We examined the effects of soil moisture changes on soybean growth, yield, and the structure of soybean-nodulating bradyrhizobial communities in cultivars with different Rj genotypes. The experiment was conducted using cultivation pots with soybean cultivars Bragg (non-Rj), CNS (Rj2Rj3), D-51 (Rj3), and Fukuyutaka (Rj4). Test strains included Bradyrhizobium diazoefficiens USDA 110T, B. japonicum USDA 6T and USDA 123, and B. elkanii USDA 31. Cultivation pots were built with 15-cm ridges, and three soil moisture conditions were generated by varying the presence and placement of drainage holes on the pots. Declining soil moisture significantly reduced shoot length, shoot dry weight, root dry weight, root length, nodule number, pod number, pod dry weight, and seed number. An occupancy analysis showed that USDA 110 dominated Fukuyutaka only; across treatments, it was the most abundant under high soil moisture, but significantly declined with reductions in soil moisture, where USDA 31 became dominant. A non-metric multidimensional scaling analysis revealed shifts in community compositions in response to soil moisture and cultivar. Collectively, these results indicate that soybean growth, yield, and symbiosis with bradyrhizobia are strongly affected by soil moisture and also that these effects vary among cultivars.
Ito et al. (Thu,) studied this question.