Regionalized concentrations and fingerprints of polycyclic aromatic hydrocarbons (PAHs) in German forest soils
Introduction
Polycyclic aromatic hydrocarbons (PAHs) are organic compounds which are released to the environment by combustion processes involving biomass or fossil fuels. Due to their persistency and recalcitrance as well as the carcinogenic potential of some homologues, their fate in different compartments of the global ecosystem has been intensely studied (e.g. Jones and de Voogt, 1999, Wilcke, 2000, Srogi, 2007, Ravindra et al., 2008, Křumal et al., 2013).
While relatively high deposition rates of PAHs are expected in vicinity to emitters, especially the low-molecular-weight homologues with enhanced volatility show a significant potential for long-range-transport (e.g. Hung et al., 2005). Consequently, a relatively large number of studies have evaluated PAH pollution in urban environments (e.g. Mielke et al., 2001, Aichner et al., 2007, Liu et al., 2010; and references therein). Studies from more remote areas mostly concentrate on grassland or agricultural areas (e.g. Heywood et al., 2006, Maliszewska-Kordybach et al., 2008, Plaza-Bolaños et al., 2012), while data from forest sites are in many cases only available from selective spots and extensive large-scale studies are relatively rare (Krauss et al., 2000, Weiss et al., 2000, Desaules et al., 2008, Holoubek et al., 2009, Aichner et al., 2013). Soils appear to act as a sink for PAHs, due to the high affinity of those compounds to soil organic matter (SOM) (Sweetman et al., 2005). At forest stands, this effect is enhanced by the canopy scavenging of trees, which increase pollutant fluxes from air to soil, e.g. by litter fall (Horstmann and McLachlan, 1998). Once in the soil, PAHs are susceptible to re-volatilization, reversible sorption to SOM, translocation to deeper soil horizons, mineralization, and aging, i.e. irreversible binding to SOM which decreases bioavailability (Brorström-Lundén and Löfgren, 1998, Cousins et al., 1999, Alexander, 2000, Haritash and Kaushik, 2009, Luo et al., 2012).
Recently, we analysed persistent organic pollutants (POPs), in O horizons on 474 forest stands distributed over Germany in 16 × 16 km grid (Aichner et al., 2013). The results clearly showed that maximum concentrations could be attributed to specific present or historic sources for most compound groups, which included PAHs, polychlorinated biphenyls (PCBs), and several pesticides. For this follow-up study, we further analysed PAH concentrations in the mineral soil, thus adding up to totally three sampling depths (Of/Oh horizon; mineral soil: 0–5 cm and 5–10 cm) and included a more in-depth evaluation of relative PAH abundances. We aim to a) evaluate if specific source-regions are identifiable based on PAH patterns, and b) decipher the processes behind potential variability in depth distribution, considering both PAH fingerprints and concentrations.
Section snippets
Study area
Germany extends between ca. 5.86 and 15.04 °E and 47.27 and 55.06 °N, covering a total area of 357,021 km2. It can be segregated in four major topographic units: the Northern German Lowlands, the Central German Uplands, the Alpine Foreland of southern Germany and the Alps in the very south with the Zugspitze as highest point (2962 m) (Fig. 1a). About 30% of the total area is covered by forest (Fig. 1b).
Due to its location in the influence of the Westerlies and the Gulf Stream, the climate is
PAH concentrations
Sum of total 16 EPA-PAH concentrations range from 105 to 14,889 ng/g d.w. in the O horizon (Aichner et al., 2013), from 20 to 9038 ng/g d.w. and from 7 to 4424 ng/g d.w. in the mineral topsoil (0–5 cm and 5–10 cm, respectively) (Table 1, Fig. 2). Thus, average concentrations are decreasing with depth. Nevertheless, ca. 25% of sampling spots show similar or higher concentrations in the mineral soil compared to the organic layer.
In the organic horizon, highest concentrations were found primarily
Influence of humus-type on depth distribution
At most sampling sites PAH concentrations are decreasing with depth (Table 1), while some plots show higher concentrations in the mineral soil compared to the organic layer. Enrichment factors (EF) were calculated as ratios between the PAH concentrations in the lower and the upper soil horizon (Krauss et al., 2000); i.e. the mineral topsoil (0–5 cm) to the O horizon. Fig. 5 illustrates average enrichment factors of PAHs classified by the number of benzene-rings and the humus types mull, moder
Conclusions
In this study, we found clear spatial trends concerning concentrations and fingerprints of PAHs in German forest soils. The spatial distributions of PAH patterns are comparable in different sampling depths while concentrations show some variability between the O horizons and the mineral soils. Generally, PAH concentrations are decreasing with depths at most sampling sites but enrichment factors are dependent from humus type. At mull stands relatively high enrichment factors were measured which
Acknowledgements
This study was conducted within the German National Forest Soil Inventory (BZE II – Bodenzustandserhebung Wald II) and was enabled by the co-operation between the federal and state authorities for forest and environment. Specifically, forest soil samples were taken by the authorities of the German states and provided for this study. We gratefully thank Katja Kaminski and Dino Berners for processing the samples in the BAM laboratory. The authors thank for fruitful discussions with the colleagues
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