An improved method for recovering and preconcentrating mercury in natural water samples for stable isotope analysis

et al. 2020

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137

Mercury (Hg) deposition through litterfall has been regarded as the main input of gaseous elemental mercury (Hg0) into forest ecosystems. We hypothesize that earlier studies largely underestimated this sink because the contribution of Hg0 uptake by moss and the downward transport to wood and throughfall is overlooked. To test the hypothesis, we investigated the Hg fluxes contributed via litterfall and throughfall, Hg pool sizes in moss covers and woody biomass as well as their isotopic signatures in a glacier-to-forest succession ecosystem of the Southeast Tibetan Plateau. Results show that Hg0 depositional uptake and pool sizes stored in moss and woody biomass increase rapidly with the time after glacier retreat. Using the flux data as input to a Hg isotopic mixing model, Hg deposition through litterfall accounts for 27–85% of the total accumulation rate of Hg0 in organic soils of glacial retreat over 20–90 years, revealing the presence of additional sources of Hg0 input. Atmospheric Hg0 accounts for 76 ± 24% in ground moss, 86 ± 15% in tree moss, 62–92% in above ground woody biomass (branch–bark–stem), and 44–83% in roots. The downward decreasing gradient of atmospheric Hg0 fractions from the above ground woody biomass to roots suggests a foliage-to-root Hg transport in vegetation after uptake. Additionally, 34–82% of atmospheric Hg0 in throughfall further amplifies the accumulation of Hg0 from atmospheric sources. We conclude that woody biomass, moss, and throughfall represent important Hg0 sinks in forest ecosystems. These previously unaccounted for sink terms significantly increase the previously estimated atmospheric Hg0 sink via litterfall.

Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Underestimated Sink of Atmospheric Mercury in a Deglaciated Forest Chronosequence

et al. 2020

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137

“…δ 202 Hg precipitation and Δ 199 Hg precipitation represent the Hg isotopic signatures in precipitation. 55 T refers to the fractional contribution of Hg II by precipitation mixed in the soil pool. Since throughfall Hg comes from rainfall Hg and additional Hg input during washout, it is assumed that the fraction of washed-out Hg contained in throughfall remains the in soil.…”

Section: Methodsmentioning

confidence: 99%

Stable Mercury Isotope Transition during Postdepositional Decomposition of Biomass in a Forest Ecosystem over Five Centuries

et al. 2020

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Organic soil is an important transient reservoir of mercury (Hg) in terrestrial ecosystems, but the fate of deposited Hg in organic forest soil is poorly understood. To understand the dynamic changes of deposited Hg on forest floor, the composition of stable Hg and carbon (C) isotopes in decomposing litters and organic soil layer was measured to construct the 500 year history of postdepositional Hg transformation in a subtropical evergreen broad-leaf forest in Southwest China. Using the observational data and a multiprocess isotope model, the contributions of microbial reduction, photoreduction, and dark reduction mediated by organic matter to the isotopic transition were estimated. Microbial reduction and photoreduction play a dominant role in the initial litter decomposition during first 2 years. Dark redox reactions mediated by organic matter become the predominant process in the subsequent 420 years. After that, the values of Hg mass dependent fractionation (MDF), mass independent fractionation (MIF), and Δ 199 Hg/Δ 201 Hg ratio do not change significantly, indicating sequestration and immobilization of Hg in soil. The linear correlations between the isotopic signatures of Hg and C suggest that postdepositional transformation of Hg is closely linked to the fate of natural organic matter (NOM). Our findings are consistent with the abiotic dark reduction driven by nuclear volume effect reported in boreal and tropical forests. We recommend that the dark reduction process be incorporated in future model assessment of the global Hg biogeochemical cycle.

“…In addition to the physical, chemical, and biological processes that induce MDF (reported as δ 202 Hg), Hg MIF signatures, reported as Δ 199 Hg and Δ 201 Hg for odd-MIF and Δ 200 Hg for even-MIF, may specifically trace reactions and determine the contribution of distinct endmembers. For example, in forest ecosystems, Hg II in precipitation shows predominantly positive odd-MIF and even-MIF and negative values in MDF. − Soil and forest biomasses display primarily negative odd-MIF and MDF, and insignificant even-MIF, ,,− while atmospheric Hg 0 vapor shows negative odd-MIF, positive MDF, and slightly negative even-MIF (only around −0.05‰ in remote sites). ,− Given the anthropogenic emissions and biomass burning, the atmosphere in Southeast Asia showed the slightly more positive in odd-MIF , (Δ 199 Hg = −0.13 ± 0.08‰ and Δ 201 Hg = −0.12 ± 0.08‰ − ) and comparable even-MIF (Δ 200 Hg = −0.04 ± 0.04‰) compared to other remote sites (Δ 199 Hg = −0.20 ± 0.08‰, Δ 201 Hg = −0.17 ± 0.10‰ and Δ 200 Hg = −0.06 ± 0.04‰ ,− , ). Furthermore, most physico-chemical processes preferentially remove lighter Hg isotopes and result in the heavier isotopes remaining in the residual. ,, While microbial reduction does not significantly affect odd-MIF, , abiotic oxidation by natural organic matter (NOM) in the dark gives a positive shift in odd-MIF of the residual pool of Hg 0 . , Moreover, abiotic dark reduction and organosulfur-mediated photoreduction produce a positive odd-MIF shift in the product Hg 0 , albeit the MIF is induced through different mechanisms, , whereas the organic matter-mediated photoreduction driven preferentially by O-donor groups cause a negative odd-MIF shift in the product Hg 0 . , …”

Section: Introductionmentioning

confidence: 98%

Quantification of Atmospheric Mercury Deposition to and Legacy Re-emission from a Subtropical Forest Floor by Mercury Isotopes

et al. 2021

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Air–soil exchange of elemental mercury vapor (Hg0) is an important component in the budget of the global mercury cycle. However, its mechanistic detail is poorly understood. In this study, stable Hg isotopes in air, soil, and pore gases are characterized in a subtropical evergreen forest to understand the mechanical features of the air–soil Hg0 exchange. Strong HgII reduction in soil releases Hg0 to pore gas during spring–autumn but diminishes in winter, limiting the evasion in cold seasons. Δ199Hg in air modified by the Hg0 efflux during flux chamber measurement exhibit seasonality, from −0.33 ± 0.05‰ in summer to −0.08 ± 0.05‰ in winter. The observed seasonal variation is caused by a strong pore-gas driven soil efflux caused by photoreduction in summer, which weakens significantly in winter. The annual Hg0 gross deposition is 42 ± 33 μg m–2 yr–1, and the corresponding Hg0 evasion from the forest floor is 50 ± 41 μg m–2 yr–1. The results of this study, although still with uncertainty, offer new insights into the complexity of the air-surface exchange of Hg0 over the forest land for model implementation in future global assessments.