Long-Term Paddy Soil Development Buffers the Increase in Arsenic Methylation and Thiolation after Sulfate Fertilization

Sulfate fertilization has been proposed to limit arsenic (As) mobility in paddy soils and accumulation in rice grains. However, As and sulfur (S) have complex biogeochemical interactions. Besides the desired precipitation of sulfides that sorb or incorporate As, S can enhance As biotic methylation and abiotic thiolation. Incubating 50- to 2000-year-old paddy soil chronosequence samples without and with S-addition showed the highest relative increases in the formation of low-sorbing, phytotoxic methylated oxyarsenates, and low-sorbing, cyto-, and phytotoxic thioarsenates in the youngest soil. These increases were related to low soil organic carbon (SOC) and iron (Fe) availability, high pH, and As methylation driven by sulfate-reducing bacteria. In older paddy soils, higher SOC and Fe availability buffered these net increases but only in healthy soils. Two paddy soils, where microbial activity and Fe availability had been anthropogenically disturbed, lacked this buffering effect. Therefore, soil history should be considered prior to sulfate fertilization.