1、第五章 稳定氮同位素,氮同位素概述氮同位素分馏自然界中稳定氮同位素分布特征稳定氮同位素应用简介,Outline,1.1 氮的地球化学,一、概述,地球上最大的氮储库是大气，近地表环境中约99%的氮以N2形式存 在于大气中和溶解在海水中。其余的氮呈与C、O、H结合的形式存在于生物圈中，含量非常少，但对生命过程起极为重要的作用。含氮矿物少见，主要有钠硝石(NaNO3)、硝石(-KNO3)、鸟粪石(NH4MgPO4.6H2O)、磷酸镁钠石(NH4MgPO4.H2O)、陨氮钛石(TiN)、氧氮硅石(Si2N2O)等。在地表条件下，氮可以气、液或固态存在，具有多变价态（由+5到-3），存在形式为NO3-、
2、NO2-、N2、NH4+、NH3、NO2、NO、N2O和氨基酸等有机氮等。,1.2 氮同位素及其丰度,Stable N Isotopes：14N： Atomic Mass: 14.0030740； Atomic Percent Abundance: 99.634%.15N: Atomic Mass: 15.0001089 ； Atomic Percent Abundance: 0.366%.%,Titan is among the most remarkable objects in our solar system. It has a remarkable atmosphere. Most
3、of the atmosphere is nitrogen gas, (N2), as is our atmosphere. it is cold on Titan : the surface temperature is a bit below 100 K, but the atmosphere is notably warmer, around 160 K, but still pretty darn cold.,The bottom line is that isotope ratios such as these are important clues into the chemica
4、l and physical processes that have evolved over time across the Universe.,二、氮同位素分馏,氮同位素的国际标准为大气N2，其“绝对”同位素比值为15N/14N=(3676.58.1)10-6 (Hayes, 1982)，定义其15 N=0。,氮同位素样品的制样方法多为燃烧法(Combustion)，15 N分析精度为0.10.2。,2.1 15N 的表示方法及测定,2.2 大陆生态系统中的氮循环,2.3 氮同位素的动力分馏,氮多价态的变化有利于同位素分馏。微生物在生物氮循环中起关键作用：,固氮过程；硝化过程；反硝化过程,
5、固氮作用(Fixation)：N2+3H2O2NH3+3/2O2,动力分馏,The term fixation is used for processes that convert unreactive atmospheric N2 into reactive nitrogen such as ammonium, usually involving bacteria. Fixation commonly produces organic materials with 15N-values slightly less than 0 ranging from 3 to +1 (Fogel and
6、Cifuentes 1993) and occurs in the roots of plants by many bacteria. The large amount of energy needed to break the molecular nitrogen bond makes nitrogen fixation a very inefficient process with little associated N-isotope fractionation .,动力分馏,2. 硝化作用(Nitrification)：,Organic N NH4 NO2 NO3,第一步基本上无氮分馏
7、，第2步和第3步伴有较大的动力学分馏。 总分馏取决于哪一步是速率控制步骤。,如有大量氨存在，第2步或第2和第3步是速率控制步骤，形成的硝酸盐比初始物质亏损15N约20 35 。土壤中的氮慢慢转化为氨，第1步为速率控制步骤，将无分馏。实验表明，NH4+同化时分馏为0 -27 ，NH4+浓度越低，分馏越大，野外观测到的分馏约为-10 （Fogel and Cifuentes, 1995)。,动力分馏,3. 反硝化作用(Denitrification)：,为富氧环境下的细菌作用，将硝酸盐还原为氮气。该过程平衡了自然界的固氮过程，否则大气中的氮将在100Ma以内消耗殆尽。反硝化作用的顺序为：NO3-
8、NO2- NO N2O-N2 。 由两个步骤组成:细胞吸收营养物质，无分馏；营养物质分解，涉及一系列与N-O键破裂有关的反应，伴随着大的氮同位素分馏。,Denitrification causes the 15N-values of the residual nitrate to increase exponentially as nitrate concentrations decrease. Experimental investigations have demonstrated that fractionation factors may change from 10 to 30, w
9、ith the largest values obtained under lowest reduction rates.,Naturally observed isotope fractionation for nitrogen assimilation (after Fogel and Cifuentes Fogel and Cifuentes 1993),dependence of fractionations on nitrogen concentrations,At low nitrogen concentrations fractionations are nearly zero
10、because virtually all the nitrogen is used.,Equilibrium exchange reactions have been demonstrated for the common inorganic nitrogen compounds. Of special importance in this respect is the ammonia volatilization reaction:,=1.0251.035 (Kirshenbaum et al. 1947; Mariotti et al. 1981),Experimental data (
11、Nitzsche and Stiehl ,1984): =1.0143 at 250C and =1.0126 at 350C . Very small 15N-enrichment of about 0.1 occurs during the solution of atmospheric N2 in ocean water (Benson and Parker 1961).,2.4 氮同位素的平衡分馏,不同含氮气体原子团间的氮同位素平衡分馏系数 (Letolle, 1980),NO NH3 N2 N2O NO2, 15N依次富集,氮同位素不同类型分馏作用的分馏因子,2.5 其他物理化学效应
12、,在扩散过程中气体运移距离愈远，愈贫15N，同时也贫13C和D。这种效应是质量相关的，=(M1/M2)0.5。,天然气的15N变异；幔源岩石与大气氮之间的氮同位素分馏系数很大，可用前寒武纪以来的 长期扩散去气来解释；火星大气特别高的15N,挥发过程中气相相对于原始物质贫15N，如氮肥中NH3的挥发。,NH4+易与土壤和软泥中的“腐泥复合物”发生离子交换。在实验室离子交换树脂与水相间交换时，固相富集15N达5 25 .。,三、氮同位素的天然分布,地外物质中的氮：月岩、火星大气、陨石；表生氮同位素：大气氮、淡水、海水、动植物、土壤；化石燃料中的氮：天然气、石油、煤；岩石圈中的氮：火成岩、沉积岩、变
13、质岩。,大气中的NO和NO2可生成硝酸盐。煤和石油制品燃烧产生的NO和NO2的同位素组成与被燃原料一致。土壤中的硝化和反硝化反应都可生成NO和NO2。但其15N很难与人工源的15N相区分。 N2O是一种重要的温室气体。它由细菌产生，在平流层中因光化学反应而破坏。各地土壤生成的N2O的15N不同，一般为负值。,地下水中氮的来源，除雨水从表层土壤带来外，还有肥料、污水、动植物排泄物、地下水蓄水层中的溶解物质等。主要以NO3-式存在，在还原条件下有少量NH4+。地下水的15N=0 25 ，取决于当地的氮源。例如：人工肥料4 +4 ；动物排泄物：5 。,海洋颗粒有机物的15N值为3 13 ，而陆地来源
14、颗粒有机物则为-6.6 5.2 ，相对贫15N。据此可研究近海区域水体的混合。,氮同位素在天然物质中的一般分布，以15 N表示,15 N变化总范围为100，从约-50到+50，大部分落在-10到+20范围内。 火成岩：-16 +31 ； 水圈中的氮以大洋水中的氮为代表，-8 +10； 植物：-10 +22； 石油和煤落在现代生物范围内，0 +15 ； 天然气的15 N变化极大：-45 +45 ； 地外物质的15 N变化范围最大，为-40 +100 ，而火星大气的值高达700.,天然气 Natural Gas,石油 Oil,地表有机物,Terrestrial Organism,海水 Oceani
15、c Water,大气 Atmosphere,岩浆岩 Magmatic Rock,金刚石 Diamond,海相有机物,Marine Organism,15N/14N ratio in some geologically important materials (15N vs. atmosphere N2),三、氮同位素的应用,生物地球化学水文循环海洋沉积气候变化环境污染地外物质考古,Nitrate Contamination in aquifer systems,Range of 15N values for various sources of nitrogen and the enrichm
16、ent effects of denitrification and volatilization processes. Ranges of 15N values from Heaton (1986) and site studies in Kansas (Townsend et al., 1996).,Can isotopes resolve relative trophic level differences at the food web scale in the complex Everglades ecosystem? Does food web structure (as dete
17、rmined by isotopes) vary over space? Does food web structure vary over time?,23,0-1,A positive slope,The carbon and nitrogen isotope compositions of various types of diets are known and can be compared to the compositions of your fingernails. Hence, the graphs of your fingernail d15N and d13C values
18、 can help to identify your diet.,Along the groundwater flow path, denitrification is demonstrated by decreasing nitrate concentrations and increasing concentrations of excess dissolved nitrogen gas.,Nitrate isotopic compositions measured in groundwater and potential source materials relevant to HE o
19、perations provided discrete markers, as compared to range values of nitrate isotopic compositions of soil, septic effluent, and precipitation.,Distribution of nitrate concentrations in drinking water and mortality of esophageal cancer in the Linzhou-Anyang areas in Henan Province. 1. Presidial borderline; 2.Rivers; 3.Isoline of nitrate nitrogen levels, 20 mg/L; 4.Mortality distributing isoline of EC, 80100/ 100 thousand.,Nitrate & Esophageal Cancer ?,Where does nitrate in groundwater come from?,
