Unignorable Role of Iron Redox and Microbial Adaptation in Mediating Arsenic-Induced Suppression of Carbon Emissions in Paddy Soils

Arsenic (As) contamination disturbs organic carbon (OC) cycling in paddy soils, potentially influenced by microbial adaptive evolution and redox-active iron (Fe). Here, the association of As with C emissions (CH4 and CO2), microbes, and Fe dynamics in flooded paddy soils was investigated via microcosm experiments across two As levels (20 and 200 mg kg–1). Results showed that As contamination markedly inhibited CH4 (>90%) and CO2 (>38%) emissions, accompanied by inhibited ferric reduction, decreased OC bioavailability, and shifted microbial communities toward As-tolerant and oligotrophic strategies. Correlation analysis and validation experiments demonstrated that Fe reduction covaried negatively with available As but positively with C bioavailability indicators (p < 0.001). The suppressed C emissions under low-dose As were closely linked to perturbed Fe redox cycling, while high-dose As appeared to amplify this inhibition through additional microbial toxicity and community restructuring. Exogenous zerovalent iron increased CH4 emissions by 0.92–19.54-fold through coupled changes in As immobilization, redox conditions, C bioavailability, and abundance of Fe-reducing bacteria and methanogens. This work indicated that Fe redox and microbial adaptation are important contributors to As-mediated suppression of C emissions, providing evidence for the As–Fe–C nexus and offering insights for balancing As remediation with C emission mitigation.