Reduced Methane Production Is a Crucial Factor Contributing to Reduced Methane Emissions from Degraded Wetlands

Abstract: Methane (CH4) is a major greenhouse gas contributing to climate change, and degraded wetlands are both sources and sinks of CH4. However, little is known about the role of microbial communities in mediating methane cycling during the process of wetland degradation. To investigate the microbial mechanisms influencing CH4 flux in degraded marsh wetlands, a metagenomic approach was used to study the processes of CH4 production and oxidation in herbaceous marsh wetlands. In this study, soil samples were collected from four degradation gradients of herbaceous marshes in the lower reaches of the Tumen River in Jilin Province, China. In situ CH4 fluxes were measured, and the microbial communities involved in CH4 metabolism, the abundance of key enzymes and KEGG pathways were analyzed. Environmental factors influencing CH4 flux were also correlated. One-way ANOVA indicated that wetland degradation significantly reduced CH4 flux and altered environmental factors. Metabolic pathway analysis showed that the CO2 pathway of methanogenesis was dominant. The abundance of methanogens and the key enzyme (mcrA; EC: 2.8.4.1) decreased, while the number of methanotrophs increased, with no significant change in the abundance of the key enzyme (pMMO; EC: 1.14.18.3). Random forest and partial least squares regression analyses（PLSR）indicated that the key environmental factors affecting CH4 flux are soil water content (SW), total carbon (TC), and soil pH. Therefore, the decrease in methanogen abundance and metabolic activity due to reduced soil moisture is the primary reason for the rapid decline in methane emissions. The main factor affecting methane flux is the methane production process, rather than methane oxidation. Further research is needed to reduce the significant uncertainty in estimating CH4 emissions from degraded wetlands.