Urbanization restructures wetland nitrogen cycling via soil properties and microbial functional communities

Urban expansion, characterized by growing impervious surface area, severely disrupts wetland ecosystem functions and nutrient cycling processes. However, a holistic understanding of how urbanization affects wetland soil nitrogen cycling and the associated functional microorganisms remains unclear. In this study, the key soil nitrogen cycling processes and their associated core microbial functional genes across 22 wetlands were investigated along a rural–suburban–urban gradient. Results showed that soil total nitrogen and ammonium content in urban sites were significantly lower than those in rural sites. Although microbial alpha diversity did not change significantly, the E/O ratio (the abundance ratio of eutrophic to oligotrophic phyla) decreased sharply along the urbanization gradient. Metagenomic analysis revealed a systematic functional restructuring: urban wetlands exhibited upregulated genetic potential for nitrate scavenging and conservation (assimilatory and dissimilatory nitrate reduction to ammonium—ANRA/DNRA— and nitrate transport) but downregulated pathways for nitrogen acquisition (fixation) and organic matter processing. This shift indicates a microbial adaptation to nitrogen limitation in urban wetlands, where enhanced DNRA and ANRA pathways compete with nitrification and denitrification, crucially regulating nitrogen transformation rates. The study identifies soil organic carbon, pH, bulk density, and the interaction between key microbial taxa and nitrogen cycle gene groups as the primary factors controlling wetland soil nitrogen status under urban pressure. We conclude that urbanization threatens wetland nutrient functions not merely by habitat loss, but by inducing a functional microbial shift. These findings underscore the need for urban planning strategies that mitigate soil property changes to preserve the microbial basis of wetland ecosystem services.