a chemical process by which nitrogen in the atmosphere is assimilated into organic compounds, by nitrogen‐fixing bacteria and made available to plants and from them to other organisms through food chains, incorporation of atmospheric nitrogen into chemicals for use by organisms. Nitrogen-fixing microorganisms (mainly bacteria and cyanobacteria) absorb nitrogen gas from the air, from air spaces in the soil or from water, and build it up into compounds of ammonia. Other bacteria can then change these compounds into nitrates, which can be taken up by plants.