A. F. Bouwman

Summary for policy makers

Over the past century humans have caused unprecedented • changes to the global nitrogen cycle, converting atmospheric di-nitrogen (N 2 ) into many reactive nitrogen (N r ) forms, doubling the total fi xation of N r globally and more than tripling it in Europe. Th e increased use of N • r as fertilizer allows a growing world population, but has considerable adverse eff ects on the environment and human health. Five key societal threats of N r can be identifi ed: to water quality, air quality, greenhouse balance, ecosystems and biodiversity, and soil quality. Cost–benefi t analysis highlights how the overall environ• mental costs of all N r losses in Europe (estimated at €70–€320 billion per year at current rates) outweigh the direct economic benefi ts of N r in agriculture. Th e highest societal costs are associated with loss of air quality and water quality, linked to impacts on ecosystems and especially on human health.

A global high‐resolution emission inventory for ammonia

A global emissions inventory for ammonia (NH3) has been compiled for the main known sources on a 1° × 1° grid, suitable for input to global atmospheric models. The estimated global emission for 1990 is about 54 Tg N yr−1. The major sources identified include excreta from domestic animals (21.6 Tg N yr−1) and wild animals (0.1 Tg N yr−1), use of synthetic N fertilizers (9.0 Tg N yr−1), oceans (8.2 Tg N yr−1), biomass burning (5.9 Tg N yr−1), crops (3.6 Tg N yr−1), human population and pets (2.6 Tg N yr−1), soils under natural vegetation (2.4 Tg N yr−1), industrial processes (0.2 Tg N yr−1 ), and fossil fuels (0.1 Tg N yr−1). About half of the global emission comes from Asia, and about 70% is related to food production. The regions with highest emission rates are located in Europe, the Indian subcontinent, and China, reflecting the patterns of animal densities and type and intensity of synthetic fertilizer use. The overall uncertainty in the global emission estimate is 25%, while the uncertainty in regional emissions is much greater. As the global human population will show considerable growth in the coming decades, food production and associated NH3 emissions are likely to increase as well.

Direct emission of nitrous oxide from agricultural soils

This analysis is based on published measurements of nitrous oxide (N2O) emission from fertilized and unfertilized fields. Data was selected in order to evaluate the importance of factors that regulate N2O production, including soil conditions, type of crop, nitrogen (N) fertilizer type and soil and crop management. Reported N2O losses from anhydrous ammonia and organic N fertilizers or combinations of organic and synthetic N fertilizers are higher than those for other types of N fertilizer. However, the range of management and environmental conditions represented by the data set is inadequate for use in estimating emission factors for each fertilizer type. The data are appropriate for estimating the order of magnitude of emissions. The longer the period over which measurements are made, the higher the fertilizer-induced emission. Therefore, a simple equation to relate the total annual direct N2O−N emission (E) from fertilized fields to the N fertilizer applied (F), was based on the measurements covering periods of one year: E=1+1.25×F, with E and F in kg N ha-1 yr-1. This relationship is independent of the type of fertilizer. Although the above regression equation includes considerable uncertainty, it may be appropriate for global estimates.

Global river nutrient export: A scenario analysis of past and future trends

[1] An integrated modeling approach was used to connect socioeconomic factors and nutrient management to river export of nitrogen, phosphorus, silica and carbon based on an updated Global NEWS model. Past trends (1970–2000) and four future scenarios were analyzed. Differences among the scenarios for nutrient management in agriculture were a key factor affecting the magnitude and direction of change of future DIN river export. In contrast, connectivity and level of sewage treatment and P detergent use were more important for differences in DIP river export. Global particulate nutrient export was calculated to decrease for all scenarios, in part due to increases in dams for hydropower. Small changes in dissolved silica and dissolved organics were calculated for all scenarios at the global scale. Population changes were an important underlying factor for river export of all nutrients in all scenarios. Substantial regional differences were calculated for all nutrient elements and forms. South Asia alone accounted for over half of the global increase in DIN and DIP river export between 1970 and 2000 and in the subsequent 30 years under the Global Orchestration scenario (globally connected with reactive approach to environmental problems); DIN river export decreased in the Adapting Mosaic (globally connected with proactive approach) scenario by 2030, although DIP continued to increase. Risks for coastal eutrophication will likely continue to increase in many world regions for the foreseeable future due to both increases in magnitude and changes in nutrient ratios in river export.

The global nitrogen cycle in the twenty- first century

Global nitrogen fixation contributes 413 Tg of reactive nitrogen (Nr) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic Nr are on land (240 Tg N yr−1) within soils and vegetation where reduced Nr contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer Nr contribute to nitrate (NO3−) in drainage waters from agricultural land and emissions of trace Nr compounds to the atmosphere. Emissions, mainly of ammonia (NH3) from land together with combustion related emissions of nitrogen oxides (NOx), contribute 100 Tg N yr−1 to the atmosphere, which are transported between countries and processed within the atmosphere, generating secondary pollutants, including ozone and other photochemical oxidants and aerosols, especially ammonium nitrate (NH4NO3) and ammonium sulfate (NH4)2SO4. Leaching and riverine transport of NO3 contribute 40–70 Tg N yr−1 to coastal waters and the open ocean, which together with the 30 Tg input to oceans from atmospheric deposition combine with marine biological nitrogen fixation (140 Tg N yr−1) to double the ocean processing of Nr. Some of the marine Nr is buried in sediments, the remainder being denitrified back to the atmosphere as N2 or N2O. The marine processing is of a similar magnitude to that in terrestrial soils and vegetation, but has a larger fraction of natural origin. The lifetime of Nr in the atmosphere, with the exception of N2O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 102–103 years), the lifetime is a few decades. In the ocean, the lifetime of Nr is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N2O that will respond very slowly to control measures on the sources of Nr from which it is produced.

Uncertainties in the global source distribution of nitrous oxide

Inventories with 1°×1° resolution were compiled of nitrous oxide (N2O) emissions from fertilized arable land, animal excreta, postclearing effects on soil emissions, fossil fuel and fuelwood combustion, and industrial N2O sources. For other sources of N2O, including soils under natural vegetation, oceans, and biomass burning, published inventories were used. From these inventories the annual N2O emission was calculated for four broad latitudinal zones covering the globe. Uncertainties were assessed by comparing variants of inventories with source estimates inferred from inverse modeling techniques. Major uncertainties occur in the tropics, where small errors in both soil and oceanic emission estimates may have large repercussions for the zonal distributions. Although there may still be many poorly known and unidentified N2O sources, the analysis has resulted in improved understanding of some sources, i.e., (1) the oceanic N2O emission may be more important than assumed in recent global N2O budgets, with a major portion stemming from the 30°–90°S zone; (2) the N2O emission from animal excreta forms a significant global source; (3) most of the N2O from arable lands and grasslands, including effects of synthetic fertilizers and animal excreta, comes from the northern hemisphere; accounting for only the synthetic-fertilizer effect on N2O emission leads to an underestimation of the emission from arable lands; (4) fossil fuel combustion and industrial N2O sources are dominant in the 30°–90°N zone, while N2O from fuelwood combustion is mainly produced in the 0°–30°N zone; (5) the estimation of enhanced N2O soil emission following tropical forest clearing that has accounted for gradually declining N2O fluxes, along with aging of the clearing leads to a global emission that is significant but lower than previous estimates; (6) most of the N2O from coastal marine and freshwater systems and soil N2O emission resulting from N deposition probably comes from the northern hemisphere.

Global air emission inventories for anthropogenic sources of NOx, NH3 and N2O in 1990

Abstract Global emission inventories with 1° × 1° resolution were compiled for nitrogen oxides (NO + NO 2 , together denoted as NO x ), ammonia (NH 3 ) and nitrous oxide (N 2 O) emissions. For NO x the estimated global anthropogenic emission for 1990 is about 31 million ton N year −1 . The major anthropogenic sources identified include fossil fuel combustion (70%, of which the major sources are road transport and power plants) and biomass burning (20%). Natural sources contribute about 19 million ton N year −1 , mainly lightning and soil processes. For NH 3 the estimated global emission for 1990 is about 54 million ton N year −1 . The major sources identified include excreta from domestic animals and wild animals, use of synethetic N fertilisers, oceans and biomass burning. About half of the global emission comes from Asia, and about 70% is related to food production. For N 2 O the major sources considered include fertilised arable land, animal excreta, soils under natural vegetation, oceans, and biomass burning. The global source of N 2 O is about 15 million ton N 2 O-N year −1 of which about 30% is related to food production. All three inventories are available on a sectoral basis on a 1° × 1° grid for input to global atmospheric models and on a regional/country basis for policy analysis.

A Global Analysis of Acidification and Eutrophication of Terrestrial Ecosystems

This paper presents an explorative, quantitative analysis of acidification and eutrophication of natural terrestrial ecosystems caused by excess sulfur (S) and nitrogen (N) deposition. The analysis is based on a steady-state approach, involving the comparison of deposition fluxes with critical loads to identify areas where critical loads are exceeded. Deposition fields for sulfur and nitrogen were obtained from the STOCHEM global chemistry-transport model, and they were combined with estimated base cation deposition to derive net acid deposition fluxes. The results indicate that the critical loads for acidification are exceeded in 7–17% of the global area of natural ecosystems. In addition, comparison of nitrogen deposition with critical loads for eutrophication yielded an exceedance in 7–18% of the global natural ecosystems. Apart from serious problems in the heavily industrialized regions of eastern USA, Europe, the former Soviet Union, and large parts of Asia, risks are also found in parts of South America, and West, East and Southern Africa. Both acidification and eutrophication risks could significantly increase in Asia, Africa and South America in the near future, and decrease in North America and Western Europe. Accounting for the effects of N in the analysis of acidification significantly enlarges the potentially affected areas and moves them away from highly industrialized areas compared to studies considering S deposition alone. Major uncertainties in the approach followed are associated with upscaling, the estimates of S, N and base cation emission and deposition fluxes, the critical loads to describe ecosystem vulnerability and the treatment of soil N immobilization and denitrification.

Global Patterns of Dissolved Inorganic and Particulate Nitrogen Inputs to Coastal Systems: Recent Conditions and Future Projections

We examine the global distribution of dissolved inorganic nitrogen (DIN) and particulate nitrogen (PN) export to coastal systems and the effect of human activities and natural processes on that export. The analysis is based on DIN and PN models that were combined with spatially explicit global databases. The model results indicate the widely uneven geographic distribution of human activities and rates of nitrogen input to coastal systems at the watershed, latitudinal, and regional-continental scales. Future projections in a business-as-usual scenario indicate that DIN export rates increase from approximately 21 Tg N yr−1 in 1990 to 47 Tg N yr−1 by 2050. Increased DIN inputs to coastal systems in most world regions are predicted by 2050. The largest increases are predicted for Southern and Eastern Asia, associated with predicted large increases in population, increased fertilizer use to grow food to meet the dietary demands of that population, and increased industrialization. Results of an alternative scenario for North America and Europe in 2050 indicate that reductions in the human consumption of animal protein could reduce fertilizer use and result in substantial decreases in DIN export rates by rivers. In another scenario for 2050, future air pollution control in Europe that would reduce atmospheric deposition of nitrogen oxides in watersheds is predicted to decrease DIN export by rivers, particularly from Baltic and North Atlantic watersheds. Results of a newly developed global PN river export model indicate that total global PN and DIN export by rivers in 1990 are similar, even though the global distribution of the two differ considerably.

Global nitrogen and phosphate in urban wastewater for the period 1970 to 2050

[1] This paper presents estimates for global N and P emissions from sewage for the period 1970–2050 for the four Millennium Ecosystem Assessment scenarios. Using country-specific projections for population and economic growth, urbanization, development of sewage systems, and wastewater treatment installations, a rapid increase in global sewage emissions is predicted, from 6.4 Tg of N and 1.3 Tg of P per year in 2000 to 12.0–15.5 Tg of N and 2.4–3.1 Tg of P per year in 2050. While North America (strong increase), Oceania (moderate increase), Europe (decrease), and North Asia (decrease) show contrasting developments, in the developing countries, sewage N and P discharge will likely increase by a factor of 2.5 to 3.5 between 2000 and 2050. This is a combined effect of increasing population, urbanization, and development of sewage systems. Even in optimistic scenarios for the development of wastewater treatment systems, global N and P flows are not likely to decline.