The objective of this study was to determine the amount of phosphorus application needed for the Bradyrhizobium inoculation to improve soybean N uptake and grain yield. Two factors were investigated: inoculation with Bradyrhizobium diazoefficiens strain USDA 110 and application of phosphorus fertilizer with different doses (0; 50; 100 and 150 kg ha-1 P2O5). Trials were carried out in two agro-ecological zones (ZAE) in Benin with two experimental sites namely Kétou and Zè. The experimental design was a Complete Randomized Block Design. Results showed that the highest mean height values were recorded T1+150P (42.22 cm at Zè and 43.82 cm at Kétou) while the lowest values were recorded with the treatment T0 + 0P. Treatment T0 +100P gave the highest aboveground biomass at 10 weeks after sowing at Kétou (2521.43 kg DM ha-1) while T1+150P recorded the highest aboveground biomass at Zè (3392.75 kg DM ha-1). At Kétou, the grain yield was increased under T1+100P by 98% compared with T0+0P. At Zè, the treatments T1+100P significantly increased the soybean grain yield by 147% compared with T0+0P. Likewise, T1+100P allowed to have the highest total nitrogen yields in the grain which is respectively 144.13 kg ha-1 N and 295.02 ha-1 kg N at Kétou and Zè. Overall, the inoculation with B. diazoefficiens had significantly increased the number of nodules for all of the phosphorus fertilizer amounts. The mycorhization frequency was significantly lower under the treatment T0 + 0P (41.46%) than the other treatments while that was higher under the treatment T1+50P (76.04%).
Keywords: Bradyrhizobium diazoefficiens, soybean inoculation, grain yield, Benin.
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