The global nitrogen cycle in the twenty-first century

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 (NO x ), 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.