The Nitrogen Isotope Research In The Changjiang River Estuary And The Adjacent Areas

Stable nitrogen isotope (δ¹⁵N) can be used as fingerprints to study sources andsinks of nitrogen in aquatic environments. To reveal nitrogen dynamics in theChangjiang River estuary, variability of nitrogen isotope and its environmentalimplications were studied. Natural zeolites were modified to enhance theirperformances in acid environments for stable nitrogen isotopic analysis in waters. Theδ¹⁵N of dissolved nitrate (δ¹⁵NO₃) and suspended particulate matters (δ¹⁵Np) in thesurface and transect water columns of the Changjiang River estuary were analyzed bythis method in2009and2010. According to the seasonal and spatial variations of15N, fate of nitrogen as well as the biogeochemical processing involved were studiedand its environmental implications were also discussed. Accordingly, the nitrogencycle and its evolution process were explored in the Changjiang River estuary.The main conclusions were listed below:Several Chinese natural zeolites were modified with multiple methods toincrease their ammonium adsorption efficiency for nitrogen isotope analyses. Beingtreated by gravity screen, sodium chloride, hydrochloric acid, microwave andultrasonic wave, their ammonium adsorption efficiency under acid conditions wereimproved even at low concentrations. The nitrogen isotope fraction factor of newlymodified zeolite was small and the modified zeolite is proper for nitrogen isotopeanalysis of nitrate in seawater.The15NO₃in surface and transect water of the Changjiang River estuary wereanalyzed in February, May, August and November of2009and2010. The distributionof15NO₃varied both spatially and temporally, which reflected that different physicaland biogeochemical processes might affect the15NO₃signatures in differentgeographic regions. In the inner estuary,15NO₃was mainly affected by the riverineinput from the Changjiang River. The15NO₃feature in the channel may reflect thedifferent pollution resources from drain outlets. In the Turbidity Maximum zone,complicated biogeochemical processes were found in different seasons. Assimilation was found in both surface and transects waters in May. Nitrification was found in thedeep waters in August. With upwelling current, surface water was influenced bynitrification and assimilation in August. In November, nitrification was found in thedeep waters while assimilation and denitrification was found in the surface water. Inthe adjacent marine area, deep waters were mainly controlled by nitrification, whilesurface waters were influenced by nitrification, assimilation and denitrification.Meanwhile, influence of environmental factors such as temperature, salinity,dissolved oxygen, nutrients of nitrification were discussed. The nitrogen isotopicfactor of nitrification was evaluated to be-8‰using Rayleigh equation. It is inferredthat newly regenerated nitrogen from mineralization and nitrification is an importantsource of eutrophication in the Changjiang River estuary.Meanwhile, theδ¹⁵Npin surface and transect water of the Changjiang Riverestuary were analyzed in February, May, August and November of2009and2010. Itwas observed that the distribution ofδ¹⁵Npvaried seasonally in different regions,which reflected that different physical and biogeochemical processes affected theδ¹⁵Npsignatures. In the inner estuary,δ¹⁵Npwas mainly affected by riverine inputfrom the Changjiang River. Chemical fertilizer and atmospheric precipitation wasbelieved to be the main source of nitrogen from the Changjiang River in February andMay, while industrial and domestic sewage were the main source of nitrogen from theChangjiang River in August and November. In the turbidity maximum zone and theadjacent marine area, different biogeochemical processes were found in the surfaceand deep waters. Normally,δ¹⁵Npvalues were high in the deep waters, whichindicated that mineralization occurred in deep waters. However, in November of2010,δ¹⁵Npvalues were low in the deep waters. Mineralization, nitrification andassimilation occurred in the deep waters and caused the lowδ¹⁵Npvalues.Assimilation was found in the surface waters throughout the whole year except inFebruary. The distribution ofδ¹⁵Npin February was influenced mainly bymineralization and mixing between river and ocean. The reciprocal transformation of dissolved nitrate and suspended particulatematters was observed in the Changjiang River estuary. The correlations betweenδ⁽15NO₃andδ¹⁵Npreflected biogeochemical processing and evolution of nitrogen. Ingeneral,15NO₃was slightly lower thanδ¹⁵Npin the surface waters. The nitrogenfractionation （） between them was positive in average. The lowest values of15NO₃andδ¹⁵Npwere both observed in February. Dissolved nitrate and suspendedparticulate materials were mainly influenced by riverine input from the ChangjiangRiver. The15N distributions were also affected by assimilation and mineralization. InMay, biogeochemical processes were significantly increased. Negative correlationswere found between15NO₃and15Np, and values were mainly negative.Assimilation contributed greatly in this season, while mineralization and nitrificationwere also found in the east area of the adjacent marine area. In August, values weremainly positive; and were higher in the deep waters than those in the surface waters.The results indicated different biogeochemical processes between surface and deepwaters. Assimilation and nitrification dominated in the surface waters whilemineralization and nitrification played important roles in the deep waters. InNovember, values were mainly positive expect some negative values in the adjacentmarine area. values increased from surface to deep layers. Surface waters weremainly influenced by assimilation and nitrification, while mineralization andnitrification were found in the deep waters.