Size-Dependent Mechanism of Selenium Nanoparticles in Regulating Cadmium Accumulation in the Soil–Rice (Oryza sativa L.) System

Selenium nanoparticles (Se NPs) have shown potential for regulating cadmium (Cd) accumulation in rice; however, their particle-size-dependent effects in the paddy soil–rice system remain unclear. In this study, a whole-growth-period pot experiment was conducted to investigate the mechanisms of Se NPs with different particle sizes (50, 100, and 200 nm) applied at different rates (20 and 50 mg/kg) in regulating Cd accumulation in rice grown in Cd-contaminated paddy soil. Se NP application significantly altered rhizosphere pH and redox potential, decreased Cd concentrations in soil solution and DTPA-extractable Cd, and increased soil solution Se and KH2PO4-extractable Se. In the 20 mg/kg treatment, Se NPs promoted the transformation of Cd from exchangeable fractions to Fe–Mn oxide-bound fractions, indicating reduced Cd mobility. Se NPs also enhanced root iron plaque formation and increased the retention of Fe, Cd, and Se on the root surface. XPS analysis showed that 50 and 100 nm Se NPs increased the proportion of Fe(III) in root iron plaque, thereby strengthening Cd adsorption and immobilization at the root–soil interface. In addition, Se NP treatments reshaped rhizosphere microbial communities. The 50 nm treatment showed stronger effects on fungal taxa related to organic matter decomposition and Se activation, whereas the 100 nm treatment more effectively enriched bacterial groups associated with Fe and S cycling, including Thermodesulfobacteriota and Sideroxydans. Among all treatments, 100 nm Se NPs at 20 mg/kg reduced grain Cd from 0.466 to 0.050 mg/kg, representing an 89.27% reduction, while maintaining grain Se at 0.384 mg/kg, which is within the acceptable range for selenium-enriched rice. Overall, 100 nm Se NPs showed the best balance among Cd mitigation, Se biofortification, and ecological safety, suggesting their potential for application in safe rice production in Cd-contaminated paddy soils.