| Mercury pollution issue has always been one of the research focuses in environmental science field since Minamata disease event, which occurred in Japan in the early1950s. Mercury can transport with atmospheric circulation from long distance because of its special physical and chemical properties, which may cause global mercury pollution. People have realized many important aspects of mercury pollution such as its sources, transportation, transformation and harm. However, researches on quantitative relation between sources and sinks of mercury in environment have not been satisfactorily obtained, especially emission of mercury from nature sources is to be estimated regionally and globally, and extensive measurement of mercury exchange flux over air-surface is of great importance. Mercury exchange flux over different environmental interfaces, such as forest, prairie, desert, waters (ocean, lake, river, wetland), and mining area were discussed by some researchers, and they have important reference values for evaluating mercury emission correctly. As the largest land ecosystem-forest, many experts and scholars had done some researches since the1990s, but mainly of them were concentrated in the temperate forest ecosystems, study about soil/air interface exchange characteristics of mercury emission and deposition in the Mid-Subtropical Forests Ecosystem, is relatively deficient. Especially areas which have regional typical forest characteristics, the field monitoring data is still lacking. Thereafter a Dynamic Flux Chamber (DFC) technique was applied to monitor air/surface exchange flux of mercury synchronous over forest land and bare land one year in a row. This study choices Jinyun mountain nature reserve-a typical mid-subtropical forest ecosystem, which located in beibei, chongqing as the research sites. Effect of environmental factors were also measured and investigated to mercury flux in this article.Results showed that the annual average Hg emission flux in soil of forest land was16.82±6.70ng·m-2·h-1, the annual average Hg emission flux of soil/air interface of bare land was21.12±14.55ng·m-2·h-1, which were significantly higher than the global nature mercury releasing fluxes (0.7-1.1ng·m-2·h-1).There were obviously seasonal differences of Hg emission in both forest and bare land of Jinyun mountain. Hg emission of warm season was significantly higher than that of the cold season. Affected by illumination, meteorological factors such as temperature, relative humidity and rules of mid-subtropical forest vegetation growth changes, Hg emission of bare land was higher than the forest in the warm season(bare land:29.19±23.89ng·m-2·h-1>forest land:22.23±13.19ng·m-2·h-1), which was totally opposite when it is in cold season(bare land:4.97±5.75ng·m-2·h-1<forest land:6.01±4.05ng·m-2·h-1).The diurnal variation of the soil Hg emission flux of both forest and bare land of Jinyun Mountain connected closely with the light intensity. In the day time, a sharp increase in flux starting in the morning and peak emission at midday, then decreased gradually with sunlight decreased, and to reach the minimum value at midnight. Hg emission of both forest and bare land was obviously different between day and night, and it was significantly higher than the night during the day, Emissions from air/soil surface was dominated by deposition in nighttime.Hg emission of the bare soil was higher than the forest during the day, but it was totally opposite at night. This was mainly affected by vegetation keeping out sunshine during the day to reduce the radiation intensity, and the effect of mercury intercepting of atmospheric precipitation at night.When there comes the land use type change, forests which fulled with vegetation would change to bare land by cutting and destruction, the inhibition effect of Hg accumulation in soil caused by vegetation would disappear, and a large number of Hg2+which adsorbed by woodland soil would transfer to bare land after reactivation and release by photoreduction and thermal recovery, finally, they would participate in the global mercury cycle and cause more serious mercury pollution.Environmental factors, including soil Hg concentration, solar radiation, soil temperature, air temperature, relative humidity and atmospheric pressure could influence air/soil mercury exchange flux. There were significantly linear positive correlation between some factors and the mercury flux, such as soil Hg concentration, solar radiation, soil temperature and air temperature. In contrary, there were negative correlation between relative humidity and mercury flux. And there were no obvious correlation between atmospheric pressure and mercury flux. Light intensity is the main influence factors of soil/air interface Hg exchange fluxe, followed by Mark Twain, the temperature and relative humidity of soil total mercury. In this study, no significantly affect between mercury flux and atmospheric pressure were found. Significant correlation relationship between mercury flux and some factors, such as soil temperature, air temperature and relative humidity might be the representation of the relation between light intensity and soil mercury emission. |
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