Contrasting effects of impervious cover on riparian plant and soil bacterial communities in a rapidly urbanising Himalayan city

Rapid urbanisation-particularly the expansion of impervious surfaces-is reshaping riparian landscapes worldwide. These areas are frequently targeted for development due to their favourable topography, abundant water resources, and aesthetic appeal. However, the impact of increasing impervious cover on soil bacterial communities in biodiverse urban riparian zones remains poorly understood, especially in developing countries, raising concerns about potential declines in essential ecosystem functions. In this study, we investigated the effects of impervious cover, quantified as the Percentage of Total Impervious Area (PTIA), on the taxonomic and functional diversity of riparian soil bacteria in Thimphu City, Bhutan. Using plot-based taxonomic profiling and metagenomic analysis across a PTIA gradient, we tested the hypothesis that bacterial diversity and functional pathways would decline beyond 40 % PTIA, mirroring patterns observed in riparian plant communities. Contrary to our hypothesis, plots with PTIA exceeding 40 % exhibited greater bacterial richness and functional diversity. These findings suggest opposing responses to impervious cover, with belowground microbial communities increasing in diversity while aboveground plant diversity declines. A significantly lower carbon-to‑nitrogen ratio in urban plots-likely driven by nutrient enrichment-emerged as the primary factor promoting bacterial diversity in high PTIA areas. This enrichment appeared to favour copiotrophic bacteria, enhancing both diversity and functional capacity. Our results suggest that bacterial communities may be effective bioindicators of riparian ecosystem health than plant communities. Incorporating microbial metrics into urban riparian management and monitoring could therefore provide valuable insights for sustaining ecosystem functions in rapidly urbanising regions.