Azolla

The practice of Azolla is an age-old technique popularized as ‘green’ nitrogen fertilizer to increase rice production.
Azolla is a freshwater water fern that lives in ponds, lakes, swamps, and streams in both tropical and sub-tropical conditions.
Azolla in association with blue-green algae anabaena can fix atmospheric Nitrogen (N) into ammonia which can be utilized by rice plant when it is incorporated into soil.

As the plant decomposes, its organic nitrogen is rapidly mineralized and released as ammonia, which then becomes available as a biofertilizer for the growing rice plants.
Azolla contains from 2−5% N, 0.3−6.0% K (dry weight). Average N-fixation through Azolla is estimated at 30–40 kg N ha⁻¹.
Less than 5% of the nitrogen sequestered by Azolla is available immediately to the growing rice plants, while rest 95% remains in the Azolla’s biomass until the plant dies.
Azolla substantially increases the amount of nitrogen fertilizer available to growing rice and hence cut-off the demand of chemical N-fertilizer.