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

Soybean is one of the world's most valuable crops and fulfills most of its nitrogen requirements by developing symbiotic associations with nitrogen-fixing rhizobia. This reduces the need for chemical fertilizers by converting atmospheric nitrogen into a plant-useable form of nitrogen. Multiple species from four rhizobial genera can nodulate soybean, and the plant's choice of rhizobial partner is reported to change depending on environmental conditions such as pH, host genotype, geographic location, and other environmental factors. This study explores how the age of the soybean plant affects its preference for two frequently reported beneficial rhizobial species (<i>Bradyrhizobium diazoefficiens</i> and <i>Sinorhizobium fredii</i>). By testing inoculation at different growth stages, we discovered that at early growth stages, plants favored <i>Bradyrhizobium</i>, while older plants increasingly selected SR for nodule formation. These findings highlight the level of complexity in plant-microbe interactions and could help optimize bioinoculant strategies for improving sustainability and crop yields.