Influence of warming, redox dynamics, and exogenous organic matter on iron plaque mineralogy in paddy soils

Rice paddy soils play a central role in global carbon (C) cycling and food security. Nevertheless, climate warming, drought stress in particular, threaten soil organic carbon (SOC) stability by modifying redox-driven biogeochemical processes. This study investigates how warming, flooding regime, and exogenous organic matter interact in controlling iron (Fe) plaque formation and root trait development in rice (Oryza sativa L.), a key interface regulating Fe-mediated SOC stabilization and mineralization. A field experiment was conducted using open-top chambers to simulate warming (+2 °C; WR) relative to ambient temperature (AM). Rice plants were grown under normal flooding (NF) or reduced flooding (RF), with or without anaerobic digestate amendment. Root Fe plaque formation was quantified to assess treatment-driven redox responses. To investigate Fe speciation changes within the plaque, Fe K-edge X-ray Absorption Spectroscopy (XAS) analyses were performed. Under reduced flooding, periodic soil reoxygenation interrupted prolonged anoxia and limited Fe reduction, promoting comparatively more stable and crystalline Fe mineral assemblages. RF treatments generally showed higher contributions of hematite and less ferrihydrite-rich assemblages, suggesting greater mineral aging and structural reorganization during oxic phases. Under warming and waterlogging conditions, plaques showed increased contributions of ferrihydrite and lepidocrocite, indicating enhanced Fe redox turnover and the formation of reactive and poorly crystalline Fe phases. Across both flooding regimes, digestate amendment modified Fe plaque abundance and Fe mineral transformation pathways, indicating that exogenous organic matter strongly influenced Fe redox dynamics and mineral formation. Moreover, pre-edge centroid positions, used as indicators of the average oxidation state of Fe atoms within the plaques, were generally lower and more variable under NF than under RF, indicating that continuous flooding promoted more reduced and dynamically cycled Fe pools. Overall, the results highlight how flooding regime, warming, and exogenous organic matter jointly regulate Fe mineral transformations and Fe redox turnover within rice root plaques, with potential consequences for Fe-mediated organic matter stabilization in paddy soils.