Nitrogen fixation is an essentially biological process and initial stage of the nitrogen cycle.
In this process, nitrogen in the atmosphere is converted into ammonia (another form of nitrogen) by certain bacterial species like the Rhizobium, Azotobacter, etc. and by other natural phenomenon.
Biological Nitrogen Fixation: Certain bacteria or prokaryotes are capable of converting atmospheric nitrogen to ammonia. This process is generally called biological nitrogen fixation.
The enzyme nitrogenase then converts dinitrogen to ammonia.
Nitrogen-fixing bacteria may be free-living or symbiotic.
Some of the free-living nitrogen fixers are Azotobacter, Beijernickia, Rhodospirillum, cyanobacteria, etc.