Arsenic redox transformation and methylation are long-recognized to control its chemical speciation, thus bioavailability to rice plants. Arsenic thiolation, the formation of soluble inorganic thioarsenates (As^VS^-II_nO_4-n^3-, with n=1-4) and methylated thioarsenates ((As^VS^-II_n(CH₃)O_3-n^3- or As^VS^-II_n(CH₃)₂O_2-n^3- with n=1-3) under sulfate-reducing conditions, has to date seldom been considered in soil environments. Inorganic thioarsenates are structural analogues to arsenate and formed from arsenite, zerovalent sulfur, and sulfide. How methylated thioarsenates form in nature has not been investigated, yet, but in synthetic solutions they are formed from the reaction between methylated arsenates and microbially produced sulfide.

Studies of 31 distinct paddy fields across China using standardized soil anaerobic incubations revealed widespread occurrence of arsenic thiolation over various environmental gradients. The magnitude of arsenic thiolation was comparable to methylation, with thioarsenates contributing in sum up to 29% to total arsenic. Sulfate addition generally increased total arsenic thiolation, but with only minor impacts on its large scale distribution. The geographic pattern of total arsenic thiolation was intrinsically determined by variations in soil parameters, and was largely correlated with total zerovalent sulfur formed anaerobically. The speciation of thioarsenates was strongly driven by soil pH values. Methylated thioarsenates dominated in acidic paddy soils, whereas inorganic thioarsenates dominated in neutral and alkaline paddy soils. Both inorganic and methylated thioarsenates were found to be partially decomposed after soil drainage, but all species were also discovered after re-flooding. Further soil oxidation experiments proved that thioarsenates oxidized the fastest the more thio-groups they had. Dimethylmonothioarsenate showed the highest stability upon prolonged oxidation. Given the ubiquitous occurrence of arsenic and sulfur, we suggest that thioarsenate formation as well as their environmental fate need to be deciphered in future paddy soil arsenic studies.