Millimeter-scale topsoil layer blocks arsenic migration in flooded paddy soil

Abstract In this study, we investigated the microscale behavior of arsenic (As) in an oxic-anoxic interface in laboratory-incubated paddy soil by combining multiple techniques, micro X-ray fluorescence spectrometry combined with micro X-ray absorption fine structure (μXRF-μXAFS), microelectrodes, and quantitative polymerase chain reaction (qPCR) analyses. A steep redox gradient with depth was observed in the topsoil millimeter layer (depth: 0−4.6 mm) after 15 days, oxidative condition in the surface soil and reductive condition in deeper soil, due to the limited oxygen supply caused by flooding. Microscale elemental mapping of the top soil layer by μXRF showed that a large amount of the As (about 10% of total) in the system strongly accumulated within the top 3 mm soil layer after 15 days of incubation. Direct As speciation by micro extended X-ray absorption fine structure (μEXAFS) indicated that the host phases of As in the accumulation layer were the Fe(III) hydroxides included in the soil, which were the dominant Fe species in the layer. Direct As speciation by micro X-ray absorption near edge structure (μXANES) showed that the dominant As species in the As accumulation and depletion zones were clearly changed. Specifically, the oxidized species As(V) increased in the As accumulation (top surface) layer, and the reduced species As(III) decreased in the As depletion (deeper) layer. Direct Fe speciation also showed that the most abundant fraction of Fe in the accumulation layer was the Fe(III) hydroxides such as ferrihydrite, the most preferable sorbent for As. These findings suggest that the As accumulations observed in the study are significantly associated with As reduction and oxidation in the oxic-anoxic interface of paddy soil. Specifically, more labile As(III) formed in the reductive deeper layer migrates to the upper layer, the migrated As(III) is oxidized to As(V) in oxidative surface millimeter layer, and the As(V) is then scavenged by Fe(III) hydroxides that have high affinity for As(V); this is a possible mechanism for the strong accumulation of As observed in the topsoil layer in this study. In addition, manganese (Mn) and iron (Fe) μXANES analyses and microbial As(III) oxidase gene (aioA) quantification by qPCR suggested that As(III) oxidation in the top layer, which acted as a trigger reaction for As accumulation, might be induced by both chemical and microbial oxidation processes. The findings in this study indicate that the paddy soil system could potentially have a strong ability to prevent As migration to the floodwater. Our findings also indicate that the millimeter-scale topsoil layer in the flooded paddy soil could biogeochemically influence the whole As behavior in the paddy fields. Thus, to fully understand and predict As fate and migration in the paddy fields, we should investigate localized physical, chemical, and, biological properties of paddy soil.