[Identification of Nitrate Sources and Transformation Processes in Midstream Areas:A Case in the Taizi River Basin]

This study analyzed the nitrate (NO₃^-), ammonia nitrogen (NH₄⁺-N), chloride (Cl^-), δ¹⁵ N-NO₃^-, δ¹⁸O-NO₃^-, and δ¹⁸O-H₂O in the surface water to identify the nitrate sources and transformation processes in midstream areas of the Taizi River basin in May and August 2016, corresponding to the dry season and wet season, respectively. The results indicated that the NO₃^-, Cl^-, and NH₄⁺-N concentrations and the δ¹⁸O-NO₃^- in the dry season were greater than those in the wet season, and the δ¹⁵ N-NO₃^- did not show significant seasonal variations based on the Man-Whitney U test. The NO₃^- was derived mainly from the mixed sources according to the ranges of NO₃^-/Cl^-, δ¹⁵ N-NO₃^-, and δ¹⁸O-NO₃^-. The nitrate sources in the surface water were synthetic fertilizer, soil nitrogen, manure, and sewage in the Xihe River (XR), Lanhe River (LR), and Xiada River (XDR) and soil nitrogen and synthetic fertilizer in the Er dao River (EDR) during the wet season. The sources were synthetic fertilizer and soil nitrogen in the XDR and soil nitrogen, manure, and sewage in the XR, LR, and EDR during the dry season. The significant negative relationship between NO₃^- and NH₄⁺ and the significant positive relationship between NO₃^- and δ¹⁵ N-NO₃^- in the wet season indicated that the volatilization and nitrification of soil N might be related to NO₃^- sources in the wet season. NH₄⁺-N and δ¹⁵ N-NO₃^- increased with decreasing NO₃^- and Cl^- in EDR and LR during the wet season, which indicated denitrification processes occurred. There was a significant relationship between NO₃^- and Cl^-, indicating that mixing processes occurred in the surface water during the two sampling periods. This study will provide a better understanding for nitrate sources related to seasonal variations and transformation processes in hilly areas.