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:

straight fertilizers

i believe it would be the lungs.