A biogeochemical cycle involving the element nitrogen. Leguminous plants, such as peas, beans, and clover, form a symbiotic association with Rhizobium bacteria, developing root nodules in which these nitrogen-fixing bacteria can live.
All plants take up nitrates from which they produce all their essential nitrogenous compounds.
Some of this nitrogen is lost when leaves, seeds, and fruits are shed, but most remains trapped until the plants die or are eaten. Animals obtain their nitrogenous substances directly or indirectly from plants and excrete excess nitrogen in their faeces and urine. These substances and the dead bodies of all organisms are acted on by decomposing and nitrifying bacteria, which convert ammonia into nitrate, ultimately resulting in the return of nitrates to the soil. Denitrifying bacteria, which live in waterlogged soils, use nitrate for respiration , producing nitrogen gas, which is released to the atmosphere to complete the cycle. Modern agriculture may disturb the natural balance by adding nitrate fertilizer to the soil, much of which is washed out into rivers.