N2- FIXATION ABILITY OF THREE MAIN SOYBEAN CULTIVARS IN SYMBIOSIS WITH BRADYRHIZOBIUM JAPONICUM USING N-15 ISOTOPE DILUTION METHOD IN IRAN

The present pot study was carried out for comparison of N2 fixation ability for three main soybean cultivars in Iran i.,e. Sahar, Williams and Clark 63. The experiment was conducted under the proper growth chamber condition, with a randomized complete block design on a soil with no indigenous rhizobia and low nitrogen content. Each treatment was replicated four times. Each pot contained 3.5-kg air dried homogenized soil, and at the time of planting each seedling was inoculated with l ml of inoculum containing approximately 9 × 10 8 cells per ml. For quantifying the fixed nitrogen, using A- value (N-15) method, two solutions of N-15 enriched ammonium sulfate containing 9.616 and 2.086% N-15 atom excess were applied in amount of 6.67 and 33.33 mg/kg N at the stage V2 for the fixing, and for the stages V2, R2 and R5 as reference pots. During 4 months of growth, the plants were irrigated with distilled water to maintain the soil moisture approximately 0.8 of the field capacity. The plants were harvested at the plant developmental stage reproductive 6 (R6) and a number of growth parameters were measured. According to the results, Sahar cultivar which is more lateness, showed a higher results in the most measured characteristics, including nitrogen derived from air, shoot dry matter, nodule dry matter and the whole dry matter of plant, significantly. All the three cultivars could supply more than 90 % of nitrogen demand through symbiosis. The cultivars were different significantly in amount of nitrogen derived from air, despite of the percent of nitrogen derived from air.