第五讲 稳定氮同位素

第五讲稳定氮同位素稳定氮同位素:14N:99.64%15N:0.36%standard=N2inair(15N/14N=0.00368)measuredas:N2典型氮库的同位素组成:Nitrate,rain:-13to+2OrganicN,soil:0to+9Fertilizer:-5to+5Animalwaste:+8to+22氮同位素及其循环

NitrogenIsotopesandNitrogenCyclingADELTA15-N=+10‰

meansthereis10partsperthousand(orpermil)more15-Ninthesamplethaninthestandard.ADELTA15-N=-10‰

meansthereis10partsperthousand(orpermil)less15-Ninthesamplethaninthestandard.Delta(d)value=Ratiosample

RatiostandardRatiostandardx1000-WhereRatioistheratioof2isotopesofanelement,suchas15N/14Nfornitrogen,forasampleorastandard.氮同位素及其循环

NitrogenIsotopesandNitrogenCyclingPeopleusuallyfindthenegativedvaluesabitconfusing,buttheyarejustaproductofwhatmaterialwaschosenasthestandard.Example:

ForN,theratioof15N/14NinAirisabout1/250.Ifasamplehas1%more15NthanAir(=1/250),itsratiowouldbe:1.01/250.This1%correspondsto+10‰.Ifasamplehas1%less15NthanAir--(=0.99/250),thisis-10‰.氮同位素及其循环

NitrogenIsotopesandNitrogenCyclingheavyvs.lightisotopes the“heavy”isotopeistheonewithmoreneutrons;itisalsogenerallythelessabundantisotope.enrichedvs.depleted remembertostatewhatisotopeisinshortsupply: does“enrichednitrogen”meanthat: thenitrogenisenrichedinheavynitrogen

ORthenitrogenisenrichedinlightnitrogen?positivevs.negative

-10‰

ismorepositivethan-20‰.氮同位素及其循环

NitrogenIsotopesandNitrogenCycling1天然气；2火成岩；3火山气；4受粪肥污染土壤中的氮；5动物粪便(厩肥)或污水中的NO3-；6沉积岩中的有机质；7石油；8非豆科植物；9垦植土壤和受生活污水污染土壤中的氮；10土壤有机氮矿化形成的NO3-

；11天然土壤中的氮；12雨水；13煤；14受化肥和工业废水污染土壤中的氮；15含氮化肥的NO3-；16豆科植物；17泥炭不同氮源的δ15N值分布图氮同位素及其循环

NitrogenIsotopesandNitrogenCycling-50-40-30-20-10010203040501152351787166410121491113一般讲，城市地下水的δ15N

值较高，而农村地区较低；在生活污水和畜禽废水为污染源的地下水层，δ15N

一般在1%以上；农业用地的地下水δ15N值为0.10%-1.08%，化肥施用地区地下水δ15N值较低，有机肥施用地区地下水δ15N

值较高，有时候超过1.0%；当然，施肥的方法、灌溉的情况及土壤的特性等都会影响同位素值。氮同位素及其循环

NitrogenIsotopesandNitrogenCyclinglightheavy0‰0.6‰-1to0‰-1to0‰20-30‰氮同位素分馏

NitrogenIsotopeFractionationKarletal.,2002表生环境中，某些作用过程会导致环境中N组分的同位素发生分馏，这些过程包括：

Duringbiologicalprocesses(e.g.,assimilation,nitrification,denitrification),thelighterisotope(14N)endsupbeingconcentratedintheproductswhiletheheavierisotope(15N)endsupbeingconcentratedintheresidualreactants.Hence,duringthereactionNO3

N2,theresultingN2hasalowerd15NthantheresidualNO3.Biologicalfractionationscanmakeitverydifficulttoidentifysourcesandquantifymixingproportions.氮同位素分馏

NitrogenIsotopeFractionation15NGlobalaverage=4.5to5.0

‰

AdditionofdiazotrophNtonitratepool

=0to-1‰0100 100 200%ofOriginalPool

20NO3Pelagicdenitrification=-20‰Nitrateuptake=-5‰Sedimentdenitrification=-0‰[Nitrification]氮同位素分馏

NitrogenIsotopeFractionation15NAdditionofdiazotrophNtoNH4pool

=0to-1‰0100 100 200%ofOriginalPool

20NH4Nitrification=0to-20‰Ammoniumuptake=(-5‰??)Ammonification=(-3to-5‰??)氮同位素分馏

NitrogenIsotopeFractionationProcessEnrichmentfactor()(w/rtoproducts)CommentsNO3assim:culture,field0to-24‰;-10‰-4to–5‰mMNO3;µMNO3µMNO3NH4assim:culturefield0to-15‰;-3to-27‰-10‰mMNH4;µMNH4µMNH4Nitrification0to-20

‰Concentration

dependentDenitrification,pelagicsediment-20to-40‰~0‰Concentration

dependentN2Fixation~-3to+1‰LittleenzymaticfractionationAmmonification-3to-5‰(?)HardtomeasureNH4+

氮同位素分馏

NitrogenIsotopeFractionationPrimaryproductioninaclosedsystem–Raleighfractionation氮同位素分馏

NitrogenIsotopeFractionationNH4的示踪:The15NofNH4derivedfromvolatilizationofNH3fromanimalwastelagoonsisverylowcomparedtothe15NofotherNsources.NitrificationofNH4producesNO3witha15Nthatisonlyabout0to2‰higherthanthe15NoftheNH4(ifmostoftheNH4isnitrified).Hog

Hog

WasteinLagoonMatureLagoon15Nhog

+3‰15Nwaste

+2‰15NNH3

-15to-25‰VolatilizationofNH315NNH4

+30‰LagoonResidualwastematuration氮来源示踪TracetheofsourcesofN氮来源示踪TracetheofsourcesofN15NNO3Cannoteasilydistinguishbetween:(1)amanuresourceand(2)afertilizersourcethathasdenitrifiedtohavethesame15Nasmanure.Justusing15-Nisotope环境介质中NO3-的示踪d15N(permil)(ModifiedfromKendall,1998)d18O(permil)Highnaturalbackgrounds,especiallyinaridandsemi-aridenvironments.Widespreaduseasfertilizer.Byproductofanimalwaste.Commonconstituentofchemicalprocessing氮来源示踪TracetheofsourcesofNMississippiRiverBasin(Changetal.,2002)SanJoaquinRiverBasin(Kratzeretal.,2003)NEUSArivers,(Mayeretal.,2002)denitrification氮来源示踪TracetheofsourcesofN地表水体中NO3Howtoestimatetheeffectsofde-nitrificationcanbeestimatedusing:analysisofdissolvedN2for15N(tocorrectfortheN2producedbydenitrification),nitrate-δ18O(thistracerusuallyisnotasusefulforthispurposeasN2-15N);andgeochemicalmodelingusingchemicaldataandperhapsDIC-13C,SO4-34S,etc.useofall3methods,alongwithage-datingofthewaterusingCFCsor3He/T,willimprovequantificationofsourcecontributions.地下水环境中NO3可能经历了反硝化作用过程，因此，当用15-N对地下水中NO3进行示踪时：对化学肥料来源？

动物排泄物？？氮来源示踪TracetheofsourcesofN

地下水系统中，NO3天然降解Modifiedfr