The total nitrogen gain from nitrogen fixation by Azospirillum has been estimated to be 2–3 kg of N/ha/annum. Supplementation of different sources of nitrogen including nitrate affect growth, ARA, NR activity and ammonia excretion, differently. Various reports have shown that NR mutants help in better mobilization of nitrate to the plants as compared to NR-mutants of Azospirillum. 15N labeled fertilizer and N measurements have shown that increase in grain yield and N content are due to increased nitrate assimilation from nitrate reductase activity. The physiology of NO3− assimilation has been well studied in both plants and fungi and till recently NO3− assimilation by bacteria had attracted less interest, but now nitrate assimilation system has been characterized in several bacterial species. Anaerobic nitrogenase activity by Spirillum lipoferum was greatly stimulated in the presence of nitrate. The time course of nitrate reduction was coincidental with the pattern of nitrate stimulated nitrogenase activity indicating that relationship exists between two process, nitrate reductase and nitrite reductase. The level of ammonia excretion under oxygen-limited conditions indicated that the excreted ammonia originated from nitrate source. Substantial yield increase have been reported for various crops like wheat, cotton, pearl millet, vegetables and oil seed crops.
