Dose-dependent control of arsenic in rice by sulfate-mediated iron-sulfide formation

Arsenic (As) contamination in paddy soils poses a potential risk to rice safety, while sulfate fertilization may reduce As bioavailability through sulfur-mediated redox processes. However, the dose-dependent effects of sulfate (SO 4 2− ) under flooded–drained conditions remain poorly understood. Here, a pot experiment was conducted with four K 2 SO 4 application rates (0, 50, 100, and 200 mg K 2 SO 4 kg −1 soil) to investigate changes in porewater chemistry, soil As fractions, bacterial communities, and As accumulation in rice during the heading, grain-filling, and maturity stages. Thermodynamic simulations were further performed based on porewater geochemical conditions. The 100 mg K 2 SO 4 kg −1 soil treatment resulted in the greatest SO 4 2− consumption (∼70%) and the highest sulfide (S 2− ) concentration, reaching 350 mg L −1 . Desulfobacca , the dominant genus associated with SO 4 2− reduction, also reached its highest relative abundance under this treatment. Concurrently, dissolved iron (Fe) and As concentrations decreased, whereas solid-phase As retention increased, consistent with enhanced As immobilization associated with Fe sulfide (Fe–S) precipitation. In contrast, the 200 mg K 2 SO 4 kg −1 soil treatment generated less S 2− and showed weaker As immobilization. Grain As concentration decreased by 48% under the 100 mg K 2 SO 4 kg −1 soil treatment, compared with only 26% under the 200 mg K 2 SO 4 kg −1 soil treatment. These results demonstrate a non-linear SO 4 2− dose response, with an intermediate application rate promoting microbial SO 4 2− reduction and Fe–S formation, thereby decreasing porewater As bioavailability and its accumulation in rice grains.