The Ratio of S2−/SO42− Induces the Transference of Cadmium in Rhizosphere Soil, Soil Pore Water and Root Iron Plaque

Rice (Oryza sativa L.) readily accumulates cadmium (Cd), posing dietary exposure risks in populations dependent on rice-based diets. This study investigated how sulfur (S) redox processes regulate Cd mobility in S-deficient, Cd-contaminated paddy soil under waterlogged conditions. A pot experiment was conducted with two S treatments (−S and +S, 30 mg kg−1) throughout the rice growing season. S addition markedly increased pore water S2− concentrations during early growth (tillering) and mid-season (booting) and suppressed the diffusion of SO42− from non-rhizosphere to rhizosphere at later stages (filling–maturity). Consequently, Cd in soil pore water was significantly lower in +S than −S treatments at all stages. Sulfur-amended soil showed a redistribution of Cd from labile fractions (exchangeable and carbonate-bound) to more stable fractions (Fe/Mn oxide-bound). Sulfur application also altered the rhizosphere microbiome: the relative abundance of sulfate-reducing bacteria (SRB) increased at the booting and filling stages, while sulfur-oxidizing bacteria (SOB) became more dominant at maturity. Additionally, +S enhanced Cd sequestration on rice root iron plaque by 32–67% during the grain-filling and maturity stages compared to −S. Throughout the rice growing period, redox-driven shifts in the S2−/SO42− ratio emerged as a key control on Cd behavior, with low pe + pH (strongly reducing conditions) promoting Cd sulfide precipitation and high pe + pH (more oxidizing conditions) causing Cd remobilization.