Tracing Nitrate Sources with Nitrogen and Oxygen Isotopes and Hydrochemical Characteristics During Wet Season in Lake Chengbi, Guangxi Province

As the core demonstration zone for mango cultivation in Baise City, the ecological condition of the Lake Chengbi Basin is of paramount importance to the development of the local specialty agriculture industry. The nitrogen and oxygen isotopic composition and hydrochemical characteristics of the water were systematically studied to identify the major sources of nitrate in Lake Chengbi watershed, and the contributions of each source were quantitatively analyzed. The ratio of NO3–/Cl– to Cl– in the lake water indicates that waters in Lake Chengbi watershed were primarily influenced by the combined effects of agricultural activities and urban sewage. The δD and δ18O values of the lake water indicate that the water in Lake Chengbi is mainly recharged by atmospheric precipitation. The values of δ15N-NO3– and δ18O-NO3– in the lake water were 5.4‰ to 8.7‰ and 0.9‰ to 6.1‰, respectively, which were more positive than in the rivers. The results of SIAR model showed that the nitrate in Lake Chengbi and its rivers is mainly from agricultural activities in the watershed, totally with a contribution rate of 81.7% from soil organic nitrogen, fertilizer application, and livestock farming. This is followed by contributions from urban wastewater discharge, with a contribution rate of 13.8% to 23.4%. The contribution from atmospheric precipitation is less than 5%.