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.