<i>Spartina alterniflora</i> invasion stimulates methane emissions in coastal wetlands by increasing labile organic matter and shifting microbial communities

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.