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 Hg0 exchange on the ecosystem scale over a long period. Particularly
important is the contribution in the subtropics/tropics, where the
bulk (∼70%) of the Hg0 deposition is considered
to occur. Using the relaxed eddy accumulation technique, this study
presents for the first time a whole ecosystem Hg0 flux
record over an evergreen hardwood forest. This tower-based micrometeorological
method gauged a cumulated net Hg0 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 Hg0. 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 Hg0 concentration
and its stable isotopic composition support the finding of year-round
Hg0 deposition. The stable Hg isotope measurements also
show that in-canopy bi-directional Hg0 exchange is prevalent.
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