No abstract

Significant knowledge gaps exist regarding the emission of elemental mercury (Hg 0 ) from the tropical forest floor, which limit our understanding of the Hg mass budget in forest ecosystems. In this study, biogeochemical processes of Hg 0 deposition to and evasion from soil in a Chinese tropical rainforest were investigated using Hg stable isotopic techniques. Our results showed a mean air−soil flux as deposition of −4.5 ± 2.1 ng m −2 h −1 in the dry season and as emission of +7.4 ± 1.2 ng m −2 h −1 in the rainy season. Hg re-emission, i.e., soil legacy Hg evasion, induces negative transitions of Δ 199 Hg and δ 202 Hg in the evaded Hg 0 vapor, while direct atmospheric Hg 0 deposition does not exhibit isotopic fractionation. Using an isotopic mass balance model, direct atmospheric Hg 0 deposition to soil was estimated to be 48.6 ± 13.0 μg m −2 year −1 . Soil Hg 0 re-emission was estimated to be 69.5 ± 10.6 μg m −2 year −1 , of which 63.0 ± 9.3 μg m −2 year −1 is from surface soil evasion and 6.5 ± 5.0 μg m −2 year −1 from soil pore gas diffusion. Combined with litterfall Hg deposition (∼34 μg m −2 year −1 ), we estimated a ∼12.6 μg m −2 year −1 net Hg 0 sink in the tropical forest. The fast nutrient cycles in the tropical rainforests lead to a strong Hg 0 re-emission and therefore a relatively weaker atmospheric Hg 0 sink.

Mercury (Hg) is a toxic metal released into the environment through human activities and natural processes and occurs in the atmosphere mainly as gaseous Hg 0 , which has a 0.5-to 1-year residence time (Amos et al., 2013;Lindberg et al., 2007). The present atmospheric Hg deposition has increased by 3-5 times compared to that during the preindustrial period due to the distinctly increasing anthropogenic Hg emissions since the 1850s (UN-Environment, 2019). To reduce anthropogenic Hg emissions across the globe, the Minamata Convention on Mercury, a legally binding international treaty, entered into force in August 2017. However, significant knowledge gaps in Hg cycling challenge our ability to assess the effectiveness of the Convention in reducing human and wildlife Hg exposure. Although the reduction of anthropogenic Hg emissions is expected to distinctly decrease the Hg concentration in the atmosphere, as has been observed at Northern Hemisphere remote sites (Tang et al., 2018;Zhang et al., 2016), it may take much longer for biotic Hg to decrease due to the complex processes associated with changes in climate and land use (