Which organisms carry out most natural nitrogen fixation?

Most natural nitrogen fixation is carried out by nitrogen-fixing bacteria, which are responsible for more than 90 percent of this process. These microorganisms transform atmospheric nitrogen into fixed nitrogen, an essential step in the nitrogen cycle that makes nitrogen available to plants.1

These bacteria are categorized into two primary types:

• Free-living (nonsymbiotic) bacteria: This group includes cyanobacteria (such as Anabaena and Nostoc) as well as genera like Azotobacter, Beijerinckia, and Clostridium.1
• Mutualistic (symbiotic) bacteria: These organisms live in association with host plants, such as Rhizobium with legumes, Frankia with certain dicotyledonous species, and Azospirillum with cereal grasses.1

Within these organisms, the enzyme nitrogenase catalyzes the reduction of molecular nitrogen into ammonia, which the host plant can then utilize for growth.12