中亚热带常绿阔叶林凋落物分解过程中汞的动态变化及迁移机理
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摘要
森林凋落物对于汞在林地土壤的生物地球化学循环中起到重要作用,为研究森林凋落物分解过程中汞的迁移转化特征,以重庆四面山风景名胜区典型林分(常绿阔叶林)作为研究对象。于2014年3月—2015年3月连续监测典型林分凋落物中各形态汞浓度和有机质变化量,同时监测周围土壤中汞浓度变化。结果表明:四面山典型林分凋落物分解过程中汞浓度整体上升,总汞浓度(初始浓度:78 ng/g)的增幅最高达53%,甲基汞浓度(初始浓度:0.32 ng/g)最高增幅达138%;在春季和夏季,水溶态和酸溶态两种活性态汞含量分别增加了851%和96%,在分解前期和末期,凋落物汞的中惰性汞比例最高,占比达75%。土壤腐殖质层中总汞和甲基汞浓度比较稳定。凋落物中活性态汞通过雨水淋洗进入土壤与有机质络合并发生甲基化/去甲基化过程,通过地表径流、地下径流进入水体。凋落物中C含量减少了22%,N含量增加了15%,总汞浓度与C/N比呈负相关,与N含量呈正相关。凋落物中微生物C、N含量整体增加,与汞浓度峰值同步,且夏季含量显著高于冬季含量(P<0.05),说明微生物与凋落物固定汞和汞的甲基化过程密切相关。
Forest ecosystem is the largest terrestrial ecosystem and plays a critical role in the biogeochemical cycling of mercury(Hg). Litterfall is confirmed to be a dominant pathway for Hg to reach the ground surface under a forest canopy. The objective of this research was to understand the characteristics of Hg migration and transformation in the litterfall and underlying soil during litter decomposition in a subtropical evergreen broad-leaf forest. Therefore, the dynamics of Hg concentrations and speciation distribution in the decomposing litterfall and underlying soils of Simian Mountain Nature Reserve were investigated for 1 year, from March 2014 to March 2015. The results indicated that initial total Hg(THg) concentration in the litterfall was 78.40 ± 1.4(standard error, SE) ng/g, which increased with the decomposition of litterfall. THg content increased up to 101.80±7.6 ng/g after decomposition for 1 year, while THg concentration increased by 30%. THg concentration reached its maximum(120.45 ng/g) after decomposing for 90 days. Compared with THg, the increase of methylmercury(MeHg) in the decomposing litter was more remarkable(P<0.01). MeHg concentration was 2.38 times that of its initial value(0.32 ng/g) at the end of the decomposition, and its content peaked(0.86 ng/g) after decomposing for 210 days. Generally speaking, the concentrations of both THg and MeHg increased with the decomposition of litterfall. The proportion of inert Hg in the decomposing litterfall was highest at the initial and advanced stages of decomposition, accounting for 75%. Moreover, Hg in the litter was relatively stable at these stages. Therefore, the mobility of Hg was slow, and thus its ecological risk were low. Both the soluble and acid soluble Hg contents increased significantly in the sfpring and summer. At this time, THg and MeHg in the decaying litterfall were inclined to be transported to the downstream water with a large amount of rainwater, thereby causing high ecological risk. On the contrary, the concentrations of THg and MeHg in the organic layer of the underlying soil were quite stable. A comparison of the THg concentrations in the litter and underlying soil revealed that Hg in the soil could enter the litter, so the soil could act as the source of Hg during litterfall decomposition. Results of MeHg content showed that MeHg in the litter could be exported to the underlying soil, and was finally stored in the soil and gradually demethylated. The volatilization of Hg could be one of the reasons why its concentration in the soil was stable. The decomposing litter was also an important source of THg and MeHg to the surface water because it could be transported by the runoff and percolate. The content of organic C in the litterfall decreased by 22%, while the organic N content increased by 15% during decomposition. THg concentration was negatively correlated with the C/N ratio but positively correlated with the N content. The increase of THg concentration was presumed to be closely related with the nitrogen fixation microorganisms. Microbial carbon and nitrogen in the litter increased and were significantly higher in summer than that in winter(P<0.05). The microorganisms activated in the decomposing litterfalls could immobilize more nutrients as decomposition rates and microbial activities peaked in the summer, and thus contributed to microbial biomass of the soil. Consequently, the concentration of THg increased during this process, especially MeHg, which increased exponentially.
Forest ecosystem is the largest terrestrial ecosystem and plays a critical role in the biogeochemical cycling of mercury(Hg). Litterfall is confirmed to be a dominant pathway for Hg to reach the ground surface under a forest canopy. The objective of this research was to understand the characteristics of Hg migration and transformation in the litterfall and underlying soil during litter decomposition in a subtropical evergreen broad-leaf forest. Therefore, the dynamics of Hg concentrations and speciation distribution in the decomposing litterfall and underlying soils of Simian Mountain Nature Reserve were investigated for 1 year, from March 2014 to March 2015. The results indicated that initial total Hg(THg) concentration in the litterfall was 78.40 ± 1.4(standard error, SE) ng/g, which increased with the decomposition of litterfall. THg content increased up to 101.80±7.6 ng/g after decomposition for 1 year, while THg concentration increased by 30%. THg concentration reached its maximum(120.45 ng/g) after decomposing for 90 days. Compared with THg, the increase of methylmercury(MeHg) in the decomposing litter was more remarkable(P<0.01). MeHg concentration was 2.38 times that of its initial value(0.32 ng/g) at the end of the decomposition, and its content peaked(0.86 ng/g) after decomposing for 210 days. Generally speaking, the concentrations of both THg and MeHg increased with the decomposition of litterfall. The proportion of inert Hg in the decomposing litterfall was highest at the initial and advanced stages of decomposition, accounting for 75%. Moreover, Hg in the litter was relatively stable at these stages. Therefore, the mobility of Hg was slow, and thus its ecological risk were low. Both the soluble and acid soluble Hg contents increased significantly in the sfpring and summer. At this time, THg and MeHg in the decaying litterfall were inclined to be transported to the downstream water with a large amount of rainwater, thereby causing high ecological risk. On the contrary, the concentrations of THg and MeHg in the organic layer of the underlying soil were quite stable. A comparison of the THg concentrations in the litter and underlying soil revealed that Hg in the soil could enter the litter, so the soil could act as the source of Hg during litterfall decomposition. Results of MeHg content showed that MeHg in the litter could be exported to the underlying soil, and was finally stored in the soil and gradually demethylated. The volatilization of Hg could be one of the reasons why its concentration in the soil was stable. The decomposing litter was also an important source of THg and MeHg to the surface water because it could be transported by the runoff and percolate. The content of organic C in the litterfall decreased by 22%, while the organic N content increased by 15% during decomposition. THg concentration was negatively correlated with the C/N ratio but positively correlated with the N content. The increase of THg concentration was presumed to be closely related with the nitrogen fixation microorganisms. Microbial carbon and nitrogen in the litter increased and were significantly higher in summer than that in winter(P<0.05). The microorganisms activated in the decomposing litterfalls could immobilize more nutrients as decomposition rates and microbial activities peaked in the summer, and thus contributed to microbial biomass of the soil. Consequently, the concentration of THg increased during this process, especially MeHg, which increased exponentially.
引文
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[5] 刘伟明,马明,王定勇,孙涛,魏世强.中亚热带背景区重庆四面山大气气态总汞含量变化特征.环境科学,2016,37(5):1639- 1645.
[6] 冯新斌,王训,林哲仁,付学吾.亚热带与温带森林小流域生态系统汞的生物地球化学循环及其同位素分馏.环境化学,2015,34(2):203- 211.
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[11] Obrist D,Johnson D W,Lindberg S E,Luo Y,Hararuk O,Bracho R,Battles J J,Dail D B,Edmonds R L,Monson R K,Ollinger S V,Pallardy S G,Pregitzer K S,Todd D E.Mercury distribution across 14 U.S.Forests.Part I:spatial patterns of concentrations in biomass,litter,and soils.Environmental Science & Technology,2011,45(9):3974- 81.
[12] Frescholtz T E,Gustin M S,Schorran D E,Fernandez G C J.Assessing the source of mercury in foliar tissue of quaking aspen.Environmental Toxicology and Chemistry,2003,22(9):2114- 2119.
[13] Millhollen A G,Gustin M S,Obrist D.Foliar mercury accumulation and exchange for three tree species.Environmental Science & Technology,2006,40(19):6001- 6006.
[14] Ma M,Wang D Y,Du H X,Sun T,Zhao Z,Wang Y M,Wei S Q.Mercury dynamics and mass balance in a subtropical forest,southwestern China.Atmospheric Chemistry and Physics,2016,15(24):35857- 35880.
[15] Wang X,Bao Z D,Lin C J,Yuan W,Feng X B.Assessment of Global Mercury Deposition through Litterfall.Environmental Science & Technology,2016,50(16):8548- 8557.
[16] Jiskra M,Wiederhold J G,Skyllberg U,Kronberg R M,Hajdas I,Kretzschmar R.Mercury deposition and re-emission pathways in boreal forest soils investigated with Hg isotope signatures.Environmental Science & Technology,2015,49(12):7188- 7196.
[17] Graydon J A,St Louis V L,Hintelmann H,Lindberg S E,Sandilands K A,Rudd J W M,Kelly C A,Hall B D,Mowat L D.Long-term wet and dry deposition of total and methyl mercury in the remote boreal ecoregion of Canada.Environmental Science & Technology,2008,42(22):8345- 8351.
[18] 万猛,田大伦,樊巍.豫东平原杨-农复合系统凋落物的数量、组成及其动态.生态学报,2009,29(5):2507- 2513.
[19] 李学斌,陈林,吴秀玲,宋乃平,李昕.荒漠草原4种典型植物群落枯落物分解速率及影响因素.生态学报,2015,35(12):4105- 4114.
[20] 李强,周道玮,陈笑莹.地上枯落物的累积、分解及其在陆地生态系统中的作用.生态学报,2014,34(14):3807- 3819.
[21] Hall B D,St Louis V L.Methylmercury and Total mercury in plant litter decomposing in upland forests and flooded landscapes.Environmental Science & Technology,2004,38(19):5010- 5021.
[22] Timperley M H,Hill L F.Discharge of mercury from the Wairakei geothermal power station to the Waikato River,New Zealand.New Zealand Journal of Marine and Freshwater Research,1997,31(3):327- 336.
[23] Lombardi G,Lanzirotti A,Qualls C,Socola F,Ali A M,Appenzeller O.Five hundred years of mercury exposure and adaptation.Journal of Biomedicine and Biotechnology,2012,2012:472858.
[24] 王定勇,牟树森.酸沉降地区大气汞对土壤-植物系统汞累积影响的调查研究.生态学报,1999,19(1):140- 144.
[25] 张成,宋丽,王定勇,张金洋,孙荣国.干湿交替条件下三峡水库消落带土壤汞形态变化.应用生态学报,2013,24(12):3531- 3536.
[26] Carpi A,Lindberg S E.Application of a teflonTM dynamic flux chamber for quantifying soil mercury flux:tests and results over background soil.Atmospheric Environment,1998,32(5):873- 882.
[27] Grigal D F,Kolka R K,Fleck J A,Nater E A.Mercury budget of an upland-peatland watershed.Biogeochemistry,2000,50(1):95- 109.
[28] Zheng W,Liang L Y,Gu B H.Mercury reduction and oxidation by reduced natural organic matter in anoxic environments.Environmental Science & Technology,2016,46(1):292- 299.
[29] Demers J D,Driscoll C T,Fahey T J,Yavitt J B.Mercury cycling in litter and soil in different forest types in the Adirondack region,New York,USA.Ecological Applications,2007,17(5):1341- 1351.
[30] 王锐萍,刘强,彭少麟,林开豪,文艳,薛宁.尖峰岭不同树种枯落物分解过程中微生物动态.浙江林学院学报,2006,23(3):255- 258.
[31] Zhou J,Wang Z W,Zhang X S,Chen J.Distribution and elevated soil pools of mercury in an acidic subtropical forest of southwestern China.Environmental Pollution,2015,202:187- 195.
[32] Laskowski R,Niklinska M,Maryanski M.The dynamics of chemical elements in forest litter.Ecology,1995,76(5):1393- 1406.
[33] Patel K,Nirmal K J I,N Kumar R,Kumar B R.Seasonal and temporal variation in soil microbial biomass C,N and P in different types land uses of dry deciduous forest ecosystem of Udaipur,Rajasthan,western India.Applied Ecology and Environmental Research,2010,8(4):377- 390.
[34] Devi N B,Yadava P S.Seasonal dynamics in soil microbial biomass C,N and P in a mixed-oak forest ecosystem of Manipur,North-east India.Applied Soil Ecology,2006,31(3):220- 227.
[35] Singh M K,Ghoshal N.Variation in soil microbial biomass in the dry tropics:impact of land-use change.Soil Research,2014,52(3):299- 306.
[2] Feng X B,Jiang H M,Qiu G L,Yan H Y,Li G H,Li Z G.Mercury mass balance study in Wujiangdu and Dongfeng Reservoirs,Guizhou,China.Environmental Pollution,2009,157(10):2594- 2603.
[3] Feng X B,Jiang H M,Qiu G L,Yan H Y,Li G H,Li Z G.Geochemical processes of mercury in Wujiangdu and Dongfeng reservoirs,Guizhou,China.Environmental Pollution,2009,157(11):2970- 2984.
[4] 王少锋,冯新斌,仇广乐,李仲根,魏中青.大气汞的自然来源研究进展.地球与环境,2006,34(2):1- 11.
[5] 刘伟明,马明,王定勇,孙涛,魏世强.中亚热带背景区重庆四面山大气气态总汞含量变化特征.环境科学,2016,37(5):1639- 1645.
[6] 冯新斌,王训,林哲仁,付学吾.亚热带与温带森林小流域生态系统汞的生物地球化学循环及其同位素分馏.环境化学,2015,34(2):203- 211.
[7] Rea A W,Lindberg S E,Keeler G J.Dry deposition and foliar leaching of mercury and selected trace elements in deciduous forest throughfall.Atmospheric Environment,2001,35(20):3453- 3462.
[8] Stamenkovic J,Gustin M S.Nonstomatal versus stomatal uptake of atmospheric mercury.Environmental Science & Technology,2009,43(5):1367- 1372.
[9] St Louis V L,Rudd J W M,Kelly C A,Hall B D,Rolfhus K R,Scott K J,Lindberg S E,Dong W J.Importance of the forest canopy to fluxes of methyl mercury and total mercury to boreal ecosystems.Environmental Science & Technology,2001,35(15):3089- 3098.
[10] Fu X W,Feng X,Dong Z Q,Yin R S,Wang J X,Yang Z R,Zhang H.Atmospheric gaseous elemental mercury (GEM) concentrations and mercury depositions at a high-altitude mountain peak in south China.Atmospheric Chemistry and Physics,2010,10(5):2425- 2437.
[11] Obrist D,Johnson D W,Lindberg S E,Luo Y,Hararuk O,Bracho R,Battles J J,Dail D B,Edmonds R L,Monson R K,Ollinger S V,Pallardy S G,Pregitzer K S,Todd D E.Mercury distribution across 14 U.S.Forests.Part I:spatial patterns of concentrations in biomass,litter,and soils.Environmental Science & Technology,2011,45(9):3974- 81.
[12] Frescholtz T E,Gustin M S,Schorran D E,Fernandez G C J.Assessing the source of mercury in foliar tissue of quaking aspen.Environmental Toxicology and Chemistry,2003,22(9):2114- 2119.
[13] Millhollen A G,Gustin M S,Obrist D.Foliar mercury accumulation and exchange for three tree species.Environmental Science & Technology,2006,40(19):6001- 6006.
[14] Ma M,Wang D Y,Du H X,Sun T,Zhao Z,Wang Y M,Wei S Q.Mercury dynamics and mass balance in a subtropical forest,southwestern China.Atmospheric Chemistry and Physics,2016,15(24):35857- 35880.
[15] Wang X,Bao Z D,Lin C J,Yuan W,Feng X B.Assessment of Global Mercury Deposition through Litterfall.Environmental Science & Technology,2016,50(16):8548- 8557.
[16] Jiskra M,Wiederhold J G,Skyllberg U,Kronberg R M,Hajdas I,Kretzschmar R.Mercury deposition and re-emission pathways in boreal forest soils investigated with Hg isotope signatures.Environmental Science & Technology,2015,49(12):7188- 7196.
[17] Graydon J A,St Louis V L,Hintelmann H,Lindberg S E,Sandilands K A,Rudd J W M,Kelly C A,Hall B D,Mowat L D.Long-term wet and dry deposition of total and methyl mercury in the remote boreal ecoregion of Canada.Environmental Science & Technology,2008,42(22):8345- 8351.
[18] 万猛,田大伦,樊巍.豫东平原杨-农复合系统凋落物的数量、组成及其动态.生态学报,2009,29(5):2507- 2513.
[19] 李学斌,陈林,吴秀玲,宋乃平,李昕.荒漠草原4种典型植物群落枯落物分解速率及影响因素.生态学报,2015,35(12):4105- 4114.
[20] 李强,周道玮,陈笑莹.地上枯落物的累积、分解及其在陆地生态系统中的作用.生态学报,2014,34(14):3807- 3819.
[21] Hall B D,St Louis V L.Methylmercury and Total mercury in plant litter decomposing in upland forests and flooded landscapes.Environmental Science & Technology,2004,38(19):5010- 5021.
[22] Timperley M H,Hill L F.Discharge of mercury from the Wairakei geothermal power station to the Waikato River,New Zealand.New Zealand Journal of Marine and Freshwater Research,1997,31(3):327- 336.
[23] Lombardi G,Lanzirotti A,Qualls C,Socola F,Ali A M,Appenzeller O.Five hundred years of mercury exposure and adaptation.Journal of Biomedicine and Biotechnology,2012,2012:472858.
[24] 王定勇,牟树森.酸沉降地区大气汞对土壤-植物系统汞累积影响的调查研究.生态学报,1999,19(1):140- 144.
[25] 张成,宋丽,王定勇,张金洋,孙荣国.干湿交替条件下三峡水库消落带土壤汞形态变化.应用生态学报,2013,24(12):3531- 3536.
[26] Carpi A,Lindberg S E.Application of a teflonTM dynamic flux chamber for quantifying soil mercury flux:tests and results over background soil.Atmospheric Environment,1998,32(5):873- 882.
[27] Grigal D F,Kolka R K,Fleck J A,Nater E A.Mercury budget of an upland-peatland watershed.Biogeochemistry,2000,50(1):95- 109.
[28] Zheng W,Liang L Y,Gu B H.Mercury reduction and oxidation by reduced natural organic matter in anoxic environments.Environmental Science & Technology,2016,46(1):292- 299.
[29] Demers J D,Driscoll C T,Fahey T J,Yavitt J B.Mercury cycling in litter and soil in different forest types in the Adirondack region,New York,USA.Ecological Applications,2007,17(5):1341- 1351.
[30] 王锐萍,刘强,彭少麟,林开豪,文艳,薛宁.尖峰岭不同树种枯落物分解过程中微生物动态.浙江林学院学报,2006,23(3):255- 258.
[31] Zhou J,Wang Z W,Zhang X S,Chen J.Distribution and elevated soil pools of mercury in an acidic subtropical forest of southwestern China.Environmental Pollution,2015,202:187- 195.
[32] Laskowski R,Niklinska M,Maryanski M.The dynamics of chemical elements in forest litter.Ecology,1995,76(5):1393- 1406.
[33] Patel K,Nirmal K J I,N Kumar R,Kumar B R.Seasonal and temporal variation in soil microbial biomass C,N and P in different types land uses of dry deciduous forest ecosystem of Udaipur,Rajasthan,western India.Applied Ecology and Environmental Research,2010,8(4):377- 390.
[34] Devi N B,Yadava P S.Seasonal dynamics in soil microbial biomass C,N and P in a mixed-oak forest ecosystem of Manipur,North-east India.Applied Soil Ecology,2006,31(3):220- 227.
[35] Singh M K,Ghoshal N.Variation in soil microbial biomass in the dry tropics:impact of land-use change.Soil Research,2014,52(3):299- 306.
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