Sulfur depletion through repetitive redox cycling unmasks the role of the cryptic sulfur cycle for (methyl)thioarsenate formation in paddy soils

Abstract

Inorganic and oxymethylated thioarsenates form through the reaction of arsenite and oxymethylated arsenates with reduced sulfur, mainly as sulfide (S^II−) but also as zerovalent sulfur (S⁰). In paddy soils, considered low-S systems, microbial reduction of the soil's “primary” sulfate pool is the principal S^II− source for As thiolation. Under anoxic conditions, this primary pool is readily consumed, and the precipitation of iron (Fe) sulfides lowers S^II− availability. Nonetheless, sulfate can be constantly replenished by the reoxidation of S^II− coupled with the reduction of Fe^III phases in the so-called cryptic S cycle (CSC). The CSC supplies a small secondary sulfate pool available for reduction and, according to previous studies, As thiolation. However, sulfate concentrations commonly found in paddy soils mask the biogeochemical processes associated with the CSC. Here, we depleted a paddy soil from excess S, Fe, and As from a paddy soil through repetitive flooding and draining (e.g., redox cycling). After 10, 20, and 30 such cycles, we followed thioarsenate formation during an anoxic incubation period of 10 days. Higher S/As ratios increased As thiolation contribution to total As up to 10-fold after 30 cycles. During the anoxic incubation, the depleted soils showed a transient first phase where the reduction of the primary sulfate pool led to inorganic thioarsenate formation. In the second phase, methylthioarsenate formation correlated with partially oxidized S species (S⁰, thiosulfate), suggesting CSC-driven sulfate replenishment, re-reduction, and thiolation. Methylthioarsenates formed even as inorganic thioarsenates de-thiolated, indicating thermodynamic preference under S-limited conditions. This study highlights the role of the CSC in sustaining thioarsenate formation in low-S systems.