| Mercury (Hg) is a hazardous heavy metal pollutant, which is easily to be bioaccumulated through food chain, leading considerous hazard to human health. Mercury emitted to the atmosphere from natural and anthropogenic sources exists mainly as gaseous elemental mercury (GEM, Hg0), accounting for over90%of the available ambient total gaseous mercury (TGM). The atmospheric residence time of GEM is as long as one year, so Hg0is involved in long range transport through atmospheric circulation, traveling far from the emission sources, being regarded as a global pollutant.Therefore, researches on the sources of gaseous mercury are key to understanding the biogeochemical cycle of mercury. Terrestrial ecological system is the main place for biogeochemical cycle of mercury, whereas forest ecosystem is the largest one and always be seen as an active sink of mercury. Forest soil is the main place for mercury accumulation, emissing a large amount of mercury to the atmosphere every year through dissusion effect. Meanwhile, mercury input to the forest soil includes dry and wet deposition. Therefore, mercury inputs, outputs, cycles and sources in the forest ecosystem are the important parts of the biogeochemical cycle of mercury.Southwest has the second largest forest coverage rate in China, with subtropical forest area accounting for three fourth, being the most representative forest ecosystem in China. Considerable work had been done on possible mercury pollution in different ecosystems previously; however, studies on mercury inputs, outputs of subtropical forest ecosystem are reported far less. In this research, national nature reserves of Mts. Jinyun and Simian were selected as study areas, with coniferous and deciduous forests selected as forest types. The characteristics of mercury of typical subtropical forest ecosystem was studied by field monitoring to investigate soil/air mecury exchange mechanism and influcing factors, results of which has significant meanings in elucidating forest soil/air mecury exchange laws under different atomospheres, evaluating global emission of natural mercury, protecting mercury pollution, researching on biogeochemical cycling of mercury, as well as forcasting environmental risks of mercury.Results of this research indicated that the highest TGM concentration of Mt. Jinyun was observed in summer, followed by winter and spring, and the lowest was in fall. TGM concentrations of Mt. Jinyun in the whole year were relatively stable, with little fluctuation. The highest average concentration of Mt. Jinyun in summer was4.99ng·m-3, which indicated that it was affected more by human activites in summer. As for Mt. Simian, the highest TGM concentration was observed in winter, followed by summer and spring, and the lowest was also in fall.The largest average concentration of Mt. Simian in winter was3.68ng·m-3. TGM concentrations of Mt. Jinyun were only lower than those in Guiyang, Beijing, Guangzhou, and Yangtze Delta, and higher than those in Shanghai, Seoul (Korean), Chicago(the United States), Mts. Changbai, Leigong, Gongga, Okinawan (Japan), Great Mountain Forest(USA), Adirondacks (New York, USA). TGM concentrations of Mt. Simian were lower than those in Guiyang, Beijing, Guanghzou, Yangtze Delta and Mt. Gongga, higher than Shanghai, Seoul (Korean), Chicago (the United States), Mts. Changbai, Leigong, Gongga, Okinawan (Japan), Great Mountain Forest(the United States), Adirondacks (New York, USA). In addition, TGM concentrations of Mts. Jinyun and Simian were both positively correlated with meteorological parameters, with obviously seasonal difference. TGM concentrations of Mts. Jinyun and Simian had remarkably positive correlation with atomospheric temperature, solar radiation, and relative humidity, but no significant correlations were found in air pressure.Cluster analysis results of TGM of Mts. Jinyun and Simian with72h back-trajectories by Hysplit identified Chongqing, Guizhou, Sichuan and Mideast areas of China as likely Hg sources. And there were both four types of72h trajectories arriving at Mts. Jinyun and Simian from March2012to Febrary2013, with cluster3making up for52%(Mt. Jinyun) and54%(Mt. Simian) in the four types of air masses. Cluster3correlated with the highest value of TGM concentrations, with4.99ng·m-3and3.68ng·m-3for Mts. Jinyun and Simian respectively. Cluster3air mass was from urban areas of Chongqing which had the most serious mercury pollution.The mercury exchange flux between forest soil and atmosphere was bidirectional, with soil mercury release as the main type. The variation of mercury exchange flux observed in four different forest floors in different seasons found that:(1) the mercury exchange flux of all the forest floors started to increase at6:00am in the morning, with the maximum value appearing at noon and the minimum values appearing at midnight;(2) the mercury exchange flux of the four forest floors were all higher by day than those by night;(3) the soil mercury released into the atmosphere was observed almost everyday of the four seasons in the four kinds of forest floors, with only individual winter time occouring mercury deposition (negative values);the bamboo forest of Mt. Jinyun in the winter midnight showed maximum mercury deposition,-4.34ng-(m2·h)-1.(4) According to the statistical results of mercury flux in the soil/air interface of the four seasons, the mercury exchange flux of the four forests were all highest in the summer, with averages of49.06±12.52,67.52±15.49,58.34±16.03and46.73±12.64ng-(m2·h)-1for coniferous and broad-leaved mixed forest, shrub forest, and bamboo forest of Mt. Jinyun, as well as coniferous and broad-leaved mixed forest of Mt. Simian.(5) the fluctuations of mercury exchange flux of shrub forest of Mt. Jinyun reached highest in the summer, having the most obvious trend. The maximum and minimum values were67.52ng·(m2·h)-1and7.10ng·(m2·h)-1individually.(6) soil/gas mercury exchange flux reached the lowest value in the winter for the four forest floors, with average flux of only-3.36ng·(m2·h)-1. The fluctuations of mercury exchange flux of the four forest floors were all smallest in one single day, having the weakest trend. Maximum and minimum values of mercury exchange flux both occurred in the shrub forest of Mt. Jinyun,67.52ng (m2·h)-1and7.10ng-(m2-h)-1individually. The conifer, shrubs, bamboo forest of Mt. Jinyun and the conifer of Mt. Simian had similar relationship with environmental factors, with positive relationship with air temperature, soil temperature and solar radiation, and negative relationship with relative humidity.The VMW concentrations of total mercury in the precipitation of coniferous and broad-leaved mixed forests of Mt.Jinyun and deciduous forests of Mt. Simian were23.40ng·L-1and24.89ng·L-1, and average VMW concentrations of total mercury in the throughfall were32.13ng·L-1and35.64ng·L-1. For the concentrations of MeHg, the VMW concentrations of MeHg in the precipitation and throughfall of coniferous and broad-leaved mixed forests of Mt.Jinyun and deciduous forests of Mt. Simian were0.32ng·L-1and0.29ng·L-1(precipitation) and0.29ng·L-1and0.43ng·L-1(throughfall). During the monitoring period, the total mercury concentration in precipitation was significantly higher than those in the rural and urban of Europe, the United States, as well as Japan, being slightly lower than those in Guiyang and Wujiang, being obviously lower than those in other cities in China, such as Beijing and Changchun. MeHg content was about7times of that in Carolina, the United States, and over twice of that in several areas of the United States. The trend of total mercury reversed with that of rainfall during the study period. When rainfall for a long time, the total mercury concentration was low, whereas when rainfall was less, the total mercury concentration was relatively high. The concentration of MeHg in precipitation varied with rainfall, with little fluctuations except individual samples. VMW concentrations in the two monitoring sites were both higher than that in precipitation. Ranges for the the mercury concentration in the throughfall was5.4-125.52ng·L-1. The volume weighted value of precipitation in Mt. Jinyun was23.40ng·L-1, and32.12ng·L-1for throughfall. For Mt. Simian, the volume weighted value of precipitation in Mt. Simian was24.89ng·L-1, and35.64ng-L’1for throughfall. The average wet deposition of total mercury in the precipitation within the12months in Mt. Jinyun was23.40μg·m-2, and0.32μg·m-2for MeHg. For Mt. Simian, the average wet deposition of total mercury in the precipitation within the12months in Mt. Jinyun was24.89μg·m-2, and0.32μg·m-2for MeHg.During the monitoring period,12-month wet deposition of total mercury and MeHg were larger than those in most areas of the USA (except North Carolina), but were lower than those in Beijing and Changchun, China. During the monitoring period, the cumulative total mercury flux in throughfall in the coniferous and broad-leaved mixed forests of Mt.Jinyun and deciduous forests of Mt. Simian were48.19and53.46μg m-2·yr-1repectively. While for precipitation, the accumulative total mercury flux in the coniferous and broad-leaved mixed forests of Mt.Jinyun and deciduous forests of Mt. Simian were38.03and38.44μg m-2·yr-1, which were higher than those in precipitation by21%and28%respectively.During the study period, both the concentration and mass of total mercury in leaf litter of deciduous and coniferous and broad-leaved mixed forests increased. The mean concentration of mercury in deciduous forests increased from46.3ng·g-1to57.88ng·g-1. And for coniferous and broad-leaved mixed forests, it increased from42.40ng·g-1to50.88ng·g-1. One year later, the average concentration of total mercury in the leaf litter of deciduous as wellas coniferous and broad-leaved mixed forests increased to125%and120%respectively. During the process of decomposition, the concentration of MeHg varied slightly, but comparing with the initial values, the concentration of MeHg increased slightly. According to the exponential attenuation model, the half-life of the mass of litter estimated as3.1and3.7years for deciduous as well as coniferous and broad-leaved mixed forests individually,which were similar with previous research results. In general, the mercury mass increased, but the mass of litter declined. During the study period, growing season mercury content accumulated gradually, while the mass of litter dropped by a relatively consistent rate. Piecewise regression model analysis showed that the accumulation of mercury varied with the season, with not significant decrease in non-growing season and clear increase in growing seasons. This indicated that seasonal accumulation of mercury was continuous, occouring in both deciduous and coniferous and broad-leaved mixed forests. |
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