Improving the Performance of Bradyrhizobium japonicum by Double Inoculation in Non-Fertilized and Fertilized Wheat–Soybean Rotation

Field experiments were conducted during the four crop seasons 2013 through 2015 on Bradyrhizobium japonicum inoculation of wheat, soybean or double inoculation of both crops under fertilization or not with NPK. Inoculation of wheat with B. japonicum gave 21 and 18% increase in grain yield and plant N uptake under fertilization regime. Pre-inoculation of wheat resulted in 23% higher nodule numbers in soybean; however, the soybean seed yield and plant N content increases were non-significant. Inoculation of only soybean increased the nodule numbers, seed yield and N content in fertilized soybean by 43, 11 and 16%, respectively; double inoculation increased it by 53, 17 and 22%. Averaged over inoculation treatments and fertilization regimes, the rhizobial numbers in soybean were 2.0× of those in wheat. The effects of chemical fertilization stimulated the rhizobial populations 2.1× over non-fertilized. Averaged over all 10-stages and both fertilization regimes, the rhizobial populations in soybean rhizosphere increased 1.8× by pre-inoculating wheat; 3.0× by inoculating only soybean and 4.3× by double inoculation over uninoculated. Overall, the symbiotic performance was: Double inoculation > Soybean inoculation > Wheat inoculation > Uninoculated. Application of Bradyhizobium inoculants to both soybean and wheat was beneficial to maximize rhizobial numbers, soybean nodulation, crop and N yield of soybean and wheat, and buildup soil organic matter and nitrogen. The additional N content in soybean and wheat by double inoculation over single soybean inoculation was ~ 40 kg N ha−1.