Diversity of Bradyrhizobium populations associated to soybean–maize rotations in Québec, Eastern Canada, and their potential to improve growth of both plant species

► Bradyrhizobial diversity of 255 strains from soybean–maize rotations in Québec. ► Six IGS types; 40% and 30% in IGS I and III; plant species: no effect on diversity. ► Reference strains and commercial inoculants in IGS III and VI. ► 25 strains more effective symbiotically than reference strain (532c) with soybean. ► 20 PGPR strains with 10–20% increase in maize SDW. Bradyrhizobial populations associated with soybean–maize rotations have not been extensively studied, especially in Canada. In this work, we estimated the diversity of 255 bradyrhizobial strains isolated from soil, rhizosphere and nodules in farms under soybean or maize grown in rotation in Québec, Eastern Canada, and we characterized them for their ability to promote growth of both plants. The population was distributed into six IGS types. The major types IGS I and III included 98 and 70 strains respectively. About 50% of strains from soils of both plants and from soybean nodules were included in IGS I. Reference strains of Bradyrhizobium japonicum and inoculant products were included in IGS III and VI. Diversity indices were similar for both plants but were lower in soil and nodules than in rhizosphere of soybean. REP-PCR analysis was congruent with IGS typing, the major REP-clusters including a majority of strains within a same IGS type. With soybean, 25 strains allowed higher symbiotic effectiveness (shoot dry yield) than the commercial strain 532c and 5 strains (out of 8 tested) were more competitive for nodule occupancy. With maize, 20 strains showed a PGP effect increasing shoot dry weight by 10–20%. There was no link between the IGS type and symbiotic effectiveness or PGP effect, neither with siderophore or IAA production. Our results indicate that diverse bradyrhizobial strains may compete with inoculant strains, but this diversity can be exploited to select superior strains able to improve growth of both soybean and maize.