David Blair is a third year PhD student studying the "Biogeochemical behaviour of Pb and Hg in peat and forest soils; degradation of organic matter in peat and forest soils" at the University of Edinburgh.
Atmospheric deposition, especially since the onset of the Industrial Revolution, has resulted in markedly increased inventories of pollutants such as lead (Pb) and mercury (Hg) in UK peats and organic-rich soils. Moreover, ombrotrophic peat bogs, which receive all their nutrients (and pollutants) from the atmosphere, provide a historical record of deposition for elements, e.g. Pb and Hg, that are immobilised within the peat solid phase. Although the same may be true for some organic-rich soil systems, vertical transport of Pb within forest soil systems can perturb temporal records. Nevertheless, forest soils still act as a sink for atmospheric Pb albeit that some is transferred to deeper horizons of the soil. The longer term picture may, however, be affected by processes which lead to degradation of the OM, e.g. warming and drying out of peat bogs and surface soils as a result of climatic change and/or natural diagenetic processes, since these may release Pb into the aqueous phase and volatile Hg species into the atmosphere. The associations and speciation of Pb and Hg within peats and organic-rich soils are not well understood but are central to an improved understanding of the potential for release of such pollutants into the hydrosphere and atmosphere.
This project has focused on Pb and Hg associations and speciation within near-surface peats and forest soils. Due to the organic-rich nature of the solid matrix in such systems, research in this area has typically focused on solid phase metal-organic matter interactions. Recent laboratory experiments, however, have shown that, in addition to organic matter, iron (Fe) oxides may also play an important role in determining the association of metals within American forest soils (e.g. Schroth et al., 2008). For our research work, two ombrotrophic peat bogs, one minerotrophic bog and one forest soil site, all in central or south-eastern Scotland, were selected. A common characteristic was the high organic matter content of the solid phase matrix but the minerotrophic bog and especially the forest soil had higher mineral contents (~10% and ~30-60% of total solids, respectively).
Preliminary work has shown that a large proportion (~40-99%, depending on vertical depth) of Pb in ombrotrophic peat was extracted in association with humic substances. The lower values for Pb-humic association were obtained for the near-surface regions where there was intact plant material which had not yet undergone the full humification process. Fe was also associated with the humic material but no Fe-rich mineral nodules were detected by SEM-EDX. In addition, it has been shown that Pb associations in the forest soils differed from those in the peat bog. With respect to Hg, between-site differences in speciation were observed. For example, Hg2+ represents <25% of the total Hg species in the top 10 cm of ombrotrophic peat but >50% of the total present in forest soil.
As outlined above, pollutant sinks such as peat bogs and forest soils may undergo long-term physical and chemical transformations that could cause previously sequestered pollutants to be released into other environmental compartments. Better predictions of the ultimate fate of heavy metal contaminants such as Pb and Hg may be made through thorough understanding of the forms in which they may be released. In this way, the risks to ecosystem and indeed human health posed by such Pb and Hg reservoirs may be more accurately assessed.
David Blair, School of Geosciences, University of Edinburgh.
A.W. Schroth, B.C. Bostick, J.M. Kaste, A.J. Friedland., Lead sequestration and species redistribution during soil organic matter decomposition, Environ. Sci. Technol. 2008;42:3627-33.