Nitrogen: the essential public enemy


  • Howard Dalton,

    1. Chief Scientific Adviser, Department for Environment, Food and Rural Affairs, Science Directorate, Cromwell House, Dean Stanley Street, London, SW1P, 3JH, UK; and
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  • Richard Brand-Hardy

    Corresponding author
    1. Department for Environment, Food and Rural Affairs, Science Directorate, Cromwell House, Dean Stanley Street, London, SW1P, 3JH, UK
      Richard Brand-Hardy.
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  • The text of the Eleventh BES Lecture delivered on 18 December 2002 at the University of York during the Winter and Annual General Meeting.

Richard Brand-Hardy.


  • 1Increased demand for food and energy is leading to changes in the global nitrogen cycle. These changes are resulting in increasing levels of nitrogen in the environment in its pollutant 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 the Department for Environment, Food and Rural Affairs (Defra) to tackle these problems.
  • 2Over 70% of the UK land area is farmland. The farmed environment is composed of a wide range of semi-natural habitats including heather moorland, chalk downland, wet grasslands farm woodlands and hedgerows. As a result, much of the UK's cherished biodiversity is an integral part of agriculture and therefore vulnerable to changes in farming practices.
  • 3Defra's overall goal is to build a sustainable future for the UK. With regard to nitrogen pollution, this involves finding ways of continuing to meet our food and energy requirements whilst causing little or no harm to the environment.
  • 4Defra's science programme has a central role to play in the development of its nitrogen pollution policies. These pollution policies provide a key input to the Department's evidence base for policy formulation, and support international negotiations on pollution targets.
  • 5The Department's science programme has addressed the major components of the nitrogen cycle associated with harmful impacts on the environment and human health. The main aims have been the understanding and quantification of impacts through monitoring and modelling and the development of abatement measures.
  • 6Synthesis and application. It is becoming increasingly apparent that whilst advances can and have been made in the reduction of emissions from combustion processes, the problem of nitrogen pollution from agriculture is far more intractable. This scientific challenge, when taken together with emerging regulatory initiatives, will require imaginative solutions if the UK Government is to forge a sustainable way forward1, 2.
Figure 1.
Figure 2.


Over 70% of the UK land area is farmland and, as such, constitutes one of the most diverse landscapes in the country. The farmed environment contains a wide range of semi-natural habitats including heather moorland, chalk downland, wet grasslands, farm woodlands and hedgerows. These habitats harbour the wildlife that we all cherish and seek to conserve and enhance. However, agricultural intensification, particularly since the 1970s, has had a damaging effect on them and their constituent flora and fauna. No one aspect of intensification has been responsible for these negative impacts but nutrient enrichment and soil acidification are recognized as having played a major role, particularly as a result of nitrogen losses to the environment. Nitrogen emissions from combustion processes have also been responsible for damaging impacts, including human health effects.

Emissions of nitrogen have long been recognized as a serious problem by Defra and its predecessor departments. As a result, policies and associated research programmes have been directed towards this topic since the 1980s. In subsequent years, the Department has faced challenging emissions targets, such as those agreed for ammonia, and far-reaching legislation, such as the Nitrates Directive. Although phosphorus has recently been identified as a significant pollutant of the environment, we wish to focus on the effects of nitrogen as an ecological problem that is deserving of research effort to mitigate its effects on the environment.

In this paper, we will cover the major problems caused by nitrogen losses to the environment. We will address some of the ways in which Defra is addressing these problems, with particular emphasis on the role of science in characterizing the problem and identifying possible solutions. We will finish by considering what we view as the major opportunities and challenges for Defra in the future. Although we are very aware these days of the problems caused by nitrogen pollution, it is important not to lose sight of the benefits that nitrogen use has brought to mankind.

Nitrogen as an essential nutrient

Nitrogen is a critical component of living systems and comprises about 3% dry weight in the human body. It is a fundamental constituent of nucleic acids and proteins, including enzymes that drive the chemical processes in each cell. In agriculture, it has been recognized as the single most important nutrient for increasing crop yields across the globe. In the UK, the 5-year average wheat yield increased from about 3 t ha−1 in the period 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 for winter wheat (MAFF 2000). Intensification of agriculture has resulted in a wide range of technological advances, particularly with regard to the development of improved varieties of crops and crop management techniques. Modern varieties require high nitrogen inputs, and the availability of highly concentrated nutrients in the form of inorganic fertilizers has enabled ever-increasing yields to be realized. These advances have resulted in increasing self-sufficiency in the UK at a time when a rising human population is placing growing demands on world food supplies.

Whilst the benefits from the use of nitrogen in fertilizers are clearly evident, intensification of agriculture has resulted in environmental problems associated with this vital nutrient. We also know that most combustion processes for energy production or transport inevitably produce significant levels of nitrogen oxides as a by-product. This is not surprising as combustion of fossil fuels occurs in an atmosphere of 80% dinitrogen. However, like agriculture, energy is a basic human need and economic growth relies heavily on fuel use. The problems related to Man's association with nitrogen can be identified by reference to the numerous pathways associated with the nitrogen cycle.

The nitrogen cycle

Reference to the nitrogen cycle shows that the effects of nitrogen from man-made sources are pervasive, affecting terrestrial and aquatic ecosystems and the atmosphere. Emissions of ammonia, oxides of nitrogen and nitrous oxide are causes of concern with regard to impacts on atmospheric processes whilst nitrate is principally associated with losses affecting water quality.

The majority of plants, animals and micro-organisms are adapted to use and retain small amounts of nitrogen efficiently. Under normal conditions, the nitrogen cycle is essentially in equilibrium and, for the most part, can tolerate change through uptake, storage and use resulting in increased biomass production. However, large additions, such as those that have accompanied the intensification of agriculture, cause imbalances in the nitrogen cycle and potential leakages.

Prior to 1860, natural biological nitrogen fixation was the dominant source of nitrogen for the terrestrial environment. Today, human-induced production and release into the environment is about 15 times greater than the contribution in 1860. A major cause of this large increase was a technological breakthrough by Fritz Haber in the early 20th century. Haber developed a method for synthesizing ammonia utilizing atmospheric nitrogen and had established the conditions for large-scale synthesis of ammonia by 1909. The process was handed over to Carl Bosch for industrial development and was subsequently known as the Haber–Bosch process. Most of us will be distantly familiar with this from school chemistry lessons, but the current international scale of nitrogen fixation may be less apparent. It is worth noting at this point that fixing nitrogen is a highly energy intensive process. This means that whatever other environmental impacts nitrogen may have, its manufacture contributes to the greenhouse effect. Nitrogen fertilizer use in the UK is equivalent to about 1% of UK greenhouse gas emissions.

Today, about 160 Mt N year−1 is released into the global environment by human activities (Fig. 1). Of this amount, up to 100 MtN is associated with the production and use of inorganic fertilizers. About 25 MtN is derived from the combustion of fossil fuels. Continued population growth is expected to significantly increase these amounts, particularly if developing countries achieve Western emission rates. At present, total nitrogen production is higher in Asia than any other region but, per capita, production is highest in North America and Europe. The human population is expected to reach a peak of about 9 billion at the end of this century. If, at that time, everyone had the same per capita production rate as today the global nitrogen production rate would be 250 MtN year−1. However, if each person had achieved the same per capita rate as North America today, the global rate would be 900 MtN year−1 (Cowling et al. 2001).

Figure 1.

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

The perturbations in the nitrogen cycle caused by man's activities, particularly by agriculture, has resulted in the large number of environmental problems associated with nitrogen use that face us all today. Trying to solve those problems requires governments to stimulate research to quantify the levels of emissions, identify the magnitude of their impacts and develop effective abatement techniques. However, it is becoming increasingly clear that to find solutions, we must take into account the effects that modifying one part of the nitrogen cycle has on other pathways. The introduction of an abatement technique may be successful in reducing losses from one part of the cycle but it could lead to increased losses of nitrogen from other pathways. This concept is known as ‘pollution swapping’ and we will return to it later in this paper. The challenge for politicians and scientists alike is to find sustainable solutions that maintain the benefits of nitrogen use for food production whilst minimizing the impacts on the environment. Recent successes with abatement policies in the energy production and transport sectors have reduced their significance as polluters of the environment leaving agriculture as the one key area where substantial in-roads still have to be made.



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

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

Figure 2.

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

In 2000–01, UK consumption of fertilizer nitrogen was 1·1 m tonnes (Fertiliser Manufacturers Association 2002). There has been a reduction over the last 10 years which is largely due to increased set-aside and greater efficiency of nitrogen use. Nonetheless, the consumption is still considerable and the losses of nitrogen from agriculture are also large and take a variety of forms. Estimates of ammonia emissions suggest that more than 80% are from agricultural sources, principally livestock and manures (Webb et al. 2002). About 70–80% of the nitrates found in English surface and groundwaters are estimated to be derived from agricultural land (Defra 2002a). Agriculture also contributed about 65% of UK emissions of nitrous oxide in 1999 (Goodwin et al. 2001). This gas contributes significantly to climate change and has a global warming potential almost 300 times greater than carbon dioxide. So how do these main nitrogen compounds impact on the environment?


Ammonia, together with oxides of nitrogen emitted from combustion processes, are major constituents of atmospheric deposition, with ammonia being a particular problem for semi-natural habitats. The publication of Defra's ammonia booklet in 2002 highlighted the problem of losses of this gas from agriculture (Defra 2002b). This pollutant is generally less well known than some air pollutants, such as sulphur dioxide, where large emission cuts have been made over the last decade. Reducing emissions of ammonia is not as straightforward as reducing many other air pollutants, where application of a single technology can bring large cuts in emissions (e.g. catalytic converters, which reduce emissions of nitrogen oxides from vehicles). Ammonia is a diffuse pollutant, emitted from a number of sources over large areas. Therefore, several approaches are needed to control its release into the environment. The complex nature of the problem implies that no one approach will provide a solution.

The principal sources of ammonia are from livestock manures, 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 of ammonia tend to be restricted to areas close to large sources of ammonia, such as intensive livestock farms. In the UK, the ammonia problem due to the indirect effects of increased nitrogen deposition is more widespread than the direct effects of ammonia concentration. Ammonia released into the atmosphere can react to form particles containing ammonium (inline image) contributing to atmospheric levels of PM10 which are fine particles with a diameter of less than 10 µm. These can damage health and can be carried long distances before being deposited on the land surface by rain.

Figure 3.

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

When ammonia in its gaseous or ionic form reaches the land, it can cause nitrogen enrichment or eutrophication of semi-natural habitats. Until recently, such habitats have existed in relatively low nutrient environments. The disruption caused by large inputs of anthropogenic ammonia has contributed to unwanted changes in the balance of plant communities. A few fast-growing common species utilize the additional nitrogen to out-compete less tolerant species, resulting in an ecologically impoverished habitat. The replacement of dwarf shrubs by grasses in heathlands is well known. It has been estimated that about a third of valuable ecosystems are threatened in the UK, including upland and lowland heath, upland bog, semi-natural grassland and some woodlands. Excesses of ammonia can also cause some upland soils, streams and lakes to become acidic through conversion of ammonia to nitrate and its subsequent leaching to ground waters. This has resultant adverse impacts on plants and aquatic biodiversity. These impacts do not solely involve ammonia. Of the 380 kt of total nitrogen deposited annually, 43% is from NOx and 57% from ammonia, even though the total emission of NOx is considerably greater than the emission of ammonia (NEGTAP 2001).

Another form of nitrogen lost from organic and inorganic fertilizers is nitrate. Although the Department's research on nitrate loss to water is now much reduced, since many of the questions have been addressed, the topic has been a key consideration for pollution policy and research since the 1980s.


Nitrate loss from agriculture has been the focus for concerns about the quality of drinking water and detrimental effects to the aquatic environment, particularly through eutrophication. A maximum allowable concentration of nitrate in drinking water of 50 mg L−1 was set by the 1980 EU Directive on the Quality of Water intended for Human Consumption. In 1991, the Government signed up to the Nitrates Directive which required the designation of Nitrate Vulnerable Zones (NVZs). These zones are water catchments where the nitrate concentration either exceeds the 50 mg L−1 limit or is at risk of doing so. Within NVZs, farmers are required to implement an Action Programme of measures to help reduce the amount of nitrate lost from agricultural land to the aquatic environment.

The cost of removing nitrates from drinking water is significant. Based on a figure of 80% of nitrate coming from agricultural land, an annual cost to the water industry 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 or inappropriately timed applications of organic nitrogen, such as animal manures derived from housed livestock. The potential for nitrate loss from manures is greater than from inorganic fertilizers because of difficulties in applying manures in a timely and accurate way and their inherent lower efficiency. The manures are often applied to arable land and grassland through autumn and winter according to farmer convenience and prevailing soil conditions or at rates well in excess of crop uptake. NVZ restrictions will limit the way in which manure is used over much of England in the future and should result in more efficient use of nitrogen in manures with a resultant reduction in inputs of mineral fertilizers.

Nitrate in soil is also subject to denitrification processes resulting in emissions of nitrous oxide to the atmosphere. Until recently, nitrous oxide emissions were almost equally derived from agriculture and other sources. However, moves to curb emissions from the adipic acid production process led to significant reductions in 1998. As a result, since 1999, agriculture has become by far the largest source of nitrous oxide entering the atmosphere. (Goodwin et al. 2001).

Although agriculture is now the major focus for action, nitrogen emissions from the combustion of fossil fuels are still a cause for concern not only for the environment but also for human health.

Combustion of fossil fuels

The combustion of fossil fuels is an important source of emissions of oxides of nitrogen (NOx). In addition, conversion of NOx in the atmosphere to nitrate can lead to their incorporation as secondary constituents in particles (PM10). In remote unpolluted regions, nitric oxide concentrations are generally only a small fraction of the NOx total. However, in polluted towns and cities where the oxidizing capacity of the air may be limited, NO concentrations often exceed those of NO2. The primary sources are the combustion of coal, oil and natural gas for energy production and use, including transportation. In the UK, total emissions of nitrogen oxides were 1·6 m tonnes in 2000. The transport sector is the major source of nitrogen oxides, responsible for about half of these emissions. However, levels of NOx have been falling, with a decrease of 46% since 1989 due to the introduction of catalytic converters and stricter emission limits from road transport and reductions in emissions from power stations (Goodwin et al. 2001).

Once emitted into the atmosphere, nitrogen oxides can contribute to a range of environmental and health problems. NOx is a key component in the formation of ground level ozone (‘photochemical smog’). Through this complex cycle of chemical reactions, the NOx is converted to nitrates which are incorporated into fine particles (PM10), often as ammonium nitrate where the ammonia has originated from the agricultural emissions. Another component of the atmospheric particles mix is ammonium sulphate, where ammonia has reacted with sulphuric acid produced by the reactions of sulphur dioxide, emitted largely from power generation and industry. These fine particles can cause visibility impairment, acid deposition, and excess nutrient inputs to semi-natural ecosystems. NOx as a precursor of ground level and tropospheric ozone is an indirect contributor to climate change and is involved in the reactions leading to stratospheric ozone depletion. In its role as a precursor in particulate formation and ozone creation at low levels in smog, NOx can cause premature death, chronic respiratory illness, such as bronchitis and asthma, and aggravation of existing respiratory problems. By increasing tropospheric ozone, NOx can also act indirectly as a greenhouse gas. In addition, it can damage plants and crops. Other environmental impacts of NOx emissions and deposition include die-back of trees, loss of biodiversity in grasslands and acidification of streams and lakes. These environmental impacts also involve nitrogen inputs from ammonia emissions.

The imbalances in the nitrogen cycle caused by Man's activities have resulted in losses to the environment causing a range of problems including eutrophication, soil acidification and greenhouse gas emissions. There have also been significant effects on human health, particularly from the combustion of fossil fuels. These are daunting problems for government, and particularly Defra, to address and science has a key role to play in informing policies to tackle these problems.

Meeting the scientific challenge

So what is Defra doing about these problems? Government 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 successful and has permanently removed previous fears of food shortages. With food security having declined as an issue, other priorities have begun to emerge. Today, society's values are more diverse and more sophisticated. With regard to agriculture they can include an attractive landscape, less pollution, protection and enhancement of biodiversity, high standards of animal welfare and disease control, improved recreational opportunities, positive impacts of agriculture in rural society, as well as meeting the objectives of food production and economic viability. For food, the values might 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 on board. They do not necessarily all match the needs and values of the agricultural industry, and particularly farmers, whose falling incomes present real difficulties in meeting the more demanding aspirations of society. Defra needs to find a way to balance these differing demands so that, for example, legislation or other controls minimize costs to farmers.

Defra's goal is to build a sustainable future for the UK and its ideas were published in June 2002 in the Department's Sustainable Development Strategy (Defra 2002c). The Department is not merely the merger of the functions of the former MAFF, and parts of DETR and the Home Office. It reflects the Government's determination to exploit the synergies that exist between environmental protection, rural affairs and food, farming and fisheries. The core aim of sustainable development involves a better quality of life for people now and for future generations. More specifically, the aim involves securing a better environment and sustainable use of natural resources together with economic prosperity for those industries primarily affected by the Department, thriving rural communities and a countryside for all to enjoy. In essence, this approach involves thinking in an integrated way about economic, environmental and social objectives. With regard to nitrogen pollution, this is a perfect illustration of the problem of sustainable development – if we are to go on producing quality agricultural goods, we must do so in a way that does little or no harm to the environment. To this end, the Government initiated a strategic review of diffuse water pollution from agriculture in June 2002. The aim of the review is to develop cost-effective and proportionate means of reducing water pollution levels to meet existing commitments and to encourage sustainable farming practices.

So how can science contribute to the Department's sustainability agenda in relation to the nitrogen problem? In particular, what science have we commissioned that has made a difference to our understanding of the problem and identified realistic solutions? Defra needs a robust evidence base upon which to develop its policies. This is where its science programmes have such a crucial role to play.

We have been commissioning research since the 1980s to address the problems caused by emissions of nitrogen in its various forms. At its height, some £6 m per year was being spent in the Nitrate research programme in the 1990s. In recent years funding on nitrates has declined as understanding of the problem has grown, though some important associated topics remain to be resolved, such as the development of abatement measures for nitrous oxide emissions. The focus also changed to address the emerging problem of phosphorus loss from agriculture.

With regard to combustion processes, the thrust of the current research on NOx is to quantify exposure to human populations and ecosystems and assess the effectiveness of abatement measures.

Whilst considerable work has been, and continues to be undertaken on these aspects of the nitrogen cycle, we would like to focus on emissions of ammonia. This topic is a good example of the complexity of the problems facing science and the difficulties of developing sustainable solutions that do not result in knock-on effects for other parts of the nitrogen cycle.

ammonia emissions

Funding for research on ammonia emissions dates from the early 1990s. About £1·4 m per year is now being spent by Defra on research on ammonia emissions and abatement measures with an extra £90 k for a monitoring network and an additional £900 k on atmospheric models and environmental impacts, much of which includes ammonia. This large commitment was in response to growing international concern about the damaging impact of ammonia on sensitive habitats through eutrophication and acidification processes. The research had a significant role in informing the UNECE Gothenburg Protocol and the National Emission Ceilings Directive. Under the Protocol and Directive, the UK has agreed to a legally binding emissions target of 297 kilotonnes of ammonia per year to be achieved by 2010. The emissions figure for the UK was 320 kilotonnes in 2000. The Protocol and Directive are due to be reviewed by signatories in 2004–05 with regard to compliance costs and the effectiveness of the agreed emission targets. It is therefore important that any future decisions on emissions targets are underpinned by robust scientific evidence for cost-effective abatement strategies. Current research on agricultural systems is aiming to improve estimates of ammonia losses from UK agriculture and assess the cost-effectiveness and practicality of abatement techniques.

One example of this research that has informed Defra's policies is the Ammonia Distribution and Effects Project known as ADEPT. This project used census data, satellite land cover data and estimates of source strength to map agricultural ammonia emissions to air across the UK. Key sources were identified as animal housing, manure stores, intensively grazed areas and landspreading of manures. Application of an atmospheric transport and deposition model indicated that up to 30% of the emitted ammonia is re-deposited locally (Sutton et al. 1998).

A subsequent Defra research project supported the findings from ADEPT on sources of ammonia (Chambers, Williams & Chadwick 2002). The project quantified ammonia fluxes for complete (housing, storage and land spreading) solid and liquid manure management systems. This showed that for beef cattle, losses were greater from liquid manure systems, at 37 kg NH3-N per 500 kg liveweight gain, than from the solid manure system, at 24 kg NH3-N per 500 kg liveweight gain. Most of the losses (70–75%) occurred during the housing phase. Therefore, targeting the housing phase, e.g. by switching from a liquid to a solid, straw-based system, has the potential to contribute to a reduction in ammonia emissions from agriculture. Ongoing work is looking at how to optimize straw additions to reduce ammonia emissions. This may include targeting additions to dirty areas within houses rather than blanket spreading.

There are also options for reducing ammonia emissions during spreading. Research has shown that application of cattle slurry by shallow injection, trailing shoe and band-spreading as compared with surface broadcast reduced ammonia emissions by a mean of 73, 57 and 26%, respectively, on grassland.

Defra's research has involved the development of models that quantify the inputs and outputs of the agricultural nitrogen cycle for different cropping systems. For manures, the Manure Nitrogen Evaluation Routine, known as MANNER, quantifies the fertilizer nitrogen value of manures and the fate of the nitrogen after spreading. As a Decision Support System, it assists farmers in planning their use of manures so as to encourage greater efficiency in the use of this valuable resource. Results from the model have achieved strong correlations with results from field experiments and many farmers and advisers are now using this tool.

Whilst modified techniques for spreading slurry may reduce ammonia emissions, the nitrogen applied to the land increases the soil mineral nitrogen pool, thereby increasing the potential for losses as nitrous oxide and nitrate. This is an example of pollution swapping, where modifications to the nitrogen cycle to reduce pollution may lead to problems elsewhere in the cycle. There are also other examples of pollution swapping that need to be resolved. For instance, if a farmer moves the date of applying manures to land from autumn to spring to satisfy nitrate leaching requirements under NVZ regulations, he could cause an increase in ammonia volatilization. Also, covering a slurry store to reduce ammonia volatilization can cause problems because the slurry will retain more nitrogen for application to fields and will therefore be at risk of creating losses following application. Pollution swapping represents a major challenge for researchers seeking to find sustainable solutions for manures use. Our approach to finding solutions must therefore avoid following a compartmentalized approach but must instead take into account the many interactions associated with the nitrogen cycle.

Atmospheric deposition of ammonia and oxides of nitrogen is largest in upland semi-natural areas, where high rainfall augments deposition, and in agricultural areas where ammonia per se is the problem. The critical loads concept has been developed to gauge the potential impacts of deposition on an ecosystem. When deposition exceeds the critical load, the ecosystem or some specified element of it, is at risk of damage. Defra supports a programme of work to quantify the critical load for a range of semi-natural ecosystems, and maps these values on a 1 × 1 km grid. By overlaying maps of deposition, produced from a combination of monitoring and atmospheric modelling, it is possible to identify those areas receiving unsustainable levels of deposition. About one-third of the area of UK ecosystems receives deposition of nitrogen above the critical load, and this is mainly due to ammonia.

In addition to funding research and monitoring, there are a number of other approaches Defra uses to strengthen its evidence base. A good example of a way in which we make the very best use of science is critical evaluation and assessment by a group of expert scientists. The National Expert Group on Transboundary Air Pollution (NEGTAP) recently produced a detailed evaluation of the scientific evidence for the effectiveness of our policies for the reduction of emissions affecting acidification, eutrophication and ground-level ozone (NEGTAP 2001). The resulting report is a definitive, high quality review of the responses of the atmosphere and of freshwaters and ecosystems to the significant changes which have occurred in emissions – particularly of sulphur and NOx– over the past 10 years or so as a result of Government policies. The group highlighted the problems caused by ammonia and ground-level ozone as a continuing cause for concern.

The sustained effects of nitrogen emissions over several decades have left a signal of nitrogen enrichment in the countryside. Defra has contributed funding to two national monitoring studies that have picked up this signal and their results were recently published.

eutrophication in the countryside

The New Atlas of British and Irish Flora published in 2002 (Preston, Pearman & Dines 2002) comprises the findings from a major survey of flowering plants and ferns undertaken in over 3800 10 × 10 kilometre squares across Britain and Ireland between 1987 and 1999. The survey is a repeat of the only previous national survey undertaken in the 1950s and published in 1962. The new survey provides the most complete record of British and Irish flora to date with 700 species mapped for the first time; many of these new species are recent introductions.

In a comparison of the results from the two surveys, species were categorized according to their nutrient requirements. The groupings were based on published Ellenberg indicator values where each species is allocated a number from 1 to 9, with 1 representing species typically found in extremely infertile sites and 9 covering species found in very fertile sites. An average change index for species with a particular indicator value was used to analyse the relative performance of that group. The results clearly show that species characteristic of less fertile sites have been less successful than those found in more fertile sites (Fig. 4). This pattern is repeated in almost all regions, although it is not as marked in Wales, Northern Ireland and eastern Scotland. The Scottish Highlands is the only region where it does not apply. This signal represents a marked change in the flora of Britain and Ireland over the last 40 years.

Figure 4.

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

The results from the Atlas survey support the findings from the Countryside Survey 2000 (CS2000, Haines-Young et al. 2000). CS2000 consisted of surveys of Britain and Northern Ireland involving a combination of detailed field recording and land cover satellite mapping. The aim of CS2000 was to provide information on the stock of habitats and landscape elements and the change in time of these elements. The survey was undertaken in 1998 and builds on earlier field surveys in Great Britain in 1978, 1984 and 1990 and satellite mapping in 1990. The survey was based on a stratified sample of 1 × 1 km squares.

Results from CS2000 show a clear increase in fertility score across a range of habitats. These results together with those from the Atlas are consistent with the changes expected from widespread eutrophication in the countryside. It is very likely that nitrogen deposition from the atmosphere will have contributed to these changes, but changes in land management practices in the last decades may also have played a role.

Although the adoption of individual abatement measures may help to reduce the impact of eutrophication, an alternative option is to take a more holistic approach by following a different system of farming, for example, organic farming.

organic farming

Defra spends about £2 m each year on organic farming research. Organic farming is considered to deliver significant benefits in terms of biodiversity and resource protection. These flow both from the strict limitation on the inputs which organic farmers are permitted to use and from the system of management which organic farmers are required to adopt. Organic farmers have available to them only a limited range of, mostly natural, pesticides and are not permitted to use synthetic fertilizers and herbicides. The rotation, which is a central feature of organic systems, encourages soil health both in terms of fertility and in terms of microbial and invertebrate activity. It also enhances local landscapes and biodiversity by preventing monoculture. The evidence base for nitrogen losses is sparse but Defra research has provided some insights.

With regard to nitrate loss, many organic farms have been found to be operating at a lower level of nitrogen intensity than conventional systems, with nitrogen inputs derived from fixation by legumes or importation of animal feed onto the farm. Data are limited, but a comparison of farms in Nitrate Sensitive Areas (forerunners to NVZs) with organic equivalents found that nitrate losses were similar on area basis if grass sites receiving more than 200 kg ha−1 of fertilizer N were excluded.

For ammonia, although intensive poultry and pig units are not permitted under organic standards, composting of manures is encouraged which leads to relatively high losses. However, organically produced manures often have a lower concentration of nitrogen than their conventionally produced equivalents, and composting reduces losses during subsequent spreading. In comparing farming systems, it is important to distinguish between losses per unit area and those per unit of yield. Whilst it seems likely that there is little difference between organic and conventional systems for losses per unit of yield, it is considered likely that there are lower losses per unit area.

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

Although organic farming may be attractive to some, conventional farming still accounts for about 97% of farming in England. As a result, abatement measures that address the requirements of conventional agriculture will have to remain the higher priority for Defra.

The future

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

There are also the over-arching demands of society for cheap, safe, high-quality food produced with less pollution of the environment, protection and enhancement of biodiversity and high standards of animal welfare and disease control. These demands are reflected in increasingly stringent emission standards agreed in international fora.

One major piece of European legislation that is expected to have profound effects on resource protection in the UK and the rest of Europe is the Water Framework Directive. The Directive is the most substantial piece of EU water legislation ever drawn up. For the first time, ecological quality objectives will be set for surface waters. There will also be a requirement to produce strategic management plans for each river basin. These plans are likely to have knock-on benefits for coastal waters. All water quality targets will have to be met by 2015 and the legislation will be transposed into domestic legislation in 2003.

The requirements of future EU legislation, such as the Water Framework Directive, will require high-quality science to inform domestic policy formulation. In addition, other future policy initiatives will need to be anticipated as early as possible to enable appropriate science programmes to be established. These will provide the necessary information for Defra to negotiate with other countries from a position of strength. For this purpose, science must provide solutions that are based on well-researched evidence and that provide sensible, cost-effective abatement strategies.

New methodologies, such as use of GM technologies, may provide opportunities for meeting future stringent targets. We are all too familiar with the development of GM crops with, for example, specific herbicide tolerance or resistance to insect pests. It is quite possible that these ‘new’ plants may become acceptable in the future but, for the present, we are still awaiting evaluation. Results from the Field Scale Evaluation project, which covers possible effects on biodiversity, are due to be published in autumn 2003. Until these results are known, we will not be able to make firm decisions on the future use of such technologies in the farming sector. The use of these technologies to develop GM products can, if applied responsibly, potentially provide a wide range of benefits for society.

With regard to addressing nitrogen losses, research teams in the UK are actively seeking a way of inducing plants to take up symbiotic bacteria that will fix nitrogen from the atmosphere. Whilst there have been some successes in the laboratory, there have been no field trials demonstrating that this is a viable option. An alternative approach is to use GM technologies to increase plant nitrogen-use efficiency rather than fixation, for example, in wheat. Some research has been funded by Defra but further work is required to demonstrate the feasibility of this approach. Overall, it is evident that additional studies are required to explore fully the potential of GM technologies for providing solutions to the nitrogen emissions problem.

Defra will continue to ensure that GM organisms are only approved for release if they are judged not to pose an unacceptable risk to public health or the environment. In this context, the Government is promoting an open debate on the issues relating to the future of genetically modified organisms in the UK. One element that will inform the debate is a review of the science surrounding GM issues.

The Science review will support the main independent public debate in its ambition to fully explore the issues relating to GM technologies. Scientists across disciplines, and members of the public with an interest in the relevant science, are encouraged to participate via the website that has been launched at

We believe this will be an exciting venture. It will enable us to take a really comprehensive and open look at the science relevant to GM with the focus on crops and foods, and will do so in a way that recognizes the interests and concerns of the public. For it to be a success we will need to harness the enthusiasm of as many scientists as possible and we hope members of the British Ecological society will take an active part.

Another new advance that is showing promise is precision farming. The electronics revolution has spawned two technologies that will impact agriculture in the next decade. These technologies are Geographic Information Systems (GIS) and Global Positioning Systems (GPS). Along with GIS and GPS, a wide range of sensors, monitors and controllers are being developed for agricultural equipment. Together they will enable farmers to use electronic guidance aids to provide precise positioning for all equipment and chemical applications. For fertilizer applications, research is developing variable rate controllers for granular, liquid and gaseous fertilizer materials. These variable rates can be automatically controlled by the farmer using an on-board computer with an electronic prescription map based on yield mapping and other data. This should result in a reduction in overall nitrogen use on a field whilst maintaining yields.

It is generally accepted that climate change is likely to have far-reaching effects on the environment in future decades. Research by Defra, in collaboration with other stakeholders, is seeking to determine the nature and extent of these effects. It is difficult to predict what the impacts might be on the nitrogen cycle. However, warmer, wetter winters in some parts of the country could result in increased nitrogen losses by leaching, together with increased emissions of nitrous oxide by denitrification. The availability of nitrogen from mineralization of soil organic matter may also increase. These changes are likely to affect the accuracy of models that are based on current conditions. A scoping study recently commissioned by Defra will be a first step towards predicting the impact of a changed climate on nutrient losses (Hossell 2003).

Finally, CAP reform could provide benefits for the environment, for example, through increased funding for agri-environment schemes and general de-coupling of subsidies from production. Such reform should encourage the adoption of less intensive farming methods, including reductions in the use of fertilizers. Indeed, the Commission's mid-term review paints an encouraging picture for sustainable farming in Europe but the UK will need to maintain pressure on the Commission to effect the changes that it is seeking. Science will have an important role to play in the success of any domestic policy initiatives flowing from CAP reform.

Looking across the many challenges for the future, it seems as though we may be at something of a crossroads. On the one hand, agriculture could follow a path where environmental protection and enhancement is at the core of farming activities. However, farmers may decide to follow another path characterized by technological innovation and involving further intensification. Of course, these approaches are not necessarily mutually exclusive. In the spirit of Sir Don Curry's report (Policy Commission on the Future of Farming and Food 2002), it may be possible for farmers to pick the best from both paths and combine them successfully into their businesses. However, their decisions will be based on what is most appropriate for their particular circumstances. Whatever the outcome of these decisions, it is evident that there are significant pressures building that could have a considerable impact on the future face of UK agriculture.

In conclusion, we hope that we have demonstrated Defra's commitment to tackling the nitrogen problem as part of its overall drive to make sustainable development a reality. We have only been able to scratch the surface of the Department's many activities in this area. Defra is striving to integrate the needs of the environment, industry and rural communities. This is the essence of sustainable development and is described in more detail in the Department's Sustainable Farming and Food Strategy which was launched in December 2002 (Defra 2002d). The dilemma of the nitrogen problem for sustainable development is to understand what level and type of agriculture/land management is economically viable whilst also meeting environmental protection and health requirements. It is a challenging target but it is the yardstick by which the Department's success will be measured. Science has much to offer in bringing about the realization of this goal and my job will be to ensure that Defra's science is at the heart of the Department's policies for sustainable development.


We would like to thank colleagues in Defra and other organizations who contributed to the preparation of this article. We are particularly grateful to Peter Costigan and Alison Vipond in Defra for their valuable comments.