Nitrogen-fixing bacteria are microorganisms present in the soil or in plant roots that change nitrogen gases from the atmosphere into solid nitrogen compounds that plants can use in the soil.
Nitrogen fixing bacteria lives in the root nodules of the plant in the soil.
Plants fix atmospheric nitrogen with the help of them. e.g. Rhizobium is a genus of Gram-negative soil bacteria that fix nitrogen. The bacteria colonize plant cells within root nodules, where they convert atmospheric nitrogen into ammonia and then provide organic nitrogenous compounds such as glutamine or ureides to the plant.