Bacteria like Azospirillum and Azotobacter can fix atmospheric nitrogen while free-living in the soil, thus enriching the nitrogen content of the soil.
Rhizobium is a nitrogen-fixing bacteria found in legumes, such as groundnut, soya beans etc. It forms a symbiotic association with the root nodules of leguminous plants. These bacteria fix atmospheric nitrogen into organic forms (ammonia and other nitrogenous compounds), which is used by plants as nutrients.
Cyanobacteria or blue-green algae (BGA) are autotrophic microbes widely distributed in both aquatic and terrestrial environments, many of which are capable of fixing atmospheric nitrogen for example, Anabaena, Nostoc, Oscillatoria. Anabaena is found commonly in paddy fields in a symbiotic association with the water fern Azolla.
Frankia (a gram-positive actinomycete) show a symbiotic association with root nodules of non-legume plants for example, Casuarina and Alnus.