Identifying nitrate sources and transformations in Taizi River Basin, Northeast China

The nitrate (NO₃ ⁻) pollution of aquatic systems in Northeast China is a severe problem. To identify NO₃ ⁻ sources and transformations in different zones with different land uses in the Taizi River Basin, ion-exchange methods were utilized to determine the concentrations and isotopic compositions (δ¹⁵N and δ¹⁸O) of NO₃ ⁻ and chloride (Cl⁻). Results showed that Cl⁻ concentrations ranged from 2.7 to 73.4 mg/L. Cl⁻ concentrations were the highest in zone 8 and the lowest in zone 1. NO₃ ⁻ concentrations ranged from 0.3 to 27.4 mg/L and were the highest in zone 1 and the lowest in zone 8. During the sampling period, δ¹⁵N–NO₃ ⁻ values varied from 3.8 to 37.2‰, and δ¹⁸O–NO₃ ⁻ values ranged from −0.5 to 10.4‰. δ¹⁵N–NO₃ ⁻ values were the highest in zone 9 and the lowest in zone 1. The differences in physicochemical parameters and NO₃ ⁻ isotopes may be affected by land use and biogeochemical nitrogen processes in different zones. The combined analysis of dual isotopes (δ¹⁵N–NO₃ ⁻ and δ¹⁸O–NO₃ ⁻) and NO₃ ⁻/Cl⁻ versus Cl⁻ showed that different sources contributed NO₃ ⁻ to different zones during the sampling period. Soil N, manure, and sewage were the main NO₃ ⁻ sources in the Taizi River Basin. In zones 1 to 6, the δ¹⁵N–NO₃ ⁻ values of almost all samples were more than 10‰, NO₃ ⁻/Cl⁻ values were high, and Cl⁻ molar concentration was low during the sampling period. These findings suggested that the volatilization and nitrification of soil NH₄ ⁺ might be related to NO₃ ⁻ sources in zones 1 to 6. A 1:1 to 2:1 linear relationship between δ¹⁵N–NO₃ ⁻ and δ¹⁸O–NO₃ ⁻ combined with the significantly negative relationship between ln (NO₃ ⁻) and δ¹⁸O–NO₃ ⁻ indicated that denitrification affected NO₃ ⁻ distribution in zones 8 to 9 during the sampling period. These results can provide useful information to control NO₃ ⁻ concentrations in different zones in Taizi River Basin.