Study On Nitrate Source Tracing Techniques Based On ¹⁵N And ¹⁸O Isotope

Nitrate is one of the important forms of bioavailable nitrogen.The study of nitrate nitrogen and oxygen isotopes has gradually become one of the most important methods of nitrogen cycle research.The source of water nitrate and its migration and transformation rules are the key factors to evaluate the quality of water environment and the mechanism of nitrogen cycle process.However,the existing analysis methods of nitrate nitrogen and oxygen isotope compositions in water still have room for improvement in terms of improving conversion efficiency,shortening reaction time,and controlling experimental cost.In this study,based on the original chemical conversion method,the purge gas improvement,the elimination of the azide reaction interference and the improvement of the nitrous oxide measuring instrument were studied.A set of methods and procedures suitable for the determination of freshwater and seawater,simplifying the experimental steps,reducing the experimental cost,shortening the measurement time and improving the efficiency of sample determination were determined.It was used to analyze the source of nitrate pollution in the Wuliangsu Sea Basin in Inner Mongolia,and used three-dimensional fluorescence spectroscopy to qualitatively identify the source of pollution.Finally,the contribution rate of each source was calculated using the IsoSource isotope mixing model.The results of method research show that:?1?At low temperature?4??,the generation of impurity gases in the NaN₃-CH₃COOH mixture can be effectively suppressed.?2?The improved chemical conversion methods?¹⁵N-NO₃^-and?¹⁸O-NO₃^-have a measurement accuracy of 3‰and 5‰,respectively.The pretreatment time required only 1.5 h,and the sample volume was only 4.5?g NO₃^--N.Compared with the other two methods,this method reduces water consumption and shortens the pretreatment time.The research results help to improve the stability of the chemical conversion method and lay the foundation for future promotion and application.The results of the analysis of pollution sources show that:protein substances,microbial metabolites,fulvic acid substances and humic acid substances in the water DOM are inherently correlated in source,and their sources are related to the migration and transformation of nitrate nitrogen It is also closely related to the sources of COD and TOC;the calculation results of the IsoSource model show that the contribution rates of nitrate sources in drains are:animal feces and sewage 69.9%,precipitation and fertilizer NH₄⁺11.6%,soil organic nitrogen 9.3%,atmospheric deposition 9.2%.Source of nitrate in the lake area:animal feces and sewage 70.6%,followed by precipitation and fertilizer NH₄⁺12.9%,soil organic nitrogen 9.9%,atmospheric deposition 6.6%.Sources of nitrate pollution in well water:animal feces and sewage67.7%,soil organic nitrogen 16.4%,atmospheric deposition 14.3%,precipitation and fertilizer NH₄⁺1.6%.This study has laid a foundation for the popularization and application of chemical conversion methods in the future,and has guiding significance for the water pollution control of the watershed in the study area.