Speciation and Possible Origins of Organosulfur Compounds in Rice Paddy Soils Affected by Acid Mine Drainage

Although sulfur cycling in acid mine drainage (AMD)-contaminated rice paddy soils is critical to understanding and mitigating the environmental consequences of AMD, potential sources and transformations of organosulfur compounds in such soils are poorly understood. We used sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy to quantify organosulfur compounds in paddy soils from five AMD-contaminated sites and one AMD-uncontaminated reference site near the Dabaoshan sulfide mining area in South China. We also determined the sulfur stable isotope compositions of water-soluble sulfate (δ³⁴S_WS), adsorbed sulfate (δ³⁴S_AS), fulvic acid sulfur (δ³⁴S_FAS), and humic acid sulfur (δ³⁴S_HAS) in these samples. Organosulfate was the dominant functional group in humic acid sulfur (HAS) in both AMD-contaminated (46%) and AMD-uncontaminated paddy soils (42%). Thiol/organic monosulfide contributed a significantly lower proportion of HAS in AMD-contaminated paddy soils (8%) compared to that in AMD-uncontaminated paddy soils (21%). Within contaminated soils, the concentration of thiol/organic monosulfide was positively correlated with cation exchange capacity (CEC), moisture content (MC), and total Fe (TFe). δ³⁴S_FAS ranged from -6.3 to 2.7‰, similar to δ³⁴S_WS (-6.9 to 8.9‰), indicating that fulvic acid sulfur (FAS) was mainly derived from biogenic S-bearing organic compounds produced by assimilatory sulfate reduction. δ³⁴S_HAS (-11.0 to -1.6‰) were more negative compared to δ³⁴S_WS, indicating that dissimilatory sulfate reduction and abiotic sulfurization of organic matter were the main processes in the formation of HAS.