Reversible Control of Microbial As(III) Oxidation by Nitrous Oxide Availability in Flooded Paddy Soils

The persistence of arsenite (As(III)) oxidation in flooded paddy soils is difficult to explain once canonical oxidants are rapidly depleted under anoxia. Here we tested whether nitrous oxide (N₂O), a prevalent nitrogen-cycle intermediate, reversibly regulates microbial As(III) oxidation and arsenic (As) partitioning in flooded soils. Using two paddy soils with low and high As contents, we conducted (i) three-generation serial-transfer enrichments with exogenous As(III) addition and (ii) continuous-cessation-readdition N₂O exposure microcosms targeting native As pools. Across transfer generations, N₂O consistently promoted As(III) oxidation under strictly anoxic conditions, while sterilized controls showed no As(III) loss, indicating biological mediation. In native-soil microcosms, porewater As(III) declined during N₂O input, rebounded upon N₂O withdrawal, and decreased again after N₂O readdition, demonstrating reversible control. N₂O exposure also shifted As toward amorphous Fe (hydr)oxide-associated operational fractions, consistent with reduced porewater mobility. Metagenomic analyses further showed enrichment of functional genes for As oxidation (aioA, aioB) and N₂O reduction (nosZ), with the strongest responses in the high-As soil at day 70 (1 mM vs 0 mM N₂O: aioA 13.9-fold, aioB 1.68-fold, nosZ 3.26-fold). These results indicate that N₂O availability can act as a reversible control point associated with microbially mediated As(III) oxidation and As redistribution under anoxia, with implications for As mobility and exposure risk in flooded paddy systems.