## 3 Research Progress in Analytical Techniques of Nitrogen and Oxygen Isotopes
When the nitrate derived from atmospheric precipitation, fertilizer, or soil transfers from soil system to rivers, influencing factors like microorganisms may cause the enrichment of \({}^{15}\)N. Consequently, the isotopic characteristics of nitrate source disappear. However, in the event that the activity of microorganisms is low, \(\delta^{15}\)N and \(\delta^{18}\)O values are relatively stable during nitrate transfer, in which case the nitrate source can be correctly identified [5; 47]. [PERSON] et al. [47] reported that \(\delta^{15}\)N and value of farmland wastewater in winter was significantly higher than those of groundwater and precipitation, but the \(\delta^{18}\)O value of farmland wastewater was much higher than that of groundwater yet was far lower than that of precipitation. The \(\delta^{15}\)N and \(\delta^{18}\)O values of rivers and farmland wastewater were close. The results of researches conducted on nitrogen and oxygen isotopes technique and geochemical methods show that the high-concentration of nitrate in farmland wastewater and in rivers is directly correlated with the use of massive chemical fertilizers [4; 48]. Besides, it has been proven that the negative impacts of fertilizer can be largely reduced by proper agricultural management [49]. Precipitation and soil are the major sources of the nitrate in rivers which run in the basin mostly covered with forestland. Nitrate from soil includes the soil organic nitrogen and the nitrate transformed from the NH\({}_{4}\)\({}^{+}\) in soil through nitrification. Though the \(\delta^{15}\)N values of these sources are quite close, the enrichment of \(\delta^{18}\)O in atmospheric precipitation makes it possible to correctly identify the nitrate source through \(\delta^{18}\)O value [4]. When the precipitation is low in amount, the nitrification in soil is the main reason for the increase of the nitrate concentration of rivers within forestland-covered basin [50; 51]. The nitrogen in precipitation is unable to go into rivers directly for being intercepted by forestland, and thus the nitrogen is absorbed by organisms where organic nitrogen is synthesized. The nitrate will be produced by the mineralization and nitrification of upper soil organic nitrogen. The nitrate in soil will eventually enter surface rivers under eluviation, infiltration, etc. [52]. After analyzing the \(\delta^{15}\)N and \(\delta^{18}\)O composition of surface water within the forestland-covered basin in Northeastern United States, [PERSON] et al. [28] found that over 80% of nitrate was generated by the nitrification of soil nitrogen. When the precipitation increased, the soil organic nitrogen and precipitation became the major contributors of nitrate in rivers within forestland-covered basin [53]. Besides, the data of \(\delta^{18}\)O show that the atmospheric nitrate will go into rivers after the nitrification of microorganisms [51]. As for the basin mostly covered with urban mixed-use land, the nitrate of surface water is mainly from domestic wastewater, industrial sewage, chemical fertilizer, manure, soil nitrogen, and atmospheric precipitation. Seasonal changes and human activities are the major influencing factors altering nitrate sources of surface water within this kind of basin. There are lots of impervious surfaces which only intercept a small amount of precipitation and thus most of the precipitation will go into surface rivers. Therefore, seasonal change of precipitation has a great impact on the nitrate in urban rivers. During rainy season, the contribute rate of atmospheric precipitation to nitrate in surface rivers can reach 50% or even 67% [28; 53]. By measuring \(\delta^{15}\)N and \(\delta^{18}\)O values, [PERSON] et al. [54] studied the sources of nitrate in Taihu Lake area. The nitrate mainly derived from precipitation, domestic wastewater, and organic fertilizer in summer, while it was from the nitrification of domestic wastewater, organic fertilizer and soil organic nitrogen in winter.