This study was carried out to determine the influence of some Bradyrhizobium japonicum strains on amount and proportion of N fixed in three soybean cultivars most commonly cultivated in lran. For this purpose five strains of Bradyrhizobium japonicum namely Helinitro, Rhizoking, Biodoz, Gold Coat, and Cb1809 on YMA- slant, obtained from Soil and Water Research Institute of Iran, were used. These strains were tested for purity, infectiveness and their symbiotic effectiveness with soybean cultivars. Three strains namely Helinitro, Rhizoking and Gold Coat were selected. A factorial experiment in 4 replications was conducted under growth chamber conditions with RCBD on a soil with no indigenous bradyrhizobia and therefore in low nitrogen level. Treatments consisted of three soybean cultivars (Sahar, Williams and Clark 63), 3 bacterial strain inoculums (Goldcoat, Rhizoking, Helinitro) and a blank (with no inoculation). Each pot contained 3.5 Kg of homogenized soil and on planting, each seed was inoculated with l ml of inoculum containing about 1×108 cells per ml. During the 4 months of growth, plants were irrigated with distilled water to maintain the soil moisture around 0.8 FC. Plants were harvested at stage R6 while some important growth parameters being measured. The data were analyzed using the MSTATC statistical package. The results indicated that simple as well as interaction effects of bradyrhizobial strains and soybean cultivars were significant. Sahar cultivar, more prominent in lateness, had significantly the best growth characteristics as compared with other cultivars. In addition, all inoculated treatments exhibited a considerable increase in yield as compared with the non-inoculated control. Helinitro strain, despite good infectiveness showed a lower effect on amount as well as proportion of N2 fixed in soybean cultivars. Calculation of proportion of N2 fixed in soybean cultivars, using Nitrogen Difference Method, showed that 80-90% of soybean nitrogen demand was supplied through symbiosis.
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