What type of organism is responsible for performing the majority of nitrogen fixation?

There are two ogranisms that are nitrogen fixers:

Free-living (non-symbiotic) bacteria, which live in the soil. This includes the cyanobacteria (or blue-green algae) Anabaena and Nostoc and such genera as Azotobacter, Beijerinckia, and Clostridium

Mutualistic (symbiotic) bacteria, which live live in nodules in the roots of plants. This includes Rhizobium,associated with leguminous plants, and Spirillum lipoferum,associated with cereal grasses.

Nitrogen fixing bacteria are microorganisms present in the soil or in plant roots that change nitrogen gas from the atmosphere into solid nitrogen compounds that plants can use in the soil.