三峡水库消落带季节性植被对土壤（底泥）汞的活化效应研究

Reservoir is an important and typical eco-sensitive areas due to its special "mercury (Hg) activation effect", which is an important source resulted in increasing methylmercury (MeHg) in water and fishes due to soils and vegetations submerged while water-level increases of reservoir. Three Gorges Reservoir (TGR) is the most distinguished extra-large reservoir, which has very large flooding areas called water-level fluctuation zones resulted in long-term wet-dry alternation after completion of impoundment. Water-level fluctuation zones is in dry period from April to September every year showing an special local characteristics that plentiful vegetations (e.g. herbaceous plants) grow in these large areas due to sufficient rainfall and appropriate temperature, especially some parts of these areas are usually used by local people for cultivation. Thus, it's proposed that vegetations in soils/sediments in water-level fluctuation could make an important impact on the environmental fate of Hg in these areas, but we still don't know the underlying mechanisms of this effect. Thus, this study is proposed to use water-level fluctuation zones as research objective, combining investigation vegetation types, biomass and Hg contents in dry period, also including in situ experiments in field and simulated experiments in lab, for understanding following aspects including: accumulation characteristics of typical vegetation and crops in these areas for Hg, activation effect of root exudates for Hg in soils/sediments and relevant mechanisms of Hg species transformation. Further, through explicitly understanding the detailed mechanisms underlying including Hg releases characteristics, transformation, and methylation trend in vegetations, and effect on neighboring waters, which definitely it will be helpful for further understanding geochemical cycle of Hg in TGR areas, and also offer scientific evaluation and a theoretical foundation for comprehensively protecting eco-environment in the sensitive areas.

水库具有"汞活化效应"，是典型的汞敏感生态区，水库淹没的土壤与植被是水体及鱼体甲基汞增高的重要来源。三峡水库是举世瞩目的特大型水库，淹没的陆地面积大，成库后形成大面积长期处于干湿交替的消落带。消落带在每年4-8月的出露期，由于雨热充沛，大片区域会生长出茂密的草本植被，部分消落带被当地农户利用种植农作物。消落带土壤（底泥）由于植物的生长，势必会影响到消落带中汞的环境化学行为，但其影响特征和机制尚不清楚。为此，本研究以三峡库区消落带为研究对象，在调查消落带土壤（底泥）出露期植被类型及其汞含量分布的基础上，采用现场定位试验和室内模拟研究相结合的方法，研究消落带典型植物和农作物对汞的富集特征，根系分泌物对土壤（底泥）中汞的活化效应、形态转化及其机制，探索淹水期消落带植物中汞释放特征、转化规律、甲基化趋势及其对水环境的影响，旨在为深入了解三峡库区汞的生物地球化学循环及其生态环境保护提供研究基础。

三峡水库消落带在每年4-9月的出露期，大片区域会生长出茂密的草本植被，部分消落带被当地农户利用种植农作物。考虑到消落带植物的生长势必会影响到消落带中汞的环境化学行为，为此，本研究在调查消落带土壤（底泥）出露期植被类型、生物量及其汞含量的基础上，采用现场定位试验和室内模拟研究相结合的方法，研究消落带典型植物和农作物对汞的富集特征，根系分泌物对土壤（底泥）中汞的活化效应、形态转化及其机制，探索淹水期消落带植物中汞释放特征、转化规律、甲基化趋势及其对水环境的影响。经过四年的野外调查与室内模拟研究，完成了既定目标任务。调查发现，分布于三峡库区重庆段消落带植物有11科28种，自然植被以狗牙根、空心莲子草、稗草和酸模叶蓼为优势种，根据植物汞含量、单位面积生物量计算，消落带优势植物汞储存量为124.3~166.4 mg/hm2，其中，甲基汞储量为1.9~3.6 µg/hm2。消落带土壤中低分子有机酸主要为草酸、酒石酸和柠檬酸，三者占总有机90%以上，总有机酸含量为23.64±23.26µmol/kg~717.42±777.14µmol/kg。模拟试验显示，低分子量有机酸均能明显地增强土壤中汞的生物有效性，同时，也能促进土壤甲基汞的生成；土壤、植物淹没后，水体甲基汞含量明显增加，并导致鱼体甲基汞含量增加。野外定点试验显示，植物生长区域由于可向土壤释放更多的有机酸类物质，提高了汞甲基化微生物的活性，使其土壤甲基汞含量要高于裸地；以分布面积较大、农业活动较为频繁的重庆市开州区渠口镇消落带为研究区域，选择种植水稻、玉米、蔬菜和未农用的草地4类消落带地块为研究对象，调查了不同利用地块表层土壤汞（Hg）的变化特征。结果表明：研究区域内土壤THg、有效态Hg（Hg-wh）和甲基汞（MeHg）平均含量分别为25.80~68.74、0.44~0.88和0.08~0.85 ng/g。未耕作土地表层土壤THg、Hg-wh和MeHg含量均高于耕作土地表层土壤，说明耕作扰动能够加速土壤Hg的流失。由此可见，三峡库区消落带植物生长，有利于土壤汞的活化与甲基化，农业活动有利于土壤Hg的释放。