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ManageMent practices to increase efficiency of fertilizer and aniMal nitrogen and MiniMize

nitrogen loss to the atMosphereand groundwater

J.R. FreneyCSIRO Plant Industry,

G.P.O. Box 1600, Canberra, A.C.T. 2601, AustraliaE-mail: john.freney@csiro.au

AbstrAct

Because of the need to feed the increasing human population, the number of livestock, (cattle, sheep, pigs and chickens) to provide animal protein, has increased tremendously from 1961 to 2009. Cereal grain production to feed the human and livestock population has increased from 877 Tg in 1961 to 2,351 Tg over the same period. These increases would not have been possible without the increased input of synthetic fertilizer (increased from 11.6 Tg in 1961 to 100.5 Tg in 2008). However, when fertilizer nitrogen is applied to soil, it is not used efficiently, and the plant seldom assimilates more than 50% of the nitrogen added. One of the main reasons for the poor efficiency of fertilizer nitrogen is that much of the nitrogen applied (up to 92%) can be lost from the plant–soil system. Fertilizer nitrogen can be lost by ammonia volatilization, during nitrification, and by denitrification, leaching, erosion, and runoff. These processes result in the release of ammonia, nitric oxide, and the greenhouse gas nitrous oxide into the atmosphere, and nitrate to rivers, lakes, and estuaries. Nitrogen emitted to the atmosphere as ammonia may be returned to the biosphere and recycled thus adding to the nitrous oxide and nitric oxide burden in the atmosphere. The relative importance of these processes varies widely depending on the agroecosystem, fertilizer form, and method of application. Livestock do not utilize the nitrogen they ingest efficiently; only 3.6-7.7% of the nitrogen in grass, silage or other feedstuff is converted into milk, meat, eggs or wool and the remainder is excreted in the form of dung and urine. Consequently with grazing animals, large quantities of nitrogen are deposited on pasture. The amount of nitrogen contained in urine patches (equivalent to 500 kg N•ha−1 for sheep and 1000 kg N•ha−1 for cattle) is much greater than the capacity of pasture plants to assimilate, therefore the nitrogen can be readily lost by ammonia volatilization, nitrification-denitrification, or leaching. Many approaches have been suggested for controlling losses of fertilizer nitrogen including optimal use of fertilizer form, rate and method of application, optimizing split application schemes, changing the form to suit the conditions, matching nitrogen supply with demand, supplying fertilizer in the irrigation water, applying fertilizer to the plant rather than the soil, and use of slow-release fertilizers, urease inhibitors and nitrification inhibitors. In addition, agronomic practices such as site-specific nitrogen management, higher plant densities, weed and pest control and balanced fertilization with other nutrients can reduce loss of nitrogen. Mitigation strategies for reducing the amount of animal nitrogen lost to the environment include; reducing the number of animals, increasing the efficiency of use of nitrogen by manipulating the diet thereby reducing the amount of nitrogen excreted, improving drainage, and use of urease and nitrification inhibitors. Feeding dairy cattle condensed tannins, low degradable protein and high starch diets, and grazing sheep and cattle on grasses high in water soluble carbohydrate results in less nitrogen excretion in urine and reduced ammonia volatilization. Mitigation options are available which could result in considerable reductions in nitrogen loss from agricultural systems if they were adopted by farmers. If the options proposed for reducing emissions from fertilizer use were implemented, they would be more likely to increase rather than decrease farmers’ income.

Key words: Fertilizer management, fertilization, inhibitors, slow release, diet manipulation

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IntroductIon

Because of the need to feed and clothe the increasing human population, cereal grain production worldwide has increased from 877 Tg in 1961 to 2079 Tg in 2009, and cotton production has increased from 9.5 to 20.9 Tg over the same period (FAO, 2011). The number of livestock used for the production of meat, milk and eggs has also increased during the same period; cattle from 9.4×108 to 13.8×108, sheep 9.9×108 to 10.8×108, pigs 4.1×108 to 9.4×108 and chickens 3.9×109 to 18.6×109 (FAO, 2011). It is expected that the number of animals will continue to increase, even without further population growth, because of the changing preference for animal protein by people in Asia and Latin America (Delgado et al., 1999). These increases would not have been possible without the increased input of plant available nitrogen. The increased input has come from synthetic fertilizer (increased from 11.6 Tg in 1961 to 100.5 Tg in 2008 (FAO, 2011; Heffer, 2009), animal wastes, biological fixation, mineralization of crop residues, atmospheric deposition, irrigation water, and seeds (Table 1). Unfortunately, the nitrogen added is not used efficiently and large losses can occur. Nitrogen may be lost by ammonia volatilization, during nitrification, by biological denitrification, by chemodenitrification, leached whenever rainfall exceeds evaporation, and lost by runoff. Investigations have shown that the predominant loss process and the amounts lost are influenced by the ecosystem, soil characteristics, cropping procedure, fertilizer techniques, and prevailing weather conditions (Peoples et al., 1995).

As most of the fertilizer nitrogen (76%) used for crop production is in the form of ammonia, ammonium salts or ammonium producing compounds such as urea (IFA, 2004), addition of fertilizer increases the amount of ammonium in soil. When urea is applied, it also acts as a source of alkalinity and ammonia can be volatilized. Loss of nitrogen by ammonia volatilization has ranged from negligible amounts to >50% of the fertilizer nitrogen applied, depending on the fertilizer practice and environmental conditions (Bacon et al., 1986; Keller and Mengel, 1986; Black et al., 1989; Freney et al., 1992). In flooded rice, ammonia volatilization can account for 20% to >80% of the total nitrogen lost from fertilizer sources (De Datta et al., 1989; Freney et al., 1990; Mosier et al., 1989; Zhu, 1992). Losses of nitrogen by denitrification can also vary widely (2% to 73% of nitrogen applied) depending on farming system and management (Galloway et al., 2004; Raun and Johnson, 1999). Denitrification is the main loss process in irrigated cotton when fertilizer is drilled into heavy clays (Freney et al., 1993) and in lowland rice when the soil is reflooded (Buresh and De Datta, 1991; Aulakh et al., 1992). During nitrification and denitrification, the greenhouse gas nitrous oxide is emitted to the atmosphere. Under aerobic conditions, ammonium is converted by nitrifying organisms to nitrite and nitrate and some nitrous oxide is formed by a side reaction involving nitrite. If the soil becomes anaerobic after irrigation or heavy rain) nitrate is reduced by denitrifying organisms and nitric oxide, nitrous oxide and dinitrogen gas are emitted into the atmosphere. Losses of fertilizer nitrogen in surface runoff have been reported to range from 1% (Blevins et al., 1996) to 13% (Chichester and Richardson, 1992) of the nitrogen applied. Leaching of nitrate is generally low when application rates are less than 200 kg N ha-1 y-1 but increase greatly when application rates approach 400 kg N ha-1 y-1 (Cameron et al., 2002). Leaching of nitrogen from fertilizer applied to the winter wheat–summer maize double cropping systems in the North China Plain varied from 4.0–7.6% of the nitrogen applied (180, 260, and 360 kg N ha−1; Huang et al., 2011). Leaching loss of nitrogen from sandy soils is normally greater than that from fine textured soils (Powell Gaines and Gaines 1994). Nitrogen can also be lost during the burning of crop residues. During burning, the nitrogen in residues is lost to the atmosphere in the form of ammonia, nitric oxide, nitrous oxide, dinitrogen and hydrogen cyanide. It has been calculated that

Table 1. Nitrogen applied annually to cro-plands for global food production during the mid 1990s1.

Source Amount added (Tg N)

Fertilizer 78Biological nitrogen fixation 33Animal manure 18Crop residues 14Atmospheric deposition 20Irrigation water 4Seeds 2Total 169

1 Smil, 1999

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a 9.3 tons per hectare rice crop loses 57 kg nitrogen per hectare by stubble burning (Batten et al., 2001) while an average dryland wheat crop yielding 1.9 tons grain per hectare could lose ~11 kg nitrogen per ha if the straw is burnt (Angus, 2001). These losses mean that an extra ~$71 and ~$14 per ha will need to be spent on fertilizer for the respective crops in the following year. Burning also results in a longer term increase in the production of nitrous oxide in soil. The nitric oxide and ammonia emitted during burning also serve as secondary sources of nitrous oxide when deposited in the soil. In Australia, 24.41 thousand tons of nitrous oxide are produced during prescribed burning of savannas and 370 tons are emitted into the atmosphere by the burning of agricultural residues (AGO, 2004).

Nitrogen loss in East Asia

In 2005 as a direct result of population growth, 27.1 Tg of nitrogen was applied to soils in East Asia to stimulate plant growth, and 23.4 Tg N was excreted from animals in the form of urine and feces (Xiong et al., 2008). Using the methodology developed for calculating nitrous oxide emissions (IPCC, 1997), Xiong et al. (2008) estimated gaseous emissions of nitrous oxide and ammonia, and leaching and run-off losses of nitrogen for 5 regions of East Asia (Table 2). Emissions of nitrous oxide from the region totalled 1,111 Gg N yr–1, with 94.4% emanating from China, 2.6% from Japan, and 1.4% from South Korea. On average, 28% came from animal systems. However, in Mongolia, the bulk of the emissions (80%) came from animal production. Xiong et al. (2008) calculated that 9.9 Tg N as ammonia was emitted into the atmosphere in 2004 as a result of agricultural activities, and that China was the main source 95%. In China, 4.5 Tg of the ammonia nitrogen came from the application of fertilizer and 4.9 Tg from excreta. Another 2.7

Tg N yr–1 was lost from East Asia by leaching and run-off to aquatic systems, and Zhu et al. (2005) estimated that 72% of this nitrogen was derived from application of fertilizer. Most of the leaching and runoff loss occurred in China and little nitrogen was lost by this pathway in Mongolia (Table 2).

Practices to increase efficiency of nitrogen fertilizer

Many approaches have been suggested for improving the efficiency of fertilizer nitrogen, including selecting plants to increase acquisition and internal use efficiencies, improving fertilizer management, and developing more efficient fertilizers (Giller et al., 2004).

Better plants

Hirel et al. (2007) maintains that in addition to the improvement of nitrogen fertilization, soil management, and irrigation practices, there is still a significant margin to improve nitrogen use efficiency in cereals by selecting new hybrids or cultivars from the available ancient and modern germplasm collections in both developed and developing countries. According to Giller et al. (2004), nitrogen use efficiency for different crops or genotypes has three components: (i) efficiency of absorbing nitrogen from the soil; (ii) internal efficiency with which plant nitrogen is used to produce biomass; and (iii) internal efficiency with which plant nitrogen is used to produce grain. There have been numerous reports of significant variations in nitrogen uptake efficiency, nitrogen utilization efficiency and nitrogen use efficiency for maize, rice, ryegrass, sorghum, soybean, tef and wheat (e.g. Singh et al., 1998; Borrell et al., 1998; Reed et al., 1980; Traore and Maranville, 1999; Le Gouis et al., 2000; Fageria and Baligar, 2003; Balcha et al.,

Table 2. Loss of nitrogen (Gg N yr-1) from East Asian agroecosystems in 20041).

Nitrous oxide emission Ammonia volatilization Leaching and runoff

China 1,049 9,419 2,601Japan 29 233 52South Korea 16 127 39North Korea 8 56 15Mongolia 9 82 5Total 1,111 9,917 2,712

1) Xiong et al. (2008)

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2006; Vinod, 2007). The genetic variation in both acquisition and internal-use efficiencies indicates that there is potential for increases in efficiency of nitrogen use through plant selection, particularly in low nitrogen environments and for crops such as vegetables (Bänziger and Cooper 2001; De Melo 2003; Giller et al., 2004). Better management

While fertilizer nitrogen use has remained constant, or declined, in Western Europe and North America over the past 25 years, yields of many crops have increased as a result of improvements in the efficiency of use of nitrogen fertilizer. For example, nitrogen use efficiency in corn production in the United States has increased from 42 kg grain per kg nitrogen in 1980 to 57 kg grain per kg nitrogen in 2000, while nitrogen fertilizer use declined from 10.8 to 10.5 million tons (Doberman and Cassman, 2002). Factors that have contributed to the increased efficiency are (i) more vigorous crop growth associated with greater stress tolerance of modern corn hybrids, (ii) improved management of factors other than nitrogen supply (for example, use of conservation tillage, better seed quality, higher plant densities, weed and pest control, balanced fertilization with other nutrients, controlled irrigation), and improved matching of the amount and timing of applied nitrogen to the indigenous soil supply and crop demand. However, while increased nitrogen use efficiency has been achieved at the national scale in the United States, current efficiencies on cereal cropping farms (20-50%; Cassman et al., 2002) are well below those reported in small-scale research plots (60-90%, Balasubramanian et al., 2004). This difference is often explained by the better management of research plots with regard to water supply, weed and pest management, and balanced nutrition. Improving farm-scale management toward matching that on research plots would increase nitrogen use efficiency and nitrogen loss to the environment. Knowledge gained on the importance and timing of the loss processes, and the factors controlling them has been utilized to develop strategies to increase efficiency of nitrogen use and decrease nitrogen loss. It is axiomatic that if nitrogen fertilizer is used more effectively, then less nitrogen will need to be supplied to meet the demand for food, and loss of nitrogen to the environment will be reduced. Approaches that have been suggested and tested for further increasing the efficiency of fertilizer nitrogen (e.g. Peoples et al.,

1995; Cole et al., 1996, 1997; Mosier et al., 1996, 1998, 2002; Cassman et al., 2002; Fixen and West, 2002; Balasubramanian et al., 2004; Dobermann and Cassman, 2004; Giller et al., 2004; Smith et al., 2008; Snyder et al., 2009) are given below.

Soil and plant testing

It has been demonstrated that plant uptake of nitrogen fertilizer can be improved by using soil and plant testing to make best use of indigenous soil nitrogen. Tremendous variation in soil nitrogen supply has been found in fields with similar soil types or in the same field in different years as a result of previous land use or land forming (Beecher et al., 1994). However, many farmers do not adjust their rates of nitrogen application to allow for the differences in the supply of indigenous nitrogen. The farmer needs to know how much mineral nitrogen is in the soil at planting, and how much will become available in the growing season from the mineralization of soil organic nitrogen, legumes, manures, or organic wastes so that he can determine how much nitrogen fertilizer is required to produce his targeted crop yield. The amount of nitrogen mineralized varies greatly depending on weather conditions. Soil testing, plant testing or modelling which would allow the farmer to predict the amount of nitrogen mineralized early so that he could make adjustments in the amount of fertilizer required would greatly increase nitrogen fertilizer use efficiency (Angus, 2001). Soil tests include assays for preplant and presidedress nitrate (Bundy et al., 1992; Magdoff et al., 1984; Williams 2007a), the Illinois Soil Nitrogen Test [which determines the amount of nitrogen present as amino sugar (Khan et al., 2001; Mulvaney et al., 2006; Williams et al., 2007b)], and potentially mineralizable nitrogen using aerobic and anaerobic incubations to determine nitrogen that will become available to the crop from soil organic matter (Bundy and Meisinger, 1994). Plant assays include plant tissue analyses and crop color tests using chlorophyll meters (Peng et al., 1996), and a simple leaf color chart (Yang et al., 2003; Buresh, 2007). During the growing season, nitrogen fertilizer is applied whenever the leaf nitrogen status falls below an empirically calibrated threshold as determined by the chlorophyll meter or leaf color chart. Match nitrogen supply with crop demand

The most effective management practice to maximize plant uptake and minimize loss is to

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synchronize nitrogen supply with plant demand for nitrogen. Therefore, an application of nitrogen fertilizer when the crop is developed, or several applications of small amounts of nitrogen fertilizer during the growing season is a more effective means of supplying nitrogen for plant growth, than one application at the beginning of the season. Thus Ortiz-Monasterio et al. (1996) showed that applying 33% of nitrogen fertilizer to an irrigated wheat crop at planting and the remainder one month later resulted in nitrous oxide emission being less than half that from the traditional method of applying 75% one month before planting and the final 25% one month after planting (2.28 v 5.08 kg nitrous oxide per hectare, respectively). Post emergent nitrogen applications to wheat also produced higher grain yields than equivalent rates of nitrogen applied at sowing (Hooper, 2004). The highest yield of 3.07 tons per hectare was obtained when 20 kg nitrogen per hectare was applied at first node and a further 20 kg nitrogen per hectare at first awn. Where irrigation is used, there is the opportunity for supplying fertilizer nitrogen in the irrigation water (Ortiz-Monasterio et al., 1996). This allows the farmer to overcome some of the limitations in supplying multiple applications of nitrogen fertilizer to crops by conventional techniques and to attune nitrogen fertilizer supply to crop requirements. When urea was applied to sunflowers and cotton in this way losses of nitrogen by ammonia volatilization were less than 1.3% of the nitrogen applied (Freney et al., 1985; Humphreys et al., 1990). This method has the advantages of simplicity, convenience and low cost (Muirhead et al., 1985).

Fertilizer placement

It has been demonstrated that plant uptake of nitrogen fertilizer can be improved, and total nitrogen losses reduced from the levels achieved with surface broadcasting, by incorporation or deep placement of the nitrogen fertilizer (De Datta et al., 1989; Zhang et al., 1992; Mohanty et al., 1999; Cai et al., 2002a; Roy and Hammond, 2004). Rees et al. (1997) found that improved fertilizer placement could increase the recovery of fertilizer nitrogen by 20-30%. Placement beneath the soil decreases ammonia volatilization by providing a barrier in the form of a layer of soil to trap any ammonia liberated. Fig. 1. shows that drilling urea into the middle of a sugar cane row, 5-10 cm below the soil surface, instead of banding it on the surface, reduced ammonia loss from 60 kg nitrogen per hectare (37% of applied nitrogen) to 9 kg nitrogen per hectare (6% of the nitrogen applied) (Prasertsak et al., 2002). Experiments on the North China plain by Zhang et al. (1992), Cai et al. (2002a), Cai et al. (2002b), and Li et al. (2002) showed that nitrogen loss by ammonia volatilization during the maize-growing season was reduced from 30–48% to 11–18% of applied nitrogen when urea was deep placed instead of broadcasting it on the soil surface. Deep placement of urea super granules for rice is gaining popularity as a management practice in Bangladesh and Vietnam, more than 20 years after the efficiency of this technology was demonstrated (Roy and Hammond, 2004). In Asia, prilled urea was conventionally surface broadcast by farmers to transplanted rice, but its use was very inefficient because much of the nitrogen was lost by ammonia volatilization. Deep placement of urea super granules was found to be superior to the broadcast

Fig. 1. Ammonia loss from urea broadcast onto trash surface and drilled 5 cm into soil Prasertsak et al. (2002).

12 16 20 24 28 2 60

10

20

30

40

50

60

70Cumulative ammonia loss (kg N / ha)

Urea drilled into soil

Urea on trash surface

November December

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applications (Savant and Stangel, 1990; Mohanty et al., 1999). Bowen (2008) reported the results of over 500 on-farm trials in Bangladesh to evaluate the performance of deep-placed urea super granules which showed that use of the super granules has resulted in increased yield of grain (1120 kg/ha) and a decreased use of fertilizer (70 kg N/ha) in the Boro season. When urea granules or prills are used as fertilizer they provide a source of alkalinity as well as a source of ammonia and the pH of the soil in the vicinity of the urea granules is ~8.2 (Sherlock et al., 1987; 1995; Wang et al., 1991). One possible way to reduce the localized pH around the granule and ammonia volatilization is to spread the urea in smaller individual particles using a fine particle spray (Quin et al., 2005; Quinfert 2011). Zaman and Blennerhassett (2010) observed greater dry matter production and uptake of nitrogen when urea was applied as a fine particle spray rather than in granular form, and Quin et al. (2005) found the urea applied in fine particle spray improved the efficiency of nitrogen. Use of the fine particle spray treatment instead of top dressing a pasture with granular urea reduced ammonia loss in autumn from 30% to 22.5% (Suter et al., personal communication).

Match fertilizer type to precipitation

Matching the type of fertilizer with rainfall and moisture conditions in the soil could result in appreciable reductions in nitrogen loss (McTaggart et al., 1994). In situations where ammonia volatilization is the main loss process, a switch from urea to ammonium nitrate would prevent ammonia loss and secondary emissions of nitrous oxide. Where nitrification is the main process responsible for nitrous oxide production, changing from urea to calcium nitrate may be beneficial (Clayton et al., 1997). Where denitrification is likely to be the main process responsible for nitrogen loss, nitrate forms of fertilizer should not be used.

Foliar application

Foliar fertilization represents an alternative means of applying supplementary nitrogen during periods of rapid plant growth and nitrogen demand, or at times of critical physiological stress. Its greatest use has traditionally been with high-value crops such as fruits and vegetables, although it has been successfully used for late applications of nitrogen to cereals, legumes and fiber crops to either increase

grain protein or yield (Woolfolk et al., 2002; Varga and Svečnjak, 2006). As urea is rapidly absorbed it is commonly used for foliar applications of nitrogen. In wheat, around two-thirds of foliar applied urea-nitrogen was incorporated into plants within four hours of application, and almost 80% of the nitrogen applied was recovered in grain at the final harvest (Smith et al., 1991). Direct measurements of gaseous emission in such systems showed that very little nitrogen was lost from foliar applied urea unless rainfall washed unassimilated urea from the plant onto the soil (Humphreys et al., 1990; Smith et al., 1991). Balanced nutrition

Balanced fertilization and soil fertility are factors, which affect crop yield and nitrogen use, that a farmer can control (Cassman et al., 2002; Snyder et al., 2009). Aulakh and Mahli (2004) point out that the significance of nutrient interactions increase as agriculture becomes more intensive, and that interaction between two or more nutrients can be positive or negative. A deficiency of any one of the other 16 essential plant nutrients can affect the absorption and function of nitrogen and reduce nitrogen use efficiency. For example, Salvagiotti et al. (2009) found that correcting sulfur deficiency increased nitrogen recovery from 42% to 70%, and Gordon (2005) showed that balanced fertilization, using the correct rate of sulphur, in addition to phosphorus and potassium, increased the recovery of applied nitrogen. The study of Salvagiotti et al. (2009) showed that the addition of sulfur increased nitrogen use efficiency by increasing the recovery of nitrogen from soil. Oberle and Keeney (1990) showed that correcting a deficiency of phosphorus increased recovery of nitrogen by corn from less than 40% to 75%. Aulakh and Mahli (2004) also show the importance of deficiencies of phosphorus, sulfur, potassium, calcium, magnesium and micronutrients on nitrogen use efficiency in a variety of crops.

Water status

Barton et al. (1999) reviewed work on denitrification rates in agricultural and forest soils which showed that denitrification rates were increased for longer periods in poorly drained soils. The review suggested that improved drainage could reduce denitrification losses from 40 to less than 1 kg N ha-1y-1. Since maximum denitrification rates are commonly observed when soil water filled pore

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space is more than 90%, minimizing the time a soil is saturated should limit denitrification (Linn and Doran, 1984). Denmead et al. (2005) found that, when the water filled pore space of a sugar cane soil in northern New South Wales was between 60% and 80%, 220 g nitrous oxide was lost per hectare per day, but when water filled pore space was between 30% and 60% only 96 g nitrous oxide was lost per hectare per day. Smith et al. (1997) concluded that under dry conditions nitrification is inhibited and emission of nitrous oxide is low. At slightly greater water contents up to ~60% water filled pore space, nitrification can proceed and nitrous oxide is evolved. At water filled pore space greater than 60% and particularly above ~75% denitrification becomes the dominant mechanism and the rate of nitrous oxide can increase dramatically. Above 90% water filled pore space the diffusivity of gases is restricted, nitrous oxide is reduced to dinitrogen, and very little nitrous oxide is evolved. Thus the aim should be to ensure that the water-filled pore space of the soil does not exceed 60% (Smith et al., 1997). Consequently in irrigated agriculture with ridges or raised beds, it would be better to place the fertilizer near the top of the ridge or bed to avoid the wet areas in the furrows which have much greater rates of nitrous oxide emission (Fig. 2; McTaggart and Smith, 1996). Application of irrigation water at the optimum rate for plant growth increased nitrogen uptake and reduced leaching of nitrate, but may increase leaching if too much water is applied (Cameron et

al., 2002). Maintaining a balance between limiting denitrification or nitrate leaching and appropriate water management is difficult. Trickle or drip irrigation systems allow the delivery of nitrogen to the area of maximum crop uptake and thus, the rate of application can be matched to the plants requirements. With careful operation, trickle systems can reduce deep percolation, runoff and denitrification (Doerge et al., 1991). It has been demonstrated that less nitrous oxide is emitted from less frequently irrigated soils; for example Rolston et al. (1982) showed that when the same amount of irrigation water was applied to a soil in either one, two or six applications during a two-week interval, less nitrous oxide was emitted from the less frequently irrigated soils.

Provide continuous plant cover

Strategies known to be effective for reducing nitrate leaching include reducing fertilizer nitrogen application rates, synchronizing fertilizer nitrogen supply to plant nitrogen demand, and growing a cover crop (Cameron et al., 2002; Di and Cameron 2002). Agricultural systems that provide continuous plant cover should be utilized whenever feasible to minimize leaching and denitrification of nitrate associated with bare soil fallow (Wagner-Riddle and Thurtell, 1998; Cameron et al., 2002; Di and Cameron 2002). The nitrate which accumulates in the soil during fallow periods between cropping seasons, as a result of mineralization of soil

Fig. 2. Emission of nitrous oxide from the ridge and furrow of an irrigated potato field (modified from McTaggart and Smith, 1996).

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organic matter and nitrification of the ammonium so formed, is more susceptible to leaching or denitrification when no plants are present. Winter cover crops reduced nitrate leaching by up to 30 kg N ha-1 compared to fallow (Cameron et al., 2002). Ryegrass and ryecorn were found to be effective winter cover crops in New Zealand as they reduced nitrate leaching by up to 30 kg nitrogen per hectare compared to fallow (McLenaghan et al., 1996). With this practice, mineral nitrogen is conserved within the system in plant tissue. The nitrogen can then be cycled for use by a subsequent crop through incorporation of the plant residues.

Site specific nitrogen management

Site-specific nitrogen management is used to synchronize the supply and demand of nitrogen, and it can be used to manage nitrogen in labor intensive small-scale farming or highly mechanized large scale production fields (Dobermann and Cassman, 2004). Optimum nitrogen rates vary spatially and seasonally, thus diagnostic tools are required to assess soil or crop nitrogen status during the growing season to make decisions on the amount of nitrogen to be applied (Schroeder et al., 2000). Several techniques are used to measure greenness including near-infrared leaf nitrogen analysis, chlorophyll meters, leaf color charts, crop canopy reflectance sensors and remote sensing (Giller et al., 2004). Significant increases in nitrogen use efficiency have been achieved through reductions in nitrogen use, but increases in yield tend to be small (Table 3).

Better fertilizers

Even though the management practices outlined above have increased the effectiveness of applied nitrogen in cropping systems, there is scope for further improvement as loss of fertilizer nitrogen

to the environment is still occurring by ammonia volatilization, denitrification, and leaching (Galloway et al., 2008). In order to further reduce loss by these processes, alternative fertilization techniques, such as the use of controlled release fertilizers, urease inhibitors, and nitrification inhibitors, need to be considered.

Controlled release fertilizers

Many different controlled release forms of nitrogen have been produced, and considerable advances have been made in the formulation of these materials (Peoples et al., 1995; Shoji and Kanno, 1995; Shaviv, 2000). According to Trenkel (1997) the main procedure for preparing these controlled release fertilizers is to coat conventional soluble fertilizers with a water insoluble, semipermeable or impermeable material with pores to control water penetration and also the rate of dissolution and nutrient release. By coating urea with a polymer, manufactures have greater flexibility in designing polymer-coated urea with release rates that match uptake of specific crops (Fan and Mylavarapu, 2010). The pattern of nutrient release from coated fertilizers can be parabolic, linear, or sigmoidal and long- or short-term (Shaviv, 2005). Because of the variety of polyolefin-coated fertilizers available, it is now possible to use computers to program fertilizer release patterns to match the specific requirements of a crop Shoji (2005). Zvomuya et al. (2003) found that NO3 leaching from a potato (Solanum tuberosum L.) crop on a coarse-textured soil during the growing season was 34 to 49% lower with a single application of polymer coated urea than three applications of urea. In addition, tuber yields and nitrogen recovery were greater when polymer coated urea was used instead of uncoated urea. Shoji et al. (2001) also showed that use of controlled-release fertilizer instead of uncoated fertilizer markedly

Table 3. Effect of using corrective site specific nitrogen management in a rice field on grain yield and nitrogen use efficiency.

Treatment Nitrogen applied Yield Nitrogen use efficiency (kg per hectare) (tonnes per hectare) (kg grain per kg nitrogen) Conventional 120 5.5 46Site specific1 90 5.6 62

1 One site, average of two varieties for 2 years. No pre-plant nitrogen, post-emergence nitrogen applications based on weekly chlorophyll meter readings (Singh et al., 2002).

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increased potato yields (from 31.4 to 46.1 tons per hectare) and nitrogen use efficiency (from 17.3 to 58.4%). Increased yields and nitrogen use efficiency have also been obtained in rice (Fashola et al., 2002) and direct-seeded onions (Drost et al., 2002). Large reductions in the emission of nitrous oxide have been achieved using polyolefin coated ammonium nitrate (Minami, 1994), polyolefin coated ammonium sulphate (Smith et al., 1997) and polyolefin coated urea (Shoji et al., 2001), instead of uncoated nitrogen fertilizer. Delgado and Mosier (1996) pointed out that use of controlled release fertilizers may result in nitrogen remaining in the soil after harvest. This nitrogen may then be lost to the environment.

Urease inhibitors

Urea has become the most widely used form of fertilizer nitrogen, because it is the least expensive form of fertilizer available and its high nitrogen content means lower transportation costs (Roy and Hammond, 2004). However, it has the disadvantage that considerable losses of nitrogen can occur if the urea is not incorporated into soil soon after application. The loss occurs by ammonia volatilization after the urea is converted to ammonia at the soil surface by reaction with the enzyme urease. One approach to reducing ammonia volatilization is to add compounds that inhibit urease activity to the fertilizer, thus allowing the urea to move into the soil before hydrolysis. The ammonia then released would be retained by the soil (Byrnes and Freney, 1995; Chen et al., 2008). A large number of compounds with differing characteristics have been tested for their ability to inhibit urease activity (Kiss and Simihaian, 2002), but the most effective compounds appear to be the phosphoryl amides (e.g. N-(n-butyl) phosphoric triamide and cyclohexylphosphoric triamide (Chai and Bremner, 1987; Keerthisinghe and Blakely, 1995; Byrnes and Freney, 1995). One compound which has been widely tested for its capacity to reduce ammonia loss from urea is N-(n-butyl) thiophosphoric triamide, (Trenkel, 1997; Watson, 2005) which is marketed as Agrotain by Agrotain International. In Australia a fertilizer (Green Urea 14) containing 45.8% N as urea and Agrotain @ 5.0 L/t can be obtained from Incitec Pivot Ltd. However, N-(n-butyl) thiophosphoric triamide it is not a urease inhibitor; it has to be converted to its oxygen analogue (N-(n-butyl) phosphoric triamide), on contact with soil before inhibition can occur (McCarty et al., 1989; Creason

et al., 1990). It would seem to be more logical to market the oxygen analogue, but it is not sufficiently stable to be packaged and distributed for commercial application (Incitec Pivot, pers. comm.). N-(n-butyl) thiophosphoric triamide seems to be quite stable (Hendrickson and Douglass, 1993), but its effectiveness is controlled by temperature (Carmona et al., 1990). Suter et al. (2011) showed that the effectiveness of N-(n-butyl) thiophosphoric triamide decreased with increasing temperature and with increasing urease activity in the soil. Turner et al. (2010) found that ammonia loss from a wheat crop treated with Green Urea 14 (containing N-(n-butyl) thiophosphoric triamide) was significantly lower than that treated with granular urea (1% and 9.5% of applied N, respectively). The reduction in ammonia loss reported by Turner et al. (2010) was greater than those observed in experiments by Gioacchini et al., (2002); Sanz-Cobena et al., (2008); and Watson et al., (2008). Applying urea with Agrotain to wheat was also found to increase productivity and improve quality of grain (Zaman et al., 2010). Its use also significantly delayed and reduced ammonia and nitrous oxide emissions from soil after application of urea, urine, and urea ammonium nitrate (Bronson et al., 1989; Schlegel, 1991; Grant et al., 1996; Wang and Douglas, 1996; Singh et al., 2004) and produced significant improvements in nitrogen use efficiency of corn following application of urea ammonium nitrate (Fox and Piekielek, 1993).

Nitrification inhibitors

Maintaining nitrogen in the ammonium form in soil would prevent its loss by both nitrification and denitrification. This can be achieved by adding a nitrification inhibitor with the fertilizer. Many chemicals have been tested as nitrification inhibitors and some, such as acetylene and the substituted acetylenes (2-ethynylpyridine and phenylacetylene) are very effective (Freney et al., 1993), but their form or price restricts their use by farmers. Commercially available nitrification inhibitors, which have been studied extensively include Nitrapyrin (2-chloro-6 trichloromethyl pyridine), Etridiazole (5-Ethoxy-3-trichloromethyl-1, 2, 4-thiadiazol), DCD (Dicyandiamide), and DMPP (3,4-Dimethylpyrazole phosphate). The most commonly used commercial product, nitrapyrin, is often ineffective because of sorption on soil colloids, hydrolysis and loss by volatilization (Hoeft, 1984; Liu et al., 1984). However, Wolt (2004) observed that addition

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of nitrapyrin to soils resulted in increased corn yields and soil retention of nitrogen, and decreased nitrogen leaching and nitrous oxide emission. Etridiazole maintained soil mineral and fertilizer nitrogen in the ammonium form for several weeks, thereby reducing soil nitrate concentration and suppressing denitrification indirectly (Rochester et al., 1994, 1996, 2000). Nitrification inhibitors vary in effectiveness depending on climatic conditions and soil type (Weiske et al., 2001; Chen et al., 2008; Singh et al., 2008). Dicyandiamide was effective in soils of temperate regions (Di et al., 2007), but was not very effective in tropical soils or other soils when temperatures were high (Shoji et al., 2001; Simpson et al., 1985). Yield increases have been obtained when DCD was applied to pastures (Smith et al., 2005), maize (Ball-Coelho and Roy, 1999), and wheat (Rao, 1996; Sharma and Kumar, 1998; Rao and Popham, 1999). Yield increases usually occurred at low fertilizer application rates (Frye, 2005). Because DCD retards nitrification when it is added to soil along with ammonium based fertilizers, emissions of nitric oxide and nitrous oxide are substantially reduced compared with fertilizer alone (Skiba et al., 1993; Majumdar et al., 2000; Shoji et al., 2001; Vallejo et al., 2001; Singh et al., 2004; Hatch et al., 2005; Merino et al., 2005). Dicyandiamide has been found to reduce nitrous oxide emissions from fertilizers applied to a cereal crop (Shoji et al., 2001), grass (Dobbie and Smith, 2003), and grazed pastures (Di and Cameron, 2003; Hoogendoorn et al., 2008). 3,4-Dimethylpyrazole phosphate is generally more effective and longer lasting than DCD in inhibiting nitrification, and inhibition has been achieved with lower rates of application. The effectiveness of DMPP, like DCD, is influenced by temperature, soil texture, and moisture (Barth et al., 2001; Pasda et al., 2001; Merino et al., 2005). European field trials showed that addition of DMPP increased yields of winter wheat, wetland rice, maize, potatoes, sugar beets, carrots, lettuce, radish, cauliflower, and onions (Pasda et al., 2001). DMPP has been found to significantly lower nitric oxide and nitrous oxide emissions, and nitrate leaching (Dittert et al., 2001; Pasda et al., 2001; Weiske et al., 2001; Zerulla et al., 2001; Chao et al., 2005; Menéndez et al., 2006). Banerjee and Mosier (1989) coated calcium carbide with layers of wax and shellac to make a slow-release source of acetylene which is a very good inhibitor of nitrification. Its use resulted in increased yield or recovery of nitrogen in irrigated

wheat, maize, cotton, and flooded rice (Banerjee et al., 1990; Bronson and Mosier, 1991; Chen et al., 1994; Freney et al., 1992, 1993; Mosier, 1994). Freney et al. (2000) developed an alternative slow-release source of acetylene. This product consisted of a polyethylene matrix containing small particles of calcium carbide and various additives to provide controlled water penetration and acetylene release. The matrix inhibited nitrification in soil for 90 days and considerably slowed the oxidation for 180 days (Randall et al., 2001). Subbarao et al. (2006, 2007) showed that some plants have the capacity to produce biological nitrification inhibitors which will modify nitrification in situ. Subbarao et al. (2007) were able to show that there was a large range in the production of biological nitrification inhibitors across the pasture species Brachiaria, but the only crops which had detectable capacity to inhibit nitrification were sorghum (Sorghum bicolour), pearl millet (Pennisetum glaucum) and peanut (Arachis hypogaea). Ledgard et al. (2008) investigated a novel approach to reduce N losses from urine nitrogen which is recognized as the main source of nitrogen loss from grazed pastures. In one experiment, they infused the nitrification inhibitors dicyandiamide and 4-methylpyrazole into the abomasum and the second, dicyandiamide was infused into the rumen of sheep. Administration of nitrification inhibitors to the sheep in this way resulted in voided urine with a markedly slowed rate of nitrification. Zaman et al. (2008) studied the effect of using a combination of urease [N-(n-butyl) thiophosphoric triamide] and nitrification (dicyandiamide) inhibitors on ammonia volatilization, nitrate leaching, nitrous oxide emission, pasture dry matter, and nitrogen uptake. The results suggest that the combination of inhibitors has the potential to reduce nitrogen losses and improve pasture production in intensively grazed systems.

Nitrogen loss from animal production

Until recently, farmers in Australia and New Zealand relied almost exclusively on fixation by clover to supply nitrogen to pastures, and the main source of atmospheric ammonia was the excreta of grazing animals. Now farmers are using organic wastes (e.g. dairy shed effluent, pig slurry) at <200 kg N ha-1 y-1 to supply nitrogen to pastures and crops (Cameron et al., 1997), and are applying up to 400 kg synthetic fertilizer nitrogen ha-1 y-1 to pastures to provide feed for dairy cows (Cameron

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et al., 2002). These three sources add urea to pastures and result in large losses of nitrogen by ammonia volatilization (Jarvis et al., 1995). This occurs because the plants and surface mat have high urease activity (McGarity and Hoult, 1971). Application of urea as synthetic fertilizer, effluent or slurry increases the dry matter yield of the pasture so that the stocking rate can be increased. Fertilization also increases the concentration of nitrogen in the pasture which results in increased intake of nitrogen by the grazing animals. The combined result is increased excretion of urinary nitrogen and increased ammonia volatilization (Bussink, 1992). Animals do not utilize the nitrogen they ingest efficiently; very little of the nitrogen ingested is converted into milk, meat, eggs or wool and the remainder is excreted in dung and urine (Table 4). Because of the inefficiency of use, large quantities of nitrogen are deposited on pasture. The amount of nitrogen contained in urine patches (equivalent to 500 kg N ha-1 for sheep and 1000 kg N ha-1 for cattle) is much greater than the capacity of pasture plants to assimilate (Jarvis et al., 1995; Silva et al., 1999; Cameron et al., 2002), therefore the nitrogen can be readily lost by ammonia volatilization, nitrification-denitrification, or leaching.

Mitigating nitrogen loss from animal production

Methods proposed for improving the efficiency of use of nitrogen and reducing the amount of nitrogen excreted by animals include manipulating the diet by (i) lowering the crude protein content and increasing the carbohydrate content of the diet so that more microbial protein is synthesized and less ammonia is lost from the rumen (Dove and Robards, 1974; Misselbrook et al., 1998; Kebreab

et al., 2001) and (ii) increasing the concentration of condensed tannins in the diet (Min et al., 2001).

Protein and carbohydrate contents

A comprehensive study on the effect of nitrogen intake, energy source, protein degradability and silage type on excretion of nitrogen in milk, feces and urine was made by Kebreab et al. (2001). They point out that the increase in nitrogen excretion by dairy cows is related to the increased consumption of protein supplements, and that the increase in protein is difficult to justify, as there are marginal gains in terms of animal production, but an exponential increase in terms of pollution. Urinary nitrogen was strongly and exponentially correlated with nitrogen uptake, with the result that 80% of the nitrogen intake above 500 g N day-1 was excreted in the urine. Kebreab et al. (2001) show that reduction of nitrogen pollution from dairy cows can be achieved in several ways, of which the most important is a reduction of nitrogen intake in the form of highly degradable protein. Cows that received lowly degradable protein excreted 24% less nitrogen in the urine than those fed highly degradable protein, without reducing milk output. Energy supplement in the form of lowly degradable starch also reduced the amount of urinary nitrogen excretion. They concluded that nitrogen pollution can be ameliorated by using grass grown with moderate fertilizer application, and maize-based energy supplements, formulated to provide lowly degradable protein and nitrogen intakes of less than 400 g day-1 for average yielding cows. Misselbrook et al. (1998) made measurements of ammonia volatilization, denitrification and nitrous oxide emission from grass/clover plots treated with slurries obtained from pigs fed a standard commercial diet (containing 205 g kg-1 crude protein) or a specially formulated diet

Table 4. Efficiency of nitrogen use in global animal production1.

Animal Intake (Tg N) Product (Tg N) Efficiency of nitrogen use (%) Goats 5.726 0.207 3.6Sheep 11.617 0.719 6.2Cattle 64.417 4.959 7.7Pigs 12.230 2.513 20.5Chickens 9.495 3.211 33.8Total 114.355 12.004 10.5

1Van der Hoek, 1998

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(containing 140 g kg-1 crude protein). Decreasing the crude protein content of the pig’s diet had no effect on the growth rate of the pigs, but resulted in a slurry with a higher dry matter content and a lower pH and total nitrogen content. Ammonia volatilization from the amended diet was less than half that of the standard diet (43 v 109 kg N ha-1), and loss by denitrification was 6 and 23 kg N ha-1, respectively. Inefficient utilization of nitrogen in ruminants grazing pasture is considered to be the result of low levels of non-structural carbohydrate relative to soluble protein (van Vuuren, 1993). Dove and Robards (1974) found that infusion of starch into sheep increased the excretion of nitrogen in feces and decreased the amount of nitrogen excreted in the urine. The rise in faecal nitrogen has been attributed to the presence of undigested microbial protein in feces as a result of the utilization of starch by caecal micro-organisms. Breeding high sugar ryegrass cultivars to improve nitrogen efficiency in dairy cattle has proved effective (Clark et al., 2001; Miller et al., 2001). Dairy cows fed grasses high in water soluble carbohydrate excreted significantly less nitrogen than those fed normal diets. The grasses tended to contain less protein, but because of a better balance between energy and protein supply, milk yields and milk protein yields were improved.

Condensed tannins

Another technique used for achieving greater excretion of dietary nitrogen in the feces is to include condensed tannins in the diet. Tannins are polyphenolic polymers of relatively high molecular weight with the capacity to form complexes with proteins due to the presence of a large number of phenolic hydroxyl groups (Patra and Saxena, 2011). The inclusion of tannins in the diet has been achieved by grazing animals on plants containing elevated concentrations of condensed tannins, e.g. Lotus corniculatus (birdsfoot trefoil) or by adding tannins to the feed (Waghorn and Sheldon, 1997; Min et al., 2001, 2003; de Klein and Eckard, 2008; Eckard et al., 2010). Condensed tannins are found in a number of other legumes including Lotus pedunculatus, Onobrychis viciifolia (sainfoin), and Lespedeza cuneata (sericea lespedeza). The condensed tannins in Lotus corniculatus (30-35g kg-1 dry matter) have reduced protein solubility and degradation in the rumen (Min et al., 2000), increased the absorption of essential amino acids from the small intestine by 62% (Waghorn et al.,

1987) and increased the flow of cysteine to body synthetic reactions (Wang et al., 1994). It also increased wool growth by 12% during summer and increased milk protein secretion by 14% from ewes during spring (Wang et al., 1996; Min et al., 1998). Feeding sheep (Carulla et al., 2005) and lactating dairy cows (Grainger et al., 2009) a diet containing condensed tannins from an extract of black wattle (Acacia mearnsii) resulted in a marked reduction in urinary N excretion and an increase in fecal nitrogen. However, the beneficial effects on animal performance have not always been observed (Makkar, 2003; Patra and Saxena, 2011). The difference in response to tannins in different studies is attributed to the different chemical structures, concentrations of tannins, and types of diet (Patra and Saxena, 2011).

Application of wastes to fields

Abatement techniques for reducing ammonia loss from dairy shed effluent and pig slurry applied to fields have the greatest impact if they are employed at the time of application when ammonia loss rates are fast. Erisman et al. (1999) showed that mitigation techniques applied after this time had no significant effect on the emission of ammonia. It is also apparent that applying the waste at night, when temperatures and wind speeds are low, or in winter would result in reduced loss of ammonia (Sommer and Olesen, 2000). Decreasing the water content of the slurry, and delaying application until a substantial canopy has developed (to reduce wind speeds) would also appear to have a large impact on ammonia loss (Sommer et al., 1997; Sommer and Olesen, 2000). Other techniques proposed for reducing loss of ammonia include applying during rainfall, incorporation or injection of the waste into the soil, application with trail hoses, applying in bands instead of broadcasting, acidification before application and matching nitrogen supply to the demand of the crop (Sommer et al., 1997; Stevens and Laughlin, 1997).

conclusIon

Mitigation options are available that could result in considerable reductions in nitrogen loss from agricultural systems if they were adopted by farmers. In many cases, there will be a cost to the farmer for the implementation of the measure, but for other farmers this could be a direct benefit to them. If the options proposed for reducing emissions from fertilizer use were implemented,

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farmers would be more likely to increase rather than decrease their income, because the amount of fertilizer applied and its cost can be reduced.

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