The nitrogen cycle is the network of processes through which nitrogen moves among the atmosphere, living organisms, soils, waters, and sediments while changing chemical form. It is a biogeochemical cycle: biological transformations interact with chemical reactions and physical transport. Nitrogen supports life as a constituent of proteins and nucleic acids, but its availability depends on its chemical form. Microorganisms convert atmospheric nitrogen into usable compounds, recycle nitrogen from organic matter, and return it to the atmosphere. (openstax.org)
Reservoirs and chemical forms
Molecular nitrogen, N₂, constitutes approximately 78 percent of dry air by volume. Its strong triple bond makes it relatively unreactive under ordinary conditions, and most organisms cannot directly incorporate it into biological compounds. The abundance of atmospheric nitrogen therefore does not guarantee an adequate supply for biological growth. (noaa.gov)
Other important forms include ammonia (NH₃), ammonium (NH₄⁺), nitrite (NO₂⁻), nitrate (NO₃⁻), and nitrogen contained in organic matter. These occur in soils, organisms, and aquatic environments. Nitrogen availability can limit primary production and decomposition within an ecosystem. Inorganic nitrogen compounds differ in mobility and biological accessibility, with nitrate particularly important because it moves readily through surface and subsurface waters. (epa.gov)
Nitrogen fixation
Nitrogen fixation converts N₂ into chemically combined nitrogen. In biological fixation, microorganisms use the enzyme nitrogenase to reduce N₂ to ammonia. The process requires energy and reducing power. Nitrogenase is sensitive to oxygen, so nitrogen-fixing organisms require mechanisms or environments that protect its activity. (openstax.org)
Fixation occurs in free-living microorganisms and in symbiotic associations. Bacteria collectively known as rhizobia inhabit root nodules of legumes, where they supply fixed nitrogen to their host plants. Certain cyanobacteria also fix nitrogen and contribute to nitrogen inputs in aquatic environments. These organisms connect the large atmospheric reservoir with nitrogen circulating through biological communities. (openstax.org)
Nonbiological fixation occurs through lightning, which enables nitrogen and oxygen to react, forming nitrogen oxides that can subsequently enter soils. Industrial fixation uses the Haber–Bosch process to combine nitrogen with hydrogen and produce ammonia under elevated temperature and pressure. This ammonia is a major starting material for nitrogen-containing fertilizers. (openstax.org)
Assimilation and ammonification
Assimilation incorporates inorganic nitrogen into organic compounds. Plants and microorganisms obtain nitrogen from available compounds, while animals acquire it through food. Nitrogen consequently passes through a food web rather than moving directly from atmospheric N₂ into most consumers. Uptake temporarily stores nitrogen in biomass, but does not remove it permanently from circulation. (epa.gov)
Ammonification returns organic nitrogen to the inorganic pool. During decomposition, microorganisms break down nitrogen-containing material in dead organisms and wastes, releasing ammonium. Bacteria and fungi participate in this recycling. The ammonium may subsequently be assimilated again, transformed by nitrification, or transported elsewhere. Microbial incorporation of inorganic nitrogen into organic matter is also called immobilization in soil nitrogen budgets. (openstax.org)
Nitrification
Nitrification is the microbial oxidation of ammonia to nitrate through nitrite. In its conventional description, ammonia oxidizers carry out the first stage and nitrite-oxidizing bacteria carry out the second. Ammonia-oxidizing organisms include both bacteria and archaea. These transformations connect reduced inorganic nitrogen with the more oxidized nitrate pool. (nature.com)
The simplified sequence is:
NH₄⁺ → NO₂⁻ → NO₃⁻
This represents changes in nitrogen form, not a fully balanced chemical equation. The functional division between two microbial groups is not universal: research published in 2015 demonstrated complete nitrification by certain Nitrospira bacteria. Such organisms, called comammox organisms, perform both ammonia oxidation and nitrite oxidation within one organism. (nature.com)
Denitrification and alternative pathways
Denitrification converts nitrate through progressively reduced intermediates into gaseous nitrogen. A common sequence is:
NO₃⁻ → NO₂⁻ → NO → N₂O → N₂
The pathway provides a route back to atmospheric N₂. Its intermediates include nitric oxide and nitrous oxide, and conversion need not proceed completely to N₂ before gaseous products escape. (pubs.usgs.gov)
Anaerobic ammonium oxidation, commonly shortened to anammox, provides another route to N₂. Anammox bacteria transform ammonium and nitrite under anoxic conditions. Measurements in river sediments show that this pathway can contribute substantially to nitrogen loss, although its importance varies among environments. (nature.com)
A contrasting pathway, dissimilatory nitrate reduction to ammonium (DNRA), converts nitrate into ammonium. Unlike pathways producing N₂, it retains nitrogen in a form that can remain biologically available. Research in rice paddies demonstrates that competition between DNRA and denitrification can influence nitrogen retention and loss. (pubmed.ncbi.nlm.nih.gov)
Transport and human modification
Chemical transformations operate alongside physical movement. Nitrate can leach into groundwater, while surface runoff transports nitrogen into streams and other receiving waters. Atmospheric deposition supplies nitrogen to land and water. Some nitrogen entering the ocean becomes incorporated into sediments, linking biological recycling with longer-term geological storage. (epa.gov)
Human activities increase and redistribute nitrogen inputs through fertilizer application, livestock manure, wastewater discharge, and fossil-fuel combustion. Nitrogen not retained or used within agricultural systems can move into downstream waters or the atmosphere. (epa.gov)
Excess nutrient inputs can cause eutrophication, stimulating algal growth and reducing dissolved oxygen available to aquatic organisms. Nitrous oxide generated within the nitrogen cycle is also a greenhouse gas. Biological wastewater treatment applies nitrogen transformations deliberately: nitrification and denitrification convert nitrogen compounds, while anammox-based systems provide an alternative nitrogen-removal pathway. (epa.gov)