Comparison of nitrogen fixation in some Bradyrhizobium Japonicum strains using the N-15 isotopic method

Several studies have demonstrated the significant contribution of atmospheric N fixation to soybean nutrition and growth. The variability in N fixing is due to the several factors including rhizobial strain. Hence, this study was conducted to compare some commercial Bradyrhizobium japonicum strains for the amount and proportion of N fixation in soybean (Cv. Williams) which is most commonly cultivated in Iran. Initially, strains were tested for purity, infectiveness and symbiotic effectiveness using Leonard jars and three of the more efficient strains Helinitro, Rhizoking and Gold Coat were selected. A pot experiment (four replicated) was carried out with RCBD on soil without indigenous rhizobia having 530-ppm total nitrogen under greenhouse condition. Treatments included three inoculum of selected strains (separately) and a set of non inoculated blank. For estimating BNF by A-value method, two solutions of N-15 enriched ammonium sulfate, containing 9.616 and 2.086% n-15 atom excess, were applied in amount of 20 and 100 kg N/ha at stage V2 in inoculated pots and stages V2, R2 and R5 to reference pots. During the 4 months of growth, plants were irrigated with distilled water to maintain 80% FC. Plants were harvested at stage R6. The results showed that the effects of rhizobial strains were significantly different. Gold Coat and Rhizoking strains had higher significant results compared with Helinitro. The percentages of fixed nitrogen in plants inoculated with these strains were 92.3, 90.7 and 84.7%, respectively. On the basis of these results, it may be necessary to pre-select more compatible rhizobial strain for supplying the N-requirements of soybean. (Author)