The Distribution Characteristic Of The Nitrogen Isotope In Different Nitrogen Forms In Changjiang River Estuary And Its Environmental Implication

The distribution characteristic of stable nitrogen isotope and its environmentalimplication were studied. A pretreatment method for measurement of the stableisotope of ammonium was established，and was applied to the field sample collectedquarterly from November，2011to August,2012.The process of nitrogen cycle wasstudied by analyzing the data of15NO3-、δ¹⁵Np and15NH4+,combining with otherenvironment factors. The result are as follows:A pretreatment method was developed to determine the isotopic composition ofammonium based on the rapid steam distillation. The optimal distillation time,appropriate zeolite, and optimal concentration range of NH4Clwere studied. Theoptimal distillation time was50min when the average recovery of samples was（97±5.8）%.The optimality zeolite was determined by measurement of theadsorption efficiency of different zeolites under acid condition. The optimalconcentration range of ammonium was2¹0μmol/L, when recovery of ammoniumwas94%¹00%, and the isotopic fractionation was0.1⁰.8‰.The fact that nitrogen varies with different areas and seasons was found byanalyzing15NO3-,δ¹⁵Np and15NH4+in changjiang river estuary.The stable isotope ofnitrogen was mainly controlled by mineralization and nitrification in bottom water ofchangjiang river estuary，but denitrification also affected the distribution when DOwas low;the fractionation factor of mineralization in spring was6.7‰.the nitrificationfractionation factor varied with seasons.15NO3-was influenced by assimilation insurface water, the discrimination fraction of assimilation was different between0m（0.96‰）and5-10m （6.7‰）;Mineralization and nitrification was found in bottom water，factor of nitrification was16.27‰. The stable isotope of nitrogen was impactedby input of runoff, but nitrification was founded in estuary. Biological assimilationgerminated in the maximum zone,while nitrogen fixtion and biological assimilationoccured in open sea in spring.The biogeochemical process of nitrogen in turbiditymaximum zone was comparatively complicated in summer, and15NO3-was mainlycontrolled by assimilation,whose assimilation discrimination factor was5.29‰.