Deposition and Re-Emission of Atmospheric Elemental Mercury over the Tropical Forest Floor

Significant knowledge gaps exist regarding the emission of elemental mercury (Hg⁰) from the tropical forest floor, which limit our understanding of the Hg mass budget in forest ecosystems. In this study, biogeochemical processes of Hg⁰ 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 Δ¹⁹⁹Hg and δ²⁰²Hg in the evaded Hg⁰ vapor, while direct atmospheric Hg⁰ deposition does not exhibit isotopic fractionation. Using an isotopic mass balance model, direct atmospheric Hg⁰ deposition to soil was estimated to be 48.6 ± 13.0 μg m^-2 year^-1. Soil Hg⁰ 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 Hg⁰ sink in the tropical forest. The fast nutrient cycles in the tropical rainforests lead to a strong Hg⁰ re-emission and therefore a relatively weaker atmospheric Hg⁰ sink.