陆地生态系统碳/水交换显著影响着大气CO2浓度变化和全球水分循环。随着全球变化趋势的日趋明显,陆地生态系统在碳素的吸收、转移、贮存和释放过程中以及区域乃至全球水分循环过程中所起的作用越来越受到人们的关注(Ciais et al.,1995; Francey et al.,1995; Conway et al.,1994; Lin et al.,1999)。利用微气象法,人们已经能够测定生态系统CO2或H2O通量,但是不能精确量化不同生态过程(碳通量中的光合作用和呼吸;蒸散通量中的蒸腾和蒸发)对碳、水通量变化的相对贡献(Baldocchi et al.,1988,1996)。稳定性同位素(Stable isotopes)贯穿于生态系统复杂的生物、物理、化学过程中,能够在时间和空间尺度上整合反映生物生理生态过程对环境条件变化的响应,并逐渐成为人们深入了解生态系统对环境变化响应的重要工具(Ehleringer,1993; Yakir & Sternberg, 2000; Dawson et al.,2002; Ehleringer et al.,2002)。
其中,δ13C、 I(13C) 、M、μCO2分别表示大气CO2的13C同位素组成、添加源的13C同位素组成、响应系数和大气CO2浓度。
Keeling曲线以生物学过程前后的物质平衡原理为基础,将稳定性同位素含量测定与物质(CO2或H2O)浓度测定相结合,利用植被不同高度样点之间同位素组成和CO2或水浓度之间的差异,构建同位素组成与CO2或水蒸气浓度倒数之间的线性关系,该直线的截距即为生态系统呼吸释放CO2或水分蒸散的同位素组成(Keeling,1958,1961; Yakir & Sternberg, 2000)。Keeling曲线法与微气象法结合,能够区分光合和呼吸对整个生态系统C通量的贡献,以及蒸腾和蒸发占生态系统蒸散的比例(Lloyd et al.,1996; Yakir & Wang, 1996;Moreira et al.,1997; Bowling et al.,1999a,1999b,2001; Ferretti et al.,2003; Yepez et al.,2003; Scott et al., 2003; Williams et al.,2004)。利用Keeling曲线法求得的生态系统呼吸释放CO2的δ13C值(δ13CR),能够将叶片尺度的同位素判别外推到生态系统尺度,通过生态系统判别(Δe),结合全球植被模型,则能够确定不同植被类型在全球碳循环中的源-汇关系(Buchmann et al.,1998; Pataki et al.,2003a)。
作为一种重要的生态学研究手段,稳定性同位素技术在生态学研究的许多领域得到广泛应用。近年来,关于稳定性同位素在生态学不同研究领域内和不同研究尺度上的应用,发表了一些综述性论文,例如Ehleringer等(2000)以及Staddon(2004)分别就碳同位素在地下碳循环及土壤生态学研究过程中所起的作用进行了总结和阐述;陈世苹等(2002)综述了稳定性碳同位素在光合途径判别、水分利用效率确定以及生态系统乃至全球碳平衡中的应用;Flanagan和Ehleringer(1998)以及Yakir和Sternberg(2000)总结了稳定性同位素在生态系统气体交换研究中的应用,并阐明了稳定性同位素技术使用存在的问题和未来的研究方向;Dawson等(2002)从物质循环角度对碳、氢、氧和氮同位素在不同尺度植物生态学研究过程中的应用进行了概要的阐述。王建柱等(2004)还综述了稳定同位素技术在陆地生态系统动-植物关系研究中的应用。随着陆地生态系统碳/水交换研究的不断深入,稳定性同位素技术和Keeling曲线法已成为进一步揭示生态系统碳/水交换对环境条件变化响应的重要手段。本文在简要介绍稳定性同位素有关术语及Keeling曲线法的由来后,着重介绍稳定性同位素技术和Keeling曲线法在陆地生态系统碳/水交换研究中的应用。
1 CO2与H2O的稳定性同位素特征
1.1 CO2中的碳、氧同位素比率
大气CO2中13C同位素比率主要取决于光合判别和呼吸释放CO2的δ13C值,以及这些过程与大气的湍流混合程度(Sternberg,1989; Lloyd et al.,1996)。光合判别指的是不同光合途径植物(C3、C4和CAM)因光合羧化酶(RuBP羧化酶和PEP羧化酶)和羧化的时空上的差异对13C有不同的识别和排斥,导致了不同光合途径植物具有显著不同的δ13C值(O'Leary,1981; Farquhar & Sharkey, 1982)。C3植物光合作用过程中,RuBP羧化酶对13CO2有较强的排斥作用,导致叶片δ13C值(-20‰~-35‰)小于C4植物(-11‰~-15‰)。CAM植物δ13C值则介于二者之间(Farquhar et al.,1989;Griffiths,1992)。Lin和Ehleringer(1997)研究发现,线粒体呼吸过程中几乎不发生同位素分馏。因此,呼吸CO2的δ13C值取决于植物和土壤有机碳的平均同位素组成(Rochette & Flanagan, 1997; Rochette et al.,1999; Lin et al.,1999)。以C3植物为主的热带和温带森林生态系统呼吸释放CO2的δ13C较低,而以C4植物为主的稀树草原和温带草原其呼吸释放CO2则具有相对较高的δ13C值(Farquhar et al.,1989)。然而,最近Duranceau等(1999)和Ghashghaie等(2001)的研究结论认为,植物叶片呼吸过程可能存在同位素分馏。无论叶片年龄和相对含水量如何变化,暗呼吸释放CO2的δ13C值始终比蔗糖高。不同光合途径植被在演替过程中,土壤有机碳在很长一段时间内保留原有植被的同位素特征,从而导致土壤呼吸释放CO2同位素组成与取样时期植被同位素组成的显著差异,这种现象被称为同位素失衡(Isotopic dis-equilibrium)(Buchmann & Ehleringer, 1998)。大量化石燃料燃烧释放的贫13C的CO2,导致大气和植物C同位素组成的显著变化(Troiler et al.,1996),进而形成现存植被与来自早期植被的土壤有机碳的平均δ13C值之间有一定的差异(Enting et al.,1995)。
在时间和空间梯度上,生态系统不同成份之间13C同位素组成存在明显的差异。热带雨林植物同位素特征在垂直剖面上存在很大的异质性,冠层上部植物组织δ13C值介于-28‰和-26‰之间,而冠层底部植物组织δ13C值明显减小(介于-36‰与-32‰之间)(Sternberg,1989)。以C3植物为主的生态系统,冠层内部和边界层大气CO2的δ13C值高于以C4植物为主的生态系统。因强烈的光合判别作用和相对较弱的湍流混合,在热带雨林和农田生态系统内部或其边界层内,CO2的C同位素比率可能高于大气CO2的C同位素比率(Yakir & Wang, 1996; Buchmann & Ehleringer, 1998)。光合判别受日变化和季节变化影响,生态系统中光合作用的同位素特征也同样存在日变化和季节变化(Quay et al.,1989)。
大气CO2中O同位素组成主要受叶水和土壤水O同位素组成的影响(Farquhar et al.,1993; Tans,1998)。C3植物光合作用过程中,扩散进入叶片的CO2并非全部被固定下来,2/3的CO2在碳酸酐酶(CA)的催化作用下,经过与叶水的同位素平衡反应后重新扩散回大气。叶水因蒸腾作用而富集18O,因而经过叶片同位素平衡反应之后的CO2,其18O含量明显升高。有些植物的碳酸酐酶活性不全,造成CO2与叶水的同位素平衡反应不彻底,经过叶片的CO2之18O含量比预测的低(Gillon & Yakir, 2000,2001)。叶片呼吸释放CO2的O同位素组成除了受叶绿体(C3植物)或叶肉(C4植物)内水分O同位素比率、叶温(影响CO2-H2O交换反应平衡分馏因子)和CO2从叶片内部向大气扩散的分馏作用影响之外,还受扩散分馏以及与土壤和植物组织中水的同位素平衡反应所影响(Farquhar et al.,1989,1993)。CO2从土壤向大气扩散的判别因子约为8.8,但该值因呼吸释放CO2量、CO2与H2O之间的平衡程度、CO2向大气的扩散程度和大气CO2向土壤扩散程度的不同而有一定程度的变异。由于植物水和土壤水O同位素组成变异程度较大,因此冠层内呼吸产生CO2的O同位素组成不如C同位素组成均衡(Farquhar et al.,1993; Flanagan & Varney, 1995)。
1.2 H2O的同位素组成
植物蒸腾与土壤蒸发能够显著影响大气水蒸气H和O同位素组成,从而使土壤水、植物水和大气水蒸气之间,以及三者之间的水分通量具有不同的同位素特征(Gat,1996)。Craig和Gordon(1965)最先描述了水蒸气同位素组成与表面水体同位素组成之间的关系。相对蒸发水体而言,水蒸气呈同位素贫化趋势。土壤水蒸发过程与水体蒸发过程基本类似。大量实验证明Craig-Gordon模型预测值与实测数据的一致性很高(Allison & Leaney, 1982; Barnes & Allison, 1988; Mathieu & Bariac, 1996)。土壤水随着深度的不断增加,重同位素含量逐渐增加,通常在0.1~0.5 m处达到最大值,即形成所谓的蒸发面(Evaporation front)(Barnes & Allison, 1988)。
植物叶片蒸发通量与其水分含量之间比率相对较高,叶片通常处在同位素稳定状态(Isotopic steady state),即输入叶片的水和通过叶片蒸发的水具有相同的同位素比率。当叶片处于同位素稳定状态时,蒸腾水蒸气的同位素组成与植物利用的土壤水的同位素组成一致(Dawson & Ehleringer, 1993; Wang & Yakir, 1995)。这就导致了经过高度分馏的土壤蒸发水与没有分馏的植物冠层蒸腾产生水蒸汽之间具有明显的同位素组成差异。二者之间同位素组成差异是利用同位素方法将生态系统蒸散通量区分为土壤蒸发通量和植物蒸腾通量的基础(Moreira et al.,1997)。当然,尽管当叶片处于同位素稳定状态时,通过叶片蒸腾的水分没有发生分馏,但土壤深度剖面变化、茎水富集和季节变化还是会显著影响叶片蒸腾水的同位素组成(Dawson & Ehleringer, 1993;Dawson & Pate, 1996)。
2 Keeling曲线法
Keeling曲线的基础是生物学过程前后的物质平衡,即群落冠层或相临边界层气体浓度是大气本底浓度与增加源的气体浓度之和,这种关系可以用公式(2)来表示(以CO2为例)。
其中:Ca,Cb和Cs分别表示生态系统中大气的CO2浓度、CO2浓度的本底值和源添加的CO2浓度。公式(1)不仅适用于CO2,也适用于生态系统其它气体,如水蒸汽或甲烷(Pataki et al.,2003a)。将公式(2)的各项组分分别乘以各自的CO2同位素比率(δ13C),就能够得到稳定性同位素13C的质量平衡方程公式(3)。
其中 δ13Ca, δ13Cb和 δ13Cs分别表示3个部分的同位素比率。将公式(1)和公式(2)合并之后,我们就可以得到公式(4)
其中δ13Cs是生态系统中,自养呼吸和异养呼吸释放CO2的整合同位素比率。由此可见,δ13Ca vs 1/Ca的直线在y轴的截距即为δ13Cs。
在冠层尺度,Keeling曲线截距表示植被和土壤呼吸释放CO2的δ13C在空间上的整合。同时,它也表示植被和土壤不同年龄C库(周转时间和δ13C值不同)在时间上的一种整合(Dawson et al.,2002)。Keeling曲线法经过不断的修改和完善,广泛应用于森林生态系统(Sternberg et al.,1989; Buchmann et al.,1997a,1997b; Flanagan et al.,1996;Bowling et al.,2001)、农田生态系统(Buchmann & Ehleringer, 1998)和草地生态系统(Ometto et al.,2002)C通量研究,以及植物蒸腾和土壤蒸发对生态系统蒸散的贡献(Harwood et al.,1999; Moreira et al.,1997;Yepez et al.,2003;Williams et al.,2004)。
应用Keeling曲线法时有两个基本假设,首先假设该模型为两部分气体的简单混合模型(源气体和本底气体的混合),其次假设观察过程中两部分气体的同位素比率不发生显著变化。在野外条件下,两种假设同时成立的情况很少,因此在实际运用该方法时,在时间和空间的选择上一定要慎重(Pataki et al.,2003a)。
3 Keeling曲线法在生态系统碳循环研究中的应用
3.1 生态系统呼吸释放CO2的同位素组成及生态系统判别
通过Keeling曲线法测得的生态系统呼吸释放CO2的碳同位素组成(δ13CR)能够反映出所有植物物种土壤根系和微生物呼吸释放CO2的13C同位素组成。在时间尺度上,生态系统呼吸所表示的不仅仅局限在一个生长季内,它是一个生态系统内不同年龄碳分解释放的整合(Bird et al.,1996; Kaplan et al.,2002)。在空间尺度上,生态系统呼吸可以表示多尺度(斑块、生态系统和区域)水平碳通量(Flanagan & Ehleringer, 1998)。Pataki等(2003a)对多个样点的研究结果表明,仅由C3植物构成的生态系统,其δ13CR的平均值为-26.2‰±0.2‰,变化范围为-19.0‰到-32.6‰。不同生物群系之间,δ13CR存在显著差异,但二者之间似乎没有清晰明了的规律,温带阔叶林δ13CR最高,热带雨林δ13CR值最低(Pataki et al.,2003a)。这主要是因为δ13CR不仅受植物光合判别的影响,还受土壤呼吸及纬度差异导致的环境条件变化影响。例如水分有效性的降低,可能导致光合速率不变的情况下,植物叶片气孔的暂时关闭,进而导致有机物质的同位素富集和δ13CR的增大。Pataki等(2003a)的研究结果表明,随着年平均降雨量(从230增至2 250 mm)的增加,δ13CR逐渐减小,88%的δ13CR空间变异是由年平均降雨量的差异造成的。Ometto等(2002)对热带常绿林的研究结果表明,受降水季节变化的影响,δ13CR存在明显的季节变化。生态系统呼吸释放CO2的O同位素组成(δ18OR)也存在明显的时空变化。Bowling等(2003a)沿一个降水梯度(年降水从227到2 760 mm)的研究结果表明,受降水O同位素组成和蒸发的影响,δ18OR存在很大变异(24.2‰~35.3‰)。此外,降水以及土壤蒸发和叶片蒸腾富集也导致了δ18OR在时间尺度上的变化(Flanagan et al.,1999;Bowling et al.,2003b; McDowell et al.,2004)。
陆地生物圈是由众多形态、结构和功能各异的生态系统构成的,不同生态系统由于物种组成、垂直剖面结构和土壤有机物质的不同,其生态系统C同位素判别(Δe)之间存在显著差异。Lloyd和Farquhar(1994)利用优势种植物的生态生理指标(气孔对叶片到大气蒸气压差值摩尔分数变异的响应)及气候因子(温度、降水和相对湿度)来计算整个冠层的同位素判别(ΔA)。这个模型为自下而上(Bottom-up)的生态系统研究方法提供了理论基础,但ΔA仅能反映少数优势种的同位素判别,且在时间尺度上仅仅是一个生长季的生态系统生理过程整合。Buchmann等(1998)将叶片尺度同位素判别(Δleaf)公式(5)外推到生态系统尺度上,从而可以利用呼吸CO2同位素组成计算生态系统同位素判别(Δe)公式(6)。
其中,Δleaf和Δe分别表示叶片和生态系统的C同位素判别;δ13Cair,δ13Cleaf, δ13Ctrop, δ13Cresp分别表示大气、植物叶片、对流层和生态系统呼吸的CO2同位素比率。Δe受降水、水分有效性、蒸气压亏缺、林龄和物种组成的影响,具有很大的时间和空间变异性(Bowling et al.,2002; Fessenden & Ehleringer, 2002; Buchmann et al.,1997b)。这些因子既影响冠层判别又影响叶片和根系呼吸的大小,同时也影响利用根系渗出物的根际微生物呼吸(Lloyd et al.,1996; Bowling et al.,2001; Buchmann et al.,1998)。Buchmann和Kaplan(2001)对26个研究样点的数据分析结果表明,Δe随着年均温度的升高而降低,随着年均降水的增大而增大。Buchmann等(1998)对不同生态系统Δe分析结果表明,北方森林生态系统(包括常绿和落叶)与温带森林之间Δe差异不大,二者平均值分别为18.2‰和18.0‰,而热带雨林Δe变化范围较窄(19.5‰~21.1‰),平均为20.4‰。生态系统呼吸δ13C变化主要受新固定CO2的影响,因此Δe可以用来估测生态系统对最近环境变化的整合响应(Pataki et al.,2003a; Bowling et al.,2002)。
3.2 森林生态系统内的CO2再循环
生态系统呼吸释放的CO2并非完全能与外界大气进行湍流混合,其中一部分气体被植物重新吸收利用,这就导致了生态系统内部的CO2再循环(CO2 recycling)。Sternberg(1989)通过对Keeling公式做了一定程度的修改,将再循环指数(fS)定义为重新固定的呼吸释放CO2与生态系统呼吸通量的比值。将CO2再循环考虑在内的Keeling公式如下:
其中,δF、δa和δR分别表示环境、大气和呼吸释放CO2的同位素组成;[CO2]a和[CO2]F分别表示对流层和周围环境CO2浓度;Δ表示光合过程对13C的判别。当没有发生再循环时(fS=0),则公式(6)就变成最基本的Keeling公式。Lloyd等(1996)则通过微气象通量测量提出了不同的再循环指数(FL),即重新固定CO2量与整个CO2同化量的比值。两个再循环指数的最大区别就是对[CO2]a和δa的定义不同,Lloyd等认为,冠层边界层(CBL)和对流层之间CO2浓度和同位素组成存在显著差异,实际进入冠层的气体浓度和同位素组成应以CBL为准。Sternberg则认为,对流层与CBL之间同位素组成和浓度差异是由呼吸、湍流混合和光合等过程造成的,因此,对流层CO2浓度和同位素组成应该是该混合模型真正的最终组分。Lloyd等(1996)对亚马逊热带雨林和西伯利亚针叶林的研究结果表明,两种森林的再循环指数分别为不到0.01和0.004,这一数值远远小于Sternberg(1989)对巴拿马热带雨林的研究结果(0.09)。我们认为这两种再循环指数表现了相似的生态学现象,只是在表示方式上有所不同而已。
3.3 生态系统碳通量研究
利用微气象法,人们已经能够测得冠层与大气之间CO2或水交换的净通量(Baldocchi et al.,1988)。但是,微气象法无法确定不同的生态过程对净通量的影响和贡献。土壤呼吸CO2释放的增加和光合吸收CO2的减少都可能导致冠层CO2净吸收的减少。Keeling曲线法与同位素测量及冠层尺度通量测量相结合,能够将冠层CO2或水的净通量区分为不同组分通量。以冠层与大气CO2交换为例,假设FN为净通量,F1和F2是两个初级通量组分(如光合CO2吸收和土壤呼吸CO2释放),三者的同位素组成分别为δN、δ1和δ2。根据同位素质量平衡法,
能够推导出通量F1和F2的计算公式:
FN可以通过微气象法测得,δN可以通过Keeling曲线计算,δ1和δ2可以分别通过土壤和植物样品测得。到目前为止,这种方法的应用在一定程度上受到取样及分析技术和仪器的限制。CO2中C和O同位素组成测量能够精确到±0.3‰(Troiler et al.,1996),观测到的冠层边界层13C和18O同位素组成梯度通常为0.3‰·m-1或(0.02‰~0.1‰)·μmol-1·mol-1 CO2(Yakir & Wang, 1996; Buchmann & Ehleringer, 1998; Harwood et al.,1998),只有在高光合速率群落冠层或者在非常理想状态下才能够产生较大的同位素梯度。Yakir和Wang(1996)采用这种方法,成功的将农田生态系统CO2净交换区分为光合同化和呼吸释放,并量化了光合吸收减少(56%)和呼吸减少(71%)对生长季晚期麦田生态系统CO2净吸收变化(从44.2到20.6 μmol·m -2·s-1)的相对贡献。利用浓度梯度-同位素法来区分生态系统净通量至少需要两个假设,首先假设两个通量组分来自同一位置(即忽略了来自土壤和来自冠层的高度差异),其次假设背景大气(通常取自上风向或冠层边界层之上)同位素组成在浓度-同位素梯度上是均一的(Yakir & Sternberg, 2000)。
Bowling等(1999a)利用气瓶取样(Flask sampling),通过实验室同位素分析和Keeling曲线法估计δN值,并进一步将同位素组成与传统的涡度相关公式相结合,从而能够计算13C和18O的通量。
其中,ρ为干燥空气的密度;ω为垂直风速组分;c为干燥空气化学要素混合比率或摩尔分压,可以通过涡度相关系统计算得到,常数m和b则可以利用与涡度相关测量同时进行的取样-同位素分析法所获得的δ与c之间的相关关系获得。Bowling等(1999b)强调了利用这种取样法获得的Keeling关系与涡度相关取样之间的差异,他们将同位素测量与条件取样技术(Conditional sampling technique)相结合,通过直接测量13C或18O通量来独立验证公式(10)的有效性,但未得出确切的结论,需要进一部更深入的研究。尽管如此,随着技术手段和分析方法的不断进步,稳定性同位素分析与涡度相关技术相结合,将能够更精确的区分光合吸收和呼吸释放通量对生态系统CO2净通量的贡献(Bowling et al.,2003c)。
生态系统通量组分之间同位素组成的显著差异是利用Keeling曲线结合微气象法区分组分通量的先决条件。当土壤呼吸释放和植物光合吸收CO2之间13C差异较小时,我们还可以利用18O来研究生态系统的气体交换过程。叶水的大量蒸腾造成其18O的富集,从而导致与叶水和土壤水进行同位素交换之后的CO2具有不同的18O同位素组成。Nakazawa等(1997)研究发现,对流层内取样梯度上,18O/CO2约为13C/CO2的二倍。Troiler等(1996)观察到大气CO2中同样的18O和13C存在同样程度的季节变化幅度。Lloyd等(1996)利用质量平衡法估测光合气体交换判别、呼吸CO2同位素比率和湍流混合对冠层CO2的O同位素比率影响。以同位素效应作为权重,3个主要通量对大气CO2中O同位素组成的影响可以表示如下:
其中,Mi为森林冠层内空气的摩尔密度(mol·m-3); [CO2]i为冠层平均CO2浓度(μmol·mol-1);δ18Oi为冠层CO2平均O同位素比率(‰); A为净CO2同化速率(μmol·m-2·s-1);ΔC18O16O为光合气体交换对C18O16O的判别(‰);R为植物和土壤的呼吸速率(μmol·m-2·s-1);δ18OR为植物与土壤呼吸CO2的O同位素组成(‰);Foi为由大气向冠层的单向CO2湍流混合通量(mmol·m-2·s-1);δ18Oo为大气CO2平均O同位素比率(‰)。Flanagan等(1997)将森林生态系统CO2的微气象测量与O同位素组成分析相结合,结果表明,白天光合CO2交换显著促进了冠层CO2的O同位素富集,夜晚呼吸释放CO2通常导致冠层CO2中O同位素的贫化。将同位素效应与响应过程CO2通量相结合,他们发现在南部加拿大短叶松(Pinus banksiana)样地,尽管7月和9月的ΔC18O16O值相近,但由于水分条件的限制,9月光合气体交换判别显著降低。Bowling等(2003b)则利用δ18OR区分俄勒冈州森林生态系统夜晚呼吸组分,结果表明土壤呼吸对总生态系统呼吸通量贡献为80%±12%。
3.4 城市生态系统碳循环
Keeling曲线法与稳定性同位素技术不仅能够区分自然生态系统中不同组分对碳通量的贡献,而且还能够用于研究城市生态系统中生物和人类活动对大气CO2浓度变化的影响和贡献(Takahashi et al.,2002)。Pataki等(2003b)对美国盐湖城的研究结果表明,利用夜晚和交通高峰时期(冬季)数据,通过Keeling曲线法求得的δ13Cs存在明显季节变化,春季和夏季明显高于冬季;δ18Os在冬季变化较小,春秋季较高,夏季相对较低。13C和18O同位素质量平衡法计算结果表明,人类活动(汽油和天然气燃烧)和生物过程对夜晚城市CO2浓度变化的影响存在明显的季节变化。汽油燃烧对大气CO2浓度变化贡献较为恒定,冬季天然气燃烧以及春季和夏季的生物呼吸对不同季节大气CO2浓度变化影响较大。目前为止,该方法还存在一定程度的不确定性,进一步量化城市生态系统地上植被和土壤呼吸对整个呼吸释放CO2的贡献并测量叶片水、茎水和土壤水同位素组成将有助于验证通过模型计算的δ13CR和δ18OR。自下而上的燃料消耗统计将有助于验证同位素质量平衡法的计算结果的可靠性以及该方法是否能在区域或全球尺度上反映出化石燃料燃烧释放CO2同位素组成在时间和空间上的变化(Kuc & Zimnoch, 1998;Pataki et al.,2003b)。由于石油资源的日益枯竭以及对大气CO2升高所带来效应的高度关注,未来城市能源结构势必出现巨大变化,Keeling曲线法与稳定性同位素技术的应用将有助于了解城市不同化石能源利用的动态变化。
4 Keeling曲线法在生态系统水循环研究中的应用
在生态系统水分循环研究中,Keeling曲线法的另一个重要用途是区分植物蒸腾(Transpriation)和土壤蒸发(Evaporation)对整个生态系统蒸散(Evapotraspiration)的贡献(Brunel et al.,1992;Yakir & Wang, 1996; Moreira et al.,1997; Harwood et al.,1999; Yepez et al.,2003; Williams et al.,2004)。采用Keeling曲线法时,除需要假设水蒸气与大气湍流混合之外,还需假设该生态系统没有其它的水蒸气散失或来源。Yakir和Wang(1996)、Moreira等(1997)及Harwood等(1999)分别在麦田、亚马逊森林和欧洲橡树林利用高度梯度上稳定性同位素组成与水蒸气浓度倒数来确定蒸腾和蒸发对蒸散的贡献,实验结果表明,蒸散水蒸气δ18O值与植物茎水的δ18O值非常接近,即大部分的蒸散来自于植物的蒸腾。Moreira等(1997)利用不同源水蒸气同位素组成差异以及通过Keeling曲线计算所得蒸散水蒸气同位素组成,进一步确定植物蒸腾对蒸散贡献的百分比:
其中,FT(%)表示蒸腾对蒸散通量贡献的百分数;δET、δE和δT分别表示蒸散、蒸腾和蒸发水蒸气的同位素组成,δET可以利用Keeling曲线计算;δT可以通过Craig-Gordon公式计算。研究结果表明,在亚马逊盆地森林,植物蒸腾对整个蒸散通量的贡献为76%~100%(Moreira et al.,1997)。Wang和Yakir(1995)在麦田生态系统的研究结果表明,96%~98%的蒸散来自于植物的蒸腾。以上研究中,均假设植物处于同位素稳定状态,即植物蒸腾水蒸气同位素组成与茎水同位素组成相同。叶尺度(Wang & Yakir, 1995)和冠层尺度(Harwood et al.,1998)研究结果表明,稳定状态假设只是一个近似值,蒸腾水蒸气δ18O值在早晨比茎水δ18O值低,而在下午则比茎水δ18O值高。因此,Harwood等(1998)建议直接测量蒸腾水蒸气同位素组成,以减小稳定状态假设所带来的误差。
最近,Yepez等(2003)还利用Keeling曲线法区分半干旱稀树草原林地上层林冠蒸发(以牧豆树(Prosopis velutina)为主)、林下冠层蒸发(以鼠尾栗(Sporobolus wrightii)为主)及地表的蒸散。该生态系统乔木层和草本层之间层次分明,在湍流混合较弱的情况下,利用Keeling曲线在接近地表层(0.1~1 m)所测得的水蒸气同位素组成(δD或δ18O)能够反映土壤蒸发和底层草本植物蒸发通量整合,而整个冠层剖面(3~14 m)所做的Keeling曲线则能够反映整个冠层蒸腾和土壤蒸发对生态系统蒸散的贡献。利用Keeling曲线和公式(9),Yepez等(2003)测量并计算得到60%的生态系统蒸散来自灌木的蒸腾,22%来自草本蒸腾,剩余的18%来自土壤蒸发。尽管利用Keeling曲线法的假设在野外条件下有时不能完全满足,但是Gat(1996)及Wang和Yakir(2000)认为由此导致的同位素组成变异对于区分蒸散通量影响较小。
5 Keeling曲线法在全球尺度碳循环研究中的应用
在全球尺度CO2循环研究过程中,同位素信号同样可以提供精确估算的方法。将13C同位素分析结合到全球碳循环模型中,我们能够有效地将全球CO2交换区分为陆地来源和海洋来源(Ciais et al.,1995; Francey et al.,1995)。而18O同位素分析则能够将陆地全球尺度CO2交换通量区分为光合固定和呼吸释放。Francey和Tans(1987)、Farquhar等(1993)及Ciais等(1997)的研究结论认为,生物圈活动的18O特征在全球大气中能够清楚地观测到(如纬度梯度和季节变化),并且全球尺度大气CO2的18O质量平衡与生态过程一致。全球尺度CO2δ18O预算中,整个生态系统判别是全球每年的整合值。
6 利用Keeling曲线法的一些问题及其解决办法
Keeling曲线法往往需要外推不同梯度上C、O或H同位素比率及CO2或水的浓度倒数之间的比例关系,从而获得生态系统呼吸释放CO2或蒸散水分的同位素比率。因这种外推远远超过实际测量范围,实际测量的微小误差可能导致外推结果的较大变异。因此,在测量过程中,CO2或H2O的变化范围应该足够大,以便降低外推截距的不确定性。Pataki等(2003a)的研究结果表明,随着取样CO2浓度差的增加,Keeling曲线截距的标准误差逐渐降低,CO2变化范围大于75 μmol·mol-1左右时,δ13CR的标准误差低于1‰。因此,Pataki等(2003a)建议CO2变化范围应不低于75 μmol·mol-1。
利用Keeling曲线法研究生态系统呼吸同位素组成,取样时间只能选择在夜间,因为白天边界层活动较为频繁,边界层厚度变化过程中背景环境同位素信号也发生较大的变化。同时,如果在测量过程中,背景大气δ13C发生变化,则来自不同生态系统的空气水平对流也会影响Keeling混合线(Pataki et al.,2003a)。白天,土壤呼吸释放的CO2可能会被底层植被重新固定,尤其是在热带雨林生态系统(Sternberg,1989; Lloyd et al.,1996)。此外,光合和呼吸通常处于同位素失衡状态(Isotopic-dis-equilibrium),即单位呼吸通量所导致的大气CO2同位素比率变化与单位光合固定所导致的变化并不相等。因此,将夜晚和白天的测量相结合,将会导致Keeling截距的不确定性。Pataki等(2003a)对(Biosphere-atmosphere Stable Isotope Network, BASIN)的数据分析表明,白天测量结果与夜晚测量最多可相差高达5‰。取样时间也是影响Keeling曲线截距有效性的重要因素。以O同位素为例,取样时间过长必然会导致水气压亏缺(VPD)和温度的较大变化,这与Keeling曲线法的基本假设相悖。Bowling等(2003a)认为,Keeling曲线法的取样时间不能超过5 h。
不同梯度水平取样代表具有不同同位素特征的不同来源的碳或水与大气背景浓度的混合,因此,取样地点的选取及取样剖面的设置均会影响Keeling曲线的结果。量化取样剖面设置对Keeling曲线截距的影响已经引起越来越多学者的关注。在较大尺度上,通常利用飞行器在混合较好的对流边界层取样。Lloyd等(2001)发现,在很小的CO2浓度变化范围情况下,对流边界层δ13C和1/CO2之间存在很好的相关性。
7 小结与展望
生源要素如碳、水、氮和氧等的稳定性同位素组成是生态系统生物过程、生态过程和生物地球化学过程的整合反映。尽管存在一些不确定性因素,稳定性同位素技术与传统方法的结合将逐渐成为进一步揭示和解释生态学问题和现象的有效工具(Yakir & Sternberg, 2000; Ehleringer et al.,2002; Dawson et al.,2002)。尤其是在全球变化背景下,Keeling曲线法与已有生态系统模型相结合,能够进一步确定陆地生态系统和海洋生态系统在整个碳循环过程中的源-汇关系(Yakir & Wang, 1996; Ciais et al.,1995),而Keeling曲线法与微气象法相结合则能够进一步区分生态系统不同组分对碳-水通量的影响和贡献(Flanagan et al.,1997; Bowling et al.,2001;Yakir & Wang, 1996; Wang & Yakir, 2000)。当然,稳定性同位素技术及其应用方法还存在一定程度的不确定性因素,例如生态系统不同组分对稳定性同位素的判别作用还需进一步确定。在大尺度条件下,利用Keeling曲线法的精确度也受测量地点、时间及高度设置的影响(Pataki et al.,2003a)。这些因素对解释稳定性同位素数据及其反映的生态学过程均有显著影响,因此进一步研究不同生态系统同位素判别,并将研究尺度从生态系统水平外推到区域,乃至全球水平,并与区域或全球植被模型相结合是未来的研究重点(Bowling et al.,2003c; Schauer et al.,2003)。
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Interseasonal comparison of CO2 concentrations, isotopic composition and carbon dynamics in an Amazonian rainforest (French Guiana)
Influence of stand structure on carbon-13 of vegetation, soils, and canopy air within deciduous and evergreen forests in Uath, United States
Carbon isotope ratios (delta(13)C) were studied in evergreen and deciduous forest ecosystems in semi-arid Utah (Pinus contorta, Populus tremuloides, Acer negundo and Acer grandidentatum). Measurements were taken in four to five stands of each forest ecosystem differing in overstory leaf area index (LAI) during two consecutive growing seasons. The delta(13)Cleaf (and carbon isotope discrimination) of understory vegetation in the evergreen stands (LAI 1.5-2.2) did not differ among canopies with increasing LAI, whereas understory in the deciduous stands (LAI 1.5-4.5) exhibited strongly decreasing delta(13)Cleaf values (increasing carbon isotope discrimination) with increasing LAI. The delta(13)C values of needles and leaves at the top of the canopy were relatively constant over the entire LAI range, indicating no change in intrinsic water-use efficiency with overstory LAI. In all canopies, delta(13)Cleaf decreased with decreasing height above the forest floor, primarily due to physiological changes affecting c i/c a (> 60%) and to a minor extent due to delta(13)C of canopy air (< 40%). This intra-canopy depletion of delta(13)Cleaf was lowest in the open stand (1 per thousand) and greatest in the denser stands (4.5 per thousand). Although overstory delta(13)Cleaf did not change with canopy LAI, delta(13)C of soil organic carbon increased with increasing LAI in Pinus contorta and Populus tremuloides ecosystems. In addition, delta(13)C of decomposing organic carbon became increasingly enriched over time (by 1.7-2.9 per thousand) for all deciduous and evergreen dry temperate forests. The delta(13)Ccanopy of CO2 in canopy air varied temporally and spatially in all forest stands. Vertical canopy gradients of delta(13)Ccanopy, and [CO2]canopy were larger in the deciduous Populus tremuloides than in the evergreen Pinu contorta stands of similar LAI. In a very wet and cool year, ecosystem discrimination (Deltae) was similar for both deciduous Populus tremulodies (18.0 +/- 0.7 per thousand) and evergreen Pinus contorta (18.3 +/- 0.9 per thousand) stands. Gradients of delta(13)Ccanopy and [CO2]canopy were larger in denser Acer spp. stands than those in the open stand. However, (13)C enrichment above and photosynthetic draw-down of [CO2]canopy below tropospheric baseline values were larger in the open than in the dense stands, due to the presence of a vigorous understory vegetation. Seasonal patterns of the relationship delta(13)Ccanopy versus 1/[CO2]canopy were strongly influenced by precipitation and air temperature during the growing season. Estimates of Deltae for Acer spp. did not show a significant effect of stand structure, and averaged 16.8 +/- 0.5 per thousand in 1933 and 17.4 +/- 0.7 per thousand in 1994. However, Deltae varied seasonally with small fluctuations for the open stand (2 per thousand), but more pronounced changes for the dense stand (5 per thousand).
Carbon isotope discrimination of terrestrial ecosystems
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CO2 concentration profiles and carbon and oxygen isotopes in C3 and C4 crop canopies
Carbon isotope discrimination of terrestrial ecosystems—How well do observed and modeled results match
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A large northern hemisphere terrestrial CO2 sink indicated by the 13C/ 12C ratio of atmospheric CO2
Measurements of the concentrations and carbon-13/carbon-12 isotope ratios of atmospheric carbon dioxide can be used to quantify the net removal of carbon dioxide from the atmosphere by the oceans and terrestrial plants. A study of weekly samples from a global network of 43 sites defined the latitudinal and temporal patterns of the two carbon sinks. A strong terrestrial biospheric sink was found in the temperate latitudes of the Northern Hemisphere in 1992 and 1993, the magnitude of which is roughly half that of the global fossil fuel burning emissions for those years. The challenge now is to identify those processes that would cause the terrestrial biosphere to absorb carbon dioxide in such large quantities.
A three-dimensional synthesis study of 18O in atmospheric CO2. 1. Surface fluxes
Applications of stable carbon isotope techniques to ecological research
Evidence for interannual variability of the carbon cycle for the National Oceanic and Atmospheric Administration/Climate Monitoring Diagnostic Laboratory Global Air Sampling Network
Deuterium and oxygen-18 variations in the ocean and the marine atmosphere
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Seasonal water uptake and movement in root systems of Australian phraeatophytic plants of dimorphic root morphology: a stable isotope investigation
A natural abundance hydrogen stable isotope technique was used to study seasonal changes in source water utilization and water movement in the xylem of dimorphic root systems and stem bases of several woody shrubs or trees in mediterranean-type ecosystems of south Western Australia. Samples collected from the native treeBanksia prionotes over 18 months indicated that shallow lateral roots and deeply penetrating tap (sinker) roots obtained water of different origins over the course of a winter-wet/summer-dry annual cycle. During the wet season lateral roots acquired water mostly by uptake of recent precipitation (rain water) contained within the upper soil layers, and tap roots derived water from the underlying water table. The shoot obtained a mixture of these two water sources. As the dry season approached dependence on recent rain water decreased while that on ground water increased. In high summer, shallow lateral roots remained well-hydrated and shoots well supplied with ground water taken up by the tap root. This enabled plants to continue transpiration and carbon assimilation and thus complete their seasonal extension growth during the long (4-6 month) dry season. Parallel studies of other native species and two plantation-grown species ofEucalyptus all demonstrated behavior similar to that ofB. prionotes. ForB. prionotes, there was a strong negative correlation between the percentage of water in the stem base of a plant which was derived from the tap root (ground water) and the amount of precipitation which fell at the site. These data suggested that during the dry season plants derive the majority of the water they use from deeper sources while in the wet season most of the water they use is derived from shallower sources supplied by lateral roots in the upper soil layers. The data collected in this study supported the notion that the dimorphic rooting habit can be advantageous for large woody species of floristically-rich, open, woodlands and heathlands where the acquisition of seasonally limited water is at a premium.
Gender-specific physiology, carbon isotope discrimination and habitat distribution in boxelder. Acer negundo
Stable isotopes in plant ecology
δ 13C of CO2 respired in the dark in relation to δ 13C of leaf carbonhydrates in Phaseolus vulgaris L. under progressive drought
Carbon and water relation in desert plants, an isotope perspective
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Carbon isotope ratios in belowground carbon cycle processes
Stable isotope and carbon cycle processes in forests and grasslands
A synthesis inversion of the concentration and δ 13C of atmospheric CO2
Stomatal conductance and photosynthesis
Carbon isotope discrimination and photosynthesis
Vegetation effects on the isotope composition of oxygen in atmospheric CO2
Partitioning evapotranspiration fluxes from a Colorado grassland using stable isotopes: seasonal variations and ecosystem implications of elevated atmospheric CO2
Age-related variations in 13C of ecosystem respiration across a coniferous forest chronosequence in the Pacific Northwest
Influence of vegetation and soil CO2 exchange on the concentration and stable oxygen isotope ratio of atmospheric CO2 within a Pinus resinosa canopy
Carbon isotopic discrimination during photosynthesis and the isotope ratio of respired CO2 in boreal forest ecosystem
Discrimination against concentration and stable oxygen isotope ratio of atmospheric CO2 in boreal forest ecosystems
Ecosystem-atmosphere CO2 exchange: interpreting signals of change using stable isotope ratios
Changes in the concentration and stable isotope ratio of atmospheric CO(2) can be used to study variations in the net exchange of carbon dioxide in terrestrial ecosystems (net difference between total photosynthesis and respiration). Changes in the timing of seasonal fluctuations in atmospheric CO(2) concentration have suggested that net uptake of carbon dioxide has been increasing in northern latitude ecosystems in association with warmer temperatures and a lengthening of the growing season. Stable isotope techniques allow a more detailed separation of differences between ecosystem photosynthesis and respiration because these two processes have contrasting effects on both the carbon and oxygen isotope ratio of atmospheric CO(2). Future applications of stable isotope analyses include documenting and monitoring the influence of global environmental change on ecosystem CO(2) exchange at regional scales (10-1000km(2)).
Spatial and temporal variation in the carbon and oxygen stable isotope ratio of respired CO2 in a boreal forest ecosystem
Latitudinal variation in O18 of atmospheric CO2
Changes in oceanic and terrestrial uptake since 1982
Carbon 13 exchange between the atmosphere and biosphere
Oxygen and hydrogen stable isotopes in the hydrologic cycle
δ 13C of CO2 respired in the dark in relation to δ 13C of leaf metabolites: comparison between Nicotiana sylvestris and Helianthus annuus under drought
Naturally low carbonic anhydrase activity in C4 and C3 plants limits discrimination against C18O during photosynthesis
Influence of carbonic anhydrase activity in terrestrial vegetation on the 18O content of atmospheric CO2
Carbon isotope discrimination and the integration of carbon assimilation pathways in terrestrial CAM plants: commissioned review
Diurnal variation of δ 13CO2, δC18O16O and evaporative site enrichment of δH218O in Piper aduncum under field conditions in Trinidad
Determinants of isotopic coupling of CO2 and water vapour within a Quercus petraea forest canopy
Concentration and isotopic composition (delta(13)C and delta(18)O) of ambient CO2 and water vapour were determined within a Quercus petraea canopy, Northumberland, UK. From continuous measurements made across a 36-h period from three heights within the forest canopy, we generated mixing lines (Keeling plots) for deltaa(13)CO2, deltaa C(18)O(16)O and deltaa H2(18)O, to derive the isotopic composition of the signal being released from forest to atmosphere. These were compared directly with measurements of different respective pools within the forest system, i.e. delta(13)C of organic matter input for deltaa(13)CO2, delta(18)O of exchangeable water for deltaa C(18)O(16)O and transpired water vapour for deltaa H2(18)O. [CO2] and deltaa(13)CO2 showed strong coupling, where the released CO2 was, on average, 4 per mil enriched compared to the organic matter of plant material in the system, suggesting either fractionation of organic material before eventual release as soil-respired CO2, or temporal differences in ecosystem discrimination. deltaa C(18)O(16)O was less well coupled to [CO2], probably due to the heterogeneity and transient nature of water pools (soil, leaf and moss) within the forest. Similarly, deltaa H2(18)O was less coupled to [H2O], again reflecting the transient nature of water transpired to the forest, seen as uncoupling during times of large changes in vapour pressure deficit. The delta(18)O of transpired water vapour, inferred from both mixing lines at the canopy scale and direct measurement at the leaf level, approximated that of source water, confirming that an isotopic steady state held for the forest integrated over the daily cycle. This demonstrates that isotopic coupling of CO2 and water vapour within a forest canopy will depend on absolute differences in the isotopic composition of the respective pools involved in exchange and on the stability of each of these pools with time.
The stable carbon isotope composition of the terrestrial biosphere: modeling at scales from the leaf to the globe
The concentration and isotopic abundances of atmospheric carbon dioxide in rural areas
The concentration and isotopic of carbon dioxide in rural marine air
Changes of the CO2 sources and sinks in a polluted urban area (southern Poland) over the last decade, derived from the carbon isotope composition
Carbon isotopic fractionation does not occur during dark respiration in C3 and C4 plants
The magnitude of possible carbon isotopic fractionation during dark respiration was investigated with isolated mesophyll cells from mature leaves of common bean (Phaseolus vulgaris L.), a C3 plant, and corn (Zea mays L.), a C4 plant. Mesophyll protoplasts were extracted from greenhouse-grown leaves and incubated in culture solutions containing different carbohydrate substrates (fructose, glucose, and sucrose) with known [delta]13C values. The CO2 produced by protoplasts after incubation in the dark was collected, purified, and analyzed for its carbon isotope ratio. From observations of the isotope ratios of the substrate and respired CO2, we calculated the carbon isotope discrimination associated with metabolism of each of these substrates. In eight of the 10 treatment combinations, the carbon isotope ratio discrimination was not significantly different from 0. In the remaining two treatment combinations, the carbon isotope ratio discrimination was 1[per mille (thousand) sign]. From these results, we conclude that there is no significant carbon isotopic discrimination during mitochondrial dark respiration when fructase, glucose, or sucrose are used as respiratory substrates.
Elevated CO2 and temperature impacts on different components of soil CO2 efflux in Douglas-fir terracosms
13C discrimination during CO2 assimilation by the terrestrial biosphere
Estimates of the extent of the discrimination against(13)CO2 during photosynthesis (DeltaA) on a global basis were made using gridded data sets of temperature, precipitation, elevation, humidity and vegetation type. Stomatal responses to leaf-to-air vapour mole fraction difference (D, leaf-to-air vapour pressure difference divided by atmospheric pressure) were first determined by a literature review and by assuming that stomatal behaviour results in the optimisation of plant water use in relation to carbon gain. Using monthly time steps, modelled stomatal responses toD were used to calculate the ratio of stomatal cavity to ambient CO2 mole fractions and then, in association with leaf internal conductances, to calculate DeltaA. Weighted according to gross primary productivity (GPP, annual net CO2 asimilation per unit ground area), estimated DeltaA for C3 biomes ranged from 12.9 per thousand for xerophytic woods and shrub to 19.6 per thousand for cool/cold deciduous forest, with an average value from C3 plants of 17.8 per thousand. This is slightly less than the commonly used values of 18-20 per thousand. For C4 plants the average modelled discrimination was 3.6 per thousand, again slightly less than would be calculated from C4 plant dry matter carbon isotopic composition (yielding around 5 per thousand). From our model we estimate that, on a global basis, 21% of GPP is by C4 plants and for the terrestrial biosphere as a whole we calculate an average isotope discrimination during photosynthesis of 14.8 per thousand. There are large variations in DeltaA across the globe, the largest of which are associated with the precence or absence of C4 plants. Due to longitudinal variations in DeltaA, there are problems in using latitudinally averaged terrestrial carbon isotope discriminations to calculate the ratio of net oceanic to net terrestrial carbon fluxes.
Vegetation effects on the isotopic composition of atmospheric CO2 at local and regional scales: theoretical aspects and a comparison between a rain forest in Amazonia and a boreal forest in Siberia
Vertical profiles, boundary layer budgets, and regional flux estimates for CO2 and its 13C/12C ratio and for water vapor above a forest/bog mosaic in central Siberia
An isotopic study (2H and 18O) of water movements in clayey soils under a semi-arid climate
Response of the carbon isotopic content of ecosystem, leaf, and soil respiration to meteorological and physiological driving factors in a Pinus ponderosa ecosystem
Measurement of 18O/16O in the soil-atmosphere CO2 flux
Contribution of transpiration to forest ambient vapour based on isotopic measurement
Aircraft measurements of the concentrations of CO2, CH4, N2Oand CO and carbon and oxygen isotopic ratios of CO2 in the troposphere over Russia
Carbon isotope discrimination in forest and pasture ecosystems of the Amazon Basin, Brazil
The application and interpretation of Keeling plots in terrestrial carbon cycle research
Seasonal cycle of carbon dioxide and its isotope composition in an urban atmosphere: Anthropogenic and biogenic effects
13C/12C of atmospheric CO2 in the Amazon basin: forest and river sources
Quatifying rhizosphere respiration in a corn crop under field conditions
Sepatating soil respiration into plant and soil components using analyses of the natural abundance of carbon-13
An automated sampler for collection of atmospheric trace gas samples for stable isotope analyses
The understory and overstory partitioning of energy and water fluxes in a semi-arid woodland ecosystem
Carbon isotope in functional soil ecology
Ecological interpretation of leaf carbon isotope ratios: influence of respired carbon dioxide
A model to estimate carbon dioxide recycling in forests using 13C/12C ratios and concentrations of ambient carbon dioxide
Diurnal variation of CO2 concentration, δ14C and δ13C in an urban forest: estimate of the anthropogenic and biogenic CO2 contributions
Oxygen isotopic equilibrium between carbon dioxide and water in soils
Monitoring the isotopic composition of atmospheric CO2: measurements from the NOAA Global Air Sampling Network
Applications of stable isotopes to study anima1-plant relationships in terrestrial ecosystems
Temporal and spatial variations in the oxygen-18 content of leaf water in different plant species
Using stable isotopes of water in evaporation studies
Evapotranspiration components determined by stable isotope, sap flow and eddy covariance technique
Fluxes of CO2 and water between terrestrial vegetation and the atmosphere estimated from isotope measurement
The use of stable isotopes to study ecosystem gas exchange
Stable isotopes are a powerful research tool in environmental sciences and their use in ecosystem research is increasing. In this review we introduce and discuss the relevant details underlying the use of carbon and oxygen isotopic compositions in ecosystem gas exchange research. The current use and potential developments of stable isotope measurements together with concentration and flux measurements of CO2 and water vapor are emphasized. For these applications it is critical to know the isotopic identity of specific ecosystem components such as the isotopic composition of CO2, organic matter, liquid water, and water vapor, as well as the associated isotopic fractionations, in the soil-plant- atmosphere system. Combining stable isotopes and concentration measurements is very effective through the use of
Partitioning overstory and understory evapotranspiration in a semiarid savanna woodland from the isotopic composition of water vapor
