Abstract:Urban sewage pipelines are significant sources of greenhouse gas (GHG) emissions. However, the mechanisms underlying GHG production and release at the sediment-water interface, particularly in real-world sewage networks characterized by highly variable water quality, remain under-researched. This study aims to clarify the dynamic patterns and driving mechanisms of these processes. From November to December 2024, sediment and overlying water samples were collected from representative sewage pipelines in Shenzhen. A comprehensive approach was adopted, incorporating water quality analysis, headspace gas chromatography measurements of dissolved GHGs (CH4, CO2, N2O), 16S rRNA gene amplicon sequencing, and quantitative PCR, to systematically investigate the physicochemical and microbial processes at the sediment-water interface. The results showed that the dissolved methane (CH4) concentration did not respond immediately to changes in water quality, exhibiting significant metabolic lag effects. This indicates that CH4 production and release at the sediment-water interface follow a delayed response to environmental changes. In contrast, the generation of dissolved carbon dioxide (CO2) showed distinct patterns: in the overlying water, CO2 was positively correlated with several water quality indicators, such as chemical oxygen demand (COD), volatile fatty acids (VFAs), and nitrogen compounds, suggesting that its source is linked to various microbial and biochemical processes. In the sediment, CO2 was primarily associated with COD and VFAs, indicating production mainly through fermentation. Nitrous oxide (N2O) was detected only in pipeline sections with relatively higher dissolved oxygen (DO) levels, confirming that DO is a critical environmental factor governing the types of greenhouse gases produced. Microbial analysis further highlighted that organic load is a central factor driving the differentiation of microbial community structure and the distribution of carbon metabolism pathways. High organic load conditions favored the enrichment of microbial communities specializing in the degradation of large organic molecules, with representative genera such as Syntrophorhabdus, leading to increased carbon flow toward CO2 production. Under moderate organic load conditions, microbial communities that utilize small-molecule substrates, including genera like Lactivibrio, became more abundant. These communities showed a significant positive correlation with the abundance of the methane-producing gene mcrA-1, which is associated with increased CH4 emissions in the overlying water. The microbial communities appear to regulate the balance of CH4 and CO2 emissions through a "synergy-competition steady-state" mechanism. This dynamic regulation is influenced by both the type and amount of organic matter present in the system. Finally, daily dynamic monitoring of GHG emission fluxes further confirmed that microbial community function plays a crucial role in regulating the timing and magnitude of greenhouse gas emissions. Close-