Ongoing element conversion from a stable, state to various organic as well as forms that are then to various organisms, along with ongoing return of the same element to the gaseous state.

is converted from its by in the course of what is known as nitrogen fixation. These are microorganisms and either free living or found in with other organisms such as plants.

can be modified into organic forms such as amino acids and nucleic acids as well as or and the inorganic . Decomposer organisms, again microorganisms (and particularly bacteria), are responsible for what is known as , which is the conversion of ammonia to as well as ions.

is also mediated by microorganisms (again particularly bacteria) and represents the conversion of bioavailable forms of nitrogen back to the N₂ gaseous form.

Though the nitrogen cycle is often depicted as occurring in association with , various aspects of conversion of nitrogen-containing compounds from one form into another occurs in all ecosystems. Indeed, nitrogen fixing can occur in as well as ones.

See also biogeochemical cycles.