Canopy-Level Flux and Vertical Gradients of Hg 0 Stable Isotopes in Remote Evergreen Broadleaf Forest Show Year-Around Net Hg 0 Deposition.

Vegetation uptake represents the dominant route of Hg input to terrestrial ecosystems. However, this plant-directed sink is poorly constrained due to the challenges in measuring the net Hg 0 exchange on the ecosystem scale over a long period. Particularly important is the contribution in the subtropics/tropics, where the bulk (∼70%) of the Hg 0 deposition is considered to occur. Using the relaxed eddy accumulation technique, this study presents for the first time a whole ecosystem Hg 0 flux record over an evergreen hardwood forest. This tower-based micrometeorological method gauged a cumulated net Hg 0 flux of -41.1 μg m -2 over 16 months, suggesting that the subtropical montane forest acts as a large and continuous sink of atmospheric Hg 0 . The monthly net fluxes were consistently negative (-7.3 to -1.0 μg m -2 month -1 ) throughout the year, with the smallest absolute values occurring during the mild and dry subseason in spring, which was also the annual lowest in vegetation activity. Colocated measurements of multilevel gradients of Hg 0 concentration and its stable isotopic composition support the finding of year-round Hg 0 deposition. The stable Hg isotope measurements also show that in-canopy bi-directional Hg 0 exchange is prevalent.