From my understanding N2 cannot be used directly by plants so nitrogen fixing bacteria convert it to something else that is all I know.
Bacteria that fix nitrogen in the soil fix nitrogen gas into nitrogen compounds.
In order to make proteins animals and plants need nitrogen, they cannot get nitrogen from the air as nitrogen is an unreactive gas.
Plants can get nitrogen compounds from ammonium salts and nitrates found in the soil and use this for protein synthesis.
Animals such as cows eat the plants and they release nitrogen through excretion(sweat, breathing) and egestion.
When these animals die, bacteria breaks them down whixh returns nitrogen to the soil.