Biochar mitigates dimethylarsenate-stimulated methane emissions mediated by DOM-microbe interactions in paddy soils

Paddy soils represent key biogeochemical interfaces linking arsenic (As) transformation and carbon (C) cycling. Dimethylarsenate (DMAs), a widespread organoarsenic contaminant in rice paddies, can alter methane (CH₄) dynamics, but the underlying mechanisms based on dissolved organic matter (DOM)-microbe interaction and the mitigation potential of biochar still remained insufficiently understood.

Here, we investigated As biogeochemistry, CH₄ emissions, DOM quantity and composition, microbial community assembly and functional gene variations through a 98-day microcosm incubation experiment using paddy soil amended with DMAs alone and DMAs combined with biochar.

We found that DMAs greatly promoted CH₄ emissions in a dose-dependent manner, with the DMAs120 treatment increasing cumulative CH₄ emissions by 60.4% relative to the control. DMAs underwent significant biotransformation, declining from 98.7% on the 7th day to 26.2% of the total As by day 98. Porewater DOC concentrations were significantly increased by up to 1.81-fold (DMAs120, day 28). PARAFAC analysis revealed that DMAs shifted DOM composition toward labile protein-like fractions, with tryptophan-like (C2) and tyrosine-like (C3) components increasing while microbial humic-like (C1) and terrestrial humic-like (C4) fractions decreased. Such labile DOM precisely matched substrate requirements of enriched anaerobic fermentative microbes, establishing synergistic metabolic networks to supply methanogenic precursors. DMAs markedly increased mcrA gene abundance by 4.2-7.6 times with negligible change in pmoA, confirming stimulated methanogenesis rather than suppressed oxidation. By contrast, biochar effectively decreased % DMAs in porewater, reducing CH₄ emissions by 54.3% after 2% biochar amendment. Biochar also decreased DOC concentrations and protein-like DOM (C2, C3), reducing bioavailable substrates for methanogens. Furthermore, biochar suppressed mcrA gene abundances by 12.7-81.3%, while increased that of pmoA gene by 7.1-7.5 times, as well as increased porewater As(V) and enriched ammonia-oxidizing Candidatus_Nitrosopelagicus to potentially or indirectly stimulate anaerobic CH₄ oxidation.

These results establish mechanistic connections between DMAs biotransformation, DOM turnover and CH₄ production in paddy soils, and verify that biochar restructures DOM-microbe interactions to mitigate DMAs-stimulated CH₄ emissions. This study deepens insights into As-C biogeochemical coupling and offers practical support for simultaneous As remediation and greenhouse gas control in paddy ecosystems.