Effect of rainfall-runoff process on sources and transformations of nitrate using a combined approach of dual isotopes, hydrochemical and Bayesian model in the Dagang River basin

The control of agricultural nitrogen through inflow rivers is crucial for lake aquatic environment conservation, while nitrate is the main form of non-point source pollution of agricultural nitrogen in watersheds. Therefore, understanding the nitrate sources and transformation in the intensive-agricultural region was particularly concerned. Nitrate sources and transformation varied largely during some crucial periods or events. However, low-resolution sampling campaigns increased some uncertainties due to without considering the effect of key driving factors for identifying nitrate transformations and sources. In our study, high-frequency sampling and analysis of nitrogen and oxygen isotope, hydrochemical and Bayesian model was conducted at the Dagang River to capture nitrate sources and transformations and identify its response to rainfall-runoff process at the event scale. In addition, the N cycle process was refined by comparing the variation and relationship of water quality factors and isotopes before, during, and after rainfall. We found that nitrate and major ions derived from similar agricultural activities caused by anthropogenic factors, such as domestic sewage from rural residents and livestock waste, through field survey and principal component analysis. The δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- in Dagang River ranged from +0.05‰ to +9.94‰ and + 1.49‰ to +11.64‰, respectively. The spatio-temporal variations of nitrate isotopes and hydrochemical compositions of river water suggested that nitrification was the dominant nitrate transformation process although the mixing effect occurred in some periods, especially during, and after the rainfall. The relationship between NO₃^-/Cl^- and Cl^- ratios suggested the occurrence of denitrification in downstream of the river basin after the rainfall. The results of Bayesian model showed that livestock manure and groundwater contributed to the most (66.4 ± 31.9%) nitrate, which indicated the necessity to establish its regulatory policy to avoid the overuse of livestock manure and groundwater in Dagang River. This study benefit for developing concrete and legible management and conservation strategies for decreasing the effect of anthropogenic nitrogen loading on lake eutrophication.