植被在大气汞收支中作用的研究进展与展望

牛振川，张晓山，陈进生，王森，王章玮，慈志佳

1. 中国科学院地球环境研究所，黄土与第四纪地质国家重点实验室，西安 710075 2. 中国科学院生态环境研究中心，北京 100085 3. 中国科学院城市环境研究所，厦门 361021 4. 国家加速器质谱中心(西安)，西安 710054 5. 西北大学，城市与环境学院，西安 710127

植被在大气汞收支中作用的研究进展与展望

牛振川1,2,3,4,*，张晓山2，陈进生3，王森5，王章玮2，慈志佳2

1. 中国科学院地球环境研究所，黄土与第四纪地质国家重点实验室，西安 710075 2. 中国科学院生态环境研究中心，北京 100085 3. 中国科学院城市环境研究所，厦门 361021 4. 国家加速器质谱中心(西安)，西安 710054 5. 西北大学，城市与环境学院，西安 710127

在汞的生物地球化学循环中，对于“源”和“汇”的认识还存在许多不确定性。大气汞收支不平衡的问题使得植被在汞循环中的作用日益凸现；开展植被在大气汞收支中作用的研究有助于为全球汞减排政策的制定提供参考。本文首先概述了植被中汞的来源和影响因素及其与大气汞的源汇关系；进而重点论述了植被参与大气汞收支的主要方式：凋落物沉降、生物质燃烧和植被表面与大气汞的动态交换，并阐述了植被在大气汞污染监测中的应用；最后在总结我国相关研究的基础上展望了未来的发展方向。

植被；大气汞收支；交换过程；生物质燃烧；凋落物沉降

汞是一种全球性污染物，会随大气传输扩散到世界各地，甚至引起偏远地区环境介质和生物体内汞浓度的升高，对人类健康和生态系统安全造成潜在威胁[1,2]。针对全球汞污染问题，联合国环境规划署已于2013年1月正式通过了旨在全球范围内控制和减少汞排放的国际公约《水俣公约》，并在当年10月由87个国家正式签约。我国是世界上汞排放量较大的国家，人为源每年的汞排放量约为609.1 t[3,4]，这给我国的环境保护和环境外交带来巨大压力。排放到大气的汞通过干湿沉降过程进入植被系统，而植被系统又可通过复杂的物理、化学及生物过程释放汞。因此，开展植被在大气汞收支中作用的研究不仅有助于正确地评价自然排放源在大气汞循环中的作用，而且能为全球汞减排政策的制定提供参考[5-7]。

在汞的生物地球化学循环过程中，对于“源”和“汇”的认识仍存在很多的不确定性。主要表现在目前研究所认识到的大气汞排放源要远多于所认识到的汇，产生大气汞收支的不平衡问题[8]；其中的一个重要原因是忽视了植被在大气汞收支中的重要作用。近年来，植被在汞循环中的重要作用得到了足够的重视；本文对植被中汞的来源、与大气汞的源汇关系、参与大气汞收支的方式以及植被在大气汞污染监测中的应用等方面的研究进展进行系统综述，并对未来的发展方向进行展望。

1 植被中汞的来源及影响因素

汞循环中的一个中心问题是植被中的汞是来自土壤还是大气。植物体内的汞主要来源于以下途径：(1)气孔对大气Hg0的吸收[9]；(2)叶对干湿沉降的大气Hg0、Hg2+以及与颗粒物结合汞的吸附[10]；(3)根通过蒸腾作用对土壤中可溶性汞的吸收。大量研究结果表明，根部的汞主要来自土壤吸收，木本、灌木以及大部分草本植物地上部分的汞主要来自大气吸收[11-15]；而对于部分草本植物，由于其独特的生理和生长方式，叶中很大一部分的汞来自土壤吸收。例如，Deschampsiaflexuosa L和Calamagrostisvillosa(Chaix ex Vill.)这两种草本植物叶中的汞来自土壤汞的比例分别为30%和93%[16]。

植物体内汞浓度受环境汞浓度和生长时期的共同影响。随着大气汞浓度的增加，叶汞浓度也随之线性增加，而其它器官汞浓度变化不明显；同样，当土壤汞浓度增加时，根汞浓度也随之增加，而其它器官汞浓度变化不明显[11-15]。总体而言，植物叶汞浓度随生长时期的延长而增加[14,17,18]。但对于一些草本植物，叶汞浓度在整个生长时期内波动[15,19]。Dunagan等对菠菜53 d试验结果表明，叶汞浓度在整个生长时期内变动较大，在28 d时浓度最高[19]。

2 植被与大气汞的源汇关系

植被通过气孔吸收Hg0和叶表面吸附干湿沉降的大气汞而成为大气汞的汇。然而，在一定条件下，植被富集的汞可通过以下方式向大气排放而成为大气汞的源：(1) 沉降在叶片表面的Hg2+在紫外线的照射下会还原为Hg0重新排放到大气中[20]；(2) 植被随呼吸作用向大气排放汞[21,22]；(3) 植被以生物质燃烧的方式向大气释放汞[33-35]。但总体而言，植被是大气汞的净汇(net sink)[13]，其富集的大气汞进而以凋落物沉降的方式释放到地表系统。因此，植被通过凋落物沉降、生物质燃烧和表面与大气汞的交换这些方式参与大气汞收支。此外，植被还通过影响土壤汞的排放来间接影响大气汞的收支。

3 植被参与大气汞收支的方式

3.1 凋落物沉降中汞的输入通量

植被富集的大气汞会以凋落物沉降的方式输送到地表，可以作为估算森林系统富集大气汞通量的一种粗略研究方法。凋落物沉降是植被参与大气汞收支的重要方式，Lindberg 等估计全球凋落物沉降中汞的年输入量可达2 400～6 000 t[23]。温带地区凋落物沉降中汞的输入通量为8～25 μg·(m2·y)-1[24-27]；热带地区由于凋落物较大的生物量，汞的输入通量可达30～122 μg·(m2·y)-1 [28-30]。在中国西南某些地区，凋落物沉降中汞的输入通量也较高，云南哀牢山为119.5 μg·(m2·y)-1[31]，贵州雷公山为119.5 μg·(m2·y)-1，重庆铁山坪为291.2 μg·(m2·y)-1[32]。

3.2 生物质燃烧中汞的释放通量

植被富集的大气汞还可以以生物质燃烧的方式参与大气汞收支，全球生物质燃烧释放的汞通量每年可达数百t。通常，可以通过 Hg/CO、Hg/CO2和Hg/fuel等排放因子来估算生物质燃烧过程中释放的汞量。Weiss-Penzias等以排放因子△TAM/△CO估计全球生物质燃烧释放的汞量为670±330 t·y-1，其中北方针叶林燃烧释放汞168±75 t·y-1[33]。Brunke等以排放因子Hg/CO (2.10±0.21)×10-7mol·mol-1和Hg/CO2(1.19±0.30)×10-8mol·mol-1分别估算全球生物质燃烧释放的汞量为930 t·y-1(510～1 140 t·y-1)和590 t·y-1(380～1 330 t·y-1)[34]。Friedli等将世界划分为不同区域，并采用不同的排放因子17～312 gHg·(kg fuel)-1，估计全球生物质燃烧释放的汞通量为675±240 t·y-1[35]。

在生物质燃烧中，汞主要以气态元素汞(Hg0)和颗粒汞(Hgp)的形式释放。燃料湿度是控制燃烧过程中汞形态的主要因素，在湿度较低的情况下，Hg0是主要的形态，可占95%以上；而在鲜样中，颗粒汞会占相当的比例，最多可达50%[36]。此外，燃烧方式对汞的形态也有影响，熏烧中颗粒汞的比例较高；而火焰比较明显时，颗粒汞的比例不显著[36]。

在森林大火中，不仅生物质燃烧会释放大量的汞，而且土壤受热也会释放一定量的汞来参与大气汞的收支。实验室测得的排放因子为14～71×10-6gHg·(kg fuel)-1，而野外森林大火中测得的排放因子略高，为112 × 10-6gHg·(kg fuel)-1，Friedli等认为差值来自土壤受热释放的汞[37]。在森林大火中，土壤释放的大量汞主要来自表层土壤[38]，占土壤总汞的79%左右[39]。

生物质燃烧会增加大气汞的沉降而影响大气汞的收支，对生态系统的汞循环造成一定影响。Witt等发现森林大火之后，美国北明尼苏达州降水中总汞和甲基汞的浓度均增加1.7～8倍[40]。而Kelly等报道森林大火引起虹鳟鱼(Oncorhynchusmykiss)体内的汞浓度增加了5倍，其它种类鱼肌肉中的汞浓度也有一定增加，这可能与大火使营养元素输入增加而导致食物链的重建，以及大火中瞬时释放出大量的汞有关[41]。

3.3 植被表面与大气汞的交换过程

植被表面与大气汞的交换过程是其参与大气汞收支的基本形式。Obrist估计全球植被的地上部分每年大约富集1 000 t的大气汞，甚至可以引起春夏之际大气汞浓度的降低，其认为植被是大气汞“丢失的汇”(missing sink)[42]。因此，植被生物量的增加将能富集更多的大气汞从而降低国际减排压力。植被表面存在富集和排放大气汞的动态双向交换过程，据此，Shetty 等[43]和Quan等[44]分别通过模型的方法估计东亚和我国陆地植被每年向大气排放630 t和79～177 t的汞。

研究植被表面与大气汞交换过程的方法有动态通量袋法(dynamic flux bags, DFB)和微气象法(micrometeorological method)两类。微气象法包括修正波文比法(MBR)、气体动力学法(AER)和弛豫涡旋积累法(REA)，其对地表无干扰，可长期大面积连续监测，但对仪器要求较高，采样复杂。微气象法是在冠层尺度上认识植被与大气汞的交换过程，其不仅包括植被与大气汞的交换通量，还包括植被覆盖土壤与大气汞的交换通量。动态通量袋法更适于在叶片尺度上认识叶汞交换的动态过程，简单方便，但也存在改变袋内气象条件[45]和夜晚有水汽冷凝[46]的缺点。动态通量袋法的影响因素包括袋体材料、体积、进出口位置和气体流速，其中气体流速的影响最大，最佳气体流速为保持出气口与进气口汞浓度之差(ΔC)为恒定值的最小流速[45]。

植被与大气汞的交换通量存在一个补偿点，当大气汞浓度高于此补偿点时，大气汞被叶片表面所富集，此时交换通量数据为负；低于补偿点时，植被向大气排放汞，此时交换通量数据为正[47]。补偿点因植物种类的不同而异，范围为2～33 ng·m-3[11,12,47]。补偿点也因环境而异，通常白天略高于夜晚[12]。因此，确立植被表面与大气汞交换通量的补偿点有助于揭示其与大气汞的源汇关系。

影响植被表面与大气汞交换通量的因素很多，主要包括植物的种类、生长环境及其生理活动。对于旱生植物，交换通量随温度(20～40°C)和大气汞浓度的增加而增加，太阳辐射(尤其紫外线)和气孔导度是控制叶汞交换通量的重要因素[10,12,20,48]；而对于水生植物，交换通量与植物蒸腾作用所产生的水汽通量显著相关[22]。

植被表面与大气汞存在复杂的交换过程，不仅有气孔的吸收和排放过程，还存在大气汞在叶片表面的沉降和光致还原等引起的再排放的非气孔过程[9,10,20]。其中，气孔的数量是气孔过程的控制因素[9]，紫外线是非气孔过程的控制因素[20]。目前，气孔和非气孔的微观过程机制及相关控制因素是植被与大气汞交换过程的研究热点。

3.4 植被对土壤汞排放的影响

此外，植被还通过影响土壤汞的排放来间接影响大气汞的收支。随着植物冠层的发育，使照射到土壤的紫外线减少，土壤中Hg2+的光致还原作用减弱，土壤汞的排放通量逐渐降低[49-53]。据报道，有森林覆盖的土壤汞排放通量为1.4±0.3～2.4±1.0 ng·(m2·h)-1，裸地土壤汞的排放通量则高达7.6±1.7 ng·(m2·h)-1[53]。植被的存在还会影响森林土壤汞排放通量的季节变化，如在美国东部森林的观测结果表明冬春季土壤汞的排放通量高于夏秋季[50]。

4 植物监测在大气汞污染中的应用

叶汞浓度与大气汞浓度的线性关系表明叶片可以用于大气汞污染的植物监测。与传统的仪器监测相比，大气污染的植物监测具有分布广泛、采样便利、监测时间长、维护费用低且能直接反应污染物对生态系统的影响等优点。自1886年Nylander用地衣的丰度来反应大气污染程度起[54]，大气污染的植物监测已有100多年的历史了。随后，苔藓、树皮、树木的年轮、植物叶片以及蕨类植物都显示出大气污染的植物监测能力[55]。

苔藓是目前应用最广泛的监测大气污染的植物材料，它主要从大气沉降中吸收水分和营养物质，具有积累大气污染物的能力[56]。苔藓不仅已用于监测市政固废焚烧厂[57]、氯碱厂[58]、热电厂[59]、温度计厂[60]周边大气汞污染情况，而且用于区域大气汞的植物监测，例如整个欧洲大陆[61]。空气凤梨属(Tillandsia genus)植物，既非苔藓又非地衣，而是一类空中附生凤梨科植物，平常缠绕在树枝上，直接从大气中吸收水分和营养物质。常见的凤梨属植物西班牙苔藓(Tillandsiausneoides L.)已用于监测巴西氯碱厂[62]和亚马逊金矿区[63]周围大气汞的污染状况。

关于高等植物叶片监测大气汞污染的研究目前也有不少报道。Kono和Tomiyasu报道日本鹿儿岛市蕨类植物瓦韦(Lepisorusthunbergianus (kaulf.) Ching)的叶汞浓度与大气汞浓度有线性关系，可以用来原位估计大气汞浓度[64]。草类植物的叶片在大气污染的植物监测中具有广泛的应用前景，是目前大气汞污染植物监测的研究热点。多年生黑麦草(Loliumperenne L.)和意大利黑麦草(Loliummultiflorum Lam.)是目前研究较多的两种植物材料，其叶片已用于德国和比利时大气汞污染的植物监测[65,66]。此外，松针[67]和一些叶菜类蔬菜的叶片[15,68]也显示了大气汞污染的植物监测能力。Laacouri等探讨了树叶用于大气汞污染被动监测的不确定性因素，如叶的生长日期和位置[9]。

5 我国相关的研究进展与展望

鉴于植被在大气汞循环中的重要作用，我国开展了农作物中汞的来源与影响因素[14,15]、农田植被与大气汞的双向动态交换过程[69]、凋落物沉降中汞的通量[26,31,32,70]以及森林植被对土壤汞挥发的影响[52]等方面的研究，但对于大气汞在植物体内的迁移转化、植被与大气汞交换的微观过程以及植被削减我国大气汞浓度的宏观作用还认识不足。植被圈上下联系着大气圈和土壤圈，在今后的研究中，微观上应重视植被表面富集和排放大气汞的动态机制和植被富集的大气汞在叶表面、表皮和叶肉组织中的分布与转化过程；宏观上应重视我国植被对较高且还在上升的大气汞浓度的降低作用和植树造林、退耕还林导致的生物量增加对汞汇的增加作用；并将植被表面与大气汞交换的微观过程与大气汞长距离迁移的宏观过程相结合来研究植被在我国大气汞跨区域传输中的捕集作用；而在大气汞污染防治上应重视大气汞污染仪器监测与植物监测的结合。

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TheRoleofVegetationinAtmosphericMercuryBudgets:ProgressesandPerspectives

Niu Zhenchuan1,2,3,4,*, Zhang Xiaoshan2, Chen Jinsheng3, Wang Sen5, Wang Zhangwei2, Ci Zhijia2

1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710075, China 2. Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China 3. Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China 4. National Center for Accelerator Mass Spectrometry in Xi'an 710054, China 5. College of Urban and Environmental Sciences, Northwest University, Xi'an 710027, China

15 May 2010accepted8 July 2010

There are some uncertainties on the sinks and sources of mercury (Hg) in the Hg biogeochemical cycle. The estimated Hg emissions to the atmosphere are significantly greater than the known sinks, resulting in the mass unbalance for atmospheric Hg. Therefore the role of vegetation in Hg biogeochemical cycle has attracted more and more attentions. The studies of the role of vegetation in atmospheric Hg budgets can provide important information for the global Hg-reduced strategies. In this paper, we briefly review the origination of Hg in vegetation and its controlling factors, as well as the source/sink relationship to atmospheric Hg. Then three main manners involved in the atmospheric Hg budgets were emphasized in detail, including litterfall, biomass burning and bi-directional exchange of Hg between the surfaces of vegetation and atmosphere. In addition, the application of vegetation as biomonitorsof atmospheric Hg pollution was discussed. Finally, the progresses of mercury in vegetationin China were presented and some suggestions on future research were put forward.

vegetation; atmospheric Hg budgets; exchange process; biomass burning; litterfall

国家自然科学基金(41303072)，中国科学院地球环境研究所青年人才项目(Y354011480)，福建省青年科学基金(2013J05063)，环境化学与生态毒理学国家重点实验室开放基金(KF2011-11)

牛振川(1982-)，男，环境科学博士，副研究员，研究方向为汞的生物地球化学循环和14C环境示踪，E-mail: niuzc@ieecas.cn

10.7524/AJE.1673-5897-20140515011

2014-05-15录用日期：2014-07-08

1673-5897(2014)5-843-07

： X171.5

： A

牛振川, 张晓山, 陈进生, 等. 植被在大气汞收支中作用的研究进展与展望[J]. 生态毒理学报，2014, 9(5): 843-849

Niu Z C, Zhang X S, Chen J S, et al. The role of vegetation in atmospheric mercury budgets: Progresses and perspectives [J]. Asian Journal of Ecotoxicology, 2014, 9(5): 843-849 (in Chinese)