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

Abstract

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, identification of the source-transformation-pathway of nitrate is limited in the karst rivers. Thus, a suit of hydrogeochemical parameters, including δ15N-NO3−, δ18O-NO3−, δ13C-DIC, δ18O-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 (δ13C-DIC, Mg2+, DIC) suggested that most nitrates were transported 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 ∼31 % of nitrate was denitrified in the wet season in this study. The reanalysis of δ15N-NO3− and δ18O-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 ∼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 inhibitors. Additionally, a suit of hydrogeochemical indicators proves to be helpful for identification of source, transformation and transport pathway of nitrate in the karst basin.