Unlocking Soil Selenium: Indigenous Microorganisms Facilitate Speciation Shifts to Enhance Plant Bioavailability

Background and Aims Selenium (Se) deficiency and toxicity pose significant health risks, with its solubility and bioavailability being regulated by chemical forms and soil environmental factors. However, the role of indigenous microorganisms in mediating Se cycling and transformation in soils remains insufficiently understood. Methods A pot experiment using Raphanus sativus L. was conducted to evaluate the role of soil-root microbial communities in Se transformation and plant Se accumulation. Soils were sampled at regular intervals for analysis of Se speciation and microbial community composition. Roots and shoots were harvested separately to determine total Se and characterize endophytic microorganisms. Results Soil sterilization increased pH by 0.10 units and organic matter by 5%, while reducing the proportion of particles <0.05 mm and the content of EXC-Se (P < 0.05). RES-Se was the dominant fraction in all Se-treated soils within the first 7 days of incubation. By day 14, OM-Se and SOL-Se became the predominant forms in selenite- and selenate-amended soils, respectively. With prolonged incubation, SOL-Se increased, while FMO-Se and RES-Se fractions decreased. Concurrently, the relative abundance of two Proteobacteria species, Massilia sp000745265 and Massilia sp001412595, increased significantly by day 14 (P < 0.05). Notably, the relative abundance of Massilia sp001412595 was positively correlated with SOL-Se and EXC-Se contents in soil (P < 0.05). Conclusion The application of exogenous selenite or selenate did not significantly alter soil physicochemical properties or microbial communities in either bulk soil or radish roots. However, distinct soil Se speciation profiles exhibited significant correlations with specific root-associated microorganisms.