Overview

The nitrogen cycle describes how nitrogen moves through the atmosphere, biosphere, lithosphere and hydrosphere, changing chemical form as it is used, stored and returned to the air and water. Nitrogen is abundant in the atmosphere as molecular nitrogen (N2) and is essential for life because it forms the backbone of amino acids, proteins and nucleic acids such as DNA and RNA. Most organisms cannot use atmospheric N2 directly; instead, nitrogen must be converted into biologically available compounds through a sequence of biochemical and chemical transformations.

Main processes of the cycle

  • Nitrogen fixation: Conversion of N2 into ammonia (NH3) or related forms. This occurs biologically via nitrogenase enzymes in free-living and symbiotic bacteria and cyanobacteria, and industrially through the Haber–Bosch process to make synthetic fertilizers.
  • Assimilation: Plants and microbes take up inorganic nitrogen (ammonium NH4+ or nitrate NO3−) and incorporate it into organic molecules such as amino acids and nucleotides.
  • Ammonification (mineralization): Decomposers break down proteins and nucleic acids from dead organisms and waste, releasing ammonia or ammonium back into soil or water.
  • Nitrification: A two-step aerobic oxidation where ammonia is converted to nitrite (NO2−) and then to nitrate (NO3−) by specialized bacteria; nitrate is highly soluble and mobile in water.
  • Denitrification: Under low-oxygen conditions certain bacteria reduce nitrate back to gaseous forms such as N2 or nitrous oxide (N2O), completing the cycle by returning nitrogen to the atmosphere.

Biological agents and chemical forms

The cycle is driven largely by microorganisms: nitrogen-fixing bacteria (including symbionts in legume root nodules), free-living diazotrophs, nitrifying bacteria and archaea, and denitrifiers. Plants obtain nitrogen mostly as nitrate or ammonium through roots and then transfer it to herbivores and the wider food web. Important chemical species include molecular nitrogen (N2), ammonia (NH3), ammonium (NH4+), nitrite (NO2−) and nitrate (NO3−). Ammonium tends to bind to negatively charged soil particles and organic matter, while nitrate remains dissolved and can leach into groundwater. Some nitrogen transformations release trace gases such as nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting substance.

History, human alteration and environmental impacts

Human activity has greatly accelerated the natural nitrogen cycle. The industrial Haber–Bosch synthesis enabled large-scale production of reactive nitrogen in fertilizers, increasing agricultural yields but also elevating losses to air and water. Excess nitrate runoff can lead to eutrophication in lakes and coastal waters, driving algal blooms and oxygen depletion that harm aquatic life. Elevated nitrate in drinking water poses risks to infants and other sensitive groups. Emissions of N2O from soils and industry contribute to climate forcing.

Management, measurement and mitigation

Because reactive nitrogen moves between air, soil and water, management aims to balance productive use with reduced pollution. Practices include improving fertilizer timing and application rates, planting cover crops and legumes to fix nitrogen biologically, restoring wetlands that denitrify excess nitrate, and treating wastewater to remove nitrogenous waste. Monitoring often measures forms such as ammonia, ammonium, nitrite and nitrate in water and soil, and tracks gaseous emissions. Soil properties, oxygen availability, moisture and pH influence which processes dominate in a given environment.

Notable distinctions and additional facts

Key distinctions include the mobility of nitrate versus the soil-retention of ammonium, and the different organisms and enzymes responsible for each step (for example, nitrogenase in fixers and distinct oxidizing bacteria for nitrification). Some bacteria form close partnerships with plants, supplying fixed nitrogen in exchange for carbohydrates. The entire network of transformations is sometimes called a nitrogen cascade because a single added atom of reactive nitrogen can pass through multiple environmental compartments and cause several ecological effects.

For concise references and definitions see: atmospheric nitrogen overview, nitrogen in nature, air composition, DNA basics, RNA basics, photosynthesis, nitrogen fixation, microorganisms, bacteria roles, enzymes, hydrogen, ammonia, plant roots and nodules, soil nitrogen compounds, soil, animals and nitrogen, ammonium, decomposers, clay minerals, humus, toxicity, fish impacts, sewage, waste, water, nitrite, nitrate, rain, irrigation, health risks, blue-baby syndrome, algal growth, eutrophication, fertilizers.