The role of afforestation species as a driver of Hg accumulation in organic horizons of forest soils from a Mediterranean mountain area in SW Europe.

Forest areas are a primary sink of atmospheric mercury (Hg) within terrestrial ecosystems, whereas forest vegetation plays a key role in atmospheric Hg transfer to soil horizons. This study assessed variations in total Hg contents (HgT) and accumulation (HgRes) in the soil organic horizons of a forest area in NE Portugal, where post-wildfire afforestation led to the substitution of the native deciduous species (Quercus pyrenaica) by fast-growing coniferous species (Pseudotsuga menziesii and Pinus nigra). The study also evaluated, for each species, the links between Hg contents and other biophilic elements of soil organic matter (C, N, S) present in organic subhorizons (OL, OF, OH). Mean HgT in the organic horizons of the different tree species follow the sequence: P. nigra (88 µg kg-1) < Q.pyrenaica (101 µg kg-1)

Comment on: "A novel approach to peatlands as archives of total cumulative spatial pollution loads from atmospheric deposition of airborne elements complementary to EMEP data: Priority pollutants (Pb, Cd, Hg)" by Ewa Miszczak, Sebastian Stefaniak, Adam Michczynski, Eiliv Steinnes and Irena Twardowska.

A recent paper by Miszczak et al. (2020) examines metal contamination of mires in Poland and Norway. The authors conclude that lead (Pb) records in ombrotrophic peatlands cannot be used to reconstruct the chronological history of anthropogenic activities due to post-depositional mobility of the metal. We contest this general conclusion which stands in contrast with a significant body of literature demonstrating that Pb is largely immobile in the vast majority of ombrotrophic peatlands. Our aim is to reaffirm the crucial contribution that peat records have made to our knowledge of atmospheric Pb contamination. In addition, we reiterate the necessity of following established protocols to produce reliable records of anthropogenic Pb contamination in environmental archives.

Mercury accumulation in upland acid forest ecosystems nearby a coal-fired power-plant in southwest Europe (Galicia, NW Spain).

This study was carried out to determine total Hg concentrations (HgT) in acid soils and main plant species in forest ecosystems located in the river Sor catchment, which is located 20 km to the NE of the biggest coal-fired power-plant in southwestern Europe (Galicia, NW Spain). Mercury enrichment factors and Hg inventories were also determined in the soils, which were regularly sampled between 1992 and 2001. The presence of elemental Hg was estimated by simple thermal desorption at 105 degrees C. The highest HgT concentrations occurred in upper soil layers (O and A horizons) with values up to 300 ng g(-1). HgT decreased with depth, achieving the lowest values in the bottommost horizons (i.e. the soil parent material, <6 ng g(-1)), except in podzolic soils. A similar trend occurred for Hg enrichment factors (HgEF) which showed values from 40 to 76 in topsoils. Upper soil mineral horizons (A or AB) made the largest contribution (>50%) to the HgT inventory despite showing lower concentrations than the organic horizons. The role of vegetation in capturing atmospheric Hg and subsequent deposition to soil agrees with the sequence of HgT in plant material: wood

Atmospheric Pb deposition in Spain during the last 4600 years recorded by two ombrotrophic peat bogs and implications for the use of peat as archive.

Two ombrotrophic peat bogs in Northwestern Spain provided a history of 4600 years of Pb accumulation. Highest Pb concentrations (84-87 microg g(-1)) were found near the bogs' surface, but there were also other significant peaks (6-14 microg g(-1)), indicating pre-industrial atmospheric pollution. The enrichment factors (EFs) in both cores show a remarkably similar record. Atmospheric Pb pollution dates back to at least approximately 2500 years ago, reaching a first maximum during the Roman period. For the last 300 years, Pb EFs significantly increased due to industrial development, but the uppermost samples of the bogs show decreasing Pb EFs, probably due to the phasing out of leaded gasoline. These results are also supported by 206Pb/207Pb isotope ratios, as they continuously decrease from ca. 3000 BP until 2000 BP (from 1.275 at 4070 14C years BP to 1.182), indicating the growing importance of nonradiogenic Pb released from Iberian ores by ancient mining. Peat samples at a 3-5-cm depth are even less radiogenic (206Pb/107Pb = 1.157), indicating the strong influence of leaded gasoline. Despite the common history shared by the two bogs, striking differences were found for Pb enrichment, whether this was calculated by normalising to the Pb/Ti ratio of the upper continental crust or to the Pb/Ti ratios of peats from pre-anthropogenic times. This effect seems to be related to differences in Ti accumulation in both bogs, possibly due to physical fractionation of the airborne dust during wind transport. Enrichment has to be carefully considered when comparing the results obtained for different bogs, since our results suggest that normalising to crustal proportions is meaningless when the bulk of the deposition in an area is strongly influenced by short- and medium-range dust transport.

Mercury in a spanish peat bog: archive of climate change and atmospheric metal deposition

A peat core from a bog in northwest Spain provides a record of the net accumulation of atmospheric mercury since 4000 radiocarbon years before the present. It was found that cold climates promoted an enhanced accumulation and the preservation of mercury with low thermal stability, and warm climates were characterized by a lower accumulation and the predominance of mercury with moderate to high thermal stability. This record can be separated into natural and anthropogenic components. The substantial anthropogenic mercury component began approximately 2500 radiocarbon years before the present, which is near the time of the onset of mercury mining in Spain. Anthropogenic mercury has dominated the deposition record since the Islamic period (8th to 11th centuries A.D.). The results shown here have implications for the global mercury cycle and also imply that the thermal lability of the accumulated mercury can be used not only to quantify the effects of human activity, but also as a new tool for quantitative paleotemperature reconstruction.