Favored by warm temperature and neutral pH. Wastes and decayed organic materials decomposed by another set of bacteria into ammonia - called ammonification. Various microbes are responsible, favored by cool temperatures, all pH's. Denitrification - closes the cycle. Returns nitrogen to the atmosphere. Note the heavy reliance on microbes for the function of this cycle.
Legumes are excellent green manure because they have high levels of nitrogen.
Nitrogen in many systems is usually in short supply so legumes are valuable soil enhancers. Although legumes provide an excellent source of protein in the diet, there are limitations to using legumes for food. For example, lima beans, especially wild types, have lots of cyanogenic glycosides in them. Lathyrism is an irreversible paralysis caused by eating a vetch and favism is a hemolytic anemia from eating Broad beans. This latter condition results in genetically susceptible individuals who lack the ability to make a critical enzyme.