Biological N2 fixation and yield performance of soybean inoculated with Bradyrhizobium

The performance of the soybean (Glycine max) inoculated with Bradyrhizobium is influenced by cropping history. However, re-inoculation with elite strains of Bradyrhizobium generally provides increases in soybean grain yield. The Amazonian savanna has an agricultural frontier area where soybeans have recently been introduced. In this study, we evaluated soybean nodulation, N accumulation, N-2 fixation, and grain yield responses to inoculation with the commercial Bradyrhizobium strains SEMIA 587 and SEMIA 5019 of B. elkanii, SEMIA 5079 of B. japonicum, and SEMIA 5080 of B. diazoefficiens. Four field experiments were performed in areas with or without previous cropping with inoculated soybean that contained preexisting bradyrhizobia populations of >= 10(3) or < 10(2) cells g(-1) soil, respectively. Inoculation of soybean in first-year cropping areas resulted in increases in several parameters compared to those in non-inoculated soybean; as expected, these effects were less pronounced when areas had been previously cropped with soybean. Despite no statistical differences, re-inoculation provided average increases in grain yield of 12 to 18% compared to that in the non-inoculated control. A dose of 200 kg ha(-1) of N-fertilizer impaired the nodulation and did not increase grain yields when compared with the inoculation. Estimates obtained with the (15) N natural abundance technique confirmed the high contribution of N-2 fixation in inoculated plants, ranging from 84 to 90%. Although soybean cropping increases the soil bradyrhizobia population size, annual inoculation is important to guarantee host infection by efficient strains of soybean Bradyrhizobium and their persistence in savanna soils.

Automated summary

The performance of soybean inoculated with Bradyrhizobium strains is influenced by cropping history, with re-inoculation generally leading to increases in grain yield. However, the effects are less pronounced in areas previously cropped with soybean. The use of N-fertilizer can impair nodulation, but annual inoculation is important to ensure efficient nodulation and persistence of Bradyrhizobium strains in savanna soils.