Native bradyrhizobia for soybean: genetic and functional diversity in Heihe soils, a major production zone of Heilongjiang, China

Soybean (Glycine max) forms symbiotic nitrogen fixation with rhizobia, and compatible, efficient rhizobia in soils are vital for its sustainable production. This study analyzed the distribution and traits of native soybean-associated rhizobia in soils from Heihe, Heilongjiang Province, to identify strains with high symbiotic nitrogen fixation efficiency and environmental adaptability, and select candidates for local microbial inoculants to boost sustainable soybean production. Seventy-four rhizobial isolates were obtained from three Heilongjiang sampling sites using a local soybean variety, and characterized genetically and symbiotically. PCR-RFLP of IGS DNA grouped them into 9 genotypes. Multilocus sequence analysis (16S rRNA, recA, atpD, gyrB genes) placed representative strains into three known species (Bradyrhizobium japonicum, B. diazoefficiens, B. ottawaense) and an uncharacterized Bradyrhizobium group (64%, dominant). Phylogeny of nodC/nifH markers showed affiliation with symbiovar glycinearum. All strains nodulated soybean with symbiotic efficiency (67-88%); about half enhanced plant biomass. Three strains (DG28, GCZ12, SH16) showing superior symbiotic efficiency. Representative strains had varied tolerance to alkalinity, high temperature, and PEG-induced drought. Strain SH16 combined high efficiency and stress tolerance. These results provide promising candidates for local soybean inoculant development.