Dose-Dependent Control of Arsenic in Rice by Sulfate-Mediated Iron-Sulfide Formation

Arsenic (As) contamination in paddy soils poses a severe threat to rice food safety and human health. Although sulfate fertilization has been proposed as a mitigation strategy, its performance is often inconsistent, and the dose-dependent controls on As mobility and plant uptake under alternating flooded-drained conditions remain poorly resolved. This uncertainty limits the optimization of agronomic practice. Here, we conducted a rice pot experiment with potassium sulfate (K2SO4) applied at 0, 50, 100, and 200 mg kg^-1. We tracked porewater geochemistry through the rice growth cycle, quantified solid-phase As speciation at the end of the experiment, and characterized microbial community responses. The system exhibited a clear threshold response. The intermediate sulfate dose (100 mg kg^-1) maximized sulfide accumulation and promoted FeS precipitation, coinciding with the lowest porewater As concentrations during flooding and a 48% reduction in grain As relative to the control. By contrast, the high sulfate dose (200 mg kg^-1) produced weaker sulfide accumulation, limited FeS formation, and yielded less effective As immobilization. Together, these results indicate that Fe-S mineral buffering provides the mechanistic link between sulfate supply and As bioavailability in rice systems, and they identify an intermediate sulfate input as the optimal range for mitigating As risk in contaminated paddy soils.