Previous Year Question

Biological Nitrogen Fixation is the process through which certain microorganisms convert atmospheric Nitrogen (N₂) into forms usable by plants, such as Ammonia (NH₃). The Nitrogenase Enzyme plays an important role in this process. Nitrogen-fixing organisms are used as Biofertilizers, which can improve soil fertility and increase nitrogen availability to plants.

Blue-Green Algae — Paddy: Blue-Green Algae are actually photosynthetic Cyanobacteria. Organisms such as Anabaena and Nostoc can fix atmospheric nitrogen. In flooded paddy fields, sufficient water, sunlight, and favourable temperatures support their growth. The nitrogen fixed by these organisms can become available to rice plants. Therefore, Blue-Green Algae are used as Biofertilizers in rice cultivation.

Rhizobium leguminosarum — Pea: This bacterium performs Symbiotic Nitrogen Fixation by forming Root Nodules in Leguminous Plants. The bacterium obtains carbohydrates from the plant and supplies nitrogen in return. Azotobacter — Wheat: It is a Free-Living Nitrogen-Fixing Bacterium that fixes atmospheric nitrogen in soil without forming root nodules. It is used as a Biofertilizer for wheat and various other non-leguminous crops.

Azolla is an Aquatic Fern that floats freely on the water surface. Its leaves contain a symbiotic nitrogen-fixing cyanobacterium known as Anabaena azollae. Azolla is primarily used as Green Manure and a Biofertilizer in Flooded Paddy Cultivation. Its decomposition releases nitrogen into the soil. Maize is generally grown under dry or well-drained conditions, so the Azolla–Maize pairing is not a conventional crop association for nitrogen fixation. Bacteria such as Azotobacter and Azospirillum are used for non-leguminous crops, including maize.