Fe plaque on rice root debris continues to stabilize As in re-flooded paddies

Arsenic (As) is a regulated environmental hazard that poses significant health risks to humans through rice (Oryza sativa L.) consumption. Iron (Fe) plaque on rice roots typically acts as a barrier, limiting As uptake by rice plants. After harvest, rice root debris (RS) containing both organic carbon (C) and As is left in the soil, potentially enhancing As release when paddy fields are re-flooded, although this has not been experimentally verified. In this study, we simulated the addition of RS with and without Fe plaque (RS+Fe and RS-Fe) into As-contaminated and uncontaminated paddy soils to investigate the post-harvest effect of Fe plaque on As dynamics. In As-contaminated soils, RS+Fe did not significantly alter porewater chemistry (pH, Eh, DOC, Fe, Mn, or As), whereas RS-Fe markedly (p < 0.05) increased porewater DOC (29.5%), Fe (25.2%), and As (31.7%) during a 60-day microcosm incubation. The rise in DOC under RS-Fe conditions strongly stimulated Proteobacteria (199%), a group of dissimilatory Fe- and As-reducing bacteria, resulting in elevated As(III) (98.1%) and exchangeable As (26.8%) fractions. Conversely, in uncontaminated soils, both RS+Fe and RS-Fe treatments promoted As immobilization, increasing the proportion of As in the more stable carbonate-bound (68.4%) and Fe/Mn-bound (32.0%) fractions. Overall, these findings demonstrate a clear effect of Fe plaque in immobilizing As in re-flooded paddy soils, and reveals that Fe plaque could be a long-lasting As sink in rice monoculture.