Nitrogen cycle

Nitrogen circulates through the atmosphere, biosphere, hydrosphere and geosphere. About 78 percent of Earth's atmosphere is molecular nitrogen (N₂), a chemically stable form that most organisms cannot use directly. Nitrogen is nevertheless essential to life: it is part of amino acids, proteins and nucleic acids. The cycle describes the transformations that make nitrogen available to organisms and move it among air, land and water.

Nitrogen fixation converts atmospheric N₂ into reactive compounds, including ammonia and ammonium. It occurs naturally through nitrogen-fixing microbes, with a smaller contribution from lightning, and is also carried out industrially to make ammonia. In soils, microbial nitrification converts ammonia or ammonium through nitrite to nitrate. Plants take up usable nitrogen compounds; animals obtain nitrogen through food webs. Waste and the decay of organisms return organic nitrogen to soils and waters, where microbes mineralize it into forms that can re-enter the cycle. In denitrification, microbes convert nitrate into gaseous nitrogen, returning it to the atmosphere. Other microbial pathways and chemical forms also operate, so the cycle is a network of processes rather than one fixed sequence.

The amount of biologically available nitrogen can limit plant growth and ecosystem productivity. Nitrogen moves into food webs when plants assimilate it, and then passes to consumers and decomposers. Microbial transformations ultimately return some of it to the atmosphere, while other nitrogen remains in soils, sediments or waters for different lengths of time. These exchanges link terrestrial and marine ecosystems with the atmosphere.

Industrial ammonia production and the use of nitrogen fertilizers add reactive nitrogen to agricultural systems; combustion of fossil fuels also creates reactive nitrogen compounds. These additions have helped raise crop production, but nitrogen that crops do not take up can be carried into groundwater, rivers and coastal waters or released to the air. Excess nutrients can spur algal growth and oxygen depletion in water bodies. Nitrogen compounds also contribute to air pollution, and microbial transformations can produce nitrous oxide, a greenhouse gas. Managing the cycle involves balancing nitrogen needed for food production against losses that harm ecosystems and human health.