Urbanization-Induced Shifts in Microbial Functional Genes of Wetland Nitrogen Cycling Promote Nitrous Oxide (N2O) Emissions

Urban wetlands are assumed to contribute to nitrous oxide (N₂O) emissions; however, the microbial mechanisms underlying enhanced N₂O fluxes in urban wetlands and differences in microbial responses between aquatic and soil compartments have not been clearly identified. Here, we characterized the nitrogen (N) cycling microbial communities and their functional metabolic pathways in urban and rural wetlands using metagenomics and N₂O flux measurements. Results showed that urbanization drove a 6~8-fold increase in N₂O fluxes from urban wetlands compared to rural wetlands. Structural equation modeling (SEM) confirmed that urbanization intensity was a primary driver (standardized coefficients: 0.72 for soil and 0.92 for water). In wetland water, N₂O emissions were negatively correlated with inorganic nutrient concentrations (coefficient = -0.62). Aquatic microbial communities exhibited substantial taxonomic shifts but preserved network connectivity, indicating adaptive strategies for surviving urban perturbations at the cost of reduced functional redundancy. In wetland soil, microbial communities maintained stability under urbanization, which was attributed to environmental buffering from heterogeneous microenvironments. Soil N₂O emissions were positively linked to microbial alpha diversity (coefficient = 0.79). Furthermore, urban wetlands enriched genes mediating nitrification and denitrification while depleting genes associated with N fixation and organic N metabolism. This functional shift reflects microbial specialization in processing elevated reactive N (Nr) inputs from urban sources, trapping urban wetlands in an "N loss loop" that reinforces high N₂O fluxes. This study elucidates the microbial mechanisms governing wetland N₂O emissions under urbanization, thereby enhancing understanding of microbially mediated N cycling in the urban wetland ecosystem.