Which microorganisms perform most nitrogen fixation in nature?

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Difference in molecular structure between ammonia/amines and quaternary ammonium ions — ammonia Ammonia and amines have a slightly flattened trigonal pyramidal shape with a lone pair of electrons above the nitrogen. In quaternary ammonium ions, this area is occupied by a fourth substituent.

Most nitrogen fixation in nature is performed by nitrogen-fixing bacteria. These microorganisms are responsible for more than 90 percent of all nitrogen fixation, a critical process in the nitrogen cycle that converts atmospheric nitrogen into fixed nitrogen usable by plants.1

These bacteria are categorized into two primary groups:

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 plants. Examples include Rhizobium, which forms nodules on the roots of legumes; Frankia, which associates with certain dicotyledonous species; and Azospirillum species, which associate with cereal grasses.1

Within these symbiotic relationships, the bacteria convert free nitrogen into ammonia, which the host plant then utilizes for its development.1