Abstract

Abstract Urban development and increased human activities impose major environmental stress on the receiving bodies of water. Although urban rivers have been recognized as hotspots of regional nitrogen (N) pollution, detailed measurements of river nutrient species in response to urbanization are rarely reported, so the impacts of urban development on N cycling processes and transport to coast remains unclear. Here we investigated the changes in N species (concentration, composition and isotope) and N functional genes between upstream and downstream sections of several rivers affected by urban development in western Taiwan Strait under various flow conditions (low, medium and high flow). Our results suggest that urban sewage (high ammonium) is the predominant substrate that stimulated nitrification and subsequently denitrification and gaseous N removal (N2O, N2). Nitrifying and denitrifying functional genes increased their abundance along the urban rivers. There were hydrological and meteorological controls on urban rivers regulating changes in nitrogen retention between seasons. Overall, the enhanced microbe-driven N retention could not balance the increase of urban N loading. Consequently, urbanization increased riverine N export and caused other changes in nutrient supply such as changing the nutrient ratio (N:P:Si ratio), increasing the potential for eutrophication both in the river and in receiving coastal ecosystems.