Foliage/atmosphere exchange of mercury in a subtropical coniferous forest in south China

Foliage/atmosphere exchange is an important pathway of deposition and loss in the biogeochemical mercury (Hg) cycle of terrestrial ecosystems. The foliage/atmosphere fluxes of Hg⁰ were observed over four seasons in a Masson pine (Pinus massoniana) forest in south China. Hg⁰ exchange showed a bidirectional process but without clear compensation point. Hg⁰ emissions peaked midday in all four seasons, probably associated with Hg photoreduction on needle surface. Peaks in Hg⁰ adsorption/deposition often occurred in the morning, especially in spring and autumn. Although current-year needles accumulated Hg at a rate of 19.4 µg m⁻² yr⁻¹, they were a net Hg⁰ source of 1.7 µg m⁻² yr⁻¹ to the atmosphere as their release of Hg exceeded inputs. In addition, previous-year needles emitted Hg⁰ at an average rate of 9.2 µg m⁻² yr⁻¹. Based on the mass balance of Hg in the forest canopy, the dry deposition of Hg was estimated 52.5 µg m⁻² yr⁻¹, much higher than the wet deposition (to 14.4 µg m⁻² yr⁻¹). Although Hg in the atmosphere is considered the main source of Hg in folia, soil water may contribute to Hg⁰ emission by plant transpiration. These processes should be further studied in the future.