Influence of warming, redox dynamics and the application of exogenous organic matter on iron plaque mineralogy

Paddy soils are critical agroecosystems with significant roles in global carbon (C) cycling and food security. However, warming and altered water management pose challenges to their productivity and soil organic carbon (SOC) stabilization. This study investigates how a moderate warming (+2°C), flooding regime variations, and exogenous organic matter (digestate) amendment affect iron (Fe) plaque formation and mineralogy on rice (Oryza sativa L.) roots, key drivers in SOC stabilization. Root Fe plaques form under anaerobic conditions as oxygen leaks from roots, oxidizing soluble Fe(II) to precipitate Fe (oxyhydr)oxides. These plaques exert a “double-edged sword” effect on SOC: they can stabilize organic C by forming Fe-SOC complexes but also mediate soil organic matter (SOM) mineralization through Fe redox cycling. Understanding how climate warming and soil amendments affect Fe plaque formation and function is essential to predict the fate of SOC and associated greenhouse gas emissions in paddy environments. The experimental setup employed open-top chambers (OTCs) to simulate a +2°C increase in temperature, with rice plants grown under two flooding regimes, normal flooding (NF) and reduced flooding (RF), and with or without digestate amendment (DS). Root samples were analyzed for Fe content, and grain yields were recorded at maturity. Root Fe concentrations varied widely across treatments (7.5 to 24 mg Fe/g dry weight), evidencing strong environmental and management controls on plaque formation. Key results show that warming caused a significant increase in root Fe plaque formation under unamended and normal flooding conditions, with root Fe content rising from 7.46 to 19.94 mg/g, confirming the hypothesis that elevated temperatures amplify plaque formation. Conversely, under digestate-amended conditions, warming had a muted effect on root Fe content, indicating that organic amendments mitigate warming-induced Fe plaque formation. Reduced flooding generally increased root Fe content under ambient temperature but interacted complexly with warming and amendments. Taken together, these outcomes support the central hypotheses posed: (i) warming intensifies root Fe plaque formation, potentially accelerating SOC mineralization, (ii) digestate amendments buffer this effect, stabilizing Fe plaque levels, and (iii) flooding regime modulates plaque dynamics and crop performance in nuanced ways.