Field Measurement Of Soil Mercury Emission Flux In Forest

Mercury is the only metal that can keep liquid state under room temperature. Because of its toxicity and long-range transportation, much more attention was paid to mercury around the world in the last50years. Compared with anthropogenic sources, studies carried out about characteristics and mechanisms of mercury emission flux from natural sources were much less in domestic, especially on mercury emission flux from forest soils.To investigate soil mercury emission flux characteristics in areas with high atmospheric mercury concentration, the emission flux of gaseous elemental mercury (Hg0) was measured for four seasons from September2012to March2014in two different areas using a dynamic flux chamber and a LUMEX RA-915+mercury analyzer. The two experiment sites were located at a Masson pine forest at Tieshanping, Chongqing in Southwestern China and a theropencedrymion forest near a Zn/Pb smelter, Zhuzhou in South China. The effects of ambient atmospheric mercury concentration and environmental factors, such as radiation intensity, air temperature, air humidity, soil temperature, soil water content, precipitation, and surface litter layer, on mercury emission flux were also studied.Result showed that there were clearly seasonal variations of the Hg0emission flux in the two sites. The value of Hg0emission flux from the Masson pine forest soil was35.3ng·m2·d)-1in the summer, but very low in other seasons, even negative in the spring and winter. While the value of Hg0emission flux from the theropencedrymion forest soil was490ng·m2·d)-1average in the spring, autumn and winter, much lower in the summer [13.7ng·m2·d)-1]. In addition to the different environmental factors in the two sites, the different variation season trend of ambient atmospheric mercury concentration in the two sites was an important impacting factor. Because the gradient difference of Hg0concentration between soil and air was the most important force to releasing mercury from soils, and ambient atmospheric mercury concentration was negatively correlated with Hg0emission flux. The equilibrium mercury concentration between soil and atmosphere was5.61ng·m-3in the Masson pine forest site, with ambient atmospheric mercury concentration only low above it in the summer, leading to the Hg0emission flux keeping in a low level around zero in a long time during the spring, autumn and winter. Because of the higher soil mercury content, the equilibrium mercury concentration between soil and atmosphere was17.3ng-m-3in the theropencedrymion forest site. On the contrary, the ambient atmospheric mercury concentration was much lower than the equilibrium concentration in a long time during the spring, autumn and winter, which made a great contribution to the soil mercury emission flux. In summer, the ambient atmospheric mercury concentration would increase to a high level around the equilibrium concentration, influenced by the wind blowing from the Zn/Pb smelter.In addition to ambient atmospheric mercury concentration, radiation intensity was another important impacting factor, which can inspire the photo-reduction, the dominant process facilitating mercury emission flux. As the canopy decreased the radiation intensity reaching soil surface, Hg0emission flux from forest soils was lower than that monitored in open fields. Soil/air temperature was positive with Hg0emission flux. Besides, precipitation and the increasing of soil water content would also lead to an immediately release of Hg0from soil surface.But the relation between air humidity and Hg0emission flux was not sure. With organic carbon enriching in the surface litter layer, which could strongly adsorb the mercury, the litter layer played a key role in reducing the mercury emission flux from forest soils.The total soil Hg0emission of the Masson pine forest was estimated to2.65μg·(m2·a)-1, which was much lower than that in similar forest in cleaner areas. High ambient atmospheric mercury concentration in Tieshanping was the main reason for the difference. And influenced by the higher ambient atmospheric mercury concentration, the total soil Hg0emission of the theropencedrymion forest was estimated to133μg·m2·a)-1, which was also lower than that in some mercury mining areas with higher mercury concentration in soils. In all, the characteristics of mercury emission flux from forest soils in areas with high atmospheric mercury concentration were different from those in other areas in our country, and further research need to be carried on in the future.