Oscillatoria is a filamentous cyanobacterium that fixes atmospheric nitrogen in paddy (rice) fields and is widely used as a biofertiliser. Other cyanobacteria like Anabaena and Nostoc also fix nitrogen in paddy fields. Rhizobium is specific to legumes; Frankia to non-leguminous trees; Azospirillum is a free-living aerobic fixer associated with grasses.