Biofertilizers (also known as bio inoculants), the organic preparations containing microorganisms are beneficial to agricultural production in terms of nutrient supply particularly with respect to N and P. When applied as seed treatment or seedling root dip or as soil application, they multiply rapidly and develop a thick population in rhizosphere. The population of Azotobacter is generally low in the rhizosphere of the crop plants and in uncultivated soils. The occurrence of this organism has been reported from the rhizosphere of a number of crop plants such as rice, maize, sugarcane, bajra, vegetables and plantation crops, (Arun, 2007). They derive food from the organic matter present in the soil and root exudates and fix atmospheric N (Maryenko, 1964). Biofertilizers can fix atmospheric N through the process of biological nitrogen fixation (BNF), solubilise plant nutrients like phosphates, and stimulate plant growth through synthesis of growth promoting substances and have have C: N ratio 20:1 indicating the stability of the biofertilizer. The isolated culture of Azotobacter fixes about 10 mg nitrogen g-1 of carbon source under in vitro conditions. They are cheaper, low capital intensive besides being eco-friendly. The first species of the genus Azotobacter, named Azotobacter chroococcum family Azotobacteriaceae, was isolated from the soil in Holland in 1901. Azotobacter represents the main group of heterotrophic free living nitrogen-fixing bacteria principally inhabiting neutral or alkaline soils. Azotobacter chroococcum and Azotobacter agilis were studied by Beijerinck (1901). In subsequent years several other types of Azotobacter group have been found in the soil and rhizosphere such as Azotobacter vinelandii, Lipman (1903); Azotobacter beijernckii, Lipman (1904); Azotobacter nigricans, Krassilnikov (1949); Azotobacter paspali, Döbereiner (1966), Azotobacter armenicus, Thompson and Skerman (1981); Azotobacter salinestris, Page and Shivprasad (1991). These nitrogen-fixing bacteria are important for ecology and agriculture. Along with nodular bacteria, Azotobacter was considered to be the most extensively studied genus among the saprophytes (Horner et al., 1942). There is a great significance of Azotobacter chroococcum in plant nutrition and its contribution to soil fertility.
Production of Growth Substances and Their Subsequent Effects
Growth substances, or plant hormones, are natural substances that are produced by microorganisms and plants alike. They have stimulatory or inhibitory effects on certain physiological-biochemical processes in plants and microorganisms. Brakel and Hilger (1965) showed that Azotobacter produced indol-3-acetic acid (IAA) when tryptophan was added to the medium. Hennequin and Blachere (1966) found only small amounts of IAA in old cultures of Azotobacter to which no tryptophan was added. Bacteria of the genus Azotobacter synthesize auxins, cytokinins, and GA-like substances, and these growth materials are the primary substance controlling the enhanced growth of tomato (Azcorn and Barea, 1975). These hormonal substances, which originate from the rhizosphere or root surface, affect the growth of the closely associated higher plants. Eklund (1970) demonstrated that the presence of Azotobacter chroococcum in the rhizosphere of tomato and cucumber is correlated with increased germination and growth of seedlings.). Puertas and Gonzales (1999) report that dry weight of tomato plants inoculated with Azotobacter chroococcum and grown in phosphate-deficient soil was significantly greater than that of non inoculated plants. Phytohormones (auxin, cytokinin, gibberellin) can stimulate root development.
Results of a greenhouse pot experiments with onion showed that application of G. fasciculatum + A. chrooccocum + 50% of the recommended P rate resulted in the greatest root length, plant height, bulb girth, bulb fresh weight, root colonization and P uptake (Mandhare et al., 1998). Elgala et al. (1995) concluded that with microbial inoculation rock phosphate could be used as cheap source of P in alkaline soils and that combined inoculation could reduce the rate of fertilizer required to maintain high productivity. A study by Govedarica et al. (1993) on the production of growth substances by nine Azotobacter chroococcum strains isolated from a chernozem soil has showed that these strains have the ability to produce auxins, gibberelins, and phenols and in association with the tomato plant, increase plant length, mass, and nitrogen content. Azotobacter chroococcum produces an antibiotic which inhibits the growth of several pathogenic fungi in rhizosphere thereby seedling mortality (Subba Rao, 2001). Under greenhouse conditions inoculation of Azotobacter chroococcum recorded a significant N and P uptake in both seed and stover in Brown sarson over the control (Wani, S A., 2012).
Possibility of Using Azotobacteria in Crop Production
Azotobacteria is used for studying nitrogen fixation and inoculation of plants due to its rapid growth and high level of nitrogen fixation. Despite the considerable amount of experimental data concerning Azotobacter stimulation of plant development, the exact mode of action by which Azotobacter enhances plant growth is not yet fully understood. Three possible mechanisms have been proposed: N2 fixation; delivering combined nitrogen to the plant; the production of phytohormone-like substances that alter plant growth and morphology, and bacterial nitrate reduction, which increases nitrogen accumulation in inoculated plants. Triplett (1996) concluded that the development of the diazotrophic endophytic association in maize appears to be the most likely route to success in the development of a corn plant which does not require nitrogen fertilization for optimum growth and yield. Yield increased ranges from 2 to 45 per cent in vegetables, 9 to 24 per cent in sugarcane, 0 to 31 per cent in maize, sorghum, mustard etc., on Azotobacter inoculation (Pandey and Kumar, 1989). Tandon (1991) estimated the fertilizer equivalent of important biofertilizers. According to the estimate, fertilizer equivalent of 19-22 kg ha-1 for rhizobium 20 kg N ha-1 for Azotobacter and Azospirillium, 20-30 kg N ha-1 for blue green algae (BGA) and 3 to 4 kg N ha-1 of Azolla. Dutta and Singh (2002) reported a significant in seed yield (7.86q ha-1) in rapeseed and mustard (var. yella) due to inoculation with Azotobacter. Similarly, positive reports on application of Azotobacter and Azospirillum on the yield of mustard (Brassica juncea) are available (Tilak and Sharma, 2007). According to Das and Saha (2007), combined inoculation of Azotobacter, Azospirillium along with diazotrophs increased grain and straw yield of rice by 4.5 and 8.5 kg ha-1, respectively.Under green house conditions plant height, leaf number/plant, number of primary and secondary branches/plant, fresh and dry weight of whole plant, number of siliqua/plant, seeds/siliqua of brown sarson increased significantly with Azotobacter inoculation than no inoculation with seed and stover yield of 10.107 g pot -1 and 22.400 g pot -1 respectively (S. A. Wani, 2012).
The challenge to the research community will be to develop systems to optimize beneficial plant-endophyte bacterial relationships (Sturz et al., 2000). In order to guarantee the high effectiveness of inoculants and microbiological fertilizers it is necessary to find compatible partners, i.e. a particular plant genotype and a particular Azotobacteria strain that will form a good association.
We are grateful to DRI and Head of the Division of SKUAST- Kashmir for their constructive comments on the manuscript.
- Arun K.S. 2007. Bio-fertilizers for sustainable agriculture. Sixth edition, Agribios publishers, Jodhpur, pp.76-77.
- Azcorn, R., Barea, J.M. 1975. Synthesis of auxins, gibberellins and cytokinins by Azotobacter vinelandi and Azotobacter beijerinckii related to effects produced on tomato plants. Plant Soil, 43: 609-619.
- Beijerinck, M.W. 1901. Über ologonitrophile mikroben. Zentralbl. Bakteriol. Parasitenkd. Infektionskr. II Abt., /: 561-582.
- Brakel, J., Hilger F. 1965. Etude qualitative et quantitative de la synthese de substances de nature auxinique par Azotobacter chroococcum in vitro. Bull. Inst. Agron. Stns. Rech. Gembloux, 33: 469-487.
- Das, A.C. and Saha, D. 2007. Effect of Diazotrophs on mineralization of organic nitrogen in the rhizosphere soils of rice (Oryza sativa L.). Journal of Crop Weed 3: 69-74.
- Döberainer, J. 1966. Azotobacter paspali sp. nov., uma bacteria fixadora de nitrogenio na rhizosfera de Paspalum. Pesquisa Agropecuaria Brasileira, 1: 357-365
- Dutta, S. and Singh, M.S. 2002. Mustard and Rapeseed response to Azotobacter Indian Journal Hill Farming 15: 44-46.
- Eklund, E. 1970. Secondary effects of some Pseudomonads in the rhizosphere of peat grown cucumber plant. In: Pharis R.P., Reid D.M., eds, Hormonal Regulation of Development. Vol 3, Springer-Verlag, N.Y., p. 613.
- Elgala, A.M., Ishac Y.Z., Abdel Monem M., ElGhandour I.A.I. 1995. Effect of single and combined inoculation with Azotobacter and VA micorrhizal fungi on growth and mineral nutrient contents of maize and wheat plants.
- Govedarica, M., Miliv, V., Gvozdenovi, Dj. 1993. Efficiency of the association between Azotobacter chroococcum and some tomato varieties. Soil plant, 42: 113-120.
- Hennequin, J.R., Blachere, H. 1966. Recherches sur la synthese de phytohormones et de composes phenoliques par Azotobacter et des bacteries de la rhizosphere. Ann. Inst. Pasteur, 3: 89-102.
- Horner, C.K., Burk, D., Allison, F.E., Sherman, M.S. 1942. Nitrogen fixation by Azotobacter as influenced by molybdenum and vanadium. J. Agric. Res., 65: 173-193.
- Lipman, J.G. 1903. Report on the New Jersey Agricultural Experiment Station, 24: 217-285
- Lipman, J.G. 1904. Report on the New Jersey Agricultural Experiment Station, 25: 237- 289.
- Maryenko, V.G. 1964. Dokt TSHA, No. 99 pp. 399-406, 2: 357-360.
- Mandhare, V.K., Patil, P.L., Gadekar, D.A. 1998. Phosphorus uptake of onion as influenced by Glomus fasciculatum, Azotobacter and phosphorus levels. Agricultural Science Digest, 18: 228-230.
- Page, W.J., Shivprasad S. 1991. Azotobacter salinestris sp. nov., a sodium-dependent, microaerophili, and aeroadaptive nitrogen-fixing bacterium. Int. J. Syst. Bacteriol., 41: 369-376. Ann. Microbiol., 51, 145-158
- Pandey, A. and Kumar, S.J. 1989. Soil beneficial bacterial and their role in plant growth promotion. Science Indian Research 48: 134-144.
- Puertas, A., Gonzales, L.M. 1999. Aislamiento de cepas nativas de Azotobacter chroococcum en la provincia Granmay evaluacion de su actividad estimuladora en plantulas de tomate. Cell. Mol. Life Sci., 20: 5-7.
- Sariiv, M.R., Sariiv Z., Govedarica, M. 1988. Efficiency of strain combination of different genera of nitrogen fixing bacteria on sunflower genotypes. In: 12th Intern. Sunflower Conference, Novi Sad, pp. 187-191
- Subba Rao, N.S. 2001. An Appraisal of biofertilizers in India. In : Biotechnology of Biofertilizers. Maximising the Use of Biological Nitrogen Fixation in Agriculture (Ed.S. Kannaiyan) Narosa Pub. House, New Delhi, p. 375.
- Sturz, A.V., Christie, B.R., Nowak J. 2000. Bacterial Endophytes: Potential Role in Developing Sustainable Systems of Crop Production. Critical Reviews in Plant Sciences, 19: 1-30.
- Tandon, H.L.S. 1991. Role of sulphur in plant nutrition. Fertilizer News 36: 69 79.
- Thompson, J.P., Skerman V.B.D. 1981.Validation list No6. Int. J. Syst. Bacteriol., 31
- Tilak, K. Sharma, K.C. 2007. Does Azotobacter help in increasing the Yield. Indian Farming Digest 9:25-28
- Sartaj, A. Wani, 2012. “Effect of balanced NPKS, biofertilizer (Azotobacter) and vermicompost on the Yield and Quality of Brown sarson (Brassica rapa L.)” , M. Sc Thesis, Sher-e-Kashmir University of Agriculture Sciences and Technology, Kashmir, Shalimar, Srinagar.