Understanding nitrate dynamics in urbanized and forested river ecosystems: A study integrating natural-abundance and...

Nitrate pollution in river ecosystems poses significant global environmental and public health challenges. However, the underlying drivers behind the elevated nitrate concentrations within river systems remain unclear. This study employed natural abundance isotopes combined with ¹⁵N-pairing techniques to elucidate the origins and transformation pathways of nitrate in two contrasting river ecosystems: a heavily urbanized river and a minimally disturbed forested river. Results revealed that domestic sewage was the dominant nitrate source in both rivers, contributing 56.9 % in the urban river and 37.8 % in the forested river. Denitrification was identified as the primary nitrate removal process, although its efficiency was constrained in the urban river due to excessive nitrate inputs and limited in-stream removal capacity. Furthermore, soil denitrification was largely regulated by moisture. The elevated nitrate loadings within urban river were attributed to substantial domestic sewage inputs coupled with limited nitrate removal capabilities. These findings highlight the impact of urbanization on nitrate dynamics and underscore the urgent need for integrated management strategies. Effective measures, such as upgrading wastewater treatment infrastructure, preserving riparian vegetation, and implementing nutrient reduction programs, are critical for mitigating nitrate pollution and enhancing water quality in both urban and natural river systems. This study provides insights applicable to riverine nitrogen management globally, contributing to the sustainable development of aquatic ecosystems.