Human activities have accelerated the earth’s nitrogen (N) cycle by increasing the natural rate of N2-fixation.

Nitrogen fixation is the transformation of the highly abundant but biologically unavailable atmospheric dinitrogen (N₂) to 'reactive' reduced and oxidised N forms such as ammonia (NH₃), nitrate (NO₃-), nitrous oxide (N₂O) and nitric oxide (NO). Anthropogenic N fixation occurs in N fertiliser (80 Tg yr^-1) and energy (30 Tg yr^-1) production and cultivation of leguminous crops in agricultural systems (40 Tg yr^-1). The total human-induced increase of global N fixation is about 150 Tg yr^-1 and about equal to the natural rate of biological N fixation. The biological fixation occurs by specialized bacteria and algae, free-living or in a symbiotic relationship with higher plants and lightning.

Accelerated global N cycle

While most of the reactive N emissions occur locally in the terrestrial system, these emissions have both regional and global influence as they move through water and air across political and geographic boundaries. Thus the accelerated N cycling has consequences for the atmosphere, biosphere and hydrosphere. Several PBL (formerly MNP, RIVM) studies have been aimed at estimating the major global fluxes of reactive nitrogen.

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For more on integral nitrogen see

• Reactive nitrogen
• Global nitrogen cycle
• Gas fluxes from N₂O, NO and NH₃
• Surface N balance
• Reactive N in the global hydrologic system
• Denitrification

For policy analyses see

• Policy analyses