What group of organisms plays the largest role in moving nitrogen between the atmosphere and plants

Nitrogen is an essential element for all living organisms. It is required for the synthesis protein and nucleic acids and other nitrogenous compounds. Although, in the atmosphere nitrogen is 78%, but it cant be used by plants as it is highly inert. It can only be used when nitrogen is converted into usable form in the process of nitrogen fixation done by certain microorganisms.

Nitrogen fixing organisms are classified into:

Symbiotic nitrogen-fixing bacteria such as Rhizobium, Sinorhizobium, Bradyrhizobium. These are collectively called rhizobia. They form symbiotic association with leguminous and non-leguminous (Frankia) plants. Non-symbiotic nitrogen fixing organisms such as cyanobacteria (Anabaena, Nostoc), Azospirillum, Azotobacter, Gluconacetobacter diazotrophicus and Azocarus.