The nitrogen cycle is a biogeochemical cycle. It describes the transformations of nitrogen and nitrogen-containing compounds in nature.

The major source of nitrogen is air, which is about 78 percent N₂ by volume. Nitrogen is essential for many biogical processes; for example, it is included in all amino acids (the stem amin derives from ammonia), is incorporated into proteins and is present in the four bases that make up nucleic acids, such as DNA. Processing is necessary to convert gaseous nitrogen into forms usable by living organisms.

All nitrogen obtained by animals can be traced to the eating of plants at some stage down the food chain. Plants get nitrogen from the soil by absorption at their roots in the form of either nitrate ions (NO₃^-) or ammonia (NH₃). Ammonia is produced in the soil by nitrogen fixing organisms such as Azobacter vinelandii which produces the enzyme nitrogenase. Nitrogenase combines gaseous nitrogen with hydrogen to produce ammonia. Some nitrogen fixing bacteria, such as Rhizobium, live in root nodules of leguminous plants (such as peas or beans). Here they form a symbiotic relationship with the plant, producing ammonia in exchange for supplies of carbohydrate. Low nutrient containing soils can be planted with leguminous plants to enrich them with nitrogen; see crop rotation.

Another source of ammonia is the decomposition of dead organic matter by saprophytic bacteria called decomposers, which produce ammonium ions (NH₄⁺). In well-oxygenated soil, these are then oxygenated first by bacteria such as Nitrosomonas into nitrites (NO₂^-) and then by Nitrobacter into nitrates. This conversion of ammonia in to nitrates is called nitrification.

Ammonium ions can bind to soils and clays, whereas nitrates, due to their negative charge, cannot. They are also very soluble in water, and, after heavy rain, leaching (the removal of nitrates into rivers and lakes) can occur. Leaching of large amounts of nitrates often results in eutrophication, a process leading to unsustainably high algae and blue-green bacteria populations and the death of other river life. Abuse of nitrate- and phosphate-containing fertilizers has caused severe problems in some rivers, and in Britain their use has been restricted. In the presence of anaerobic (low oxygen) conditions in soils, denitrification by bacteria such as Thiobacillus denitrificans can happen. This is the reverse of nitrification and results in nitrates being converted to nitrogen gas and lost to the atmosphere.

There are three major ways to convert N₂ into a chemically more reactive species:

1. Biological fixation: some bacteria (associated with certain plants, leguminosae) and certain blue-green algae are able to fix nitrogen and assimilate it as organic nitrogen
2. Technical N-fixation: in the Haber-Bosch process N₂ is converted to-gether with hydrogen gas (H₂) into ammonia (NH₃).
3. Combustion of gasoline and fossil fuel (automobile engines and thermal power plants), which transfers elemental nitrogen gas into NOx.

Important for the atmospheric chemistry, but not for the nitrogen turnover on earth, are also the formation of NO from N₂ and O₂ due to photons and lightning.

As a result of extensive cultivation of legumes (particularly soy), use of the Haber-Bosch process in the creation of chemical fertilizers, and pollution emitted by vehicles and industrial plants, human beings have more than doubled the annual transfer of atmospheric nitrogen into a biologically available form. This has occurred to the detriment of aquatic and wetland habitats (through eutrophication).