The common nitrogen-fixer in paddy fields is

Azospirillum is a nitrogen fixing bacterium in paddy fields. It is very useful soil and root bacterium. It is an associative symbiotic $\mathrm{N}_2$-fixing bacteria. When it is added to the soil, it multiplies in millions and can supply 20-40 $\mathrm{Kg}$ of nitrogen per hectare per season. It also produces growth promoting substances like Indole Acetic Acid (IAA), gibberellins and promotes root proliferation. These substances improve the plant growth and yield.