DGT and kinetic analyses differentiate Se and Cd bioavailability in naturally enriched paddy soils

Naturally selenium (Se)-rich soils often contain elevated cadmium (Cd) levels, complicating safe production of Se-enriched rice. This study employed diffusive gradients in thin-films (DGT) and DGT-induced fluxes in soils (DIFS) model to determine Se and Cd bioavailability in paddy soils. We investigated desorption kinetics and accumulation patterns in rice using paired rhizosphere and grain samples from 65 field sites in Guangxi, China, encompassing Se-enriched karst and non-karst soils. Despite greater total Se and Cd contents in karst soils, their elevated pH, along with greater soil organic matter and total Fe, Mn, and Ca contents, constrained Se and Cd bioavailability, resulting in similar accumulation levels in rice grains from both soil categories. DIFS-derived kinetic data revealed that Se was replenished 75.4 times faster than Cd, but Cd had an 83.2 times larger labile pool, leading to a stronger overall Cd resupply capacity. DGT-based labile Se:Cd molar ratios showed that rice Cd content declined sharply as the ratio increased from 0.7 to 4.0, stabilizing at its lowest level when exceeding 20. Moreover, DGT measurements demonstrated stronger correlations with grain Se and Cd concentrations compared to traditional methods. Our findings highlight the effectiveness of DGT and kinetic analyses in determining Se and Cd bioavailability in high-background paddy soils, offering insights for balancing Se fortification and Cd risk mitigation in rice production.