Question

what is mean by nitrogen fixation ? how does it happen ?

Answer 1

The chemical process by which atmospheric nitrogen is assimilated into organic compounds especially by certain microorganisms ( like rhizobium) as a part of the nitrogen cycle
Hope it helps

Answer 2

nitrogen fixation is the chemical process by which atmospheric nitrogen is assimilated into organic compounds, especially by certain microorganisms as part of the nitrogen cycle.
Some bacteria can convert nitrogen into ammonia by this process.
Hope it is correct.