## 3 Research Progress in Analytical Techniques of Nitrogen and Oxygen Isotopes
Moreover, it is also applicable for samples with a concentration of nitrate lower than 1 M. The denittifier method has been used as a standard technique by the United States Geological Survey to determine the \(\delta^{15}\)N and \(\delta^{18}\)O of nitrate [41]. The nitrogen is completely transformed during the denitrification, so \(\delta^{15}\)N only needs a blank correction. However, the oxygen in nitrate is not completely transformed into N\({}_{2}\)O and there is oxygen isotopic fractionation, so \(\delta^{18}\)O needs correction of oxygen isotopic fractionation, exchange, and blank. The limitation of this method lies in that it fails to distinguish the nitrate in water sample from nitrite, and thus the presence of nitrite in water samples can bias the isotopic composition of product N\({}_{2}\)O. For this reason, if the water sample contains nitrite, the nitrite should be removed first [42]; otherwise, the measured results should be corrected [43]. In addition, it needs a long time to cultivate bacteria, in which the bacteria are likely to be affected by toxicity of the sample. Therefore, the rapid and complete transformation of nitrate into N\({}_{2}\)O should be considered in the selection and culture of bacteria. [PERSON] et al. [44] modified the classical denittifier method with _P. aureusforeiens_ for robust and reliable conversion of nitrate to N\({}_{2}\)O without prior induction of denitrification with extraneous nitrate. Cd-azide reduction method, proposed by [PERSON] et al. [45], is a new method which is able to measure \(\delta^{15}\)N and \(\delta^{18}\)O. In this method, Cd is added into water sample to reduce nitrate into nitrite and then azides are added as well so that the nitrite can be further reduced into N\({}_{2}\)O. Subsequently, high-purity N\({}_{2}\)O can be obtained through separation, enrichment, and purification. Finally, the compositions of \(\delta^{15}\)N and \(\delta^{18}\)O are measured through IRMS. Apart from not being susceptible to organic substances, advantages of this method also include simple preparation procedure, inexpensive, small sample volumes, and automation with high sample throughput. However, this method is dangerous due to the toxicity of the adopted reagents [46]. Furthermore, the nitrite in water sample will lead to the deviation of \(\delta^{15}\)N and \(\delta^{18}\)O values, so the measured results need further correction. Even though the accuracies of the above mentioned three simultaneous testing techniques for \(\delta^{15}\)N and \(\delta^{18}\)O have been significantly improved, they still have certain limitations in tracing nitrate pollution sources of surface water. The preparation procedure of water samples will directly affect the accuracy of final results. So far, no single simple method has been able to automatically extract the nitrate in water samples and meanwhile to completely avoid the isotope contamination and fractionation. Application of Nitrogen and Oxygen Isotopes in Nitrate Source Tracing of Surface Water under Different Land-Use Types  
Chemical fertilizer, manure, precipitation, soil nitrogen, and nitrogen fixation are the main sources of nitrate in rivers which run in the basin mostly covered with agricultural land. The crop species cultivated in farmland, the fertilizing methods, and the activities of microorganisms in soil vary in different seasons, so these factors have great influences on the source identification of nitrate in surface water within farmland area. The nitrate in farmland soil which comes from different sources is transferred to surface water through processes like eluviation [5]. [PERSON] et al. [22] found that the \(\delta^{15}\)N value in the wastewater of farmland applied with pig manure was much higher than that in the wastewater of farmland applied with inorganic fertilizer. It was also higher than that of pig manure. This evidence indicates that the \(\delta^{15}\)N in farmland soil has been fractionated and inorganic fertilizer and animal manure are the major sources of the nitrate in farmland wastewater.