How do biofertilisers enrich the fertility of the soil?

Explanation

Biofertilisers are the organisms which increase the nutrient availability to the crop plants either directly or through soil enrichment. They play role in increasing soil fertility and soil productivity. They are of three types-bacteria, cyanobacteria and mycorrhizal fungi. Bacteria and cyanobacteria function as biofertilisers because of their property of nitrogen fixation while mycorrhizal fungi preferentially withdraw minerals from decaying organic matter for the plant with which they are associated. Nitrogen fixation is the process of conversion of molecular or dinitrogen of the air into nitrogenous compounds.
Nitrogen fixing bacteria and cyano-bacteria may be free-living or form symbiotic association with the roots, stem & leaves of higher plants. For example Azolla-Anabaena association is of great importance to agriculture. Azolla pinnata is a free floating fresh water fern which multiplies rapidly doubling every 5-7 days. The fem can co-exist with rice plants because it does not interfere with their growth. In some south eastern countries especially. China and in southern states in India rice fields are regularly provided with Azolla. Anabaena azollae residing in the leaf-cavities of fern, fixes nitrogen. A part of fixed nitrogen is excreted in cavities and available to the fern. The decaying fern plants, release the same for utilisation of the rice plants. When field is dried at the time of harvesting the fer functions as green manure, decomposing and enriching the field for the next crop.