Nitrogen: the essential public enemy

Journal of Applied Ecology

2003

40

, 771–781

© 2003 British Ecological Society

Blackwell Publishing Ltd.Oxford, UKJAPPLJournal of Applied Ecology0021-8901British Ecological Society, 200310 2003405

ELEVENTH BES LECTURE

Nitrogen: the essential public enemyH. Dalton & R. Brand-Hardy

Nitrogen: the essential public enemy

HOWARD DALTON* and RICHARD BRAND-HARDY†

*

Chief Scientific Adviser, Department for Environment, Food and Rural Affairs, Science Directorate, Cromwell House, Dean Stanley Street, London, SW1P, 3JH, UK; and

†

Department for Environment, Food and Rural Affairs, Science Directorate, Cromwell House, Dean Stanley Street, London, SW1P, 3JH, UK

Summary

1.

Increased demand for food and energy is leading to changes in the global nitrogencycle. These changes are resulting in increasing levels of nitrogen in the environment in itspollutant forms with consequences for both biodiversity and human health. In this paper,we discuss the impacts in the UK and give examples of the steps that are being taken by theDepartment for Environment, Food and Rural Affairs (Defra) to tackle these problems.

2.

Over 70% of the UK land area is farmland. The farmed environment is composed of a widerange of semi-natural habitats including heather moorland, chalk downland, wet grasslandsfarm woodlands and hedgerows. As a result, much of the UK’s cherished biodiversity isan integral part of agriculture and therefore vulnerable to changes in farming practices.

3.

Defra’s overall goal is to build a sustainable future for the UK. With regard to nitro-gen pollution, this involves finding ways of continuing to meet our food and energyrequirements whilst causing little or no harm to the environment.

4.

Defra’s science programme has a central role to play in the development of its nitrogenpollution policies. These pollution policies provide a key input to the Department’s evidencebase for policy formulation, and support international negotiations on pollution targets.

5.

The Department’s science programme has addressed the major components of thenitrogen cycle associated with harmful impacts on the environment and human health.The main aims have been the understanding and quantification of impacts throughmonitoring and modelling and the development of abatement measures.

6.

Synthesis and application.

It is becoming increasingly apparent that whilst advancescan and have been made in the reduction of emissions from combustion processes, theproblem of nitrogen pollution from agriculture is far more intractable. This scientificchallenge, when taken together with emerging regulatory initiatives, will require imag-inative solutions if the UK Government is to forge a sustainable way forward.

Key-words

: agriculture, ammonia, combustion processes, pollution, sustainability.


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Introduction

Over 70% of the UK land area is farmland and, as such,constitutes one of the most diverse landscapes in thecountry. The farmed environment contains a wide rangeof semi-natural habitats including heather moorland,chalk downland, wet grasslands, farm woodlands andhedgerows. These habitats harbour the wildlife that weall cherish and seek to conserve and enhance. However,agricultural intensification, particularly since the 1970s,has had a damaging effect on them and their constituentflora and fauna. No one aspect of intensification has beenresponsible for these negative impacts but nutrientenrichment and soil acidification are recognized ashaving played a major role, particularly as a result ofnitrogen losses to the environment. Nitrogen emissionsfrom combustion processes have also been responsiblefor damaging impacts, including human health effects.

Emissions of nitrogen have long been recognized as aserious problem by Defra and its predecessor depart-ments. As a result, policies and associated research pro-grammes have been directed towards this topic sincethe 1980s. In subsequent years, the Department hasfaced challenging emissions targets, such as thoseagreed for ammonia, and far-reaching legislation, suchas the Nitrates Directive. Although phosphorus hasrecently been identified as a significant pollutant of theenvironment, we wish to focus on the effects of nitrogenas an ecological problem that is deserving of researcheffort to mitigate its effects on the environment.

In this paper, we will cover the major problemscaused by nitrogen losses to the environment. We willaddress some of the ways in which Defra is addressingthese problems, with particular emphasis on the role ofscience in characterizing the problem and identifyingpossible solutions. We will finish by considering whatwe view as the major opportunities and challenges forDefra in the future. Although we are very aware thesedays of the problems caused by nitrogen pollution, it isimportant not to lose sight of the benefits that nitrogenuse has brought to mankind.

Nitrogen as an essential nutrient

Nitrogen is a critical component of living systems andcomprises about 3% dry weight in the human body. It isa fundamental constituent of nucleic acids and pro-teins, including enzymes that drive the chemical pro-cesses in each cell. In agriculture, it has been recognizedas the single most important nutrient for increasingcrop yields across the globe. In the UK, the 5-year aver-age wheat yield increased from about 3 t ha

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1

in theperiod 1950–54 to 7·5 t ha

−

1

in 1992–96 whilst fertilizer

nitrogen applications increased from about 20 kg ha

−

1

in the late 1940s to over 200 kg ha

−

1

in the mid-1990s forwinter wheat (MAFF 2000). Intensification of agricul-ture has resulted in a wide range of technologicaladvances, particularly with regard to the developmentof improved varieties of crops and crop managementtechniques. Modern varieties require high nitrogeninputs, and the availability of highly concentrated nutri-ents in the form of inorganic fertilizers has enabledever-increasing yields to be realized. These advanceshave resulted in increasing self-sufficiency in the UKat a time when a rising human population is placinggrowing demands on world food supplies.

Whilst the benefits from the use of nitrogen in fer-tilizers are clearly evident, intensification of agriculturehas resulted in environmental problems associatedwith this vital nutrient. We also know that most com-bustion processes for energy production or transportinevitably produce significant levels of nitrogen oxidesas a by-product. This is not surprising as combustion offossil fuels occurs in an atmosphere of 80% dinitrogen.However, like agriculture, energy is a basic human needand economic growth relies heavily on fuel use. Theproblems related to Man’s association with nitrogencan be identified by reference to the numerous path-ways associated with the nitrogen cycle.

The nitrogen cycle

Reference to the nitrogen cycle shows that the effects ofnitrogen from man-made sources are pervasive, affectingterrestrial and aquatic ecosystems and the atmosphere.Emissions of ammonia, oxides of nitrogen and nitrousoxide are causes of concern with regard to impactson atmospheric processes whilst nitrate is principallyassociated with losses affecting water quality.

The majority of plants, animals and micro-organismsare adapted to use and retain small amounts of nitrogenefficiently. Under normal conditions, the nitrogen cycleis essentially in equilibrium and, for the most part, cantolerate change through uptake, storage and use re-sulting in increased biomass production. However,large additions, such as those that have accompaniedthe intensification of agriculture, cause imbalances inthe nitrogen cycle and potential leakages.

Prior to 1860, natural biological nitrogen fixationwas the dominant source of nitrogen for the terrestrialenvironment. Today, human-induced production andrelease into the environment is about 15 times greaterthan the contribution in 1860. A major cause of thislarge increase was a technological breakthrough byFritz Haber in the early 20th century. Haber developeda method for synthesizing ammonia utilizing atmos-pheric nitrogen and had established the conditions forlarge-scale synthesis of ammonia by 1909. The processwas handed over to Carl Bosch for industrial developmentand was subsequently known as the Haber–Bosch process.Most of us will be distantly familiar with this from schoolchemistry lessons, but the current international scale of

Correspondence: Richard Brand-Hardy. E-mail: [email protected] text of the Eleventh BES Lecture delivered on 18 December2002 at the University of York during the Winter and AnnualGeneral Meeting.

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nitrogen fixation may be less apparent. It is worth not-ing at this point that fixing nitrogen is a highly energyintensive process. This means that whatever other envi-ronmental impacts nitrogen may have, its manufacturecontributes to the greenhouse effect. Nitrogen fertilizeruse in the UK is equivalent to about 1% of UK green-house gas emissions.

Today, about 160 Mt N year

−

1

is released into the glo-bal environment by human activities (Fig. 1). Of thisamount, up to 100 MtN is associated with the produc-tion and use of inorganic fertilizers. About 25 MtN isderived from the combustion of fossil fuels. Continuedpopulation growth is expected to significantly increasethese amounts, particularly if developing countriesachieve Western emission rates. At present, total nitro-gen production is higher in Asia than any other regionbut, per capita, production is highest in North Americaand Europe. The human population is expected toreach a peak of about 9 billion at the end of this century.If, at that time, everyone had the same per capita pro-duction rate as today the global nitrogen productionrate would be 250 MtN year

−

1

. However, if each personhad achieved the same per capita rate as North Americatoday, the global rate would be 900 MtN year

−

1

(Cowling

et al

. 2001).The perturbations in the nitrogen cycle caused by

man’s activities, particularly by agriculture, has resultedin the large number of environmental problems associ-ated with nitrogen use that face us all today. Trying tosolve those problems requires governments to stimulateresearch to quantify the levels of emissions, identify themagnitude of their impacts and develop effective abate-ment techniques. However, it is becoming increasinglyclear that to find solutions, we must take into accountthe effects that modifying one part of the nitrogencycle has on other pathways. The introduction of anabatement technique may be successful in reducing lossesfrom one part of the cycle but it could lead to increased

losses of nitrogen from other pathways. This conceptis known as ‘pollution swapping’ and we will return toit later in this paper. The challenge for politicians andscientists alike is to find sustainable solutions thatmaintain the benefits of nitrogen use for food produc-tion whilst minimizing the impacts on the environment.Recent successes with abatement policies in the energyproduction and transport sectors have reduced theirsignificance as polluters of the environment leavingagriculture as the one key area where substantial in-roads still have to be made.

Agriculture

Agriculture is a significant source of nitrogen losses thatcan have far-reaching consequences for the environment.Nitrogen can be supplied to agricultural systems fromthe soil mineral nitrogen pool, organic matter by miner-alization, the atmosphere by deposition and fixation bylegumes, inorganic fertilizers and organic manures.

Where not taken up by crops, it can be lost by leaching,ammonia volatilization, denitrification and incorpora-tion into soil organic matter (Fig. 2). When calculatingapplication levels of fertilizers, farmers need to be awareof these nitrogen supply and loss pathways.

In 2000–01, UK consumption of fertilizer nitrogenwas 1·1 m tonnes (Fertiliser Manufacturers Associ-ation 2002). There has been a reduction over the last10 years which is largely due to increased set-aside andgreater efficiency of nitrogen use. Nonetheless, the con-sumption is still considerable and the losses of nitro-gen from agriculture are also large and take a varietyof forms. Estimates of ammonia emissions suggestthat more than 80% are from agricultural sources,principally livestock and manures (Webb

et al

. 2002). About70–80% of the nitrates found in English surface and

Fig. 1. Global population growth and N production. (A) Haber-Bosch = N produced via the Haber-Bosch process and includesproduction of NH3 for non-fertilizer purposes. (B) C-BNF = N production from cultivation, e.g. legumes. (C) Fossil Fuel = Nproduction from fossil fuel combustion. (D) Population. Based on Cowling et al (2002).

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groundwaters are estimated to be derived from agricul-tural land (Defra 2002a). Agriculture also contributedabout 65% of UK emissions of nitrous oxide in 1999(Goodwin

et al

. 2001). This gas contributes signific-antly to climate change and has a global warmingpotential almost 300 times greater than carbon dioxide.So how do these main nitrogen compounds impact onthe environment?

Ammonia, together with oxides of nitrogen emittedfrom combustion processes, are major constituents ofatmospheric deposition, with ammonia being a partic-ular problem for semi-natural habitats. The publicationof Defra’s ammonia booklet in 2002 highlighted the prob-lem of losses of this gas from agriculture (Defra 2002b).This pollutant is generally less well known than someair pollutants, such as sulphur dioxide, where largeemission cuts have been made over the last decade. Redu-cing emissions of ammonia is not as straightforward asreducing many other air pollutants, where applicationof a single technology can bring large cuts in emissions(e.g. catalytic converters, which reduce emissions ofnitrogen oxides from vehicles). Ammonia is a diffusepollutant, emitted from a number of sources over largeareas. Therefore, several approaches are needed tocontrol its release into the environment. The complexnature of the problem implies that no one approach willprovide a solution.

The principal sources of ammonia are from livestockmanures, namely, cattle (44%), poultry (14%) and pigs(9%), with other livestock contributing about 7%

(Fig. 3). Direct damage to plants – including lichens,mosses and heather – caused by high concentrations ofammonia tend to be restricted to areas close to largesources of ammonia, such as intensive livestock farms.In the UK, the ammonia problem due to the indirecteffects of increased nitrogen deposition is more wide-spread than the direct effects of ammonia concentra-tion. Ammonia released into the atmosphere can reactto form particles containing ammonium ( ) con-tributing to atmospheric levels of PM10 which are fineparticles with a diameter of less than 10

µ

m. These candamage health and can be carried long distances beforebeing deposited on the land surface by rain.

When ammonia in its gaseous or ionic form reachesthe land, it can cause nitrogen enrichment or eutro-phication of semi-natural habitats. Until recently, suchhabitats have existed in relatively low nutrient envir-onments. The disruption caused by large inputs ofanthropogenic ammonia has contributed to unwantedchanges in the balance of plant communities. A fewfast-growing common species utilize the additionalnitrogen to out-compete less tolerant species, resultingin an ecologically impoverished habitat. The replace-ment of dwarf shrubs by grasses in heathlands is wellknown. It has been estimated that about a third of valu-able ecosystems are threatened in the UK, includingupland and lowland heath, upland bog, semi-naturalgrassland and some woodlands. Excesses of ammoniacan also cause some upland soils, streams and lakesto become acidic through conversion of ammonia tonitrate and its subsequent leaching to ground waters.This has resultant adverse impacts on plants and aquaticbiodiversity. These impacts do not solely involve ammonia.

Fig. 2. A simplified agricultural N cycle. Source: Davies (2000).

NH4+

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Of the 380 kt of total nitrogen deposited annually,43% is from NO

x

and 57% from ammonia, even thoughthe total emission of NO

x

is considerably greater thanthe emission of ammonia (NEGTAP 2001).

Another form of nitrogen lost from organic and in-organic fertilizers is nitrate. Although the Department’sresearch on nitrate loss to water is now much reduced,since many of the questions have been addressed, thetopic has been a key consideration for pollution policyand research since the 1980s.

Nitrate loss from agriculture has been the focus forconcerns about the quality of drinking water and detri-mental effects to the aquatic environment, particu-larly through eutrophication. A maximum allowableconcentration of nitrate in drinking water of 50 mgL

−

1

was set by the 1980 EU Directive on the Quality ofWater intended for Human Consumption. In 1991, theGovernment signed up to the Nitrates Directive whichrequired the designation of Nitrate Vulnerable Zones(NVZs). These zones are water catchments where thenitrate concentration either exceeds the 50 mg L

−

1

limitor is at risk of doing so. Within NVZs, farmers arerequired to implement an Action Programme of measuresto help reduce the amount of nitrate lost from agriculturalland to the aquatic environment.

The cost of removing nitrates from drinking water issignificant. Based on a figure of 80% of nitrate comingfrom agricultural land, an annual cost to the waterindustry of £16·4 m was estimated for the period 1992–97 for the removal of nitrate pollution from agriculture(Pretty

et al

. 2000).

Considerable losses can arise due to excessive orinappropriately timed applications of organic nitrogen,such as animal manures derived from housed livestock.The potential for nitrate loss from manures is greaterthan from inorganic fertilizers because of difficulties inapplying manures in a timely and accurate way andtheir inherent lower efficiency. The manures are oftenapplied to arable land and grassland through autumnand winter according to farmer convenience and prevai-ling soil conditions or at rates well in excess of crop uptake.NVZ restrictions will limit the way in which manureis used over much of England in the future and shouldresult in more efficient use of nitrogen in manures witha resultant reduction in inputs of mineral fertilizers.

Nitrate in soil is also subject to denitrification pro-cesses resulting in emissions of nitrous oxide to theatmosphere. Until recently, nitrous oxide emissionswere almost equally derived from agriculture and othersources. However, moves to curb emissions from theadipic acid production process led to significant reduc-tions in 1998. As a result, since 1999, agriculture hasbecome by far the largest source of nitrous oxide enter-ing the atmosphere. (Goodwin

et al

. 2001).Although agriculture is now the major focus for

action, nitrogen emissions from the combustion offossil fuels are still a cause for concern not only for theenvironment but also for human health.

Combustion of fossil fuels

The combustion of fossil fuels is an important source ofemissions of oxides of nitrogen (NO

x

). In addition,conversion of NO

x

in the atmosphere to nitrate canlead to their incorporation as secondary constituents

Fig. 3. UK ammonia emissions by source. Source: Webb et al. (2002).

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in particles (PM10). In remote unpolluted regions,nitric oxide concentrations are generally only a smallfraction of the NO

x

total. However, in polluted townsand cities where the oxidizing capacity of the air may belimited, NO concentrations often exceed those of NO

2

.The primary sources are the combustion of coal, oil andnatural gas for energy production and use, includingtransportation. In the UK, total emissions of nitrogenoxides were 1·6 m tonnes in 2000. The transport sectoris the major source of nitrogen oxides, responsible forabout half of these emissions. However, levels of NO

x

have been falling, with a decrease of 46% since 1989 dueto the introduction of catalytic converters and stricteremission limits from road transport and reductions inemissions from power stations (Goodwin

et al

. 2001).Once emitted into the atmosphere, nitrogen oxides

can contribute to a range of environmental and healthproblems. NO

x

is a key component in the formation ofground level ozone (‘photochemical smog’). Throughthis complex cycle of chemical reactions, the NO

x

isconverted to nitrates which are incorporated into fineparticles (PM10), often as ammonium nitrate wherethe ammonia has originated from the agriculturalemissions. Another component of the atmosphericparticles mix is ammonium sulphate, where ammoniahas reacted with sulphuric acid produced by the reac-tions of sulphur dioxide, emitted largely from powergeneration and industry. These fine particles can causevisibility impairment, acid deposition, and excessnutrient inputs to semi-natural ecosystems. NO

x

as aprecursor of ground level and tropospheric ozone is anindirect contributor to climate change and is involvedin the reactions leading to stratospheric ozone deple-tion. In its role as a precursor in particulate formationand ozone creation at low levels in smog, NO

x

cancause premature death, chronic respiratory illness,such as bronchitis and asthma, and aggravation ofexisting respiratory problems. By increasing tropo-spheric ozone, NO

x

can also act indirectly as a green-house gas. In addition, it can damage plants and crops.Other environmental impacts of NO

x

emissions anddeposition include die-back of trees, loss of biodiversityin grasslands and acidification of streams and lakes.These environmental impacts also involve nitrogeninputs from ammonia emissions.

The imbalances in the nitrogen cycle caused by Man’sactivities have resulted in losses to the environmentcausing a range of problems including eutrophication,soil acidification and greenhouse gas emissions. Therehave also been significant effects on human health, par-ticularly from the combustion of fossil fuels. These aredaunting problems for government, and particularlyDefra, to address and science has a key role to play ininforming policies to tackle these problems.

Meeting the scientific challenge

So what is Defra doing about these problems? Govern-ment responds to, and is accountable to, the society it

governs. In the UK, the drive to make the nation self-sufficient in food in the post-war years has been suc-cessful and has permanently removed previous fears offood shortages. With food security having declined asan issue, other priorities have begun to emerge. Today,society’s values are more diverse and more sophist-icated. With regard to agriculture they can include anattractive landscape, less pollution, protection andenhancement of biodiversity, high standards of animalwelfare and disease control, improved recreationalopportunities, positive impacts of agriculture in ruralsociety, as well as meeting the objectives of food pro-duction and economic viability. For food, the valuesmight include competitive prices, safety, high quality,convenience and choice.

There may well be other values that could be added.These are challenging issues for industry to take onboard. They do not necessarily all match the needs andvalues of the agricultural industry, and particularlyfarmers, whose falling incomes present real difficultiesin meeting the more demanding aspirations of society.Defra needs to find a way to balance these differingdemands so that, for example, legislation or other con-trols minimize costs to farmers.

Defra’s goal is to build a sustainable future forthe UK and its ideas were published in June 2002 inthe Department’s Sustainable Development Strategy(Defra 2002c). The Department is not merely themerger of the functions of the former MAFF, and partsof DETR and the Home Office. It reflects the Govern-ment’s determination to exploit the synergies that existbetween environmental protection, rural affairs andfood, farming and fisheries. The core aim of sustainabledevelopment involves a better quality of life for peoplenow and for future generations. More specifically,the aim involves securing a better environment andsustainable use of natural resources together witheconomic prosperity for those industries primarilyaffected by the Department, thriving rural commun-ities and a countryside for all to enjoy. In essence,this approach involves thinking in an integrated wayabout economic, environmental and social objectives.With regard to nitrogen pollution, this is a perfectillustration of the problem of sustainable develop-ment – if we are to go on producing quality agriculturalgoods, we must do so in a way that does little or noharm to the environment. To this end, the Governmentinitiated a strategic review of diffuse water pollutionfrom agriculture in June 2002. The aim of the reviewis to develop cost-effective and proportionate meansof reducing water pollution levels to meet existingcommitments and to encourage sustainable farmingpractices.

So how can science contribute to the Department’ssustainability agenda in relation to the nitrogen prob-lem? In particular, what science have we commissionedthat has made a difference to our understanding of theproblem and identified realistic solutions? Defraneeds a robust evidence base upon which to develop its

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policies. This is where its science programmes have sucha crucial role to play.

We have been commissioning research since the1980s to address the problems caused by emissions ofnitrogen in its various forms. At its height, some £6 mper year was being spent in the Nitrate research pro-gramme in the 1990s. In recent years funding onnitrates has declined as understanding of the problemhas grown, though some important associated topicsremain to be resolved, such as the development ofabatement measures for nitrous oxide emissions. Thefocus also changed to address the emerging problem ofphosphorus loss from agriculture.

With regard to combustion processes, the thrust ofthe current research on NO

x

is to quantify exposure tohuman populations and ecosystems and assess theeffectiveness of abatement measures.

Whilst considerable work has been, and continues tobe undertaken on these aspects of the nitrogen cycle, wewould like to focus on emissions of ammonia. Thistopic is a good example of the complexity of the prob-lems facing science and the difficulties of developingsustainable solutions that do not result in knock-oneffects for other parts of the nitrogen cycle.

Funding for research on ammonia emissions dates fromthe early 1990s. About £1·4 m per year is now being spentby Defra on research on ammonia emissions and abate-ment measures with an extra £90 k for a monitoringnetwork and an additional £900 k on atmospheric modelsand environmental impacts, much of which includesammonia. This large commitment was in response togrowing international concern about the damaging impactof ammonia on sensitive habitats through eutrophica-tion and acidification processes. The research had asignificant role in informing the UNECE GothenburgProtocol and the National Emission Ceilings Directive.Under the Protocol and Directive, the UK has agreedto a legally binding emissions target of 297 kilotonnesof ammonia per year to be achieved by 2010. The emis-sions figure for the UK was 320 kilotonnes in 2000. TheProtocol and Directive are due to be reviewed by sig-natories in 2004–05 with regard to compliance costsand the effectiveness of the agreed emission targets. It istherefore important that any future decisions on emis-sions targets are underpinned by robust scientific evid-ence for cost-effective abatement strategies. Currentresearch on agricultural systems is aiming to improveestimates of ammonia losses from UK agriculture andassess the cost-effectiveness and practicality of abate-ment techniques.

One example of this research that has informedDefra’s policies is the Ammonia Distribution andEffects Project known as ADEPT. This project usedcensus data, satellite land cover data and estimates ofsource strength to map agricultural ammonia emis-sions to air across the UK. Key sources were identified

as animal housing, manure stores, intensively grazedareas and landspreading of manures. Application of anatmospheric transport and deposition model indicatedthat up to 30% of the emitted ammonia is re-depositedlocally (Sutton

et al

. 1998).A subsequent Defra research project supported the

findings from ADEPT on sources of ammonia (Chambers,Williams & Chadwick 2002). The project quantifiedammonia fluxes for complete (housing, storage andland spreading) solid and liquid manure managementsystems. This showed that for beef cattle, losses weregreater from liquid manure systems, at 37 kg NH

3

-Nper 500 kg liveweight gain, than from the solid manuresystem, at 24 kg NH

3

-N per 500 kg liveweight gain.Most of the losses (70–75%) occurred during the housingphase. Therefore, targeting the housing phase, e.g. byswitching from a liquid to a solid, straw-based system,has the potential to contribute to a reduction in ammoniaemissions from agriculture. Ongoing work is lookingat how to optimize straw additions to reduce ammoniaemissions. This may include targeting additions to dirtyareas within houses rather than blanket spreading.

There are also options for reducing ammonia emis-sions during spreading. Research has shown that appli-cation of cattle slurry by shallow injection, trailingshoe and band-spreading as compared with surfacebroadcast reduced ammonia emissions by a mean of73, 57 and 26%, respectively, on grassland.

Defra’s research has involved the development ofmodels that quantify the inputs and outputs of theagricultural nitrogen cycle for different cropping sys-tems. For manures, the Manure Nitrogen EvaluationRoutine, known as MANNER, quantifies the fertilizernitrogen value of manures and the fate of the nitrogenafter spreading. As a Decision Support System, itassists farmers in planning their use of manures so as toencourage greater efficiency in the use of this valuableresource. Results from the model have achieved strongcorrelations with results from field experiments andmany farmers and advisers are now using this tool.

Whilst modified techniques for spreading slurry mayreduce ammonia emissions, the nitrogen applied to theland increases the soil mineral nitrogen pool, therebyincreasing the potential for losses as nitrous oxide andnitrate. This is an example of pollution swapping,where modifications to the nitrogen cycle to reduce pol-lution may lead to problems elsewhere in the cycle.There are also other examples of pollution swappingthat need to be resolved. For instance, if a farmer movesthe date of applying manures to land from autumn tospring to satisfy nitrate leaching requirements underNVZ regulations, he could cause an increase inammonia volatilization. Also, covering a slurry storeto reduce ammonia volatilization can cause problemsbecause the slurry will retain more nitrogen for applica-tion to fields and will therefore be at risk of creatinglosses following application. Pollution swapping repre-sents a major challenge for researchers seeking to findsustainable solutions for manures use. Our approach to

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finding solutions must therefore avoid following a com-partmentalized approach but must instead take intoaccount the many interactions associated with thenitrogen cycle.

Atmospheric deposition of ammonia and oxides ofnitrogen is largest in upland semi-natural areas, wherehigh rainfall augments deposition, and in agriculturalareas where ammonia

per se

is the problem. The criticalloads concept has been developed to gauge the poten-tial impacts of deposition on an ecosystem. Whendeposition exceeds the critical load, the ecosystemor some specified element of it, is at risk of damage.Defra supports a programme of work to quantify thecritical load for a range of semi-natural ecosystems,and maps these values on a 1

×

1 km grid. By overlayingmaps of deposition, produced from a combination ofmonitoring and atmospheric modelling, it is possibleto identify those areas receiving unsustainable levels ofdeposition. About one-third of the area of UK ecosys-tems receives deposition of nitrogen above the criticalload, and this is mainly due to ammonia.

In addition to funding research and monitoring,there are a number of other approaches Defra uses tostrengthen its evidence base. A good example of a wayin which we make the very best use of science is cri-tical evaluation and assessment by a group of expertscientists. The National Expert Group on TransboundaryAir Pollution (NEGTAP) recently produced a detailedevaluation of the scientific evidence for the effective-ness of our policies for the reduction of emissionsaffecting acidification, eutrophication and ground-level ozone (NEGTAP 2001). The resulting report is adefinitive, high quality review of the responses of theatmosphere and of freshwaters and ecosystems to thesignificant changes which have occurred in emissions –particularly of sulphur and NO

x

– over the past 10 yearsor so as a result of Government policies. The grouphighlighted the problems caused by ammonia andground-level ozone as a continuing cause for concern.

The sustained effects of nitrogen emissions overseveral decades have left a signal of nitrogen enrich-ment in the countryside. Defra has contributedfunding to two national monitoring studies that have

picked up this signal and their results were recentlypublished.

The

New Atlas of British and Irish Flora

published in2002 (Preston, Pearman & Dines 2002) comprises thefindings from a major survey of flowering plants andferns undertaken in over 3800 10

×

10 kilometresquares across Britain and Ireland between 1987 and1999. The survey is a repeat of the only previousnational survey undertaken in the 1950s and publishedin 1962. The new survey provides the most completerecord of British and Irish flora to date with 700 speciesmapped for the first time; many of these new species arerecent introductions.

In a comparison of the results from the two surveys,species were categorized according to their nutrientrequirements. The groupings were based on publishedEllenberg indicator values where each species is allo-cated a number from 1 to 9, with 1 representing speciestypically found in extremely infertile sites and 9 cover-ing species found in very fertile sites. An average changeindex for species with a particular indicator value wasused to analyse the relative performance of that group.The results clearly show that species characteristicof less fertile sites have been less successful thanthose found in more fertile sites (Fig. 4). This patternis repeated in almost all regions, although it is not asmarked in Wales, Northern Ireland and easternScotland. The Scottish Highlands is the only regionwhere it does not apply. This signal represents a markedchange in the flora of Britain and Ireland over the last40 years.

The results from the Atlas survey support thefindings from the Countryside Survey 2000 (CS2000,Haines-Young

et al

. 2000). CS2000 consisted of surveysof Britain and Northern Ireland involving a combinationof detailed field recording and land cover satellite map-ping. The aim of CS2000 was to provide informationon the stock of habitats and landscape elements and thechange in time of these elements. The survey wasundertaken in 1998 and builds on earlier field surveys

Fig. 4. The mean change index for species with different Ellenberg indicator values for soil fertility. Low Ellenberg values indicateplants associated with infertile soils. Source: Preston et al. (2002).

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in Great Britain in 1978, 1984 and 1990 and satellitemapping in 1990. The survey was based on a stratifiedsample of 1

×

1 km squares.Results from CS2000 show a clear increase in

fertility score across a range of habitats. These resultstogether with those from the Atlas are consistent withthe changes expected from widespread eutrophica-tion in the countryside. It is very likely that nitrogendeposition from the atmosphere will have contributedto these changes, but changes in land managementpractices in the last decades may also have played arole.

Although the adoption of individual abatementmeasures may help to reduce the impact of eutroph-ication, an alternative option is to take a more holisticapproach by following a different system of farming,for example, organic farming.

Defra spends about £2 m each year on organic farmingresearch. Organic farming is considered to deliver sig-nificant benefits in terms of biodiversity and resourceprotection. These flow both from the strict limitationon the inputs which organic farmers are permitted touse and from the system of management which organicfarmers are required to adopt. Organic farmers haveavailable to them only a limited range of, mostly natu-ral, pesticides and are not permitted to use syntheticfertilizers and herbicides. The rotation, which is a cen-tral feature of organic systems, encourages soil healthboth in terms of fertility and in terms of microbial andinvertebrate activity. It also enhances local landscapesand biodiversity by preventing monoculture. The evid-ence base for nitrogen losses is sparse but Defra researchhas provided some insights.

With regard to nitrate loss, many organic farms havebeen found to be operating at a lower level of nitrogenintensity than conventional systems, with nitrogeninputs derived from fixation by legumes or importationof animal feed onto the farm. Data are limited, but acomparison of farms in Nitrate Sensitive Areas (fore-runners to NVZs) with organic equivalents found thatnitrate losses were similar on area basis if grass sitesreceiving more than 200 kg ha

−

1

of fertilizer N wereexcluded.

For ammonia, although intensive poultry andpig units are not permitted under organic standards,composting of manures is encouraged which leads torelatively high losses. However, organically producedmanures often have a lower concentration of nitrogenthan their conventionally produced equivalents, andcomposting reduces losses during subsequent spread-ing. In comparing farming systems, it is important todistinguish between losses per unit area and those perunit of yield. Whilst it seems likely that there is little dif-ference between organic and conventional systems forlosses per unit of yield, it is considered likely that thereare lower losses per unit area.

For nitrous oxide, there is insufficient evidence toreach any sort of reliable conclusion.

Although organic farming may be attractive tosome, conventional farming still accounts for about97% of farming in England. As a result, abatementmeasures that address the requirements of conven-tional agriculture will have to remain the higher prior-ity for Defra.

The future

So what of the future? We have already mentioned theproblem of pollution-swapping which requires greaterunderstanding of the impact of tackling one form ofnitrogen loss on other parts of the nitrogen cycle. Solu-tions to this complex problem still need to be found andtherefore we rank this issue as a major challenge for thefuture. In recognition of the significance of this problem,Defra has commissioned new research to address it.

There are also the over-arching demands of societyfor cheap, safe, high-quality food produced with lesspollution of the environment, protection and enhance-ment of biodiversity and high standards of animal wel-fare and disease control. These demands are reflectedin increasingly stringent emission standards agreed ininternational fora.

One major piece of European legislation that isexpected to have profound effects on resource protec-tion in the UK and the rest of Europe is the WaterFramework Directive. The Directive is the most sub-stantial piece of EU water legislation ever drawn up.For the first time, ecological quality objectives will beset for surface waters. There will also be a requirementto produce strategic management plans for each riverbasin. These plans are likely to have knock-on benefitsfor coastal waters. All water quality targets will have tobe met by 2015 and the legislation will be transposedinto domestic legislation in 2003.

The requirements of future EU legislation, such asthe Water Framework Directive, will require high-quality science to inform domestic policy formulation.In addition, other future policy initiatives will need tobe anticipated as early as possible to enable appropriatescience programmes to be established. These will pro-vide the necessary information for Defra to negotiatewith other countries from a position of strength. Forthis purpose, science must provide solutions that arebased on well-researched evidence and that providesensible, cost-effective abatement strategies.

New methodologies, such as use of GM technolo-gies, may provide opportunities for meeting futurestringent targets. We are all too familiar with the develop-ment of GM crops with, for example, specific herbi-cide tolerance or resistance to insect pests. It is quitepossible that these ‘new’ plants may become accept-able in the future but, for the present, we are still await-ing evaluation

.

Results from the Field Scale Evaluationproject, which covers possible effects on biodiversity,are due to be published in autumn 2003. Until these

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results are known, we will not be able to make firmdecisions on the future use of such technologies inthe farming sector. The use of these technologies todevelop GM products can, if applied responsibly,potentially provide a wide range of benefits forsociety.

With regard to addressing nitrogen losses, researchteams in the UK are actively seeking a way of inducingplants to take up symbiotic bacteria that will fix nitro-gen from the atmosphere. Whilst there have been somesuccesses in the laboratory, there have been no field trialsdemonstrating that this is a viable option. An altern-ative approach is to use GM technologies to increaseplant nitrogen-use efficiency rather than fixation, forexample, in wheat. Some research has been funded byDefra but further work is required to demonstrate thefeasibility of this approach. Overall, it is evident thatadditional studies are required to explore fully thepotential of GM technologies for providing solutionsto the nitrogen emissions problem.

Defra will continue to ensure that GM organisms areonly approved for release if they are judged not to posean unacceptable risk to public health or the environ-ment. In this context, the Government is promoting anopen debate on the issues relating to the future of gene-tically modified organisms in the UK. One elementthat will inform the debate is a review of the sciencesurrounding GM issues.

The Science review will support the main inde-pendent public debate in its ambition to fully explorethe issues relating to GM technologies. Scientistsacross disciplines, and members of the public with aninterest in the relevant science, are encouraged toparticipate via the website that has been launched atwww.gmsciencedebate.org.uk.

We believe this will be an exciting venture. It will en-able us to take a really comprehensive and open look atthe science relevant to GM with the focus on crops andfoods, and will do so in a way that recognizes the inter-ests and concerns of the public. For it to be a success wewill need to harness the enthusiasm of as many scien-tists as possible and we hope members of the BritishEcological society will take an active part.

Another new advance that is showing promise isprecision farming. The electronics revolution hasspawned two technologies that will impact agriculturein the next decade. These technologies are GeographicInformation Systems (GIS) and Global Positioning Sys-tems (GPS). Along with GIS and GPS, a wide range ofsensors, monitors and controllers are being developedfor agricultural equipment. Together they will enablefarmers to use electronic guidance aids to provideprecise positioning for all equipment and chemicalapplications. For fertilizer applications, research isdeveloping variable rate controllers for granular, liquidand gaseous fertilizer materials. These variable ratescan be automatically controlled by the farmer using anon-board computer with an electronic prescriptionmap based on yield mapping and other data. This

should result in a reduction in overall nitrogen use on afield whilst maintaining yields

.

It is generally accepted that climate change is likelyto have far-reaching effects on the environment infuture decades. Research by Defra, in collaborationwith other stakeholders, is seeking to determine thenature and extent of these effects. It is difficult to pre-dict what the impacts might be on the nitrogen cycle.However, warmer, wetter winters in some parts of thecountry could result in increased nitrogen losses byleaching, together with increased emissions of nitrousoxide by denitrification. The availability of nitrogenfrom mineralization of soil organic matter may alsoincrease. These changes are likely to affect the accuracyof models that are based on current conditions. A scopingstudy recently commissioned by Defra will be a first steptowards predicting the impact of a changed climate onnutrient losses (Hossell 2003).

Finally, CAP reform could provide benefits for theenvironment, for example, through increased fundingfor agri-environment schemes and general de-couplingof subsidies from production. Such reform shouldencourage the adoption of less intensive farming methods,including reductions in the use of fertilizers. Indeed, theCommission’s mid-term review paints an encouragingpicture for sustainable farming in Europe but the UKwill need to maintain pressure on the Commission toeffect the changes that it is seeking. Science will have animportant role to play in the success of any domesticpolicy initiatives flowing from CAP reform.

Looking across the many challenges for the future, itseems as though we may be at something of a cross-roads. On the one hand, agriculture could follow a pathwhere environmental protection and enhancement is atthe core of farming activities. However, farmers maydecide to follow another path characterized by techno-logical innovation and involving further intensifica-tion. Of course, these approaches are not necessarilymutually exclusive. In the spirit of Sir Don Curry’sreport (Policy Commission on the Future of Farmingand Food 2002), it may be possible for farmers to pickthe best from both paths and combine them success-fully into their businesses. However, their decisions willbe based on what is most appropriate for their par-ticular circumstances. Whatever the outcome of thesedecisions, it is evident that there are significant pres-sures building that could have a considerable impact onthe future face of UK agriculture.

In conclusion, we hope that we have demonstratedDefra’s commitment to tackling the nitrogen problemas part of its overall drive to make sustainable develop-ment a reality. We have only been able to scratch thesurface of the Department’s many activities in thisarea. Defra is striving to integrate the needs of the envir-onment, industry and rural communities. This is theessence of sustainable development and is described inmore detail in the Department’s Sustainable Farmingand Food Strategy which was launched in December 2002(Defra 2002d). The dilemma of the nitrogen problem

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for sustainable development is to understand whatlevel and type of agriculture/land management is eco-nomically viable whilst also meeting environmentalprotection and health requirements. It is a challengingtarget but it is the yardstick by which the Department’ssuccess will be measured. Science has much to offer inbringing about the realization of this goal and my jobwill be to ensure that Defra’s science is at the heart ofthe Department’s policies for sustainable development.

Acknowledgements

We would like to thank colleagues in Defra and otherorganizations who contributed to the preparationof this article. We are particularly grateful to PeterCostigan and Alison Vipond in Defra for their valuablecomments.

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Documents

Nitrogen: the essential public enemy