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E-International Scientific Research Journal ISSN: 2094-1749 Volume: 3 Issue: 2, 2011 Azospirillum brasilense (Galal and Thabet 2002). This field trial concluded that soybean residue as enriched N material, and Azospirillum brasilense inoculation enhanced growth, grain and N yields of wheat cultivars grown in poor fertile sandy soil. All studies on lowland rice reported a positive N balances but no one determine what proportion of this N may be derived from free-living N 2 -fixing cyanobacteria in the flood water, heterotrophic N 2 -fixers in the soil or those associated with the plant (Boddey et al. (1995). Against the acetylene reduction (AR) assay, the 15N isotope dilution technique has the potential to estimate contribution of BNF to the plants over the whole growth season and unlike the N balance and acetylene reduction techniques, it estimates fixed N actually incorporated into the plant tissue (Chalk 1985). The main problem with this technique lies in labelling the soil with 15 N, if the enrichment varies with area, depth or time, different plants (the control and different rice varieties) may have different N uptake patterns and do not obtain the same 15 N enrichment in the soil derived N, an assumption essential to the application of the technique (Boddey 1987). Many diazotrophs has been isolated from the rhizospherte and roots of rice such as Azotobacter, Azospirillum, Pseudomonas, Klebsiella and Enterobacter, but the presence of these microorganisms in association with rice roots does not necessarily mean that the plants obtain significant contribution from biological fixation. In this respect, Boddey et al. (1986) counted numbers of Azospirillum brasilense above 10 6 cells g fresh root -1 of wheat plants grown in 15 N- labelled soil and in the same time, plant N uptake was significantly increased by inoculation, but 15 N enrichment data showed that the response was not due to BNF inputs. Galal and El-Ghandour (2000) examined the effect of inoculation of Azospirillum brasilense on grain yield, biological nitrogen fixation and NPK uptake by two rice cultivars (Giza 172 and IR 28), grown under greenhouse conditions (pot experiment). 15 N data confirmed the enhancement of N derived from fertilizer and 15 N recovery due to inoculation with Azospirillum as compared to the uninoculated treatment. The proportion of N derived from air not exceeds 28% indicating that the effective mechanism is the promotion of plant growth and nutrients uptake rather than BNF. Similar findings were observed when comparative study was held between Azospirillum brasilense, Azolla pinnata and arbuscular mycorrhizae fungi, as individual inoculum, using 15 N tracer technique in pot experiment with japonica rice variety, Giza 171 (Galal; 2000). Azolla The aquatic fern Azolla is probably used as a green manure on < 2% of the world’s rice crop, but this still represents around 2 to 3 million ha (Giller and Wilson 1991), Under optimal conditionsa Azolla doubles in biomass every 3 to 5 days and one crop can be expected to accumulate between 70 and 110 kg N ha -1 (Ventura and Watanabe 1993). With experimental values of Pfix commonly >70% (Kumarasinghe and Eskew 1993; Roger and Ladha 1992).Azolla represents a potentially important source of N for flooded rice. However, there is little information available concerning inputs of N by Azolla in farmer’s fields. Since growth and N 2 -fixing capacity of Azolla can be affected by many environmental variables, mineral nutrition (particularly phosphorus), insect predators and pathogens, it is uncertain whether experimental potentials are ever realized in farmer’s paddies (Giller and Wilson 1991). 113
E-International Scientific Research Journal ISSN: 2094-1749 Volume: 3 Issue: 2, 2011 Estimation of N 2 fixed by Azolla and utilized by rice plants using 15 N techniques indicated that N derived from Azolla pinnata was identical to those derived from 15 N-labelled urea and more than 80% of the N utilized by rice was gained from fixation. This result was true under both sterilized and nonsterilized soils (Galal 1997). Azolla have a potential to fix atmospheric air in adequate quantities (more than 60% of total N uptake) which compensated a considerable amount of N requirements for rice production (Galal, 2000; Galal and El-Ghandour, 2000). References Abdel Monem, MAS, Khalifa, HE, Beider, M, El-Ghandour, IA and Galal, YGM (2001) Using biofertilizers for maize production: Response and economic return under different irrigation treatments. J. sustainable Agric. 19: 41-48. Azcon-Aguilar, C. and Barea, J.M. (1996). Arbuscular mycorrhizas and biological control of soil-borne plant pathogens: an overview of the mechanisms involved. Mycorrhiza 6: 457-464. Barea, J.M. (1997). Mycorrhiza / bacteria interactions on plant growth promotion. In: Ogoshi A., Kobayashi L., Homma Y., Koclama E., Kondon N., Akino S. (eds) Plant growth-promoting rhizobacteria, present status and future prospects. OECD, Paris, pp. 150-158. Barea, J.M. (2000). Rhizosphere and mycorrhiza of field crops. In: Toutant J.P., Barazs E., Galante E., Lynch J.M., Schepers J.S., Werner D., Werry P.A. (eds) Biological resource management: connecting science and policy, (OECD) INRA Editions and Speringer, Berlin Heidelberg New York, pp. 110-125. Boddey, RM (1987) Methods for quantification of nitrogen fixation associated with gramineae. CRC Crit. Rev. Plant Sci. 6: 209-266. Boddey, RM, de Oliveira, OC, Urquiaga, S, Reis, VM, de Olivares, FL, Baldani, VLD and Döbereiner, J (1995) Biological nitrogen fixation associated with sugar cane and rice: Contribution and prospects for improvement. Plant and Soil 174: 195-209. Boddey, RM, Baldani, VLD, Baldani, JI and Döbereiner, J (1986) Effect of inoculation of Azospirillum spp. on the nitrogen assimilation of field grown wheat. Plant and Soil 95: 109- 121. Brockwell, J., Bottomley, P.J. Thies, J.E. (1995). Manipulation of rhizobia microflora for improving legume productivity and soil fertility: a critical assessment. Plant and Soil 174: 143- 180. Burns, RC and Hardy, RWF (1975) Nitrogen fixation in bacteria and higher plants. Springer Verlag, Berlin, 189 p. Chalk, PM (1985) Estimation of N 2 fixation by isotope dilution: An appraisal of techniques involving 15 N enrichment and their application. Soil Biol. Biochem. 17: 389-410 114
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