Texture-associated DOM transformation links methane accumulation with arsenic mobilization in paddy soils

Arsenic (As)-contaminated paddy soils are important interfaces where methane (CH₄) production and As mobilization occur concurrently, yet the role of soil texture in coupling these processes through dissolved organic matter (DOM) transformation and microbial activity remains unclear. Loam and silt paddy soils from the Jianghan Plain, China, were examined using geochemical analyses, DOM characterization, microbial profiling, metagenomics, quantitative PCR (qPCR), partial least squares path modeling (PLS-PM), and anaerobic microcosms. Loam soils contained less solid-phase As and Fe but more porewater As and Fe than silt soils. Their mean CH₄ concentration was 8.7-fold higher (91.66 vs 10.59 μmol/kg). Loam porewater DOM showed greater humification and aromaticity, and humic-like components were positively correlated with As(III), whereas highly aromatic, polyphenolic, and highly unsaturated molecules were negatively correlated with CH₄, suggesting preferential transformation of these compounds during CH₄ emissions and As mobilization. Methanogenic and CH₄-cycling archaea, including Methanobacterium, Methanosaeta, Methanosarcina, and Candidatus Methanoperedens, together with Fe/As-reducing taxa such as Geobacter, were more abundant in loam soils. Genes related to CH₄ cycling and As metabolism were also detected. In microcosms, CH₄ concentrations were 84.9% higher in loam soils and positively correlated with As(III) (R = 0.337, p < 0.001). Higher mcrA, ANME-mcrA, and arrA copy numbers and their positive intercorrelations further indicated greater functional potential for methanogenesis, anaerobic methane oxidation, and As reduction. Overall, soil texture indirectly regulates CH₄ emissions and As mobilization by reshaping soil physicochemical conditions, DOM reactivity, and microbial functional niches.