Canopy-Level Flux and Vertical Gradients of Hg⁰ Stable Isotopes in Remote Evergreen Broadleaf Forest Show Year-Around Net Hg⁰ Deposition

Abstract

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.

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

“…The direct Hg 0 deposition flux to the forest floor is nearly 1.5 times the litterfall Hg flux (48.6 ± 13.0 versus 34.1 ± 6.5 μg m –2 year –1 ) at this tropical site . The Hg 0 exchange flux on the ecosystem scale using the microclimatological measurements is 2–3 times higher than the litterfall deposition flux, implying that the other deposition pathways are not considered fully. , Therefore, litterfall Hg deposition is insufficient to represent the total Hg 0 deposition to the forest floor.…”

“…The Mt. Ailao subtropical forest is a larger atmospheric Hg 0 sink with a flux of −53.9 μg m –2 year –1 by micrometeorological measurements, although the litterfall Hg deposition in the Mt. Ailao subtropical forest is comparable to the deposition in this tropical rainforest (34.1 ± 4.5 μg m –2 year –1 versus 31.6 ± 6.5 μg m –2 year –1 ). , The weaker atmospheric Hg 0 sink in the tropical rainforest also explains its relatively lower soil Hg concentration compared to the values in subtropical forests (70–90 ng g –1 in tropical rainforests versus 200–300 ng g –1 ). ,, …”

“…17 The Hg 0 exchange flux on the ecosystem scale using the microclimatological measurements is 2−3 times higher than the litterfall deposition flux, implying that the other deposition pathways are not considered fully. 67,68 Therefore, litterfall Hg deposition is insufficient to represent the total Hg 0 deposition to the forest floor.…”

Mercury Uptake, Accumulation, and Translocation in Roots of Subtropical Forest: Implications of Global Mercury Budget

“…The studied site is located at the Ailaoshan Station for Subtropical Forest Ecosystem Research Studies (ASSFERS, 24°32′ N, 101°01′ E, 2476 m elevation), Yunnan Province, Southwest China, which has been described in detail elsewhere. ,− Briefly, ASSFERS has a subtropical monsoon climate, with an annual mean temperature of 11.3 °C and a precipitation depth of ∼1800 mm yr –1 . , The dominant tree species are Lichocarpus xylocarpus (LX), Scima noronhae (SN), Castanopsisi wattii (CW), and Manglietia insignis with an average canopy height of ∼23 m. , The forest surface soil is mainly alfisols with a pH of 3.5–4.0, containing ∼70% sand, 20%–25% silt, and 5%–10% clay. , In addition, the bedrock is dominated by slate.…”

“…6,39−41 Briefly, ASSFERS has a subtropical monsoon climate, with an annual mean temperature of 11.3 °C and a precipitation depth of ∼1800 mm yr −1 . 39,42 The dominant tree species are Lichocarpus xylocarpus (LX), Scima noronhae (SN), Castanopsisi wattii (CW), and Manglietia insignis with an average canopy height of ∼23 m. 39,43 The forest surface soil is mainly alfisols with a pH of 3.5−4.0, containing ∼70% sand, 20%−25% silt, and 5%−10% clay. 28,44 In addition, the bedrock is dominated by slate.…”