Subtropical Forests Act as Mercury Sinks but as Net Sources of Gaseous Elemental Mercury in South China

Comprehensive mercury (Hg) budgets were constructed in two typical subtropical forests in southern China in 2014 to quantify Hg (gaseous elemental Hg, Hg⁰, and reactive Hg, Hg^II) input and output fluxes and Hg retention in forests, consequently exploring the roles of subtropical forests in the global Hg cycle. At site Qianyanzhou, representing a background region with an enhanced atmospheric Hg⁰ concentration, the total Hg^II deposition (67.7 μg·m^-2·year^-1, 73% as dry Hg^II deposition) was found to be slightly higher than the Hg⁰ emission above the canopy (58.5 μg·m^-2·year^-1), indicating that the forest is a minor Hg sink but a significant net Hg⁰ source on a yearly basis. In contrast, the forest in the moderately polluted region (site Huitong) acted as a significant Hg sink but a minor net Hg⁰ source with a higher Hg^II deposition (73.7 μg·m^-2·year^-1) and relatively negligible Hg⁰ emission (2.65 μg·m^-2·year^-1). The decreasing atmospheric Hg⁰ concentrations declined the total Hg sink based on the Hg budgets synthesized of this and previous studies and may promote forest Hg⁰ emissions. Consequently, it was expected that the re-emission of historically deposited Hg may be enhanced from subtropical forests by recent decreases in atmospheric Hg⁰ concentrations throughout China.