Air-soil exchange of elemental mercury vapor (Hg-0) is an important component in the budget of the global mercury cycle. However, its mechanistic detail is poorly understood. In this study, stable Hg isotopes in air, soil, and pore gases are characterized in a subtropical evergreen forest to understand the mechanical features of the air-soil Hg-0 exchange. Strong Hg-II reduction in soil releases Hg-0 to pore gas during spring-autumn but diminishes in winter, limiting the evasion in cold seasons. Delta Hg-199 in air modified by the Hg-0 efflux during flux chamber measurement exhibit seasonality, from -0.33 +/- 0.05% in summer to -0.08 +/- 0.05 parts per thousand in winter. The observed seasonal variation is caused by a strong pore-gas driven soil efflux caused by photoreduction in summer, which weakens significantly in winter. The annual Hg-0 gross deposition is 42 +/- 33 mu g m(-2) yr(-1), and the corresponding Hg-0 evasion from the forest floor is 50 +/- 41 mu g m(-2) yr(-1). The results of this study, although still with uncertainty, offer new insights into the complexity of the air-surface exchange of Hg-0 over the forest land for model implementation in future global assessments.