Mercury isotopes in a forested ecosystem: Implications for air-surface exchange dynamics and the global mercury cycle
Mercury isotopes in a forested ecosystem: Implications for air-surface exchange dynamics and the global mercury cycle
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DOI：
10.1002/gbc.20021
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发表时间：
2013-01-01
影响因子：
5.2
通讯作者：
Zak, Donald R.
中科院分区：
文献类型：
--
作者：
Demers, Jason D.;Blum, Joel D.;Zak, Donald R.
森林调节大气和陆地生态系统之间汞（Hg）的生物地球化学循环；然而，我们对这些过程的理解仍然存在许多差距。我们在这项研究中的目标是表征森林内的汞同位素组成，并利用自然丰度的稳定汞同位素来追踪汞的来源并揭示汞循环的机制。我们在美国威斯康星州东北部的莱茵兰德 FACE 实验中，量化了 10 年生白杨森林中树叶、森林地面、矿质土壤、降水以及大气中和土壤逃逸中的总气态汞 (THg(g)) 的稳定汞同位素组成。相对于森林生态系统组成部分之间的差异，大气中 CO2 和 O-3 浓度增加对汞同位素组成的影响很小。降水样品的 Delta Hg-202 值为 -0.74 至 0.06 ppm，Delta Hg-199 值为 0.16 至 0.82 ppm。大气 THg(g) 的 Delta Hg-202 值为 0.48 至 0.93 ppm，Delta Hg-199 值为 -0.21 至 -0.15 ppm。叶子对 THg(g) 的吸收导致 Delta Hg-202 值发生较大变化（-2.89 ppm）；叶子显示 Delta Hg-202 值为 -2.53 至 -1.89 ppm，Delta Hg-199 值为 -0.37 至 -0.23 ppm。森林地面样品的 Delta Hg-202 值为 -1.88 至 -1.22%，Delta Hg-199 值为 -0.22 至 -0.14%。汞同位素区分了土壤中汞的地质来源和大气来源的汞，并表明降水汞仅占大气汞输入的 16%。森林地面逃逸汞的同位素组成与大气 THg(g) 相似；然而，甚至同位素（Delta Hg-200 和 Delta Hg-204）的 Delta Hg-202 值和 MIF 也存在系统性差异。森林地面的汞逃逸可能是由于大气 THg(g) 的空气-表面交换引起的，但不是土壤中遗留汞的排放，也不是湿沉积的再排放。这意味着森林土壤中有净大气 THg(g) 沉积。此外，在叶子吸收和大气THg(g)的空气-表面交换过程中，汞同位素的MDF导致汞以非常正的δHg-202值释放到大气中，这是模拟全球汞循环同位素平衡的关键信息，并且可能表明大气汞池的停留时间比之前认识的要短。
Forests mediate the biogeochemical cycling of mercury (Hg) between the atmosphere and terrestrial ecosystems; however, there remain many gaps in our understanding of these processes. Our objectives in this study were to characterize Hg isotopic composition within forests, and use natural abundance stable Hg isotopes to track sources and reveal mechanisms underlying the cycling of Hg. We quantified the stable Hg isotopic composition of foliage, forest floor, mineral soil, precipitation, and total gaseous mercury (THg(g)) in the atmosphere and in evasion from soil, in 10-year-old aspen forests at the Rhinelander FACE experiment in northeastern Wisconsin, USA. The effect of increased atmospheric CO2 and O-3 concentrations on Hg isotopic composition was small relative to differences among forest ecosystem components. Precipitation samples had delta Hg-202 values of -0.74 to 0.06 parts per thousand and Delta Hg-199 values of 0.16 to 0.82 parts per thousand. Atmospheric THg(g) had delta Hg-202 values of 0.48 to 0.93 parts per thousand and Delta Hg-199 values of -0.21 to -0.15 parts per thousand. Uptake of THg(g) by foliage resulted in a large (-2.89 parts per thousand) shift in delta Hg-202 values; foliage displayed delta Hg-202 values of -2.53 to -1.89 parts per thousand and Delta Hg-199 values of -0.37 to -0.23 parts per thousand. Forest floor samples had delta Hg-202 values of -1.88 to -1.22% and Delta Hg-199 values of -0.22 to -0.14%. Mercury isotopes distinguished geogenic sources of Hg and atmospheric derived sources of Hg in soil, and showed that precipitation Hg only accounted for similar to 16% of atmospheric Hg inputs. The isotopic composition of Hg evasion from the forest floor was similar to atmospheric THg(g); however, there were systematic differences in delta Hg-202 values and MIF of even isotopes (Delta Hg-200 and Delta Hg-204). Mercury evasion from the forest floor may have arisen from air-surface exchange of atmospheric THg(g), but was not the emission of legacy Hg from soils, nor re-emission of wet-deposition. This implies that there was net atmospheric THg(g) deposition to the forest soils. Furthermore, MDF of Hg isotopesduring foliar uptake and air-surface exchange of atmospheric THg(g) resulted in the release of Hg with very positive delta Hg-202 values to the atmosphere, which is key information for modeling the isotopic balance of the global mercury cycle, and may indicate a shorter residence time than previously recognized for the atmospheric mercury pool.