What is nitrogen fixation?

Conversion of answering gas into Nigerian containing compounds. Lightning can cause this, but there are also two types of bacteria which perform this role:
1) Mutualistic N-fixing bacteria: live in nodules on the roots of leguminous plants e.g. clover, beans and produce ammonia( bacteria obtain carbohydrate in return).
2) Free-living N-fixing bacteria- produce amino acids from N₂, which is returned to the soil as nitrates when the bacteria die and decay.