Abstract: The invasion of Spartina alterniflora poses a significant threat to coastal wetlands in China. The large biomass and organic substrates introduced by this species are likely to alter soil microbial communities and drive methane (CH₄) and other greenhouse gas emissions; however, the underlying mechanisms remain poorly understood. To address this, we conducted a one-year in situ monitoring of CH₄ emission rates, soil properties, dissolved organic matter (DOM) fractions, and CH₄-cycling microbial communities in invaded wetlands and adjacent native mangroves. Our results showed that S. alterniflora invasion increased soil CH₄ emissions by 8.7-fold relative to mangrove soils. Redundancy analysis and structural equation modeling revealed that this increase was closely linked to invasion-induced shifts in soil conditions, including elevated water content and pH, enrichment of labile DOM fractions (lipids and protein/aliphatic compounds), and decreases in sulfate, soil organic carbon, and total nitrogen. These changes reduced DOM molecular stability and collectively facilitated CH₄ production. Moreover, quantitative PCR showed an increase in the absolute abundance of methanogens and a decrease in both the abundance and diversity of methanotrophs in invaded soils. Amplicon sequencing further indicated a higher relative abundance of Methanococcoides and a reduction in type II methanotrophs, weakening methane oxidation capacity. Overall, S. alterniflora invasion enhances wetland CH₄ emissions by altering soil physicochemical properties, providing more labile substrates, and restructuring CH4-related microbial communities, thereby weakening the carbon-sink function of coastal wetlands. Integrated management approaches are needed to mitigate invasion-driven methane production while sustaining wetland ecosystem resilience.

Key words: Wetland, Mangrove, Spartina alterniflora, Dissolved organic matter, Methane, Methanogen