Nitrogen cycle

The nitrogen cycle is a fundamental biogeochemical cycle through which nitrogen moves between the atmosphere, terrestrial environments and marine ecosystems. Nitrogen exists in multiple chemical forms and undergoes numerous transformations driven by biological, chemical and physical processes. Although atmospheric nitrogen gas constitutes roughly 78 per cent of the Earth’s atmosphere, its inert nature makes it biologically inaccessible without conversion. As a result, usable nitrogen is often a limiting nutrient in many ecosystems, affecting rates of primary production, decomposition and overall ecosystem functioning.
Human activities—particularly the combustion of fossil fuels, industrial fertiliser production and the release of nitrogen-rich wastewater—have altered the global nitrogen cycle considerably. Such disruptions lead to environmental consequences including eutrophication, soil acidification and air pollution, as well as impacts on human health through contaminated water sources or nitrogen-related atmospheric pollutants.

Forms of Nitrogen and General Pathways

Nitrogen in the environment occurs in various organic and inorganic forms, including:

• organic nitrogen in living tissue, humus and decomposition intermediates
• ammonium ions
• nitrite and nitrate ions
• gaseous nitrogen compounds such as nitric oxide and nitrous oxide
• molecular nitrogen, which dominates the atmosphere

Microorganisms play a critical role in mediating transformations between these forms. These reactions support energy acquisition, nutrient assimilation and broader ecological interactions.

Nitrogen Fixation

Nitrogen fixation is the conversion of atmospheric nitrogen gas into biologically usable forms such as ammonia, nitrites or nitrates. This process can occur through:

• Lightning, fixing an estimated five to ten billion kilograms of nitrogen annually
• Biological fixation, largely by diazotrophic bacteria and archaea equipped with the nitrogenase enzyme
• Industrial fixation, mainly via the Haber–Bosch process, which now produces about thirty per cent of the total fixed nitrogen globally

Biological fixation is dominated by molybdenum nitrogenase, a two-component enzyme complex with metal cofactors. Free-living diazotrophs such as Azotobacter operate independently, while symbiotic nitrogen-fixing bacteria such as Rhizobium inhabit the root nodules of legumes. In these mutualistic associations, bacteria supply ammonia in exchange for plant-derived carbohydrates. Some non-leguminous plants also form analogous relationships.

Assimilation by Plants and Other Organisms

Plants absorb nitrogen from soil predominantly in the form of nitrate or ammonium. When nitrate is taken up, it is reduced first to nitrite and then to ammonium before incorporation into amino acids, nucleotides and chlorophyll. Leguminous plants with rhizobial symbionts assimilate significant quantities of ammonium directly from root nodules.
Nitrogen metabolism is tightly regulated among organisms. Heterotrophic animals, fungi and many bacteria typically obtain nitrogen by consuming amino acids or other organic compounds, whereas some bacteria can utilise inorganic ammonium as their sole nitrogen source.

Ammonification (Mineralisation)

Ammonification is the process through which organic nitrogen from dead organisms and metabolic waste is converted into ammonia or ammonium. This transformation is conducted by soil bacteria and fungi using enzymes such as proteases, dehydrogenases and deaminases. Rates of ammonification depend on soil organic matter content, microbial biomass and climatic factors such as temperature and precipitation. High carbon-to-nitrogen ratios in plant material can slow ammonification, whereas warmer and moister conditions typically enhance it.

Nitrification

Nitrification is the biological oxidation of ammonium to nitrate through a two-step process:

1. Oxidation of ammonium to nitrite, typically by bacteria such as Nitrosomonas
2. Oxidation of nitrite to nitrate, primarily by Nitrobacter species

Although nitrate is essential for plant growth, it is highly soluble and prone to leaching into groundwater. Elevated nitrate levels in drinking water can cause methemoglobinemia in infants and contribute to eutrophication of water bodies when nitrate-rich groundwater feeds surface waters. Regulatory controls on nitrogen fertiliser use have therefore increased in recent decades.
Related processes include complete ammonium oxidation (comammox), in which a single microorganism performs both steps of nitrification.

Denitrification

Denitrification converts nitrate back into nitrogen gas, completing the nitrogen cycle. It occurs under anaerobic conditions when bacteria such as Pseudomonas and Paracoccus denitrificans use nitrate as an alternative electron acceptor in respiration. This reduces bioavailable nitrogen and returns gaseous nitrogen to the atmosphere. Denitrification is common in waterlogged soils and sediments and can also occur in symbionts of anaerobic ciliates.

Dissimilatory Nitrate Reduction to Ammonium (DNRA)

DNRA is an anaerobic microbial process in which nitrate is reduced to nitrite and further to ammonium. This pathway conserves nitrogen within ecosystems, as ammonium remains bioavailable. In environments where organic carbon is abundant, DNRA competes with denitrification for nitrate, influencing whether nitrogen is retained or lost to the atmosphere.

Anaerobic Ammonia Oxidation (Anammox)

The anammox process is a microbial reaction in which ammonia serves as a reducing agent and nitrite as an oxidising agent, producing nitrogen gas and water. This comproportionation reaction is a major component of marine nitrogen cycling, especially in oxygen-depleted zones. Anammox bacteria carry out this process within specialised cellular compartments and contribute significantly to nitrogen loss from the oceans.

Human Impacts and Ecological Significance

Human activities have increased the global pool of reactive nitrogen, accelerating nitrogen fixation through fertiliser production and fossil-fuel combustion. While this has supported agricultural productivity, excessive reactive nitrogen has created environmental issues, including:

• eutrophication and harmful algal blooms
• soil and freshwater acidification
• atmospheric pollution and greenhouse gas emissions
• health impacts from nitrate-contaminated water