Rhizobium converts atmospheric nitrogen into usable compounds in many legume root nodules.
Rhizobium is a genus of soil bacteria that forms symbiotic relationships with legumes such as peas, beans, and clover. The bacteria enter root tissues and stimulate the formation of nodules, specialized structures where nitrogen fixation occurs.
Inside nodules, the enzyme nitrogenase converts atmospheric nitrogen gas into ammonia, which the plant can incorporate into nitrogen-containing molecules. The plant supplies the bacteria with carbon compounds made by photosynthesis. This exchange is a major example of mutualism.
Rhizobium is not the name of one single species, and different legumes often associate with different bacterial partners. Bradyrhizobium is a related genus that also fixes nitrogen, so it is a plausible source of confusion. Legume crops can enrich agricultural soils, though available nitrogen depends on the whole ecosystem and farming system.