Investigation of factors affecting mercury emission from subtropical forest soil: A field controlled study in southwestern China

Recent studies demonstrated that subtropical forest in China was considered as a large pool of atmospheric mercury and soils of forested watershed is a large reservoir of atmospherically deposited mercury. However, forest ecosystems not only act as sinks but also as sources of previously deposited mercury emitted back to the atmosphere. In this study a field controlled method was performed in Tieshanping National Forest Park (TNFP) to identify the effects of the most important parameters that controlled mercury emissions from soil surfaces, including chamber flushing flow turnover times (TOTs), soil water content and watering, total gaseous mercury (TGM) in air and understory. Flushing flow rates significantly affected the calculation of mercury flux and the optimal TOTs were 0.94 min in the forest. TGM in atmosphere was significantly inhibited mercury emission from soils, and the deposited mercury was not absorbed firmly by the soils in a short time and emitted back to atmosphere rapidly when TGM concentration decreased. Higher soil moisture reduced the emission of mercury and initial watering produces a spike in the mercury emissions due to the interstitial soil gas mercury displaced by infiltrating water physically. However, subsequent watering was reducing the fluxes, because surface soil was saturated and soil pores were blocked by water film and inhibited the soil mercury emission. Soils under the understory had a higher mercury concentrations and deep organic layers. However, the fluxes of soil under the understory significantly were inhibited in daytime because solar radiation was blocked by the understory and the higher litter layer.