The global nitrogen cycle in the twenty-first century

Abstract

Global nitrogen fixation contributes 413 Tg of reactive nitrogen (N r) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic N r are on land (240 Tg N yr−1) within soils and vegetation where reduced N r contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer N r contribute to nitrate (NO 3−) in drainage waters from agricultural land and emissions of trace N r compounds to the atmosphere. Emissions, mainly of ammonia (NH 3) 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 (NH 4 NO 3) and ammonium sulfate (NH 4)2 SO 4. Leaching and riverine transport of NO 3 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 N r. Some of the marine N r is buried in sediments, the remainder being denitrified back to the atmosphere as N 2 or N 2 O. 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 N r in the atmosphere, with the exception of N 2 O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 10 2–10 3 years), the lifetime is a few decades. In the ocean, the lifetime of N r is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N 2 O that will respond very slowly to control measures on the sources of N r from which it is produced.