Soybean genotype determines functional symbiotic outcomes with phylogenetically diverse Bradyrhizobium

Soybean can meet much of its nitrogen demand through biological nitrogen fixation (BNF). However, yields in sub-Saharan Africa (SSA) remain constrained by nitrogen deficiency and inconsistent responses to rhizobial inoculation. Despite widespread promotion of inoculation, the influence of host genotype on symbiotic effectiveness in African soybean cultivars remains is not well characterized. We assessed nodulation, nitrogen fixation, and growth responses of three widely cultivated Ghanaian soybean cultivars inoculated with ten phylogenetically diverse Bradyrhizobium strains under controlled, nitrogen-free conditions. Symbiotic performance was assessed using nodulation traits, acetylene reduction assay, shoot biomass, and relative symbiotic effectiveness (RSE) relative to mineral nitrogen treatment. Symbiotic outcomes were strongly dependent on the host. Two cultivars exhibited high nitrogen fixation and growth with multiple strains, whereas one showed consistently weak fixation and growth despite nodulation, indicating host-imposed post-infection constraints. Nodule weight and nitrogenase activity, but not nodule number, reliably predicted symbiotic benefits. Notably, several non-classical soybean Bradyrhizobium strains performed comparably or better to recognized soybean symbionts when paired with compatible hosts. These results demonstate that host genotype is a key determinant of soybean BNF effectiveness and highlight the need to integrate symbiotic performance traits into breeding and inoculant design for reliable BNF in low-input SSA farming systems.