Research on the sources of nitrate in agricultural watershed in karst areas of Southwest China based on stable isotopes

The agricultural watersheds in the karst regions of Southwest China face severe nitrate pollution, but research on its sources and transport mechanisms remains limited. This study focuses on the Mudong River watershed in Guilin, Guangxi. Water samples were collected from 2020 to 2025 and analyzed for hydrogen, oxygen, nitrogen, and oxygen isotopes as well as nitrogen concentrations, in combination with the SIAR model. Results show that total nitrogen is mainly in the form of nitrate, with N-NO and O-NO values ranging from -1.651 to 14.993 and 0.037 to 19.266, respectively. Nitrification dominates nitrate transformation, with soil nitrogen and animal manure/domestic wastewater as the primary sources. This research provides a scientific basis for nitrate pollution control in agricultural watersheds of karst regions.