## 7 Conclusions and Suggestions  
When combining with hydrochemistry, hydrogeology, other isotopes, and mathematical models, the \(\delta^{15}\)N and \(\delta^{18}\)O of nitrate are able to solve such problems as the partial overlapping of isotopic composition of various sources and isotopic composition reduced by isotopic fractionation. Thus, they are promising in confirming identification sources of nitrate pollution of surface water as well as in analyzing the transformation mechanisms. However, it should be pointed out that the following limitations lie in the current researches. (1) The isotopic compositions of nitrate from different sources have large spatial and temporal differences. The previous studies on isotope of different-form nitrogen are limited to scattered regions, and there is not any comprehensive study on the systematical changes of the nitrogen in global surface water or of the \(\delta^{15}\)N and \(\delta^{18}\)O of nitrate. Therefore, a global isotopic database which could reflect the change of nitrogen and oxygen isotopes is missing. (2) Some researches only analyze nitrate sources by comparing the \(\delta^{15}\)N and \(\delta^{18}\)O values of nitrate with the eigenvalues of \(\delta^{15}\)N and \(\delta^{18}\)O of different sources and they fail to go deep into the isotope values of nitrate during migration and transformation. (3) The pretreatment methods for testing the compositions of nitrogen and oxygen isotopes are not cost-effective and have complex operating steps. It is quite important to make the method automatic, simple, and cost-effective. (4) The mechanisms of relevantprocesses are still unclear. For instance, there are only methods for the qualitative judgment of isotopic fractionation and there is lack of quantitative basis. That is, these methods can only ascertain whether isotopic fractionation occurs. They are unable to determine the extent of fractionation caused by biochemical reactions. As a result, the change degree of the compositions of original nitrogen and oxygen stable isotopes of nitrate as well as the influence of the change on research results cannot be analyzed thoroughly. In order to overcome the limitations in the current researches and to make research progress, the following issues should be taken seriously. (1) The stable isotope values of nitrogen from different countries or regions are different owing to the various regional conditions. For the quick and accurate analysis on the nitrogen pollution source of the water bodies, the stable isotope values of nitrogen from different pollution sources should be gathered to establish a global and regional isotope database. (2) Nitrogen will go through multiple biochemical reactions when it transfers from the pollution source to water body, and these biochemical reactions are affected by such factors as land-use types, climate, and hydrogeological conditions. It is of great significance to study the influencing factors for identifying the pollution sources and tracing the migration and transformation ways of nitrogen. Besides, because of the complexity of river system, the geochemical investigation of isotope alone is not enough for revealing the geochemical cycle process of nitrate in rivers. Thus, nitrate sources of water can be identified more accurately if data involved in different fields are adopted. For example, the atmospheric precipitation, rivers, groundwater, and soil water are regarded as a whole when establishing a hydrological model, through which the hydrological cycle and nitrogen cycle can be better understand. (3) On the premise that the accuracy for measuring the isotope values is ensured, current methods should be optimized or new methods need to be explored. Thus, the isotope testing methods characterized by small sample size, high accuracy, automation, and low cost are the main focuses of the future researches on nitrogen pollution source.