Environmental Studies Program, Dartmouth College, Hanover, NH, 03755, USA.
Earth and Atmospheric Sciences Department, Cornell University, Ithaca, NY, 14850, USA.
Environ Sci Pollut Res Int. 2017 Dec;24(36):27994-28005. doi: 10.1007/s11356-017-0356-9. Epub 2017 Oct 8.
Mercury (Hg) is a globally distributed pollutant trace metal that has been increasing in terrestrial environments due to rising anthropogenic emissions. Vegetation plays an important role in Hg sequestration in forested environments, but increasing tree removal for biofuels and wood products may affect this process. The long-term effect of clear-cutting on forest soil Hg remains uncertain, since most studies are limited to measuring changes for < 10 years following a single harvest event. The chronosequence approach, which substitutes space for time using forest stands of different ages since clear-cutting, allows for investigation of processes occurring over decades to centuries. Here, we utilized three clear-cut forest soil chronosequences across the northeastern USA to understand Hg accumulation and retention over several decades. Total Hg concentrations and pools were quantified for five soil depth increments along three chronosequences. Our results showed Hg concentrations and pools decreased in the initial 20 years following clear-cutting. Mineral soil Hg pools decreased 21-53% (7-14 mg m) between 1-5-year-old stands and 15-25-year-old stands but mineral soil Hg pools recovered in 55-140-year-old stands to similar values as measured in 1-5-year-old stands. Our study is one of the first to demonstrate a decrease and recovery in Hg pool size. These changes in Hg did not correspond with changes in bulk density, soil C, or pH. We utilized a simple two-box model to determine how different Hg fluxes affected organic and mineral soil horizon Hg pools. Our simple model suggests that changes in litterfall and volatilization rates could have caused the observed changes in organic horizon Hg pools. However, only increases in leaching could reproduce observed decreases to mineral soil Hg pools. Further studies are needed to determine the mechanism of Hg loss from forest soils following clear-cutting.
汞 (Hg) 是一种在全球范围内分布的污染物痕量金属，由于人为排放的增加，在陆地环境中的含量不断增加。植被在森林环境中汞的固定中起着重要作用，但为生物燃料和木制品而增加的树木采伐可能会影响这一过程。由于大多数研究仅限于在单次收获事件后测量 <10 年的变化，因此，对皆伐对森林土壤汞的长期影响仍不确定。利用不同年龄的森林林分代替时间的定年序列方法，可以研究几十年来发生的过程。在这里，我们利用美国东北部的三个皆伐森林土壤定年序列来了解几十年内汞的积累和保留情况。我们对三个定年序列中的五个土壤深度增量进行了总汞浓度和储量的定量测量。结果表明，在皆伐后的最初 20 年内，汞浓度和储量均有所下降。在 1-5 年生林分和 15-25 年生林分之间，矿物土壤汞库减少了 21-53%（7-14mg m），但在 55-140 年生林分中，矿物土壤汞库恢复到与 1-5 年生林分相似的水平。我们的研究是第一个证明汞库大小减少和恢复的研究之一。这些汞的变化与容重、土壤 C 和 pH 没有对应关系。我们利用一个简单的两箱模型来确定不同的汞通量如何影响有机和矿物土壤层的汞库。我们的简单模型表明，凋落物和挥发速率的变化可能导致了有机层汞库中观察到的变化。然而，只有淋溶的增加才能解释观察到的矿物土壤汞库的减少。需要进一步的研究来确定皆伐后森林土壤中汞损失的机制。
