Exogenous selenium addition alters key community composition and decreases the abundance of diazotrophs in soybean rhizosphere

Exogenous selenium addition is an effective strategy to enhance selenium content in legume plants. However, its impact on the composition and nitrogen-fixing potential of rhizosphere diazotrophic communities remains poorly understood. To elucidate the effects of exogenous selenium on rhizosphere nitrogen-fixing potential and the underlying mechanisms, we conducted pot experiments with treatments including selenium addition (Se), chemical fertilization (CF), and their combination (CFSe). Our results showed that CF and CFSe significantly reduced diazotrophic community richness (~39.36 % and 40.69 %, respectively) and diversity (~10.55 % and 20.29 %, respectively), whereas Se had no significant effect. Furthermore, Se, CF, and CFSe treatments significantly reduced nifH gene copy numbers by 54.18 %, 83.96 %, and 91.06 %, respectively. Random Forest and redundancy analyses identified total nitrogen (TN), available nitrogen (AN), total phosphorus (TP), and available selenium as critical abiotic factors influencing the α- and β-diversity of diazotrophs. Co-occurrence network analysis revealed that diazotrophs within module 1 were particularly sensitive to selenium addition and fertilization, with their community stability being crucial for maintaining diazotrophic α diversity. Notably, Se and CF treatments appeared to suppress rhizobia-legume symbiosis by increasing the abundance of non-symbiotic Cyanobacteria in module 1, thereby reducing nodule density in legumes. These findings indicate that exogenous selenium addition can profoundly alter the composition and function of diazotrophic communities, reducing their nitrogen-fixing potential and modulating plant-microbe interactions.