A chemical process in which atmospheric nitrogen is assimilated into organic compounds in living organisms and hence into the nitrogen cycle. The ability to fix nitrogen, by means of nitrogenase enzymes, is limited to certain bacteria (e.g. Azotobacter, Anabaena), and, in some anaerobic environments, such as swamps, to methane-producing archaea (see methanogen). Some bacteria (e.g. Rhizobium, Bradyrhizobium) are able to fix nitrogen in association with cells in the roots of leguminous plants, such as peas and beans, in which they form characteristic root nodules (see bacteroid); cultivation of legumes is therefore one way of increasing soil nitrogen. Certain nonleguminous plants are also hosts to nitrogen-fixing bacteria. For example, alder trees develop root nodules containing Frankia, a streptomycete-like organism. Various chemical processes are used to fix atmospheric nitrogen in the manufacture of fertilizers. These include the Birkeland-Eyde process, the cyanamide process, and the Haber process.