Hydrological regulation of nitrate sources, transformation and transport pathway in a karstic river

A comprehensive understanding of nitrate dynamics in rivers is vital for nitrogen management, especially for the karst zone with high leaching of nitrate producing potential risk to environmental quality. However, identifi-cation of the source-transformation-pathway of nitrate is limited in the karst rivers. Thus, a suit of hydro-geochemical parameters, including delta N-15-NO3-, delta O-18-NO3-, delta C-13-DIC, delta O-18-H2O, soil-related parameters (DOC) and geogenic ions (Mg2+, DIC), were used to identify nitrate sources and dynamics in the Wujiang River, a typical karstic river as a tributary of Changjiang River. The combined use of a conceptual model of normalized NO3- and DOC, and other hydrogeochemical parameters (delta C-13-DIC, Mg2+, DIC) suggested that most nitrates were trans-ported through the soil/epikarst pathway in the wet season, whereas >70 % of nitrates were transported through groundwater pathway in the dry season for Wujiang River. Nitrification and mixing were the dominant processes, meanwhile similar to 31 % of nitrate was denitrified in the wet season in this study. The reanalysis of delta N-15-NO3- and delta O-18-NO3- values for reported karstic rivers in China and worldwide also revealed the predominant nitrification and denitrification in karst regions. The Bayesian model indicated that chemical fertilizer (CF, 42 %) and soil organic nitrogen (SON, 32 %) contributed the most in the wet season, while similar to 60 % of nitrate originated from SON (31 %) and manure and sewage wastes (M&S, 30 %) in the dry season in the Wujiang River. The sustained contribution of non-point sources from nitrification through the soil/epikarst pathway might be related to the widespread porosities in the karst rivers. Thus, measures to reduce nitrogen leaching and improve fertilizer use efficiency should be considered, like water and soil conservation, slow-release fertilizer and nitrification in-hibitors. Additionally, a suit of hydrogeochemical indicators proves to be helpful for identification of source, transformation and transport pathway of nitrate in the karst basin.

Automated summary

A comprehensive understanding of nitrate dynamics in rivers is crucial for effective nitrogen management, particularly in karst regions where high leaching of nitrate poses potential risks to environmental quality. However, the identification of nitrate sources and transformation pathways in karst rivers is limited. This study used a combination of hydro-geochemical parameters to identify nitrate sources and dynamics in the Wujiang River, showing that nitrates were mainly transported through the soil/epikarst pathway in the wet season and through the groundwater pathway in the dry season.