Co-utilization of manganous nitrate and pyrolusite enables concurrent mitigation of arsenic mobilization and greenhouse gas emissions in paddy-field wetlands.

Constructed wetlands face the dual challenge of arsenic (As) mobilization and greenhouse gas (GHG) emissions under flooded anoxic conditions. Moving beyond single-process remediation, this study developed a synergistic strategy through the co-application of manganous nitrate (Mn(NO3)2) and pyrolusite (MnO2) in microcosms simulating As-contaminated paddy-field wetlands. The results demonstrated that the Mn(NO3)2+MnO2 treatment achieved near-complete As(III) immobilization in the overlying water, significantly outperforming other amendments (KNO3-alone, MnO2-alone and KNO3+MnO2) during a 24d-incubation period. Concurrently, it substantially suppressed cumulative emissions of CH4 and N2O by approximately 35% and 61% than that of the KNO3-alone treatment. Metagenomic analysis revealed that this dual amendment reshaped the microbial community and metabolism. It enriched key taxa such as the dissimilatory nitrate reduction to ammonium (DNRA)-associated archaeon Candidatus Methanoperedens nitroreducens and nitrate-reducing coupled with Fe/Mn-oxidizing bacteria (e.g., Propioniciclava, Zoogloea, and Bryobacter). Meanwhile, the combined amendment also significantly increased the abundance of critical functional genes, including the N2O-reductase gene nosZ, DNRA marker gene nrfA and CH4-oxidation genes (pmoA and reverse methanogenesis-associated mcrA). The underlying mechanism relies on a regenerative Mn(II)/Mn(IV) cycle driven by the biotransformation of Mn(NO3)2. This cycle strategically redirects electron flow from pollutant‑mobilizing pathways, e.g., methanogenesis and dissimilatory Fe/As reduction toward As(III) immobilization and low‑carbon‑emission processes. These processes specifically include anaerobic oxidation of methane coupled to Mn(IV) reduction (AOM‑MnR), nitrate-dependent anaerobic methane oxidation (n-DAOM) and complete denitrification. Overall, this work provides a novel "mineral-electron switch coupled with nitrate-metabolic trigger" framework, offering an effective and sustainable synergy-based approach for the co-management of metalloid and GHG pollution in flooded anoxic environments.