Bioavailability, accumulation, translocation and transformation of Cd depend on the molar ratio of soil Se-to-Cd: Evidence from a natural SeCd rich soil-corn system

The existing understanding of the critical interactions between selenium and cadmium in crops grown in naturally Se–Cd-rich soils remains limited. To address this knowledge gap, this study investigated agricultural systems in naturally Se–Cd-rich areas within the Wumeng Mountain region of Guizhou, China. Samples of roots, stems and leaves, along with the corresponding rhizosphere soils of corn (Zea mays L.) in the grain-filling stage, were collected and analysed for Se and Cd concentrations and speciations. Results demonstrated significant inverse relationships among rhizosphere-soil Cd bioavailability, Cd accumulation in corn plant compartments, and increasing bioavailable Se:Cd molar ratios in soil (p < 0.05). Notably, the soil Se:Cd molar ratio was inversely correlated with Cd translocation from soil to roots (p < 0.01) while showing no significant effects on aboveground translocation (root-to-stem and stem-to-leaf, p > 0.05). These outcomes suggest that the soil Se:Cd molar ratio primarily regulates Cd accumulation at the soil–root interface, with limited impact on subsequent translocation within plant compartments. Moreover, the transformation of Cd into Cd–SeCys and Cd–SeMet occurred more frequently in the stem and leaf compared with that in the root. Furthermore, the underlying mechanism was conceptualised as comprising ‘three barriers’: SeCd complex formation in the rhizosphere (Barrier 1), Cd sequestration by the selenohydryl group (-SeH) in plant cells (Barrier 2) and Cd inhibition by phloem and nodes (Barrier 3). These findings support the use of Se:Cd molar ratios as indicators for sustainable crop safety in SeCd co-contaminated regions. This study advances the understanding of the potential mechanisms underlying the Se:Cd molar ratio and its influence on Cd toxicity in natural environments. Based on these findings, the Se:Cd molar ratios serve as an effective indicator for sustainable management of Cd contamination in agriculture.