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

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