Ruiz‐Ramos et al. 2018). Models predictions estimate a substantial drying and warming of this region, with a pronounced decrease in precipitation, mainly in the warm season, and increase in temperature, especially in the summer season (Lionello et al. 2014). Moreover, interannual variability is forecasted to increase also mainly in the summer, leading to a greater occurrence of extremely high temperature events (Giorgi and Lionello, 2008). These authors calculated a precipitation decrease higher than ‐25%/‐30% along with an increase in temperature (higher than the global average) (Giorgi and Lionello, 2008). These shifts in climate conditions can affect relevant aspect of Mediterranean agriculture. Negative impacts on water availability (Iglesias et al. 2011), crop yields (Moriondo et al. 2010) or harvest index of cereal (Moriondo et al. 2011) have been predicted for Mediterranean croplands due to climate change. Drier and hotter conditions of regions under this climate could led to specifically dramatic effects on crops (Moriondo et al. 2010) and on soil organic matter (SOM) content and soil quality (Lavee et al. 1998). The low and erratic distribution of rainfall, along with extremely high temperatures coinciding with the end of the wheat cycle (Royo et al. 2014), could be responsible of the even worse negative impact of climate change on cereal yields in the Mediterranean Basin (Ferrise et al. 2011) when comparing to other regions worldwide. As for drylands in general, it is relevant to seek and evaluate the ability of the Mediterranean agriculture to adapt to climate change (Iglesias et al. 2011), and to mitigate its negative impacts. The seek for these strategies would require of specific research under these climatic conditions in order to design suitable practices under the sustainability approach needed to cope with the agricultural challenges facing climate change. 1.2.3. The abandonment of Mediterranean drylands. Threats and possibilities
Drylands are lands where the limited water availability is the main factor constraining crop yields (MEA 2003). Between 1989 and 1999, dry farming decreased more than 25% in Spain (Serra et al. 2008), with socioeconomic factors leading the reasons for the abandonment of this agrarian practice (Bielsa et al. 2005; Saurí and Breton, 1998). Concretely, during the 1970s, Andalusian economy changed direction towards an increased relevance of industry and tourism‐economy,
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motivating the loss of land value and, thus, the loss of traditional soil uses (Bermejo et al. 2011). The modernization of agriculture in this region entailed the loss of employment and farm income in pro of productivity increments, involving the abandonment of the agrarian activity and the depopulation of rural areas (González de Molina, 2011). In semi‐arid regions, land abandonment first affected rainfed fields with low yields and fields degraded by irrigation (Romero‐Díaz et al. 2007). Areas with little or none irrigation possibilities are left for growing fodder and grazing, while irrigated agriculture concentrates the most profitable cropping (Verheye and De la Rosa, 2005). Indeed, the population migration during the 1960s was also a relevant factor for land abandonment and Mediterranean ecosystems conservation (Cerdà et al. 2010). In semi‐arid environments, farmland abandonment is a major problem due to development of structural and sedimentary crusts, which reduce infiltration and increase overland flow and soil erosion (García‐Ruiz and Lana‐Renault, 2011). In Mediterranean agroecosystems, land abandonment is specially concerning due to their low resilience (Bielsa et al. 2005). The main consequences of the abandonment of traditional agricultural systems are the homogenization of the landscape, the fragmentation of agricultural land, the increased risk of wildfires, the reduction or extinction of flora and fauna species, and the increased risk of soil erosion (Bielsa et al. 2005). The limited annual rainfall of the Mediterranean climate restricts the choice of crops for rainfed conditions: trees like olives, figs, almonds and pistachios, along with grapes, are suitable to grow under these conditions. Likewise, annual crops must run a growing period corresponding “in length and moisture requirements with the rainy season” (Verheye and De la Rosa, 2005), which has led to wheat being the most common annual crop under Mediterranean rainfed conditions (Soriano et al. 2016). However, modern agriculture adoption entailed the abandonment of this crop and the change towards irrigated cropping (Guzmán and González de Molina, 2008; Verheye and De la Rosa, 2005). Despite all of this, rainfed Mediterranean cereal fields are expanding, and their importance for Mediterranean agriculture sustainability is increasingly acknowledged (Perniola et al., 2015). For example, Tahmasebi et al. (2018) recently found that rainfed Mediterranean wheat cropping systems are more sustainable than irrigated ones due to a lower input intensity. They concluded that the higher yield of irrigated systems did not compensate for the unproportioned increase in GHG emissions. Linked to this, the researcher community should be tackling the opportunities for building resilience and sustainability in rainfed wheat cereals, opening the door to the recuperation of abandoned drylands while enhancing and supporting decent farmers’ incomes and generational turnover. The increase in population, along with the obtainment of the product and by‐products with higher added value (e.g. straw for bioconstructions, u others) can lead to an increase in demand and, therefore, be an incentive to increase the farmers’ wage or generational turnover. Regarding the concern about the abandonment of rural areas and the lack of generational turnover, the adoption of organic farming has been suggested as a promising strategy to curb that rural exodus phenomena in the Mediterranean basin, mainly because of higher incomes perceived by the farmers (Testa et al. 2015), the greater market opportunities and the subsidies they received (González de Molina, 2011). These advantages of organic farming adoption have been especially relevant for rainfed traditional crops in Spain (González de Molina, 2011). However, the support by government institutions is an essential step for this reversion (Testa et al. 2015).
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1.3. WHEAT LANDRACES 1.3.1. Definition of “landraces”
Camacho‐Villa et al. (2005) defined landraces as crop cultivars with an historical origin and grown before modern genetic breeding programs. “Farmer varieties”, “local varieties”, “primitive varieties” or “landraces” (Negri, 2005) are different manners to make reference to these old cultivars. Landraces are named, selected and maintained by farmers to meet their “social, economic, cultural and ecological needs” (Teshome et al. 1997). Steps in the process of landraces evolution have been widely described (Harlan, 1992). From the domestication of wild plants and the emergence of the first crops, these forms kept crossing with wild relatives, developing new forms and spreading this process through others places thanks to human migration (Negri, 2005). “Wheat domestication was responsible for the increase in human population by enabling humans to produce food in large quantities, thereby contributing to the emergence of the human civilization” (Zohary and Hopf, 2000). With the process of migration from the Fertile Crescent (assumed centre of origin and current centre of distribution and diversity) into new environments (new climatic, edaphic and biotic conditions) and, thus, new selection pressures, cultivated wheat responded with increased variation and formation of many endemic forms. This process of migration, along with human and natural selection, resulted in local landraces adapted to a diversity of conditions (Feldman et al. 2001). For example, humans have selected useful traits for agriculture, such as the nutritional value and the adaptability to agronomic practices. On the other hand, the environment, in the form of biotic and abiotic factors, have selected best competitors, disease and pest resistant plants. These adaptations led to a wide variation of these races through the millennia or, in other words, this long and wide evolution process modulated them (Negri, 2005). Landraces are genetically diverse, and they express this diversity within an inter and intra‐population level (Negri, 2005). They are heterogeneous populations of crops (Brown, 2000), able to maintain a diverse gene pool essential for their coevolution and adaptation to the environment (Namkoong et al. 1996). In other words, they are mixtures of genotypes that evolved under the environmental conditions in which they grew. In self‐pollinated crops like wheat, landraces harbour relevant genetic variation for qualitative and quantitative traits (Ehdaie and Waines, 1989). The traditional management by farmers of wheat landraces led to the conservation of a general level of diversity, rather than of genetically stable and distinct populations. Wheat landraces are not genetically, nor phenotypically stable, distinct or uniform (Jaradat, 2011). This diversity prevented the disappearance of landraces due to epidemic nor climatic stresses. Landraces have phenotypic variability (Mercer and Perales, 2010), allowing for a wide adaptation to local conditions and the biotic and abiotic stresses of the area (Negri, 2005), as well as to the agronomic practices of their regions of origin. They are a “valuable tool for identifying different strategies of adaptation and grain yield formation” (Moragues et al. 2006a). According to Jaradat (2011),
The genetic structure of wheat landraces is an evolutionary approach to survival and performance, especially under arid and semi‐arid growing conditions. The combined effects of natural and human selection have led to architecture of genotypes representing different combinations of traits, such as growth habit, cold, heat or drought tolerance, early growth vigor, time to heading and maturity, seed filling duration, and quality traits. As a result, wheat landraces developed into complex, variable, genetically dynamic and
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diverse populations, in equilibrium with both biotic and abiotic stresses in their environment.
Under adverse conditions such as semiarid and arid environments, landraces evolution did not lead to an individual genotype possessing “one trait” with superior performance, or an individual genotype with a specific architecture of different traits (Ceccarelli et al. 1991). Instead, the combined effects of natural and artificial selection involved the build of a collective genetic structure by different combinations of traits in genotypes. Landraces, contrarily to modern cultivars, and due to their genetic heterogeneity, benefit from the population buffering capability that give them stability against variable conditions. The genetic structure of landraces then, may be considered as an evolutionary approach to survival and performance under arid and semi‐arid conditions (Schulze, 1988). Their individual stability “is sacrificed to maximize the stability of the population” (Ceccarelli et al. 1991). It is the interaction among specific traits of landraces that determines their performance under Mediterranean stressful conditions, and “efforts to identify individual traits causally associated with yield stability under stress is unlikely to be successful” (Ceccarelli et al. 1991). The landrace’s diversity is tightly related to the diversity of the material sown in adjacent areas, and to the frequency of short‐ and long‐ distance seed exchange among farmers (Jaradat and Shahid, 2014). Old cultivars have been told to be associated to the traditional agrarian systems where they have coevolved, and their identity makes them highly recognizable among other varieties (Camacho‐Villa et al. 2005). The traditional knowledge and culture have developed in association with local varieties (more information regarding this issue can be found in section 3.1.4.). Landraces and farmers have an interdependent relationship (Teshome et al. 1997), and this tight relationship is well expressed in Jaradat (2014):
Whether man was domesticated by wheat, or wheat was domesticated by man is but two faces of the same coin; both incidents marked a turning point in human history and led to the emergence of human civilization in the Fertile Crescent of the Old World.
Other ideas at this respect can be found in Grando et al. (2001):
Landraces are adapted to levels of inputs farmers can afford, yet are variable. So, they can be improved without requiring additional inputs. Landraces are adapted to their environment; they fit in the farming systems of their area of adaptation; they are often essential in the diet; in many cases they are the only food or feed available: the welfare of people depending on landraces should and can be improved not by replacing landraces but by improving them.
1.3.2. Genetic erosion
According to FAO (2010), genetic vulnerability is the “condition that results when a widely planted crop is uniformly susceptible to a pest, pathogen or environmental hazard as a result of its genetic constitution, thereby creating a potential for widespread crop losses”, while genetic erosion, is defined as “the loss of individual genes and the loss of particular combinations of genes” such as those present in locally adapted landraces. Genetic erosion can, thus, make reference both to the loss of genes or alleles and to the loss of cultivars. In addition, while genetic erosion “does not necessarily entail the extinction of a species or subpopulation, it does signify a loss of variability and thus a loss of flexibility” (FAO, 2010). Ultimately, genetic erosion can be expressed as the disappearance and displacement of diverse, local populations of crops (Brush, 2000).
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The huge decrease in landraces cultivation is undoubtedly related to the modernization of agriculture (Altieri and Merrick, 1987) and modern plant breeding approaches (Brush, 2000). Since modern breeding started, at the end of 19th century, “the wheat field has become genetically uniform and no longer conducive of spontaneous gene exchange” (Feldman et al. 2001). With this process, individual genotypes, rather than mixtures, became the unit of selection. Consequently, modern agriculture carried the expansion of monocultures of genetically uniform varieties (Altieri and Merrick, 1987), with the consequent loss of biodiversity of agroecosystems and traditional knowledge associated to them (Altieri, 2004). For all of this, the conservation of local wheat landraces and old varieties for agrobiodiversity conservation is It is a concern that has to be addressed both at farm, country and global levels (FAO, 2018e). Modern varieties are the result of “scientific breeding” by professional plant breeders working in private companies or publicly‐funded research institutes (FAO, 1997). The rapid displacement of landraces by modern cultivars (Ehdaie and Weines, 1989; FAO, 1997) reached a great amount and diversity of cropping systems worldwide (Brush, 2000). In the Mediterranean basin, it resulted in a decrease of the varietal diversity and the near extinction of on‐farm genetic variability (Skovmand et al. 2005). The narrowing of the genetic base due to modern plant breeding (Ceccarelli et al. 1991; Grandon et al. 2001), the high selective pressure of breeding programs (Nazco et al. 2012) and the small number of varieties cultivated nowadays (Khoury et al. 2014) are some of the factors underlying the reduction in the genetic diversity of farms (Skovmand et al. 2005). Moreover, the structural changes in wheat farming systems also have a responsibility in the loss of genetic diversity and fragmentation of meta‐population structures of wheat landraces (Jaradat, 2011). The loss of these varieties also involves the loss of indigenous and farmers’ knowledge associated to them (Karagöz, 2014). It has been estimated that approximately 75% of crop genetic diversity has been lost during 20th century (Jaradat, 2013; Pretty 1995), which has reduced valuable resources for future crop improvement (Feuillet et al. 2008; Newton et al. 2010). While 4% of the 250000‐300000 plant species are edible, only 12 provide about 75% of food worldwide and, in turn, only 3 species from the 150 commercial crops (rice, maize, wheat) provide 60% of plant‐derived calories or food energy supply (FAO, 1999). This number rises up to 30 crops that “feed the world”, providing 95% of the world’s calorie intake (FAO, 1997). In other words, global food security relies on a very small number of major crops, whose diversity conservation, availability for use and proper management is of great relevance (FAO, 1997). Mediterranean landraces are considered a relevant source of genetic resources because of their extensive genetic variability and their documented resilience to pests, resistance to diseases and tolerance to abiotic stresses (Pecetti et al. 1994). The genetic diversity of landraces may involve an advantage under the high environmental variability typical of semi‐arid regions (Blum et al. 1989). In this sense, the narrowed genetic base of high yielding cultivars could be related to the stagnation of yields in less favourable areas from the Mediterranean basin (Pecetti et al. 1994). Landraces play an essential role in agroecosystem biodiversity and the local communities (Rocchi et al. 2016). Therefore, their disappearance involves genetic and cultural erosion and their loss is of a growing concern (Negri, 2005). The loss of genetic diversity associated to landraces is not only a concern to farmers, but also to agriculture in its wide acceptation, as this diversity is essential to the continuation of coevolution (Brush, 2000). The conservation and maintenance of diverse crop species and varieties enables farmers to cope with environmental constraints and risks “by matching varieties to diverse production conditions” (van Etten, 2011). Farming systems harbouring higher levels of diversity are more resilient to face adverse conditions, more productive and are linked to better nutrition and
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people’s health (Frison et al. 2011). Therefore, diverse farming systems enhance food security of local communities (Frison et al. 2011; Jaradat, 2011). Jaradat (2011) highlighted that genetic erosion of landraces due to the substitution with modern varieties could threat the future sustainability of agriculture by reducing the diversity available for crop evolution and for future breeding‐efforts. Organic and low‐input farming systems could be specially affected by the loss of genetic diversity (Jaradat, 2011), as their management practices and conditions are far from the easily achievable uniformity under conventional farming. Finally, the crop response to altered conditions derived from climate change is highly dependent on the genetic variation of the crops (Mercer and Perales, 2010) and the resilience of farming systems (Frison et al. 2011), thus landraces conservation should be central in developing climate change adaptation scenarios.
Plant genetic diversity also has the potential to provide traits that can help to meet future challenges, such as the need to adapt crops to changing climatic conditions or outbreaks of disease (Bruinsma, 2017).
The use of landraces with a breeding purpose to broaden the existing genetic base of crop improvement and to detect and profit from desirable traits have been widely described under Mediterranean conditions. The improvement of yield (Pecetti et al. 1994), yield stability (Boggini et al. 1997), grain quality (Boggini et al. 1997; Pecetti and Annicchiarico 1993), grain protein content (Blum et al. 1987; Koç et al. 2000), gluten strength (Nazco et al. 2012; Pecetti and Annicchiarico 1993), drought resistance (Blum et al. 1989; Mohammadi et al. 2015; Reynolds et al. 2007), etc., could benefit from the introgression of landraces alleles relative to traits adapted to local conditions. However, the relevance of agrobiodiversity conservation is not only the source of traits, but as the underpinnings of more resilient farming systems (Frison et al. 2011). 1.3.3. Genetic material conservation
Agricultural genetic resources are essential for humanity and their conservation is indispensable for future food security (CBD, 1992). The conservation of landraces and wild relatives of crops is a key issue for the maintenance of the gene pool required for future breeding programs and to ensure the adaptability of crops under changing climatic conditions (Frese et al. 2011). Without genetic diversity, the evolution of crop species would not be possible and, therefore, neither would environmental adaptation (Namkoong et al., 1996). Genetic conservation can take place in two different levels. On the one hand, in situ conservation promotes the maintenance of inter‐ and intra‐population diversity of species directly used in agriculture or as a source of genes in the habitat where this diversity came from or where it still growing (Brown, 2000); while, on the other hand, ex situ conservation is based on off‐farm representative collections of the genetic diversity from cultivated species (Marshal, 1989 in Brown, 2000). Ex situ conservation consists in maintaining genetic resources out of their natural habitat, mostly in gene banks, botanical gardens or research stations (Plucknett and Horne, 1992). This conservation strategy has been the material source for high‐yielding varieties of the Green Revolution (Toledo and Barrera‐Bassols 2008). Some constrains of this type of conservation are the difficulty in maintaining a wide base of variability from field sampling, the lack of representation of the broad genetic diversity of a crop and its cultivars in the gene banks (Plucknett and Horne, 1992) and the genetic variations due to storage and controlled growing phase (Altieri and Merrick, 1987). Concretely about wheat, FAO’s 2010 report emphasized the lack of regeneration progress in wheat genetic resources collections because of the lack of
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funding. The main objection to ex situ conservation is that the coevolutionary process between the plant and its environments is stopped, hindering adaptive responses to evolutionary pressures, such as environmental biotic or abiotic changes (Simmonds, 1962). So much so that it has been called “the freezing way of conservation” (Toledo and Barrera‐Bassols 2008). Excessive paperwork needed to use the gene bank material (Pinheiro de Carvalho et al. 2013), the lack of international laws to protect intellectual property rights of local producers and the increase in patents’ creation (Toledo and Barrera‐Bassols 2008) can hinder the access of local producers and promote the privatization of genetic resources. Last but not least, gene banks are subject to institutional decisions and willingness, thus institutional changes can be detrimental to their fate and functioning (Plucknett and Horne, 1992). Despite these objections, gene banks must share their germplasm collections, fostering an adequate use of their genetic material and the future development of sustainable agriculture (Pinheiro de Carvalho et al. 2013). Regarding in situ conservation, two types can be identified (Brush, 2000). Firstly, this conservation refers to the maintenance of genetic resources in their habitats, including farmers’ fields and their practices. This conservation has taken place historically, and it is the background where old varieties have evolved. Nowadays, this type of conservation is still done by some farmers. Farmers who chose to grow traditional cultivars often prefer to diversify their cropland by growing more than one traditional variety (Jarvis et al. 2008). Although this can be called in situ conservation, often farmers continue to grow traditional varieties not due to conservation purposes, but due to “tradition and preferences, risk avoidance, local adaptation, niche market opportunities or simply the lack of a better alternative” (FAO, 2010). Secondly, in situ conservation is also linked to projects and programs dedicated to diversity conservation in croplands. With governments’ support, international programs or private organizations, these projects search to persuade farmers to keep this local crop diversity (Brush, 2000). In situ conservation in farms allows the coevolution and local adaptation of cultivars and, ultimately, its preservation and connection with social processes responsible of traditional breeding of crops (Brush, 1999). Nevertheless, despite its benefits, in situ conservation has been widely rejected by conservation programs, due to the “misconception about farming systems that produce landraces” (Brush, 1995). In the last decades of the 20th century, in situ conservation has been recognized as an essential complement to ex‐situ conservation (Brush, 1995; Maxted et al. 2000). In addition, many relevant traits of landraces are, or have been, under evaluation for their use in modern breeding programs (Eagles and Lothrop, 1994; Lopes et al. 2015), as it has been briefly described above. However, landraces should not only be seen “as genetic resources to conserve for the needs of tomorrow, but as breeding material to be used today, particularly in breeding programmes for stress environments and for poor farmers” (Grando et al. 2001). The introduction of old varieties in croplands, along with their wild relatives, has been highlighted as essential for the conservation of genetic resources in developing countries (Altieri and Merrick, 1987). “The main challenges of on‐farm conservation of wheat landraces are non‐biological, but involve a complex of ethno‐anthropological processes, including legal, economic and social factors, superimposed on interacting ecological and genetic processes” (Jaradat, 2011). In 2013, a research on global gene banks material and their role unveiled that, from 7.4 million entries, 24% were wheat local cultivars (Pinheiro de Carvalho et al. 2013). To emphasize the sustainable use of traditional varieties out of the gene banks, it is necessary to recognize farmers’ work and to become aware of their value for society (Pinheiro de Carvalho et al. 2013).
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The maintenance of crop genetic resources under the modern agricultural paradigm “is not only technically complicated, but a politically sensitive issue” (Altieri and Merrick, 1987). One of the big challenges for the conservation of this genetic material is the convincement of agronomists, researchers and decision makers of its relevance and our dependence on them for the future of agriculture (Plucknett y Horne, 1992). In front of “economic forces” that led to farmers to use modern varieties (Altieri and Merrick, 1987), it is crucial that scientists, farmers and researchers from other linked fields cooperate to evaluate the role, potential and benefits of biodiversity conservation in agrarian landscapes (Jackson et al. 2007). Finally, for local cultivars conservation is also relevant the recognition of plant genetic resources as an open access resource (van Etten, 2011). And for this challenge, we must wonder which tradeoffs involved the varietal substitution. The continuation of the evolution process of landraces in cultivated fields is essential to increase the probabilities of overcome the main negative effects of climate change on future production (Merecer and Perales, 2010), because in situ conservation helps to preserve their adaptive potential and contributes to stablish old cultivars as valuable resources for sustainable agriculture under a climate change context (Bellucci et al. 2013). However, the effect that climate change could exert on landraces and their success in farmer fields has been poorly evaluated (Mercer and Perales, 2010). In this line, Coromaldi et al. (2015) highlighted a better performance of local varieties against modern ones in climate change vulnerable areas. 1.3.4. The Green Revolution high‐yielding varieties. Two sides of the same coin
The rise in agrarian productivity since the middle of the 20th century is mainly due to the use of high‐yielding varieties of the Green Revolution relying on high fertilizers and pesticides, and fossil fuel use (Jackson et al. 2007). Additionally, the Green Revolution has also heavily relied on the intensification of water use (Postel, 1999). Regarding cereal cropping, the rise in cereal yield in the last century was due to the higher harvest index, but also to a denser planting pattern and the use of chemical inputs (Smil, 2011). With reference to wheat, breeding programs carried out during the 20th involved great increases in grain yields, due to both physiological and morphological changes. As Loss and Siddique (1994) stated “breeders have changed the structure of cereals considerably, both indirectly through changes in phenology and directly through the introduction of dwarfing genes”. Many studies have been conducted to elucidate the main changes in wheat that led to the yield increases. A higher harvest index and a shorter size, a longer post‐anthesis green area duration, and the increase in grains spikelet‐1, grain ear‐1 and grain m‐2 were related to higher yields of Mediterranean modern cultivars (Siddique et al. 1989). Similarly, Austin et al. (1980) described genetic yield improvement for winter wheat in the U.K. They also found that the newer cultivars were shorter and reached anthesis earlier than the older ones, and related their higher yields with an increase in harvest index, not to an increase in aboveground biomass. Another mechanism to increase yield in wheat was breeding for stronger and shorter stems to reduce lodging (Reitz, 1970 in Eagle et al. 2012). In Spain, the wheat genetic improvement during the 20th century led to the increase in yield in a similar way. Sánchez‐Garcia et al. (2013) reported an average increase in the number of grains spike‐1 and the number of spikes m‐2 of 0.6% y ‐1 and 0.3% y‐1 respectively, without any significant increase in grain weight. These authors concluded that the genetic gains of Spanish bread wheat were highly dependent on the environmental conditions, and they reported two main episodes:
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(i) During the first stage, in the 1950s, it took place a transition from traditional landraces to the first cultivars released from the earliest breeding programs in Spain and the improved cultivars introduced from France and Italy. The intraspecific crosses led to yield increases of 30% compared to landraces. This genetic gain was due to 58% higher number of grains spike‐1. Grain weight was slightly reduced by 16%. These new varieties harboured higher harvest index (from 0.25 to 0.40), and kept the same aboveground biomass at maturity, due to an elevated number of tillers bearing spikes. Contrarily, height and the number of tillers per plant were diminished.
(ii) In the second phase, during the late 1960s and the early 1970s, the semi‐dwarf germplasm from CIMMYT (International Maize and Wheat Improvement Centre) and some French cultivars were introduced. Plant height was further reduced, consequently harvest index was raised up to 0∙45, so did the number of tillers/plant. A new increment in yield of c. 37% was achieved.
(iii) The cultivars released during the last decade of the century did not contribute to significant yield nor harvest index improvements, suggesting a plateau for yield gains in Spain achieved during the last 30 years of the century.
Accordingly, the grain yield improvements of durum wheat in Spain were based on similar genetic gains: increases in the number of grains m‐2 and harvest index, without significant changes in grain weight or aboveground biomass (Royo et al. 2007). However, a decrease in total crop dry matter of modern cultivars has been also found (Álvaro et al. 2008). According to Sánchez‐Garcia et al. (2013), harvest index and plant height of durum wheat in Spain remained unchanged from 1980 to 2000 (Royo et al. 2007). Studies assessing the role of modern breeding during the 20th century have drawn conclusions in the same direction for Australian (Perry and D’Antuono, 1989) and Italian (Motzo et al. 2004) wheats. Overall, the great rise in the grain yield that took place in the second half of the 20th century in Spain (De Vita et al. 2007; Royo et al. 2007) was a consequence of the introgression of CIMMYT semi‐dwarf genes, and involved a gradual replacement of traditional tall cultivars by semi‐dwarf and fertilizer‐responsive varieties (García del Moral et al. 2005). However, there were already increases in grains spike‐1 and biomass partitioning to spike before the introgression of Rht genes (De Vita et al. 2007; García del Moral et al. 2005), due to breeders’ improvement since the early 1900s. The consequences of breeding programs for the Mediterranean regions have been widely described. In addition to the higher harvest index (Austin et al. 1980) and the reduction in straw production (Bektas et al. 2016), the lodging resistance (Heisey et al. 2002), the shortening in the cycle length (Álvaro et al. 2008; Motzo et al. 2004; Pecetti and Annicchiarico 1993), the increase in gluten content (Motzo et al. 2004) and the reductions in green area index (Álvaro et al. 2008), grain filling rate and grain weight (Nazco et al. 2012; Royo et al. 2007), grain protein content (Guarda et al. 2004; Nazco et al. 2012; Motzo et al. 2004) and grain nutrient content (Sciacca et al. 2018) are consequences of breeding stations programs. There can be some discrepancies in these findings. For example, Koç et al. (2000) found that grain protein content of the high‐yielding varieties was almost as high as that of the best landraces. However, breeding programs activities entailed some trade‐offs that should be considered. The introgression of dwarfing genes brought some pleiotropic effects such as the reduction in root size and depth (Song et al. 2009; Waines and Edhaie, 2007), a poorer nutrient use efficiency and an increased dependence on high inorganic N input (Foulkes et al. 1998), an increased susceptibility to climate events (Heinemann et al. 2013) and to diseases and pests (Oerke 2006), the decrease in yield stability (Acreche et al. 2008; Annicchiarico et al. 2005; Calderini and Slafer,
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1999; Koç et al. 2000; Pecetti and Annicchiarico 1993) and in weed competition ability (Coleman and Gill, 2003 ; Lemerle et al. 2001a; Murphy et al. 2008a). The use of high‐yielding new varieties less adapted to local conditions, with higher susceptibility against diseases together with a greater reliance on herbicides (Lemerle et al. 2001b), could underlie the rise in pest affectations during the last decades (Oerke, 2006). Nazco et al. (2012) found that modern cultivars had lost phenotypic variability, compared to old ones, which could be useful in breeding programs to improve gluten strength, grain weight and accelerate grain filling rate. Modern wheat varieties show a high adaptation to high‐input environments (De Vita et al. 2010), while this is not true for low‐input or organic conditions (Wolfe et al. 2008). Changes in agronomic practices such as the increased use of N and P fertilizer, the fitting of sowing density and crop phenology, the use of herbicides for weed control, and the implementation of irrigation and mechanization account for a high yield improvement during last decades (Araus et al. 2004). Modern breeding led to modern varieties that are more responsive to changes in environmental conditions (Koç et al. 2000; Slafer and Kernich, 1996) and management improvement (Calderini and Slafer, 1999; Feldman et al. 2001) than the older ones. For example, Fang et al. (2017) found modern wheat cultivars to be more responsive to irrigation than older ones and Foulkes et al. (1998) stated that modern cultivars could show a higher dependency on an easy access to nutrients. Accordingly, under low N conditions, modern cultivars suffered from higher yield losses than old ones (Gaju et al. 2016). Modern varieties are commonly referred as widely adapted varieties (e.g. Martos et al. 2005), which means that the cultivar is adapted over space, that is, the performance of the cultivar in several locations is better than that of a reference cultivar (Ceccarelli and Grando, 2002). The opposite is adaptation over time or stability. Wheat landraces were not “specifically selected for high grain yield because the priority was yield stability under local conditions” (Bektas et al. 2016). The question that should be tackled here is if farmers would be worried for varieties being adapted to many environments, or varieties being stable over time (Ceccarelli et al. 2004). Finally, after the improvement in yield potential during the mid‐ twentieth century, responsiveness to environmental amelioration has been limited (Araus et al. 2004). The yield improvement in the Mediterranean region has been seen to be lower than that of other regions due to a high interannual variation in rainfall. In this context, some authors suggest that modern agricultural management practices as well as technological innovations, such as those focused on the increase of irrigated areas, should be reinforced to overcome that constraint (Sener et al. 2009). 1.3.5. The performance of genetically improved varieties under unfavourable conditions
It is widely accepted that yields have grown mainly because of the Green Revolution technological progress (plant breeding, agrochemicals use, etc.). Nevertheless, genetic and environment interaction is one of the main limiting factors in assessing genetic gains, especially in regions with a high variability in environmental conditions and large yield variations between years, as is the Mediterranean case (Sánchez‐Garcia et al. 2013). Under semiarid conditions, cereal yield relies more on rainfall distribution than any other factor (Angás et al. 2006; Díaz Ambrona and Mínguez 2001). Accordingly, under Mediterranean climate conditions, Loss and Siddique (1994) acknowledged that water stress is a major constraint to wheat growth and yield. Therefore, breeding yield gains under rainfed conditions are harder to achieve than under wetter conditions or where irrigation is applied, due to the highly variable seasonal rainfall (Richards et al. 2002).
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This fact has entailed that the increase in yields has not been distributed uniformly across environments. The improved‐seed fertilizer technologies had greater effect in irrigated and more favourable rainfed areas, where breeders were able to overcome the most limiting factor in these areas (the poor response of traditional varieties to improved fertility by inputs use), while marginal environments with moisture as the main limiting factor remained relatively unaffected (Byerlee and Morris, 1993; Ceccarelli et al. 2004). At this regard, a global simulation well correlated with observations from FAO by Bondeau et al. (2007) found that temperate dry climate regions with limited irrigation possibilities and low fertilizers inputs performed lower cereal yields than regions with favourable climatic conditions. Since the introduction of semidwarf wheat varieties in the 1960s, their adoption grew steadily. By the end of the last century, semidwarf wheat varieties covered over 80% of all developing country wheat area (Heisey et al. 2002). Adoption of modern cultivars has been very high in irrigated lands, while it has been lower in rainfed areas (Byerlee and Moya, 1993). Nevertheless, despite the modest yield gains compared with irrigated counterparts (Byerlee and Morris, 1993), the adoption of semidwarf varieties grew quickly in rainfed areas during the 1980s. It has been estimated that, since the initial Green Revolution varieties were released, wheat yields have risen by about 1% annually, roughly 50 kg ha‐1 yr‐1, and less than 0.5% per year, less than 10 kg ha‐1 yr‐1, in favourable and marginal areas, respectively (Byerlee and Morris, 1993). This fact has involved an increased in inequality between regions, as marginal have become food‐scarce areas and have had to import higher amounts of cereals produced in favourable environments (Byerlee and Morris, 1993). In plant breeding programs, only a small step of the process of selection is carried out under farmers’ field conditions. Thus, it is highly uncertain whether the material discarded could have been useful in the real conditions of farmers’ fields (Ceccarelli and Grando, 2002). These authors emphasized that this is specially affecting varieties grown under environmental conditions that are not represented by experimental stations (Tekle et al. 2000), as water stress conditions. Thus, it is likely that modern high‐yielding varieties are not the best option for low‐input and marginal farming conditions (Ceccarelli et al. 2004). Accordingly, the selection of varieties under optimum conditions such as those found on research stations, tends to produce superior varieties regarding landraces under improved management, but not under low‐input conditions or stressful environments (Ceccarelli and Grando, 2002). Frequently, landraces yield more than modern varieties under low‐input and stress conditions (Ceccarelli et al. 2004). Although it is possible to find modern cultivars with similar yields than landraces under stressful conditions, this is not that common. According to that, modern varieties have not been as widely adopted in marginal environments as in favourable environments, and four main factors have been depicted to explain why (Byerlee and Morris, 1993):
(i) Lower yield gains of modern varieties as compared to more favorable environments. (ii) Low use of inputs. Low and erratic rainfall makes the use of fertilizers a risky task,
and, thus, modern varieties are less attractive. (iii) Higher grain quality of traditional varieties is not compensated with the low yield
increments of marginal areas; thus, farmers prefer to develop their own market with high quality grains.
(iv) The dual‐purpose role of wheat in marginal regions where livestock is still important in farming systems.
Therefore, old varieties could constitute a suitable alternative for growing environments far from optimum yields conditions. Pecetti and Annicchiarico (1993) compared the performance
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of recently bred cultivars and landraces in Northern Syria under water stressful conditions, and found that some of the landraces evaluated showed similar yield responses than recently bred cultivars, and even some landraces proved to be more stable without significantly jeopardizing the yield performance. That is, landraces were more tolerant to unfavourable conditions performing higher yield stability than genetically improved cultivars, while harboring a satisfactory yield potential under better environmental conditions. In Australia, the negative effect of a limited water supply on the rate of genetic yield improvement has also been reported (Siddique et al. 1989). In addition, Betkas et al. (2016) found that wheat bred during the Green Revolution tended to fail under low input conditions at higher elevations and colder winter temperatures. 1.3.6. Adaptations of old cereal varieties
Many evaluations of landraces have shown high adaptation to unfavourable conditions, as changing climate conditions and stressful environments due to their genetic structure, buffering capacity and a combination of morpho‐physiological traits conferring adaptability to stressful conditions (Jaradat, 2011). Landraces germplasm has been recognized as a potential source for drought‐adaptive traits in breeding programmes (Reynolds et al. 2007). For example, Mohammadi et al. (2015) evaluated a collection of 380 landraces entries under rainfed conditions, showing relevant adaptations to biotic and abiotic stresses. In addition, under typical Mediterranean conditions such as low soil fertility and high environmental stress, the grain yield of tall durum wheat landraces is not penalized compared to that of modern cultivars (Giunta et al. 2017).
1.3.6.1. Root biomass
Root system is the major plant organ for water and nutrient uptake, and it is linked to plant growth and grain productivity (Palta and Yang, 2014). On the one hand, it is thought that a vigorous root system can increase wheat yields under water‐limited conditions by being able to absorb more water, which is especially relevant in environments such as the Mediterranean type, where crops rely on seasonal rainfall (Palta et al. 2011). On the other hand, there is also the argument that, since roots are a major sink for assimilates (Passioura 1983), reducing root biomass would leave a higher amount of assimilates for aboveground biomass formation, including grain yield (Siddique et al. 1990). Landraces have a greater root:aerial biomass ratio (Bektas et al. 2016; Siddique et al. 1990; Subira et al. 2016) and larger root systems (Waines and Edhaie, 2007) than semi‐dwarf wheat varieties, whose genes introduction during 20th century in the Mediterranean area provoked a greater decrease in root biomass than in aerial biomass (Subira et al. 2016). This could be due to the fact that most breeding programs only involved the observation and selection of aboveground parts, along with little available genetic variation among parent lines, all of which hinders the ability to select for a larger root system (Waines and Edhaie, 2007). It has been suggested that the small root systems of some “green‐revolution wheats” allow them to perform well under optimum conditions, “but poorly in drought and heat‐stressed conditions” (Waines and Edhaie, 2007). The higher total root biomass of landraces has been positively related to shoot biomass and plant height (Bektas et al. 2016). Larger root systems allow to explore deeper soil layers in searching for water (Richards et al. 2002; Song et al. 2009), N and other nutrients (Feil, 1992). Song et al. (2009) found that belowground biomass is a determining factor for crop competition, and the higher root biomass of old varieties confers them a higher competition ability than modern ones when grown under mixture. Likely, greater root systems of Mediterranean
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landraces might enable a higher soil water uptake in deep soil layers and thus a better adaptation to rainfed conditions (Annicchiarico and Pecceti, 2003; Bektas et al. 2016). In this respect, seminal roots of barley landraces are considered a useful adaptation to stress conditions, as they represent the only roots that plant produces in dry years (Grando et al. 2001). Contrastingly, under no water stress, enhanced root growth of old varieties during grain filling can result in the reallocation of resources during grain development with a reduced grain yield as a consequence (Stoppler et al. 1990). In addition, higher root biomass involves higher belowground C input, constituting a relative advantage for C sequestration of old wheat varieties. It is known that root‐derived C has a longer time of residence in soil than shoot‐derived C, due to the higher recalcitrance, physico‐chemical protection in deep soil horizons and physical protection through mycorrhiza and root‐hair activities (Rasse et al. 2005). Promoting root growth can lead to increased SOC storage (Paustian et al. 2016). Root‐derived C constitutes 30‐90% of total organic input of agroecosystems (Kätterer et al. 2011), and the need to include roots in cropland C assessments has recently been highlighted (Aguilera et al. 2018).
1.3.6.2. Aerial biomass production
As stated before, modern wheat varieties experienced an unprecedented harvest index increase throughout the past century (Smil, 1999b). The increase in harvest index was related to an increase of biomass partitioning to the grain and a decrease in straw yield associated with a reduction in height (De Vita et al. 2007; Giunta et al. 2007; Motzo et al. 2004). Although some authors have reported no changes in total aerial biomass due to the increase in harvest index (Royo et al. 2007), a decrease has also been reported. For example, Subira et al. (2016) and Álvaro et al. (2008) found lower total aerial biomass for modern wheat varieties due to the introduction of dwarf genes. While total plant biomass of modern genotypes may not be reduced when water is not limited (Miralles and Slafer, 1977), old cultivars produce higher amounts of aboveground biomass than modern ones when grown under water stress conditions (Álvaro et al. 2008; Blum, 2005). Likely, the higher production of biomass may be related to a greater ability to acquire water from deeper soil layers (Bektas et al. 2016), their greater early vigour (Annicchiarico and Pecetti, 1995) and their tolerance to terminal drought stress (Ali Dib et al. 1992). In addition, old wheat cultivars have a greater inter‐cultivar competitive ability, and are able to produce higher yields and aboveground biomass than modern ones when grown in mixtures cropping (Song et al. 2009). Crop residues are a valuable renewable resource, essential for a sustainable agriculture and providers for important ecosystem services (Smil, 1999b). Producing higher amounts of crop residues such as straw can be of great relevance for soil quality and for agriculture sustainability under a climate change scenario. In section 1.5. some benefits of straw and crop residues incorporation to the soil after the harvest are described.
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Figure 4. Old (left) and modern (right) wheat varieties cultivated under organic and rainfed Mediterranean conditions. Differences in straw production can be identified. Picture was taken at our field experiment in Sierra de Yeguas (studies 2 and 3).
In Mediterranean rainfed agroecosystems where cereal‐livestock integration is still relevant for farmers and rural economies, wheat straw is still an important resource, and farmers choose to grow old wheat cultivars instead of semi‐dwarf varieties due to their higher straw production (e.g. Ben Amar, 1997). Low harvest indexes can allow for higher biomass production in a context of low N availability (Sinclair, 1998), being functional in a context where straw had a great value. Along with the higher straw production, the grain protein content of old wheat varieties also contributes to their suitability as dual‐purpose varieties with respect to modern ones under Mediterranean semi‐arid farming systems (Giunta et al. 2017). Indeed, their disadvantage related to the susceptibility to lodging has been suggested to be reduced by grazing (Christiansen et al. 1989), and thus could be overcome under the dual‐purpose context. The greater availability of straw can help to avoid overgrazing of marginal land, hindering land degradation in some semi‐arid Mediterranean areas (Annicchiarico et al. 2005), and contributing to the sustainability enhancement needed in these areas. From an economic point of view, the higher straw production of tall wheat varieties can offset the loss of benefits from a reduced grain yield, providing the farmers with a higher economic stability (Annicchiarico et al. 2005). Finally, even the seek for new energy feedstocks can recapitalize the ability of dual‐purpose old cereal varieties to produce more straw biomass without significantly reducing grain yield (Townsend et al. 2017).
1.3.6.3. Nitrogen uptake and grain nitrogen content
Improving N use efficiency in semi‐arid systems has been said to enhance sustainable farming systems (Liu et al. 2016). Following Barraclough et al. (2010), there are two ways of improving N efficiency in wheat fields, improving N‐fertilizer management, or growing better crop varieties. For this latter purpose, the inclusion of landraces in the germplasm pool for breeding processes has been proposed (Hawkesford, 2014). Breeding activities have resulted in changes in N partitioning with an increase in nitrogen harvest index (NHI) but without changes in total N uptake (Barraclough et al. 2010; Motzo et al. 2004). In this regard, Guttieri et al. (2017) recently suggested that genetic yield gains of modern cultivars comparing to old ones is due to increased dry matter, rather than increased N efficient uptake and remobilization. In wheat, the remobilization of assimilates stored before anthesis to grain is a relevant trait to adaptation to dry Mediterranean environments (Moragues et al. 2006b), and the proportion of pre‐anthesis accumulated N remobilized to grain can reach 73‐
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82% of grain N (Masoni et al. 2007). Wheat landraces can uptake and translocate more N into the grain than modern ones, presumably due to higher pre‐anthesis uptake and an increased buffering capacity in genotypes with high vegetative biomass (Jaradat, 2011). Modern cultivars are adapted to uptake the surface fertilizer N, and their lower root system hinder their acquisition of deeper soil N making them less adapted at recovering soil N than old wheat cultivars (Foulkes et al., 1998). Additionally, Baresel et al. (2005) suggested that the higher root of Mediterranean landraces can be advantageous for N uptake efficiency. In addition, old cereal cultivars typically have higher N concentration in grains than modern wheat (e.g. Nazco et al. 2012) likely due to a negative correlation between grain N and grain yield and a dilution effect (Calderini et al. 1995; Barraclough et al. 2010). In other words, modern cultivars have been bred for higher harvest indixes, and this is not necessarily correlated to higher translocations of N into the grain because of a genetic dilution effect: the selection for higher grain yield dry weight was related to the selection for lower nutrient grain concentrations (since nearly 80‐90% of the grain dry weight is carbohydrate) (Fan et al. 2008; Davis 2009). That is, grain N concentration dilution could be a negative effect derived from the introgression of dwarf genes (Casebow et al. 2016). This tradeoff could have deeper negative impacts in grain protein concentration considering the global context of diminished N concentration in terrestrial vegetation, linked to the increase in atmospheric CO2 concentration (Craine et al. 2018). At this respect, rising atmospheric CO2 concentration has been proved to have an effect on the decrease of protein concentration of many crops (Taub et al. 2008), including wheat (Fernando et al. 2012, 2015) as mentioned before. Regarding organic production, a higher protein content in grain is especially relevant, as it is usually lower than under conventional farming systems (e.g. Nitika et al. 2008). Under organic fertilization conditions of slow N release, it is important to identify wheat cultivars able to meet these conditions of N availability (Büchi et al. 2016). Modern wheat, less adapted to acquire soil N and more dependent on N fertilizer, could be less suitable than old ones under organic and low‐yielding conditions, unsatisfying the necessary levels of crude protein to milling industry (Baresel et al. 2005).
1.3.6.4. Grain micro‐ and macronutrient content
The Green Revolution could also have another unexpected consequence, as the increase in prevalence of micronutrient malnutrition coincides with the global expansion of the high‐yielding and high‐input cultivars (Welch and Graham, 2002). Several evaluations of micro‐ and macro‐ nutrient contents of old and modern wheat cultivars have been carried out (e.g. Murphy et al. 2008b), also under Mediterranean conditions (Sciacca et al. 2018). Although it has been found that there is not a genetic trade‐off between yield and mineral concentrations (Murphy et al. 2008b), the selection by breeders had a pleiotropic effect on the grain micronutrient content of modern wheat (Sciacca et al. 2018). An experimental comparison of 56 old wheat varieties and 7 modern cultivars under rainfed conditions by Murphy et al. (2008b) revealed that concentration of copper (Cu), iron (Fe), magnesium (Mg), manganese (Mn), phosphorus (P), selenium (Se), and zinc (Zn) had decreased with modern breeding. Garvin et al. (2006) found similar conclusions for Fe, Zn and Se, and Hussain et al. (2010) found higher concentration of Fe and copper (Cu) in old cultivars than in modern ones under organic conditions. This advantage has been translated into the number of wheat‐based product portions made from modern and old wheat cultivars necessary to achieve the same levels of micronutrients. A study of the nutritional value of whole wheat bread unveiled that a higher amount of bread made from modern varieties is needed to equalize the micronutrient consumption from old wheat bread (Murphy et al. 2008b). These authors stated
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that “although the increased yield of modern cultivars could potentially increase the mineral content per acre of grain production, the mineral concentration per seed or loaf of bread is reduced” (Murphy et al. 2008b). Grain nutrient concentration can be related to the year of release of the variety, but also with the water regime (Bassiri and Nahapetian, 1977) and the cropping system management (e.g. Ryan et al. 2004). Ryan et al. (2004) found that conventionally grown wheat had lower Zn and Cu but higher Mn and P than organically grown grain. They linked these differences to soluble P fertilizers, that are able to increase P uptake, but consequently reducing mycorrhizal colonization and thereby reducing Zn uptake and enhancing Mn uptake; and past lime applications on the organic farm of their experiment decreasing Mn availability. In this regard, Jaradat (2011) highlighted that arbuscular mycorrhiza could increase the active absorbing root surface with minor cost to the wheat plant, fostering the uptake of P, in particular, and other macro‐ and micro‐nutrients, in general. Additionally, mineral nutrients’ concentration in the grain can be negatively affected by an increment of CO2 concentration (Beleggia et al. 2018), and this has also been reported under Mediterranean conditions (Fernando et al. 2012). Therefore, similarly to grain N concentration, the effects of GHG emissions go beyond grain yield and can impact grain quality and, therefore, human nutrition (Fernando et al. 2012). Old wheat varieties can constitute an alternative under rainfed Mediterranean conditions to overcome negative effect of climate change in human nutrition and counteract the lower grain mineral nutrient and protein concentration of modern wheat cultivars. Provided that the most suitable genotype to maximize grain nutrient content of wheat is selected (Hussain et al. 2010), organic farming conditions can provide for high wheat quality with lower harmful inputs (Mäder et al. 2007). From now on, the research paradigm should be redirected and focus, instead of maximizing yields at any expense, towards enhancing crop diversity and “nutritional quality of agricultural products to meet human dietary demands” (Welch and Graham, 2002).
1.3.6.5. Drought resistance
Limited water availability is the main factor limiting crop production in Mediterranean drylands. Therefore, under a climate change scenario, the increase in crop drought resistance is increasing in relevance among researchers, farmers and breeders (Ceccarelli et al. 2004). Under semi‐arid Mediterranean conditions, yield losses related to drought events can be of greater relevance than in irrigated agriculture, and measures should be taken to avoid negative impacts (Tigkas and Tsakiris, 2015). Cereal landraces have been said to have a great tolerance to water stress (Blum et al. 1989; Grando et al. 2001; Pecetti and Annicchiarico 1993; Pecetti et al. 1994). They are able to extract water from deeper soil layers, and they show relatively higher biomass production under drought conditions than modern cultivars (Reynolds et al. 2007). Many traits of old varieties have been linked to this drought adaptation. Root dry weight at depth (Lopes and Reynolds, 2010), phenology and tiller production and survival (Blum et al. 1989) and early vigour (Moragues et al. 2006b) are traits of landraces with a positive effect on yield under drought stress. In a similar way, the early ground cover (Reynolds et al. 2007; Sener et al. 2009) due to early crop development of old wheat varieties contributes to their adaptation to Mediterranean drylands (Moragues et al. 2006a), increasing grain yield because of the shade driven diminishment of soil water evaporation (López‐Castañeda and Richards, 1994) and, therefore, increasing water use efficiency (Moragues et al. 2006b; Richards et al. 2002). In this regard, Giunta et al. (2017) suggested that the choice of vigorous varieties under rainfed Mediterranean environments could be relevant because sowing cannot take place before October, as low and
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erratic rainfall of September hinder it. A higher value of stem carbohydrates concentration shortly after anthesis of landraces have also been related to drought resistance (Reynolds et al. 2007). Interestingly, Cecarelli et al. (2004) highlighted that “limitation and/or competition for resources, and the need to avoid further environmental degradation, suggest to many scientists that fitting crops to the environment is a more sustainable strategy than modifying the environment to fit the crops, particularly for drought‐affected areas”. Thus, adaptive traits to drought stress of old cereal varieties can constitute and advantage for sustainability under rainfed and semiarid conditions.
1.3.6.6. Weed competition
Modern genetic breeding made modern varieties more dependent on the presence of herbicides (Lemerle et al. 2001b), and cultivars grown before historical herbicide employment expansion are often more competitive against weed (Murphy et al. 2008a; Vandeleur and Gill, 2004; Wicks et al. 2004). The higher yield of modern cultivars has been linked to a decrease in inter‐genotype competition (Donald, 1981; Reynolds et al. 1994) and, thus, lower competitiveness against weeds (Lemerle et al. 2001a). The result is that a greater yield loss in the presence of weeds is usually observed with modern cultivars (Vandeleur and Gill, 2004), while the competitive ability of old cereal cultivars is related to reductions in above ground weed biomass (Bertholdsson, 2005; Christensen, 1994: Drews et al. 2009; Lemerle et al. 1996; Murphy et al. 2008a; Olesen et al. 2004). The weed suppression ability of old cereal cultivars could be due to the high early biomass accumulation, the large number of tillers (Lemerle et al. 1996), lodging (Wicks et al. 2004) and the higher height (Lemerle et al. 1996; Murphy et al. 2008a; Piliksere et al. 2013; Vandeleur and Gill, 2004). Weed infestation is a major concern for organic production, and organic fields should be handled with care to avoid important weed‐related reduction in yields (José‐María and Sans, 2011). Many studies have reported higher weed biomass production for organic fields (Sadowski and Tyburski, 2000; Hyvönen et al. 2003) and lower weed biomass production have been associated to herbicide application (Armengot et al. 2013; Tørresen et al. 2003). Nevertheless, a fast nutrient release, characteristic of chemical fertilizers, is advantageous to weeds (Liebman and Davis, 2000), and can be responsible of their high competitiveness under conventional managements (Scursoni et al. 2012). All in all, weed suppression ability is one of the most required traits under organic systems (Bertholdsson, 2007; Lammerts van Bueren et al. 2011), particularly since this ability may continue to decline as newer wheat cultivars (whose selection might ignore weed competition traits) are released to fields and farms (Murphy et al. 2008a).
1.3.6.6.1. Allelopathy
Allelopathy is the mechanism explaining inhibitory and/or stimulatory interactions in soil–plant interface through bioactive products produced by biochemical pathways (Khaliq et al. 2011). This trait has been described for several crops (Singh et al. 2003), and also for wheat under Mediterranean conditions (Bensch et al. 2009; Oueslati, 2003), relating its effect with a reduction of weed biomass or an increase in wheat competition against weed. In wheat, three families of compounds are responsible of phytotoxicity: hidroxamic acids and their derivates, phenolic acids and short chain fat acids (Aslam et al. 2017; Wu et al. 2000, 2001a, 2001b). Many researches have shown wheat ability to control weeds due to its allelopathic capacity, through aqueous extract (Khaliq et al. 2012; Mahmood et al. 2013; Ma, 2005), mulching (Mahmood et
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al. 2013), growing crops in field trials (Bertholdsson, 2012; Mahmood et al. 2013) or as a cover crop (Wu et al. 2001b). Nevertheless, this trait could have been lost during breeding for higher yields (Wu et al. 1999), as the research stations conditions of breeding programs were characterized, among others, by the use of herbicides (Worthington and Reberg‐Horton, 2013). The growing concern about developing sustainable cereal production systems has led to the study and search for these compounds in many cereal varieties during last decades, giving a special role to old cereal cultivars (Bouhaouel et al. 2018; Vasilakoglou et al. 2009). In this respect, some studies have found higher allelopathy activity from old wheat extracts than for modern ones (Bertholdsson, 2005; Bouhaouel et al. 2015; Oveisi et al. 2008), which could underlie the higher weed suppression ability of old wheat varieties described in last section, or constitute a relevant proportion of it (Reiss et al. 2018). Clearly, weed competition ability of old wheat varieties in general, and allelopathic capacity in particular, represent an interesting trait for both organic and conventional systems (Mason and Spaner, 2006), especially under semi‐arid rainfed conditions, where water stress reduces the yield penalty comparing to modern cultivars. In conventional farming systems, this trait could help reducing herbicide dependence (Worthington and Reberg‐ Horton, 2013; Weiner et al. 2001; Hoad et al. 2012; Wolfe et al. 2008) and the associated environmental damage (Freemark and Boutin, 1995; José‐María and Sans, 2011; Pimentel et al. 1996). Particularly, the appearance of herbicide resistance in weeds (Heap, 2018; Hoad et al. 2012) can also increase the interest for alellopathy, as allelochemicals from wheat have been reported to inhibit some herbicide‐resistant weed species (Wu et al. 2001). Weed suppression ability is of special interest for organic farming systems (Lammerts van Bueren et al. 2011; Murphy et al. 2008a), since organic farmers are not allowed to use chemical herbicides and one of the main challenges during conversion to organic production is weed control (Sumption et al. 2004). In addition, under organic farming the aim is not to eradicate weeds as synthetic herbicides do, but to maintain their population under an acceptable threshold that do not jeopardize crop yields. Weeds, besides being competitors for light, nutrients and water, also offer ecological and agronomic services, such as nutrient recycling (Clergue et al. 2005), biodiversity (Chamorro et al. 2016) and SOC sequestration (Aguilera et al. 2018; De Sanctis et al. 2012). In this respect, cultivars with higher weed suppression ability could complement cultural methods for weed control, allowing the coexistence of weed plants with reduced vigor (Fitter, 2003) without eradicating them and benefits they provide. Besides organic farming, allelopathic ability can result in relevant benefits for marginal areas with small‐scale and low‐input agriculture, where it has been considered a key player in the sustainability of farming systems and to enlarge profit margins for farmers (Makoi et al. 2012). 1.4. ORGANIC FARMING
1.4.1. Benefits, challenges and limits of Organic Agriculture
In this part, the main benefits and constraints of organic agriculture will be addressed. I will focus on the production phase, and not on the whole agri‐food production systems, since it escapes the aims of this dissertation. In the same way, issues regarding organic livestock will not be covered. Organic Agriculture has been defined as a “production system that sustains the health of soils, ecosystems and people; relies on ecological processes, biodiversity and cycles adapted to local conditions, rather than the use of inputs with adverse effects; and combines tradition,
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innovation and science to benefit the shared environment and promote fair relationships and a good quality of life for all involved” (IFOAM, 2005). Organic farming systems should rely on preventive rather than curative methods and focus on indirect, long‐term strategies of enhancing systems resilience (Lammerts van Bueren and Myers, 2012). It aims to strengthen ecological processes that foster plant nutrition and soil and water resources conservation, reducing agrochemicals use and improving environmental and economics benefits of the farm (Pimentel et al. 2005). Organic farming is widely regulated at an international level, with many similarities among different states (Seufert et al. 2017). A recent analysis showed that, together with non‐synthetic inputs utilization, organic farming practices make use of natural processes to manage the agroecosystem such as “the use of crop and animal species with high resistance to pests and diseases, or to use crop rotations and cover crops for crop nutrient management”. It is relevant, thus, that the application of non‐prohibited inputs “should only be considered a last resort, when other measures have failed to achieve the intended management goal” (Seufert et al. 2017). Many studies report that organic agriculture yields lower than conventional one (Reganold and Wachter, 2016). In Spain, the average yield gap is 23% for crops cultivated under both farming systems (Alonso and Guzmán, 2010). However, once an organic system has matured, that is, the soil quality and the community of natural enemies are restored and as farmer’s skills have had enough time to develop and get used to the new organic practices (Jordan, 2002; Pimentel et al. 2005), production can be comparable or even higher than that of conventional farms (Jordan, 2002). A research of a global dataset of 293 cases of yields comparing organic versus conventional production estimated that organic farming could produce enough food on a global per capita basis to sustain current human population, and an even larger one if production comes from the developing world, without incrementing the land devoted to agriculture (Badgley et al. 2007). Nevertheless, these studies comparing crop to crop usually do not include the land cost of generating the nutrients’ flows destined for crops in organic agriculture, nor the territory dedicated to biodiversity that organic farms must incorporate (González de Molina and Guzmán, 2017). In organic systems, N crop supply highly depends on the N fixation by legumes in crop rotations (Watson et al. 2002), and this leguminous input could meet N requirements if organic production supplied global food demands (Badgley et al. 2007). Nonetheless, this strategy has a land cost that must be considered (González de Molina and Guzmán, 2017). The adoption of agroecological techniques such as crop rotations, the use of legumes as green manure, the utilization of agroindustry wastes as fertilizers and more efficient water management (González de Molina and Guzmán, 2018; Ponisio et al. 2015; Pretty et al. 2003), can diminish and even nullify the yield gap between both farming systems. These strategies for agroecological intensification reduce the land cost of sustainability (LACAS), making feasible on the large scale conversion of conventional farming to organic one (González de Molina and Guzmán, 2017). On the other hand, many studies have shown that organic cropping systems present lower long‐term yield variability (Smolik et al. 1995; Lotter et al. 2003). Contrastingly, Benincasa et al. (2016) found higher grain yield variability across years in organic versus conventional low input systems. A recent meta‐analysis did not show significant differences in yield stability when comparing organic and conventional horticulture, concluding that long‐term trials are needed to elucidate this (Lesur‐Dumoulin et al. 2017). Pimentel et al. (2005) found that for dry years, organic practices resulted in higher yields than conventional farming ones, thus environmental conditions can also influence on the size and sign of the yield gap. Finally, when comparing the productivity of both systems, all “yield‐forming and reducing factors” should be accounted for in the sustainability evaluation of organic systems (Van Bruggen and Finckh, 2016).
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What is undoubtedly is that organic practices have more benefits than conventional management, both in a social and environmental basis (Figure 5).
The crop yields and economics of organic systems, compared with conventional systems, appear to vary based on the crops, regions, and technologies employed in the studies. However, the environmental benefits attributable to reduced chemical inputs, less soil erosion, water conservation, and improved soil organic matter and biodiversity were consistently greater in the organic systems than in the conventional systems (Pimentel et al. 2005).
Figure 5. A flower diagram comparing production, environmental sustainability, economic sustainability and wellbeing between organic and conventional farming systems. The length of the petals indicate the level of performance of each sustainability metric relative to the four concentric cicles representing 25, 50, 75 and 100%. Source: Reganold and Wachter, 2016.
Organic farming has been linked with valuable environmental benefits (Mondelaers et al. 2009; Tuomisto et al. 2012) and to ecosystem services protection (Sandhu et al. 2010). Organic practices have been related to the preservation of biodiversity (Bengtsson et al. 2005; Ponce et al. 2011; Puig‐Montserrat et al. 2017; Romero et al. 2008; Tuck et al. 2014), a reduction in plant root diseases due to higher soil quality (Van Bruggen and Finckh, 2016), a higher SOM content (Bai et al. 2018), an increase in microbial biomass, activity and diversity (Birkhofer et al. 2008; Hartmann et al. 2015; Lori et al. 2017), a decrease in soil erosion through the augmentation of aggregates stability (Siegrist et al. 1998), an enhanced nutrient crop uptake through the positive effect on arbuscular mycorrhizal fungi (Gosling et al. 2006; Hildermann et al. 2010) ‐unlike conventional farming, where application of N fertilizer has been related to negative impact to arbuscular mycorrhizal (Avio et al. 2013)‐, a lower potential risk of soil N lossess and higher long‐term soil N storage (Migliorini et al. 2014), an increment of natural enemies in agroecosystems (Birkhofer et al. 2008), a higher energy efficiency and renewable energy use (Alonso and Guzmán, 2010; Smith et al. 2015a) and a better food quality (Lairon and Huber, 2014). Indeed, organic farming has been linked to better social wellbeing and economic performance (Crowder et al. 2015; Reganold and Wachter, 2016) than conventional agriculture. Some of these factors related to organic and conventional farming comparison are represented in Figure 5. Organic manures have a major role in organic farming benefits, showing complex interactions with soil fertility and soil biota diversity (Hartmann et al. 2015). So much so that is the relevance of organic ammendments shaping the soil microbiota, that the plant protection measures appear to be of subordinate importance as factor impacting soil organisms (Hartmann et al. 2015). These C‐based amendments, along with the use of crop rotations and cover crops, are indispensable practices to build soil fertility: they help to increase biologically available SOM and
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beneficial soil microbe and invertebrate activities and to improve soil physical properties (Reeve et al. 2016). Even under chemical fertilizer applications, Büchi et al. (2016) found from a 40‐year long term experiment that manure application had positive effects on crop (wheat) performance mainly due to indirect long term effects on soil properties. The role of legumes rotation and manure availability in organic farms has been related to the yield gap between organic and conventional farming (De Ponti et al. 2012). Farm diversification through the use of crop rotations and the care of soil fertility through organic matter amendments have been identified as practices enhancing the climate change adaptation potential of organic farming (Scialabba and Müller‐Lindenlauf, 2010). Contrarily to these mentioned benefits, a major challenge has been described for organic farming in last decades. The growth of the organic sector as an industry has entailed it “conventionalization”, a process whose implications are under debate (Constance et al. 2015) and whose reach depends on the context and territorial realities (García‐Ramos et al. 2018). It is agreed upon that conventionalization lead organic agriculture to mirror the industrial approach of conventional farming regarding the scale, the structure, the specialization and the profit‐oriented against environmental‐oriented concerns of farmers (Best, 2008; Cakirli et al. 2017; Constance et al. 2015). It affects the whole agri‐food chain and distances organic farming from environmental, social and economic sustainability (García‐Ramos et al. 2018). This process occurs under the umbrella of Institutions (Wit and Verhoog, 2007), which are favorable to conventional farming and, ultimately, responsible for the reliance of organic farming on public funding and international markets, both proposed sources of conventionalization (Ramos‐García et al. 2018). Public funding and regulations are not focused on the design of sustainable agroecosystems (García‐Ramos et al. 2018). Additionally, and albeit in the short‐term markets can promote positive changes in the agri‐food system, the capitalist market imposes dynamics such as the reductionism of organic standards, the limitations of private organic certification and the prices competitiveness (Allen and Kovach, 2000), responsible for the distancing of organic farming from sustainable approaches. Ultimately, these constraints push farmers to the reduction in crop rotations and diversification and the decrease in functional biodiversity (García‐Ramos et al. 2018), which entails negative effects on energy use and nutrient recycling (De Wit and Verhoog, 2007). At the end, the value of a more sustainable way of food production is reduced to the mere substitution of conventional inputs by other ones allowed by the organic standards (De Wit and Verhoog, 2007), risking economic and environmental sustainability (García‐Ramos et al. 2018). Connected to what has been previously mentioned, the substitution of inputs and the simplifications of organic practices under minimum standards, lead to outsource the land cost of sustainability, since farmers become less attached to their local resources to sustain the production (García‐Ramos et al. 2018). Although conventionalization does not nullify benefits of organic practices, it can constrain its sustainability and positive effects (García‐Ramos et al. 2018). Therefore, it is essential to develop strategies to mitigate conventionalization progress through the reduction of external inputs (De Wit and Verhoog 2007). In a study assessing organic farming in Andalusia, García‐Ramos et al. (2018) recommended some measures to counteract the conventionalization of organic farming in Spain and to help focusing on a more agroecological approach, such as “the growth of organic food (processing) industries, the continuation of policies of public purchase of organic products, the increased domestic production of inputs (organic fertilizers and seeds), better crop/livestock integration, or a revision of the contradictions in current organic regulations”.
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1.4.2. Climate change mitigation and adaptation
There can be found three broad types of climate change mitigation strategies, namelly, the GHG emissions’ redution, the C sequestration potential and the emissions’ replacement (Smith et al. 2008). The improvement of soil C storage and N use efficiencies are good indicartors of the synergies between adaptation and mitigation strategies in agriculture (Smith and Olesen 2010), as both of them are key for mitigation and adaptation to climate change of agricultural lands. Concretely, besides the environmental benefits related above, organic farming can offer some climate change mitigation opportunities. The reduction in N2O through a thoughtful use of nutrients, the increase in SOC sequestration and the avoidance of mineral fertilizers are main factors contributing to the mitigating role of organic farming (Scialabba and Müller‐Lindenlauf, 2010). When comparing organic farming with conventional farming GHG emissions, different performance has been reported. For example, some authors have reported lower GHG emissions per unit production area for organic farming (Chiriaco et al. 2017; Gutiérrez et al. 2017; Skkiner et al. 2014). However, if lower yields under organic management are considered (Seufert et al. 2012), higher yield‐based emissions could be found (Chiriaco et al. 2017; Fedele et al. 2014; Skinner et al. 2014). On the contrary, Aguilera et al. (2015a,b) found lower yield‐based emissions under organic management for most crop types under Mediterranean conditions, with the production of fertilizer as a main responsible for higher emissions under conventional management. At this regard, “more research is needed to better explore the potential of organic farming and to improve organic food production, optimizing the balance between the use of resources and yields, to ensure sufficient organic food supply at global levels” (Chiriaco et al. 2017). Beyond GHG emissions reduction, the increase in SOC due to organic farming (Aguilera et al. 2013a; Parras‐Alcántara and Lozano‐García, 2014) can contribute to reduce the C footprint of food products. The use of catch crops, mulches and organic ammendments can lead to the increase in C sequestration rates over time (Keesstra et al. 2018), contributing to climate change mitigation through the reduction of C loss due to soil erosion, the avoidance of OM decomposition during sediment transport (Novara et al. 2016) and the offset of GHG emissions from agricultural soil (Gattinger et al. 2012). Some authors are sceptics whether increased SOC stocks due to organic farming derivated from imports of organic matter that would have had another fate constitute an aditional transfer of C from the atmosphere to land (Powlson et al. 2011). However, even comparing organic practices that avoids external C inputs, conventional farming shows lower SOC concentrations and stocks than organic ones (Gattinger et al. 2012). Recently, García‐Palacios et al. (2018) suggested that crop residues traits related to higher decomposability (higher N concentration in leafs and roots) and soil C losses under conventional farming systems can also be responsible for the lower SOC sequestration in conventional and organic farming comparissons, beyond the intensity on the use of manure. Thus, the focus on certain traits of crop species and cultivars should be of interest for the development of SOC sequestration strategies (García‐Palacios et al. 2018) and, consequently, for the climate change mitigation goal. In this context, the use of varieties with higher residue production would represent a net increase in C input respect to lower residue yielding varieties, thus constituting a additional transfer of atmospheric C into the soil. Additionally, and following findings by García‐Palacios et al. (2018), the use of varieties that are less easily decomposed can have a relevant role aswell.
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Finally, Gattinger et al. (2012) concluded that C sequestration should not be the only mitigation practice. There is a need for reducing GHG emissions, to which purpose there should be a reduction of the emissions from inputs production, irrigation, farm machinery utilization and manure manipulation. It has been highlighted that the level of affectation by climate change effects relatively depends on the variety chosen (Wang et al. 2007), indicating the need for reevaluating cultivated varieties that are already been grown. Likewise, crop diversification can buffer crop yield from the effect of greater climate variability and biotic stresses, constituting a relevant adaptation strategy to climate change by farmers (Lin, 2011). Nevertheless, some circumstances such as the economic support for a few selected crops, the biotechnological paradigm and the belief that croplands are more productive under monoculture cultivation (Lin, 2011), have relegated this practice to small‐scale and organic farmers. Some other adaptations practices, although not especifically destined to cereal fields, can be relevant to cereal cropping system. For instance, the introduction of water‐conserving tillage practices are assumed to be of especial interest for cereal in dry zones (Olesen et al. 2011). Some authors are confidence on agronomy and modern breeding to be able to overcome main adverse effect on the reduction of crop yields due to climate change (e.g. Anwar et al. 2007). Nevertheless, even if this is achievable to some extent, the depletion of natural resources and the degradation of agroecosystems pose great challenges on changing our bussiness as usual framework to cope with global change in the future. 1.4.3. Organic cereal under Mediterranean rainfed conditions Studies comparing organic versus conventional wheat under Mediterranean conditions generally report higher yields for conventional one (Benincasa et al. 2016; Campiglia et al. 2018; Fagnano et al. 2012; Kitchen et al. 2003), although yield gap size and sing has been found to depend on site (Deria et al. 2003) and weather conditions (Campiglia et al. 2015). For instance, Olesen et al. (2009) found under different climate conditions that yield gap between organic and conventional systems can also vary with soil texture, and Campiglia et al. (2015) found that low rainfall and soil water availability, combined with high temperatures in spring, can be responsible for the grain yield reduction in both organic and conventional Mediterranean wheat. Contrastingly, Migliorini et al. (2014) found similar grain yield in organic and conventionally managed wheat in a 16‐years long term field experiment, thus the field experiment duration can be a relevant factor for organic cereals outputs as well. Accordingly, Lacasta and Meco (2011) run a long‐term trial for more than ten years comparing rainfed organic and conventionally grown cereal in Spain, concluding that water deficit hindered the response to chemical fertilizers, thus leading to similar yields between both farming systems. In the Mediterranean regions, N availability for organic winter cereal production is usually very low (Tosti and Guiduci, 2010), and many studies agree that this lower N availability underlies the lower yields of organic wheat compared to conventional one (Benincasa et al. 2016; Campiglia et al. 2015b; Deria et al. 2003; Kitchen et al. 2003; Mazzoncini et al. 2015). Under more humid climatic conditions, a requirement to improve organic N availability and synchrony between nutrient supply and demand in organic wheat production has also been highlighted (Mayer et al. 2015). Accordingly, lower P supplies have also been reported as relevant factors underlying the mentioned yield gap (Deria et al. 2003; Kitchen et al. 2003). Appropriate N soil availability for organic wheat has to be achieved by an adequate crop rotation and the direct supply of organic fertilizers (Hawkesford, 2014). Migliorini et al. (2014) agreed that the use of green manure and organic fertilizers can help to lower the N deficit in organic
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cereal systems (Migliorini et al. 2014). At this regard, Campiglia et al. (2015) suggested that it might be recommendable to apply organic fertilizers with high available N, such as slurries or blood meal, in the reproductive period of the wheat. However, slurries can be affected by N leaching and has been proved to contribute more to grain yield than to grain protein (Olesen et al. 2009), contrastingly to what has been fond for legume green manure. Additionally, the application of slurries leads to higher GHG emissions than solid organic fertilizer amendments (Aguilera et al. 2013b). Conversely, the use of legumes in cereal croplands can reduce GHG emissions through lowering N fertilizer inputs (Liu et al., 2016) and contribute to rise the grain yield of organic cereals under Mediterranean conditions (Dalias et al. 2012; Ryan et al. 2008a; Tosti and Guidicci, 2010). A temporary cereal‐legume intercropping has been proved to contribute to a late supply of N for organically cultivated wheat (Benincasa et al. 2016), and Migliorini et al. (2014) found comparable grain yields between wheat cultivated with chemical fertilizers and wheat whose N came from legumes biological fixation. However, the land cost of legumes cultivation under typical dry climatic conditions of Mediterranean areas warn us about the need for a deep analysis of the suitability of different strategies to face N deficiencies of organic farming (González de Molina and Guzmán, 2017). The higher weed infestation of organic cereal drylands is another factor that contributes to reduce organic wheat yields (Campiglia et al. 2015; Deria et al. 2003; Kitchen et al. 2003), since mechanical means of organic systems are less effective in reducing weeds than herbicide application of conventional ones (Campiglia et al. 2018). Contrastingly, Mazzoncini et al. (2015) found that this factor was not responsible of yield reductions in organic versus conventional wheat production systems, since weed biomass data were similar for both of them. Campiglia et al. (2015) concluded that an efficient weed control strategy is determinant for an optimum grain yield when weather conditions during the reproductive period of wheat is particularly favorable to weed development (high rainfall in spring). Organic farming requires a proper design of crops rotation and proper cultivars to contribute to improve weed control (Campiglia et al. 2018). Weed infestation can be affected by the origin of organic amendments with differences found between grass‐clover green manure and animal slurries (Olesen et al. 2009), therefore, the strategic design of organic agroecosystem must be done from a comprehensive perspective of factors determining yield and environmental sustainability. Related to previous focus on legumes rotation, some examples such as barley‐legume rotations (Díaz‐Ambrona and Mínguez, 2001) and wheat‐subclover intercropping (Radicetti et al. 2018) have been said to reduce weed biomass of organic cereal under Mediterranean conditions, strengthening the environmental benefits of the inclusion of pulses in cereals rotations under these climatic conditions. However, these cropping systems could need of management strategies aimed to avoid the possible yield gap between them and pure cereal cropping systems (Radicetti et al. 2018). On the contrary, the use of mineral fertilizers can have an adverse effect on the environmental benefits delivered from the use of legumes in cereal systems (Radicetti et al. 2018), while can change weed species composition towards a higher presence of nitrophilous species (Campiglia et al. 2015). Finally, the use of herbicides can lead to a higher frequency of grasses in conventional cereal croplands (Campiglia et al. 2018), likely because they are less efficient on monocotyledons than on dicotyledons. Other factors are said to contribute to this yield disadvantage of organic cereal in Mediterranean drylands. Grain yield and quality are sensitive to interannual variations in temperatures and to the rainfall levels during the growing season (Campiglia et al. 2015). In this sense, the occurrence of low temperatures during the reproductive phase of wheat can hinder N mineralization rate of organic fertilizers, thus supplying less N in organic compared to conventional wheat (Campiglia et al. 2015). These authors found that the yield limitation due to weed competition and N supply strongly depend on weather conditions, relating an excess of rainfall and low temperatures during the reproductive phase of wheat to a higher influence of those constraints.
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They also found that high rainfall levels during grain filling period benefited conventional wheat yield more than organic one, due to weed infestation in latter (former did not showed relevant weed presence due to herbicide). In addition, rainfall distribution and severity can affect N availability due to nitrate leaching. Benincasa et al. (2016) observed that grain yield can be negatively correlated to fall‐winter rainfall for its effects on soil N availability. In this case, rainfall distribution can affect organic wheat yield more than conventional one, if chemical fertilizer are applied more than once during the growing season while organic fertilizers only once before sowing (Mazzoncini et al. 2015). Contrastingly, Kitchen et al. (2003) and Deria et al. (2003) discarded rainfall levels as a cause for lower organic wheat biomass and yield, respectively, that is, they concluded that rainfall do not limit organic systems in a greater level than conventional ones. Rainfall and temperatures interannual variability under Mediterranean conditions underlies the requirement of long term field evaluations to determine crop suitability in organic vs conventional farming systems, especially for rainfed crops, since water and N soil availability is highly affected by the season’s weather (Benincassa et al. 2016). Finally, previous land use of croplands can be responsible for differences between organic and conventional wheat yield due to SOM content, soil structure, weeds and pathogens (Deria et al. 2003). Nevertheless, there are other differences between organic and conventionally grown wheat in the Mediterranean region that should be considered. For example, the higher weed biodiversity in organic cereal fields (Campiglia et al. 2018). Regarding weed seed bank, José‐Maria and Sans (2011) reported higher species richness for organic fields, and highlighted the importance of cleaning crop seeds properly to reduce seedbank size and using complex rotations, especially as this tends to conserve species richness while reducing seed abundance. Additionally, higher N use efficiency (NUE) (Benincasa et al. 2016; Migliorini et al. 2014), higher long‐term soil fertility (Benincassa et al. 2016), or the higher presence of natural enemies, such as aphid predators (Moschini et al. 2012), have been related to organic cereal fields in the Mediterranean region when compared to conventional ones. In this last case, Gosme et al. (2012) showed that even the presence of organically managed wheat fields in the neighborhood had a positive effect on natural enemies of both organic and conventional French cereal fields. Likewise, lower efficiency in distributing dry matter to grain relative to vegetative growth with water stress of conventional wheat is linked to its higher susceptibility to N oversupply, that can lead to a higher vegetative growth and, thus, soil water depletion for the grain filling phase, depressing grain yield (Deria et al. 2003). Contrastingly, lower grain protein levels (Benincasa et al. 2016) and grain quality (Campiglia et al. 2015) have been linked to higher weed infestation and lower N availability in wheat under organic farming conditions in Mediterranean drylands. Contrarily, others studies did not find significant differences in grain protein levels between organically grown and conventionally grown wheat (Deria et al. 2003; Mazzoncini et al. 2015). At this regard, the use of a low decomposing source of N in organic amendments and a suitable amendment timing can help to acquire appropriate protein levels in organic wheat (Olesen et al. 2009). Moreover, the inclusion of a leguminous in rotation with organic cereal (Dalias, 2012; Ryan et al. 2008b) or through the intercropping of both crops, provided an early legume soil incorporation to avoid competition (Tosti and Guiducci, 2010), contributes to improve quality, yield and protein content of grains organically grown wheat under rainfed Mediterranean conditions. With regards to pasta and bread making quality, organic cropping systems usually show lower values. Mazzoncini et al. (2015) suggested that poorer bread making quality of flour from organic wheat than that of conventional wheat under Mediterranean conditions can be due to both its lower protein content and proportions of gliadins and glutenins in gluten. In the same way, low N availability during reproductive phases and the reduction in grain protein of organic wheat can led to lower pasta making quality of wholemeal and semolina comparing to those from conventional wheat (Fagnano et al. 2012). Again, the presence of legumes in long‐term rotations is proposed to
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guarantee the necessary N availability at sustainable cost in organic systems (Fagnano et al. 2012). Interestingly, Mazzoncini et al. (2015) found higher antioxidant properties in bran from organically cultivated wheat, related to N shortages, and similar total phenolics and total phenolic acid in conventional and organic wheat. They concluded that an organic cropping system can maintain or even increase the health properties of end‐use wheat products, provided a reduction in grain yield is accepted (Mazzoncini et al. 2015). Beyond agronomic and quality traits studies, organic farming has been pointed out as a more efficient system to cultivate cereal under Mediterranean rainfed conditions. To this respect, Gutiérrez et al. (2017) recently found that “for the specific Southern Spain region considered (in their study), organic production is technically ad environmentally more efficient than conventional farming system”. These authors considered agronomic and environmental aspects of both farming conditions, and concluded that the higher inputs consumption of conventional farms, although harboring higher yields than organic ones, led to a less efficient overall production process. These authors gathered many studies from the Mediterranean region evaluating conventional and organic farming efficiency, and reported that studies in favor of conventional farming efficiency could have drawn their conclusions only based in economic, overlooking environmental issues. Particularly, fuel consumption and GHG emissions were major factors determining differences between these farming systems (Gutiérrez et al. 2017). In this sense, higher rates of N fertilizer use of conventional wheat cultivation has been correlated to higher GHG emission (Ali et al. 2017). Accordingly, organic cereal production has been proved to be more efficient in terms of energy use (Alonso and Guzmán, 2010) and GHG emissions (Aguilera et al. 2015a) under Mediterranean conditions. In the long‐term, organic farming can provide for relevant advances in environmental sustainability of Mediterranean cereal drylands respect to conventional methods (Migliorini et al. 2014), therefore, the development and study of strategies aimed at improving the performance of organic cereal drylans are essential for the Mediterranean region. Particularly, since grain yield and quality gap between organic and conventional systems can differ between wheat cultivars (Fagnano et al. 2012; Gevrek and Atasoy, 2012) under Mediterranean conditions, the next section 1.4.4. is dedicated to the need of appropriate varieties for organic farming, especially regarding old wheat cultivars as the main focus of this thesis disertation. 1.4.4. Organic farming needs of appropriate varieties
The development of conventional agriculture since the mid‐20th century, among other conditions, has depended on a great investment in plant breeding, which has produced varieties adapted to conventional agriculture conditions (Wolfe et al. 2008). Contrarily, organic farming does not account for suitable genetic material, as it relies on such varieties selected under conventional high‐input agriculture conditions (Murphy et al. 2007). It is estimated that 95% of organic production is based on varieties bred for conventional high‐input agriculture in conventional breeding programs (Lammerts van Bueren et al. 2011). Although there are many traits identically required for both conventional and organic agriculture, the former has the possibility to compensate for the lack of certain traits utilizing chemical inputs such as fertilizers and herbicides. Contrarily, some traits required for conventional agriculture lack such relevance under organic conditions, as they are needed as aconsequence of the intensive use of agrochemicals in conventional agriculture (Lammerts van Bueren et al. 2011). Conventional breeding methods have been successful in developing cultivars with high yields and other desirable traits, but modern crops often require intensive management to avoid being outcompeted by weeds, infected by diseases, or eaten by insects (Andersen et al. 2015).
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Modern varieties can serve to farmers whose field growing conditions are not so different from those of breeding stations. However, farmers of organic and low‐input and marginal agriculture lacks of specific adapted varieties (Ceccarelli, 1996; Lammerts van Bueren et al. 2002; Murphy et al. 2007; Wolfe et al. 2008). The use of unsuitable genetic material could be partially responsible for lower yields generally linked to organic systems (Murphy et al. 2007) when compared to conventional ones (e.g. Seufert et al. 2012), since the highest yielding genotypes in conventional farming could not be the highest yielding genotypes under organic conditions (Hildermann et al. 2008; Murphy et al. 2007; Newton et al. 2017). This is true also for organic wheat production, where varieties selected under organic conditions can perform better than those seleted under conventional farming conditions (Kirk et al. 2012) . Van Bruggen and Finckh (2016) stated that organic farmers lose potential yield due to a lack of relevant traits for organic production as weed suppression, pest and disease resistance traits. Therefore, there is a need for selecting varieties better adapted to organic farming and low input conditions (Fagnano et al. 2012; Sassi et al. 2014; Wolfe et al. 2008). Organic farming requires traits related to yield stability and cultivars bred to focus on optimizing soil processes relevant for plant nutrition, soil fertility and crop disease resistance (Lammerts van Bueren et al. 2011). Indeed, organic farming needs cultivars adapted to variable conditions, able to compensate for unfavorable conditions and independent from external inputs (Lammerts van Bueren and Myers, 2012). The need for adaptation to diverse environmental conditions is more relevant in organic than in conventional agriculture (Wolfe et al. 2008). In such way, the varietal selection is a usefull tool of organic producers to foster system and yield stability and quality of organic production (Fontaine et al. 2008; Lammerts van Bueren and Myers, 2012; Wolfe et al. 2008). Table 1. Differences in trait requirements of high input conventional and low‐input and organic cropping systems.
Conventional Organic
Above‐ground traits
Performs well at high population density Optimal performance at lower densities Increased harvest index Increased harvest index, but not as dramatic as for
conventional production Erect architecture and leaves, shortened plantstature
Taller plants, spreading canopy to be productive in lowinput situations
Weeds controlled by herbicides Weeds limited by competition (plant height,spreading architecture), plants tolerate cultivation
Yield is maximized with high level of inputs Maximized sustainable yield achievable with input ofnutrients from organic sources
Pest and disease resistance to specific complexof organisms; need for resistance to diseases of monoculture systems
Pests and pathogens of monoculture potentially lesssevere, pathogen and pest complex differ; induced resistance relatively important; secondary plant compounds important for pathogen and pest defense
Rhizosphere traits
Root architecture unknown Exploratory root architecture; able to penetrate tolower soil horizons
Adapted to nutrients in readily available form Adapted to nutrients from mineralization – notreadily available; need for nutrient use efficiency; responsive to mycorrhizae
Legume‐specific traits
Nitrogen production by rhizobia of lesserimportance
Rhizobia more important; discrimination againstinfective rhizobia important for N acquisition
Harvest and marketing traits
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Improved labor efficiency Incorporate traits that improve working conditionsImproved processing, packing, and shippingefficiency
Improved nutrition, taste, aroma, and texture
Crop shaped by mechanical harvestconstraints
Traits priorities set jointly by researcher and farmer
*Source: Lammerts van Bueren and Myers, 2012.
There is a wide consensus on which traits are specially relevant for organic farming (Table 1). Nutrient use efficiency, resistance to diseases, weed competition, drought and heat stress tolerance and nutritional quality are some examples of such traits (Lammerts van Bueren et al. 2011; Wolfe et al. 2008). However, the wide range of organic farming conditions between years and environments led to the need to “select for specific adaptation in target environments” (Wolfe et al. 2008). Particularly, for low input and marginal areas, Ceccarelli (1996) arrived to the same conclusion, and reported the need for breeding under these conditions in order to develop adapted varieties, as genetic gains are achievable despite the environmental variation typical of these areas. Indeed, agronomic practices carried out are also relevant for the development of suitable varieties under organic prescriptions. Overall, an holistic approach instead of a narrow one is needed to integrate selection for different priorities in the farming systems and for traits adapted to local conditions without jeopardizing other relevant traits for varieties performance (Wolfe et al. 2008). Although organic farming legislation do not expressly favor the use of landraces, the higher adaptation of these varieties (regarding modern ones) has entailed that certain sectors maintain, and even recover, their use in Mediterranean countries. This is the case of the sector of vegetables grown outdoors in Spain, in which 74% of organic farmers cultivate at least one landrace. Mainly, such farmers have the ability and the posibility of reproducing their own seed and of directing their product to the local market (Martín et al., 2018). The linkage of in situ conservation of landraces with local organic food markets has been evidenced by several authors (Guzmán et al., 2000; Soriano and Thomas, 2010). 1.5. CLIMATE CHANGE, SOIL QUALITY AND CARBON SEQUESTRATION
Soil is a natural resource playing a key role in environmental sustainability with relevant functions for human welfare (Blum, W.E., 2005). It is recognized as a non‐renewable resource on our human life scale, since the regeneration process after its degradation is extremely slow (Lal, 2015). Soil quality is defined as “the capacity of a soil to function within ecosystem boundaries to sustain and promote plant and animal health” (Doran and Parkin, 1994). Maintaining soil quality is an essential issue because of its relevance in supporting ecosystems services and natural capital (Smith et al. 2016). The link between soil and ecosystem services is essential for sustainable development and human well‐being (Bouma, 2014), and it will be tackled in section 2.2. Ultimately, soil quality is linked to long‐term soil productivity and environmental benefits, thus soil conservation is relevant for present and future generations (Reeve et al. 2016). Soil quality is tightly related to soil organic C (SOC) and SOM dynamics and contents. The accumulation of SOM is an important process for soil formation, especially in semi‐arid conditions where organic matter dynamics are limited, and a determinant factor of soil fertility on cultivated lands (De Groot et al. 2002). Benefits linked to SOM are associated to the improvement in soil structure, retention of water and plant nutrients, the increase in soil biodiversity and decrease in risk of soil erosion and degradation (Lal, 2009a). The depletion on SOM is responsible for the reduction in moisture retention, soil workability and increasing CO2
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emissions (Lal, 2009b; Wood et al. 2000). The loss of SOM is usually related to the decline of soil quality and, because it is highly susceptible to agricultural practices (Ding et al. 2002), its appropriate management is the key aspect of sustainable agriculture (Weil, 1992). SOM increases have been suggested as a strategy to adapt to climate change and increase soil resilience (Lal, 2009a). In croplands, the increase in SOM is linked to the increase in agronomic productivity and thus, contributing to ensure global food security (Lal, 2006). Strategies to enhance the SOM pool are widely described in Lal (2009a). Either by increasing C input or by decreasing SOM losses soil quality would benefit from the resulting increase in SOM. A recent review of 72 long‐term experiments has found “promising practices” such as no tillage, organic matter additions, crop rotations or organic farming practices to be positively correlated to SOM increase (Bai et al. 2018). Particularly, crop residues incorporation as organic amendments can benefit soil fertility through the increase in microbial activity (Cayuela et al. 2009). Some factors determining SOM formation related to plant input are the amount of litter input, the above and belowground plant part and different plant tissues proportion (Kögel‐Knaber, 2002). Regarding cereal crops, straw incorporation after harvest can boost soil activity and nutrient availability, promoting higher yields for the next crops (Shah et al. 2003; Wei et al. 2015), decreasing CO2 emissions (Liu et al. 2014), and contributing to reduce the use of chemical fertilizers (Yin et al. 2017). 1.5.1. Carbon sequestration
SOC is the largest C stock in most terrestrial ecosystems and plays and important role in the C cycle, comprising 58% of SOM (Mann, 1986). Its functions are as a reservoir of major and minor plant nutrient, soil aggregation and reduction of soil erosion, energy source for soil biota and mitigation of GHG emission from soil (Lal, 2004b). SOC has been recognized as a key factor for climate change adaptation and mitigation processes in agriculture systems (Lal, 2004b) and it has been widely studied in cereal fields (Blanco‐Moure et al. 2016). Increasing the SOC content improves physical, chemical and biological quality of the soil (Lal et al. 2011), crucial for sustaining and enhancing crop productivity in a context where climatic conditions become more extreme. Likewise, ecosystem services such as nutrient cycling and C sequestration (Bhogal et al. 2009; Blanco‐Canqui, 2013) are improved with SOC increases (section 2.2.3.). Regarding cereal production, the increment in SOC can contribute to increase in yields in the medium and long‐term (Bauer and Black, 1994; Díaz‐Zorita et al. 2002; Kanchikerimath and Singh 2001; Lal, 2002). Contrastingly, SOC decrease has been said to negatively affect ecosystem services such as soil fertility, biomass productivity and water quality, as well as to enhance global warming (Lal, 2004b). SOC stock is the result of the balance between net C inputs into the soil and net soil C losses. In agricultural soils, input of C is determined by NPP and the proportion of this remaining on the field. Whenever, the loss of C is determined by decompositions (loss of CO2) and loss of topsoil by erosion (Freibauer et al. 2004). The rate of decomposition depends on mean temperature and physic‐chemicals soil conditions. In a broad sense, low crop yields, high SOC content and high SOM decomposition rates speed the loss of SOC from agrarian soils (Freibauer et al. 2004). In semi‐arid areas, soil erosion is a relevant factor for C loss (Martínez‐Mena et al. 2008). This stock can be modified positively or negatively by changes in land use and land cover (Guo and Gifford, 2002). According to these authors, negative changes are difficult to revert, since it needs long periods of time to recover original levels of SOC stocks (Guo and Gifford, 2002). In Andalusia, land use and land cover changes led to an estimated soil organic C loss of 16.8 Tg during the second half of the past century (Muñoz‐Rojas et al. 2015). The expansion of subsidized herbaceous crops, the abandonment of olive trees and vineyards in transitional areas and the forest restoration in mountainous regions are important driving forces in land use
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changes in the Mediterranean region (Serra et al. 2008). In Europe, croplands are identified as the largest biospheric source of C lost, with a net loss of 300 Mt C per year (Smith,2004). Soil C sequestration is the term referring to the effect of practices that increase the photosynthetic input of C or slow the return to the atmosphere of stored C via respiration or fire, increasing the C stored in the soil. Rising temperatures due to global warming can stimulate the loss of soil C to the atmosphere, “driving a positive land C‐climate feedback that could accelerate climatic change” (Crowther et al. 2016). In Spain, a recent study has estimated that the higher temperatures in 20th century are a relevant driver of the soil C loss of Spanish croplands during that period (Aguilera et al. 2018). For all of this, strategies to enhance SOC sequestration are undoubtedly needed to face the climate change challenge. The maximal capacity of soils to sequester C relies on intrinsic abiotic soil factors such as topography, mineralogy and texture, but soil C dynamics are also driven by biota and their interaction with climate (De Deyn et al. 2008). Lal (2004b) referred to C sequestration as a “win‐win strategy”, since it contributes to restore degraded soils and water bodies, or increment biomass production, while reducing atmospheric CO2 through the C storage. The increase in SOC, therefore, can boost the two‐fold related challenge of agriculture, to know, to reduce emissions and to adapt to an increasingly changing climate (Smith and Olesen, 2010). Nevertheless, C sequestration has some limitations. The quantity of C that can be stored in soil is finite, and there is a sink saturation point (Smith, 2004) in which soils reach a new SOC equilibrium, estimated to happen after 20‐100 year after new management practice(s) implementation. It can lead to higher fluxes of other relevant GHG (Freibauer et al. 2004; Powlson et al. 2011), offsetting relevant mitigation opportunities. In addition, C sequestration in soils is not permanent and it has been called as a “riskier long‐term strategy for climate mitigation than direct emission reduction with a minor role in closing C emission gaps by 2100” (Smith et al. 2004). Indeed, the need to enhance the residence time of SOC requires for the identification and study of management practices that foster recalcitrance against microbial decomposition (Lal, 2009a). However, strategies to increase C sequestration could extend the time to be able to introduce other measures with longer‐term benefits (Bruinsma, 2017). Although SOC sequestration can offset a proportion of GHG emissions from agriculture, its finite effect clash with the infinite effect of the reduction of fossil fuel combustion on the decrease in GHG emissions (West and Marland, 2002). These limitations above warn us about considering SOC sequestration a substitute for emissions reduction (Smith, 2004). SOC sequestration management practices should be accompanied by many other alternative practices with a GHG emissions reduction aim, in order to effectively reduce the large proportion of anthropogenic GHG emissions that represent the agricultural activity. 1.5.2. Strategies to improve soil organic carbon content
Many agronomic practices enhancing SOC sequestration have been assessed. For instance, the reduction in the intensity of tillage has been widely recognized as a successful strategy to reduce SOC losses, to raise the C sequestration and improve soil quality and functioning especially in Mediterranean dryland agroecosystem (Álvaro‐Fuentes et al. 2009; Barbera et al. 2012). However, the potential for no‐till management to sequester atmospheric C in SOC has been said to be limited (Baker et al. 2007), as no‐till can enhance SOC stratification leading to higher SOC content in the top layers but declining in the bottom ones (Hernanz et al. 2009). Luo et al. (2010) found that conversion from tillage to no‐tillage resulted in significant topsoil SOC enrichment, but did not increase the total SOC stock in the whole soil profile. Cereal‐legume rotation under semi‐arid Mediterranean conditions has also been assessed as an important practice for SOC
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sequestration (Hernanz et al. 2009), although some authors do not coincide (López‐Bellido et al. 2017). A revision of 66 long‐term Mediterranean experimental comparisons on SOC by Francaviglia et al. (2017) reported that monoculture cropping systems and mineral fertilization could be related to SOC losses of ‐0.28 Mg C ha‐1 year‐1 and ‐0.17 Mg C ha‐1 year‐1, respectively. The elimination of bare fallows and the consequent reduction in SOC loss, is also a SOC positive strategy (Álvaro‐Fuentes et al. 2009). Typically, SOC increases in proportion to increases in C inputs (Paustian et al. 2000), and this is also true for Mediterranean systems (Aguilera et al. 2013a). Concerns about climate change have led to many studies discussing the C storage potential of crop residues incorporation to agricultural soils (Bhogal et al. 2009; Blanco‐Canqui, 2013; Lehtinen et al. 2014). Particularly, wheat straw incorporation (Liu et al. 2014) has been unveiled as a promising strategy to enhance SOC sequestration in soils and soil quality. On the other hand, Aguilera et al. (2013a) found that the application of organic amendments can increase the C sequestration rate by 1.31 Mg C ha‐1 year‐1, and with cover crops by 0.27 Mg C ha‐1 year‐1. The application of stabilized manures and the inclusions of legumes in rotation under organic management have evaluated as practices enhancing SOC stocks (Romanyá et al. 2012; Freibauer et al. 2004). The use of stables source of organic matter, as composted manures, could reduce priming effect of native pools of SOM (Romanyá et al. 2012) and the consequent loss of SOC in the short term when applying easily available C sources that enhance “intensive microbial mobilization of nutrients from the soil organic matter” (Kuzyakov, 2002). Although there is a wide research corpus identifying SOC sequestration management practices, there are factors as the initial SOC stock or the length of the experiment that influence the studies output (Francaviglia et al. 2017) that should be taken into account for the conclusions we come to.
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2. OBJECTIVES
In general, the objective of this project has been to find advantages of the cultivation of old wheat cultivars under rainfed Mediterranean conditions against the cultivation of modern ones. The central issue has revolved around the dismantling of the wide spread belief that old cultivars and traditional farming are backward, and to try to demonstrate that their cultivation has environmental opportunities that should not be disregarded, such as the sustainability of drylands of the Mediterranean region. More concretely, particular objectives of each study are as follows:
Study 1:
a) To compare straw production and C‐to‐N ratio of the straw of old and modern varieties under contrasting soil conditions (nutrient rich and nutrient poor soil conditions).
b) To assess if differences in straw quality can entail differences in soil respiration and N mineralization after residue incorporation in both types of soil.
c) To determine if potential differences in straw quality can lead to affect soil quality and soil nitrate leaching in two contrasting soil nutrient conditions.
Study 2:
a) To compare old and modern wheat varieties production of residues under Mediterranean rainfed conditions.
b) To test if differences in residues production entail relevant differences in C and N inputs after the harvest and if these latter result in differences in SOC sequestration and N2O emissions.
c) To calculate final C footprint of both types of cultivars under contrasting managements (organic and conventional farming).
d) To identify hotspots in the GHG emissions profiles of both farming systems and cultivars to focus future climate change mitigation strategies in rainfed Mediterranean cereal agroecosystems.
Study 3:
a) To supply data corresponding to all NPP components for old wheat varieties grown under traditional organic management, which are not found in the scientific literature.
b) To provide similar data for modern wheat varieties under conventional farming conditions, through bibliographic review.
c) To model the NPP of Spanish wheat croplands throughout the 20th century from the mentioned data of old and modern wheat cultivars.
d) To compare the destinations of the total biomass produced by both old and modern cultivars, and to assess if the biomass that is not extracted from the agroecosystem is enough to maintain and reproduce the fund elements of the agroecosystem.
e) To detect the main impacts in Spanish rainfed agroecosystem after the replacement of old wheat by modern wheat cultivars due to the reduction in non‐harvested biomass.
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3. THEORETICAL FRAMEWORK 3.1. AGROECOLOGY Modern agriculture extremely depends on fossil fuel and external inputs, and this industrialization of agriculture has led to the overuse and degradation of basic resources essential for agriculture since the beginning of the activity: soil, water and genetic and cultural resources (Gliessman, 2005). Consequently, modern agriculture has lost the balance required to reach sustainability in the long‐term (Kimbrel, 2002) and has shown its incapacity to make a proper use of nature. During the first decades of Green Revolution, the blind faith on industrialization of agriculture was easily spread, and it was in 1962 with the book of Rachel Carson The Silent Spring when a claim for a conscience about ecological crisis due to the massive use of chemical products in agriculture raised. By that time, Carson was already aware of the harmful effects due to the spread of chemical substances on croplands to increase production. She called this human intervention “a war of men against nature”:
[…] the main problem of our time is then the total environment pollution by the man by means of [those] substances with incredible harmful potential, substances that, accumulated in plant and animal tissues or even penetrating in germinal cells, can alter or destroy the same inheritable germs on which the specie future depends on (Carson, 1962:21).
The transformation of agrarian activity under the industrialization paradigm entailed not only environmental impacts, but also negative social consequences. Social inequalities, as the loss of food self‐sufficiency, the loss of traditional farming knowledge, the permanence of rural poverty, the agrarian land property concentration and the expulsion of millions of traditional farmers are some examples of the social negative consequences (Toledo et al. 1985, in Altieri, 1989; Toledo and Barrera‐Bassols, 2008). In front of industrial food systems production and its negative impacts at several levels, the main challenges of food systems are to provide enough and healthy food for the growing population, to conserve natural resources and reduce food waste, to mitigate and adapt to climate change, to eradicate social inequalities and to prevent from the loss of traditional knowledge (Migliorini and Wezel, 2017). By contrast to what is widely extended, the model of modern agriculture and the biotechnological paradigm on which is based do not respond to social or environmental needs, but they respond to private interests and profits (Altieri and Rosset, 1996). The innovations promoted by this paradigm have failed to provide enough food for the global population, at the time that have entailed the increase of agriculture footprint, threatening the capacity of nature to provide ecosystem services. In addition, the problem of hunger is still an issue of inequalities in production, distribution and consumption (Pollock et al. 2008). So we should be wondering which is the point of keeping practicing an agrarian model that does not promote the equality in access to food, while risking the environmental resource upon which it depends. 3.1.1. Agroecology as a new agriculture and food production approach During last decades of 20th century it grew the concern about the need for a sustainable alternative for food production and a way of resistance to Green Revolution model. Agroecology appeared at the end of the 1970s as a response to ecological crisis of rural areas (Guzmán et al. 2000) and as a new approach aimed at reversing both the industrialization of all phases of the food chain (from production to processing and distribution) and the increased control of private corporations over the food system (Gliessman, 2018). It entailed a re‐discovery of extremely useful peasant cultures knowledge about interactions that took place in agricultural practices
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(Guzmán et al. 2000), a co‐evolutionary process between natural and social systems that results in successful practices for ecosystem appropriation (Altieri, 2004). A broad analysis of the evolution of the concept Agroecology and papers published in this discipline can be found in Wezel and Soldat (2009). Since the beginning of the 1980s, agroecology started to be considered as a science with distinct methodology and conceptual framework for the study of agroecosystems (Wezel and Soldat, 2009). Altieri (1989) defined the term as a new scientific discipline that defines, classifies and studies agricultural systems from an ecological and socio‐economic perspective. During that decade, many research papers contributed to this corpus of knowledge, considered at that moment as the global study of agroecosystems protecting natural resources with the aim of designing and managing sustainable agroecosystems (Altieri, 1989). Together with the consolidation and the enlargement of the agroecological research (e.g. Altieri, 1995; Gliessman, 1998), publications on sustainability and sustainable agriculture increased during the decade of 1990s (Wezel and Soldat, 2009), linked to the United Nations Conference on Environment and Development held in Rio de Janeiro, Brazil, in 1992, where this topic acquired global awareness. Agroecology was then broadly defined as “the application of ecological concepts and principles to the design and management of sustainable agroecosystems, or the science of sustainable agriculture” (Altieri, 1995; Gliessman, 1990, 1998). By the end of the last century, Agroecology started to focus on the “the integrative study of the ecology of the entire food systems, encompassing ecological, economic and social dimensions, or more simply the ecology of the entire food system” (Francis et al. 2003). Following Wezel et al. (2009), in the beginning of the 21th century, Gliessman (2007) gave a broader dimension on his definition of agroecology as “the science of applying ecological concepts and principles to the design and management of sustainable food systems”, broadening the scale and the object of study from the agroecosystem to the entire food chain. The interest in agriculture sustainability reached many social sectors, who started to worry about resource depletion, environmental degradation, population growth, economic growth at all cost, etc., and the consequences on agricultural expansion (Altieri, 1989). In both developed and developing countries, the sustainable provision of agrarian products became a relevant issue, and a change in the framework of research and analysis was required. There was a need for rethinking the agricultural management of systems that were producing the food for people, but also other issues that matters beyond strictly agrarian ones. Beyond these concepts, agroecology was not only contemplated as a different approach to study agrarian systems, but it aimed at constituting an alternative to face social and environmental problems that capital‐intensive agriculture model was generating (Guzmán et al. 2000). Researchers and rural communities, along with other social agents, realized we could not keep ignoring the consideration of social and environmental factors being influenced and influencing the production process. In order to obtain the maximum production with the minimum economic and energetic effort over time, society should become aware about environmental issues and the way it appropriate from nature during the production process and its consequences (Toledo et al. 1985). However, the intensification of agrarian systems and its consequences are not only a matter of which practices are implemented on the agroecosystems and which shifts should be experienced to lead to a more sustainable food production. Agroecology means not only to acquire low‐input technologies to reach a sustainable way of producing food (Altieri, 1989). It is also a matter of global dimensions, with economic and socio‐political issues knocking at the door. Since relatively recently in time, economic rationality became the dominating element
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over the productive process, that is, determining what production is done for (Toledo et al. 1985) and leading the different stages of the food production process. Contrastingly, Agroecology stablishes that facing the problem of non‐sustainability of agricultural systems require the modification of “the socio‐economic determinants that govern what is produced, how it is produced and for whom it is produced”, strengthening the need for scientists to be aware of who will be profiting the advances in sustainable technologies and if steps are being made on the way towards socially equitable agriculture (Altieri, 1989). Therefore, agroecological strategies and designs must be aware of poverty, beyond the concern of increasing yields while conserving natural resources, and consider employment and equal access to local inputs and markets (Altieri, 2009). Against the strictly economic approach of the agro‐food busyness, there were emerging concepts related to agroecology as food sovereignty, for example, that “emphasizes farmers’ access to land, seeds, and water while focusing on local autonomy, local markets, local production‐consumption cycles, energy and technological sovereignty, and farmer‐to‐farmer networks” (Altieri, 2009). To rethink and reorganize food production systems either from an ecological or socio‐economic perspective, it is required a relevant structural change that cannot be but with the participation of social movements that pressures governments and institutions to change production systems from the root. Deepening the issue, agroecology is also a social and political movement that fight for a social and political change (Migliorini and Wezel, 2017). In order to strengthen this change, some authors have proposed the development of Political Agroecology as a priority work and research area of Agroecology (González de Molina, 2013). “A more radical transformation of agriculture is needed, one guided by the notion that ecological change in agriculture cannot be promoted without comparable changes in the social, political, cultural, and economic arenas that help determine agriculture” (Altieri, 2009). In other words, the aim of agroecology of producing food in an environmental, social and political sustainable way should be tackled from three fronts, which have been resumed in Agroecology as a science, practice and social movement (Gliessman, 2018). Perhaps, mirroring the way in which agroecosystems are dynamic structures in constant change and evolution with their environment, agroecology is a term that is shifting along the time, as we have briefly described here. Recently, Gliessman (2018) gave a new evolved definition of Agroecology as follows:
Agroecology is the integration of research, education, action and change that brings sustainability to all parts of the food system: ecological, economic, and social. It’s transdisciplinary in that it values all forms of knowledge and experience in food system change. It’s participatory in that it requires the involvement of all stakeholders from the farm to the table and everyone in between. And it is action‐oriented because it confronts the economic and political power structures of the current industrial food system with alternative social structures and policy action. The approach is grounded in ecological thinking where a holistic, systems‐level understanding of food system sustainability is required.
3.1.2. Agroecology and traditional varieties Agroecosystem sustainability depends on many diversity scales, from the genome to the food system scale (Gliessman, 2000). The ultimate objective of the agroecological paradigm is to develop and design sustainable farming systems and, among many other practices aimed at this target, it proposes and defends the use of old cultivars due to many reasons. Traditional varieties have been related to the optimization of long‐term diversity and stability of agroecosystems, as
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well as to the small‐scale farmers’ knowledge system, harboring a relevant role in the development of sustainable agriculture (Cleveland et al. 1994; van Etten et al. 2017). In the first place, traditional cultivars represent a unique source of local diversity since they harbor many traits absent in modern breeding materials (e.g. Bertholdsson, 2004). The replacement of traditional cultivars by modern ones has entailed a string of changes that has led to the reduction of agroecosystems sustainability (Gliessman, 2000). The diversity expressed in traditional cultivars represents a mechanism of flexibility against changing environment and resistance to pest (Mukhupadhyay and Roy, 2015), contributing to increase resilience of agroecosystems, that is, the capacity to respond to unexpected events (Toledo and Barrera‐Bassols, 2008). Their loss represents the loss of material for further crop improvement, the loss of resistance to pest, diseases and abiotic stresses and the loss of adaptation to local conditions, being of special concern under a climate change scenario (Altieri, 2009). Farmers that keep this genetic resource in their farms do it as an “insurance to meet future environmental change”, therefore, their loss could entail dramatic consequences for local subsistence and food‐security in the medium and long‐term (Toledo and Barrera‐Bassols, 2008). Actually, the diversity conferred by traditional varieties can also be important to the future of industrial agriculture, as they enable to adapt to evolving pests and changing climate and soils (Altieri, 2004), problems that this agriculture is already trying to face. On the other hand, the use of these varieties lessens the farmer dependence on external inputs. As they are locally adapted cultivars that do not require great amount of external inputs to hold their yield, their cultivation contributes to the sustainability of the agroecosystem (Altieri, 2009; Gliessman, 2000). Contrastingly, new bred varieties require from a technological package intrinsically related to negative effect on environment, such as the intensive use of fertilizers, pesticides, irrigation or fossil fuels (Gliessman, 2000). The need for such an amount of external inputs clearly makes modern cultivars –and, in turn, the whole agroecosystem‐ more dependent on human intervention. In a certain way, this fact is due to that seed systems are focused “on their contribution to agricultural productivity, rather than to food system sustainability” (van Etten et al. 2017). As Gliessman (2000) reported, the adoption of newly bred varieties and the technological solutions that their adoption entails, provoked many changes at a greater scale in farms. Farmers had to increase their production to be able to afford new expenses increasing the use of inputs, while they concentrated their production in few crops, becoming more dependent on centralized market structures (Gliessman 2000). The expansion of monocultures entailed an agroecosystem simplification not seen before, furthering the agroecosystem from its auto‐regulating capacity. Within the vortex of a global agri‐food system, decisions that farmers could make were reduced, since they had just to follow a recipe. Nevertheless, before the Green Revolution and the concomitant dependence on scientific breeders and public and private seed companies, agro‐food systems relied on traditional germplasm and the farmers’ experience that enabled its cultivation and evolution (Boege, 2008). Nowadays, in small scale farming systems, these varieties represent a relevant resource for subsistence of communities (Brush, 2000). The adoption of modern varieties led to negative externalities, as it has been seen before. Either natural resources damage or the economic dependence of farmers are considered as externalities derived from the spread of modern farming. The economic pressure on farmers to adopt a “cash agricultural economy” involved the loss of biodiversity of many rural areas, but also the loss of autonomy with respect to the reproduction of their growing material. In this regard, traditional varieties confer to farmers a high level of economic independence, as they do not have to buy commercial seeds if reproducing traditional ones (Alonso‐Mielgo, 2000; Mooney, 1983). Together with ecological and economic factors that lead farmers to continue
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growing these cultivars (Altieri, 2009), there are also social factors, as could be the taste (Reyes‐Garcia et al. 2014), their cooking and processing qualities, together with “historical and cultural reasons such as dietary diversity and their use in traditional foods or religious ceremonies” (Cleveland et al. 1994). In general, the “farmers’ access to seeds has an impact on the sustainability of food production and consumption” (van Etten et al. 2017). Finally, traditional varieties are linked to the traditional farmers’ knowledge of incalculable value. Their conservation has relied on generations of farmers that passed the seeds to the followings (Altieri, 2009), and under whose management practices traditional varieties have evolved and conserved their diversity. In following sections there can be found an extended definition and connection among agroecology, traditional varieties and traditional knowledge. 3.1.3. Traditional knowledge and Agroecology An important base for the emergence of agroecology was the interest in traditional farming managements and their conservation (Altieri, 1987). Agroecologists started to notice that within ancient knowledge it could be found useful experiences that it was worth studying in order to face present and future agriculture challenges (Guzmán et al. 2000). Traditional agroecosystems have been able to hold agrarian productivity through time, being the living evidence that sustainable production is achievable and constituting a relevant reference for understanding ecological bases of sustainability and for designing sustainable management practices useful for nowadays agriculture (Gliessman, 2000; González de Molina and Guzmán, 2006). These systems represent farmers’ interaction with their environment without accesses to external input, capital or scientific knowledge, providing an useful “long‐term perspective on successful agricultural management” (Altieri, 2004). Contrastingly to modern production systems, traditional systems rather display ecologically suitable practices for natural resources appropriation (Toledo, 1993), based mainly on energetic efficiency, productivity (relative to a variety of products and to the whole annual cycle, in contrast to conventional approach that refers productivity exclusively as volume extracted from ecosystems), the long‐term reach of that productivity and the minimum use of external inputs (Toledo and Barrera‐Bassols, 2008). Sustainable practices carried out by traditional farmers respond to an ecological rationality inherent to traditional production systems, through which farmers “perceive, conceive and conceptualize ecosystem they depend on to live” (Toledo, 1993). Traditional farmers have been able to understand the agrarian potential of ecosystems through a process of trial, error, selection and cultural learning that has lasted for centuries (Noorgard and Sitkor, 1999). This process provided them of that ecological rationality that would be very useful to face nowadays ecological and agrarian crisis. Therefore, the observation of these agrarian systems may help at understanding the mechanisms and complex interrelations underlying sustainability, it being necessary to translate “these principles into practical strategies for natural resource management” (Altieri, 2004). It is not about the romantic return to past agrarian ways of production, but to show that new agrarian technologies could be improved if basing them on ecological principles that underlie peasant farming (Alonso‐Mielgo, 2000). As Reyes‐Garcia et al. (2014) reported, traditional does not mean archaic or pre‐modern, as traditional knowledge is a dynamic and adaptive system “capable of incorporating new knowledge while at the same time maintaining the bulk of the accumulated body of knowledge in a process of continuity and change”. Many researches have encompassed the shifts in the linkage between traditional knowledge and cultivars through decades, uncovering social, ecological and production factors (e.g. van Etten, 2006). Local ecological knowledge of the environment is used to interpret and respond to feedbacks from the environment to guide the direction of resource management, contributing to adapt to “uncertainty and unpredictability intrinsic to all ecosystems” (Berkes et al. 2000). Its potential
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to adapt to changes is part of its idiosyncrasy, and enables the traditional knowledge system to contribute to the resilience of socio‐ecological systems (Berkes et al. 2000; Reyes‐Garcia et al. 2014). Agroecology stablishes that through the study of traditional agriculture relation with agrarian ecosystems it can be achieved the most accurate knowledge of agroecosystems potential and the most appropriate way for natural resources appropriation (Guzmán et al. 2000). Although this knowledge system and scientific one are not necessary excluding, but they can be complementary and both attend to understand natural processes, the omission of traditional knowledge by the scientific community underlies the failure of industrial civilization to properly manage nature (Toledo and Barrera‐Bassols, 2008), losing a broad and essential knowledge system that could allow us to better manage and relate with nature.
3.1.4. Traditional varieties and knowledge need each other Traditional cultivars are the product of centuries of coevolution between farmers and environment, that is, they are both the product of farmers’ selection and practices done through the time and the result of certain local conditions. Therefore, both human and natural selection pressures are responsible of the maintenance of genetic variation of traditional cultivars (Toledo and Barrera‐Bassols, 2008). Some authors have called the environment under both pressures in which traditional varieties evolved and diversified a “biocultural laboratory” (Boege, 2008:23). On the one hand, this biodiversity “culturally created is a product of a long‐term process of exchange and systemic cultural selection” (Boege, 2008:20). The conscious cultivars selection by farmers entails the existence of a certain knowledge system about the crop and its environment, which has been said to be included within a more general traditional knowledge system (Ellen and Harris 1998, in Brush 2005). The main traits of traditional knowledge have been described: “(1) localness, (2) oral transmission, (3) origin in practical experience, (4) emphasis on the empirical rather than theoretical, (5) repetitiveness, (6) changeability, (7) being widely shared and collective, (8) fragmentary distribution, (9) orientation to practical performance, (10) holism (11) diachronic (Ellen and Harris, 1998 in Brush, 2005; Toledo and Barrera‐Bassols, 2008). The interrelation among human communities and nature acquire special relevance in this corpus:
It is a cumulative body of knowledge, practices, and beliefs evolving by adaptive processes and handed down through generations by cultural transmission, about the relation of living beings (including humans) with one another and with their environment (Berkes et al. 2000).
It is the intellectual system through which farmers appropriate nature, based in local experiences and in constant adaptation to technological and socioeconomic dynamics (Toledo and Barrera‐Bassols, 2008). Within this knowledge system, there can be found essential processes for the evolution of cultivars, such as the selection and the exchange with others farmers, therefore, the conservation of this knowledge is crucial for ongoing crop evolution (Brush, 2005). In addition, technical knowledge linked to plant material manipulation, such as, for instance, how to reproduce and conserve the seeds and how to promote their germination is also included within this system (Alonso‐Mielgo, 2000). So much so that, the conservation of traditional systems is considered the only successful strategy to preserve this germplasm (Altieri and Merrick, 1987). The environmental knowledge through the observation and the empirical practice by traditional cultures, together with the coevolution between both culture and nature diversity, has given rise to what it is called biocultural memory (Toledo and Barrera‐Bassols, 2008). Biocultural memory can be understood as “an expression of the [natural and cultural] diversity reached” and it harbors a great value to understand the present and to think about an alternative future (Toledo and Barrera‐Bassols, 2008:190).
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On the other hand, and not less interesting, besides that traditional seeds are the product of farmer work and knowledge, they are also the raw material that allows the traditional knowledge to develop and evolve. Their reproduction, which is under the control of farmers, can be considered as the basis for the reproduction of material conditions required for the next growing seasons (together with the rest of inputs needed to grow a crop). Without these traditional seeds, farmers would be stripped of the base of their agrarian knowledge that, at the end, lies on the seeds they grow in their agroecosystem. Without these seeds, there is no possibility of continuing the multidirectional process of knowledge answering how to carry out agrarian practice. In other words, both traditional knowledge and cultivars are mutually dependent, as the loss of one of them makes impossible the other one’s survival (Mesa‐Jiménez, 1996). That is, we could stablish that landraces and traditional knowledge have a bidirectional relation. In the same manner that traditional knowledge is relevant for the conservation of old cultivars (FAO, 2010), the abandonment of traditional varieties has been linked to the loss of the traditional knowledge (Bruinsma, 2017; Toledo and Barrera‐Bassols, 2008). By keeping old varieties in our fields, we are enabling the traditional knowledge to keep evolving and resolving the environmental constrains, stresses and resistance that crops must overcome. Traditional cultivars provide the framework to keep developing and valuing this farmer knowledge. Although the main contribution to crops evolution has been this conscious and decentralized selection by farmers, in last decades of 20th century crop breeding programs have ignored this contribution and have given “an important role in crop evolution to scientist, public agencies and seed companies” (Poehlman, 1995, in Brush 2005). The strong influence of modern science, responsible of industrialized agriculture, has relegated other forms of knowledge to the margins, losing a really useful information that has been gathered through centuries and that now is regarded as obsolete and backward (Toledo and Barrera‐Bassols, 2008). Nonetheless, “the hyperbolic growth of agricultural production may now rely on formal science, but it is built on foundations developed by traditional farmers” (Brush, 2005), who still depend on traditional knowledge rather than on formal scientific knowledge. The application of agroecological approaches and technologies, together with traditional knowledge, have been shown to foster food security of communities, at the same time that conserving biodiversity and soil and water resources (Altieri, 2009; Pretty et al. 2003). Such is that relevance, that the livelihood and food supply of millions of farmers and communities still rely on traditional farming systems in small‐scale farms, as it has been for centuries (Altieri, 2004, 2009) and even nowadays in a climate change context (Uphoff and Altieri, 1999). In contrast to what could be thought under the modern agriculture paradigm, most of the agriculture products that are consumed are derived from 1.9 million peasant‐bred and locally adapted plant varieties donated to gene banks and mostly cultivated with the absence of chemical fertilizers (ETC, 2009). While peasants have bred 7000 domesticated plant species, commercial breeders work with only 137 crop species (ETC, 2017). In this sense, small farms applying traditional values and practices have been proved to be more productive than large farms when accounting for the whole farm production, and not only one single crop yield (Altieri, 2009). In addition, processes in contemporary world such as population growth, market development, technology diffusion and cultural change (Brush, 2005), together with the expansion of industrial agriculture (Toledo and Barrera‐Bassols, 2008), are compromising traditional agricultural knowledge. The globalization of industrial civilization is leading to the depletion of any form of diversity in pro of the uniformity of processes and living beings, contributing to the disappearance of “the main component of biocultural complex of human beings” (Toledo and Barrera‐Bassols, 2008). The loss of practices and knowledge hold by rural communities can hinder the possibilities to face climatic variability, contributing to increase the vulnerability against climate change and the reduction of the potential to develop climate change mitigation and adaptation strategies (Mukhupadhyay and Roy, 2015).
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Overall, the conservation of traditional varieties and diversity associated to them can help to enhance the sustainability of the agroecosystems in many different levels. By the conservation of these local resources, we would be reducing the affectation risk from pest and diseases, enhancing the food self‐sufficiency and the economy stability of farmers while reducing the external input dependence and enabling the adaptability to environmental variability and constrains (Altieri, 2004). Indeed, traditional varieties do not represent only the farmers’ knowledge, but also they can be considered “the heritage of historical and cultural peculiarity of regions harbored in its agriculture” (Guzmán et al. 2000). In addition, the conservation of old varieties contributes to the conservation of values that represent a way of interpret the agroecosystem beyond its economic or utilitarian practicality (Reyes‐Garcia et al. 2014). According to all of this, their conservation is not only relevant for the design of sustainable agroecosystems, but also for the biocultural memory accumulated during centuries and for the history of agrarian landscapes. It represents a different interpretation of nature and our relation with the food production systems, elevating their reintroduction and cultivation to a high level of the hierarchy in the path to sustainability. But it also entails the recognition of different forms of understanding farmers’ right to nature appropriation. Rural communities have the right to find available the local resource they have grown and conserved during centuries (Montecinos and Altieri, 1991 in Toledo and Barrera‐Bassols, 2008), maintaining a dialogue between their traditional knowledge and the landrace. Cultural and biological diversity should be considered a right of rural communities, but also a richness that the whole population can profit from. There is a lot at stake. In Spain, almost 100% of cereal varieties cultivated are modern bred cultivars (Guzmán et al. 2000). Guzman et al. (2000) called attention to the urgency of recovering and conserving both traditional varieties and traditional agricultural knowledge of the Mediterranean region, given that it is one of the areas with the greatest heritage and agricultural tradition on the planet. If we take into account that 98% of food production of major crops in Mediterranean countries is based on species originated from other regions (Kloppenburg and Kleinmann, 1987 adapted in FAO, 1997), the urgency for varieties conservation is even striking. As Wezel and Soldat (2009) stated in their bibliographic revision of the evolution of the term agroecology, “to really consider agroecology as a new scientific discipline, the basic considerations for future agroecological research should be always to ask what effect, impact or change creates an innovation on the plot level, for example, a new crop rotation or a new type of biological control on the agroecosystem level, but also in the food systems level”. Following this concern, we have aimed to evaluate the role played by old wheat varieties in the past and the advantages that could be found from their cultivation in the present. Trying to avoid the bias around the adequacy and suitability of traditional varieties from the conventional agronomic perspective, we propose an extensive analysis from several perspectives. All of them with the aim of seeking sustainability in this varietal recovery. We have evaluated old wheat cultivars performance under different management practices, in order to understand how they function and relate to other biotic and abiotic factors of the agroecosystem. 3.2. ECOSYSTEM SERVICES Ecosystem services (ES) are defined as the benefits that human beings derive from nature, either if it is in the tangible form of goods or in the intangible form of services (MEA 2003). Daily (1997:3) delimited the term as “the conditions and processes through which natural ecosystems, and the species that make them up, sustain and fulfill human life. They maintain biodiversity and
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the production of ecosystem goods, such as seafood, forage timber, biomass fuels, natural fiber, and many pharmaceuticals, industrial products, and their precursors”. ES can be beneficial to the population in a direct or indirect way, and the scale (in space and time) at which they impact human welfare is variable. With regards to ES delivered from agricultural practice, as we will see below, an example of direct ES could be the increase of crop yields due to the action of pollinators; while an indirect benefit could be when wetlands reduce the burden of N in water from croplands contributing to reduce eutrophication of estuaries receiver of that water (Dale and Polasky, 2007). The change in gaseous atmospheric composition and the loss of a local crop variety could be examples of the different scales where ES have an effect. The variety of spatial and time scales through which ES can be studied makes a “multisectoral approach” essential to evaluate changes in ES and their impacts on people (MEA, 2003). In this sense, the ways and scales in which ES operate are that large, intricate and little‐explored that they have been said to cannot be replaced by technology (Daily, 2003). ES can be grouped into different categories, as described the Millennium Ecosystem Assessment (2003) (Figure 6):
(1) Provisioning services. Products obtained from ecosystems: food, fresh water, wood, fiber, fuel, biochemicals, genetic resources; (2) Regulating services. Benefits from the regulation of ecosystem processes: air quality maintenance, climate and water regulation, erosion control, water purification and waste management, regulation of human diseases, biological control, pollination and storm protection. (3) Cultural services. Intangible gains that humans obtain from nature through spiritual enrichment, cognitive development, reflection, recreation, and aesthetic experiences: cultural diversity, spiritual, religious and educational values, knowledge systems, social relations, sense of place, cultural heritage values, recreation, etc. (4) Supporting services. These ES are necessary for the production of the other ones, and people are less aware of them as their effect is indirect and take place in the long‐term. Some examples are nutrient and water cycling, primary production, habitat, soil formation and retention and biodiversity.
Figure 6. Categories and examples of ecosystem services. Source: MEA, 2003.
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Many assessments have been conducted in order to value ES, under the slogan that only what is economically valuable will be cared for by people and will be the center of attention of policy makers (e.g. Boyd and Banzhaf, 2007; Costanza et al. 1997). It is not the moment or the place to argue against this utilitarian, economic and anthropocentric interpretation of nature, but it is maybe for emphasizing that we must understand the true value of nature and “the richness it provides to our lives in ways much more difficult to put numbers on” (MEA, 2005). Even if they were not priced, ES are essential for the economy, as nature provide much more goods and services than can be identifiable in a first moment. Further, ES are barely substituted by human designed processes or products. For instance, if life cycles of predators controlling most crop pests were disrupted, pesticides manufactured by humans would not be able to replace them (Daily, 1997:5). Even if some kind of technological substitution is available, it is often parallel to a high cost and probably it would not fully replace the ES loss (MEA, 2003). And what is more, there are not only economic reasons, but also social and cultural conditions to bear in mind when valuing if it is possible the human replacement of an ES (MEA, 2003:94). 3.2.1. Ecosystem services and sustainability Needless to say that the sustainability of human life and activities rely on the well‐being of ES because, as part of nature, our existence depends on it (Swift et al. 2004). Until nowadays, no other place in the Universe is available to support our life, thus, we should take care of the Earth and preserve from the naïve idea that a bright future could wait for us elsewhere. Just like the Millennium Ecosystem Assessment (2005) recognized, every person worldwide depends on nature and ES to enjoy a decent, healthy and secure life, thus, the care of ES is an essential task for the sustainability of life, and denying the relevance of ES can compromise the sustainability of human beings (Costanza et al. 1997). Although ethically questionable to some extent, natural ecosystems can be seen as “factors of production” whose services/goods sustain and protect human activities and well‐being, and that this continues like this is a great challenge (Folke et al. 1996). The sustainability of the production/provision of an ES by natural or managed ecosystems makes reference to the maintenance of the ecosystems biological potential to maintain the yield of that service (MEA, 2003). Although this conception leads to conceive the condition of an ecosystem as its ability to produce a certain service or good to humans (MEA, 2003), there are too many processes and traits of ecosystems that are out of our reach and knowledge and, thus, management and actions addressed to obtain benefits from nature (whether they are goods or services) must be tackled with care in order to maintain ecosystems in good conditions. Complex relations among components and processes of natural ecosystems are responsible of the specific ecosystem functions on which the provision of ES depend ultimately (De Groot et al. 2002). Accordingly, if we want these natural functions to be maintained over time and space, it is necessary to keep the use of “associated goods and services” under a sustainable threshold (De Groot et al. 2002). “The capacity of ecosystems to provide goods and services depends on the related ecosystem processes and components providing them and the limits of sustainable use are determined by ecological criteria such as integrity, resilience and resistance” (De Groot et al. 2002). These criteria, and not exclusively those related to economic profit, must govern the natural resources management by society. Therefore, our relationship with the environment should be based on premises that go beyond our own benefit, and bear in mind the support of the material base that makes the provision of ES possible. ES can be interpreted as a flow of “materials, energy and information from natural capital stocks” that, ultimately, help to provide human welfare (Costanza et al. 1997). These “lateral flows” are based on movements of biomass, organisms and their genes, nutrients,
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water, etc. through the landscape (Swift et al. 2004). However, despite their relevance to society, human practices are altering these flows (Daily, 2003), and the extent of the consequences is not clear yet. If we follow this point of view, we could describe the sustainable use of ES as the use that maintain the resource base that allows the flow of ES: in other words, it is relevant to keep safe a resource base that ensure the future exchange of flows (or the future harvest of a certain ES) (MEA, 2003). Considering all of this, the assessment of sustainability and the state of the services should bear in mind stocks and flows of ES and ecosystem resilience, as the ability of a system to return to its original state after a perturbation (MEA, 2003). That is, sustainability should not only focus on the state of the flow, but also on the state of the resource base that make possible the flow of natural services. As noted the MEA (2003), the flow of provisioning services do not exactly represent their condition, as the given flow may or may not be sustainable in the long‐term. In fact, almost as a rule, the production of useful resources for humans has been maintained in the short term at the price of compromising its sustainability in the long‐term, however, both the resource we benefit from and the productive potential of the ES that provides that resource should be evaluated (MEA, 2003). That is the case of conventional agricultural management, which has produced huge amounts of foods at the price of degrading the land sustaining that overproduction, that is, at the price of degrading the “capability of the ecosystem to maintain production” (MEA, 2003). And this is applicable for the rest of categories of ES. Albeit each group of ES has its particularities, the point is that sustainability does not only depend on the short term provision of the service, but on whether this provision is not overconsuming the stock and compromising the future provision of the service. ES are not easily quantifiable in monetary terms and they are not included/recognized in commercial markets, what has been said to underlie the little attention that ES have been given by policy makers (Costanza et al. 1997). Even when economic valuation is carried out, it usually fails to reflect the long‐term value of ES (from agroecosystems) (Wood et al. 2000:9). Connections among ES and natural processes are complex and multilateral. ES can be both positively realted, if they show a synergistic relationship, and negatively related, if the provision of one ES jeopardize the provision of a second ES (Trabucchi et al. 2014). Therefore, the study of ES must be tackled under a comprehensive approach (ES should be considered in conjunction) instead of focusing in a single ES, because if focusing on one service, trade‐offs among services can create declines in some ES (MEA, 2005). Indeed, “ecosystem processes and services do not always show a one‐to‐one correspondence: sometimes a single ES is the product of two or more processes, whereas in other cases a single process contributes to more than one service” (De Groot et al. 2002). In this sense, sustainability, in the broad sense of the term, relies on a balanced use of the ES that humans benefit from. The conservation of that equilibrium is of vital relevance for ecological and managed systems. Marketable or not, ES constitute a natural capital without which we could not cover our necessities, neither nourishment necessities nor social ones. Researching and working to understand processes that ensure them and the links among different ES, as well as the negative effect that human activities have on them should constitute a major concern among scientific community, institutions and policy makers. Linking with the next section regarding agriculture activity, and considering the negative effects of agriculture industrialization on ES (Bommarco et al. 2013; Kremen et al. 2002; Matson et al. 1997), the sustainability of ES related to the agrarian issue must be tackled through the replacement of “the reliance on external‐inputs by the re‐establishment of ES generated in the soil and in the landscape surrounding the cultivated field”, at the same time that keeping a high and stable yield (Bommarco et al. 2013).
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3.2.2. Ecosystem services and agriculture Environmental sustainability is necessary linked to a way of food production that “protects, uses and regenerates ecosystem services” (Ponisio et al. 2015). Defined as a “a biological and natural resource system managed by humans for the primary purpose of producing food as well as other socially valuable nonfood goods and environmental services” (Wood et al. 2000) it remains clear that agroecosystems management impacts human well‐being beyond the food that we eat. Agriculture activity is both a provider and a recipient of ES: it supplies provisioning, regulating and cultural services while relying on services provided by the environment to maintain its productive capacity (Swinton et al. 2007). Dale and Polasky (2007) stablished three connections between ES and agriculture that could be summarized in the “existence of ES both from and to agriculture”: (1) agro‐ecosystems provide from ES such as soil retention, food production, and aesthetics; (2) agro‐ecosystems receive ES from other ecosystems from non‐agricultural ecosystems (such as pollination); and (3) ES from non‐agricultural systems may be impacted by agricultural practices. Swinton et al. (2007) concreted this relation by stablishing that agriculture is not a “field‐based enterprise” but a “landscape‐based enterprise”. That is, the relation between agroecosystem and its environment is bidirectional, and have both negative and positive impacts: ecosystem services and diservices can be delivered from ecosystems to agroecosystems and viceversa (Figure 7) (Zhang et al. 2007). The provision of ES by farms is directly determined by their design and management, and the impacts of those ES can be on‐farm and off‐farm (Garbach et al. 2014; Zhang et al. 2007). This is of great relevance, as recognizing the good and harmful effects on the environment of agriculture should lead to notice the reach of a well‐designed agroecosystem. If ES help us to be aware of benefits that we are given by nature, under an agricultural context it is necessary to think thoroughly about how management practices can impact on the function of ecological systems and, thus, on the ES provided (Dale and Polasky, 2007). We should focus efforts on how agriculture “produces rather than consumes ES” (Boody et al. 2005). That is, it is necessary to evaluate the flow of ES form agro‐ecosystem over time and how these ES have been altered by the agrarian practices (Dale and Polasky, 2007). As previously and specifically mentioned for ES derived from nature, under an agriculture context, there must be an integrated approach to determine the sustainability of the flow of ES to society. And this approach must focus not only in the good we are directly working for, but for all the surroundings ES that make it possible. If we want to benefit from the flow, it is clear that we have to take care of the stock. This approximation will be handled in the next section 2.3., where the agrarian metabolism and the main theoretical aspects of this proposal will be drawn. Off‐farm benefits and harms are externalities that should be considered, either to try to avoid them or to enhance the positive impacts. At this respect, sustainable farming practices such as those promoted by organic farming principles have been compared to those of conventional farming and proved to maintain and enhance the ecological value of ES (Sandhu et al. 2008).
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Figure 7. Ecosystem services (solid arrows) and diservices (dashed arrows) flows from and to the agroecosystem. Source: Zhang et al. 2007
Besides production of food and materials for human consumption, some other ES related to agricultural management are water, air and soil quality, water quantity, C sequestration, pollination, seed dispersal, pest mitigation and biodiversity (Dale and Polasky, 2007). Inversely, the long‐term productivity of agroecosystems is influenced by natural processes such as soil formation, nutrient and water cycling, pollination and water purification (Wood et al. 2000). In the side of diservices from agriculture, degradation and loss of habitats and degradation of water and soil quality, along with other off‐site negative impacts have been unveiled (Garbach et al. 2014). In order to generate and maintain a positive balance between services and diservices from agriculture, it rises the necessity for giving the relevance they deserve to ES and the role that farmers play in their provision as agroecosystem managers. Both agriculture and farmers must turn to the production of ES, besides their role in the cultivation of species relevant for human consumption (Porter and Steen, 2003). Sustainability of agroecosystems relies to some extent on the condition that farmers start to “look upon themselves as ecosystem services providers” (Porter and Jensen, 2003). And this is inextricably linked to the “multifunctional agriculture” approach, that considers agricultural activity not only from its food and fiber producer role, but also from the relevant functions linked to agrarian practices “such as the management of renewable natural resources, landscape, conservation of biodiversity and contribution to socioeconomic viability of rural areas” (Renting et al. 2009). This new paradigm of multifunctionality contributes to build new models of agricultural activity in response to the crisis of the dominant model of agricultural modernization (Van der Ploeg and Roep, 2003) and to strengthen the central position of agroecosystems within the relationship between society and environment (Guzmán and González de Molina, 2017). A recent meta‐analysis has shown that conservation farming practices such as minimal soil disturbance, mulching or cover cropping can help to preserve regulating services in
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Mediterranean agroecosystems (Lee et al. 2019). In Spain, 31.3% of total land area is occupied by arable land (INE, 2016), that is, a third of Spanish area is under the farmers’ hands, and its conservation or degradation depends on farmers’ management and work. Obviously, with this we should not restrict responsibilities of the environment degradation to agricultural practices from farmers. Nevertheless, they are the agent closer to environment conditions, and their role should not be ignored. This condition does not seem to be easily discardable in the development of new agricultural designs. There is a great challenge for converting these fields into the basis to propel the transition towards a new model of farmer‐nature relationship, as well as the recognition and valuation of farmers’ position in society. The more easily identifiable ES delivered from agriculture are crop and livestock production, which at the same time are the best quantified services form agriculture (Dale and Polasky, 2007). The provision of these services entails large impacts on others ES, especially after the industrialization of agriculture and the dramatic increase in chemical inputs and irrigation in farming systems (Tilman et al. 2002), without which modern agriculture would not be productive (Altieri, 1999). To illustrate this increase in productivity wheat yield increments are shown in Figure 8. In countries from the Mediterranean basin, wheat yield greatly increased from 1961 to 2017. On average, wheat yield in 2017 was 3 times higher than that of 1961, and Albania (5.2 times) and Israel (1.5 times) harbor the highest and lowest increments, respectively. As reported in the Millennium Ecosystem Assessment (2005), many ES related to agriculture are in decline due to over‐exploitation, the use of chemical inputs, the fossil‐fuel combustion, etc., and many other practices linked to agricultural intensification. By the beginning of the 21th century, approximately 62% of ES assessed were degraded or misused because of agricultural management and other human activities (MEA, 2005). The increase in food outputs has been met at the expense of negative impacts on the provision of other ES (Wood et al. 2000), being a major trade‐off between agriculture and ES (Power, 2010; Wood et al. 2000). The simplification of ecosystems, that enabled the enhancement of provisioning services, also involved the decline in regulating ones (Cardinale et al. 2012), such as biodiversity, C storage, soil, air and water quality, pest regulation, and climate regulation at local and regional scales (MEA, 2005; Wood et al. 2000). Additionally, the provision of wood fuel and natural medicines, genetic resources and fresh water stocks are provisioning services in decline due to agricultural intensification (MEA, 2005).
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Figure 8. Wheat yield evolution from 1961 to 2017 in countries from the Mediterranean basin. Average is calculated from data of all countries in the Mediterranean basin, even those not represented in the graphic. Source: FAOSTAT
Paradoxically, current external input‐based agriculture, excused on the premise of the need of higher food production, can reduce the ability of these systems to provide some ES, offsetting their ability to produce large amounts of food and fiber (Matson et al. 1997; Tilman et al., 2001). That is, the negative impacts of agricultural intensification can have negative feedbacks on crops yield (Matson et al. 1997), risking the entrance in a vicious circle that does not seem very logic, as we are more and more reducing the capacity of food systems to precisely produce so, food. For instance, as Bommarco et al. (2013) highlighted, agricultural productivity of developed countries is near to its maximum level, but not without negative impacts such as the appearance of pesticide resistance or the reduction in SOC, trade‐offs of the high levels of external inputs use that make the stability and resilience of agrarian production to wobble. “A major concern is that the increased agricultural production over the past 50 years has come at the cost of the ecological sustainability that will be necessary to maintain productivity in the future” (Dale and Polasky, 2007). The need to address the threats to ES is more acute in agroecosystems than in other ecosystems (Robertson and Swinton, 2005) so that agricultural land can increase the rate at which it provides ES besides the production of food and fiber (Sandhu et al. 2010). 3.2.3. Soil and ecosystem services
The soil is a supplier and regulator of many essential provisioning, regulating, cultural and supporting services for people’s and environmental health (Adhikari and Hartemink, 2016), partially by virtue of two characteristics: the wide range of biogeochemical processes that it harbors and the richness and functionality of soil biodiversity (Smith et al. 2015b). However, soil disturbance with a human origin like agricultural practices, deforestation, pollution and global change affects its provision of ES (Brussaard, 1997). Albeit soil health relevance, Green Revolution technologies and approach “exacerbated the problem of soil and environmental degradation” due to (Lal, 2009b): (i) simplification of cropping systems and the reduction of legume‐based rotations, (ii) excessive use of irrigation, (iii)
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
1961
1963
1965
1967
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
2007
2009
2011
2013
2015
2017
kg ha‐
1
AVERAGE Albania Algeria EgyptFrance Greece Italy LebanonMorocco Spain Tunisia Turkey
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unbalanced use of fertilizers comprising mostly N input with little input of P, potassium (K) and micro‐nutrients and (iv) crop residues removal for fodder and uncontrolled grazing without the benefits of mulch farming, and the excessive use of plow tillage. Lal (2009b) drew many situations that unveiled the soil degradation process in croplands: (i) depletion of SOM, (ii) nutrient imbalance, (iii) soil erosion, (iv), waterlogging and salinity in irrigated lands, and excessive withdrawal of water in regions with tube well irrigation, (v) degradation of soil structure (crusting and compaction), and (vi) decline in soil water and nutrient retention capacities. One of the most severe effect of soil degradation is the increase in GHG emissions, and the shift from being a sink to a GHG source (Lal, 2015). According to all of this, for future sustainability of food production systems, it is compulsory to restore quality and productivity of degraded soils and ecosystems by increasing SOM, improving soil structure, conserving water in the root zone, enhancing soil fertility through avoiding C and nutrient losses, strengthening nutrient cycling processes, and improving soil biology (e.g., earthworm activity), etc. (Lal, 2009, 2015). Soil fertility, as the ability of soils to provide nutrients to plants (Brady, 1990), is regarded as the most important supporting service related to agroecosystems, as it is fundamental for the agrarian productivity (Swinton et al. 2007). Nevertheless, the soil is not only essential for the production of food, fodder, renewable energy and raw materials, but the good health state of soils underpins the provision of other relevant ES. Many studies have reported the role of soils in atmospheric C sequestration (Lal, 2004b), water filtration and purification (Lal, 2015; Smith et al. 2015b), food security (Lal, 2009b) and biodiversity conservation (Pulleman et al. 2012). Likewise, the preservation of soil biodiversity and soil biota is essential for nutrient cycling, organic matter decomposition, bioturbation and the suppression of soilborne diseases and pests (Brussaard, 1997). Moreover, it harbors filtering, buffering and transformation capacities relevant for the ground water quality and the maintenance of the food chain. Likewise, these capacities rule the gasses exchange with the atmosphere and the soil, contributing to the consideration of the soil as a relevant actor in the climate change mitigation and adaptation strategies (Blum, W.E., 2005; Lal 2004a,b, 2015). SOM is the most relevant component of soil quality and the base of a proper agroecosystem functioning (Lal, 2009a; Wood et al. 2000), as described in section 1.5. Regarding ES, soils with an adequate content on SOM are more productive, store C and water, regulate surface water flows and improve water quality, and are more protected against the erosion risk (Pan et al. 2013; Wood et al. 2000). Additionally, SOM provides for the energy (in the form of C) necessary to maintain soil food webs (Barrios, 2007) and soil biodiversity (Swinton et al. 2007). The contribution of SOM to ES such as crop yield or nutrient cycling is behind the support to sustainability in the long term (Barrios, 2007), and it plays an important role in soil resilience to environmental stress (Pan et al. 2013). Therefore, in order to ensure soil provision of ES, there must be a care design of management practices that enhance SOM content and soil quality. Wood et al. (2000) summarized the links between SOM and ES as follows:
SOM stabilizes and holds together soil particles, reducing soil erosion risk,
supplies a source of C and energy for soil organisms,
improves water retention and air diffusion,
stores and provide of N, P, and sulphur (S),
avoid soil compaction, making easier to work the soil,
C sequestration,
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retains nutrients (e.g., calcium (Ca), Mg, K) by providing ion exchange capacities,
serves to reduce the negative environmental effects of pesticides and other pollutants.
Concretely, SOC has been unveiled as a relevant indicator of the supporting service soil fertility, especially under Mediterranean conditions (Trabucchi et al. 2014). It is the soil property that is connected to the highest number of ES, based on a bibliography research by Adhikari and Hartemin (2016) in which a list of soil properties is linked with many ES. These authors found SOC to have an influence on each one of the four groups of ES: the provision of food, fuel, fiber, raw material, and fresh water (provisioning services); the climate, water and pest and disease regulation, the control of erosion and flood processes and the C sequestration (regulating services); recreation and ecotourism and esthetic and sense of place (cultural services); the soil formation, nutrient cycling and the provision of habitat are the supporting services found related to SOC (Adhikari and Hartemin, 2016). In Marks et al. (2009) it can be found a table with the main SOC related benefits: Table 2. Ecosystem services linked to soil organic carbon (SOC) pool.
On‐site benefits Off‐site benefits
Improvement in soil quality Improvement of water quality
Increase in available water capacity Decrease in transport of pollutants Increase in nutrient retention Biodegradation and denaturing of pollutants and
contaminants Improvement in soil structure and tilth Reduction in sediment load and siltation of water
bodies Buffering against changes in pH Decreased in non‐point source pollution Enhancement of soil biotic activity Reduction in hypoxia risk in water bodies Improvements in soil moisture and temperature regimes
Less damage to coastal ecosystems
Low risk by floods and sedimentation Decrease in transport of pollutants out of the
ecosystem
Increase in agronomic/forest productivity Improvement in air quality
Increase in crop yield Reduction in rate of enrichment of GHG Increase in use efficiency of input (fertilizer and water)
Decrease in wind‐borne sediments
Decrease in losses of soil amendments by runoff, erosion and leaching Improvements in soil conditions
Sustainability and food security Improvement in biodiversity
Increase in sustainable use of perturbed soil and water resources
Increase in soil biodiversity
Food security increase Improvement in wildlife habitat and species diversity on restored ecosystems
Additional income from trading C credits Improvement in aesthetic and cultural value Improvement in nutritional value of food and avoidance of hidden hunger
Desertification control
Restoration of desertified lands Reversal of degradation trends Strengthening elemental recycling mechanisms
*Source: Marks et al. 2009.
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C storage is a relevant ecosystem function of soils that is gaining attention in recent years (Smith et al. 2015b). C sequestration could be an example of the provision of mitigation services, that is, when a well‐designed agrarian management can help to reduce the harm derived from the agriculture activity (Swinton et al. 2007), and it has been addresed in section 1.5.1. Together with C sequestration, nutrient cycling is another relevant soil‐based ES, with a direct effect on biogeochemical cycles and GHG emissions (Ghaley et al. 2014). N, P, S and other macronutrients such as calcium (Ca), Mg, K and sodium (Na) are essential for life in the biosphere, and also for productivity of soils (De Groot et al. 2002). The cycling of these nutrients is, thus, a supporting ES important for food production and impacts on this process should be receiving attention from farmers and the research community. For instance, food production in agroecosystems is related to previously unknown changes in N and P cycles, with not insignificant repercussions in other ES (Garbach et al. 2014). An excessive use of synthetic fertilizers can disrupt this cycling (Cullen et al., 2004), likely due in part to the reduction in SOM and soil organism dedicated to decompose it (Paul et al. 1997). Contrarily, as stated before, organic farming and the use of manure amendments have been reported as strategies to enhance nutrient cycling in soils. In addition, nutrient cycling is related to climate and water regulation functions (De Groot et al. 2002). For example, an excess of soil N mineralization can lead to groundwater or surface water pollution and the increase in GHG emissions through nitrous emission. Also, an unbalanced increase in C respiration and mineralization can also determine the rise in CO2 emissions to atmosphere from the soil. The cycling of C, linked to the cycling of N and P (Smith et al. 2015b), is also relevant in agroecosystems, as it is the source of energy for microorganism responsible for soil fertility. The increase in plant aboveground and belowground C inputs increase microbial populations and the biological processes they mediate (Altieri, 1999; Gupta et al. 2011). The decomposition and mineralization of SOM and N fixation by soil biota, along with the P supply by mycorrhizal fungi, are biotic soil processes supporting the nutrient cycling essential for supporting the food chain (De Groot et al. 2002; Ghaley et al. 2014; Gupta et al. 2011). Factors as residue inputs quality and amount can alter the nutrient cycling of soils (Blanco‐Canqui and Lal, 2009). 3.2.4. Biodiversity and ecosystem services Ecosystems diversity is present at different levels: population (genetic diversity), community (species richness) and ecosystem (functional groups) levels (Lavelle, 1996; Moonen and Barberi, 2008). The relationship between biodiversity and ES is complex and dependent on the type of ecosystem (Garbach et al. 2014). Therefore, the understanding of the species and functional groups contribution to ecosystem functions (and their interactions), considering different ecosystems, is essential (Fanin et al. 2018). The proper functioning of ecosystems relies on a minimum composition of organisms that ensure the relations “between primary producers, consumers and decomposers that mediate the flow of energy, the cycling of elements, and spatial and temporal patterns of vegetation” (Folke et al. 1996). Although the impacts of biodiversity loss on ecosystems functioning are difficult to assess and broader approaches are needed in these investigations (Wardle, 2016), changes in biodiversity have been found to impact ecosystems functioning and their “ability to provide society with the goods and services needed to prosper” (Cardinale et al. 2012). In general, the reduction of ecosystems biodiversity involves the reduction on biomass production due to a loss of resource catchment efficiency (Cardinale et al. 2007) and a general reduction in ecosystem stability (Campbell et al. 2011) and resilience (Oliver et al. 2015). The loss of biodiversity also reduces the resilience of ecosystems to perturbations such as pest
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outbreaks and climate change, risking the long‐term provision of goods and services (Wood et al. 2000). A meta‐analysis by Hooper et al. (2012) unveiled that biodiversity loss could be considered as a major driver of global environmental changes, together with factors like CO2 increase, ultraviolet radiation, climate warming, ozone depletion or acidification. The reduction in the system resilience can threats the functions of the ecosystem and the economic activities relying on it (Perrings et al. 1995), thus it is an issue of great import in agriculture. Both the expansion of lands dedicated to agriculture and the intensification of the activity are responsible for the decline in above and belowground biodiversity and its adverse repercussions (Altieri, 1999; Lavelle, 1996; Wood et al. 2000). Regarding agriculture intensification, the use of pesticides and herbicides and intensive mechanical practices can affect agroecosystems biodiversity, risking the capacity of agroecosystems to self‐regulate and be self‐sustainable (Puig‐Montserrat et al. 2017). Conventional farming practices in agroecosystems lead them to a spatial uniformity, entailing a reduction in functional diversity and a higher sensitivity to disturbances, that is, entailing the loss of system resilience (Folke et al. 1996). Then, the need for external supplies arise because, “deprived of basic regulating functional components”, agroecosystems “lack the capacity to sponsor their own soil fertility and pest regulation”, resulting in the increase of the need for human intervention (Altieri, 1999). The effect of the loss of systems biodiversity and resilience is not perceived in the short‐term, as ecosystems can continue to function when the resilience is declining (Folke et al. 1996). However, the loss of ES due to the impacts of agriculture intensification on biodiversity has an economic and environmental cost (Altieri, 1999). Unfortunately, this “natural insurance capital” (Barbier et al. 1995) and human activities that compromise it are not included in market prices but considered externalities, hindering the understanding of the true cost of its loss (Folke et al. 1996). In agroecosystems, the increase in genetic and species diversity of both crops and beneficial or pest species can interact and impact the system processes related to production and environmental protection (Moonen and Barberi, 2008). The cycling of nutrients (Handa et al. 2014), the regulation of microclimate and local hydrological processes, crop protection and the enhancement of soil fertility, together with the elimination of noxious organisms and toxic chemicals have been related to biodiversity (Altieri, 1999). Additionally, biodiversity is responsible for providing of genetic material for breeding and diversity of species for pollination (Wood et al. 2000:3). Generally, the improvement in functional biodiversity in agroecosystems is closely related to their sustainability (Moonen and Barberi, 2008) through the improvement in soil fertility, crop protection and productivity (Altieri, 1999,). “Correct biodiversification results in pest regulation through restoration of natural control of insect pests, diseases and nematodes and also produces optimal nutrient recycling and soil conservation by activating soil biota, all factors leading to sustainable yields, energy conservation, and less dependence on external inputs” (Altieri, 1999). A revision of more than 1700 papers that summarizes the linkage between biodiversity and ES can be found in Cardinale et al. (2012). These authors found the following effects of biodiversity on provisioning and regulating services: Regarding provisioning services, they found that (1) intraspecific genetic diversity involved the increase of the yield of commercial crops; (2) tree species diversity enhances production of wood in plantations; (3) plant species diversity in grasslands enhances the production of fodder; and (4) increasing diversity of fish is associated with greater stability of fisheries yields. For regulating processes and services, (1) increasing plant biodiversity increases resistance to invasion by exotic plants; (2) plant pathogens, such as fungal and viral infections, are less prevalent in more diverse
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plant communities; (3) plant species diversity increases aboveground C sequestration through enhanced biomass production (but see statement 2 concerning long‐term C storage); and (4) nutrient mineralization and SOM increase with plant richness (Cardinale et. al 2012). As Southwood and Way (1970) enumerated (in Altieri, 1999), the degree of biodiversity of agroecosystems depends on 4 factors:
1. The diversity of vegetation within and around the agroecosystem. 2. The permanence of the various crops within the agroecosystem. 3. The intensity of management. 4. The extent of the isolation of the agroecosystem from natural vegetation.
Biodiversity of weeds in agroecosystems Related to the first factor of the enumeration, weed biodiversity in croplands can result in benefits to the whole system. Weed competition for light, nutrient and water can reduce crop production by limiting access to these crucial resources (Welbank, 1963), and that is the reason why farmers have to maintain that populations under control. The use of herbicides and chemical fertilizers is a major factor underlying the decline in weed biodiversity of conventional fields (Hald et al. 1999), related to the reduction of relevant functional groups and the spread of certain weed species much harder to eradicate, such as grasses (Chamorro et al. 2016). Weed biodiversity relevance within croplands is not receiving attention from conventional farming, but it is related to relevant processes in agroecosystems that should be borne in mind. On the one hand, the reduction in above ground plant diversity can affect belowground diversity and abundance of soil biota (Hooper et al. 2000), which in turn can affect soil ES. On the other hand, concretely, weed biodiversity has been shown to benefit the presence of pollinators through the provision of food and refuge (Nicholls and Altieri, 2013). Different weed species attract different pollinators, and the presence of alternative floral resources when crops are not flowering ensures the presence of pollinators in farmlands (Carreck and Williams, 2002). Pollination is essential for most modern crops and the productivity of croplands would be severely compromised without it (De Groot et al. 2002). Following Klein et al. (2006), 35% of crop production in the world and 60‐90% of plant species rely on pollinators activity, making this service crucial for agriculture. Together with the use of pesticides, the use of broad‐spectrum herbicides to reduce weed infestation in croplands can jeopardize pollinators populations in farms due to the reduction in habitat diversity (Kearns et al. 1998; Richards, 2001). Contrarily to the proposal of modern agriculture of total weed eradication through the use of herbicides, the maintenance of weed population under a threshold, instead, can help to enhance the floral diversity needed to provide nesting habitat and floral resources that sustain the pollination service (Fussell and Corbet, 1992). Carvalheiro et al. (2011) found that the conservation of weed diversity and the consequent enhancement of pollinators population, had positive impacts on yield, contributing to the sustainability of croplands due to the lower necessity of increasing the area of land under cultivation. Regarding the present study, even if cereals are not animal pollinated plants, pollination enhancement can benefit adjacent crops and wild plants present in the landscape, promoting the conservation of biodiversity across the “agro‐natural landscape” (Kremen et al. 2002). Weed biodiversity is also positively related to biological pest control through the presence of natural enemies (Altieri and Nicholls, 2004; Norris and Kogan, 2005). For example, weed biodiversity can indirectly enhance biological control through the presence of the food for natural enemies when their prey has disappeared, allowing them to stay in the field (Caballero‐López et al. 2012). Concretely, the presence of weeds from the botanical families Umbelliferae,
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Fabaceae and Asteraceae has been said to harbor and support beneficial arthropods that reduce pest populations (Altieri, 1999). On the other hand, higher trophic levels, such as birds, can also be affected by the reduction of vegetal biodiversity mainly through the reduction in the provision of shelter and food (Chamorro et al. 2016; Marshall et al. 2003). Summarizing, agricultural intensification has led to the reduction of important ES such as pollination and natural pest control through the decrease in weed biodiversity within fields, among others factors such as landscape complexity reduction or the elimination of rotations and polycultures, etc. (Altieri, 1999). The avoidance of weed eradication through the use of herbicide should be tackled, together with the premise of conserving weed biodiversity in croplands in order to maintain ES linked to it. Soil biota diversity Soil has been said to be the main basis of biodiversity which human depends on, as it harbors more species than all the aboveground biodiversity components together (Blum, W.E., 2005), although both above and belowground biodiversity are inextricably linked (De Deyn et al. 2005; Hooper et al. 2000). Soil organisms and their biodiversity promote many ecosystem functions simultaneously, and the enhancement of soil biodiversity can increase agroecosystem sustainability and stability (Bender et al. 2016). Concretely, species richness of soil organism is not as relevant as the presence of certain species, the number of trophic levels, the community composition or functional dissimilarities in nutrient and C cycling. Thus, the focus should be put on the groups of soil organisms that play major roles in ecosystem functioning and the delivery of ES, such as symbionts, decomposers, transformers, engineers, etc. (Bommarco et al. 2013). Soil biota is determinant for SOM turnover, together with other ES as biological control, the maintenance of a good soil structure, crops yield, N fixation and the control of roots diseases (Gupta et al. 2011). Good quality and nutrient content of soils (Ghaley et al. 2014), together with changes in C and N inputs, soil physical properties and harmful effects of chemical fertilizers (Altieri, 1999) are determinant for plant growth, soil biota and soil organism biodiversity. In turn, nutrient cycling and C sequestration “are dependent on above‐ and belowground flora and fauna diversity” (Ghaley et al. 2014). For example, the loss of soil organisms diversity can exert a negative influence on C global dynamic, since soil organism are directly related to soil C dynamic (Nielsen et al. 2011). That is, the correct functioning of agroecosystems and the maintenance of ES they provide, as well as those they depend on, is a complex issue that require for complex agroecosystem management and practices. Their simplification, and the simplification of the relations among their components, lead to the decline in sustainability and resilience. Since soil C is a source of energy for soil organisms, plant root and plant aboveground C inputs have great influence on the population of soil biota and the biological processes it mediates (Gupta et al. 2010). Plants also affect soil C cycling by their growth rate, affecting both C loss through the respiration process and C input (De Deyn et al. 2008); by the loss of C through volatile organic C (Kesselmaier et al. 2002); and by the “whole‐plant structure and partitioning of C and nutrients between plant organs” (De Deyn et al. 2008). Very relevant plant‐traits related to C cycling are metabolic, morphological and physiological root traits, which have strong influence on the soil C loss and gain balance (Bardgett et al. 2014). Beside impacting C cycling, root traits such as root exudation play an essential role in soil nutrients cycling (Bardgett et al. 2014). In this sense, the rhizosphere processes mediated by microbes are relevant drivers of
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SOM decomposition and nutrient cycling with potential consequences for global C stocks (Finzi et al. 2014). Rearding soil biota, roots mycorrhizal colonization can help to improve growth, nutrient uptake and yield of crops (Gosling et al. 2006), while showing positive impacts on soil C cycling (Brzostek et al. 2015). This simbiosis can also ameliorate water stress tolerance of plants and contribute to a better soil structure (Schreiner and Bethlenfalvay, 1995). Mycorrhizas can contribute to higher nutrient use efficiency under limited nutrient conditions (Hildermann et al. 2010). Factors affecting this simbiosis are, thus, relevant for low‐input and organic farming conditions. On the one hand, the use of organic manures, compost and slow‐release mineral fertilisers, together with a carefull design of crop rotations and the absence of biocides, can benefit the association (Gosling et al. 2006). In this sense, the use of water‐soluble fertilizers by conventional farming can adversely impact root colonization by arbuscular mycorrhizal fungi (Mäder et al. 2000), while the use of biocides impoverishes abundance and species diversity of mycorrhizal (Goslling et al. 2006). Generally, mycorrhizal diversity has been found to be lower under conventional farming conditions than under organic ones (Oehl et al. 2003). Interestingly, some studies have found that wheat landraces display stronger growth responses to the mycorrhizal simbionts than modern ones (Hetrick et al. 1992; Zhu et al. 2001). These authors suggested that breeding programs are partially responsible for this lack of response (Hetrick et al. 1992; Zhu et al. 2001), since the use of mineral fertilizers by such programs could have led to the loss of the need for stablishing the simbiosis by modern cultivars and, thus, to the reduction in the frequency of genes for mycorrhizal dependence in wheat (Hetrick et al. 1992). Contrastingly, Hildermann et al. (2010) did not find a correlation between the year of release of varieties and the mycorrhizal colonization of roots. Biodiversity and traditional cultivars One of the most relevant impacts of intensification was the replacement of traditional varieties by modern ones, whose genetic uniformity has jeopardized the climate change adaptation possibilities of crop production in marginal areas and the traditional knowledge linked to landraces, among others. The loss of this diversity is associated with an extreme vulnerability of productive systems (Altieri, 1999) due to the loss of resistance to diseases and the possibilities for cultivating in a variety of soil types and under different micro‐climates (Brush, 1982). In words of van Etten et al. (2017), “diverse seeds are needed to support the diversification of agriculture, which in turn may contribute to more diverse diets, and to using species and varieties for the integrity of ecosystem services”. A more detailed information regarding traditional varieties can be found in the Introduction and sections 3.1.2. and 3.1.4. Ecosystem services can be seen as the result of complex interactions between biotic and abiotic ecosystem factors through the universal driving forces of matter and energy (De Groot et al. 2002). This, linked to what has been exposed in last section regarding the sustainability of stocks and fluxes of ecosystem services (the resource base that ensure the provision of ES), can help to introduce the next section. The need for maintaining and considering stocks and flows of biomass and energy when thinking agrarian sustainability cannot be pushed into the background anymore, and must be face in order to ensure future food production and the health of the environment.
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3.3. SOCIAL AND AGRARIAN METABOLISMS. THE NECESSARY APPROACH TO SUSTAINABILITY
3.3.1. Humans and Nature relationship. Social metabolism
From the entry of the modern thinking, the conceptual and cultural framework that structured our conception and relationship with nature has been based on the domination of nature by man (Merchant, 1980). The change from an organicist perspective in which human being and nature were part of a whole, to a mechanistic perspective in which nature became an inert subject to our needs, is the cultural and philosophical basis of the environmental crisis that we suffer nowadays (Merchant, 1980).
While the organic framework was for many centuries sufficiently integrative to override commercial development and technological innovation, the acceleration of such changes throughout western Europe during the sixteenth and seventeenth centuries began to undermine the organic unity of the cosmos and society (Merchant 1980:31).
According to Bruno Latour, modern thinking and science dissociated natural and social realities as object and subject, respectively (Fischer‐Kowaski, 2003). As one of the consequences of this framework change, social and natural sciences grew in the distance, without any interaction and considering their study objects as separated and independent ones. The environmental crisis during 1970s, which altered both scientific community and social movements, constituted a turning point in the historical narrative with the inclusion of the concerns related to natural environment (González de Molina and Toledo, 2011). In the search of a new way of thinking our relation with nature and a different assessment of the historical link between human being and its environment, Environmental History entails a new paradigm for interpreting the evolution of societies without ignoring the environment that has contained them.
Essentially, environmental history deals with the material basis of social relationships. In this sense, it is consequently materialistic. This qualification does not imply that it opts for a materialist theoretical approach to another idealist, as the dichotomy artificially posed in the social sciences during Modernity. Materiality here refers to the object of environmental history, the flows of energy and materials to which all human practice can be reduced, but also to the also material nature of any cultural dimension of human practice (González de Molina and Toledo, 2011:28).
Just like Sieferle (2001) stated, there are evidence from last decades showing changes occurred in nature with a historic time dimension, making evident “an interaction between cultural and material dynamics”. The emergence of this new paradigm allowed to establish physical limitations to the action of human beings, determined by the exchange of materials and energy with nature (González de Molina and Toledo, 2011). The main driver of environmental history becomes the study of the sustainability of societies over time, in "coherence with its materialistic vocation and with the material condition of all social relationships" (González de Molina and Toledo, 2011: 42). In the light of a new attempt at historical interpretation based on reciprocity in the interaction between human beings and nature (Sieferle, 2001), the concept of social metabolism, previously introduced in the social sciences and Marx's historical materialist analysis, was taken up again. A brief description of the evolution of the term and its use can be found in Fischer‐Kowalski (2003).
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People and societies appropriate natural resources (whether in the form of materials or energy), circulate, transform, consume and excrete them. Just as organisms exchange materials and energy with the environment to maintain their growth and reproduction, societies extract materials from the environment, manufacture them to make them useful to societies and return wastes and emissions to nature (Fischer‐Kowalski and Haberl, 1997). The links that exist between human beings and nature have a double meaning or a "reciprocal determination": the forms and functions that societies operates determine their nature appropriation, and nature, in turn, influences the configuration of societies (González de Molina and Toledo, 2011: 60). This approach, which tries to overcome parcelarian determinisms, is recognized within the complex thinking, since it tries to “recognize and apprehend the complexity of reality” against the “simplification paradigm” that governs current thinking system (Morin, 1994:14, 15). The world of “simple systems” or “mechanisms” does not correspond to the world of natural systems, the world of the complex, which cannot be understood through the Newtonian model (Mikulecky, 2000). Understanding nature as a complex system necessarily bring us to the conception of a complex relationship between humans and nature. Moreover, if we applied this new paradigm of complex systems to this relationship, we must discard the idea that these components are parts of the system, understanding the system as the sum of the parts. The complex paradigm leads us to comprehend both component of the relationship as functional components (instead of parts) “totally dependent on the context of the whole system” and “with no meaning outside the context” (Mikulecky, 2000). In other words, both humans and nature are dependent on the context of the relationship, and it is hardly untestable the existence of each one outside this relationship (this context), that is, both are inextricably linked. The study of human beings and nature relationship through the social metabolism approach imply the exploration and identification of five processes: appropriation, transformation, circulation, consumption and excretion (González de Molina and Toledo, 2011). From a metabolic point of view, each metabolic arrangement endures sustainability constrains, to some extent, related to certain metabolic processes. For example, if the scarcity of resources was the main problem of hunter‐gatherer societies (appropriation), agrarian societies found ecological constraints in the limited possibilities to increase biomass production and industrial societies are mainly constrained by the excess of wastes and pollution (the output side, or excretion) (Fischer‐Kowalski and Haberl, 1997). Nowadays, the drivers of global environmental change are wastes and by‐products of human activity (Vitousek et al. 1997). The important question that arises from this revelation is given by the existence of a “risk spiral” (Sieferle and Müller‐Herold and, 1997, in Haberl et al. 2010): within the dynamic interaction of societies with the environment, the overcoming of a certain risk confronts us with new risks that should be also overcome. Currently, there are many methodologies used to quantify the social metabolism at different space and time scales. The Material and Energy Flow Accounting (MEFA) (Haberl et al. 2004), the Multi‐Scale Integrated Analysis of Societal and Ecosystem Metabolism (MuSIASEM) (Giampetro et al. 2009), Human Appropriation of NPP (HANPP) (Imhoff et al. 2004; Haberl, 1997) or the ecological footprint (Wackernagel and Rees, 2002). Social metabolism is easily reduced to the measurement of inputs and outputs of energy or materials between a society and its environment, without considering internal and immaterial processes (González de Molina and Toledo, 2011:64). Nevertheless, the metabolic process takes place within a certain institutional and cultural organization, therefore it is affected by this immaterial articulation. Likewise, if we have already crossed the borders of complex thinking when considering the relation between the human being and nature as a reciprocal one, it is not less important to bear in mind that this relationship is complex in itself and whose descriptors are interconnected in such a way that it is not possible to modified one without altering the others.
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The formulation of the Laws of Thermodynamics led to the return to the complexity of reality in the scientific thinking (Morin, 1994:18) and entailed the acceptance of a new paradigm that had a strong influence on the study of the relation between human beings and nature: human beings cannot create matter nor energy (Georgescu‐Roegen, 1993). Georgescu‐Roegen expanded and applied the concept of entropy within the world of economics recognizing, despite the difficulties of the term, that the human being (and the economic process) is subject to the same natural laws as the rest of the living beings on the planet. In this context, he tried to make clear and explain what he identified as myths of the dominant economic thinking:
Once, man believed that he could move things without consuming any energy, which is the myth of perpetual motion of the first kind‐‐certainly, an essentially economic myth. The myth of perpetual motion of the second kind, which is that we may use the same energy over and over again, still lingers on in various veiled forms. Another economic myth‐that man will forever succeed in finding new sources of energy and new ways of harnessing them to his benefit (Georgescu‐Roegen, 1975:349).
In the same manner, dynamics within ecosystems also obey the laws of conservation of matter and energy (Burkhard et al. 2011; Ulanowicz, 2004). Within an agricultural context, sustainability of production must be based on thermodynamic principles, that is, must bear in mind that agroecosystems are inserted in a finite world and that materials flows are not infinite. This appreciation became very relevant, since it resulted incompatible the defense of a continuous sustainable development within a finite world. Human beings appropriate both goods and services from their environment, therefore, the survival of societies is based on the maintenance of the material base on which it is based. If population and material consumption per capita grows following the predictions of the scientific community, the development would be incompatible with “the perpetuation of a well‐functioning biosphere” (Smil, 2013:190). Additionally, it is not only that the world is not infinite and we can use resource unconsciously because they will not finish, but also that human beings share the planet with many other species whose survival depend also on the same material base. 3.3.2. Agrarian metabolism and a biophysical approach to sustainability
Agriculture can be regarded as an activity to colonize nature and make it “more exploitable to social needs” (Fischer‐Kowalski and Haberl, 1993), being considered responsible for the greatest large‐scale transformations of terrestrial ecosystems due to the human activity (Smil, 2013). Therefore, it seems necessary to deepen the impact of this relationship between human beings and nature. In this context, rural or agrarian metabolism would be the realm of social metabolism focused on the appropriation process (González de Molina and Toledo, 2014:71). In other words, the study of the material, energy and information exchange between society and its agrarian environment (Guzmán et al. 2018), which is specialized in producing biomass and ecosystem services for human consumption (Guzmán and González de Molina, 2017). A systemic perspective of the relationship of human beings with nature must necessarily lead us to reject the idea that we can alter the outputs of a system without changing both the inputs to the system and their internal movement within it (Fischer‐Kowalski, 2003). That is, when seeking for sustainable ways of food production, it is important not only to consider the level of natural resources usage, that is, to recognize the productive potential of agroecosystems (González de Molina and Toledo, 2011), but also how to leave room to a self‐regeneration process within nature (Fischer‐Kowalski and Haberl 1997). Like other colonizing activities, agriculture faces the problem of how to maintain the “basic self‐regenerating quality” and “how to set limits to exploitation beyond production” (Fischer‐Kowalski and Haberl, 1993).
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From an agroecological point of view, sustainability of farming systems should be based both on the efficiency of the management and the health state of the agroecosystems components (Guzmán and González de Molina, 2017). To this purpose, it is crucial to distinguish between flows and fund elements. According to Georgescu‐Roegen (1971), flows make reference to the materials and energy that are consumed or dissipated by the fund elements or structures that generate goods and services. In this sense, a biophysical approach is needed to address agrarian sustainability, and it lies on the need of shifting the focus away from the energy and material flow analysis towards the assessment of the fund elements state. We should focus on whether flows of energy and materials that enter and exit the agrarian sector are capable of reproducing (or improving) fund elements in successive production cycles (Guzmán et al. 2018). According to Georgescu‐Roegen (1971), the aim of the agrarian activity is not exclusively the production of biomass, but also the reproduction of the fund elements that are required to produce it. “The evaluation of the maintenance of fund elements allows us to recognize which of them are undergoing processes of deterioration and which are being improved and restored. In addition, it allows us to predict whether the modification of a flow to improve a fund element will harm another fund element and the extent to which there can be compensations between both uses. This is so because flows of energy and materials are interconnected” (Guzmán et al. 2018). 3.3.3. The role of biomass in agroecosystems and societies
Contrarily to ecosystems, agroecosystem depends on societal external energy for their maintenance and reproduction (Gliessman, 1998). The industrialization of agriculture brought the higher and higher dependency on abiotic sources of energy and materials because, among other factors, modern agriculture has relied on the idea that soil fertility can be maintained through the use of chemical fertilizers, forgetting the fundamental role that the replenishment of fertility through organic amendments had in the past (Garrabou et al. 2010). For example, the recycling of crop residues was a relevant source of organic matter and plant nutrients in traditional agriculture while nowadays they are unfortunately considered as wastes (Smil, 2013). Accordingly, agrarian activity changed from being an energy supplier to became a net energy consumer (Guzmán and González de Molina, 2017), due to the reliance of industrial agriculture on fossil fuel, which make of agrarian activity a profound inefficient one (Pimentel et al. 2005). So much so that although productivity multiplied by four during the last century, energy inputs did by eight (Smil, 2001:256). Net primary production (NPP) is defined as the net amount of solar radiation absorbed and transformed in plant organic matter through the process of photosynthesis, and constitutes “the primary food energy source for the world ecosystems” (Imhoff et al. 2004). The human appropriation of NPP is the difference between the NPP of undisturbed ecosystems and the amount of biomass currently available in ecological cycles (Haberl, 1997), that is, “the amount of terrestrial NPP required to derive food and fibre products consumed by humans, including the organic matter that is lost during the harvesting and processing of whole plants into end products (Imhoff et al. 2004). A global estimate showed a percentage range of 3‐39% of human appropriation over NPP (HANPP) (Vitousek et al. 1986). A deeper analysis of the attempts made to account for NPP in the biosphere can be found in Smil (2013). Although biomass is no longer the major supplier of energy required by humans, it still have an indispensable role within societies (Kraussmann et al. 2008). The appropriation of NPP by societies varies spatially, with many regions of the world overconsuming its local primary production and being forced to import large quantities of biomass from other places, unveiling an unequal footprint and environmental impact (Imhoff et al. 2004). In 2009, it was estimated that every year up to 1.7 PgC of embodied HANPP was transferred from sparsely populated
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regions devoted to its production to densely populated and consumer ones (Erb et al. 2009), showing differences in environmental impacts among those regions. Extensive environmental impacts related to the biomass consumption by human populations have been reported, such as alterations of land surface and biogeochemical cycles (Erb et al. 2009; Vitousek et al. 1997), biodiversity losses (Haberl, 1997) or the reduction of energy flows of food webs (Haberl, 1997). NPP is the nutritional base of heterotrophic organism (Haberl, 1997; Wright, 1990), thus, human appropriation of biomass can affect ecosystem biodiversity through the reduction of biomass available to heterotrophic levels and nature processes (Fischer‐Kowalski and Haberl, 1997). Besides human appropriation of biomass, heterotrophic levels survival is affected by habitat destruction and degradation (Wright, 1990), all of them related to some extent to the agrarian activity and its industrialization.
3.3.3.1. Biomass, internal cycles and fund elements
Fund elements of agrarian metabolism can be reduced to land (soil, water, biodiversity), livestock, human labor and traction. Due to their different nature, it is easily understandable that they are “partially interchangeable” and flows intended to reproduce and maintain them are of different identity. The first and most important consequence of this is that the reproduction of the fund elements requires energy in the form of biomass and human labor, only partially replaceable by external energy (due to its different nature) (Guzmán and González de Molina, 2017). Both goods and ecosystem services that are appropriated by human beings are, ultimately, based on biomass flows, which underlie the correct functioning of food chains. Thus, the use of external abiotic energy inputs (such as fossil‐based one) hinders the reproduction of the fund elements of the system in the long‐term and, therefore, its sustainability (González de Molina and Guzmán 2017). This means that sustainability of agroecosystems and the ecosystem services it provides relies on the dedication of biomass flows that recirculate within the agroecosystem, since not all sources of energy and materials share the capacity to reproduce and maintain the fund elements of the agroecosystem (Guzmán and González de Molina, 2017). In other words, it is not only relevant the amount of biomass extracted, but also the biomass that is left in the agroecosystem and stays available for the rest of the heterotrophic components maintaining the provision of ecosystem services (Guzmán et al. 2018). Thermodynamics and sustainability Entropy, as a “measure of the unavailable energy in a thermodynamic system” (Georgescu‐Roegen 1993:77), is a natural law that govern any exchange of matter or energy, thus it also determines the irreversibility of the productive process of agroecosystems. Within the metabolic process, there can be a qualitative distinction between “inputs of valuable resources (low entropy) and the final outputs of valueless waste (high entropy)” (Georgescu‐Roegen, 1975). Thus, within this process, organisms or components of the metabolism strive to compensate the entropic degradation by importing low entropy and exporting high entropy (Georgescu‐Roegen, 1975), that is, generating order or negative entropy. In the same way that organisms feed on negative entropy to keep alive (Schrödinguer, 1948), ecosystems appropriate of low‐entropy energy from the environment and return dissipated energy (Guzmán and González de Molina, 2017). In agroecosystems, fund elements are the dissipative structures responsible for transforming the input energy flows into output fluxes, maintaining their integrity in the production process through the exportation of entropy to the environment (Giampietro et al. 2012:184).
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Ho and Ulanowicz (2005) and Ho (2013) explained a new thermodynamic proposal relating the sustainability of ecosystems to the energy flow, to which I will try to approach as much as possible in the following lines. As stated above, ecosystems and organisms capture energy and materials from the environment. But the key issue is not the amount of energy flowing in the system, but the amount of energy (and matter) that is stored and used efficiently by the system: “energy flow is of no consequence unless the energy can be trapped and stored within the system, where it is mobilized to give a self‐maintaining, self‐reproducing life cycle coupled to the energy flow” (Figure 9) (Ho and Ulanowicz, 2005; Ho, 2013). Thus, the captured and storaged energy from the energy flow is a key point for thermodynamic sustainability (Ho, 2013).
Figure 9. Scheme representing differences between energy flows and the energy stored. Within the system, energy stored is mobilized allowing a self‐maintaining internal structure. Source: Ho and Ulanowizc, 2005.
Dynamics in ecosystems, or ecodynamics, are not governed by mechanical relationships, but cyclic ones (Ulanowicz, 2004). Within systems, cycles structures provide for autonomy (Ulanowicz, 1983) and allow activities to couple, in such a way that the energy captured goes from one place that uses and dissipates it to another place that needs it, and this is reciprocal (Ho and Ulanowicz, 2005). Therefore, the more cycles in the system (the more activities coupled) the more energy is moving within it and the more energy is stored (higher energy residence time); this is amplified when there is a space and time scale differentiation among cycles (Figure 10) (Ho and Ulanowicz, 2005). This dynamic closure enables to store as much energy and materials as possible, and to use the energy and materials most efficiently, i.e., with the least waste and dissipation (Ho, 2013). “This complex nested dynamical space‐time structure of the organism is the secret of its sustainability”, that is, the way through which sustainable systems produce and export minimum entropy. This approach has been called “the thermodynamics of organized complexitiy” (Ho and Ulanowicz, 2005).
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Figure 10. Nested cycles spanning all space and time scales. In agroecosystems, in the same way as in organisms, the more internal cycles interconnected and exchanging entropy and negentropy, the more sustainable will result in time. Source: Ho and Ulanowicz, 2005.
Positive entropy generated in one place is compensated by negative entropy generated elsewhere within the system due to internal cycles. Thus, when the internal structure of agroecosystems is simplified because of a reduction in internal cycles (or internal loops), the system needs to import energy to generate internal order, compromising the sustainability of the agroecosystem (Guzmán et al. 2018). Not every kinds of flows are capable of feeding internal cycles. Ulanowicz (1983) stablished the difference between the cycled flows (which are conserved and non‐dissipative) and acyclic or transit flows (which are dissipative and export entropy to the outside). In agroecosystems, the energy stored can be estimated as standing biomass (Ho and Ulanowicz, 2005), therefore, low entropy systems rely on the internal recirculation of biomass; contrarily, high entropy agroecosystems are heavily dependent on external input, since their internal processes have been reduced (Guzmán and González de Molina, 2017). At this respect, the example of synthetic fertilizers could be useful to understand this idea. Chemical fertilizers do not feed any energy cycle; they represent a transit flow. They do not stay in the systems since they do not feed any trophic level. They only enter the systems, generate biomass, and exit the system through the harvestable biomass or in the form of wastes (nitrate leaching, NH3 volatilization, nitrous emission). Summarizing, from a thermodynamic perspective, agroecosystems are dissipative structures designed and managed by farmers, through which flows of energy and materials enter, exit, and recirculate (Guzmán et al. 2018) to dissipate energy and compensate for the law of entropy, creating order in the form of biomass and ecosystem services (Guzmán and González de Molina, 2017). In the words of Guzmán et al. (2018), “the sustainability of agroecosystems correlates positively with the quantity and quality of its internal loops or cycles and, to that extent, with the energy flows that circulate within them and whose function is to reproduce the fund elements. In short, the maintenance of internal loops in agroecosystems is directly related to the use of a significant part of net primary production to fuel them”. One relevant consequence of what has been exposed in these lines is that traditional farming, which relied on biomass fluxes and whose internal loops where maximized when comparing to modern farming, can be highly productive and harbors high level of biodiversity in a more sustainable way.
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Biomass beyond its economic utility From an agroecological point of view, to evaluate the long‐term sustainability of agroecosystems, that is, to determine whether the biomass flows are sufficient for the reproduction of the fund elements responsible for producing them, it is not only necessary to account for the total amount of biomass produced and appropriated by humans, but the total NPP and how much of this biomass remains in the agroecosystem. In this sense, the proposal of Guzmán and González de Molina (2015, 2017) goes beyond the HANPP concept because, despite it is a relevant indicator to get an idea of the impact of society on the natural environment, the HANNP is based on precepts that do not fully adapt to the agroecological approach. The main drawbacks of this indicator are that it masks the effects of agrarian intensification, it is not well adjusted for areas with water stress and, finally, it accounts only for aerial biomass that is useful for the society, therefore it cannot be used to assess the impacts on agroecosystems (which depends on all the biomass produced, not only that that society can buy) (Guzmán and González de Molina, 2017). Related to this latter item, crop residues “are an extremely valuable resource that provides a number of indispensable and irreplaceable agroecosystem services that range from the recycling crop of the three macronutrients as well as many micronutrients and the replenishment of SOM to the retention of moisture, and prevention of both wind and water erosion” (Smil, 2013:143). Recapitulating, HANPP is a reductionist approach that does not account for the biomass left to decomposers and heterotrophs (Smil, 2013). Contrarily, Guzmán and González de Molina (2015, 2017) developed a new methodological framework to assess the sustainability of agroecosystems through the study of the total biomass produced and the destination of the biomass flows that circulate within the agroecosystem. According to these authors, agroecosystem sustainability is inextricably linked to an optimal level of biomass production over the time without deteriorating the basis of its fund elements while maintaining an optimal provision of ecosystem services. The degradation of the fund elements can have a negative impact in the ecosystem services supply (Burkhard et al. 2011), therefore, “the study of these flows and their final destination allows us to ascertain whether or not they are of sufficient quantity and quality for the functioning, maintenance, and reproduction of the fund elements of the agroecosystem” (Guzmán and González de Molina, 2017). Finally, they highlighted that it is not only necessary to determine the cost “of the production of socially useful biomass”, but also to assess whether the provision of this supply service is jeopardizing the provision of the rest of ecosystem services provided by an agroecosystem (Guzmán and González de Molina, 2015). They propose to divide NPP into the following categories, according to if it is used by humans, animals or by the agroecosystem itself (Figure 11). Socialized vegetable biomass (SVB) and socialized animal biomass (SAB) are the phytomass and animal biomass directly appropriated by humans, respectively. The sum of SVB and SAB gives the socialized biomass (SB), which is the total biomass appropriated by society. Recycled biomass (RcB) is that one that is recycled through the agroecosystem. If it is intentionally returned by the farmer through human labor, because it is agronomically relevant for the agroecosystem, it is called reused biomass (RuB). This category does not cross the boundaries of the agroecosystems, and together with the socialized vegetable biomass they constitute the domestic extraction from the agroecosystem (DE).
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Nevertheless, if the recycled biomass is simply left in the agroecosystem without any labor or specific purpose, it is called unharvested biomass (UhB). In turn, it can be divided into aboveground unharvested biomass (AUhB) and belowground unharvested biomass (BUhB). Finally, accumulated biomass (AB) is the portion of phytomass that accumulates annually in the aerial structure and in the roots of perennial species, i.e., forest trees, woody crops, and shrubs. NPPact of agroecosystems is the sum of SVB + RuB+ UhB + AB.
Figure 11. Biomass flows within agroecosystem and between agroecosystem and society. Source: Guzmán and González de Molina, 2017.
As these authors state, from an agroecological point of view, it is relevant to consider both the unharvested biomass and the accumulated biomass, since their flows are directly related to the reproduction of the fund elements. The decrease in biomass that is not harvested has been related with the degradation of Spanish agroecosystems in the last century (Guzmán et al. 2017, 2018). For example, weed biomass plays relevant roles in the dynamics of agroecosystems (e.g. for C sequestration: De Sanctis et al. 2012 and Aguilera et al. 2018), and its measure is crucial for capturing the impacts of agrarian intensification (Guzmán and González de Molina, 2015). In the same way, the belowground biomass, which is typically obviated in metabolic assessments due to the difficulty of its measurement (Smil, 2013), is of paramount relevance for the functioning of the ecosystem both regarding the maintenance of the food webs and its role in the nutrient and C cycling and storage (Guzmán and González de Molina, 2015, 2017). The study of this network of components of biomass has to be carried out jointly, and the evolution of a component isolated from the rest cannot be interpreted as the evolution of the system. Since the agroecosystem can be considered as an organism (Ho and Ulanowicz, 2005), "they are organic in composition and behavior" and single tendencies presented in the ecosystem are not isolated from other tendencies in the system. In this sense, "the observation of any component in isolation (if possible) reveals regressively less about how it behaves within the ensemble" (Ulanowicz, 2004).
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3.3.3.2. The land cost of sustainability
The maintenance of ecological processes and services indispensable for the sustainability of agroecosystems requires of extra pieces of land devoted to the incorporation of sustainable managements (such as the internalization of renewable energy and recyclable materials flows, the land reserved to biodiversity promotion, etc.), that is, sustainability is inextricably linked to a land cost (Guzmán and González de Molina, 2009; Guzmán et al. 2011). From a metabolic point of view, each regime presents a land cost according to the source and quality of the energy and materials flows that feed them: for example, agrarian systems based on solar energy and dependent on biomass circulation occupy more land than agrarian systems relying on fossil fuels (Guzmán and González de Molina, 2009) and mechanization, which allows the totality of the land area to be allocated to food production (Georgescu‐Roegen, 1993). In fact, when the increase in productivity is not correlated with the increase in land devoted to agricultural metabolism, is because agriculture is decoupling from its agroecosystem, and the intensification of the system is relying on land imported from elsewhere, for example in the form of fertilizers (Guzmán and González de Molina, 2009; Lassaletta et al. 2014b). From a thermodynamic point of view, the increase in low‐entropy internal loops can help to reduce the land cost of sustainability (González de Molina and Guzmán, 2017), since the maximization of these cycles is linked to the increase in the carrying capacity (or biomass) and biodiversity of the ecosystems (Ho and Ulanowicz, 2013). 3.3.4. Experimental history
Ecosystems, as the rest of biotic systems and organisms, are not reversible, but historical. They have a memory, and time goes in only one direction (Ulanowizc, 2004). With regards to this feature, agroecosystems from the present are undoubtedly different from those in the past, and their evolution and adaptations will transform them in the future. This irreversible process makes it impossible to go back to past forms of agriculture that turned out to be sustainable. However, the observation and study of how agroecosystems were managed and designed in the past can help us to improve the sustainability of agroecosystems in the present (González de Molina and Guzmán, 2006; Guzmán and González de Molina, 2009). Many times it is possible to obtain this information through the experimental recreation of past conditions, which allow us to make direct measurements that can be extrapolated. This approach is called “experimental history” (Guzmán and González de Molina, 2017), and can be very relevant in providing data regarding traditional agroecosystems functioning, since most of agricultural experimentation is made under conventional farming conditions and managements, making it impossible to extrapolate data. This theoretical framework is the basis of the research carried out for this doctoral thesis, which is divided into three papers. The thread of this dissertation revolves around the three blocks presented in this theoretical framework: from an agroecological perspective, we have tried to approach the cultivation of traditional varieties as a sustainable alternative for Mediterranean drylands, an alternative which includes the provision of ecosystem services beyond productivity, based mainly on the biomass production, the C:N ratio of the straw and the higher weed competition of these varieties.
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4. STUDIES
4.1. FIRST STUDY
C and N mineralisation of straw of traditional and modern wheat varieties in soils of contrasting fertility
Roberto Garcia‐Ruíz, Guiomar Carranza‐Gallego, Eduardo Aguilera, Manuel González de Molina, Gloria Isabel Guzmán Nutrient cycling in Agroecosystems, (2019) Abstract Incorporation of crop residues can increase SOC stocks, but the extent of this depends on their C:N ratio and soil nutrient availability. Traditional wheat varieties (TWV) typically produce high straw biomass with high C:N ratio. We hypothesised that C:N ratio of straw of TWV are higher than those of modern (MWV) ones, resulting in lower carbon (C) mineralisation potential, especially in nutrient‐poor (NP) soils. Furthermore, soil nitrogen (N) retention is expected to be higher during decomposition of straw of TWV with high C:N ratio. Straw productivity of six TWV and six MWV was measured during a 2‐year field experiment, in nutrient‐rich (NR) and NP soils. Cumulative CO2 emissions and soil N availability were also examined in these soils amended with straw residues with C:N ratios of 89.2, 148.6 and 202.7 during an 84‐day lab experiment. Straw production of TWV was 1.31–1.74 times higher compared to MWV. Straw C:N ratio of TWV in NP soil averaged 152.1, greater than that of MWV (119.8). Straw‐derived CO2 emissions in NR soils were 2.5–4.3 times higher than NP and were the lowest in straw C:N ratio of TWV. After the addition of straw, immobilised N was partially re‐mineralised in the NR soil with lower values at higher straw C:N ratio. N immobilisation also occurred in straw amended NP soil independently of the straw residues C:N ratio. The higher straw productivity and higher C:N ratio of TWV can contribute to C accumulation and prevent N losses after its incorporation in soils. 1. Introduction
Small changes in soil carbon (C) content due to changes in land use or management practices may result in a significant net exchange of C between the soil reservoir and the atmosphere (Houghton 2003) because soils are considered the largest C reservoir of the terrestrial C budget (Lal 2004). Conversion of natural ecosystems to agroecosystems causes a significant depletion of the soil organic carbon (SOC) pool (Lal 2004), mainly because C output exceeds the input and this is exacerbated when soil degradation is severe. However, agricultural soils have the potential to sequester C and can thus be important as CO2 sinks (Smith et al. 2000). When sequestering C in arable soils, the aims are to increase SOC concentration and to stabilize SOC through different management interventions (Lal 2004). An increase in SOC can be achieved either by increasing the carbon input, by decreasing the output or by combining both (Freibauer et al. 2004).
The incorporation in the soil of crop residues, such as straw, is a well‐known agricultural management practice with positive effects including improved nutrient availability and water retention, better soil structure and less risk of erosion (Blanco‐Canqui and Lal 2009). In addition, many other studies consider straw incorporation as a management practice that increase SOC stocks. Triberti et al. (2008) identified this management practice as one of the most sustainable and economical C sequestration strategies. This was supported by Lehtinen et al. (2014) who
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reviewed 50 long‐term experiments in Europe, finding that SOC concentration increased by 7% due to straw incorporation and by Liu et al. (2014) whose meta‐analysis revealed an average increase of 12.8%.
Modern wheat cultivars showed an unprecedented harvest index increase during the second half of the twentieth century, leading to a yield increase and an important decrease in straw production (Lammerts van Bueren et al. 2010). This modern wheat is genetically uniform and it is adapted to high‐input conventional agriculture (Guarda et al. 2004), while it is often selected in favourable environments that do not represent the diversity of local conditions.
TWV, on the other hand, are adapted to low nutrient inputs and perform relatively well under changing environmental and biological conditions. For instance, Coromaldi et al. (2015) recently found improved performance of local traditional cultivars, compared with modern ones, for a traditionally cultivated set of crops in climate change vulnerable areas of Uganda. Therefore, it is not surprising that TWV are proposed for nutrient‐poor (NP) marginal areas or for organic and traditional low input farming systems, especially in climate change high‐risk areas (Newton et al. 2010), such as the Mediterranean region. In addition, TWV might be considered as dual‐purpose varieties because they not only produce acceptable grain yields but also high quantities of straw for on‐site or off‐site uses. Recently, the cultivation of traditional wheat cultivars under Mediterranean rainfed conditions has been suggested as a climate change mitigation strategy (Carranza‐Gallego et al. 2018), as it could enhance C sequestration while not significantly affecting grain yields, compared to modern wheat cultivation. Accordingly, the climate change adaptation potential of some traits of traditional varieties is under examination, based on their tolerance to drought and high temperature (Bellucci et al. 2013).
Different varieties might show differences in tissue quality or nutrient contents which could influence soil C and nutrient cycling in cropping systems. For instance, the higher capacity for C assimilation with relatively low N in straw of TWV led to a higher C:N ratio of these compared to their modern counterpart (Guzmán et al. 2010). This is not exclusive to wheat, but it has also been shown in other crops. For instance, Prior et al. (2006) found lower N concentration and higher C:N ratio for residues of traditional soybean cultivars when compared to modern ones and concluded that the breeding selection process may have impacted residue quality (i.e. C:N ratio) and thus affected soil C and N cycling. The specific effects depend on the direction of the change of the crop residue quality under a specific breeding program. N content of the different parts of the plants usually mirrors soil N availability and typically plant tissue N content under NP soil conditions is relatively low. Di Palo and Fornara (2015) found that the C:N ratio of different plant tissues increased with the severity of growth limitation by available soil N. The higher C:N ratio of crop residues of TWV can exert an important influence on nutrient soil cycling and C mineralisation rates (Raiesi 2006), thus entailing differences in C sequestration and CO2 emissions (Huang et al. 2004). Breakdown of complex organic C compounds of crop residues with high C:N ratio, such as wheat straw, requires the release of extracellular enzymes. Due to this additional energy requirement and the N shortage needed to build microbial biomass, microbial decomposition is slower. Therefore, wheat residues with high C:N ratio are expected to decompose more slowly and this might promote increases in SOC (Powlson et al. 2011).
However, the effects of the application of crop residues differing in C:N ratios on soil C accumulation and soil N availability also depend on the soil N availability itself. There is some evidence that soil mineral N enhances microbial decomposition of maize crop residues (Chen et al., 2014). Also, mineral N fertilizer applications have been related to a decline in SOC in continuous corn, corn‐soybeans and corn‐oats‐alfalfa cropping systems (Mulvaney et al. 2009), tillage practices and geographic regions of the USA Corn Belt (Khan et al. 2007). For instance,
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Wang et al. (2017) found that high soil N contents can be unsuitable for retaining plant residues‐derived C, as inorganic N availability can foster C‐cycling enzymes of soil microorganisms, increasing CO2 emission to the atmosphere. Thus, the effects of contrasted C:N ratios of traditional and modern wheat residue varieties on soil N availability and soil C accumulation of the residue‐derived C, will differ between nutrient‐rich (NR) and NP soils.
Overall, the aims of this study were to evaluate: (1) straw residue production and C:N ratio of TWV and MWV under high and low fertility conditions, and (2) the effects of the decomposition of wheat straw residues of contrasting C:N ratio on cumulative CO2 production and soil N availability. For this purpose, a 2‐year field experiment was carried out to evaluate traditional and modern wheat cultivars straw production performance under nutrient‐rich and nutrient‐poor sites. In addition, a wheat straw residue decomposition incubation experiment was conducted in the laboratory with NR and NP soils using the same wheat straw residues of contrasting C:N ratios. 2. Material and methods 2.1. Wheat straw and grain production of traditional and modern wheat varieties Straw and grain production of three durum (Rubio, Blanco Verdial and Recio) and three soft (Barbilla Roja, Rojo Pelón and Sierra Nevada) traditional wheat varieties and three durum (Avispa, Simeto, and Vitrón) and three soft (García, Marius and Artur Nick) modern wheat varieties were measured under two contrasting fertility conditions in two different locations in Andalusia (Spain) during two growing seasons (2013–2014 and 2014–2015). TWV were wheat landraces grown during the first third of the twentieth century in Andalusia, whereas MWV were chosen among lately released cultivars. Table 1 shows the main properties of the high (Calcic cambisol) and low (Calcic regosol) fertility soils 2 weeks before wheat sowing. In the nutrient‐poor (NP) site (Ronda, Málaga; W5°09′53″N36°44′14″), TWV and MWV were grown under rainfed and low input conditions, with no fertilisation or weed control. This site has been under organic management for more than 15 years. In the nutrient‐rich (NR) site (La Zubia, Granada; W3°35′2.4″N37°7′8.62″), TWV and MWV were grown under rainfed conventional farming conditions, receiving 45.6 kg N ha−1, 37.2 kg P ha−1 and 85.5 kg K ha−1 from a complex synthe c fertilizer 2 weeks before seeding. Weeds were controlled by means of a broad‐leaf herbicide (MCPA 40%, 2 l ha−1). Annual rainfall during the 2013–2014 and 2014–2015 growing seasons were 662 mm and 448 mm, and 309 mm and 359 mm, in the NP and NR sites, respectively. These values were 85.4% and 57.8%, 66.8% and 77.7% of the 1982–2012 mean for the NP and NR sites, respectively. Table 1. Main soil properties of the top 0.3 m of nutrient‐poor (Ronda) and nutrient‐rich (La Zubia) soils
used for the experiment. Values are the mean standard deviation of 5 replicates. Different letters within the same row stand for significant differences (one way ANOVA, p < 0.05).
Nutrient‐rich soil (La Zubia)
Nutrient‐poor soil (Ronda)
Organic matter* 2.6a0.4 1.03b0.2 Organic carbon* 2.0a0.2 0.73b0.2 CEC** 16.8a1.9 10.5b1.8 Exchangeable Ca** 13.8a2.0 8.23b1.2 Exchangeable Mg** 2.05a1.8 1.76a0.6 Exchangeable Na** 0.50a0.1 0.34a0.01 Exchangeable K** 0.48a0.04 0.21b0.03 Available K*** 208.4 a13.9 76.2b3.7 Carbonate* 18.6a0.46 2.07b0.31 P Olsen*** 27.1a14.6 3.98b1.19
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Total N* 0.17a0.02 0.07b0.01 pH 7.99a0.11 7.66b0.19 Clay* 16.4a2.6 14.3a2.33 Sand* 28.7a7.8 75.6b3.77 Silt* 54.8a5.4 10.1b1.9 Texture Silt‐loam Sand‐loam Soil type (FAO) Calcic Cambisol Calcic Regosol
*%, **meq 100 g‐1, ***ppm.
A complete, randomised block design with four blocks separated by a non‐seeded stripe 1 m in width, was set up in each location. Plots were 4 × 6 m2 in size. Wheat cultivars were hand sown at a sowing rate of 200 kg ha−1 between 25 October and 12 November. Each plot was sown with the same wheat cultivars during the two growing seasons. Harvest was done by hand between 5 June and 25 June, at the end of the cereal cycle. Straw and grain of two randomly selected squares of 0.5 × 0.5 m2 near the centre of each plot were collected, dried (70 °C, 48 h) and weighed. 2.2. Soil and wheat straw collection, and straw residues decomposition experimental set up Dried straw and grain samples were ground (< 1 mm) in a rotary mill and analysed for total N and C concentrations using a CNOH‐S analyser (Flash EA1112 CHNS‐O, Thermo Finnigan). Wheat straw of contrasting C:N ratios was collected from our own field experiment. C:N ratios for the two growing seasons averaged 152.5 and 91.2 for TWV and 119.9 and 82.45 for MWV in the NP and NR sites, respectively. Wheat straw samples selected for the laboratory decomposition experiment had C:N ratios of 89.2 and 148.6, corresponding to MWV in the NR and NP soils, respectively, and 202.7 corresponding to TWV in the NP soil. The experiment was established in 250 ml Kilner jars. 150 g of < 2 mm soil from NR and NP sites were weighed into each jar with distilled water added to reach 60% of soil field capacity which is within the optimal range of soil water content for microbial mineralization (Ilstedt et al. 2000). To initiate microbial activity, soil was conditioned at this water holding capacity for 7 days prior to establishment of the experiment. Dried straw residues were ground (< 1 mm) in a rotary mill, added and mixed with the NR and NP soils at a rate of 4 mg C g−1 dry soil on day 0. This amount of straw‐C approximately represented the grand mean of wheat straw production for both sites and growing seasons added to the top 0.05 m of soil. Unamended control treatments were also established. Both the treatments and controls were replicated 3 times. The experiment was conducted at 25 °C in the dark over 84 days after wheat straw residue application, as by day 84 soil CO2 emission rates in the amended and unamended soils were not significantly different. Time periods between CO2 measurements were dependent on the daily rates; short at the beginning and longer as the incubation progressed. To test the effects of nitrate (NO3
−) availability on production, 50 μg N–NO3− g−1 was added to
the soil after 84 days of incubation, and daily CO2 production rate was measured during the following 8 days.
2.3. Potential CO2 production and soil mineral N analysis During the incubation period, C– CO2 production was measured following the NaOH trap method described by Anderson (1982). Briefly, small test tubes (12 mL) were filled with 10 mL of NaOH
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(1 N) and inserted into the Kilner jars, which were then hermetically closed. CO2 production was recorded after day 3, 7, 14, 30, 44, 63 and 84, and 8 days after NO3
− addition by titration with HCl (0.5 N). Apparent CO2 emissions from wheat straw‐C were estimated by subtracting the C– CO2 emission of the unamended control soil from the C‐CO2 emission of the amended soils. For NO3
− and ammonium (NH4+) analyses, three additional replicates of each treatment were
established and destructively sampled on days 0 (before wheat straw amendments), 7, 24, 42, 65 and 84. Subsamples (2.0 g) of the fresh soil were extracted in 1 M KCl in a 1‐to‐10 ratio of soil‐to‐extractant and filtered through Whatman No. 44 filter paper. Concentrations of NH4+–N and NO3
−–N in the extract were colorimetrically determined following the work of Keeney and Nelson (1982). Net N mineralisation (NNM) and net nitrification (NN) rates were calculated using the equations below
NNM μgNg d NO N NH N NO N NH N
Incubationtime days
NN μgNg d NO N NO N
Incubationtime days
2.4. Statistical analysis All data was analysed using the STATISTICA 6.0 statistical package. Data was tested for normality and log‐transformed when appropriate (Parkin and Robinson, 1993), prior to the analysis of variance. Tukey’s HSD test was used after ANOVA, to test for significant differences in the measured variables among levels of treatments. Differences in properties between soils of the NP and NR sites were tested with one‐way ANOVA. Differences in straw and grain productions, C and N concentrations and C:N ratio of wheat straw and grain (due to site, varieties and growing season and their interactions) were tested using three‐way ANOVA. Differences in the C and N variables measured in the incubation experiment due to the C:N ratios of wheat straw residues in the NR and NP, were tested using two‐way ANOVA. 3. Results 3.1. Straw and grain production of traditional and modern wheat varieties Straw productivity of TWV was significantly higher than that of MWV in both growing seasons in the NP site, whilst at the NR site significant differences were only found for the second year (Table 2). Overall for the two growing seasons, straw productivity at the NR site was 14.0 and 18.7 times higher than that of the NP site for TWV and MWV, respectively. TWV produced on average 1.74 and 1.31 times more straw biomass than modern varieties in the NP and NR sites, respectively. Overall, grain productivity was not significantly different between varieties, although values for the TWV were higher (2013–2014 growing season) or similar (2014–2015) than those of MWV in the NP site, whereas there were not significant differences between varieties in the NR site during both growing seasons (Table 2).
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Table 2. Straw and grain production (Mg dry weight ha−1), straw nitrogen (%) and carbon (%) concentrations, and C:N ratio of traditional and modern wheat varieties in the nutrient‐poor and nutrient‐rich sites during the 2013–2014 and 2014–2015 growing seasons. Data are the mean ± standard deviation (n = 6). Different lowercase in a growing season and site stands for significant differences between traditional and modern wheat varieties (one way ANOVA, p< 0.05). Probability levels of the effects of site (nutrient‐poor and nutrient‐rich), varieties (traditional; T and modern; M) and growing season (2013‐2014; A and 2014‐2015; B) and their interactions on straw and grain production, C and N concentrations and C:N ratios as determined by means of three‐way ANOVA.
Site Growing season
Wheat variety
Straw production (Mg dry
matter ha‐1)
Straw C concentration
(%)
Straw N concentration
(%)
Straw C:N ratio
Grain production (Mg dry
matter ha‐1)
Grain C concentration
(%)
Grain N concentration
(%)
Nutrient‐poor, traditionally managed
2013‐2014 Traditional 0.597a0.194 43.7a0.67 0.34a0.16 151.4a58.6 0.255a0.218 42.1a4.1 0.68a0.24 Modern 0.374b0.186 43.7a1.14 0.50b0.36 111.4b44.1
0.153b0.174 41.8a3.2 0.70a0.25
2014‐2015 Traditional 1.247a0.356 42.8a0.52 0.30a0.08 152.9a37.3 0.623a0.285 41.3a1.84 1.72a0.23 Modern
0.684b0.258 42.3b0.58 0.36b0.13 128.3b39.9 0.569a0.225 40.4b0.13 1.46b0.15
Average Traditional 0.922a0.429 43.3a0.72 0.31a0.12 152.1a48.1 0.439a0.308 41.7a3.14 1.19a0.56 Modern 0.529b0.269 43.0a1.12 0.43b0.27 119.8b42.0 0.361a0.286 41.1a2.35 1.08a0.43
Nutrient‐rich, conventionally
managed
2013‐2014 Traditional 13.80a4.87 41.5a0.53 0.84a0.21 52.9a14.5 2.40b0.807 37.3b1.6 3.05a0.47 Modern 12.30a4.85 39.4b1.65 0.81a0.23 53.5a18.3
3.30a1.611 38.4a2.1 2.50b0.43
2014‐2015 Traditional 12.08a2.82 42.3a0.60 0.39a0.17 129.4a54.1 1.42a0.993 40.9a0.4 2.23a0.49 Modern 7.47b2.34
41.4b0.67 0.44a0.19 111.4a46.7 1.39a1.291 40.7b0.2 2.00b0.52
Average Traditional 12.94a3.99 41.9a0.66 0.61a0.29 91.1a54.5 1.91a1.01 39.1a2.13 2.64a0.62 Modern 9.88b4.44 40.4b1.60 0.62a0.27 82.4a45.2 2.35a1.71 39.5a1.86 2.25b0.52
Site (S) <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001Varieties (V) <0.001 <0.001 0.039 <0.001 0.156 0.822 <0.001
Growing season (GS) <0.001 0.288 <0.001 <0.001 <0.001 0.004 0.021S x V <0.001 <0.001 0.081 0.052 0.043 0.090 0.013S x GS <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001V x GS 0.032 0.132 0.932 0.892 0.085 0.107 0.790
S x V x GS 0.084 <0.001 0.132 0.162 0.055 0.586 0.006
Straw C concentration in the NP site was higher than in the NR site (p < 0.05), and values of TWV
were higher than those of MWV independently of the growing season. At the NP site, straw C
concentration of traditional cultivars was higher in 2015 but similar than that of modern ones in
2014, while it was higher for both years at the NR site (Table 2). Straw N concentration of MWV was
higher than that of TWV, independently of site and growing season, although this was especially
true in the NP site for both years. Straw‐N uptake, calculated from wheat straw biomass and straw
N concentration, for TWV was between 9 to 50% higher than that of MWV for the two growing
seasons and the two sites (data not shown). The straw C:N ratio of TWV was significantly higher than
that of MWV across sites and growing seasons, although this difference was statistically significant
only at the NP site (Table 2).
Overall, grain production and grain C concentration were not significantly different between
varieties (Table 2). Conversely, grain N concentrations of TWV were significantly higher than those
of MWV, especially in the NR site.
Although we did not measure lodging intensity, it affected both types of variety in the NR site in the
2013–2014 growing season, with slightly higher impacts on old varieties.
3.2. Potential CO2 production and C accumulation of straw residues of traditional and modern
wheat varieties of contrasting C:N ratios
Potential cumulative CO2 production in the unamended NR soil was significantly higher than that of
the unamended NP soil (p < 0.05) (Fig. 1). However, when data was expressed by per milligram soil
organic carbon instead of per gram of soil, CO2 emissions in the NR soil were 51.0% lower (12.6 μg
C–CO2 mg organic carbon−1 on average) than that of the NP soils (24.7 μg C–CO2 mg organic carbon−1)
after 84 days of incubation.
Fig. 1. Cumulative 84 days CO2 production (μg C–CO2 g−1) in unamended nutrient‐rich and nutrient‐poor
soils. Different letters stand for significant differences (one‐way ANOVA, p < 0.05)
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For both NP and NR soils, the addition of wheat straw resulted in greater CO2 production compared
to those with unamended controls (Fig. 2). Cumulative CO2 production after 84 days of wheat straw
application in the NR soil was between 2.0 and 2.4 times higher than that of NP soil (p < 0.01).
Fig. 2. Cumulative 84 days CO2 production (μg C–CO2 g−1) in unamended and after wheat straw addition of
low (89.2), medium (148.6) and high (202.7) C:N ratios in nutrient‐rich (a) and nutrient‐poor (b) soils.
Different letters within the same graph stand for significant differences (one‐way ANOVA, p < 0.05)
There were significant effects of the C:N ratio of the wheat straw residue on the magnitude of the
84 days cumulative CO2 production, and they were highly dependent on the nutrient status of the
soil (p < 0.05; two‐way ANOVA). For NR soils, the highest cumulative CO2 production (918.5 μg C–
CO2 g−1) was achieved when wheat straw residues with lowest C:N ratio were added (Fig. 2).
Differences in cumulative CO2 production between medium and high C:N ratios of the wheat straw
residues were not significant, and values ranged from 866.6 to 877.9 μg C– CO2 g−1 on average during
the 84 days (Fig. 2). For the NP soil, the lowest cumulative CO2 production was found when the straw
residues from TWV (straw C:N ratio of 202.7) were added to the soil with an average of 362.5 μg C–
CO2 g−1 throughout the 84 days (Fig. 2b). The highest cumulative CO2 production was achieved for
the straw residues of modern varieties with low and medium C:N ratios. The percentage of the
84
added straw C that was apparently emitted as CO2 differed for the two soils and the C:N ratios (Fig.
3). Higher percentages were found for the NR soil (between 18.9 and 22.9% for straw of traditional
and modern varieties, respectively), which were between 2.5 and 4.3 times higher than those found
for the NP soil (Fig. 3).
Fig. 3. Apparent straw residue carbon emitted as C–CO2 (%) after 84 days of incubation of wheat straw
addition of low (89.2), medium (148.6) and high (202.7) C:N ratios in nutrient‐rich and nutrient‐poor soils.
Different letters within the same graph stand for significant differences (one‐way ANOVA, p < 0.05).
In the NR soil, the straw with the lowest C:N ratio showed the highest percentage of C–CO2 emission
(22.9 ± 0.6%), while no differences between the medium (19.8 ± 2.1%) and highest (18.9 ± 1.9%)
straw C:N ratios were found (Fig. 3). For the NP soil, the lowest percentage (4.4 ± 0.9%) was found
for wheat straw of highest C:N ratio, and no significant difference was found between the wheat
straw residues of medium and low C:N ratios (Fig. 3).
Depending on soil nutrient status and C:N ratio of the straw residues, the addition of NO3− increased
the daily CO2 production (Fig. 4). In the NR soil, rates 1 week after NO3− amendments were between
2.7 to 12.9 times higher than those before NO3− addition, and no differences among wheat straw
samples of different C:N ratios were observed. However, in the NP soil, the increase in daily CO2
production with NO3− was over 41.4 times higher than that prior to NO3
− addition and the magnitude
of the increase was the greatest for low and medium C:N of the wheat straw (Fig. 4).
85
Fig. 4. Daily CO2 production rate (μg C–CO2 g−1 day−1) response 1 week after nitrate addition (50 μg N–NO3−
g−1) in unamended and after wheat straw addition of low (89.2), medium (148.6) and high (202.7) C:N ratios
in nutrient‐rich (a) and nutrient‐poor (b) soils. Different letters within the same graph stand for significant
differences (one‐way ANOVA, p < 0.05)
3.3. N dynamics during decomposition of straw residues of traditional and modern wheat varieties
of contrasted C:N ratios
Concentrations of available soil NH4+ were low (< 14.2 μg NH4
+–N g−1 and < 5.4 μg NH4+–N g−1 for NR
and NP soils, respectively) in all treatments over the 84 days of incubation compared to NO3−
concentrations, and therefore only concentrations of the latter are discussed.
Concentrations of NO3− in the unamended NR soil were between 4.0 to 5.6 times higher than in the
unamended NP soil during the incubation period (Fig. 5). NO3− levels in both unamended soils
increased steadily, resulting in averages in the net N mineralisation and net nitrification rates of 0.81
μg N‐inorganic g−1day−1 (or a daily percentage of 0.047% compared to soil total N) and 0.74 μg N–
NO3− g−1day−1, for NR soils respectively, and 0.37 μg N‐inorganic g−1day−1 (or 0.053% day−1 of the soil
total N) and 0.23 μg N– NO3− g−1day−1, for NP soils respectively (Fig. 6).
86
Fig. 5. Soil nitrate content (μg N–NO3− g−1) in unamended and after wheat straw addition of low (89.2),
medium (148.6) and high (202.7) C:N ratios in nutrient‐rich (a) and nutrient‐poor (b) soils. Different letters
stand for significant differences (one‐way ANOVA, p < 0.05) at the end of the incubation period
87
Fig. 6. Soil net N mineralization (μg available N g−1 day−1) (a) and net nitrifica on (μg N–NO3− g−1 day−1) (b)
rates in unamended and after wheat straw addition of low (89.2), medium (148.6) and high (202.7) C:N
ratios in nutrient‐rich (NR) and nutrient‐poor (NP) soils. Different small and capital letters stand for
significant differences (one‐way ANOVA, p < 0.05) among treatments for nutrient‐rich and nutrient‐poor
soils, respectively.
The addition of wheat straw caused profound changes in the levels of soil NO3−, especially for NR
soils. NO3− levels of NR soil, which initially averaged 90.5 ± 3.4 μg N– NO3
− g−1, decreased to lower
than 6.7 μg N–NO3− g−1 during the first week of incubation, independently of the C:N ratio of the
straw residues (Fig. 5). After addition of the wheat straw, NO3− concentration in the NP soil also
decreased to considerably low or undetectable levels during the first week of incubation (Fig. 5).
Thereafter, NO3− levels of NR soil increased steadily and linearly, but they were always lower than
those of the unamended control (Fig. 5). At the end of the incubation, soil NO3− concentration under
straw of MWV with the lowest C:N ratio was significantly higher (52.1 ± 4.3 μg N– NO3− g−1) than that
measured in the soil incubated with straw of medium (32.5 ± 5.8 μg N– NO3− g−1) and the highest
(27.3 ± 2.5 μg N–NO3− g−1) C:N ratios. For the NP soil, NO3
− levels at the end of the incubation were
similar or slightly higher than at the beginning (Fig. 5). N re‐mineralisation on day 84 in comparison
to that immobilised during the first week in the NR soils, averaged 72.4, 51.6 and 44.3% for straw
C:N ratios of 89.2, 148.6 and 202.7, respectively (Fig. 5).
88
Net N mineralisation and net nitrification were negative during decomposition of wheat straw
residues in the NR soil (Fig. 6) and there was a clear trend of more negative values at higher straw
C:N ratios. This implies that N immobilisation was significantly lower when straw from MWV with
low C:N ratio was added, in comparison with straw of TWV with high C:N ratio. In the NP soil, both
net N mineralisation and net N nitrification were very low, although positive (< 0.1 μg N‐inorganic
g−1 day−1 and < 0.02 μg N–NO3− g−1 day−1, respectively).
4. Discussion
4.1. Straw production of traditional and modern wheat varieties
The overall higher wheat straw production of TWV in both NR and NP soils is in line with the results
of Townsend et al. (2017). Marked decreases in straw, relative to grain production of wheat varieties
during the last century, reflects the replacement of draught animals by automotive machinery and
the subsequent loss of functionality of the straw. Indeed, wheat varieties with relatively high straw
production have been desirable traits in the past because straw was highly valued for animal feeding
and bedding. Currently, the search for renewable lignocellulosic energy sources and soil C
sequestration without compromising grain yields, has made high straw production wheat traits
desirable again (e.g. Lorenz et al. 2010). This is particularly true for low productivity cropland areas
or large areas of subsistence‐based low inputs systems with high demands for alternative use of
crop residues (e.g. fodder, fuel). This was the case in our study; straw and grain productivity of TWV
were higher than those of MWV in the NP site. In these environments, wheat breeding efforts should
focus on increasing residue production without decreasing grain yield (Lorenz et al. 2010).
The high soil N availability of the NR site, partially due to mineral fertilisation, was likely a main
driver of the higher N concentration and lower C:N ratio of wheat straw at this site (Agren and Weih
2012), especially for MWV. The overall higher straw biomass of TWV but similar N concentration
with respect to modern ones, indicates a higher straw‐N uptake. This suggests that cultivation of
TWV could be advantageous under low N available conditions typical of marginal and low
productivity arable land.
4.2. Potential CO2 production and C accumulation of straw residues of traditional and modern
wheat varieties of contrasting C:N ratios
Soil CO2 emissions per mg of organic C at the NR site were half that of the NP site. This was likely
due to the higher physical and chemical capacities to protect SOC compared to the NP soil. The
physical protection exerted by macro‐ and/or microaggregates on SOC is attributed to the
compartmentalisation of substrate and microbial biomass (Killham et al. 1993) and to the reduced
diffusion of oxygen into macro and, especially, microaggregates resulting in a reduced microbial
activity within the aggregates. Although neither the amount or the stability of soil aggregates were
measured in our study, it is relatively well documented that the higher the organic matter content
of a soil (which was the case of the NR soil) the higher its capacity to serve as binding agent to hold
soil particles together, forming aggregates (Jastrow et al. 1998). Furthermore, the much higher silt
89
and clay content of the NR soil may also contribute to the lower specific CO2 emissions as it has been
long recognised the role of silt and clay to chemically protect SOC from microbial decomposition (Six
et al. 2002).
In this study we cannot elucidate the contribution of the priming effect (i.e. the increase in soil
organic matter decomposition rate after fresh organic matter input to soil) on cumulative CO2
emissions after the amendment of the straw residues. However, using similar residues, Gómez‐
Muñoz et al. (2016) found that the priming effect typically peaked in the early stages (first week) of
the incubation period, followed by a rapid decline, resulting in the contribution of the priming effect
to cumulative CO2 emissions being highly diluted when the incubation period was higher than 20
days. The incubation period of our experiment was nearly 12 weeks and we have assumed little
contribution of the priming effect on 84 days cumulative CO2 production.
In the NR soil, the apparent straw‐C emitted as CO2 (i.e. the amount of straw‐C which was
transformed to CO2 in comparison to that added) was between 2.5 to 4.3 times higher than in the
NP soil. The higher overall fertility and nutrients content of the NR soil is likely to be responsible for
this fact. During decomposition of crop residues, a substantial part of the C is used to build microbial
biomass, which also needs available N to maintain the low C:N ratio typical of soil microorganisms.
Therefore, a higher crop residue C derived CO2 is expected under the presence of high N availability
(Chen et al. 2014), as it was the case for the NR soil. This was confirmed by the fact that NO3− addition
increased the daily CO2 production rate in the NP soil by 41.4 times compared to the daily CO2
production rate just before NO3− addition, while relatively little increases were found for the NR soil.
There were significant effects of the C:N ratio of the wheat straw residues in the magnitude of the
84 days cumulative CO2 production. The lowest apparent straw‐C emitted as CO2 was achieved for
the straw residues with the highest C:N ratio, typical of the TWV. This result confirms the previously
reported pattern that wheat straw with relatively high N concentration and low C:N ratios can
accelerate the initial C mineralisation in comparison to residues with low N content and high C:N
ratios (Raiesi 2006). There is a vast number of studies showing that for a wide range of C:N ratios
and residues, the lower the C:N ratio, the higher the CO2 emitted or other C mineralisation
indicators. For instance, Huang et al. (2004) showed a significant negative correlation between the
C:N ratio of the plant residues and cumulative CO2 production in a 21‐day lab experiment for a range
of C:N ratios of 8 to 118. The activity of soil microorganism depends mainly on mineralizable C and
N availability. Total C concentrations of the wheat straw residues assayed (Table 2) were relatively
similar (39.4 to 43.7%) whereas variability of the N concentrations was very large (from 0.30 to
0.84%). Assuming similar lignin, cellulose and polyphenols contents of the wheat straw samples, the
higher N concentration in the MWV straw of low C:N, can contribute to better fulfil the N
requirements of the decomposers, stimulating C mineralisation (Manzoni and Porporato 2009). Our
results suggest that soil application of straw with high C:N ratio of TWV, in both NR and marginal NP
soils, might lead to a higher short term SOC accumulation efficiency (i.e. proportion of the wheat
straw‐C which remains in the soil at the short‐term), since slowing decomposition rates can promote
SOC accumulation (Powlson et al. 2011), and reduce CO2 emissions (Wang et al. 2017). However,
because of limited microbial transformation of wheat straw residues due to available nutrient
90
shortage, contribution of this wheat straw‐C to the formation of a more stabilised, slowly
decomposing pool of SOC is limited and should be further investigated. Beyond concerns about SOC
accumulation on the long term, the higher soil C accumulation in the NP soil with straw of TWV
could contribute to increase soil fertility by improving physical, chemical and biological soil
properties, thus benefitting the following crops. In this sense, amendment crop residues with high
C:N ratios applied in soils with low N availability have been proposed as possible strategies to
increase soil fertility and C accumulation in marginal low productivity croplands (Wang et al. 2017).
4.3. N dynamics during decomposition of straw residues of traditional and modern wheat varieties
of contrasted C:N ratios under soils differing in nutrient status
The net soil N immobilisation in both NR and NP soils (e.g. negative net N mineralisation and
nitrification) after addition of wheat straw residues with C:N ratios ranging 89.2 to 202.7, was not
unexpected. Indeed, rapid N immobilisation shortly after application of residues with a relatively
high C:N ratio, followed by a moderate release of N, has also been observed by others (e.g. Garcia‐
Ruiz and Baggs 2007). The C:N ratio of plant residues is most often used as an index to assess
whether the residues will release or immobilize inorganic N. Vigil and Kissel (1991) integrated N
immobilisation data from several medium to long‐term experiments with residues having a wide
range of C:N ratios and showed that the break‐even point between net N immobilisation and N
mineralisation of residues was at a C:N ratio of 41, half the minimum C:N ratio assayed in this study.
Nutrient immobilisation during decomposition of our wheat straw residues with a large C:N ratio
was mainly due to the fact that, when soil microorganisms decompose these sources of organic
matter, some of the C, N and P are assimilated and transformed into microbial biomass (Herron et
al. 2009). Therefore, the application of wheat straw residues to soils promotes temporary
immobilisation of the available N, which increases soil N retention and offers the potential to lower
losses of N by denitrification and leaching when soil available N is not required by crops.
Along the 84 days of incubation, N immobilisation was significantly lower when straw from MWV
with low C:N ratio was added, in comparison with straw of TWV with high C:N ratio. Furthermore,
part of the immobilised N was re‐mineralised, especially in the NR soil. This partial N re‐
mineralisation was likely due to that after the death of the microorganisms, due to labile C shortage
or depredation by other microfauna, some of the microbial N components are mineralised and
nitrified, resulting in an increase in soil N availability. The lower the C:N ratio of the wheat straw,
the greater percentage of N was re‐mineralised. These results suggest that the cultivation (and the
subsequent burial of straw residues by plowing) of TWV with high C:N ratio is a sound and
comprehensive strategy for N retention. This is particularly important to preserve available N in soils
with high N availability after harvesting, due to a surplus of mineral N fertilizer.
5. Conclusions
The higher straw production and straw‐N uptake indicate that cultivation of traditional wheat
varieties is advantageous under rainfed and low N availability conditions typical of marginal, low
productivity arable land or low input farming, especially when a dual‐purpose role for the wheat is
91
required. In addition, the high straw production of traditional wheat varieties allows to incorporate
more straw residues to the soil, promoting soil fertility and other alternative uses such as livestock
feed and bioenergy feedstock.
The higher straw C:N ratio of traditional wheat varieties lead to a lower apparent straw residue
derived CO2 emission and higher short term SOC accumulation efficiency, especially in the nutrient‐
poor soils.
The higher soil N retention in the nutrient‐rich soil after addition of straw of traditional wheat
varieties due to their high C:N ratio (lower percentage of N re‐mineralisation) offers the potential to
lower N losses.
Acknowledgements
This work springs from the international research project on Sustainable Farm Systems: Long‐Term
Socio‐Ecological Metabolism in Western Agriculture funded by the Social Sciences and Humanities
Research Council of Canada (SSHRC 895‐2011‐1020) and Spanish research projects HAR2012‐38920‐
C02‐01 and HAR2015‐69620‐C2‐1‐P funded by Ministerio de Economía y Competitividad (Spain).
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4.2. SECOND STUDY
Contribution of old wheat varieties to climate change mitigation under contrasting
managements and rainfed Mediterranean conditions
Guiomar Carranza‐Gallego, Gloria Isabel Guzmán, Roberto García‐Ruiz, Manuel González de Molina,
Eduardo Aguilera
Journal of Cleaner Production, 195 (2018) 111‐121
Abstract
Agriculture represents about 11% of global anthropogenic greenhouse gas emissions (GHGe). Many
climate change mitigation strategies have been evaluated in Mediterranean agroecosystems,
including their soil organic carbon sequestration potential. High residue yielding old varieties could
constitute a useful alternative, especially for organic farming, which lacks specific genetic material.
In this study, old and modern wheat varieties were evaluated under organic (ORG) and conventional
(CON) management during a 3‐year field experiment under rainfed Mediterranean conditions. Field
measurements of biomass components, literature emission factors, and soil organic carbon
modeling were combined in an attributional Life Cycle Assessment, in order to estimate GHGe from
“cradle to farm gate”. The resulting yield‐based carbon footprints of old wheat varieties were
significantly lower than those of modern varieties both under CON management, decreasing from
263 to 144 g CO2e kg_1, and under ORG management, decreasing from 29 to _43 g CO2e kg_1. Our
results indicate that climate change mitigation strategies in Mediterranean rainfed cereal cropping
systems should focus on diminishing GHGe from machinery and fertilizer use, and promoting carbon
sequestration. The combination of organic management and old cereal varieties can constitute a
promising climate change mitigation strategy in these systems, as low area‐scaled GHGe of organic
management are combined with enhanced carbon sequestration and a good yield performance of
old varieties under these conditions
1. Introduction
The agricultural sector represents about 11.2% of global anthropogenic greenhouse gas emissions (GHGe) in 2010 or up to 21.2% when including land use changes (Tubiello et al., 2015). Mediterranean cropping systems have specific pedoclimatic conditions that affect their GHGe pattern, conditioning the effectiveness of climate change mitigation strategies and their wider environmental impacts (Sanz‐Cobena et al., 2017). Rainfed Mediterranean cereal fields are expanding, and their importance for Mediterranean agriculture sustainability is increasingly acknowledged (Perniola et al., 2015). For example, Tahmasebi et al. (2018) recently found that rainfed Mediterranean wheat cropping systems are more sustainable than irrigated ones due to a lower input intensity. They concluded that the higher yield of irrigated systems did not compensate for the unproportioned increase in GHGe. The major contributor to GHGe in Mediterranean rainfed cereal cropping systems is the use of industrial inputs, such as machinery and fuel for organic farms and these together with fertilizer in conventional ones (Aguilera et al., 2015a). For instance, Ali et al. (2017) found that the production and application of N fertilizer represented up to 60% of the C footprint of rainfed wheat in Italy. Many practices have been evaluated for their climate change
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mitigation potential, including conservation tillage (Alvaro‐Fuentes et al., 2007), biochar application (Castaldi et al., 2011), or the use of organic fertilizers (Meijide et al., 2010). In addition, the inclusion of legumes in rotations can reduce GHGe through lowering N fertilizer inputs (Liu et al., 2016). Soil organic carbon (SOC) is key for climate change adaptation and mitigation processes in agricultural systems (Lal, 2004). Changes in SOC have been widely studied in cereal fields (Blanco‐Moure et al., 2016), and under Mediterranean conditions (Aguilera et al., 2013), and specifically in Mediterranean rainfed cereal systems (e.g. Guardia et al., 2016). SOC increases can offset an important proportion of agricultural GHGe (Parton et al., 2015), thus not including SOC sequestration in the GHGe accountability can lead to underestimations of the GHGe abatement potential of agriculture (Rodríguez‐Entrena et al., 2014). Concerns about climate change have led to many studies discussing the C storage potential of organic material additions to agricultural soils (Lehtinen et al., 2014). Typically, SOC increases inproportion to increases in C inputs (Paustian et al., 2000), and this is also true for Mediterranean systems (Aguilera et al., 2013). In Mediterranean areas (Aguilera et al., 2013), and particularly in Spain (Rodríguez‐Martín et al., 2016), cropland soils have low SOC concentrations. Mediterranean cropland soils are far from their potential for SOC storage (Aguilera et al., 2013), and SOC depletion is a major vulnerability factor for the sustainability of Mediterranean agriculture in a climate change context (Iglesias et al., 2011). Organic farming can offer valuable environmental benefits (Tuomisto et al., 2012), including those regarding climate change mitigation and adaptation (Scialabba and Muller‐Lindenlauf, 2010). In Mediterranean cereal systems, organic farming has been found to lower GHGe per unit area (Gutierrez et al., 2017), but not always per unit product. Aguilera et al. (2015a) found lower C footprint per unit of product for organic systems, while Fedele et al. (2014) found the opposite when comparing with conventional farming. Likewise, organic farming can increase SOC (Aguilera et al., 2013) as compared to conventional management. Unfortunatelly, there is no specific genetic material for organic farming, so it relies on varieties selected under conventional high‐input agriculture conditions (Murphy et al., 2007). There is a need for selecting varieties better adapted to organic farming and low input conditions Murphy et al., 2007) and some authors have proposed old varieties as a valuable source for sustainable agriculture in a climate change context (Bellucci et al., 2013), more suitable to adapt to future scenarios (Ceccarelli and Grando, 2000). Wheat varieties experienced an unprecedented harvest index increase throughout the past century (Smil, 1999) leading to a great increase in grain yield without a significant change in total aboveground biomass (De Vita et al., 2007). However, a future decrease in harvest index under Mediterranean conditions has been modelled (Moriondo et al., 2011), mainly due to heat stress caused by climate change. In this context, growing varieties with high residue production can result in an effective climate change mitigation strategy. Our hypothesis is that old wheat varieties cultivation, specially under organic management, can contribute to decrease the C footprint of rainfed Mediterranean cereal cultivation through a higher production of residues. In this study, we evaluated the C footprint of rainfed Mediterranean cereal production as affected by varieties (old versus modern) and managements (organic versus conventional). We carried out a field experiment in Southern Iberian Peninsula to estimate the C footprint of old and modern wheat varieties under organic and conventionall farming practices. The 3‐years field data were processed in a life cycle assessment (LCA) to calculate the full GHG balance of the cropping systems and their products, including C sequestration.
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2. Material and methods
2.1. Site description and experimental design Two field experiments under different management were carried out at two locations 142 km away in Southern Iberian Peninsula (Figure S1), Sierra de Yeguas (Malaga province) and La Zubia (Granada province). To cover for interannual variability, the field experiments were carried out during three consecutive growing seasons (2013e2016). The main soil properties of the experimental sites are shown in Table 1. Temperature and precipitation data are shown in Table 2. Mean annual temperature for the experiment locations during the study period are close to their corresponding mean values for the last 30 years. However, annual precipitation was 56.4% and 68.9% of the mean value, for Sierra de Yeguas and La Zubia, respectively. The same old (OV, Rubio, Recio, Sierra Nevada, Barbilla Roja, Rojo Pelon, Blanco Verdial) and modern (MV, Avispa, Simeto, Vitron, García, Marius, Artur Nick) wheat varieties were sown at both locations. OV were landraces grown during the first third of the 20th century in the region. Their seeds came from the Phytogenetic Resource Centre of the National Agrarian Research Institute of Spain (CRF‐INIA). MV were chosen among lately released, currently used varieties, considering their good reputation among farmers in the area.
Table 1. Soil physico‐chemical properties of the field trials at the beginning of the experiment. Mean values
and standard error are shown.
CEC=cation exchange capacity; SOC= soil organic carbon.
The farmland at Sierra de Yeguas had been under organic management (ORG) (Table 2) for the previous 15 years. Crop rotation consisted of wheat‐legume (Vicia faba). Both species were grown in adjacent plots, interchanging cultivation plots each year. Before wheat seeding in the first year of the rotation, 3Mg ha‐1 of poultry manure was applied, but none to faba bean crop. Poultry manure mineralization was assumed as two thirds and one third for the first and second year of the rotation scheme, respectively. Weeds were controlled by hand. The farmland at La Zubia was conventionally managed (CON), based on monoculture wheat cropping and synthetic inputs. Complex synthetic fertilizer was applied before seeding (Table 2) and a broad‐leaf herbicide was applied before stem elongation. At both farms, wheat was usually grown before the field experiment. Although we do
Properties Sierra de Yeguas
La Zubia
CEC (meq/100g) 31.190.93 16.860.85 Ca exchangeable (meq/100g) 21.940.79 13.830.93 Mg exchangeable (meq/100g) 5.800.54 2.050.72 Na exchangeable (meq/100g) 1.340.07 0.500.07 K exchangeable (meq/100g) 2.120.05 0.480.02 Carbonate (%) 12.273.17 18.620.20 Limestone (%) 4.611.71 4.710.43 Olsen P (ppm) 33.74.44 27.06.55 SOC (%) 1.390.11 1.510.17 N org (%) 0.160.01 0.170.01 pH 8.180.02 7.990.05 pH in ClK 7.460.02 7.460.03 Assimilable K (ppm) 927.027.25 208.46.23 Clay (%) 42.21.14 16.41.17 Sand (%) 18.61.40 28.73.52 Silt (%) 39.10.85 54.82.40 Texture Clay Silt‐loam
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not have available data from previous years, farmers from both locations identified yield levels within typical ranges in the region drylands. Both fields were planted between October 25 and November 12. Sowing rate was 200 kg ha1 for wheat and 110 kg ha1 for faba bean. Harvest took place between June 5 and June 25. Wheat and faba bean were seeded and harvested at the same time. Each trial consisted of a complete randomized block design with four blocks separated with a non‐seeded stripe 1mwidth. Plots were 6 x4 m2 size.
Table 2. Annual temperature and rainfall and management practices of the field experiments.
ORG CON
Location Mean temperature (ºC)
Sierra de Yeguas La Zubia
2013‐2014 16.0 15.3 2014‐2015 16.6 17.0 2015‐2016 16.9 15.4
1982‐2012 average 16.3 15.2 Rainfall (mm)
2013‐2014 2014‐2015 2015‐2016
433 344 363
309 359 288
1982‐2012 average 673 462 Farming system Organic Conventional Rotation Wheat‐Faba bean Monoculture Fertilization Poultry manure
(3.6% N, d.m.) (3.0 Mg ha‐1, f.m.) NPK
(8:15:15) (570 kg ha‐1) N (kg ha‐1) 54 (wheat) + 27 (faba bean) 45.6 P (kg ha‐1) n.d. 85.5 K (kg ha‐1) n.d. 85.5
Weed control Manual weeding MCPA 40% (2 l ha‐1) Irrigation Rainfed Rainfed
f.m. and d.m. stands for fresh and dry matter respectively, whereas n.d. means not determined.
MCPA= 2‐methyl‐4‐chlorophenoxyacetic acid, active ingredient of applied herbicide
2.2. Sampling methods
Grain yield, aboveground wheat and weed biomass were measured by harvesting two 0.5 x 0.5m2
squares at each subplot that were previously randomly thrown at the centre of the subplot. Wheat
and weed biomass were dried at 70 °C to obtain dry weight. Fresh spikes were threshed to separate
grain and grain husk before they were dried in the oven. Data for OV and MV were the average for
the six old and six modern varieties, respectively.
Root biomass was sampled at anthesis for cultivars planted at the ORG trial, at the third year of the
experiment. Two soil cubes of 25 x 25 x 25 cm3 were extracted at the centre of each plot, washed
and sieved (2 mm) at farm‐gate. At the laboratory, root biomass was washed, extracted and
estimated following Metcalfe et al. (2007). The extrapolated value of root biomass, on the basis of
the logarithmic equation obtained through this method, increased by 20% and 17% the extracted
root biomass of OV and MV, respectively. For CON root biomass, it was assumed the same absolute
data obtained for ORG.
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2.3. C and N determinations
Dried samples of grain, straw, husk, weed and root biomass were milled (<1 mm) and analysed for
C and N content with an elemental autoanalyser CNOH‐S (Flash EA1112 CHNS‐O, Thermo Finnigan).
2.4. LCA analysis
2.4.1. Goals and scope
The goal was to compare the C footprint of old and modern wheat varieties under ORG and CON
managements and Mediterranean climate conditions. For this purpose, a global warming impact
assessment was performed following the standards of ISO (2006) guidelines for LCA methodology.
Our major aim was to identify if the cultivation of high residue producing varieties (OV) could
constitute an advantage to climate change mitigation strategies, taking into account their potential
benefits for soil carbon sequestration. An additional goal was to highlight the hotspots in the GHG
profile of these systems, helping to focus efforts for reducing their C footprint. The temporal
boundaries were adjusted to 100 years by selecting the 100‐year GWP of nitrous oxide (N2O) and
employing a 100‐year averaged C sequestration rate. The system boundaries were stablished from
a “cradle to farm gate” approach, including the production of farming inputs and machinery, on‐
farm operations and off‐farm emissions due to N losses from the agroecosystem. We chose 1 ha of
land and 1 kg of product as functional units. An attributional life cycle assessment was also applied,
and allocation of emissions to production followed an economic criterion, based on market prices
of products and coproducts. For ORG trial, both crops products (wheat and faba bean grains) were
accounted for, while only wheat straw was considered as co‐product.
2.4.2. Inventory analysis
Based on “the cradle to farm gate” approach, we considered inputs and outputs for the production
of one kg of wheat per hectare from the inputs production phase to the emissions derived from N
losses from the field (Fig. 1). The pre‐farm step covered the emissions from production of inputs:
seeds, fertilizer, machinery and fuel. The on‐farm step included two sources of emissions. On the
one hand, emissions derived from the use of machinery for field labors. On the other hand, the
emissions derived from the N applied to soil in form of chemical and organic fertilizer and crop
residues. The off‐farm emissions were indirect emissions due to ammonia (NH3) volatilization and
nitrate (NO3) lixiviation processes.
2.4.3. Impact assessment
Total GHGe were the results of calculating emissions from inputs production and on‐farm activities
and direct and indirect N2O emissions from chemical/organic fertilizer application and crop residues
incorporation to soil. Emissions from N2 fixation by the legume were not considered (Barton et al.,
2011). N2O emissions were transformed to kg CO2eq with a GWP of 265 (IPCC, 2014). The final C
footprints were calculated as the total GHGe minus the CO2eq related to C sequestration.
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2.4.3.1. Emissions from inputs production.
GHGe from industrial inputs production were calculated from energy consumption data of Aguilera
et al. (2015b). This database includes all life cycle processes related to agricultural inputs production
from the extraction of raw materials to the transport of commercial inputs to the farm. We applied
a conversion factor of 62.8 kg CO2e GJ‐1, based on the energy mix data of 2010 (Koppelaar, 2012)
and emissions factors of each type of energy from IPCC (2006). Total machinery GHGe was the sum
of emissions from fuel production, and the emissions from machinery and implements manufacture
and maintenance. Emission factors for industrial inputs are detailed in Table S1. Final GHGe from
machinery and fuel production was expressed as kg CO2e ha‐1 emitted during each machinery task
(cultivation, harvesting, fertilizer application, spraying).
Figure 1. Life Cycle Inventory and system boundaries of wheat production from inputs manufacture to
wheat harvest considered in this study.
2.4.3.2. On‐farm emissions.
On‐farm emissions include direct emissions from fuel combustion and direct N2O emissions from
soil. On‐farm emissions from fuel combustion were calculated based on fuel consumption. Fuel
energy consumption data for each task from Aguilera et al. (2015b) was converted to GHGe using
IPCC (2006) direct fuel emission factors for farm machinery. On‐farm operations were calculated
from machinery and fuel production emissions and on‐farm fuel use emissions. Data from previous
section were multiplied by the number of tasks needed for each labor.
Following IPCC (2006), direct N2O emissions from the soil were calculated based on N applications,
including organic and chemical fertilizer and crop/weed residues (aboveground and belowground)
incorporated to the soil. Total N (kg ha‐1) from these inputs was multiplied by a specific emission
factor (EF) for Mediterranean rainfed systems (0.27% of N inputs) (Cayuela et al., 2017) to calculate
direct N2O ‐N emissions.
2.4.3.3. Indirect N2O emissions.
Indirect emissions were calculated from soil NO3 leaching and NH3 volatilization following IPCC
(2006) schedule. N loss by NO3 leaching was assumed to be zero. This assumption was based on the
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fact that annual evapotranspiration was 3.4 times higher than annual rainfall (data not shown). Only
from January to February 2013 and in November 2014, monthly rainfall was greater than
evapotranspiration, but soil water accumulation never exceeded soil water holding capacity. N loss
as NH3 volatilization was estimated as a 2.5% and 17% of the N applied for synthetic and organic
fertilizer, respectively (EEA, 2007). Indirect emissions due to NH3 volatilization were calculated only
for poultry and chemical fertilizer. N losses were transformed into N2O emissions using IPCC (2006)
indirect N2O EFs for NO3 leaching and NH3 volatilization.
2.4.3.4. Soil carbon balance.
SOC balance entails negative emissions when C from the atmosphere is sequestered, and positive
when C is emitted to the atmosphere. To estimate SOC balance, we applied the dynamic SOC model
Humified Soil Organic Carbon (HSOC) model (Aguilera et al., 2018), which is a simplified version of
RothC model (Coleman and Jenkinson, 2014) that has been successfully applied to Mediterranean
cropping systems. HSOC is a dynamic SOC model, in which the amount of decomposed C depends
on that year's stock, which changes over time. Thus, when a new management that promotes SOC
accumulation is implemented, C sequestration rate is maximum the first year and decreases over
the time, as SOC stock asymptotically approaches to the new equilibrium. HSOC has one inert SOC
pool (IOM), and two active pools (FOM and HUM). Decomposition rates for HUM and FOM C pools
were calculated using the modifying factors of Coleman and Jenkinson (2014). At Sierra de Yeguas,
the resulting rates were 0.98% and 23.48% for fast and long‐turnover compartment, respectively.
At La Zubia trial, these rates were 0.91% and 21.92%, respectively.
The inputs to HUM are calculated from annual soil C inputs using humification coefficients (Hi),
which are the result of multiplying hi (basal humification coefficients, specific for each C input type,
i) by a soil texture modifying factor (d), specific for each soil texture. For belowground C inputs, we
considered an increment of 65% of root C due to extra‐root C from rhizodeposition and root
turnover (Bolinder et al., 2007). hi of aboveground residues, roots, extra‐root C and manure were
11.5%, 21.8%, 8% and 25.4%, respectively (Aguilera et al., 2018), and the soil texture modifying
factors d were 1.10 and 0.92 for Sierra de Yeguas and La Zubia, respectively. Inputs to FOM are
calculated as the total C inputs applied to the soil minus the humified inputs. IOM is calculated
following Falloon et al. (1998) equation, which we applied to the measured SOC levels of the studied
soils.
Following usual management practices from the area, it was assumed that 80% of MV straw is
harvested for out farm uses and 20% is incorporated to the soil. For OV, harvested straw was
assumed to correspond to the absolute value of harvested straw in MV. The aim of this assumption
was to emphasise the potential for C sequestration of OV against MV, due to the higher straw yield
of the former. Assumption of incorporated biomass of weeds growing in wheat plots followed the
same rule. In the case of faba bean,100% of crop residue and weeds were recycled into the soil,
following the typical practice in the area.
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SOC changes in the ORG rotation are the result of both faba bean and wheat. However, the effects
of the two crops on SOC are considered separately in this study, as well as the other components of
the GHGe balance, which avoids the attribution of faba bean impacts to wheat.
2.5. Statistical analysis
A complete randomized block variance analysis of field measured variables, C and N inputs and C
footprints was carried out within each growing season at a significance level of 0.05. Treatment
means were compared using Tukey‘s test at 0.05 probability level. Shapiro‐Wilk's normality test was
run previously to check for normal distribution. Statistix software (Analytical Software, Version 10)
was used for all statistical tests.
Comparisons between ORG and CON were not carried out due to the fact that they would not only
be influenced by management but also by site characteristics (e.g. soil properties, water balances,
etc.).
3. Results and discussion
3.1. Grain yield, straw and weed production
OV had a significantly higher grain production than MV under ORG management in 2014, while the
opposite was true in 2015 and no significant differences were found in 2016 (Table 3). Under CON
management, grain yield of MV was only significantly higher than that of OV in 2014. There were
not significant differences between varieties under ORG or CON management across years. The fact
that MV did not significantly outyield OV under ORG management was likely due to having been
bred under high nutrient availability conditions, which can make them dependent on an easy access
to nutrients (Foulkes et al., 1998). Under relatively slow nutrient release typical of manure
application, MV could not yield more than old cultivars, as happened in 2014 and 2016. Murphy et
al. (2008) found lower mean yields for old varieties under organic fertilization, but when comparing
individually, some landraces outyielded modern cultivars. On the other hand, there is a wide
consensus on the better yield performance of modern cultivars under high soil nutrient availability
of CON management (e.g. Fang et al., 2011), but our results did not show this pattern. This could be
due to the low rainfall in the study years (Table 2), as water availability is a major factor affecting
grain yield (Ayadi et al., 2016). This factor could also contribute to the absence of differences under
ORG. Ferrante et al. (2017) also found that under stressful conditions (0‐2.5 Mg ha‐1), there were no
significant differences between cultivars of different year of release under Mediterranean
conditions. Overall, grain production of OV proved to be more stable than that of MV, for both
managements. Previously, less stable yields of modern cultivars across environments have been
reported (Acreche et al., 2008), although the opposite has been also found (Slafer and Kernich,
1996).
Across the three growing seasons, straw production of OV was 40% and 18% significantly higher
than that of MV under ORG and CON managements, respectively (Table 3). These results are in line
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with those of other authors (Townsend et al., 2017). In the past, relative high straw production, such
of those of the OV tested in this study, was a desirable wheat trait because straw was valuable for
livestock feeding and bedding. Currently, the search for soil C sequestration without compromising
grain yields have made straw production desirable again (Lorenz et al., 2010), and therefore
cultivation of OV, specially under ORG management, is a promising strategy for C sequestration.
Finally, weed biomass under ORG management was between 90% and 63% lower for OV than for
MV. Under CON management, OV plots produced 54% lower weed biomass than MV plots. This
indicates that OV showed higher competitiveness against weed. Previously, authors have pointed
out that wheat varieties released before herbicide expansion are more competitive against weeds
(e.g. Murphy et al., 2008). In this study, higher weed production led to higher final coproduct (data
not shown) of MV.
3.2. Carbon inputs to the soil
Wheat aboveground C inputs ranged between 1.2Mg ha‐1 for MV under CON management and
1.5Mg ha‐1 for OV under ORG (Table S5), values around those found by Alvaro‐Fuentes et al. (2009)
for continuous barley. Crop root residues applied to the soil contributed with 1.3Mg ha‐1 C and 0.8
Mg ha‐1 C for OV and MV, respectively (for both ORG and CON, see Section 2.2), and these values
were higher than those found by Novara et al. (2016) from a wheat monoculture. The belowground
C input to aboveground C input ratio for wheat is of the same order of the global revision by Mathew
et al. (2017). Total C allocated belowground represented 50% of total C inputs, indicating its
relevance for SOC balance accountings. The highest values for total belowground C input of MV
under CON management (2.2 Mg ha‐1) were due to higher weed root C input, accordingly to higher
weed biomass (Table 3). In this sense, the share of weed C contribution to the total C inputs
oscillated between 62% of MV under ORG, to 18% of OV under CON. This highlights the need to
include weeds in C balance studies. Indeed, the key role of weeds as a source of OC input to the soil
has also been reported for Spanish cropland throughout the 20th century (Aguilera et al., 2018), and
in a 50‐year simulation study in a Mediterranean rainfed cereal system (De Sanctis et al., 2012).
Overall for the three growing seasons, total C inputs of OV were 32% and 27% significantly higher
than those of MV under ORG and CON, respectively (Fig. 2d), mainly due to the higher straw
production and roots C inputs. Regarding the legume‐wheat rotation, 3‐year averaged values for
wheat C inputs were higher than those
from faba bean (Fig. 2d).
3.3. Nitrogen inputs to the soil
Wheat AG residues N inputs ranged from 42.5 kg ha‐1 of OV under CON to 12.3 kg ha‐1 of MV under
ORG (Table S5). Values for OV under ORG and MV under CON were similar to the 21.1 kg N ha‐1
measured in a conventionally tilled barley field (Plaza‐Bonilla et al., 2014) under similar rainfed
Mediterranean conditions. Crop roots N inputs reached maximums of 18 kg N ha‐1 for OV under CON
and minimums of 9 kg ha‐1 for MV under ORG, and averaged values ranged those found by Plaza‐
Bonilla et al. (2014). Although OV root N inputs were higher than that of MV for both managements,
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higher weed root N inputs of MV counteracted that trend, and total belowground N inputs ranged
from 29 kg N ha‐1 of OV under ORG to 41 kg N ha‐1 of MV under CON. Across years, total N inputs of
OV were 8% higher than those of MV under CON management, while no significant differences were
found under ORG (Fig. 3d).
Table 3. Grain yield, straw biomass and weed biomass (kg ha‐1, fresh matter) of old (OV) and modern (MV)
wheat varieties in organic (ORG) and conventional (CON) trials. Mean and standard error of the mean.
2014 2015 2016
OV MV OV MV OV MV
ORG Grain yield
2343a±248.8 1326b±214.9 2706b±116.6 4245a±212.7 1448a±148.8 1484a±162.4
Straw 7899a±641.1 4970b±369.6 8709a±330.9 6576b±314.9 5258a±286.5 4053b±198.8
Weed 7366a±1480.9 9516a±1550.9 6b±3.1 65a±19.1 137b±32.9 371a±67.8
CON Grain yield
2590b±176.9 3560a±352.9 1534a±218.9 1506a±284.3 1732a±207.3 1538a±228.3
Straw 18193a±1170.6 16805a±1168.8 13637a±720.7 8693b±634.1 11002a±755.3 10770a±629.9
Weed 1114b±193.4 2443a±263.9 1074b±172.7 1974a±251.0 1153b±297.9 2911a±253.6
*Different letters represent significant differences between OV and MV within each growing season at a level of 0.05
(Tukey test).
Figure 2. Total carbon inputs to the soil (Mg C ha‐1) due to aboveground crop (AG crop) and weed (AG weed)
residues, belowground crop (BG crop) and weed (BG weed) residues, and manure for organic (ORG) and
0
1
2
3
4
5
6
7
OV MV Faba OV MV
ORG CON
C in
puts
(M
g ha
-1)
a)a
b A
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2
3
4
5
6
7
OV MV Faba OV MV
ORG CON
C in
puts
(M
g ha
-1)
b)
A
a
b
B
0
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2
3
4
5
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7
OV MV Faba OV MV
ORG CON
C in
puts
(M
g ha
-1)
c)
A Ba
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OV MV Faba OV MV
ORG CON
C in
puts
(M
g ha
-1)
d)
a
Bb
A
B
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conventional (CON) managements, in 2014 (a), 2015 (b) 2016 (c), and average values for the three growing
seasons (d). OV=old variety; MV=modern variety. Different letters indicate significant differences between
OV and MV for total carbon inputs (ANOVA, p<0.05). Comparisons between both systems do not appear in
the figure, as they may be affected by “site effect” (see Section 2.5.). Bars stand for standard error of the
mean.
3.4. Soil organic carbon balance
Although C input of OV were higher under CON (Fig. 2d), SOC sequestration rate was higher under
ORG. This was because total humified C inputs were higher under ORG due to the relatively high
humification coefficient of manure. Taking into account only the cereal phase in the case of ORG,
the highest simulated SOC sequestration rate (‐999 kg CO2 ha‐1y‐1) was achieved with OV cultivation
under ORG management (Fig. 4d, Table S9), and it was higher than values estimated for rainfed
organic cereal in Aguilera et al. (2015a) and for conventional cereal in Alvaro‐Fuentes et al. (2009)
and Novara et al. (2016). MV under CON showed the lowest rate (‐292 kg CO2 ha‐1y‐1), with similar
values than those measured in a 16‐year continuous barley field under reduced tillage by Alvaro‐
Fuentes et al. (2009), but lower than those measured in a continuous wheat cropping system by
Novara et al. (2016). OV under CON management showed a relatively high C sequestration rate of ‐
659 kg CO2 ha‐1y‐1, indicating a relevant potential of OV to increase C sequestration also under CON
management, without a significant decrease in yield (Table 3). SOC increments are central for soil
quality and protection against erosion, which are important factors in desertification‐prone
Mediterranean agroecosystems (Aguilera et al., 2013), hence OV cultivation could help to soil
sustainability under Mediterranean conditions. Sequestration rate of legume was lower (2014),
intermediate between OV and MV (2015) and higher (2016) than sequestration rate of wheat (Fig.
4). Accordingly, the 3‐years averaged C sequestration rate of ORG taking into account the faba bean
phase was lower than that exclusively for wheat (Fig. 4d), in line with experiments comparing wheat
in rotation with legume with monoculture wheat (Tellez‐Rio et al., 2017).
3.5. Nitrous oxide (N2O) emissions
N2O emissions are an important source of uncertainty in agricultural GHGe balances, mainly because
EFs are climatic‐specific, and they show a distinct pattern in Mediterranean cropping systems
(Cayuela et al., 2017). In our calculations, 3‐year average N2O emissions were very similar among all
of the studied wheat treatments. However, when wheat and legume emissions were averaged, the
ORG rotation showed lower emissions than CON management.
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Figure 3. Total nitrogen inputs to the soil (kg N ha‐1) due to aboveground crop (AG crop) and weed (AG weed)
residues, belowground crop (BG crop) and weed (BG weed) residues, manure for organic (ORG) and
synthetic fertilizer for conventional (CON) management, in 2014 (a), 2015 (b) 2016 (c) and average values
for the three growing seasons (d). OV=old variety; MV=modern variety. Different letters indicate significant
differences between OV and MV for total nitrogen inputs (ANOVA, p<0.05). Comparisons between both
systems do not appear in the figure, as they may be affected by “site effect” (see Section 2.5.). Bars stand
for standard error of the mean.
3.5.1. Direct N2O emissions
Direct N2O emissions represented the second largest source of GHGe in ORG, with an average of
19.0% and 17.8% for OV and MV (Fig. 4d, Table S9), respectively. For the legume phase, they
represented a 17.0% of GHGe and were third in relevance, very near from 17.4% of machinery and
fuel production. Under CON management, they were the third source of emissions with a share of
13.0% and 12.0% for OV and MV. Previous studies found direct N2O emissions as the greatest source
of GHGe in Mediterranean wheat cropping systems (Ali et al., 2017), likely due to the use of higher
EFs, higher residue N content and higher N fertilizer rates than those of this study. Relatively low
direct N2O emissions estimated in this work are due to low N application rates and low N2O EFs, in
line with most Mediterranean rainfed systems (Cayuela et al., 2017). Biswas et al. (2008) found low
proportions of N2O emissions from direct emissions from a conventional wheat field (9%), although
this share increased up to 36% of the total when applying IPCC values. This study, in agreement with
Biswas et al. (2008) and Aguilera et al. (2015a), highlights that the use of specific EFs for N2O
0
20
40
60
80
100
120
140
OV MV Faba OV MV
ORG CON
N in
puts
(kg
ha-
1)
a) Aaa A
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40
60
80
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120
140
OV MV Faba OV MV
ORG CON
N in
puts
(kg
ha-
1)
b)
ab
A
B
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20
40
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ORG CON
N in
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ha-
1)
c)
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OV MV Faba OV MV
ORG CON
N in
puts
(kg
ha-
1)
d)
aa
AB
107
emissions estimates rather than default values is recommended for LCA under Mediterranean
rainfed conditions.
Considering the legume phase, we calculated higher average emissions for the cereal than for the
legume phase, due to higher N inputs from poultry. Previously, field measurements did not report
an increment in N2O emissions during the legume phase in a cereal‐legume rotation (Barton et al.,
2013).
3.5.2. Indirect emissions
N2O emissions from N losses through volatilization are shown in Table S5. Rainfall scarcity during
the field experiment period, along with high evapotranspiration and soil texture lead to dismiss NO3
leaching processes, following IPCC (2014). Under ORG, the contribution of indirect emissions to the
overall GHGe was 5.9% and 3.2% for wheat and the legume, respectively; while proportion was only
0.4%, under CON (Fig. 4). Ali et al. (2017) found this contribution to be much higher for a
Mediterranean cereal field (5%), due to the leaching losses emissions, higher fertilization rates and
higher NH3 volatilization proportion considered.
Crop residue management can influence the GHGe balance in opposing ways. Here, crop residues
and roots incorporation increased direct N2O emissions of OV, but they also promoted C
sequestration. In addition, non‐incorporated residues also contribute to GHGe when they are burnt
or used for animal feed or bedding (Lehtinen et al., 2014), although this has not been considered in
this balance.
-2000
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-1200
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1200
OV MV Faba OV MV
ORG CONkg C
O2e
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a)A
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1200
OV MV Faba OV MV
ORG CON
kg C
O2e
ha-1
b)
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OV MV Faba OV MV
ORG CON
kg C
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400
800
1200
OV MV Faba OV MV
ORG CON
kg C
O2e
ha-1
d)
b a
B A
b
B
a
108
Figure 4. Greenhouse gasses emissions (GHGe, kg CO2e ha‐1) from soil, on‐farm operations and inputs
manufacturing for old (OV) and modern (MV) wheat varieties cultivation under organic (ORG) and
conventional (CON) management and wheat‐faba bean rotation under ORG management in 2014 (a), 2015
(b) and 2016 (c) and the average of the three years (d). Mean, and standard error (bars) area‐based carbon
footprint are also shown. Different letters within each location represent significant differences in the C
footprint between OV and MV (ANOVA, p<0.05). Comparisons between both systems do not appear in the
figure, as they may be affected by “site effect” (see Section 2.5.).
3.6. On‐farm operations and inputs production emissions
Emissions from fuel use and machinery and fuel production were higher under ORG than under CON
management (Fig. 4, Table S9), contrary to other studies for Mediterranean rainfed cereal croplands
(Gutiérrez et al., 2017). While this emission source only accounted for 29% and 30% of the total
GHGe in the CON trial (for OV and MV), it was the major source of GHGe for ORG (69%), similar to
the pattern observed by Aguilera et al. (2015a). Therefore, machinery use should be a relevant
target for GHGe mitigation in organic Mediterranean rainfed systems.
Synthetic fertilizer production accounted for up to 50% of the GHGe in the conventionally managed
wheat, in agreement with other conventional wheat GWP evaluations (Biswas et al., 2008). In this
sense, Tuomisto et al. (2012) found that the high energy consumption and emissions associated with
the production of synthetic N fertilizers were responsible for the high energy inputs of conventional
farming. With a similar pattern, a recent environmental assessment of the whole process of bread
manufacture in a Mediterranean region showed that the production of chemical fertilizers
contributes with more than 50% to the C footprint of the cultivation phase (Ingrao et al., 2018).
Pesticide production only represented 1% of GHGe, very near to the 0.5% observed for crop
protection in rainfed wheat under similar Mediterranean conditions (Tahmasebi et al., 2018). The
share represented by seed production was also small (6.5% and 7% for ORG and CON trial,
respectively), and slightly higher than the averaged proportion of Tahmasebi et al. (2018) (2.8%).
3.7. Carbon footprint: area and yield‐scaled GHG emissions
Highest annual area‐scaled GHGe (811 kg CO2e ha‐1) was found for MV under CON management,
while the lowest (‐345 kg CO2e ha‐1) was for the cultivation of OV (without including faba bean
footprint) under ORG (Fig. 4d). Across years and managements, C footprint was significantly lower
for OV than for MV (Fig. 4). On average, the C footprint of faba bean was positive and higher than
that of wheat in ORG (Fig. 4, Table S9), thus slightly augmenting the C footprint of the rotation when
averaging wheat and faba bean data. Our finding for organic MV C footprint was lower than that by
Aguilera et al. (2015a) (361 kg CO2e ha‐1), mainly due to our higher SOC sequestration rates. Likewise,
our C footprint under CON is lower than those from other studies of modern cereal varieties under
conventional management. For instance, Ali et al. (2017) found a higher C footprint of 1481 kg CO2e
109
ha‐1, because of their higher N2O emissions, GHGe related to inputs production and the lack of
inclusion of the SOC balance.
Averaged GHGe for wheat‐legume rotation in our study was lower than findings for a long‐term
wheat under rotation (Tellez‐Rio et al., 2017), likely because, on average, their SOC contribution to
the GHG balance was positive, rather than negative. This fact further highlights the relevance of the
SOC balance in C footprint assessments. Comparing C footprint components, C sequestration and
farm inputs and operations were more relevant than N2O fluxes, in agreement with Guardia et al.
(2016) and Aguilera et al. (2015a).
Trends for the yield‐scaled C footprint were similar to those of the area‐scaled analysis (Fig. 5b,
Table S9). Mean value for the three growing seasons of OV was significantly lower than for MV under
both managements. Our data for MV under CON management, which represents conventional
cereal production, agrees well with the values reported by Biswas et al. (2008) and Ali et al. (2017)
for conventional wheat under Mediterranean conditions. Although not statistically comparable, a
trend of lower yield‐scaled C footprint under ORG management can be depicted from our results,
which matches with previous findings under Mediterranean conditions (Aguilera et al., 2015a) and
with previous organic and conventional European farming systems comparisons (Tuomisto et al.,
2012). This lower C footprint on a product basis could lead to lower GWP of final wheat products,
such as bread (Meisterling et al., 2009). Contrastingly, Chiriacò et al. (2017) found higher GHGe from
an organic bread due to the lower yields compared to conventional whole meal bread, and Tricase
et al. (2018) observed a higher environmental impact of organic barley on a product basis when
compared to conventional barley. Tahmasebi et al. (2018) found a correlation between yield and
GHG emissions for rainfed wheat, but also lower product C footprints for higher wheat yields. Our
results do not follow this relation, as higher C footprint was estimated for MV while such trend was
not always true for yield. This difference can be due to the inclusion in our study of the soil C balance
in the emission assessment and the higher C inputs for OV.
The higher aboveground and belowground residue production of OV led to a large C sequestration
rate, responsible for a lower C footprint for OV. SOC balance contribution to C footprint reduction
ranged from ‐146% in OV‐ORG plots to ‐25% in MV‐CON plots. C sequestration of OV in the ORG trial
offsets all other GHGe, while this was not true for MV. Our results strengthen the hypothesis that
SOC balance is a major contributor to the variation of the C footprint of agricultural products (Gan
et al., 2014), so it is important to account for it in GHGe balances (Rodríguez‐Entrena et al., 2014).
In Mediterranean agroecosystems, where soil C shows great responses to organic inputs (Aguilera
et al., 2015a) and contribution to N2O emissions is low (Guardia et al., 2016), C sequestration as an
option to climate change mitigation should be fostered (Tellez‐Rio et al., 2017). It is worth noting
that SOC changes are highly dependent on specific site pedo‐climatic conditions and agronomic
practices (Francaviglia et al., 2017), and variation between C footprints estimates should warn us
about the relevance of more detailed and local studies to determine specific site and cropping
conditions and management‐related mitigation strategies.
As happened with area‐scaled C footprint, the legume phase harbored higher C footprint than the
wheat phase. Thus the C footprint of the rotation scheme was higher than that of wheat, contrary
110
to the findings of Gan et al. (2014) for wheat grown after a legume instead of a cereal. In the legume
phase of the rotation, the contribution of SOC accumulation to the reduction in the C footprint
averaged ‐60%.
Crop residues incorporation to soils can have several advantages to mitigate climate change impacts
in dry and hot environments (Liu et al., 2017). Provided that an adequate quantity of the additional
residue production of OV is returned to the soil, our results show that it can benefit the sustainability
of rainfed cereal production under Mediterranean climate, particularly under organic management,
where their cultivation resulted in a negative C footprint.
These results could potentially be applicable in other semiarid systems beyond the Mediterranean
climate. In any case, further field investigation is needed to confirm our conclusions and ensure that
both systems could benefit from the results of the present study. In addition, introducing a legume
in rotation with cereal has several advantages, such as weed control (Díaz‐Ambrona and Mínguez,
2001) or as a valuable protein source (Christiansen et al., 2015). The advantages of organic cereal‐
legume rotations could be boosted by introducing genetic material better suited to organic farming,
like old wheat cultivars in comparison to modern breeding cultivars, as has been shown in this study.
A higher level of specificity and regionalized data are needed to provide more valuable and deeper
LCA analyses of food production systems (Notarnicola et al., 2017). Policy makers searching for
options to face the climate change challenge could find new insights if adopting a regional approach.
In addition, strategies may be developed through the collaboration of farmers and researchers in
order to better identify the potential constraints to their expansion. In this respect, we have
supported our LCA with field experiment data as well as with specific Mediterranean emission
factors and SOC modeling, strengthening the validity and applicability of the results. This has
allowed us to observe that SOC sequestration, the on‐farm use of fuel and machinery and the
application of chemical fertilizers should be taken as key aspects for the development of climate
change mitigation strategies in Mediterranean rainfed systems.
Figure 5. Yield‐scaled C footprint (GHGe, g CO2e kg grain‐1) for old (OV) and modern (MV) wheat varieties,
and faba bean, under organic (ORG) and conventional (CON) management for the three growing seasons
(a), and for the 3‐year average (b). Different letters within each location represent significant differences in
the C footprint between OV and MV (ANOVA, p<0.05). Comparisons between both systems do not appear
-300
-200
-100
0
100
200
300
400
2014 2015 2016
g C
O2
kg-1
ORG OV ORG MVORG Faba CON OV
a)
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50
100
150
200
250
300
OV MV Faba OV MV
ORG CON
g C
O2
kg-1
b)
a
b
B
A
111
in the figure, as they may be affected by “site effect” (see Section 2.5.). Error bars stand for standard error
of the mean.
4. Conclusions
The results of this study show relevant differences in the carbon footprint of old and modern wheat
varieties under organic and conventional rainfed Mediterranean conditions. Fertilizers production,
in conventional systems, and machinery use, in organic systems, were identified as the major GHGe
hotspots of wheat cultivation, indicating the need for mitigation efforts. Carbon
sequestration was also an important component of the GHGe balance, and it was responsible for
the significant reduction in the C footprint observed with old varieties, due to the higher straw and
root biomass. Old varieties also suffered of a lower weed infestation than modern ones, whereas
grain yield was not significantly reduced. Overall, higher biomass incorporation to the soil with old
varieties was responsible of higher negative values for C balance and higher N2O emissions, but the
former broadly offset the latter, resulting in a lower C footprint both area and yield‐scaled, and even
in a negative C footprint for old varieties under ORG. Therefore, the results stress the relevance of
C sequestration in climate change mitigation and the importance of including it in C footprint
accountings. In conclusion, old varieties have shown a large potential for enhancing C sequestration
and reducing the C footprint through high residue production, which makes them particularly
promising for climate change mitigation in organic and low input system
Conflicts of interest
None.
Acknowledgements
Funding was provided by the international research project SSHRC 895‐2011‐1020 granted by the
Social Sciences and Humanities Research Council of Canada, and the Spanish research projects
HAR2012‐38920‐C02‐01 and HAR2015‐69620‐C2‐1‐P granted by the Spanish Ministry of Economy
and Competitiveness. The first authoress held a FPU scholarship from the Spanish
Government.
Appendix A. Supplementary data Supplementary data related to this article can be found at
https://doi.org/10.1016/j.jclepro.2018.05.188.
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Supplementary data
Table of contents
S1. Trials location ............................................................................................................... 2
S2. Emission factors .......................................................................................................... 3
S3. Carbon and nitrogen inputs ......................................................................................... 4
S4. Global warming potential ............................................................................................ 7
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S1.
TRIALS LOCATION
Figure S1. Trials location in Southern Iberian Peninsula.
S2. EMISSION FACTORS
Table S1. Emissions factors (kg CO2 ha‐1) for machinery production and fuel use and production (kg CO2 ha‐
1). fertilizer (kg CO2 ha‐1) and pesticide (kg CO2 kg‐1 active matter) manufacture and transport.
Machinery
Fuel use Fuel production
Machinery production
Implement production
Cultivating 38.53 6.35 3.71 1.92
Fertilizing 9.05 1.49 2.30 0.32
Spraying 3.65 0.60 0.93 0.13
Subsoiler 72.69 11.98 6.16 2.73
Harvest 63.16 10.41 4.73 ‐
Synthetic fertilizer
N P2O5 K2O 7.28 1.28 1.22
Pesticide
12.9
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S3. CARBON AND NITROGEN INPUTS
Table S2. Soil C input and N input for old (OV) and modern (MV) wheat varieties under organic (ORG) and
conventional (CON) management, and the legume phase of the organic rotation (faba bean) in 2014.
AG=aboveground; BG=Belowground.
ORG CON
OV MV Faba bean
OV MV
C input (Mg ha‐1yr‐1)
Manure 0.6 0.6 0.3 ‐ ‐
AG Crop 1.8 0.5 0.6 2.7 1.6
AG Weed 0.6 0.4 0.4 0.2 0.4
BG Crop 1.3 0.6 1.4 1.3 0.6
BG Weed 1.8 2.4 0.6 0.7 1.4
Total 6.0 4.6 3.4 4.8 4.0
N input (kg ha‐1 yr‐1)
Fertilizer 0.0 0.0 0.0 45.6 45.6
Manure 54.0 54.0 27.0 0.0 0.0
AG Crop 14.4 4.6 20.1 57.7 36.9
AG Weed 13.9 8.6 9.4 4.7 6.2
BG Crop 15.5 6.9 16.6 17.7 8.0
BG Weed 37.8 52.2 14.0 14.6 31.1
Total 135.5 126.3 87.1 140.3 127.8
Table S3. Soil C input and N input for old (OV) and modern (MV) wheat varieties under organic (ORG) and
conventional (CON) management, and the legume phase of the organic rotation (faba bean) in 2015.
AG=aboveground; BG=Belowground.
ORG CON
OV MV Faba bean
OV MV
C input (Mg ha‐1yr‐1)
Manure 0.6 0.6 0.3 0.0 0.0
AG Crop 1.8 1.0 0.6 3.0 1.0
AG Weed 0.0 0.0 0.3 0.2 0.3
BG Crop 1.6 1.2 1.4 1.6 1.2
BG Weed 0.0 0.1 0.4 0.7 1.2
Total 4.1 2.8 3.0 5.5 3.6
N input (kg ha‐1 yr‐1)
Fertilizer 0.0 0.0 0.0 45.6 45.6
Manure 54.0 54.0 27.0 0.0 0.0
AG Crop 33.9 25.4 20.1 34.7 14.2
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AG Weed 0.3 1.0 13.2 4.8 3.6
BG Crop 19.1 13.7 16.6 21.9 15.9
BG Weed 0.4 1.2 9.3 14.3 26.2
Total 107.7 95.3 86.2 121.2 105.5
Table S4. Soil C input and N input for old (OV) and modern (MV) wheat varieties under organic (ORG) and
conventional (CON) management, and the legume phase of the organic rotation (faba bean) in 2016.
AG=aboveground; BG=Belowground.
ORG CON
OV MV Faba bean
OV MV
C input (Mg ha‐1yr‐1)
Manure 0.6 0.6 0.3 0.0 0.0
AG Crop 1.0 0.5 0.6 1.9 1.0
AG Weed 0.1 0.2 0.4 0.2 0.3
BG Crop 1.0 0.6 1.4 1.0 0.6
BG Weed 0.2 0.4 0.6 0.6 1.5
Total 2.9 2.4 3.3 3.6 3.4
N input (kg ha‐1 yr‐1)
Fertilizer 0.0 0.0 0.0 45.6 45.6
Manure 54.0 54.0 27.0 0.0 0.0
AG Crop 13.1 6.9 20.1 35.1 19.1
AG Weed 2.0 9.3 15.8 3.7 7.2
BG Crop 11.4 7.0 16.6 13.0 8.2
BG Weed 3.7 9.7 13.0 12.7 32.3
Total 84.3 86.9 92.5 110.1 112.3
Table S5 Soil C input and N input for old (OV) and modern (MV) wheat varieties under organic (ORG) and
conventional (CON) management, and the legume phase of the organic rotation (faba bean) for a 3‐year
average. AG=aboveground; BG=Belowground.
ORG CON
OV MV Faba bean
OV MV
C input (Mg ha‐1)
Manure 0.6 1.0 0.6 0.6 1.4
AG Crop 1.5 0.7 0.6 2.5 1.2
AG Weed 0.2 0.2 0.4 0.2 0.3
BG Crop 1.3 0.8 1.4 1.3 0.8
BG Weed 0.6 1.0 0.6 0.6 1.4
Total 4.3 3.3 3.2 4.7 3.7
N input (kg ha‐1)
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Fertilizer 0.0 0.0 0.0 45.6 45.6
Manure 54.0 54.0 27.0 0.0 0.0
AG Crop 20.5 12.3 20.1 42.5 23.4
AG Weed 5.4 6.3 12.8 4.4 5.7
BG Crop 15.3 9.2 16.6 17.5 10.7
BG Weed 14.0 21.0 12.1 13.8 29.8
Total 109.2 102.8 88.6 123.9 115.2
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S4. GLOBAL WARMING POTENTIAL
Table S6. Global warming potential for old (OV) and modern (MV) wheat varieties under organic (ORG) and
conventional (CON) management, and the legume phase of the organic rotation (faba bean) in 2014 on a
100‐year time horizon. SE=Standard error of the mean.
ORG CON
OV MV Faba bean
OV MV
Soil emissions (kg CO2 ha‐1yr‐1)
Direct N2O Fertilizer 0.0 0.0 0.0 57.7 57.7
Manure 68.3 68.3 34.1 0.0 0.0
AG Crop 18.2 5.8 25.4 72.9 46.6
AG Weed 15.6 9.7 10.6 5.3 7.0
BG Crop 17.4 7.7 18.7 19.9 9.0
BG Weed 42.5 58.7 15.7 16.4 34.9
Total 162.0 150.2 104.6 172.2 155.3
Indirect N2O Fertilizer 0.0 0.0 0.0 4.7 4.7
Manure 38.2 38.2 19.1 0.0 0.0
Total 38.2 38.2 19.1 4.7 4.7
Carbon balance ‐1878.4 ‐1259.4 ‐487.9 ‐740.7 ‐399.6
On‐farm operations and inputs production emissions (kg CO2 ha‐1yr‐1)
Fuel use 347.5 347.5 347.5 249.3 249.3
Fuel production 57.3 57.3 57.3 41.1 41.1
Machinery production 46.9 46.9 46.9 33.9 33.9
Fertilizer production
N 0.0 0.0 0.0 332.2 332.2
P2O5 0.0 0.0 0.0 109.1 109.1
K2O 0.0 0.0 0.0 104.3 104.3
Pesticide production 0.0 0.0 0.0 10.3 10.3
Nursery 38.6 46.3 25.5 70.6 84.7
Total 490.2 498.0 477.1 950.7 964.8
C footprint
Area‐based (kg CO2 ha‐1yr‐1) ‐1188.0b ‐573.0a 112.9 387.0B 725.3A
SE 173.7 218.2 174.2 143.0 94.0
Yield‐based (g CO2 kg‐1) ‐174.7b ‐75.8a 59.4 94.1A 152.4A
SE 28.1 29.8 51.8 35.2 24.6
Table S7. Global warming potential for old (OV) and modern (MV) wheat varieties under organic (ORG)
and conventional (CON) management, and the legume phase of the organic rotation (faba bean) in 2015
on a 100‐year time horizon. SE=Standard error of the mean.
ORG CON
OV MV Faba bean
OV MV
Soil emissions (kg CO2 ha‐1yr‐1)
Direct N2O Fertilizer 0.0 0.0 0.0 51.3 51.3
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Manure 60.7 60.7 30.4 0.0 0.0
AG Crop 38.1 28.6 22.6 39.0 15.9
AG Weed 0.4 1.1 14.8 5.4 4.1
BG Crop 21.5 15.4 18.7 24.6 17.9
BG Weed 0.5 1.4 10.4 16.1 29.5
Total 121.1 107.1 96.9 136.3 118.6
Indirect N2O Fertilizer 0.0 0.0 0.0 4.7 4.7
Manure 38.2 38.2 19.1 0.0 0.0
Total 38.2 38.2 19.1 4.7 4.7
Carbon balance ‐849.1 ‐265.2 ‐298.4 ‐1049.4 ‐290.0
On‐farm operations and inputs production emissions (kg CO2 ha‐1yr‐1)
Fuel use 347.5 347.5 347.5 249.3 249.3
Fuel production 57.3 57.3 57.3 41.1 41.1
Machinery production 46.9 46.9 46.9 33.9 33.9
Fertilizer production
N 0.0 0.0 0.0 332.2 332.2
P2O5 0.0 0.0 0.0 109.1 109.1
K2O 0.0 0.0 0.0 104.3 104.3
Pesticide production 0.0 0.0 0.0 10.3 10.3
Nursery 38.6 46.3 25.5 70.6 84.7
Total 490.2 498.0 477.1 950.7 964.8
C footprint
Area‐based (kg CO2 ha‐1yr‐1) ‐200b 378a 295 42B 798A
SE 89.1 45.1 175.3 133.3 82.1
Yield‐based (g CO2 kg‐1) ‐27b 50a 108 ‐5B 336A
SE 13.9 6.6 62.2 57.1 54.1
Table S8. Global warming potential for old (OV) and modern (MV) wheat varieties under organic (ORG)
and conventional (CON) management, and the legume phase of the organic rotation (faba bean) in 2016
on a 100‐year time horizon. SE=Standard error of the mean.
ORG CON
OV MV Faba bean
OV MV
Soil emissions (kg CO2 ha‐1yr‐1)
Direct N2O
Fertilizer 0.0 0.0 0.0 51.3 51.3
Manure 60.7 60.7 30.4 0.0 0.0
AG Crop 14.8 7.8 22.6 39.5 21.4
AG Weed 2.3 10.5 17.7 4.2 8.1
BG Crop 12.9 7.9 18.7 14.6 9.2
BG Weed 4.2 10.9 14.6 14.2 36.3
Total 94.8 97.7 104.0 123.8 126.3
Indirect N2O Fertilizer 0.0 0.0 0.0 4.7 4.7
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Manure 38.2 38.2 19.1 0.0 0.0
Total 38.2 38.2 19.1 4.7 4.7
Carbon balance ‐270.5 ‐58.5 ‐444.9 ‐187.8 ‐186.8
On‐farm operations and inputs production emissions (kg CO2 ha‐1yr‐1)
Fuel use 347.5 347.5 347.5 249.3 249.3
Fuel production 57.3 57.3 57.3 41.1 41.1
Machinery production 46.9 46.9 46.9 33.9 33.9
Fertilizer production
N 0.0 0.0 0.0 332.2 332.2
P2O5 0.0 0.0 0.0 109.1 109.1
K2O 0.0 0.0 0.0 104.3 104.3
Pesticide production 0.0 0.0 0.0 10.3 10.3
Nursery 38.6 46.3 25.5 70.6 84.7
Total 490.2 498.0 477.1 950.7 964.8
C footprint
Area‐based (kg CO2 ha‐1yr‐1) 353b 575a 155 891A 909A
SE 77.6 41.8 175.2 101.8 93.9
Yield‐based (g CO2 kg‐1) 74b 113a 71 343A 302A
SE 15.6 9.9 54.3 40.8 38.3
Table S9. Global warming potential for old (OV) and modern (MV) wheat varieties under organic (ORG)
and conventional (CON) management, and the legume phase of the organic rotation (faba bean) on a 100‐
year time horizon for a 3‐year average. SE=Standard error of the mean.
ORG CON
OV MV Faba bean
OV MV
Soil emissions (kg CO2 ha‐1yr‐1)
Direct N2O Fertilizer 0.0 0.0 0.0 53.4 53.4
Manure 63.2 63.2 31.6 0.0 0.0
AG Crop 23.7 14.1 23.5 50.5 28.0
AG Weed 6.1 7.1 14.4 4.9 6.4
BG Crop 17.2 10.3 18.7 19.7 12.0
BG Weed 15.7 23.6 13.6 15.6 33.5
Total 126.0 118.4 101.8 144.1 133.4
Indirect N2O Fertilizer 0.0 0.0 0.0 4.7 4.7
Manure 38.2 38.2 19.1 0.0 0.0
Total 38.2 38.2 19.1 4.7 4.7
Carbon balance ‐999.4 ‐527.7 ‐410.4 ‐659.3 ‐292.1
On‐farm operations and inputs production emissions (kg CO2 ha‐1yr‐1)
Fuel use 347.5 347.5 347.5 249.3 249.3
Fuel production 57.3 57.3 57.3 41.1 41.1
Machinery production 46.9 46.9 46.9 33.9 33.9
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Fertilizer production
N 0.0 0.0 0.0 332.2 332.2
P2O5 0.0 0.0 0.0 109.1 109.1
K2O 0.0 0.0 0.0 104.3 104.3
Pesticide production 0.0 0.0 0.0 10.3 10.3
Nursery 38.6 46.3 25.5 70.6 84.7
Total 490.2 498.0 477.1 950.7 964.8
C footprint
Area‐based (kg CO2 ha‐1yr‐1) ‐345b 127a 188 440B 811A
SE 102.4 95.3 94.3 83.4 52.1
Yield‐based (g CO2 kg‐1) ‐43b 29a 79 144B 263A
SE 16.7 14.1 30.0 31.1 25.1
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4.3. THIRD STUDY
Modern Wheat Varieties as a Driver of the Degradation of Spanish Rainfed
Mediterranean Agroecosystems throughout the 20th Century
Guiomar Carranza‐Gallego *, Gloria Isabel Guzmán, David Soto, Eduardo Aguilera, Inma Villa, Juan Infante‐Amate, Antonio Herrera and Manuel González de Molina Sustainability, 10:3724, (2018), doi:10.3390/su10103724
Abstract
The high grain yield of modern varieties (MV) respond to the increase in fossil‐based inputs, and the widespread belief that they are more productive than old varieties (OV) is biased. This belief focuses only on marketable biomass, without considering the consequences on agroecosystem sustainability of the reductions in other portions of NPP. Additionally, field comparisons of OV and MV were normally conducted under industrialized farming conditions, which is detrimental for OV performance. Both trials carried out in this study comparing wheat OV and MV show that, under Mediterranean rainfed conditions and traditional organic management, aerial and belowground biomass production of OV is higher than that of MV, without significantly decreasing yield and enabling a better competition against weeds. From the data of our trials, bibliographic review and information from historical sources, we have reconstructed the NPP and destinations of biomass of Spanish wheat fields (1900–2000). Varietal replacement entailed the reduction in residues and unharvested biomass (UhB), which involved soil degradation in rainfed cereal fields and undermining heterotrophic trophic webs. Our results suggest that OV can increase the sustainability of rainfed Mediterranean agroecosystems at present through the improvement of soil quality, the reduction of herbicides use, and the recovery of biodiversity.
1. Introduction
The replacement of old varieties (OV) by modern (MV) ones was inextricably linked to the agrarian industrialization process, and can be considered the cornerstone of the Green Revolution, because new varieties were specifically designed to respond with yield increases to the incorporation of inputs of fossil origin [1–3]. The varietal change was especially intense in cereals. The high proportion of global cropland area dedicated to these crops and their relevance in human diet placed them at the center of breeding programs by the experimental stations that led the Green Revolution. With regards to wheat, the effect of varietal replacement on grain yield has been outstanding [4] and has led to the widespread belief that MV are more productive than OV [5–8]. However, this statement must be put into context. In the first place, because it assumes one part (the grain) to be the whole (net primary production); Secondly, because most of the comparative tests between types of varieties are carried out under conditions of industrialized agriculture. The conception of the marketable biomass (mainly grain) as the only important one is reductionist and has entailed the abandonment of the functions that the rest of the biomass plays in the agroecosystems. It is well known that the sustainable management of an agroecosystem depends on the levels of biodiversity and soil organic matter (SOM), the appropriate replenishment of soil fertility, and the possibilities of closing biogeochemical cycles on a local scale, among other factors
127
[9]. This implies that a significant part of the biomass generated must recirculate to perform the basic productive and reproductive functions of the agroecosystem: seeds, animal labor, SOM, biodiversity, and so forth. In other words, in order to be sustainable, the agroecosystem functioning must be based on internal loops of biomass with low dependence on the external incorporation of energy. High density of internal biomass loops (and, therefore, of energy) maintains an adequate quality of the agroecosystem with low entropic costs and, consequently, maintaining the long‐term social extraction of biomass and the provision of ecosystem services [10–13] (pp. 41, 45). In short, the maintenance of internal loops in agroecosystems is directly related to the use of a significant part of net primary production to fuel them. This has major implications when it comes to calculating net primary productivity (NPP), which must then be broken down into different categories according to its productive or reproductive functionality [10]. Regarding wheat, there is a broad consensus that the grain yield increase of the Green Revolution cultivars is explained by the increase in Harvest Index (HI) and not by the increase in aerial biomass with respect to OV, the latter remaining stable [5,6,14,15]. Therefore, the increase of biomass for human society has been done at the expense of biomass that fed other chains of heterotrophic organisms and that allowed both the accumulation of edaphic organic matter and the recycling of nutrients. However, these studies frequently do not account for all the NPP and do not consider the root and weed biomass. In accounting for these, the hypothesis that varietal substitution has been a direct cause of the deterioration of agroecosystems, and not only the use of inputs from fossil origin associated with MV, could be reinforced or discarded. On the other hand, most field experiments comparing old and modern wheat varieties are carried out under conditions of industrialized agriculture [5–8,14,15]. This situation introduces a strong bias in the comparison, since OV were selected under very different management conditions. Therefore, it is convenient and necessary to explore the potential of OV biomass production in their proper context. That is, applying an Experimental History approach that recreates the traditional organic management in which these varieties were selected. The information obtained would have two obvious uses. The first one, for those researchers (ecologists, agrarian historians, etc.) that need to estimate the NPP of traditional agroecosystems from historical sources, these sources usually only provide information of the harvested biomass. In this case, the calculation of the NPP should be based on adequate partitioning indices for OV under traditional management. In the second place, agroecologists, agronomists, etc. involved in the design and evaluation of sustainable agroecosystems need to assess whether the biomass that is not extracted from the agroecosystems is enough to maintain the quality of the agroecosystem fund elements (soil, biodiversity, etc.). In this sense, an appropriate varietal choice must respond not only to productivity criteria, but also to sustainability. Conversely, the performance and partitioning indices of MV under modern management are better documented in the scientific literature, and it is not essential to carry out additional trials. Our hypothesis is that, under conditions of traditional organic agriculture, OV are equal or more productive than MV, whether considering the grain or the NPP. However, they would have lower HI than MV, both under organic traditional management and under conventional practices. Both aspects, higher NPP and lower HI, would have enabled OV to recycle larger quantities of biomass in the agroecosystem in the past and, with it, the maintenance of the quality of the agroecosystems fund elements, with respect to MV in the present. To test our hypothesis, we have focused on rainfed wheat under Mediterranean agro‐climatic conditions. Wheat are fundamental crops in the Mediterranean culture consist of the Mediterranean Trinity: wheat, vine, and olive. The first complete statistical analysis of agrarian
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production in Spain shows that wheat occupied 4.2 Mha in 1922, and was 94% cultivated under rainfed conditions (22% of the cropland approx.) [16]. Currently, it occupies 2.2 Mha, and is 86% under rainfed conditions (14% of the cropland approx.) [17], reaching 25% of the harvested area of the coastal Mediterranean countries (excluding France) [4]. This remarkable occupation of the territory confers to wheat varietal change a great capacity to modify Mediterranean agroecosystems. The Mediterranean climate is characterized by mild, wet winters and hot, dry summers, and it is localized in five areas around the world. Most of its subtypes can be classified as semi‐arid, and rainfed crops are usually exposed to water deficit conditions. Cropland soils in Mediterranean areas have low SOM contents due to climate limitations to NPP. The water deficit and the low soil organic carbon content (SOC) make Mediterranean agriculture very vulnerable, both to the cyclic drought events and to the current process of global climate change. The low NPP also hinders the balance between alternative uses: human consumption, maintenance of the soil quality, livestock feeding, and wild fauna feeding. This fragile balance could be substantially modified due to changes in the morphology and physiology of cereals promoted by the Green Revolution, altering the quality of the fund elements of Mediterranean agroecosystems. In particular, our objectives have been the following: (a) to quantify the NPP of old and modern wheat cultivars, under two representative traditional organic management types; (b) to quantify biomass allocation in wheat plant and within cultivated plots (grain, straw, husk, roots and weed biomass) under these management conditions, as well as the related partitioning indices; c) to define the biomass partitioning indices of MV under industrialized management in Spain, through a bibliographic review; (d) to apply the biomass partitioning indices obtained by both routes to the estimate of the NPP of wheat fields in Spain throughout the 20th century; (e) to model the biomass flows according to their destination in the same period of time; (f) to discuss to what extent the alteration of the production and end‐uses of the biomass has been able to alter the fund elements (soil, biodiversity) quality in these agroecosystems. 2. Materials and Methods 2.1. Data Collection
The selection and definition of the main traditional organic management of wheat in the early 20th century in Spain has been based on the review of historical sources [18]. The selected organic managements are those applied to rainfed wheat under contrasting soil quality conditions. In most fertile soils, wheat was planted in rotation with a legume and amended with manure. At the other end of the typical soil quality range, in lower quality lands, wheat was cultivated in “one third rotation” (wheat‐fallow‐fallow), without application of manure. Under intermediate intensity management (e.g., wheat‐fallow, wheat‐legume‐fallow...) the behavior of wheat varieties should be also understood as intermediate between both extremes. Table 1 summarizes tasks performed in trials under both selected managements. From these trials we have averaged the biomass partitioning indices of OV under traditional management. We have reconstructed the evolution of yield and harvested straw from the construction of eleven points in time between 1900 and 2000, using 5‐year averages to buffer year‐to‐year variability. Sources used in our study are the statistics provided by the Spanish government, with different quality and frequency from 1900 to 2000 [16–21]. The statistical information refers to fresh matter. HI and aerial weed biomass of MV under modern management come from a bibliographic review of scientific literature. Data selected come from field experiments under rainfed Mediterranean agroclimatic conditions (see Tables S1 and S2, Supplementary Materials). Aerial weed biomass
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values found in this revision show a high dispersion, which gives rise to a high standard deviation of the mean. Therefore, a sensitivity analysis with a weed‐free scenario was performed. Direct estimates of root biomass comparing OV and MV are very scarce. The few studies found indicate that wheat OV has a more developed root system and increases the root—shoot (R:S) ratio with respect to MV [22–24]. On the other hand, there are many other factors (edapho‐climatic, management, etc.) that affect the ratio between root and aerial biomass [25]. This means that shoots and roots respond differently to changes in environmental conditions. For example, in areas with a Mediterranean climate, the R:S ratio is usually larger than in areas of higher precipitation, due to the need to spread roots over a larger soil volume to capture enough water [26]. On the other hand, organic management seems to favor root development compared to industrialized management [27]. In general, this ratio decreases with management changes that promote yield, including synthetic nutrient inputs [27–29] and irrigation [30]. Consequently, using a fixed amount of root mass (“Fixed root mass”) may lead to better fit with experimental data than a using a fixed R:S ratio [27,29,31]. However, Wiesmeier et al. [32] suggest that the R:S ratios of crops may have even increased in recent history. Given this controversy, we have built a R:S ratio throughout the 20th century that tempers the impact of changes in aerial biomass on root biomass that reflects the percentage differences in this index between OV and MV and organic and industrialized management (Table S5). For 1900–1933, the root: shoot ratio (0.36) comes from our own trial. This ratio has been multiplied by aerial biomass of wheat for those years (calculated from the grain yield from historical sources already mentioned and HI from our experimental field), to obtain root biomass. In 1940–50, the ratio applied was greater, to temper the fall in aerial biomass caused by the Spanish Civil War (1936–39), since under conditions of lower productivity root biomass remains more stable. For MV in 2000, the R:S ratio comes from data provided by recent trials conducted in Spain under rainfed conditions [33–35]. This ratio has been multiplied by wheat aerial biomass for that year to obtain root biomass. Ratios for 1980 and 1990 have been modulated so that root biomass maintained similar values to the year 2000. The period between 1960–70 has been considered as a transitional period, and the R:S ratio has been calculated considering that the varietal substitution was 50% for 1960 and 75% for 1970. Weed root biomass has not been considered in this study because it lacks minimally reliable data on which to base it. In the absence of trials in which this variable has been measured, regardless of the agroclimatic conditions, it must be added that the weeds complex present in plots cultivated with OV under organic management is different from that of MV under industrialized management [36] and, therefore, it is not possible to attribute the same R:S ratio for weeds in this study. Lastly, indices applied to calculate wheat biomass destinations (human, animal, unharvested biomass) throughout the 20th century come from FAOSTAT [4] and the Spanish statistical yearbooks [17]. FAOSTAT [4] offers annual data on grain destinations since 1961, while the Spanish yearbooks [17] offer straw harvest data. In the case of grain, for previous times we have assumed the same percentages as for 1960, since the industrialization of Spanish agriculture was just beginning. The share of straw burned comes from Soto et al. [37] (Table S3). 2.2. Trials: Site Description and Experimental Design Two field experiments were carried out at two locations in Southern Iberian Peninsula, Sierra de Yeguas (37°07′26″ N 4°52′07″ W) and Ronda (36°44′14″ N 5°09′53″ W) (Málaga province). Both farmlands had been under organic management for the previous 15 years. To cover for interannual variability, the field experiments were carried out during three consecutive growing seasons (2013–2016). The main soil properties of the experimental sites are shown in Table S4. Annual precipitation was 56.4% and 81.9% of the mean value, for Sierra de Yeguas and Ronda, respectively (Table 1). The
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same old (Rubio, Recio, Sierra Nevada, Barbilla Roja, Rojo Pelon, Blanco Verdial) and modern (Avispa, Simeto, Vitron, García, Marius, Artur Nick) durum and bread wheat varieties were sown at both locations. OV were landraces grown during the first third of the 20th century in the region. Their seeds came from the Phytogenetic Resource Centre of the National Agrarian Research Institute of Spain (CRF‐INIA). MV were chosen among lately released, currently used varieties, considering their good reputation among farmers in the area.
Table 1. Annual rainfall and management practices of the field experiments.
Sierra de Yeguas
(Wheat‐Faba Bean Rotation) Ronda
(One‐Third Rotation)
Rainfall (mm) 2013–2014 433 612 2014–2015 344 448 2015–2016 363 846
1982–2012 average 673 775 Rotation Wheat‐faba bean Wheat‐fallow‐fallow
Fertilization Manure ‐ (3.6% N, d.m.) (3.0 Mg ha−1, f.m.)
Weed control Manual weeding ‐ Irrigation Rainfed Rainfed
Note: f.m. and d.m. stand for fresh and dry matter, respectively.
The farmland at Sierra de Yeguas was cultivated with a two‐year crop rotation called ruedos, consisting of wheat and faba bean (Vicia faba). Both species were grown in adjacent plots, interchanging cultivation plots each year. Before wheat seeding in the first year of the rotation, 3 t ha−1 of manure was applied, but none to the faba bean crop. Weeds were controlled by hand. The farmland at Ronda was cultivated with a wheat‐fallow‐fallow rotation (one‐third rotation). No weed control or fertilization were applied to the soil. Both fields were planted between October 29 and November 19. Sowing rate was 200 kg ha−1 for wheat and 110 kg ha−1 for faba bean. Harvest took place between May 21 and June 23. Wheat and faba bean were seeded and harvested at the same time. Each trial consisted of a completely randomized block design with four blocks separated by a non‐seeded stripe 1m in width. Plots were 6 × 4 m size. 2.3. Sampling method. At the end of the wheat cycle, samples of aerial weed and wheat biomass were taken from the plots. Aerial net primary productivity (NPPa), which includes total crop dry matter and total weed dry matter sampled at the end of the cycle. Grain yield, straw and husk biomass production, and weed biomass were determined in 0.5 × 0.5 m squares randomly thrown at the center of each plot, following the simple random sampling (SRS) method. For wheat and weed biomass, plants inside the square were cut at ground level. Wheat plants were separated into spike and stem. Wheat and weed biomass were dried at 70 °C to obtain dry weight. Fresh spikes were previously threshed to separate grain and grain husk. The ratio between weed biomass and NPPa (weed:NPPa ratio), i.e., the share of agroecosystem biomass allocated to weed, was also calculated. Root biomass was sampled at post‐anthesis for cultivars planted at Sierra de Yeguas, at the third year of the experiment (2016). Two soil cubes of 25 × 25 × 25 cm were extracted at the center of each plot, washed, and sieved (2 mm) at farm‐gate. At the laboratory, root biomass was washed, extracted, and estimated following Metcalfe et al. [38]. The extrapolated value of root biomass, on the basis of the logarithmic equation obtained through this method, increased by 20% and 17% the extracted root biomass of OV and MV, respectively. The R:S ratio was calculated as the ratio between root and aboveground wheat biomass dry matter.
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2.4. Modelling of NPP and Biomass Fluxes According to Their Destiny
From wheat yield and cultivated area in each point in time, and from the information (HI, root: shoot ratio and weed biomass) obtained from trials, for OV, and the bibliographic review, for MV, we have modeled the impact of the varietal change on the NPP per hectare in wheat fields throughout the 20th century. Based on Pujol‐Andreu [39] and Sánchez‐García et al. [8], we have considered that until 1950, OV under traditional management represented 100% of the cultivated area. From 1980, the same happens with MV under modern management. In the transition period, a progressive substitution of 50% for 1960 and 75% for 1970 has been considered (Table S5). Subsequently, we have broken down the NPP into biomass flows according to their destination at each moment (Table S6). 3. Results
3.1. Experimental Fields Results
3.1.1. Net Primary Productivity (NPP) and Its Components of Both OV and MV under Organic
Managements
Aerial Net Primary Productivity (NPPa) ranged from 1249 kg ha−1 produced in old wheat plots in the one‐third rotation in 2014, to 11,807 kg ha−1 of old wheat plots in the wheat‐faba bean rotation in 2014 (Table 2). In the wheat‐faba bean rotation, plots cultivated with OV had significantly higher NPPa than those cultivated with MV in 2014 and 2016 (27% and 17% higher, respectively). Straw productivity was significantly higher for OV during the three growing seasons (62%, 31% and 30% higher, respectively). Husk productivity was significantly higher for OV only in 2014 (86% higher). While significant differences for grain yield were found in 2014 and 2015, with 79% higher and 36% lower yield for old wheat comparing it to the modern one, respectively. However, weed biomass was significantly higher in MV plots during the three growing seasons (53%, 1233% and 102% higher, respectively). Regarding root biomass, OV produced significantly more biomass tan MV in 2016 (41% higher), the only year in which root production was measured. In the one‐third rotation, OV plots had significantly higher NPPa than modern ones for the three years (40%, 39% and 22% higher, respectively for 2014, 2015 and 2016). Straw productivity was also significantly higher for OV during the three growing seasons (60%, 82% and 55%, higher, respectively). Likewise, husk productivity was significantly higher for OV (111%, 70%, and 55% higher in 2014, 2015 and 2016). Contrarily, significant differences for grain yield were only found in 2014, with a 67% higher yield for old wheat compared to the modern one. Regarding weed biomass, we only found significant differences in 2016, when plots cultivated with MV produced 34% higher weed biomass.
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Table 2. Aerial and total Net Primary Productivity (NPPa and NPP, respectively), grain yield, straw, and husk production, and weed biomass (kg ha−1, dry matter) of old and modern wheat varieties in wheat‐faba bean rotation (W‐FB R) and in one third rotation (O‐T R) trials. Mean and standard error of the mean. Different letters represent significant differences between OV and MV within each growing season at a level of 0.05 (Tukey test).
2014 2015 2016 Average
Old Modern Old Modern Old Modern Old Modern
W‐FB R
NPPa 11,807a ± 581 9292b ± 697 11,570a ± 428 11,078a ± 483 7000a ± 395 5957b ± 315 10,126 ± 377 8776 ± 389
NPP 9486a ± 395 7713b ± 315 Grain 2187a ± 240 1223b ± 198 2523b ± 108 3966a ± 198 1374a ± 143 1414a ± 154 2028 ± 114 2201 ± 182 Straw 6186a ± 492 3812b ± 312 7767a ± 305 5946b ± 288 4967a ± 271 3831b ± 189 6307 ± 250 4530 ± 194 Husk 1014a ± 120 544b ± 43 1276a ± 63 1126a ± 60 532a ± 52 458a ± 33 941 ± 60 709 ± 44 Root 2486a ± 252 1757b ± 166 Weed 2421b ± 417 3712a ± 609 3b ± 2 40a ± 14 126b ± 30 254a ± 50 850 ± 191 1335 ± 283
O‐T R
NPPa 1249a ± 115 893b ± 88 2553a ± 175 1841b ± 110 1957a ± 139 1611b ± 121 1920 ± 104 1448 ± 78 Grain 256a ± 45 153b ± 36 623a ± 58 569a ± 46 385a ± 47 438a ± 61 421 ± 34 387 ± 35 Straw 597a ± 40 374b ± 38 1247a ± 73 684b ± 53 1095a ± 77 708b ± 51 980 ± 50 589 ± 33 Husk 139a ± 29 66b ± 19 342a ± 28 201b ± 14 251a ± 21 162b ± 15 244 ± 18 143 ± 11 Weed 257a ± 35 300a ± 36 342a ± 41 387a ± 44 226b ± 24 302a ± 33 275 ± 20 330 ± 22
Table 3. Harvest index (fresh matter) and Weed:NPPa ratio (aerial dry matter) of old (OV) and modern (MV) wheat varieties in wheat‐faba bean rotation (W‐FB R) and one third rotation (O‐T R) trials. Mean and standard error of the mean. Different letters represent significant differences between OV and MV within each growing season at a level of 0.05 (Tukey test).
2014 2015 2016 Average
OV MV OV MV OV MV OV MV
Harvest index W‐FB R 0.235a ± 0.021 0.206a ± 0.028 0.238b ± 0.006 0.393a ± 0.011 0.211a ± 0.017 0.262a ± 0.020 0.228 ± 0.009 0.287 ± 0.015 O‐T R 0.209a ± 0.026 0.219a ± 0.037 0.308b ± 0.013 0.389a ± 0.020 0.193b ± 0.012 0.287a ± 0.023 0.237 ± 0.012 0.298 ± 0.018
Weed:NPPa W‐FB R 0.210b ± 0.038 0.371a ± 0.041 0.0003b ± 0.000 0.004a ± 0.002 0.019b ± 0.004 0.041a ± 0.006 0.076 ± 0.017 0.139 ± 0.024 O‐T R 0.195b ± 0.020 0.358a ± 0.042 0.130b ± 0.011 0.212a ± 0.020 0.124b ± 0.013 0.194a ± 0.022 0.150 ± 0.010 0.255 ± 0.019
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3.1.2. Biomass Allocation and Partitioning Indices
Biomass allocation has been proved to be different for OV and MV in both organic trials. Considering only aerial wheat biomass, the grain represented 22% to 26% of the NPPa in OV, and 30% to 35% in MV (for wheat‐faba bean and one third rotations, respectively). In both cases, the increase is 35% in MV with respect to OV. In the wheat‐faba bean rotation, if we take root biomass into account, the grain goes from representing 17% of the total wheat biomass in OV, to 24% in MV (39% more for MV) (Figure 1a). Considering the whole plot, that is, including weeds, the accumulation of aerial biomass in the grain of MV is still higher than that of OV. However, the increase is reduced to 25 and 22% (wheat‐faba bean and one third rotations, respectively). In wheat‐faba bean, if we include root biomass, the increase is reduced from 39% to 30% (Figure 1b). The reduction of the difference between varieties in biomass allocation to grain (whether accounting for root biomass or not) is due to the greater contribution of weeds to biomass of plots cultivated with MV, when comparing to OV. This indicates a lower capacity of MV to compete with weeds. Averaged weed biomass from both organic managements (562 kg d.m. ha−1) will be used in the modelling of NPP of OV under organic farming in Spanish wheat fields (Section 3.2).
Figure 1. Biomass allocation of (a) wheat varieties (OV, old varieties; MV, modern varieties), and (b) net primary productivity of plots cultivated with OV and MV, under different management.
As a consequence of the different allocation of biomass, the partitioning indices vary between types of varieties and with management (Table 3). In the wheat‐faba bean rotation, HI ranged 0.211–0.238 and 0.206–0.393 for OV and MV, respectively. No significant differences were found in 2014 and 2016, while it was 39% significantly lower for OV in 2015. Finally, OV had significantly lower Weed:NPPa ratio values than modern ones (44%, 93% and 55% lower values in 2014, 2015 and 2016, respectively). In the one‐third rotation, HI ranged from 0.193–0.308 and 0.219–0.389 for OV and MV, respectively. The difference was significant in favor of MV in 2015 and 2016, with an increase of 26% and 49%, respectively, with respect to OV. Lastly, OV plots showed a significantly lower Weed: NPPa ratio for 2014, 2015, and 2016 (44%, 38% and 37% lower, respectively). Mean HI of OV is similar under both organic managements (0.228 and 0.237 for the wheat‐faba bean rotation and the one‐third rotation, respectively), being the average of both (0.232) used in calculations of the NPP in Section 3.3. The intensity of the organic management does not seem
0,30
0,11
0,61
0,30
0,10
0,61
1,00
1,35
0,930%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
OV MV OV MV OV MV
On third rotation Wheat‐fava beansrotation (rootless)
Wheat‐fava beansrotation (with root)
a)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
OV MV OV MV OV MV
On third rotation Wheat‐fava beansrotation (rootless)
Wheat‐fava beansrotation (with root)
b)
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to have affected the HI of MV either (0.287–0.298), being higher the influence of the growing season. Lastly, the R:S ratio was 0.36 for OV in the wheat‐faba bean rotation in 2016 (Table 2).
3.2. Total Wheat Production and Cultivated Area for Spanish Wheat Fields in the 20th
Century
The wheat area cultivated under rainfed conditions in Spain increased from 3.4 to 4.3 Mha in the first third of the 20th century (Figure 2). This trend was broken with the Spanish Civil War (1936–1939), which meant the loss of almost 1 Mha. From 1940, the surface recovered and reached 3.9 Mha in 1950. The next two decades show a great stability, but since the 80s, the surface area falls sharply due to the abandonment of the less productive rainfed croplands [40] and the replacement of cereals by olives. After the entry of Spain into the European Union [40] many cereal drylands were abandoned due to the subsidies that the EU started to give through the common agrarian policies (CAP) to farmers who dedicated land to the production of olive oil. Total grain production followed a similar trend until 1960. Varietal substitution with cultivars from foreign origin and other selected in Spanish research centers had begun with some entity in the 1950s. These varieties were, in general, long‐stemmed, with some exceptions harboring Rht dwarfing alleles, such as Mara, Impeto or Estrella varieties [15,39]. These varieties, despite being long‐stemmed, produced more grain than OV, so their HI was higher than those of OV (0.40 on average according to Sánchez‐García et al. [15]). Their presence in the Spanish fields was progressive, and contributed to the increase in yield (Figure 3) and total grain production in the 1960s and 1970s (Figure 2). But it was from the mid‐70s that semidwarf varieties from CIMMYT were introduced, with varietal substitution being very rapid [8,39]. As a result, wheat yield doubled in the 1980s compared to 1950, and continued to grow until the year 2000 (Figure 3), so that, despite the drastic fall in area, total grain production grew to reach 5.2 Mt (Figure 2). Likewise, harvested residues followed a similar evolution than surface and grain production until 1970 (Figure 2). As of this date, they fell sharply due to the disappearance of traction animals and the intensification of animal husbandry in Spain, which relied on compound feed [40].
Figure 2. Evolution of the surface area (Mha), total grain production and harvested residues (Mt fresh matter) of wheat in Spain throughout the 20th century. Source: [16–21].
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3.3. Modelling of NPP and Biomass Destinies for Spanish Wheat Fields in the 20th Century
On the basis of yield, we have calculated the NPP of wheat fields in the 20th century (Figure 3a). During the first third of the century, the average wheat yield remained fairly stable, with a slight upward trend (it rose from 725 kg d.m. ha−1 in 1900 to 796 kg d.m. ha−1 in 1933), derived from a slight degree of rotations intensification and the incipient use of fertilizers [37]. The different NPP components reflect this stability, since neither the cultivated genetic material nor the weed control technology were altered. In the 1940s, the yield loss associated with the Spanish Civil War would also be reflected in aboveground wheat residues (Figure 3b) and NPP (Figure 3a), but foreseeably not in the rest of the components. As we explained in the methodology section, the R:S ratio tends to decrease under conditions of higher fertilizer and water use intensity, conditions that did not occur here. Weed biomass should not have changed significantly either, since the capacity of OV to maintain weeds in relatively low populations remained unchanged. If so, it could have increased slightly, especially in 1940, due to the difficulties of carrying out the manual weeding work traditionally practiced. Between 1960 and 1980, from the beginning and end of the varietal replacement process, wheat yield increased by 62%. However, NPP did not exceed that of 1933, because the moderate increase in yield and the higher weed biomass under modern management did not compensate for the reduction of root and stem biomass due to the incorporation of the new varieties. Finally, we identify a last period (1990–2000) in which the NPP in wheat fields exceeded what was reached in the first third of the century (Figure 3a). In this period, the yield increase (160% with respect to 1960) and the biomass associated with this increase did compensate for the relative reduction in residue production. In 1990, for the first time, the residues produced exceeded those of the first third of the century (Figure 3b).
Figure 3. Evolution of (a) Net Primary Productivity (NPP) of wheat fields and (b) of wheat residues, in Spain throughout the 20th century.
Given that a part of the residues produced was extracted from the agroecosystem for animal feed, we must disaggregate it by use in order to quantify the residue biomass actually available to maintain the biophysical quality of the agroecosystems and allocate it above or below ground. The residues harvested for use by livestock have remained fairly stable in absolute terms (Figure 4a). However, in relative terms, the residue harvest increased during the 1960–70 period (Figure 4b), due to the fall in total residue production caused by the varietal replacement (Figure 4a). Subsequently, the relative extraction was reduced by the increase in residue production due to the yield increase (Figure 4b). Initially, this residue overproduction is partially burned by farmers, until legal restrictions were imposed [41] (Figure 4b). Given the stability of the straw biomass extracted per hectare, the aboveground unharvested biomass (AUhB) evolves following the trend of the aerial biomass (Figures 3b and 4c). The slight increase in the first third of the century is interrupted, first by the armed conflict and, subsequently (1960–70), by the fall in the residue production due to varietal replacement. In 1980, aerial residue production was recovered at the
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1933 level, if we consider weed biomass (Figure 4c), and it was surpassed in 1990–2000 due to the significant increase in residue production and the stability of consumption by livestock. If weed biomass is not considered (Figure 4d), the recovery of the AUhB is delayed, and it does not reach the previous records until 1990. Nevertheless, the belowground unharvested biomass (BUhB) falls and does not recover, once the varietal change occurs (Figure 4c,d). The impact of unharvested biomass (UhB) changes on fund elements (soil and biodiversity) are discussed in the following section.
Figure 4. Reconstruction of biomass destinations (kg d.m. ha−1) of Spanish wheat fields (1900–2000), (a) of NPP, including aerial weed biomass, (b) of residue production in relative terms, aerial weed biomass included, (c) of unharvested residues, aerial weed included, (d) of unharvested residues, weed biomass excluded. (AUhB: Aboveground Unharvested Biomass, BUhB: Belowground Unharvested Biomass).
4. Discussion
4.1. Comparison of Old and Modern Varieties Under Two Traditional Organic Managements
Results from the trials show that OV are more productive than MV under those management conditions in which the former were selected, that is, they produce more biomass than modern ones. And this was true for both trials, whose grain yield differences were similar to those found in the historical data of both rotations [18,19,42]. That is, under rainfed conditions in semi‐arid regions and with organic management, MV do not compensate for the loss of straw production with the increase in grain. On the contrary, MV decreased aerial biomass production in 68% in one third rotation and 80% in wheat‐faba bean rotation, when compared to OV. Therefore, there is a net reduction in the productivity of wheat aerial biomass with MV, which gets higher with tighter agroclimatic limitations. Results obtained in the most intensive organic management are similar to those found under Mediterranean dryland conditions under industrialized management, in which Giambalvo et al. [43] found reductions of 13–22% in the NPPa of MV with respect to OV. Carranza‐Gallego et al. [44] found 17%. Annicchiarico et al. [45] and Motzo et al. [46] found 9% lower biomass for MV than that of OV. In terms of grain yield, there are almost no differences between types of variety. However, differences can be found depending on the organic management intensity and soil conditions.
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The inclusion of the legume in the rotation and the application of manure multiplied by more than five times the yield of wheat. In Ronda, the coarse soil texture surely limited water retention and SOM accumulation, which probably contributed to the low yield observed. On the other hand, averaged grain yield (421 kg d.m. ha−1 for OV and 387 kg d.m. ha−1 for MV) in the one‐third rotation is similar to the average yield of conventionally managed wheat in the most arid provinces of Southeastern Spain (Murcia, Almería, Alicante), during the same years of the trials [17], with average rainfall lower than 350 mm. The average grain yield in the wheat‐faba bean rotation (2028 kg d.m. ha−1 for OV and 2201 kg d.m. ha−1 for MV) is slightly higher than that of conventionally managed wheat in a rainfed regime in the province of Málaga, where both trials are located, for the years of the study (1753 kg f.m. ha−1) [17]. Higher rainfall during the second growing season in Sierra de Yeguas could have been responsible of the higher yield of MV in 2015, as rainfall is a major factor determining the grain yield of wheat cultivated under rainfed conditions [47]. Additionally, the lower weed biomass of this growing season could have helped MV to produce more grain, as they are not a good competitor against weed. The yield obtained both for OV and MV is within the normal range for the region with industrialized management, in relatively dry years (Table 1). This is probably due to the fact that, under these agroclimatic conditions, increases in grain productivity due to the increase of external inputs are not very relevant, or even nonexistent [48,49]. Specifically, the application of nitrogen fertilizer does not necessarily contribute to higher yields under stressful conditions [50]. The higher root biomass of OV agrees with that obtained in other field trials in the Mediterranean environment [23]. Lopes et al. [51] state that Mediterranean wheat landraces are adapted to low input farming systems and present traits related to drought and heat stress. For instance, their higher root systems might perform higher soil water uptake in deep soil layers [52], allowing for a better adaptation to rainfed conditions. On the other hand, the higher root development of Mediterranean landraces can foster N uptake efficiency [53], as it makes them more capable of recovering soil N from deeper soil than modern cultivars [54]. Higher root biomass would also contribute to a higher accumulation of organic matter in the soil [43], since root biomass is more efficient than aerial biomass for this purpose [55,56]. More SOM increases the water holding capacity of the soil [57] and, thus, water availability for the crop which, under Mediterranean rainfed conditions, implies a higher NPP, including grain. On the other hand, Junaidi et al. [58] show that there is a genotype × environment interaction in relation to root size, and suggest that genotypes with the highest R:S ratios also have the greatest increase in root and aboveground biomass when they are organically fertilized. This suggests that organic management could stimulate root growth in OV, and reinforces the need for further trials under the conditions in which these varieties were generated and not, as usual, under industrialized management conditions. The weed biomass of plots cultivated with MV is higher in both trials. This result is expected, since wheat improvement is based on the Donald’s ideotype, which is a phenotype with low intra and interspecific competitive ability, but with great capacity to produce more seeds. That is, the increase in wheat grain yield is associated with a reduced competitive ability against weeds [59–61]. Plants have different mechanisms to compete with other plants. One of them is the production of higher vegetative biomass, which occupies space and subtracts resources (water, light, nutrients) for the rest. Another mechanism is allelopathy, which negatively affects the germination and growth of other plants. Both mechanisms are present in the OV included in this study [62]. The allelopathic capacity of cereal crops could have been lost in modern cultivars due to breeding programs [63]. Therefore, the assumption of Donald’s ideotype as a model for the design of wheat MV involved the loss of their ability to compete with weeds by their own means, relying on the intensive use of tillage and/or herbicides to perform this function. Differences in weed biomass between the first growing season and the other ones in Sierra de
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Yeguas could be due to the crop rotation sequence, since faba bean can have a suppressive effect on weeds [64] and its morphology and in row planting pattern make its weeding very effective. Finally, the precedent uncompetitive crops or a deficient mechanical weed management in the farmland previous to the experiment establishment could have also enhanced the weed population in the first year. 4.2. Evolution of NPP of Wheat Fields in Spain throughout the 20th Century
The NPP evolution in Spanish wheat fields is complex, as technological and social factors intermingle and have an impact on it. Our results show that the Spanish Civil War and agriculture policies from the first decades of the Franco dictatorship had a relevant impact on the production, not only of grain, but also of aerial residues. The reduction in the availability of human and animal work, as well as the interruption of the incipient importation of fertilizers, mainly phosphorus, was behind this fall [37]. The lower availability of residues probably initiated a process of deterioration of the fund elements and environmental services of these agroecosystems, as we will discuss in Section 4.3. This decrease in NPP may have been temporary. However, it was prolonged over time due to the biological change that led to the introduction of varieties with “Reduced Height Genes” (Rht) from CIMMYT in the 70–80s. The average increases in grain yield obtained were far from those documented in other European regions with higher rainfall [39,65]. The drop in residue production did not generate, in principle, any detectable problem in the short term. On the one hand, straw had lost functionality due to the substitution of labor animals by automotive machinery. On the other hand, in the incipient stages of agrarian mechanization, shorter varieties should have greatly facilitated the harvest. The limited capacity of the harvesters lengthened the collection period and facilitated the lodging of OV, with the consequent losses of grain quantity and quality. In addition, lodging risk limited the possibility of increasing fertilizer doses. Therefore, this biological innovation was, in general, well received [39]. Finally, at the end of the 20th century (1990–2000), the upturn in yield observed enabled it to overcome the residue production of OV of the first third of the century. This increase in yield has a complex explanation, which we can only venture. We do not have reliable data on the inputs applied to wheat in this period. Total consumption for Spanish agriculture shows a slight upward trend in the period 1980–2000 [40]. A slight increase (12%) of nitrogen fertilizer applied to wheat between 1990 and 2000 is considered from the calculation of nitrous oxide emissions by Spanish inventory of emissions to the atmosphere [66]. Given the lack of response to mineral nitrogen supply under water stress conditions, the increase in N fertilizer application and the associated response, has possibly been parallel to the shift of the cultivation of rainfed wheat to areas with higher rainfall and the abandonment in more arid regions. To prove this, in Figure 5 we have represented the provinces that have increased or reduced their representativeness in cultivated surface of rainfed wheat since 1980. Between 1980 and 1990, four provinces (Cádiz, Sevilla, Burgos, and Navarra) increased more than 1% their wheat surface in relation to the national surface (Figure 5a). Together, they went from representing 19.6% of the national surface of rainfed wheat to 25.5%. At the other extreme, the other four provinces (Cáceres, Valladolid, Huesca and Teruel) reduced their representativeness by more than 1% and, overall, went from representing 10.8% of the national surface of rainfed wheat to 6.0% (Figure 5a). The average pluviometry of the former reaches 735 ± 133 mm year−1, compared to 592 ± 135 mm year−1 of the latter. Following the same trend, provinces that gained between 0.3 and 1% of representativeness have an average rainfall of 637 ± 127 mm year−1 compared to 543 ± 91 mm year−1 of the provinces that lost between 0.3–1% of representativeness [67]. Between 1980–2000 (Figure 5b), the trend is the same: concentration of rainfed wheat cultivation in more
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humid provinces and abandonment in drier ones. Therefore, this increase in residue in recent decades does not contradict our hypothesis, but rather reinforces it, since the abandonment in more arid provinces may be related to the degradation of the fund elements of the agroecosystem caused by the decrease in unharvested residues. We will address this in Section 4.3.
Figure 5. Evolution of the distribution of the surface of rainfed wheat in Spanish provinces. (a) 1980–1990; (b) 1980–2000.
4.3. Impact on the Fund Elements and Environmental Services of Spanish Wheat Fields
The varietal substitution had unforeseen consequences on the quality of the biophysical fund elements (soil and biodiversity) of cereal‐based agroecosystems and, consequently, on the ecosystem services they provide. Several studies show the low level of soil organic carbon (SOC) in cereal Spanish drylands at present [68,69], which is very close to the degradation threshold [69]. Crop management, and particularly varietal choices, are important drivers of SOC levels, which change in response to soil C inputs [70,71]. It has been estimated that average SOC levels in Spanish rainfed croplands have been continuously decreasing since 1940 due to decreasing C inputs to the soil and, in the last decades, to global warming [41]. In the case of cereals, this fall in soil C inputs was mainly due to varietal change. The depletion of these soils is an important vulnerability factor for the sustainability of Mediterranean agriculture in a context of climate change [72]. Organic matter does not only constitute a store of nutrients, but also of water, at the same time that it controls the common erosive processes under Mediterranean conditions. Therefore, the decrease of AUhB and, mainly, of BUhB, ends up triggering complex processes of degradation that undermine the productive capacity of these soils and promote their abandonment. In Spain, 2.5 Mha of rainfed agricultural land (12% of cropland) were abandoned since the mid‐1980s to the year 2000 due to its low productivity. 88% of this land was previously dedicated to extensive crops (cereal and legumes, mainly) and fallow [17]. The abandonment affected mainly those rotations that were less intensive, which manifested into a fallow abandonment annual rate of 2.4% against the 0.8% for cultivated land. This situation would be in line with the displacement of wheat crop to provinces with higher rainfall as we have shown in this article. Therefore, the increase in average yields of rainfed wheat between 1980–2000 cannot be strictly attributed to varietal change. The higher quality of lands dedicated to wheat at the end of the century compared to previous decades would have enabled a higher production intensification, raising the average yield.
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The recovery of degraded soils requires the implementation of uses and managements that leave more unharvested residues on the soil [68,69,73]. According to our results, OV could contribute to this, without reducing grain yield. The decrease in UhB could also have seriously affected biodiversity. If we consider bird species threatened in Spain as an indicator of biodiversity, 17.5% of them are associated with cereal, and 5% to agricultural spaces with high diversity (vegetable gardens, irrigated orchards, etc.) ([74] p. 80). Cereal does not constitute a group of crops especially intensive in the use of fertilizers and pesticides under Mediterranean rainfed conditions. However, UhB debacle reaches extremes in cereals. The scarce production of straw from modern varieties, mostly of short size and short cycle, and the burning of stubble, make these crops scarcely useable for heterotrophic species, which affects the size of the populations that can maintain them and trophic chains of which they are part. This is the case of populations of predatory birds, such as the lesser kestrel (Falco naumanni), whose decline is linked to the need to invest a much greater effort to obtain their prey (arthropods and small vertebrates) in cereal fields as they have been modernized [75]. AUhB also offers food and shelter for the reproduction of other birds linked to cereal fields. Hence, part of the recommended actions in the framework of the Life Project for the conservation of steppe birds proposed by the Consejería de Medio Ambiente de Andalucía [76] is to use varieties with a longer cycle and more straw than current ones. That is, varieties more similar to traditional ones. The application of herbicides also produces damage to biodiversity, both flora and fauna [77–80], which would be avoided with the cultivation of OV. As a result of the varietal change and the related intensification in inputs use, the sustainability of wheat fields has been undermined and its recovery is not viable if it is not through the production of biomass at low cost and without competing with those uses of biomass that generate income for the farmer [81]. Therefore, the evaluation of varietal suitability can not only be based on the capacity to produce more marketable biomass, but also on the size of the non‐harvested portion, which recirculates through the agroecosystem. In the light of these results, it would be of great interest to obtain data from long‐term experiments, with different varieties and in other pedo‐climatic situations under Mediterranean and other semi‐arid climates, so that we could corroborate the hypothesis presented here. Additionally, the study of the economic viability of old and modern wheat varieties cultivation under both organic and conventional management in Mediterranean drylands could also be accomplished. 5. Conclusions
The idea that wheat MV are more productive than OV is based on a double bias. The first is produced by considering a part (grain) as the whole (NPP), thus neglecting the functions of residues in maintaining the productive capacity of agroecosystems. The second bias is produced by generalizing the reach of results mostly obtained under industrialized management, which clearly benefits MV, to other contexts. However, both trials carried out here show that, under Mediterranean rainfed conditions and traditional organic management, biomass production of OV is significantly higher than that of MV. This is due to a higher straw and root biomass production, without significantly decreasing yield. Different biomass allocation of varieties results in a lower HI and a higher R:S ratio for OV. However, weed biomass is higher in plots planted with MV, due to the loss of competitive capacity that accompanied the selection for grain yield.
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Combining information obtained from trials for OV with traditional organic management, bibliographic review for MV under industrialized management, and information from historical sources, we have reconstructed the NPP (and the uses of the different portions) of Spanish wheat fields throughout the 20th century. Results show a strong decline of NPP and UhB during wars in the 1940s–1950s, which lasted over time and was exacerbated by the progressive replacement of OV from the 1960s, a process that accelerated in the 1970–1980s by the rapid introduction of “Rht” varieties from CIMMYT. The fall of UhB has meant less biomass available to maintain the levels of organic matter in the soil and to sustaining food chains of heterotrophic species, leading to the degradation of rainfed cereal agroecosystems. The massive abandonment of these degraded agroecosystems and the displacement of the wheat crop towards provinces with more rainfall have increase the yield and NPP of this crop at the end of the century. The results obtained suggest that OV under organic management can increase the sustainability of rainfed Mediterranean agroecosystems at present. Specifically, they can improve soil quality, increase carbon sequestration in the soil, reduce the need for agrochemicals, especially herbicides, and promote the recovery of biodiversity. All without significant decreases in grain production under organic management. Supplementary Materials The following are available online at www.mdpi.com/xxx/s1. Table S1: Harvest Index of modern varieties cultivated in rainfed Mediterranean environment under industrialized management, Table S2: Weed biomass of modern varieties under industrialized management, Table S3: Share of burned residues for wheat (1900–2000), Table S4: Field trials soil physic‐chemical proprieties, Table S5: Evolution of Net Primary Productivity and partitioning indices, Table S6: Reconstruction of biomass destinations of Spanish wheat fields (1900–2000). References [82–127] are cited in Supplementary Materials Author Contributions Conceptualization, G.I.G.; Data curation, G.C.‐G. and G.I.G.; Formal analysis, G.C.‐G. and G.I.G.; Funding acquisition, M.G.d.M.; Investigation, G.C.‐G. and G.I.G.; Methodology, G.C.‐G. and G.I.G.; Project administration, M.G.d.M.; Resources, G.C.‐G., G.I.G., D.S., E.A., I.V., J.I.‐A., A.H. and M.G.d.M.; Supervision, G.C.‐G., G.I.G., D.S., E.A., I.V., J.I.‐A., A.H. and M.G.d.M.; Validation, G.C.‐G. and G.I.G.; Visualization, G.C.‐G., G.I.G., D.S., E.A., I.V., J.I.‐A., A.H. and M.G.d.M.; Writing—original draft, G.C.‐G. and G.I.G.; Writing—review & editing, G.C.‐G., G.I.G., D.S. and E.A. Funding Funding was provided by the international research project SSHRC 895‐2011‐1020 granted by the Social Sciences and Humanities Research Council of Canada, and the Spanish research projects HAR2012‐38920‐C02‐01 and HAR2015‐69620‐C2‐1‐P granted by the Spanish Ministry of Economy and Competitiveness. The first authoress held a FPU scholarship from the Spanish Government. Conflicts of Interest The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
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Supplementary material Table of contents S1. Harvest Index of modern varieties cultivated in rainfed Mediterranean environment under industrialized management ................................................................................................ 2 S2. Weed biomass of modern varieties under industrialized management ..................... 5 S3. Share of burned residues for wheat (1900‐2000) ....................................................... 6 S4. Field trials soil physic‐chemical proprieties ................................................................. 7 S5. Evolution of Net Primary Productivity and partitioning indices .................................. 8 S6. Reconstruction of biomass destinations...................................................................... 9
151
Table S1. Harvest Index (HI) of modern varieties cultivated in rainfed Mediterranean environment under industrialized management. Authors Country Cultivar Year of release HI
[82] Italy Quadrato (wheat) and Ponente (barley) 0.428
[83] Jordania Cham 1, Acsad665, Ammon 1988, 1988, 2004 0.303
[84] Italy Iride 0.430
[44] Algeria Hebda/Gerardo, Bidi/Waha/Bidi/GTA Dur, Eider, Chen's, Sahel 77, Mexicali 75, Kebir, Om Rabi 9, Belikh 2, Waha, INRAT 69, Ardente, Vitrón, B.Dur 1.94, Ofanto, Simeto, Duilio
0.411
[85] Spain 0.390
[86] Italy Creso, Simeto and Svevo 0.330
[87] Spain Kilopondio and Bologna in 2013, Ingenio, Sublim and Nogal in 2014 and Ingenio, Nogal, Botticelli and Idalgo in 2015
0.318
[88] Turkey 112 RILs derived from the cross Lahn×Cham1 using the single seed descent (SSD) method from the cross identification number ICDMN91–0015
0.252
[89] Algeria Vitrón, Waha, Chen 1993, 1987, 1990 0.293
[90] Italy 0.361
[91] Turkey Bayraktar 0.290
[92] Chipre Hekabe, purified variety enlisted in the national cata‐ logue of varieties of Cyprus
0.359
[6] Italy Duilio, Simeto, Ofanto 1984‐1990 0.400
[93] Italy Gargano 0.273
[94] Italy Appio and Creso 0.300
[95] Italy Claudio, Creso, Duilio, Simeto and Svevo 0.420
[96] Italy Matt, Karalis, Pablo, San Carlo, and Saragolla)
0.280
[97] Australia Halberd, Heron, Insignia and Gabo 0.315
[98] Spain Anton 0.330
[99] Australia Gutha 0.400
[7] Italy Marzotto, Libellula, Irnerio, Manital, Centauro, Eridano, Lampo 1969‐1994 0.499
152
[100] Pakistan Tijaban‐10 2010 0.30‐0.38
[101] Lebanon Waha and Haurani 0.365
[102] Turkey and Syria Bread: Chinese Spring (origin: China), Norin 61 (Japan), Thatcher (UK) and Selkirk (UK)]; Durum: [Pentad (Russia), Golden Ball (UK or France), Langdon (USA) and AC Navigator (Canada)]
0.267
[103] Turkey Dicle‐74, Gediz‐75, Balcali‐85, Ege‐88, Cham‐1, Diyarbakır‐81 0.320
[104] Spain Gazul 0.440
[105] Spain Gazul 0.380
[106] Algeria 56 genot durum 0.359
[107] Serbia Libellula, Sava, Zlatna Dolina, Partizanka, NS rana 2, KG 56, Balkan, Yugoslavia, Skpljanka, Lasta, Evropa 90, Pobeda, NS rana 5, Renesansa, Pesma, Ljljiljana, Cipovka, Dragana, Simonida, NS 40
1962‐2006 0.422
[108] Turkey Kırik, Tir, Doğu’88, Gerek’79, Hawk, Karasu’90, Lancer, Palandöken’97
0.490
[109] Australia Falcon, Gamenya, Darkan, Halberd, Bokal, EgretA, MaddenA, WarimbaA, TincurrinA, Miling
0.354
[35] Spain Bokaro, García 0.505
[110] Australia 0.385
[111] Italy Capeiti 8, Creso, Simeto, Valbelice, Iride, Claudio 1955‐1998 0.326
[58] Australia Heron (1958), Gamenya (1960), Halberd (1969), Condor (1973), Warigal (1978), Spear (1984), Machete (1985), Janz (1989), Frame (1994), Krichauff (1997), Yitpi (1999), Wyalkatchem (2001) and Gladius (2007).
1958‐2007 0.390
[112] Australia Gamenya, Halberd, Tincurrin, Miling, Gutha, Eradu, Kulin 1960‐1984 0.354
[113] Australia Gamenya, Condor, Tincurrin, Miling, Aroona, Bodallin, Gutha, Kulin
from 1960 0.361
[23] Australia Gamenya, Condor, Miling, Kulin, 79W783, KCD0, KCD1, KCD2 from 1960 0.346
[114] Spain https://www.researchgate.net/publication/307473993_S1_Table 0.430
[115] Turkey Sardary 0.310
[116] Turkey Karahan 0.351
[117] Italy 0.473
153
[118] Turkey Karatopak 0.308
[119] Australia 0.370
[120] Syria Flk/Hork, Katya, Maya/Sap, Mexipak, Nesser, Rbs/Anza,Seri 82, Vulture
0.341
[121] Spain Spanish (Vitrón, Regallo, Gallareta, Bolo, Don Pedro, Sula, Bólido,Dorondón, Murgos, Pelayo, Don Sebastian, Don Ricardo and Kiko Nick) and three European (Simeto, Claudio and Iride from Italy
0.417
[122] Syria Cham 4 and Goman, for the first and second growing season 0.340
MEAN 0.364±0.061
154
Table S2. Aerial weed biomass (kg ha‐1) of MV under industrialized management.
Authors Country Aerial weed biomass
[123] Australia 373
[124] Italy 606
[96] Australia 2420
[97] Spain 60
[125] Spain 252
[126] Spain 149,6
[127] Turkey 1336
[36] Spain 1966
MEAN 895±893
Table S3. Share of burned residues for wheat (1900‐2000).
1900 1910 1922 1933 1940 1950 1960 1970 1980 1990 2000
0.0% 0.0% 0.0% 0.0% 0.0% 0.4% 1.0% 2.2% 3.0% 3.4% 1.1%
Table S4. Field trials soil physic‐chemical proprieties at the beginning of the experiment in 2013. Mean values and standard deviation of the mean.
Sierra de Yeguas(Wheat‐faba bean rotation)
Ronda (One‐third rotation)
CEC* 31.192.09 10.551.82 Ca exchange * 21.941.79 8.231.26 Mg exchange* 5.81.21 1.760.67 Na echange* 1.340.16 0.340.01 K echange* 2.120.11 0.210.03 Carbonate (%) 12.277.09 2.070.31 Limestone (%) 4.613.82 0.120.14 Assimilable P (ppm) 33.769.92 3.981.19 MO (%) 2.390.24 1.030.23 N org (%) 0.160.01 0.070.01 pH 8.180.04 7.660.19 ph in ClK 7.460.04 6.530.23 Assimilable K (ppm) 92760.93 76.23.77 Clay (%) 42.222.54 14.282.33 Sand (%) 18.663.12 75.62.91 Silt (%) 39.121.89 10.121.88 Texture Clay Sandy‐loam
Different letters in the same raw represent significant differences for each propriety at a significant level of 0.05 (Tukey test) CEC=cation exchange capacity; OM=organic matter; *(meq/100g).
Table S5. Evolution of Net Primary Productivity (NPP) and partitioning indices of wheat fields in Spain throughout the 20th century
1900 1910 1922 1933 1940 1950 1960 1970 1980 1990 2000
Yield (kg f.m. ha‐1) 825 885 846 906 651 755 950 1.203 1.538 2.219 2.472
Harvest index 0.232 0.232 0.232 0.232 0.232 0.232 0.298 0.331 0.364 0.364 0.364Aboveground wheat residues (kg f.m. ha‐1) 2730 2929 2799 2999 2155 2501 2238 2431 2687 3878 4320
Yield (kg d.m. ha‐1)* (a) 725 778 743 796 572 664 835 1057 1352 1951 2173
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Aboveground wheat residues (kg d.m. ha‐1)** (b)
2367 2539 2427 2600 1868 2168 1940 2108 2330 3362 3745
Root:shoot ratio 0.36 0.36 0.36 0.36 0.42 0.40 0.24 0.19 0.20 0.14 0.13
Root biomass (c) 1113 1194 1141 1223 1025 1133 676 587 736 744 754
Aerial weed biomass (d) 562 562 562 562 562 562 729 812 895 895 895Net Primary Productivity (kg d.m. ha‐1) (a+b+c+d)
4768 5073 4874 5180 4028 4527 4180 4565 5313 6952 7568
*87.9% grain dry matter; ** 86.7% straw dry matter (Guzmán et al. 2014)
Table S6. Reconstruction of biomass destinations (kg d.m. ha‐1) of Spanish wheat fields (1900‐2000).
1900 1910 1922 1933 1940 1950 1960 1970 1980 1990 2000
Yield 725 778 743 796 572 664 835 1057 1352 1951 2173
Harvested residues 1078 1157 1052 1133 841 1004 1124 1188 1063 1369 1111Aboveground Unharvested Biomass 1851 1945 1937 2029 1589 1715 1518 1668 2065 2744 3478Belowground Unharvested Biomass 1113 1194 1141 1223 1025 1133 676 587 736 744 754Aboveground Unharvested Biomass (without weeds) 1289 1383 1375 1467 1027 1153 789 856 1170 1848 2583
Burned residues 0 0 0 0 0 11 27 64 97 145 51Net Primary Productivity (with weed)
4768 5073 4874 5180 4028 4527 4180 4565 5313 6952 7568
© 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the
terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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5. DISCUSSION AND GENERAL CONCLUSIONS
Finally, the last section of this dissertation is devoted to relating the different conclusions of the articles presented, as well as discussing new conclusions generated from the discursive assembly of the three studies. While a mere repetition of the recently published conclusions has been avoided, it is necessary to reiterate the main and outstanding ideas of the studies to deepen into what are the main issues that can be extracted from the project about traditional wheat varieties. 5.1. THE AGRONOMIC PERFORMANCE OF OLD AND MODERN WHEAT VARIETIES UNDER MEDITERRANEAN RAINFED CONDITIONS
Although the agronomic traits of old and modern varieties have been discussed in the different published papers, in the following sections I will summarize the most interesting results in order to relate them to environmental sustainability and to the consequences beyond the farm. 5.1.1. Grain, straw and total biomass
According to previous studies (e.g. Giambalvo et al. 2010), the cultivation of old wheat varieties has proved to be suitable under Mediterranean semi‐arid conditions. Throughout the three studies it can be seen that the cultivation of traditional wheat varieties under these conditions presents advantages that should be considered by both farmers and institutions. In the first place, although it is widely spread that modern cultivars yield more than old ones (Ayadi et al. 2016; Guarda et al. 2004), in our study, under both organic ‐either wheat fallow and wheat faba bean rotation‐ and conventional management, grain yield of old and modern wheat cultivars was not significantly different. In the experiments of Sierra de Yeguas and Ronda, under organic and traditional management, this result could be due to the breeding process, that would have led to modern varieties to be more dependent on an easy access to nutrients (Foulkes et al., 1998) typical of synthetic fertilizers, as well as on the presence of herbicides (Lemerle et al. 2001b). Studies commonly focus on maximizing the grain yield response to N supply, instead of maintaining or increasing grain yields at a given rate of available N (Dawson et al. 2011). Our results suggest that modern wheat could not outyield old wheat under circumstances of relatively low weed control, available nutrients and water. Similarly, Murphy et al. (2008a) found that some of the landraces tested under organic fertilization outyielded modern cultivars under higher precipitation conditions than those of our trials (500 mm/year). The lack of superiority of modern wheat varieties with respect to old ones under high soil nutrient availability typical of conventional management, disagree with other studies under Mediterranean rainfed conditions (Guarda et al. 2004; Motzo et al. 2004; Ruisi et al. 2015). Some studies found that the higher grain yield of modern cultivars is due to the increase in harvest index due to breeding programs selection (Perry and D’Antuono, 1989; Siddique et al. 1990). In our study, the disagreement could be due to specific edaphoclimatic conditions of the experiment, such as the low rainfall of those years. Under Mediterranean conditions, rainfed crops are highly depend on rainfall (Flower et al. 2017), since water stress is a major factor influencing grain yield (Ayadi et al. 2016), and increases in grain productivity due to the increase of external inputs are not very relevant, or even inexistent (Ferrante et al. 2017; Lacasta, 2007). Our results are in line with studies showing that yield gains achievements in modern wheat cultivars during the last century can be underexpressed if wheat is grown under stressful conditions like semiarid rainfed conditions (Royo et al., 2007). Recently, no significant differences between cultivars of different year of release were reported from fields under Mediterranean stressful conditions (Ferrante et al. 2017).
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The higher presence of weeds in plots cultivated with modern cultivars could also underlie the absence of significance differences in grain yield between both types of varieties, since weed biomass (Lazzaro et al. 2018), and specifically grasses (Lemerle et al. 1996), have been found responsible of grain yield reductions in modern varieties. That is, in Mediterranean drylands, the presence of weeds, along with rainfall, is considered a major factor affecting grain yield (Armengot et al. 2013), and both could be responsible of our grain yield results. Regarding other agronomic traits, such as total crop aerial dry matter (CDM), old varieties were more productive than modern ones across all environments and growing seasons. That is, under rainfed conditions in semiarid regions, modern cultivars do not compensate for the loss of straw production with the increase in grain yield. Previous studies have not found a consensus on this issue (e.g. De Vita et al. 2007; Giambalvo et al. 2010; Kitchen et al. 2003; Ruisi et al., 2015). In our case, the greater straw production of traditional varieties led to their greater aerial biomass. On the contrary, in other assessments, the higher grain yield of modern varieties offset the lower straw production compared to traditional varieties (e.g. Royo et al. 2007). Just as grain yield, biomass of modern cultivars is more affected by interspecific competition than that of old ones (Ruisi et al. 2015), thus, the higher weed presence in modern wheat plots (see below) can also be responsible for their lower biomass. Regarding straw biomass, previous studies have also reported higher straw biomass production of old varieties (Townsend et al. 2017), which has been valued as a relevant source of organic material for increasing SOM (Giambalvo et al. 2010). Regarding root biomass, although it could only be sampled in Sierra de Yeguas in 2016, results of higher root:shoot ratio of old varieties agree with previous studies in the Mediterranean region (Siddique et al. 1990). In next sections 5.3. and 5.4. I will discuss the environmental benefits and ecosystems services linked to the higher root and straw biomass production of old varieties. 5.1.2. Weed competition
The comparison between old and modern wheat cultivars in terms of weed competition has been previously evaluated (Giambalvo et al. 2010; Lazzaro et al. 2018; Murphy et al. 2008a). Since the competitive ability of cereal cultivars is related to variations in aboveground weed biomass (Lemerle et al. 1996; Murphy et al. 2008a), data from this study reveal that old wheat varieties were better competitors against weeds, through the three growing seasons and across all the environments. This result agrees well with many authors who have found a higher weed suppression ability of traditional cultivars compared to modern ones (Lemerle et al. 1996), both under organic (Lazzaro et al. 2018; Murphy et al. 2008a) and inorganic fertilization (Giambalvo et al. 2010). Cultivars grown before the expansion of herbicide application are often the most competitive against weeds (Wicks et al. 2004), whereas the design of modern wheat cultivars involved the loss of their ability to compete with weeds by their own means, relying on the intensive use of tillage and/or herbicides to carry out this function. The competitive ability of old cultivars is related to increased height (Murphy et al. 2008a), aboveground biomass and leaf area index (Lazzaro et al. 2018) and higher competition for N availability (Giambalvo et al. 2010) when compared to modern varieties. In addition, some authors have related weed suppression ability with the presence of allellopatic compounds in these older genotypes (Bertholdsson, 2005), and analyses of the cultivars from this study show this same trend (forthcoming). The higher weed competition ability is of great interest for both organic and conventional farming. While conventional farming relies mainly on herbicide application, but also on mechanical methods, for weed control, sustainable low‐external input and organic farming relies on distinct practices (Lammerts van Bueren and Myers, 2012). Under organic or low input farming conditions, weeds constitute a major factor of yield reductions (Casagrande et al. 2009; Clark et al. 1999), and cultivars
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with higher weed competition are required (Wolfe et al. 2008). The inclusion of old wheat varieties into Mediterranean rainfed cereal fields can help to keep weeds under acceptable thresholds, reducing the weed seed bank in the soil for the next crop (Giambalvo et al. 2010). This is of special relevance under organic farming conditions, where herbicide use is forbidden. On the other hand, under conventional farming conditions, the cultivation of old wheat varieties can mitigate the requirements of herbicide, contributing to the sustainability of farming systems and to the reduction of the environmental damage related to the use of these chemicals (Pimentel, 1996). Weed suppression ability could have been lost during modern breeding processes throughout 20th century (Bertholdsson et al. 2004). As discussed in Study 3, the lower ability of modern cultivars to compete against weeds is due to the genetic improvement, which was based on the Donald’s ideotype, a phenotype with low intra and interspecific competitive ability, but with great capacity to produce more grain. In other words, this result was expected in breeding programs (Reynolds et al. 1994; Sadras and Lawson, 2011; Sukumaran et al. 2015). However, the relevance of this finding must be considered in consonance with two facts. On the one hand, this higher weed competition of old varieties has not been related to lower grain yields in the present study, thus it cannot be considered as a trade‐off of old varieties according to our results and in these specific climatic conditions. On the other hand, the lower weed biomass must be considered in studies regarding C sequestration strategies in croplands, as well as in historical studies on the evolution of agrarian ecosystems and their metabolism. We have successfully introduced this weed data in Study 2 and 3 regarding these topics. 5.2. SOCIO‐ENVIRONMENTAL IMPLICATIONS
5.2.1. Genetic breeding under Mediterranean rainfed conditions
Genetic breeding in cereal crops is responsible for varieties able to produce higher yields in monoculture cropping systems (Reynolds et al. 1994). Trade‐offs of this yield improvement impacted weed competition ability (section 5.1.2.), straw production (section 5.1.1.) and grain protein content and quality (sections 1.3.6.4. and 1.3.6.3). This fact is due to the control conditions under which breeding programs have been carried out in experimental stations. For example, weed suppression has not been a priority in plant breeding (Lammerts van Bueren, 2002) and these programs usually use herbicide in their trials (Worthington and Reberg‐Horton, 2013). In addition, modern breeding contributed to erode the genetic diversity of staple cereals (Vernooy and Song, 2004). However, in this study it is showed that modern cereal cultivars could not perform their yield advantage under Mediterranean rainfed conditions, thus jeopardizing agroecosystem sustainability due to higher requirements of chemical inputs and the worsening of soil quality, without a grain production benefit. Sener et al. (2009) evaluated the genetic improvement of wheat during last decades of the 20th century under Mediterranean conditions, showing a lower ability for improving yields compared to other regions due to specificities of Mediterranean climate. Acevedo et al. (1999) pointed out the low rainfall as a factor partially responsible of the lower success of breeding under these conditions. Modern varieties might lack clear adaptive traits for performing particularly well under stressful conditions (Acevedo et al., 1999) and organic farming systems (Wolfe et al. 2008). Accordingly, our results indicate that the breeding improvement of cereal varieties destined to rainfed and organic farming conditions could not be as successful as expected under optimum climate conditions. In this context, landraces could constitute an alternative plant type better adapted to farming systems
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of less favourable areas (Eyzaguirre and Iwanaga, 1995), like semiarid Mediterranean drylands (Annicchiarico and Pecetti, 2003), specifically under organic management. 5.2.2. Seeds and farmers’ right to their autonomy
As it has been widely described above, organic and low‐input farming systems do not have specific genetic material adapted to their local conditions, and farmers have to grow varieties bred under high‐input conditions. However, it is likely that future agricultural stability will depend on the yield increase in these low‐input and organic farming systems (Murphy et al. 2005). The results presented here indicate that the cultivation of old cereal varieties can be suitable under Mediterranean rainfed conditions, specially under organic and traditional farming. These outcomes highlight the success of evolutionary breeding done by farmers along the centuries (Ceccarelli et al. 2013; Murphy et al. 2005). However, breeding programs and seed companies have overcome those evolutionary processes and are partially responsible for the abandonment of local and old varieties. The research on landraces is important to contribute to a more solid decision‐making by farmers, mostly from marginal areas or croplands with a low yielding potential. Farmers that still grow landraces have not only an agronomic criterion, but socio‐economic and geographical factors, along with farm characteristics, are also influencing their decisions (Kan et al. 2016). Since the Green Revolution, seed companies have phagocytized the seed market, forcing farmers to buy the seeds year after year. In collusion with companies, legislative projects at the state and European community (EU) levels have propelled this farmers’ dependence on reproductive material. Against this privileged position of seed companies, a new ecological and knowledge‐intensive approach is needed, which should support farmers’ autonomy rather than seed companies (Evans 2009:8, in Horlings and Marsden 2011).
The recuperation of old wheat varieties for their cultivation under these Mediterranean conditions will increase farmers’ autonomy, in agreement with a new paradigm of “sustainable agriculture that support decentralization, diversity and democracy rather than centralization, uniformity and control” (Pimbert, 1994). In the agricultural realm, the research “is dominated by the search for marketable input commodities rather than by ecological and social knowledge geared to reducing the need for inputs” (Pimbert, 1994). A paramount consequence of the experiments presented here is that there is a need for a change in the way scientific community develop improvements destined to farmers’ fields. There is a need for accounting for specific management and climatic conditions when evaluating varieties, and a further step would be farmers and researchers to work together. Against the breeding method dominated by the industry and the private sector, participatory breeding entails the collaboration and share of knowledge between farmers and breeders in order to improve cultivars beyond conventional crop breeding programs, and it can profit from old varieties and landraces produced before the spread of chemical fertilizers and pesticides (Murphy et al. 2005). A key aspect of participatory breeding is that it focuses on a change in the breeding method itself, rather on breeding for specific traits, as the need for certain traits may change in the context of adverse climate change conditions. Thus, this dynamic method of varieties deployment under farmers’ field conditions can be more suitable to face future (and no so future) climate change effect on crop performances (Ceccarelli et al. 2013), which fits within proposals to manage disaster risk through knowledge coproduction (Reyers et al. 2015). The union of this and evolutionary breeding (evolutionary participatory breeding) has been suggested as the best strategy for supplying of varieties to farmers’ whose conditions are not considered by conventional crop breeders (Murphy et al. 2005). Concretely, it has been proposed as the unique suitable method to breed populations of small grain crops destined to low‐input and organic farming systems (Murphy et al. 2005). In last decade, a different approach for local varieties conservation has been proposed for marginal land farmers. “Crowdsourcing” would assemble these approaches
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mentioned with technologies within the reach of farmers (internet, mobiles phones, etc.), upscaling farmer‐participatory seed innovations beyond field scale (van Etten, 2011). This study shows that the analysis of the performance of old varieties must focus on ecosystem services and traits related to the adaptation to certain climatic conditions, such as Mediterranean rainfed conditions. Likewise, the conservation and use of genetic diversity is inextricably linked with the concept of “farmers’ rights” (Pimbert 1994), thus old varieties can be seen as the base upon which to build knowledge by democratizing research process as a basic farmer right. 5.2.3. A profound change in other links of the food chain
After these results that indicate the suitability of the cultivation of old varieties in Mediterranean drylands, the process of reintroduction must keep climbing steps through the study of the socio‐economic impact and feasibility beyond the farm. If old varieties can have positive on‐farm and environmental results, we have to look for social and political motivations that support their cultivation. In this sense, we should identify barriers to their distribution and try to ameliorate their diffusion. Probably, one of the reasons for the farmers of marginal or low yielding areas to prefer growing traditional varieties is their greater grain yield stability (Acreche et al. 2008; Annichiaricco et al. 2005), although the opposite has also been found (e.g. Slafer and Kernich, 1996). Likewise, environmental benefits linked to their reintroduction found in this work help to argue in favor of political and institutional support to farmers that choose to cultivate these seeds. However, the cultivation of old varieties entails positive consequences not only in the side of producers, but also in the side of the food industry and consumers. For example, the use of modern varieties under organic and low input conditions is associated to insufficient levels of protein for the milling industry (Baresel et al. 2005), as breeding programs selected for yield improvement with the trade‐off of decreasing grain N (Calderini et al. 1995) in a context of high N availability under conventional management. Contrarily, the use of old wheat cultivars with higher percentages of N in grain can improve the levels of protein of the final product from organic production. Similarly, although not assessed here, it is widely known that modern wheat cultivars have higher gluten index (Motzo et al. 2004), so wheat breeding programs may have contributed to the increased prevalence of celiac disease in Western societies (van den Broeck et al. 2010). Thus the reintroduction of varieties grown before breeding programs could also bring benefits associated to their lower gluten index. Additionally, poor taste or cooking quality are factors pushing farmers to maintain landraces and avoid the high‐yielding cultivars from scientific breeding programs (Pimbert, 1994). Traditionally, food taste and the way of cooking products are local factors linked to the terrain and the local identity. The cultivation of old varieties is related to local consumption and a certain trade relocation that entails the promotion of short distribution (Soriano and Thomas, 2010). These new channels are dedicated to the connection between urban and rural environments and the proximity between consumers and farmers (Lopez‐García, 2011). They are also more appropriate for the distribution and valuation of local varieties (Soriano and Thomas, 2010) and their derived products, contributing to the good economic performance necessary for agriculture as for any other economic activity. Within local markets, organic ones are more receptive to local varieties since their consumers are more concerned about organoleptic qualities, nutritional value and farming systems autonomy (Soriano and Thomas, 2010). Organic farming organizations, together with agroecological ones, are playing an essential role in the conservation and promotion of landraces and local varieties (Parrot and Marsden, 2002). The reintroduction of old varieties is inextricably linked to the re‐evaluation of the product and its artisanal transformation (Soriano and Thomas,
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2010) and the generation of alternatives to the large retailers, typical of the agri‐food industry. However, this may be put into context, as more and more consumers that are aware of environmental or health issues are demanding local and organic products at large stores. The promotion of short channels of commercialization is a relevant step for economic viability in the process of recovering local varieties. However, in the case of wheat, the processing of wheat‐derived food products makes the situation more complex. Throughout the years in which I have been involved in this project, the different visits and talks with producers who have cultivated traditional varieties, as well as people owning mills and various processing factories of the food industry, I have learned that not only on‐farm evaluation is necessary. Traditional varieties can be advantageous in the farm, and also obtain promising results in topics as important as climate change mitigation or the improvement of our soils. But if there is no industry capable of absorbing the product, there will be no possibility for farmers to cultivate them. In the case of wheat, the existence of mills or pasta and bread factories dedicated to the study of innovative methods and recipes adapted to these varieties is fundamental. And in the same way, the industry needs a mass of conscientious consumers who are willing to support the different products derived from these traditional wheat varieties. In addition, the recovery of these varieties necessarily implies the involvement of the institutions at the legislative level, strengthening and reducing the obstacles imposed during decades on the trade and exchange of seeds. In the second term of 2018, the European Parliament approved a package of measures, which will come into force in 2021, aimed at collecting the recurrent demand of farmers to be able to put on the market their own popular varieties, even if they are not homogeneous, and without going through a long and expensive registration process (as happens in the current Spanish seed legislation1). This political initiative seeks to "establish criteria for the description of the characteristics of the ecological varieties suitable for organic production"2. Therefore, this proposal could fit well with the need of “a new type of regionally embedded agri‐food eco‐economy”, which will need of a new consensus around market mechanisms and institutional structures, along with the re‐direction of science investments (Horlings and Marsden, 2011). The cultivation of traditional wheat varieties should not only be considered because it can be competitive against modern varieties under rainfed or organic conditions, but also because it entails several important benefits both at socioeconomic and environmental levels, as the results of this study show. However, in order to achieve these benefits, their cultivation must be supported by institutional and political incentives, as well as the reorientation as far as possible of certain sectors of consumers concerned about their health and the environment. From the pre‐farming phase, with the need to change the relationship of farmers with seeds, to the consumption of wheat varieties with different flavors and qualities, old wheat varieties can constitute an important advance in the sustainability of our agri‐food system.
1LEY 30/2006, de 26 de julio, de semillas y plantas de vivero y de recursos fitogenéticos.
https://www.boe.es/boe/dias/2006/07/27/pdfs/A28165‐28178.pdf
2UE, 2018. Legislative resolution P8_Ta (2018) 0180. http://www.europarl.europa.eu/sides/getDoc.do?pubRef=‐
//EP//NONSGML+TA+P8‐TA‐2018‐0180+0+DOC+PDF+V0//Es
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5.3. OLD VARIETIES AND ECOSYSTEM SERVICES BEYOND PRODUCTIVITY
It is doubtless that there is a growing interest in enhancing ecosystem services while maintaining or increasing yields (Garbach et al. 2014), which is likely related to the fact that climate change is not only threatening the maintenance of crop yields, but also the provision of other ecosystem services (Handmer et al. 2012). In this context of ample risks and concerns, it is necessary to promote and support multifunctionality within the agrarian activity, which involves the adjustment of the productive value of agriculture with the ecosystem services that can be derived from it (Jordan and Warner, 2010). Multifunctionality should be accounted for when evaluating sustainability of agroecosystems, since these systems provide many beneficial services to society, beyond the amount of final product directly directed to the agrarian market (Ripoll‐Bosch et al. 2013). For example, there is a need for taking into account other indicators of sustainability, such as the quality of grain and the number of varieties per crop (Vernooy and Song, 2004), or the quantity of biomass left in agroecosystem destined to support food webs and soil quality (Guzmán and González de Molina, 2017). In this regard, the evaluation of the performance of old wheat varieties is relevant not only in terms of grain production, but also through its linkage to the provision of other ecosystem services. Old varieties could be then regarded as an agent for multifunctionality in Mediterranean drylands, and the varietal selection should respond to these multifunctionality and sustainability criteria, not only to yield related criteria. By cultivating old wheat varieties, we could also meet some “sustainable crop intensification” (SCI) practices described for a climate‐resilient and sustainable agriculture (Adhikari et al. 2018), like the use of plants with a vigorous early growth (Moragues et al. 2006b) and the reduction in the needs of herbicide application. This research contributes to a comprehensive approach for the study of old varieties under rainfed Mediterranean conditions, showing its benefits from an agronomic perspective (maintaining yields while increasing residue production) but also from a sustainability approach (contributing to soil quality, weeds reduction and cropland biodiversity). In next sections I will discuss more deeply some of the main findings of the multifunctionality of old varieties. 5.3.1. Straw biomass and ecosystem services
Across environments and growing seasons, straw biomass production was higher for old varieties, in agreement with other studies, such as that by Townsend et al. (2017). Sharp decreases in straw relative to grain production of wheat varieties during the last century reflects the replacement of draught animals by automotive machinery and the subsequent loss of functionality of the straw. Indeed, wheat varieties with relatively high straw production have been desirable traits in the past because straw was highly valued for animal feeding and bedding. The seek for new energy feedstocks and sources of organic matter to promote soil C sequestration can recapitalize dual‐purpose old cereal varieties, which produce more straw biomass without reducing significantly grain yield (Townsend et al. 2017, Lorenz et al. 2010). Concretely, the higher straw production of old varieties can have potential sustainability benefits regarding its management and destiny. Cereal straw mulching has been proved to effectively reduce soil erosion and increase soil quality, biological activity and soil aggregate stability (Garcia‐Orenés et al. 2012) under Mediterranean rainfed conditions. In this sense, higher straw production of old cereal has been valued as a relevant source of organic material for increasing SOM (Giambalvo et al. 2010). More SOM is related to the increase in the water holding capacity of the soils (Diacono and Montemurro, 2010) and, thus, water availability for the crop, which, under Mediterranean rainfed conditions, implies a higher biomass and grain production. Accordingly, the increase of SOM can contribute to enhance yield without significant increases in soil GHG emissions under semi‐arid conditions (Barton et al. 2016) and to prevent soil structure degradation (García‐Orenés et al. 2012). In addition, crop residue incorporation contributes to ameliorate the predicted
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yield losses under climate change scenarios in semi‐arid areas by improving nutrient availability (Wei et al. 2015) and to increase cereal’s water use efficiency (Liu et al. 2017). All of these benefits can be higher for old varieties without diminishing the farmers’ income related to the sale of straw. Additionally, Annicchiarico et al. (2005) highlighted another relevant advantage from higher straw production of tall cereal varieties, since the feed increase due to higher straw production can “limit the overgrazing of marginal land, thereby hindering the land degradation”. As it has been described, the Mediterranean region suffers from a low SOC content, which is close to the degradation threshold (Romanyá et al. 2007). The possibility of higher straw incorporation to the soil without decreasing the straw destined for other uses or for its sale, together with the increase in belowground residues of old wheat cultivars (section 5.3.2.), can help overcoming this soil degradation process. In Study 2 we found that the increase of C inputs after old varieties cultivation can promote higher C sequestration when compared to modern ones, which was due to higher residues production of old varieties. Regarding this data, the replacement of old varieties by modern ones has proved to be a relevant driver of SOC dynamics in Spain during the last century (Study 3), making old varieties cultivation a promising strategy for drylands’ soil recovery without reducing grain yield. In Study 1 we found that, besides the higher production of straw biomass, old wheat varieties could positively affect SOC accumulation through the increase in straw C:N ratio, compared to modern varieties. Their higher ratio was related to lower CO2 emissions and, thus, with lower C mineralization after straw incorporation to the soil. Accordingly, Huang et al (2004) showed a significant negative correlation between the C:N ratio of the plant residues and cumulative CO2 production in a 21‐day lab experiment for a wide range of C:N ratios. Our results suggest higher C accumulation in soils with the incorporation of straw from old varieties, as compared to that of modern ones. This fact could be of special relevance in low productivity arable lands such as Ronda, where old varieties produced higher quantities of straw with similar straw N content respect to modern ones, indicating a higher N‐uptake capacity and allowing the increase in soil fertility through its incorporation after the harvest. However, because of limited microbial transformation of wheat straw residues due to available nutrient shortage, the contribution of this wheat straw‐C to the formation of a more stabilized, slowly decomposing pool of SOC is limited and should be further investigated, as we proposed in Study 1. Beyond concerns about SOC accumulation on the long term, the higher soil C accumulation in the nutrient poor soils of Ronda amended with straw of old varieties could contribute to increase soil fertility by improving physical, chemical and biological soil properties, thus benefitting the following crops. Previously, the amendment with crop residues of high C:N ratio to soils with low N availability have been proposed as possible strategies to increase soil fertility and C accumulation in marginal low productive croplands (Wang et al. 2017). Contrarily, the high soil N availability of La Zubia, partially due to mineral fertilization, was likely a main driver of the higher N content and lower C:N ratio of wheat straw (Agren and Weih, 2012), especially for modern wheat, and the consequent higher CO2 soil respiration. From Study 1, it can be suggested that the incorporation to soils of old wheat varieties straw can help to lower N losses due to a certain N immobilisation. This is particularly important in soils with high N availability due to a surplus of mineral N fertilizer, where N immobilisation can preserve available N and lower N losses by denitrification and leaching when soil available N is not required by crops. Finally, the decrease in unharvested biomass inputs can also negatively impact biodiversity. On the one hand, SOM is related to soil organisms’ biodiversity and the heterotrophic levels of the food chain it sustains (Mäder et al. 2002). On a larger scale, the scarce straw production of modern cereal makes these crops scarcely useable for heterotrophic species, which affects the size of the populations that can maintain them and trophic chains of which they are part. That is the case of
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populations of predatory birds, such as the lesser kestrel (Falco naumanni) in Andalusia, whose decline is linked to the need to invest a much greater effort to obtain their prey (arthropods and small vertebrates) in cereal fields as they have been modernized (Conserjería de Medio Ambiente, 2004). Both benefits are specially linked to organic systems, which exhibit greater soil organism activity and biodiversity than conventional ones (Mäder et al. 2002; Marshall et al. 2003). 5.3.2. Higher belowground inputs to agroecosystems
The inclusion of dwarfing alleles in wheat germplasm during the 20th century led to a reduction in total root biomass (section 1.3.6.1.), but also to a reduction in root sections from different soil depths (Subira et al. 2016). In agreement with Subira et al. (2016), other studies under rainfed conditions have found a better development of old cultivars root biomass in the whole soil profile, including shallow soil layers (Fang et al. 2011; Liu et al. 2007). Regarding Mediterranean rainfed conditions, Siddique et al. (1990) found higher root biomass of a wheat landrace in the first 40 cm of the soil profile. In rainfed cereal lands from the Mediterranean region, deep water percolation is limited, since rainfall are usually bellow evaporative demands (Izzi et al. 2008). In this sense, Motzo et al. (1993) concluded that a “large root system with high root density in the upper layers of the soil profile may be beneficial in Mediterranean environments, which are characterized by low and erratic late spring rainfalls, by ensuring that a greater proportion of water is transpired rather than lost by evaporation from the soil surface”. Annicchiarico and Pecetti (2003) reported that higher root biomass in both deep and shallow soil layers for landraces could be advantageous for water uptake, allowing for a better adaptation to semiarid rainfed conditions. Accordingly, Bektas et al. (2016) found that wheat landraces harbored higher total root biomass, higher shallow root weight and higher deep root weight in experimental tubes, concluding that under Mediterranean climate conditions root systems should be both large and well distributed through the soil profile to capture nutrients and water from light rains at the end of the growing season. They concluded that the deep, but also the dense rooting characteristics of old varieties, are relevant for rainfed conditions, since it can reduce the effects of moderate water stress and drought. Likewise, differences in seminal root morphology in early stages of landraces is a relevant adaptive trait to drought conditions in semiarid environments (Grando and Ceccarelli (1995). Sanguineti et al. (2007) stated that the length of lateral roots and their response to soil drying constitute adaptive traits to water deficiencies and, although these authors did not include landraces in their research, these conclusions could be useful for our purpose. Albeit morphology and root biomass distribution have not been evaluated in this study, they could have constituted relevant adaptations under our experimental field conditions. Further investigations would be relevant to elucidate differences in adaptive traits of roots under Mediterranean rainfed conditions. On the other hand, the higher root development of Mediterranean landraces can also foster N uptake efficiency (Baresel et al. 2005), as it makes them more capable of recovering soil N from deeper soil than modern cultivars (Foulkes et al. 1998). Besides this agronomic relevance, there are ecosystem services related to the higher root biomass and soil quality. Together with the incorporation of higher straw biomass, the higher belowground residues production of old wheat contribute to a higher accumulation of organic matter in the soil (Giambalvo et al. 2010), and this could be even of greater relevance since root biomass is more efficient than aerial biomass for the increase in SOC (Kätterer et al. 2011; Rasse et al. 2005). Thus, the higher root biomass can also be a relevant factor in C sequestration and climate change mitigation strategies supporting the cultivation of old wheat varieties (Study 2, section 5.4.). Additionally, Junaidi et al. (2018) have shown that there is a genotype x environment interaction in relation to root size, and suggest that genotypes with the highest root:shoot ratios have also the greatest increase in root and aboveground biomass when they are organically fertilized. This
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suggests that organic management could stimulate root growth in old varieties, and reinforces the need for further trials under the conditions in which these varieties were generated and not, as usual, under industrialized management conditions. 5.3.3. Reduction in herbicide application
The higher competition against weeds of old varieties is an advantage beyond the likely reduction of yield penalties. By the end of the 20th century, the amount of herbicide active ingredients applied reached 950 Gg per year (Pretty, 2008). The effects of herbicides are not unresponsive to environment conditions and their interactions can be unpredictable, hindering their control (Zimdahl, 2013). Herbicides can escape from its application sites by air, soil and water, or runoff from decomposition of treated plants (Radosevich et al. 2007), hence the environmental damage related to the use of these chemicals (Freemark and Boutin, 1995; Pimentel et al. 1992) not only affects treated fields but also the surroundings. Freemak and Boutin (1995) carried out an extensive review of the impacts on wildlife due to the spread of herbicide in agricultural fields of temperate areas. Some of their major conclusions were that the use of herbicide entailed the increased damage from insect pest in crops, reductions in weed diversity and simplification of agricultural landscapes (Freemak and Boutin, 1995). More concretely, in Mediterranean fields, herbicide is one of the agricultural intensification factors behind the depletion of weed functional biodiversity (Chamorro et al. 2016) and of weed seedbank biodiversity (Jose‐María and Sans, 2011). In turn, the effect of herbicides on weed diversity and abundance has been indirectly related to impacts on beneficial insects and birds due to the reduction in habitat, refuge and food resources, negatively affecting the trophic web and other agronomic ecosystem services like pollination, pest control and nutrient cycling (Marshall et al. 2003). The improvement in below and aboveground biodiversity and biological activity related to systems that avoid the use of pesticides “provide a positive contribution toward the development of higher food web levels including birds and larger animals” (Mäder et al. 2002).
The increased herbicide resistance (Duke and Powles, 2008; Heap, 2018) in last decades should also warn us about the necessity of changing the focus in weed control strategies. Weed management methods should diversify to tackle with ecosystem sustainability challenges (Jordan et al. 2016), dealing with comprehensive approach of interactions between crop and weed community, seeking for less favorable environment for weeds and fostering crop competition (Hartzler and Buhler, 2007). Maintaining biodiversity within field crops is an important factor for sustainable agriculture (Paoletti et al. 1992), therefore, weed control should not aim to eradicate weeds, but to reduce grain yield losses, and the maintenance of weeds within a threshold without affecting grain yield nor weed diversity is a growing concern in Mediterranean cereal fields (e.g. Armengot et al. 2013). Wheat is one of the crops in which the intensity in the use of herbicides has increased the most in recent decades globally (Kniss, 2017). In this regard, the cultivation of old varieties could help to reduce weed biomass without the need of herbicide spraying and, thus, reducing the environmental impacts of this practice. 5.3.4. The effect of varietal replacement
Recently, Guzmán et al. (2018) evaluated the impacts on ecosystems of Spanish agriculture industrialization (1960‐2008) applying a biophysical criterion through the analysis of the capacity to reproduce the fund elements without increasing the use of external energy. From a biophysical approach, the industrialization of Spanish agriculture based on the increase in external input, instead of on the increase of low‐entropy internal loops, entailed the degradation of the Spanish agroecosystems, due to both the degradation of the productivity capacity and the decline in quality of its fund elements biodiversity and soil (Guzmán et al. 2018).
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According to findings in Study 3, the industrialization of agriculture during the second half of the last century had a two‐side impact in Spanish cereal drylands. On the one hand, the lack of response of modern wheat cultivars to the increase in N fertilizers utilization under rainfed conditions led to the abandonment of more arid regions and the concentration of rainfed wheat crop in provinces with higher pluviometry. Contrarily to what is commonly assumed, the increase in yield and residue production found at the end of the century was due to the concentration of wheat crop in better croplands. The higher quality of lands dedicated to wheat at the end of the century compared to previous decades would have enabled the response of modern varieties to the increase in fertilizer use, raising the average yield. Thus, the widely spread myth that modern breeding cultivars are responsible of higher yields must be discussed and put in doubt for rainfed low yielding areas. On the other hand, the cultivation of semi‐dwarf wheat cultivars with increased harvest indexes and decreased root:shoot ratios involved the reduction of non‐harvested biomass in cereal drylands. This reduction of the recirculation of residues, boosted the soil degradation process and the concomitant depletion in their productive capacity, promoting the abandonment of many less productive rainfed areas. That is, the design of modern varieties, not adapted to the Mediterranean drylands’ conditions, led to the deterioration of the biophysical fund elements (soil and biodiversity) and environmental services they provide through the great reduction of biomass recirculation within the agroecosystem. As a result of the varietal change and the related intensification in inputs use, the sustainability of wheat fields has been undermined and its recovery is not viable if it is not through the production of biomass at low cost and without competing with those uses of biomass that generate income for the farmer (Tittonell et al. 2012). The abandonment and deterioration of low productive rainfed agricultural land could be partially reversed thanks to the reintroduction of old cereal varieties. The improvement of the edaphic quality due to the higher incorporation of aerial and belowground residues in the soil in these degraded zones, without decreasing yields, as well as the increase in the possibilities for climate change mitigation options (section 5.4.) are important potentials to consider the incorporation of currently abandoned and uncultivated drylands. Their reintroduction can improve rainfed wheat performance, which is of paramount relevance since it can lower GHG emissions when compared to irrigated wheat (Tahmasebi et al. 2018) and concordant with the forecasted human population increase, wich is projected to grow to 9.3 billion by 2050 (Bruinsma, 2017). Ultimately, the evaluation of varietal suitability cannot only be based on the capacity to produce more marketable biomass, but also on the size of the non‐harvested portion, which recirculates through the agroecosystem. The increase in internal loops due to the cultivation of old varieties, may constitute a relevant strategy for the sustainable intensification of Spanish agroecosystems under the needed agroecological criteria (González de Molina and Guzmán, 2017). Ideally, sustainable agriculture should produce good yields with minimal environmental impact on ecosystem services (Mäder et al. 2002), and old varieties cultivation under organic farming conditions could constitute a good framework to develop. 5.4. AN OPPORTUNITY FOR CLIMATE CHANGE MITIGATION/ADAPTATION STRATEGIES
Climate change impacts on the Mediterranean region are been detected at higher rates than those on a global scale, and are seriously exacerbating existing problems such as the urbanization and agricultural intensification impacts on ecosystems, the increase in pollution and the decline in biodiversity (Cramer et al. 2018). Therefore, there is a need to generate environmental policies
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focused on mitigation and adaptation strategies for this highly vulnerable region (Cramer et al. 2018). Regarding agriculture, the synergy between mitigation and adaptation strategies needed for facing the climate change should be extrapolated to the entire food system, and not only be straitened to the production phase (Niles et al. 2018). In Study 2 we carried out a life cycle assessment from “cradle to farm gate” to evaluate the C footprint of the cultivation of old and modern wheat varieties under contrasting farming systems and Mediterranean rainfed conditions. Beyond the production phase, we identified that the synthesis of chemical fertilizer was a key point to focus on the mitigation of GHG in conventional cereal production. Contrastingly, under organic farming conditions, the tasks carried out at the farm with machinery were the responsible of the higher GHG emissions. Interestingly, we found as well that SOC sequestration related to the cultivation of old varieties was higher compared to modern ones under both organic and conventional farming conditions, while it was the main factor contributing to the final C footprint. Again, the higher straw and root biomass production were responsible for relevant environmental benefits from the rainfed cultivation of old wheat under Mediterranean conditions. Indeed, the lower weed biomass incorporation (due to the higher old wheat competition against weeds), and the higher N2O emissions after the residue application to soils were not enough to counteract the higher SOC sequestration due to straw and root C input to soil of old cultivars. The result of Study 2 agrees with previous findings unveiling the paramount incidence of the SOC incorporation in C footprints assessments (Rodríguez–Entrena et al. 2014), together with the necessity for considering the incorporation of both weeds (De Sanctis et al. 2012) and belowground inputs (Aguilera et al. 2018) in these type of accountings. Organic farming has previously been related to lower cereal C footprint in a yield and area scales under Mediterranean conditions. From these results we find that the juxtaposition of old wheat cultivars and organic farming was responsible for the lower C footprint, both in area and yield scales, compared to modern wheat and conventional farming conditions, respectively. In fact, the C footprint of old varieties under organic management resulted to be negative, entailing the offset of the GHG emissions of the whole process (Study 2). Considering previous ecosystem benefits related to old wheat cultivars without reducing yields, together with this plausible reduction of GHG emissions, it can be easily suggested that the cultivation of old varieties, over all under organic farming conditions, can constitute a promising nexus between climate change mitigation and adaptation strategies for Mediterranean drylands. Rural policies should support on‐farm conservation of this germplasm, as landraces can constitute relevant resources, better than modern cultivars, “for local communities to cope with climate change while improving food security and food quality” (Migliorini et al. 2016). 5.5. OLD WHEAT VARIETIES AND ORGANIC CEREAL DRYLANDS
Summarizing, old wheat varieties are specially a good option for organic farming under Mediterranean rainfed conditions. As reported throughout this discussion section, old wheat cultivars under these conditions do not have the yield penalty commonly associated to them, but, contrarily, entails relevant benefit for organic production. Their high residue production and weed supression involves relevant direct and indirect environmental benefits, which added to those of organic farming, can enhance the sustainability of rainfed cereal lands. Their higher weed competition would be of great relevance for organic systems where weeds entails commonly very important disadvantages and discouragements for the organic conversion. The negative C footprint
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of organic old wheat is a great finding from this study, and should receive attention from institutions and policy makers. The positive effects associated to the increase in C accumulation and soil fertility is paramount, and could be of special relevance for marginal and low‐yielding agroecosystems, such as Ronda’s. Social sustainability could also be related to these varieties and their relation with farmers and consumers, especially in organic systems where concerns about quality and environmental and social health are more commonly depicted. Some technical constraints traditionally linked to tall cereal cultivars are lodging and mechanical difficulties during harvest because of their heterogeneity and height. However, these constraints could be reduced under certain contexts. For instance, the risk of lodging could be reduced in farms under grazing conditions (section 1.3.6.2.), or in organic farms under Mediterranean rainfed conditions, since drought, together with limited N applications, reduce the risk of lodging (Annicchiarico and Pecetti, 2003). In this sense, mineral fertilizers can increase the susceptibility of wheat to lodging when compared to composted manures (Rempelos et al. 2018), and yield penalties due to lodging have also been found for modern wheat under high‐productive environments in the Mediterranean region (Acreche and Slafer, 2011). With respect to mechanical difficulties during harvest, higher investment in technological advances, including machinery implements, would be of great relevance. Generally, agroecological approaches for agroecosystem designs lack appropriate technological research, thus farmers that cultivate under agroecological schemes are at a disadvantage against to farmers that follow mainstream cultivation methods. Even, some authors have found that “semi‐dwarf types may become too short for easy mechanical harvest in low‐rainfall seasons” (Annicchiarico and Pecetti, 2003). Regarding annual seed reproduction, since not all farmers are able to or want to spend time in the reproduction of seeds (Soriano and Thomas, 2010), institutional help through technical, economic and research support would be essential. Regarding economic issues, dual purpose varieties are relevant in regions where livestock is still economically relevant (section 1.3.6.2.). Actually, farmers from these regions do not grow modern cultivars because of their low straw production, and profit higher benefits from the cultivation of tall cereal varieties (Annicchiarico and Pecetti, 2003; Annicchiarico et al. 2005). As previously mentioned, Annicchiarico et al. (2005) showed that, on average, tall cereal cultivars had a significantly higher economic advantage than semi‐dwarf cultivars, mainly due to their higher straw production. Summarizing, technical constraints of old cereal fields could be relatively overcome under Mediterranean conditions, while higher public funding for research in this field would improve farmers work. To our concern, the cultivation of old cereal cultivars harbor more environmental advantages (without yield reductions) than technical constraints, which, when found, could be overcome with research. 5.6. LIMITATIONS AND PROJECTIONS OF THE STUDY
Although the results of these experiments and studies are promising for the adaptation and mitigation of climate change in the Mediterranean drylands, as well as for the improvement of their sustainability, there are some factors that need to be deepened. On the one hand, the field study only received funding for three years, so it could not be extended over time. Given the interannual climatic variability in the region and the climate change context, the continuity over time of the field trials would be advisable and desirable to assessed the feasibility of old varieties. Results from
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long‐term experiments have greater robustness, and more solid data could be extrapolated in favor of the reintroduction of these varieties in the Mediterranean region. In this sense, the effect of fallow and faba bean rotations could be analyzed in depth, and determine the differences between both types of wheat varieties under the specific rotation schemes. On the other hand, these results have important projections for academic research in two ways. Firstly, data obtained from field experiment under organic and traditional farming systems (applying an Experimental History approach) are very useful for the academic community, since no data regarding traditional systems in the past (beyond grain yield) are easily found. Secondly, agroecologists, agronomists, etc. involved in the design and evaluation of sustainable agroecosystems need to assess whether the biomass that is not extracted from the agroecosystems is enough to maintain the quality of the agroecosystem fund elements (soil, biodiversity, etc.). In this sense, and as it has been repeated through the whole of this discussion section, an appropriate varietal choice must respond not only to productivity criteria, but also to sustainability ones. 5.7. GENERAL CONCLUSIONS
Study 1
The high straw production of traditional wheat varieties allows to incorporate more straw residues to the soil, promoting soil fertility and other alternative uses such as livestock feed and bioenergy feedstock.
The higher straw production and straw‐N uptake indicate that the cultivation of traditional wheat varieties is advantageous under rainfed and low N availability conditions typical of marginal, low productivity arable lands or low input farming, especially when a dual purpose role for the wheat is required.
The higher straw C:N ratio of old wheat varieties lead to a lower apparent straw residue derived CO2 emission and higher short term SOC accumulation efficiency, especially in the nutrient‐poor soils.
The higher soil N retention in the nutrient‐rich soil after addition of straw of traditional wheat varieties due to their high C:N ratio (lower percentage of N re‐mineralisation) offers the potential to lower N losses and preserve available N in soils.
Study 2
Climate change mitigation efforts in rainfed wheat cultivation should focus on fertilizers production, in conventional systems, and machinery use, in organic ones.
Old wheat varieties promoted biomass production without decreasing yields. The higher biomass incorporation to the soil was responsible of higher negative values for the C balance and higher N2O emissions, but the former broadly offset the latter, resulting in lower area and yield‐scaled C footprints of old wheat cultivars, and even in a negative C footprint of old varieties under organic management.
C sequestration was a very important component of the GHGe balance, and it was responsible for the significant reduction in the C footprint of old varieties when compared to modern ones. The results stress the relevance of C sequestration in climate change mitigation strategies and the importance of including it in C footprint accountings.
Old varieties have shown a large potential for enhancing C sequestration and reducing the C footprint through high residue production, which makes them particularly promising for climate change mitigation in organic and low input systems.
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Study 3
The idea that modern wheat cultivars are more productive than old ones is based on a double bias. The first is produced by considering a part (grain) as the whole (NPP), and it ignores the functions of residues in maintaining the productive capacity of agroecosystems. The second bias is produced by generalizing the reach of results mostly obtained under industrialized management, which clearly benefits modern cultivars, to other contexts.
Under Mediterranean rainfed conditions and traditional organic managements, biomass production of old varieties is significantly higher than that of modern ones. This is due to the higher straw and root biomass production, without significantly decreasing yield. However, modern cultivars resulted less competitive against weeds due the selection for grain yield in modern breeding programs.
Results from the reconstruction of the NPP (and the uses of the different portions) of Spanish wheat fields throughout the 20th century show a strong decline of NPP and UhB during wars in the 1940s–1950s, which lasted over time and was exacerbated by the progressive replacement of old cultivars from the 1960s, a process that accelerated in the 1970–1980s by the rapid introduction of “Rht” varieties from CIMMYT.
The fall of UhB has meant less biomass available to maintain the levels of organic matter in the soil and to sustaining food chains of heterotrophic species, leading to the degradation of rainfed cereal agroecosystems. The massive abandonment of these degraded agroecosystems and the displacement of the wheat crop towards provinces with more rainfall have increase the yield and NPP of this crop at the end of the century.
Old wheat cultivars under organic management can increase the sustainability of rainfed Mediterranean agroecosystems at present. Specifically, they can improve soil quality, increase C sequestration in the soil, reduce the need for agrochemicals, especially herbicides, and promote the recovery of biodiversity. All without significant decreases in grain production under organic management.
6. AKNOWLEDGEMENTS
Muchas cosas han cambiado en mi vida desde que comencé este proyecto en 2014. Este apartado de agradecimiento no puede ser solo para quienes han hecho que este trabajo haya sido posible, sino que ha de ser forzosamente para todas las personas que se han mantenido a mi lado y me han dado el mejor apoyo posible. A los directores de este proyecto, Gloria y Manolo, que con su trabajo y experiencia hacen de la Agroecología una disciplina seria, digna y respetable. Sin su investigación y tesón el mundo de la agricultura sería un campo yermo. Les agradezco la oportunidad que me brindaron al confiar en mi para este proyecto, así como el apoyo que me han dado a muchos niveles. En especial, gracias a Gloria por todo lo que me ha enseñado y sus tan valiosas aportaciones a este texto e investigación. A las personas del Laboratorio de Historia de los Agroecosistemas, que han contribuido con su trabajo y consejos a que las publicaciones y escritos de este proyecto sean mejores. A Inma, Juan, David y Eva. Que siempre estuvieron dispuestos a levantarse de madrugada para sembrar y cosechar el trigo sembrado, y que siempre han mostrado gran interés y apoyo a la tesis. A Roberto, quien siempre estuvo dispuesto a enseñarme todo lo necesario para el trabajo de laboratorio y que ha dedicado tanto trabajo en las publicaciones de este compendio. Mi participación en este grupo de investigación me ha enseñado que la transdisciplinaridad y el apoyo entre compañeros es un valor de gran importancia dentro de la academia.
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También me gustaría dedicar unas frases de agradecimiento a las personas del proyecto Sustainable farm systems: long‐term socio‐ecological metabolism in western agriculture, financiado por el Canadian Social Sciences and Humanities Research Council. El apoyo económico de este proyecto ha sido imprescindible para esta tesis. A Jaume y María les deseo lo mejor en sus proyectos. Serán tan buenos como ellos. También quiero agradecer a Jose Luis Vicente su apoyo, porque supo animarme y ayudarme en los momentos en los que lo necesité. Del mismo modo, quiero dedicar unas palabras a Inés, Roc y Claudio. Muchas de nuestras conversaciones me hicieron comprender otras formas de vivir este proceso. Creo que su forma de hacer investigación y de vivir enriquecen en gran medida este mundo académico, y me alegro de habérmelos cruzado en este camino. Además, especial mención merecen Juan, Miguel y María. Ellos son las personas responsables de los terrenos en los que hemos llevado a cabo los experimentos de campo en Andalucía. Sin su experiencia y buen hacer, esta tesis no habría sido posible. Siempre estuvieron dispuestos a colaborar con la investigación y a mejorarla con sus consejos. El trabajo con estos agricultores me ha enseñado lo importante que es que fomentar los diálogos y el aprendizaje mutuo entre agricultores e investigadores. Gracias también a las técnicas de laboratorio de la Universidad de Jaén, Rafi y Gala. Por supuesto, no se me olvida lo que aprendí durante mi estancia en la Universidad Nacional Autónoma de México. Gracias a Marta Astier, que me apoyó mucho durante la misma y me ofreció todo lo que estaba en su mano para llevar a cabo el proyecto diseñado. También, mi agradecimiento a Ana Clara, que desde el principio estuvo en contacto conmigo cuidando del experimento mientras yo no estaba, y que trabajó muchísimo en sacar adelante el proyecto con el maíz. Especial dedicación para Carmen, Francisco y Juan por su trabajo con las variedades criollas de maíz y su lucha por el mantenimiento del conocimiento y cultura asociado a ellas. A Edu, para quien no tengo palabras para agradecer todo lo que me ha enseñado y la paciencia que ha tenido conmigo. Un compañero que ha dedicado tanto esfuerzo como el que más en que este trabajo saliera adelante. Sin su apoyo, sin duda, esta tesis sería mucho peor. (Y por todas las fotografías que tomó de los ensayos y que compartió siempre conmigo, incluida la fotografía de la portada de la mariquita en la espiga). A mis amigos y amigas, que han sabido entender mis momentos duros y me han apoyado al máximo. A mi familia, que son lo mejor del mundo y que siempre me dan su apoyo. Y a Jose, que lo da todo sin esperar nada a cambio y que me ha ensañado las cosas más bonitas que sé de la vida. Por ser esa persona que quieres siempre a tu lado. Many things have changed in my life since I started this project in 2014. This thank‐you section cannot be only for those who have made this work possible, but must necessarily be for all those who have remained by my side and have given me the best possible support. To the directors of this project, Gloria Guzmán and Manuel González de Molina, who with their work and experience make Agroecology a serious, dignified and respectable discipline. Without their research and tenacity, the world of agriculture would be a barren field. I thank you for the opportunity you gave me by trusting me for this project, as well as the support you have given me
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on many levels. In particular, thanks to Gloria Guzmán for all that she has taught me and her valuable contributions to this text and research. To the people of the Agroecosystems History Laboratory, who have contributed with their work and advice to make the publications and writings of this project better. To Inma, Juan, David and Eva. They were always willing to get up at dawn to plant and harvest the wheat, and have always shown great interest and support for the thesis. Roberto, who was always willing to teach me everything necessary for the laboratory work and who has dedicated so much effort to the publications of this compendium. My participation in this research group has taught me that transdisciplinarity and mutual support is a value of great importance within the academy. I would also like to say a few words of thanks to the people of the Sustainable farm systems: long‐term socio‐ecological metabolism in western agriculture project, funded by the Canadian Social Sciences and Humanities Research Council. This financial support has been essential in this dissertation. I wish Jaume and María the best in their projects. They will be as good as them. I also want to thank Jose Luis Vicente for his support, because he knew how to encourage me and help me in the moments when I needed it. In the same way, I want to dedicate a few words to Inés, Roc and Claudio. Many of our conversations made me understand other ways of living this process. I believe that their way of doing research and living greatly enrich this academic world, and I am happy I met them. Juan, Miguel and María deserve a special mention. They are the people responsible for the land in which we have carried out the field experiments in Andalusia. Without their experience and good work, this thesis would not have been possible. They were always willing to collaborate with the research and improve it with their advice. Working with these farmers has taught me how important is to encourage dialogue and mutual learning between farmers and researchers. Thanks also to the laboratory techniques of the University of Jaén, Rafi and Gala. Of course, I do not forget what I learned during my stay at the National Autonomous University of Mexico. Thanks to Marta Astier, who supported me a lot during my stay and offered me everything she could to carry out the designed project. Also, my gratitude to Ana Clara, who from the beginning was in contact with me taking care of the experiment while I was not there and who worked a lot on the maize project. Special dedication to Carmen, Francisco and Juan for their work with the maize landraces and their struggle to maintain the traditional knowledge and culture associated with them. To Edu, for whom I do not have words to thank everything he has taught me and the patience he has had with me. He has dedicated as much effort as the most for this work to go ahead. Without his support, undoubtedly, this thesis would be a worse work. (And for all the pictures he took of the field trials and that he always shared with me, including the picture on the cover of the ladybird on the spike). To my friends, who have known how to understand my hard moments and have supported me to the fullest. To my family, who are the best in the world and who always give me their support. And to Jose, who gives everything without expecting anything in return and who has taught me the most beautiful things I know about life. For being that person that you always want by your side.
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