Soil-atmosphere exchange of gaseous elemental mercury in three subtropical forests with different substrate Hg concentrations

Abstract

The subtropical forest soil in southern China has accumulated a large amount of historically deposited mercury (Hg), which may result in Hg re-emission to the atmosphere and contribute to the global Hg cycle. In this study, the soil-atmosphere exchange of gaseous elemental mercury (Hg0) was monitored using a dynamic flux chamber (DFC) method at three forest sites, with different levels of soil Hg content and atmospheric Hg0 concentration. Specifically, the background testing site, located in Qianyanzhou, showed a net annual soil Hg0 emission (2.12 ng m−2 h−1 during the day and 2.23 ng m−2 h−1 at night), with significant diurnal and seasonal variations. In contrast, the moderately polluted site, which was in Tieshanping (suburb of Chongqing city), showed a much lower annual soil Hg0 emission (0.74 ng m−2 h−1 during the day) and even negative values in spring and winter. The last site, located in Zhuzhou (near a zinc smelter) with heavy Hg contamination, showed an extremely high soil emission all through the year (37.1 ng m−2 h−1 during the day), but relatively lower value in summer (1.37 ng m−2 h−1 during the day). Overall, these results clearly indicated the stimulate effect of high soil Hg content and the suppression effect of high atmospheric Hg0 concentration on soil Hg0 emission. By considering the results, we would suggest that the future Hg emission abatement from anthropogenic sources should enhance the re-emission of historically deposited Hg from subtropical forests, thus delay the decrease in atmospheric Hg0 concentration.