Water and sulfate management manipulates soil arsenic cycling at the fine scale

Abstract

Arsenic (As) is a carcinogenic environmental contaminant whose dissolution and speciation are strongly related to sulfate reactions, including reduction-oxidation, precipitation, complexation, and the microbial activities of sulfate-reducing bacteria. In lowland rice paddy fields, the interaction between As and sulfate are highly variable due to the sharp oxic/anoxic transition in the upper soils and the strongly reducing condition in the lower soils. Both water and sulfate managements have been demonstrated to reduce As bioavailability. However, interactions between these processes at sub-centimeter scales within paddy soil profiles are poorly understood and could even lead to enhanced As mobilization. To address this knowledge gap, we established mesocosms with mine-impacted soil under continuous or intermittent flooding, with and without sulfate addition. Millimeter-scale in situ profiles of dissolved AsIII, S-II and FeII were obtained using diffusive gradients in thin-films (DGT) and diffusive equilibration in thin-films (DET), and these were complemented by centimeter-scale soil DNA sampling to quantify the abundance of functional microbial groups. Our results show that high sulfate concentrations enhance AsIII in the near-surface 0-2 cm layer of the soil profile (near the soil-water interface) under continuous flooding, while in the anaerobic zone (below 4 cm), sulfate inhibits AsIII mobilization by facilitating the reduction of FeIII and SO4 2- to FeII and S-II through the enhanced activity of iron- and sulfate-reducing bacteria. The subsequent FeS precipitation adsorbs As, thereby reducing AsIII availability by 40%. Additionally, in this mesocosm experiment, differences in As mobilization between continuous and intermittent flooding were evident only in the near-surface 0-2 cm layer, with similar As profiles observed below 2 cm depth. This study provides insights into As migration and transformation mechanisms across soil depths under varying redox conditions and sulfate levels. Under flooded conditions, high-concentration sulfate increase AsIII mobility in this near-surface 0-2 cm layer, whereas intermittent flooding reduces its mobility. These findings inform remediation strategies for As contamination in high-sulfate soils