Figure 9 Sensitivity analysis of four potential nitrate sources (SN-NF indicates the isotopic composition of altered SN, contribution of NF; a to d altered nitrogen isotope means, e to h altered oxygen isotope means). (a) Surface water in November 2021; (b) groundwater in November 2021; (c) surface water in April 2022; (d) groundwater in April 2022; (e) surface water in November 2021; (f) groundwater in November 2021; (g) surface water in April 2022; (h) groundwater in April 2022.
The uncertainty of nitrate sources mainly includes the spatiotemporal uncertainty of non-point source pollution inputs from the watershed and the complex biogeochemical processes that occur during nitrate migration in water, including nitrification and denitrification processes etc. In the water cycle, from atmospheric precipitation to the process of production and confluence, changes in river and groundwater levels alter the interaction between surface water and groundwater, which in turn changes the isotopic composition of nitrate in water and brings uncertainty to the interpretation of nitrate sources. This is also the inherent reason why soil nitrogen has been highly uncertain in Figure 8. By comparing the sensitivity analysis results with the values of isotopic end members input in Table 1, we found that for nitrogen isotopes, the standard deviation of nitrogen isotopes in MS is the largest, and the model is also most sensitive to nitrogen in MS, while oxygen isotopes are most sensitive to AD. Therefore, we recommend that it is necessary to establish the isotopic characteristics of different end members in the research area. If limited by practical conditions, sampling and analysis of the most sensitive sources should be prioritized.