In the nitrogen cycle, nitrogen fixation is primarily carried out by?
A. bacteria Nitrogen fixation is the process of making available nitrogen compounds in the air (mainly molecular nitrogen N2) Two main groups of microorganisms carry out nitrogen fixation. The more common of the two groups is made up of organisms living in soil and water—a few species of bacteria (chiefly of the genera Azotobacter and Clostridium) and some blue-green algae. The second group, consisting of bacteria of the genus Rhizobium, lives in plants, primarily legumes such as peas, clover, and alfalfa. The bacteria cause the roots of legumes to form root nodules (swellings) in which the organisms live. The plants supply the bacteria with food. In return, the bacteria secrete ammonium compounds that are absorbed and used by the legumes and by other plants that are grown in the same soil. Products of nitrogen fixation are ammonia, nitrites or nitrates. &