title: The global nitrogen cycle in the twenty-first century.
abstract: 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 10(2)-10(3) 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.
authors.affiliation: NERC Centre for Ecology and Hydrology, Penicuik, UK. dfo@ceh.ac.uk
authors.forename: David
authors.initials: D
authors.lastname: Fowler
authors.forename: Mhairi
authors.initials: M
authors.lastname: Coyle
authors.forename: Ute
authors.initials: U
authors.lastname: Skiba
authors.forename: Mark A
authors.initials: MA
authors.lastname: Sutton
authors.forename: J Neil
authors.initials: JN
authors.lastname: Cape
authors.forename: Stefan
authors.initials: S
authors.lastname: Reis
authors.forename: Lucy J
authors.initials: LJ
authors.lastname: Sheppard
authors.forename: Alan
authors.initials: A
authors.lastname: Jenkins
authors.forename: Bruna
authors.initials: B
authors.lastname: Grizzetti
authors.forename: James N
authors.initials: JN
authors.lastname: Galloway
authors.forename: Peter
authors.initials: P
authors.lastname: Vitousek
authors.forename: Allison
authors.initials: A
authors.lastname: Leach
authors.forename: Alexander F
authors.initials: AF
authors.lastname: Bouwman
authors.forename: Klaus
authors.initials: K
authors.lastname: Butterbach-Bahl
authors.forename: Frank
authors.initials: F
authors.lastname: Dentener
authors.forename: David
authors.initials: D
authors.lastname: Stevenson
authors.forename: Marcus
authors.initials: M
authors.lastname: Amann
authors.forename: Maren
authors.initials: M
authors.lastname: Voss
mesh_terms: D000383:Agriculture / Q000379:methods; D000397:Air Pollution / Q000032:analysis* / Q000266:history; D001272:Atmosphere / Q000737:chemistry*; D017753:Ecosystem; D049674:History, 21st Century; D058458:Nitrogen Cycle; D009586:Nitrogen Fixation / Q000502:physiology*; D010084:Oxidation-Reduction; D026361:Reactive Nitrogen Species / Q000032:analysis* / Q000737:chemistry; D012623:Seawater / Q000737:chemistry*
chemical_list: D026361:Reactive Nitrogen Species
keywords: denitrification; deposition; emissions; global budgets; nitrogen fixation