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

Abstract

Comprehensive mercury (Hg) budgets were constructed in two typical subtropical forests in southern China for the duration of 2014 to quantify Hg (gaseous elemental Hg, Hg⁰ and reactive Hg, Hg^Ⅱ) input and output fluxes and Hg retention in forests, consequently explore 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 HgⅡ deposition (67.7 μg·m-2·yr-1, 73% as dry HgⅡ deposition) was slightly higher than the Hg0 emission above the canopy (58.5 μg·m-2·yr-1), indicating 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 HgⅡ deposition (73.7 μg·m-2·yr-1) and relatively negligible Hg0 emission (2.65 μg·m-2·yr-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.