植被在大氣汞收支中作用的研究進展與展望

牛振川，張曉山，陳進生，王森，王章瑋，慈志佳

1. 中國科學院地球環境研究所，黃土與第四紀地質國家重點實驗室，西安 710075 2. 中國科學院生態環境研究中心，北京 100085 3. 中國科學院城市環境研究所，廈門 361021 4. 國家加速器質譜中心(西安)，西安 710054 5. 西北大學，城市與環境學院，西安 710127

植被在大氣汞收支中作用的研究進展與展望

牛振川1,2,3,4,*，張曉山2，陳進生3，王森5，王章瑋2，慈志佳2

1. 中國科學院地球環境研究所，黃土與第四紀地質國家重點實驗室，西安 710075 2. 中國科學院生態環境研究中心，北京 100085 3. 中國科學院城市環境研究所，廈門 361021 4. 國家加速器質譜中心(西安)，西安 710054 5. 西北大學，城市與環境學院，西安 710127

在汞的生物地球化學循環中，對于“源”和“匯”的認識還存在許多不確定性。大氣汞收支不平衡的問題使得植被在汞循環中的作用日益凸現；開展植被在大氣汞收支中作用的研究有助于為全球汞減排政策的制定提供參考。本文首先概述了植被中汞的來源和影響因素及其與大氣汞的源匯關系；進而重點論述了植被參與大氣汞收支的主要方式：凋落物沉降、生物質燃燒和植被表面與大氣汞的動態交換，并闡述了植被在大氣汞污染監測中的應用；最后在總結我國相關研究的基礎上展望了未來的發展方向。

植被；大氣汞收支；交換過程；生物質燃燒；凋落物沉降

汞是一種全球性污染物，會隨大氣傳輸擴散到世界各地，甚至引起偏遠地區環境介質和生物體內汞濃度的升高，對人類健康和生態系統安全造成潛在威脅[1,2]。針對全球汞污染問題，聯合國環境規劃署已于2013年1月正式通過了旨在全球范圍內控制和減少汞排放的國際公約《水俁公約》，并在當年10月由87個國家正式簽約。我國是世界上汞排放量較大的國家，人為源每年的汞排放量約為609.1 t[3,4]，這給我國的環境保護和環境外交帶來巨大壓力。排放到大氣的汞通過干濕沉降過程進入植被系統，而植被系統又可通過復雜的物理、化學及生物過程釋放汞。因此，開展植被在大氣汞收支中作用的研究不僅有助于正確地評價自然排放源在大氣汞循環中的作用，而且能為全球汞減排政策的制定提供參考[5-7]。

在汞的生物地球化學循環過程中，對于“源”和“匯”的認識仍存在很多的不確定性。主要表現在目前研究所認識到的大氣汞排放源要遠多于所認識到的匯，產生大氣汞收支的不平衡問題[8]；其中的一個重要原因是忽視了植被在大氣汞收支中的重要作用。近年來，植被在汞循環中的重要作用得到了足夠的重視；本文對植被中汞的來源、與大氣汞的源匯關系、參與大氣汞收支的方式以及植被在大氣汞污染監測中的應用等方面的研究進展進行系統綜述，并對未來的發展方向進行展望。

1 植被中汞的來源及影響因素

汞循環中的一個中心問題是植被中的汞是來自土壤還是大氣。植物體內的汞主要來源于以下途徑：(1)氣孔對大氣Hg0的吸收[9]；(2)葉對干濕沉降的大氣Hg0、Hg2+以及與顆粒物結合汞的吸附[10]；(3)根通過蒸騰作用對土壤中可溶性汞的吸收。大量研究結果表明，根部的汞主要來自土壤吸收，木本、灌木以及大部分草本植物地上部分的汞主要來自大氣吸收[11-15]；而對于部分草本植物，由于其獨特的生理和生長方式，葉中很大一部分的汞來自土壤吸收。例如，Deschampsiaflexuosa L和Calamagrostisvillosa(Chaix ex Vill.)這兩種草本植物葉中的汞來自土壤汞的比例分別為30%和93%[16]。

植物體內汞濃度受環境汞濃度和生長時期的共同影響。隨著大氣汞濃度的增加，葉汞濃度也隨之線性增加，而其它器官汞濃度變化不明顯；同樣，當土壤汞濃度增加時，根汞濃度也隨之增加，而其它器官汞濃度變化不明顯[11-15]。總體而言，植物葉汞濃度隨生長時期的延長而增加[14,17,18]。但對于一些草本植物，葉汞濃度在整個生長時期內波動[15,19]。Dunagan等對菠菜53 d試驗結果表明，葉汞濃度在整個生長時期內變動較大，在28 d時濃度最高[19]。

2 植被與大氣汞的源匯關系

植被通過氣孔吸收Hg0和葉表面吸附干濕沉降的大氣汞而成為大氣汞的匯。然而，在一定條件下，植被富集的汞可通過以下方式向大氣排放而成為大氣汞的源：(1) 沉降在葉片表面的Hg2+在紫外線的照射下會還原為Hg0重新排放到大氣中[20]；(2) 植被隨呼吸作用向大氣排放汞[21,22]；(3) 植被以生物質燃燒的方式向大氣釋放汞[33-35]。但總體而言，植被是大氣汞的凈匯(net sink)[13]，其富集的大氣汞進而以凋落物沉降的方式釋放到地表系統。因此，植被通過凋落物沉降、生物質燃燒和表面與大氣汞的交換這些方式參與大氣汞收支。此外，植被還通過影響土壤汞的排放來間接影響大氣汞的收支。

3 植被參與大氣汞收支的方式

3.1 凋落物沉降中汞的輸入通量

植被富集的大氣汞會以凋落物沉降的方式輸送到地表，可以作為估算森林系統富集大氣汞通量的一種粗略研究方法。凋落物沉降是植被參與大氣汞收支的重要方式，Lindberg 等估計全球凋落物沉降中汞的年輸入量可達2 400～6 000 t[23]。溫帶地區凋落物沉降中汞的輸入通量為8～25 μg·(m2·y)-1[24-27]；熱帶地區由于凋落物較大的生物量，汞的輸入通量可達30～122 μg·(m2·y)-1 [28-30]。在中國西南某些地區，凋落物沉降中汞的輸入通量也較高，云南哀牢山為119.5 μg·(m2·y)-1[31]，貴州雷公山為119.5 μg·(m2·y)-1，重慶鐵山坪為291.2 μg·(m2·y)-1[32]。

3.2 生物質燃燒中汞的釋放通量

植被富集的大氣汞還可以以生物質燃燒的方式參與大氣汞收支，全球生物質燃燒釋放的汞通量每年可達數百t。通常，可以通過 Hg/CO、Hg/CO2和Hg/fuel等排放因子來估算生物質燃燒過程中釋放的汞量。Weiss-Penzias等以排放因子△TAM/△CO估計全球生物質燃燒釋放的汞量為670±330 t·y-1，其中北方針葉林燃燒釋放汞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分別估算全球生物質燃燒釋放的汞量為930 t·y-1(510～1 140 t·y-1)和590 t·y-1(380～1 330 t·y-1)[34]。Friedli等將世界劃分為不同區域，并采用不同的排放因子17～312 gHg·(kg fuel)-1，估計全球生物質燃燒釋放的汞通量為675±240 t·y-1[35]。

在生物質燃燒中，汞主要以氣態元素汞(Hg0)和顆粒汞(Hgp)的形式釋放。燃料濕度是控制燃燒過程中汞形態的主要因素，在濕度較低的情況下，Hg0是主要的形態，可占95%以上；而在鮮樣中，顆粒汞會占相當的比例，最多可達50%[36]。此外，燃燒方式對汞的形態也有影響，熏燒中顆粒汞的比例較高；而火焰比較明顯時，顆粒汞的比例不顯著[36]。

在森林大火中，不僅生物質燃燒會釋放大量的汞，而且土壤受熱也會釋放一定量的汞來參與大氣汞的收支。實驗室測得的排放因子為14～71×10-6gHg·(kg fuel)-1，而野外森林大火中測得的排放因子略高，為112 × 10-6gHg·(kg fuel)-1，Friedli等認為差值來自土壤受熱釋放的汞[37]。在森林大火中，土壤釋放的大量汞主要來自表層土壤[38]，占土壤總汞的79%左右[39]。

生物質燃燒會增加大氣汞的沉降而影響大氣汞的收支，對生態系統的汞循環造成一定影響。Witt等發現森林大火之后，美國北明尼蘇達州降水中總汞和甲基汞的濃度均增加1.7～8倍[40]。而Kelly等報道森林大火引起虹鱒魚(Oncorhynchusmykiss)體內的汞濃度增加了5倍，其它種類魚肌肉中的汞濃度也有一定增加，這可能與大火使營養元素輸入增加而導致食物鏈的重建，以及大火中瞬時釋放出大量的汞有關[41]。

3.3 植被表面與大氣汞的交換過程

植被表面與大氣汞的交換過程是其參與大氣汞收支的基本形式。Obrist估計全球植被的地上部分每年大約富集1 000 t的大氣汞，甚至可以引起春夏之際大氣汞濃度的降低，其認為植被是大氣汞“丟失的匯”(missing sink)[42]。因此，植被生物量的增加將能富集更多的大氣汞從而降低國際減排壓力。植被表面存在富集和排放大氣汞的動態雙向交換過程，據此，Shetty 等[43]和Quan等[44]分別通過模型的方法估計東亞和我國陸地植被每年向大氣排放630 t和79～177 t的汞。

研究植被表面與大氣汞交換過程的方法有動態通量袋法(dynamic flux bags, DFB)和微氣象法(micrometeorological method)兩類。微氣象法包括修正波文比法(MBR)、氣體動力學法(AER)和弛豫渦旋積累法(REA)，其對地表無干擾，可長期大面積連續監測，但對儀器要求較高，采樣復雜。微氣象法是在冠層尺度上認識植被與大氣汞的交換過程，其不僅包括植被與大氣汞的交換通量，還包括植被覆蓋土壤與大氣汞的交換通量。動態通量袋法更適于在葉片尺度上認識葉汞交換的動態過程，簡單方便，但也存在改變袋內氣象條件[45]和夜晚有水汽冷凝[46]的缺點。動態通量袋法的影響因素包括袋體材料、體積、進出口位置和氣體流速，其中氣體流速的影響最大，最佳氣體流速為保持出氣口與進氣口汞濃度之差(ΔC)為恒定值的最小流速[45]。

植被與大氣汞的交換通量存在一個補償點，當大氣汞濃度高于此補償點時，大氣汞被葉片表面所富集，此時交換通量數據為負；低于補償點時，植被向大氣排放汞，此時交換通量數據為正[47]。補償點因植物種類的不同而異，范圍為2～33 ng·m-3[11,12,47]。補償點也因環境而異，通常白天略高于夜晚[12]。因此，確立植被表面與大氣汞交換通量的補償點有助于揭示其與大氣汞的源匯關系。

影響植被表面與大氣汞交換通量的因素很多，主要包括植物的種類、生長環境及其生理活動。對于旱生植物，交換通量隨溫度(20～40°C)和大氣汞濃度的增加而增加，太陽輻射(尤其紫外線)和氣孔導度是控制葉汞交換通量的重要因素[10,12,20,48]；而對于水生植物，交換通量與植物蒸騰作用所產生的水汽通量顯著相關[22]。

植被表面與大氣汞存在復雜的交換過程，不僅有氣孔的吸收和排放過程，還存在大氣汞在葉片表面的沉降和光致還原等引起的再排放的非氣孔過程[9,10,20]。其中，氣孔的數量是氣孔過程的控制因素[9]，紫外線是非氣孔過程的控制因素[20]。目前，氣孔和非氣孔的微觀過程機制及相關控制因素是植被與大氣汞交換過程的研究熱點。

3.4 植被對土壤汞排放的影響

此外，植被還通過影響土壤汞的排放來間接影響大氣汞的收支。隨著植物冠層的發育，使照射到土壤的紫外線減少，土壤中Hg2+的光致還原作用減弱，土壤汞的排放通量逐漸降低[49-53]。據報道，有森林覆蓋的土壤汞排放通量為1.4±0.3～2.4±1.0 ng·(m2·h)-1，裸地土壤汞的排放通量則高達7.6±1.7 ng·(m2·h)-1[53]。植被的存在還會影響森林土壤汞排放通量的季節變化，如在美國東部森林的觀測結果表明冬春季土壤汞的排放通量高于夏秋季[50]。

4 植物監測在大氣汞污染中的應用

葉汞濃度與大氣汞濃度的線性關系表明葉片可以用于大氣汞污染的植物監測。與傳統的儀器監測相比，大氣污染的植物監測具有分布廣泛、采樣便利、監測時間長、維護費用低且能直接反應污染物對生態系統的影響等優點。自1886年Nylander用地衣的豐度來反應大氣污染程度起[54]，大氣污染的植物監測已有100多年的歷史了。隨后，苔蘚、樹皮、樹木的年輪、植物葉片以及蕨類植物都顯示出大氣污染的植物監測能力[55]。

苔蘚是目前應用最廣泛的監測大氣污染的植物材料，它主要從大氣沉降中吸收水分和營養物質，具有積累大氣污染物的能力[56]。苔蘚不僅已用于監測市政固廢焚燒廠[57]、氯堿廠[58]、熱電廠[59]、溫度計廠[60]周邊大氣汞污染情況，而且用于區域大氣汞的植物監測，例如整個歐洲大陸[61]。空氣鳳梨屬(Tillandsia genus)植物，既非苔蘚又非地衣，而是一類空中附生鳳梨科植物，平常纏繞在樹枝上，直接從大氣中吸收水分和營養物質。常見的鳳梨屬植物西班牙苔蘚(Tillandsiausneoides L.)已用于監測巴西氯堿廠[62]和亞馬遜金礦區[63]周圍大氣汞的污染狀況。

關于高等植物葉片監測大氣汞污染的研究目前也有不少報道。Kono和Tomiyasu報道日本鹿兒島市蕨類植物瓦韋(Lepisorusthunbergianus (kaulf.) Ching)的葉汞濃度與大氣汞濃度有線性關系，可以用來原位估計大氣汞濃度[64]。草類植物的葉片在大氣污染的植物監測中具有廣泛的應用前景，是目前大氣汞污染植物監測的研究熱點。多年生黑麥草(Loliumperenne L.)和意大利黑麥草(Loliummultiflorum Lam.)是目前研究較多的兩種植物材料，其葉片已用于德國和比利時大氣汞污染的植物監測[65,66]。此外，松針[67]和一些葉菜類蔬菜的葉片[15,68]也顯示了大氣汞污染的植物監測能力。Laacouri等探討了樹葉用于大氣汞污染被動監測的不確定性因素，如葉的生長日期和位置[9]。

5 我國相關的研究進展與展望

鑒于植被在大氣汞循環中的重要作用，我國開展了農作物中汞的來源與影響因素[14,15]、農田植被與大氣汞的雙向動態交換過程[69]、凋落物沉降中汞的通量[26,31,32,70]以及森林植被對土壤汞揮發的影響[52]等方面的研究，但對于大氣汞在植物體內的遷移轉化、植被與大氣汞交換的微觀過程以及植被削減我國大氣汞濃度的宏觀作用還認識不足。植被圈上下聯系著大氣圈和土壤圈，在今后的研究中，微觀上應重視植被表面富集和排放大氣汞的動態機制和植被富集的大氣汞在葉表面、表皮和葉肉組織中的分布與轉化過程；宏觀上應重視我國植被對較高且還在上升的大氣汞濃度的降低作用和植樹造林、退耕還林導致的生物量增加對汞匯的增加作用；并將植被表面與大氣汞交換的微觀過程與大氣汞長距離遷移的宏觀過程相結合來研究植被在我國大氣汞跨區域傳輸中的捕集作用；而在大氣汞污染防治上應重視大氣汞污染儀器監測與植物監測的結合。

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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

國家自然科學基金(41303072)，中國科學院地球環境研究所青年人才項目(Y354011480)，福建省青年科學基金(2013J05063)，環境化學與生態毒理學國家重點實驗室開放基金(KF2011-11)

牛振川(1982-)，男，環境科學博士，副研究員，研究方向為汞的生物地球化學循環和14C環境示蹤，E-mail: niuzc@ieecas.cn

10.7524/AJE.1673-5897-20140515011

2014-05-15錄用日期：2014-07-08

1673-5897(2014)5-843-07

： X171.5

： A

牛振川, 張曉山, 陳進生, 等. 植被在大氣汞收支中作用的研究進展與展望[J]. 生態毒理學報，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)