Spatial Heterogeneity of Iron-Reducing and Se-Reducing Bacteria between Rice Rhizosphere and Bulk Soil Enhances Se Bioavailability

Transformation of selenium (Se) in soil is crucial for its bioavailability and subsequent transfer within the soil-plant system. However, key microbial taxa and processes mediating Se transformation in rice rhizospheres remain poorly understood. Here, we investigated spatial distribution patterns of Se in rice rhizospheres and the underlying mechanisms. Results showed that Se bioavailability was jointly regulated by iron-bound Se release mediated by iron-reducing bacteria and subsequent Se reduction. Microbial results identified that sulfate-reducing bacteria carrying dsr gene were important contributors mediating Se reduction. Heterogeneous spatial distribution of functional genes involved in iron reduction and Se reduction results in varying intensities of Se release and reduction between the rhizosphere and bulk soil. Se bioavailability in the rhizosphere is promoted by the enhanced release of iron-bound Se and suppressed Se(IV) reduction. Our findings offer new insights into microbial processes driving Se transformation and provide practical implications for improving Se bioavailability in soil-plant systems.