Effects of Manganese Redox Cycling on Arsenic (Im)mobilization in Rice Paddy Soils

Growing concerns about arsenic (As) uptake by rice and food chain contamination necessitates research into the biogeochemical processes that regulate As mobilization in rice paddy soils and mitigation techniques to control As mobilization. Alternate wetting and drying (AWD) methods for irrigating paddy fields can effectively reduce As concentrations in rice. However, a mechanistic understanding of how AWD affects the coupled cycling of As-iron (Fe)-manganese (Mn) in the subsurface remains incomplete. This contribution focuses on the role of Mn oxides in controlling the timing and extent of AWD As mobilization. Mn oxides are ubiquitous redox-active minerals that co-exist with Fe oxy(hydroxide) in sediments. Under AWD, soil Mn may play a more prominent role in controlling As through direct (oxidant or adsorbent) and indirect (redox buffer) control over As (im)mobilization.

Here we present results from laboratory microcosm experiments that probe the direct and indirect effects of Mn and Fe on As (im)mobilization in paddy soils under a simulated AWD event. Soil from Arkansas rice paddies was amended with synthetic birnessite (δ-MnO₂) to yield soil Fe:Mn ratios between 22—5, and an azide-sterilized control was used to evaluate the contribution of abiotic processes to Fe-Mn-As mobilization. All conditions were tested in triplicate. Soil redox potential, pH, and soil solution chemistry were monitored before and after the AWD event. We conducted 16S rRNA qPCR to asses Mn toxicity and used Visual Minteq to evaluate potential mineral precipitation that could impact Fe and Mn solubility. Lower Fe:Mn ratios inhibited Fe and As mobilization. However, lower Fe:Mn ratios did not result in significant differences in soil redox potential during flooded, drying, or re-wetting conditions. Post-dry down aqueous samples suggest that Fe and not Mn mineral phases are the dominant source of mobilized As. These insights into paddy soil Fe-Mn-As redox chemistry will help inform AWD methods to limit As availability for rice plant uptake.