Executive summary

Nature of the problem

• Reactive nitrogen (Nr) is of fundamental importance in biological and chemical processes in the atmosphere–biosphere system, altering the Earth's climate balance in many ways. These include the direct and indirect emissions of nitrous oxide (N2O), atmospheric Nr deposition and tropospheric ozone formation (O3), both of which alter the biospheric CO2 sink, Nr supply effects on CH4 emissions, and the formation of secondary atmospheric aerosols resulting from the emissions of nitrogen oxides (NOx) and ammonia (NH3).

• Human production and release of Nr into the environment is thus expected to have been an important driver of European greenhouse balance. Until now, no assessment has been made of how much of an effect European Nr emissions are having on net warming or cooling.

Approaches

• This chapter summarizes current knowledge of the role of Nr for global warming. Particular attention is given to the consequences of atmospheric Nr emissions. The chapter draws on inventory data and review of the literature to assess the contribution of anthropogenic atmospheric Nr emissons to the overall change in radiative forcing (between 1750 and 2005) that can be attributed to activities in Europe.

• The use of Nr fertilizers has major additional effects on climate balance by allowing increased crop and feed production and larger populations of livestock and humans, but these indirect effects are not assessed here.

Executive summary

Nature of the problem

• Anthropogenic releases of reactive nitrogen (Nr) can disturb natural systems and affect human health and welfare in many different ways. Scientific and policy views of the nitrogen cycle have typically addressed these problems from separate perspectives, looking in each case at only part of the overall issue.

• Given the multi-faceted nature of the nitrogen cycle, it is a major challenge to develop a more-integrated understanding of how different areas of nitrogen science and policies fit together.

Approaches

• Observations from the first part of the European Nitrogen Assessment (ENA Part I) are summarized, considering the distinctive character of Nr in Europe, the benefits and threats, and the current policies. Approaches to developing the following parts of the Assessment are discussed with an emphasis on how to draw out the key issues.

Key findings

• Recognizing the multi-pollutant, multi-phase complexity of the nitrogen cycle, it is concluded that it is essential to focus on a limited set of priority issues to allow effective communication between nitrogen scientists and policy makers.

• A pathway is developed for prioritization of the key environmental concerns of excess Nr. Starting with around twenty environmental effects, the list is reduced down, first to nine main concerns, and then to five key societal threats.

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Executive summary

Nature of the problem

• Most regional watersheds in Europe constitute managed human territories importing large amounts of new reactive nitrogen.

• As a consequence, groundwater, surface freshwater and coastal seawater are undergoing severe nitrogen contamination and/or eutrophication problems.

Approaches

• A comprehensive evaluation of net anthropogenic inputs of reactive nitrogen (NANI) through atmospheric deposition, crop N fixation, fertiliser use and import of food and feed has been carried out for all European watersheds. A database on N, P and Si fluxes delivered at the basin outlets has been assembled.

• A number of modelling approaches based on either statistical regression analysis or mechanistic description of the processes involved in nitrogen transfer and transformations have been developed for relating N inputs to watersheds to outputs into coastal marine ecosystems.

Key findings/state of knowledge

• Throughout Europe, NANI represents 3700 kgN/km²/yr (range, 0–8400 depending on the watershed), i.e. five times the background rate of natural N2 fixation.

• A mean of approximately 78% of NANI does not reach the basin outlet, but instead is stored (in soils, sediments or ground water) or eliminated to the atmosphere as reactive N forms or as N2.

• N delivery to the European marine coastal zone totals 810 kgN/km²/yr (range, 200–4000 depending on the watershed), about four times the natural background. In areas of limited availability of silica, these inputs cause harmful algal blooms.

A new challenge from a past solution

Human perturbation of the nitrogen cycle represents a major example of global geo-engineering. Historically, the limited availability of reactive nitrogen compounds has provided a key constraint to human activities. Although the element nitrogen is extremely abundant, making up 78% of the Earth's atmosphere, it exists mainly as unreactive di-nitrogen (N2). By contrast, to be useable by most plants and animals, reactive nitrogen (Nr) forms are needed. These include oxidized and reduced nitrogen compounds, such as nitric acid, ammonia, nitrates, ammonium and organic nitrogen compounds, each of which is normally scarce in the natural environment.

The two main historical needs for reactive nitrogen have been to provide fertilizers to increase food production and as a basis for the manufacture of munitions. Biological nitrogen fixation has always added new reactive nitrogen into the system, but the inputs have been barely sufficient for human needs. As a result, traditional agricultural production was highly dependent on effective recycling of nitrogen in manures.

By the end of the nineteenth century, an increasing human population combined with expanding military needs required that large amounts of extra reactive nitrogen be added into circulation. These demands were met by increased mining of reactive nitrogen deposits, including Chile saltpetre and guano, supplemented by the extraction of reactive nitrogen from coal and peat (Vincent, 1901; Clow and Clow, 1952; Watt, 2003; Sutton et al., 2008).

Main messages

Too much nitrogen harms the environment and the economy

• Over the past century humans have caused unprecedented changes to the global nitrogen cycle, converting atmospheric di-nitrogen (N2) into many reactive nitrogen (Nr) forms, doubling the total fixation of Nr globally and more than tripling it in Europe.

• The increased use of Nr as fertilizer allows a growing world population, but has considerable adverse effects on the environment and human health. Five key societal threats of Nr can be identified: to water quality, air quality, greenhouse balance, ecosystems and biodiversity, and soil quality.

• Cost–benefit analysis highlights how the overall environmental costs of all Nr losses in Europe (estimated at €70–€320 billion per year at current rates) outweigh the direct economic benefits of Nr in agriculture. The highest societal costs are associated with loss of air quality and water quality, linked to impacts on ecosystems and especially on human health.

Nitrogen cascade and budgets

• The different forms of Nr inter-convert through the environment, so that one atom of Nr may take part in many environmental effects, until it is immobilized or eventually denitrified back to N2. The fate of anthropogenic Nr can therefore be seen as a cascade of Nr forms and effects. The cascade highlights how policy responses to different Nr forms and issues are inter-related, and that a holistic approach is needed, maximizing the abatement synergies and minimizing the trade-offs.

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Executive summary

Nature of the problem

• Environmental problems related to nitrogen concern all economic sectors and impact all media: atmosphere, pedosphere, hydrosphere and anthroposphere.

• Therefore, the integration of fluxes allows an overall coverage of problems related to reactive nitrogen (Nr) in the environment, which is not accessible from sectoral approaches or by focusing on specific media.

Approaches

• This chapter presents a set of high resolution maps showing key elements of the N flux budget across Europe, including N2 and Nr fluxes.

• Comparative nitrogen budgets are also presented for a range of European countries, highlighting the most efficient strategies for mitigating Nr problems at a national scale. A new European Nitrogen Budget (EU-27) is presented on the basis of state-of-the-art Europe-wide models and databases focusing on different segments of Europe's society.

Key findings

• From c. 18 Tg Nr yr−1 input to agriculture in the EU-27, only about 7 Tg Nr yr−1 find their way to the consumer or are further processed by industry.

• Some 3.7 Tg Nr yr−1 is released by the burning of fossil fuels in the EU-27, whereby the contribution of the industry and energy sectors is equal to that of the transport sector. More than 8 Tg Nr yr−1 are disposed of to the hydrosphere, while the EU-27 is a net exporter of reactive nitrogen through atmospheric transport of c. 2.3 Tg Nr yr−1.

• The largest single sink for Nr appears to be denitrification to N2 in European coastal shelf regions (potentially as large as the input of mineral fertilizer, about 11 Tg N yr–1 for the EU-27); however, this sink is also the most uncertain, because of the uncertainty of Nr import from the open ocean.

Executive summary

Nature of the problem

• Anthropogenic increase of nitrogen in water poses direct threats to human and aquatic ecosystems. High nitrate concentrations in drinking water are dangerous for human health. In aquatic ecosystems the nitrogen enrichment produces eutrophication, which is responsible for toxic algal blooms, water anoxia, fish kills and habitat and biodiversity loss.

• The continuous nitrogen export to waters reduces the capacity of aquatic ecosystems to absorb, reorganise and adapt to external stress, increasing their vulnerability to future unexpected natural or climate events.

Key findings/state of knowledge

• Nitrogen concentrations in European rivers, lakes, aquifers and coastal waters are high in many regions. In addition nitrate concentrations are increasing in groundwaters, threatening the long term quality of the resource.

• In Europe, nitrogen pressures occur over large areas, implying elevated costs for meeting the long-term good chemical and ecological water quality requirements. A significant part of the European population could be potentially exposed to high nitrate values in drinking water if adequate treatments were not in place. Furthermore many of European aquatic ecosystems are eutrophic or at risk of eutrophication.

• Nitrogen pressures have reduced biodiversity and damaged the resilience of aquatic ecosystems and continue to pose a threat to the aquatic environment and to the provision of goods and services from the aquatic ecosystems.

• Even under favourable land use scenarios the nitrogen export to European waters and seas is likely to remain significant in the near future. The effects of climate change on nitrogen export to water are still uncertain.

Executive summary

Nature of the problem

• Freshwater ecosystems play a key role in the European nitrogen (N) cycle, both as a reactive agent that transfers, stores and processes N loadings from the atmosphere and terrestrial ecosystems, and as a natural environment severely impacted by the increase of these loadings.

Approaches

• This chapter is a review of major processes and factors controlling N transport and transformations for running waters, standing waters, groundwaters and riparian wetlands.

Key findings/state of knowledge

• The major factor controlling N processes in freshwater ecosystems is the residence time of water, which varies widely both in space and in time, and which is sensitive to changes in climate, land use and management.

• The effects of increased N loadings to European freshwaters include acidification in semi-natural environments, and eutrophication in more disturbed ecosystems, with associated loss of biodiversity in both cases.

• An important part of the nitrogen transferred by surface waters is in the form of organic N, as dissolved organic N (DON) and particulate organic N (PON). This part is dominant in semi-natural catchments throughout Europe and remains a significant component of the total N load even in nitrate enriched rivers.

• In eutrophicated standing freshwaters N can be a factor limiting or co-limiting biological production, and control of both N and phosphorus (P) loading is often needed in impacted areas, if ecological quality is to be restored.

Part I Nitrogen in Europe: the present position

Nitrogen inheritance

1. Gaseous di-nitrogen (N2) constitutes 78% of the earth's atmosphere. It is a rather inert chemical, being nearly unavailable for the biological cycle. The other nitrogen forms are much more reactive; these include nitrate (NO3−), ammonium (NH4+) and ammonia (NH3), gaseous nitrogen oxides (NOx), nitrous oxide (N2O) and many other inorganic and organic nitrogen forms. Collectively, they are termed ‘reactive nitrogen’ (Nr). They are normally scarce in natural environments, with their low availability limiting the productivity of natural ecosystems. This was also the case for agricultural production before 1900, which long remained dependent on the recycling of Nr in human waste and manure, and the capacity of legumes to fix atmospheric N2 biologically.

2. With a growing human population through the nineteenth century and the need for more Nr, Europe increasingly operated a ‘fossil nitrogen economy’, dependent on the addition of nitrogen fertilizers from mined sources, including from guano, coal and saltpetre. The ‘nitrogen problem’ of the time was that these sources were fast becoming insufficient to meet Europe's escalating need for fertilizer Nr, and its military need for Nr in explosives [1.1].