A novel passive exchange meter (EM) device was developed to assess air-surface exchange and leaching of Hg in a forest floor. Flux measurements were carried out in a subtropical forest ecosystem during a full year. Over 40% of the Hg fixed in fresh forest litter was remobilized in less than 60 days in warm and humid conditions as a response to rapid turnover of labile organic matter (OM). A two-block experiment including understory and clearing showed that losses of Hg covaried with seasonal conditions and was significantly affected by forest coverage. The process controlling the bulk loss of total Hg from the litter was volatilization, which typically represented 76-96% of the loss processes (Floss). The Floss ranges were 520-1370 and 165-942 ng m-2 d-1 in the understory and clearing, respectively. On a yearly basis, deposition of airborne Hg exceeded total losses by a factor of 2.5 in the clearing and 1.5 in the understory. The vegetation litter in this subtropical forest therefore represented a net sink of atmospheric Hg. This study provided a novel approach to Hg air-soil exchange measurements and further insights on the link between Hg remobilization and OM turnover along with its environmental drivers.

CAS Center for Excellence in Quaternary Science and Global Change Xi'an 710061 China

Department of Biosciences University of Oslo NO 0316 Oslo Norway

Norwegian Institute for Water Research NO 0349 Oslo Norway

Research Centre for Toxic Compounds in the Environment Masaryk University 601 77 Brno Czech Republic

State Key Laboratory of Environmental Geochemistry Institute of Geochemistry Chinese Academy of Sciences 99 Lincheng West Road Guiyang 550081 China

State Key Laboratory of Organic Geochemistry Guangzhou Institute of Geochemistry Chinese Academy of Sciences Guangzhou 510640 China

University of Chinese Academy of Sciences Beijing 100049 China

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