Invasion by the exotic species Spartina alterniflora, which has high net primary productivity and superior reproductive capacity compared with native plants, has led to rapid organic carbon accumulation and increased methane (CH₄) emission in the coastal salt marsh of China. To elucidate the mechanisms underlying this effect, the methanogen community structure and CH₄ production potential as well as soil organic carbon (SOC), dissolved organic carbon, dissolved organic acids, methylated amines, aboveground biomass, and litter mass were measured during the invasion chronosequence (0-16 years). The CH₄ production potential in the S. alterniflora marsh (range, 2.94-3.95 μg kg(-1) day(-1)) was significantly higher than that in the bare tidal mudflat. CH₄ production potential correlated significantly with SOC, acetate, and trimethylamine concentrations in the 0-20 cm soil layer. The abundance of methanogenic archaea also correlated significantly with SOC, and the dominant species clearly varied with S. alterniflora-driven SOC accumulation. The acetotrophic Methanosaetaceae family members comprised a substantial proportion of the methanogenic archaea in the bare tidal mudflat while Methanosarcinaceae family members utilized methylated amines as substrates in the S. alterniflora marsh. Ordination analysis indicated that trimethylamine concentration was the primary factor inducing the shift in the methanogenic archaea composition, and regressive analysis indicated that the facultative family Methanosarcinaceae increased linearly with trimethylamine concentration in the increasingly sulfate-rich salt marsh. Our results indicate that increased CH₄ production during the S. alterniflora invasion chronosequence was due to increased levels of the non-competitive substrate trimethylamine and a shift in the methanogenic archaea community.