Soybean root nodule occupancy: competition between Bradyrhizobium and Sinorhizobium strains inoculated at different plant growth stages

Abstract

Soybean is frequently nodulated by species from the Bradyrhizobium (BR) and/or Sinorhizobium (SR) genera. Several factors, such as soil pH, host genotype, geographic location, and other environmental variables, are reported to influence the preferential selection between BR and SR species within soybean root nodules. However, it remains unclear whether the age of the host plant at the time of inoculation affects preferential rhizobial selection. To investigate this, we inoculated soybean plants with different cell densities of BR and SR strains at three time points: at sowing (T0), 2 weeks after germination (T2), and 4 weeks after germination (T4). We used 16S rRNA gene amplicon sequencing of root nodules and rhizosphere samples to assess the relative abundance of BR and SR in nodules and rhizosphere. We observed a clear shift in nodule occupancy that favored BR at the time of seed sowing (T0) but increasingly favored SR when plants were inoculated at T2 and T4 stages. Specifically, at T4, SR dominated in nodules across all treatments, representing 88%–99% of total sequences, regardless of applied inoculum ratio. In contrast, a similar number of sequences for both strains was detected in the rhizosphere at the time of the final harvest. These results highlight host age as an important ecological driver in legume–rhizobium interactions and suggest that inoculation time strongly influences microsymbiont selection. This information is important in understanding rhizobial competition and optimizing the timing of inoculation for soybeans.

Department(s)

Biology

Document Type

Article

DOI

10.1128/aem.02489-25

Keywords

Bradyrhizobium diazoefficiens, host age, nodule occupancy, rhizobial competition, Sinorhizobium fredii, soybean

Publication Date

3-1-2026

Journal Title

Applied and Environmental Microbiology

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