Soil–atmosphere exchange flux of total gaseous mercury (TGM) at subtropical and temperate forest catchments

Abstract

Evasion from soil is the largest source of mercury (Hg) to the atmosphere from terrestrial ecosystems. To improve our understanding of controls and in estimates of forest soil–atmosphere fluxes of total gaseous Hg (TGM), measurements were made using dynamic flux chambers (DFCs) over 130 and 96 d for each of five plots at a subtropical forest and a temperate forest, respectively. At the subtropical forest, the highest net soil Hg emissions were observed for an open field (24 ± 33 ng m−2 h−1), followed by two coniferous forest plots (2.8 ± 3.9 and 3.5 ± 4.2 ng m−2 h−1), a broad-leaved forest plot (0.18 ± 4.3 ng m−2 h−1) and the remaining wetland site showing net deposition (−0.80 ± 5.1 ng m−2 h−1). At the temperate forest, the highest fluxes and net soil Hg emissions were observed for a wetland (3.81 ± 0.52 ng m−2 h−1) and an open field (1.82 ± 0.79 ng m−2 h−1), with lesser emission rates in the deciduous broad-leaved forest (0.68 ± 1.01 ng m−2 h−1) and deciduous needle-leaved forest (0.32 ± 0.96 ng m−2 h−1) plots, and net deposition at an evergreen pine forest (−0.04 ± 0.81 ng m−2 h−1). High solar radiation and temperature during summer resulted in the high Hg emissions in the subtropical forest and the open field and evergreen pine forest at the temperate forest. At the temperate deciduous plots, the highest Hg emission occurred in spring during the leaf-off period due to direct solar radiation exposure to soils. Fluxes showed strong positive relationships with solar radiation and soil temperature and negative correlations with ambient air TGM concentration in both the subtropical and temperate forests, with area-weighted compensation points of 6.82 and 3.42 ng m−3, respectively. The values of the compensation points suggest that the atmospheric TGM concentration can play a critical role in limiting TGM emissions from the forest floor. Climate change and land use disturbance may increase the compensation points in both temperate and subtropical forests. Future research should focus on the role of legacy soil Hg in reemissions to the atmosphere as decreases in primary emissions drive decreases in TGM concentrations and disturbances of climate change and land use.