Arsenic Reduces Methane Emissions from Paddy Soils: Insights from Continental Investigation and Laboratory Incubations

Arsenic (As) contamination and methane (CH₄) emissions co-occur in rice paddies. However, how As impacts CH₄ production, oxidation, and emission dynamics is unknown. Here, we investigated the abundances and activities of CH₄-cycling microbes from 132 paddy soils with different As concentrations across continental China using metagenomics and the reverse transcription polymerase chain reaction. Our results revealed that As was a crucial factor affecting the abundance and distribution patterns of the mcrA gene, which is responsible for CH₄ production and anaerobic CH₄ oxidation. Laboratory incubation experiments showed that adding 30 mg kg^-1 arsenate increased ¹³CO₂ production by 10-fold, ultimately decreasing CH₄ emissions by 68.5%. The inhibition of CH₄ emissions by As was induced through three aspects: (1) the toxicity of As decreased the abundance and activity of the methanogens; (2) the adaptability and response of methanotrophs to As is beneficial for CH₄ oxidation under As stress; and (3) the more robust arsenate reduction would anaerobically consume more CH₄ in paddies. Additionally, significant positive correlations were observed between arsC and pmoA gene abundance in both the observational study and incubation experiment. These findings enhance our understanding of the mechanisms underlying the interactions between As and CH₄ cycling in soils.