Co-inoculation of Rhizobium and Bradyrhizobium promotes growth and yield of common beans

Abstract

Some strains of Bradyrhizobium benefit the biological N2 fixation (BNF) and growth of common beans (Phaseolus vulgaris L.) when co-inoculated with Rhizobium tropici under greenhouse conditions. Here, we evaluated the benefit of Bradyrhizobium diazoefficiens strain CPAC 7 co-inoculated with Rhizobium tropici strain CIAT 899 on common bean in the field. Three Bradyrhizobium strains were co-inoculated with R. tropici CIAT 899 in one experiment in the greenhouse and two in the field. The greenhouse experiment combined each of three Bradyrhizobium strains (29w, USDA 110, and CPAC 7) in co-inoculation with CIAT 899 in Leonard jars. Then, two of these strains, 29w and CPAC 7, were co-inoculated with Rhizobium on-seeds (1.2 × 106 CFU seed−1) or both on-seed and in-furrow (1.08 × 108 CFU m−1) in a field experiment. Finally, a second field experiment evaluated strain CPAC 7 at doses ranging from 1.5 × 105 to 1.2 × 106 CFU seed−1. Co-inoculation with strain CPAC 7 improved shoot biomass accumulation in Leonard jars. In the field, co-inoculation of CPAC 7 with CIAT 899 on-seed enhanced grain yield by 20% compared with inoculation of CIAT 899 only (p < 0.10). The percentage of N derived from the air (%Ndfa) was between 50 and 60%, with only a tendency towards co-inoculation benefit. Given that the %Ndfa of the inoculated plants was not statistically different between treatments, the increased grain yield was not entirely due to BNF but also due to an increase of soil N uptake. In the second field trial, larger doses of inoculum increased nodulation and root biomass, but significant differences in grain yield were not observed. Our results show that the Rhizobium-Bradyrhizobium co-inoculation can enhance growth and grain yield of common beans and indicate that a plant growth-promoting mechanism besides BNF may be involved in the beneficial co-inoculation effect. However, further adjustments are needed to ensure a better response under field conditions.