Responses of Methane Emissions to Different Soil Amendments in Paddy Soil: Soil Properties, Microbial Communities,...

Paddy soils are important contributors to agricultural greenhouse gas emissions, particularly methane, and soil amendments may regulate methane production by altering soil physicochemical properties and microbial methane cycling. However, the effects of different amendment types on methane emissions from anaerobic paddy soils remain uncertain. In this study, an anaerobic microcosm experiment was conducted to evaluate the effect of microbial inoculants, biochar, humic acid, and montmorillonite on CH₄ and CO₂ emissions from paddy soil. Changes in acetate concentration, pH, electrical conductivity, microbial community structure, and methane cycling functional genes were further analyzed to explore the underlying mechanisms. The results showed that microbial inoculants had stronger effects on CH₄ emissions than the other amendments, but their effects were contrasting. The Chabeijian (CB) inoculant significantly increased methane emissions by 100.8%, whereas the Duojun-360 (DJ) inoculant reduced cumulative methane by 57.1%. The stimulation of CH₄ emissions under Chabeijian was associated with enhanced acetate turnover, enrichment of methanogenic taxa including Methanosarcina, Methanobacterium, Methanocella, and Methanosaeta, and a 48.7% increase in mcrA abundance. In contrast, Duojun 360 markedly increased soil electrical conductivity, reduced methanogen abundance, decreased mcrA abundance by 26.9%, and lowered the mcrA/pmoA ratio, indicating a shift away from methane production. Although both inoculants increased methanotroph abundance and pmoA abundance, methane production remained the dominant factor controlling net CH₄ emissions. These findings may provide preliminary mechanistic support for the targeted selection of soil amendments to mitigate CH₄ emissions in rice cultivation by regulating soil properties, methanogenic communities, and the balance between methane production and oxidation.