Air is a very rich source of nitrogen. The use of micro-organisms is one way to make atmospheric nitrogen available to the soil.
Only a few microbes can fix this nitrogen. These microbes have a special enzyme called nitrogenase, which reduces nitrogen to ammonia. This is called biological nitrogen fixation. Microbes that possess this enzyme are called nitrogen fixers.Nitrogen fixers live independently or as symbionts, such as azotobacter and blue-green algae. Nitrogen fixation is slow in free-living microbes due to the energy requirements. Rhizobium is known for its symbiotic association in the root nodules of leguminous plants. Rhizobium in the soil multiplies around the roots and gets attached to root hair. These bacteria reach the cortical cells through this root hair. Then the bacteria stimulate the inner cortical and pericycle cells to divide. These cells get differentiated as nitrogen-fixing cells and a nodule is thus formed. Through vascular supply, the nodule is ensured of nutrients and energy from the plant. The enzyme secreted by rhizobium, nitrogenase, is a molybdenum-ferrum protein and acts as a catalyst in the conversion of nitrogen to ammonia. Leg-haemoglobin, the iron-containing protein in the root nodule, acts as an oxygen scavenger and prevents its exposure to nitrogenase. Nitrogen requires eight electrons, eight protons and 16 ATP molecules of energy for the formation of two molecules of ammonia. These eight ATP molecules are provided by the ATP released during respiration. Nitrogenase first binds with substrate nitrogen and finally forms two molecules of ammonia. Ammonia molecules take up protons to form ammonium (NH4+) ions that are used in the synthesis of amino acids. Amino acid synthesis in plants takes place in two ways: reductive amination and transamination. When ammonium ions react with alpha ketoglutaric acid, it reduces it to an amine and forms glutamic acid and water in the presence of the enzyme glutamate dehydrogenase. The energy required for this reaction is obtained by the oxidation of NADPH to NADP. Trasamination involves the transfer of one amino group from an amino acid to the keto group of a keto acid. The reaction is catalysed by a family of enzymes called transaminases. This results in the formation of asparagine and glutamine from asparatic acid and glutamic acid. This process takes place due to the replacement of the hydroxyl group of the carboxyl group of aspartic and glutamic acids with amino group. Amino acids are usually transported through phloem but compounds such as amides and ureides, owing to a greater nitrogen-to-carbon ratio, are easily transported through xylem.