A strong enrichment of potentially toxic elements (PTEs) in Nord-Trøndelag (central Norway) forest soil

Science of the Total Environment 536 (2015) 130–141

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

A strong enrichment of potentially toxic elements (PTEs) in Nord-Trøndelag (central Norway) forest soil C. Reimann a,⁎, K. Fabian a, J. Schilling a, D. Roberts a, P. Englmaier b a b

Norges geologiske undersøkelse (NGU), P.O. Box 6315 Sluppen, N-7491 Trondheim, Norway Faculty of Life Science, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria

H I G H L I G H T S • • • •

Concentrations of 53 elements in soil O and C horizons are compared. Several PTEs are strongly enriched in Nord-Trøndelag soil O horizons. Natural processes are responsible for the enrichment of PTEs in the soil O horizon. Weathering, lithology & the impact of vegetation control topsoil element composition.

a r t i c l e

i n f o

Article history: Received 23 April 2015 Received in revised form 1 July 2015 Accepted 6 July 2015 Available online xxxx Editor: F.M. Tack Keywords: Vegetation Soil horizons Enrichment factor Top/Bot ratio

a b s t r a c t Analysis of soil C and O horizon samples in a recent regional geochemical survey of Nord-Trøndelag, central Norway (752 sample sites covering 25,000 km2), identified a strong enrichment of several potentially toxic elements (PTEs) in the O horizon. Of 53 elements analysed in both materials, Cd concentrations are, on average, 17 times higher in the O horizon than in the C horizon and other PTEs such as Ag (11-fold), Hg (10-fold), Sb (8-fold), Pb (4-fold) and Sn (2-fold) are all strongly enriched relative to the C horizon. Geochemical maps of the survey area do not reflect an impact from local or distant anthropogenic contamination sources in the data for O horizon soil samples. The higher concentrations of PTEs in the O horizon are the result of the interaction of the underlying geology, the vegetation zone and type, and climatic effects. Based on the general accordance with existing data from earlier surveys in other parts of northern Europe, the presence of a locationindependent, superordinate natural trend towards enrichment of these elements in the O horizon relative to the C horizon soil is indicated. The results imply that the O and C horizons of soils are different geochemical entities and that their respective compositions are controlled by different processes. Local mineral soil analyses (or published data for the chemical composition of the average continental crust) cannot be used to provide a geochemical background for surface soil. At the regional scale used here surface soil chemistry is still dominated by natural sources and processes. © 2015 Elsevier B.V. All rights reserved.

1. Introduction All life on Earth is supported by the thin cover of soils, plants and water on our planet, often named the “Critical Zone” (Brantley et al., 2007). Soils form via physical, chemical and biological weathering from rocks, glacial and aeolian sediments, a process that leads to an almost complete change of mineralogy. By growing on soils plants will themselves alter the chemical composition of soils via exclusion of certain elements from uptake or their accumulation or even hyperaccumulation (e.g., Brooks, 1972, 1998; Baker, 1981). Furthermore, plants shed their ⁎ Corresponding author. E-mail addresses: [email protected] (C. Reimann), [email protected] (K. Fabian), [email protected] (J. Schilling), [email protected] (D. Roberts), [email protected] (P. Englmaier).

http://dx.doi.org/10.1016/j.scitotenv.2015.07.032 0048-9697/© 2015 Elsevier B.V. All rights reserved.

leaves and die at some point. Decaying plant material leads to the development of litter and forms the O horizon of a complete soil profile in forested areas. Water acts as a weathering agent and transport medium for elements between the different spheres of the ecosystem. Geochemical and biological processes in the critical zone provide energy, nutrients and resources for the sustenance of life. Humans interact with the natural element cycles via land-use and contamination, often without any clear perception of the long-term consequences. Vernadsky (1883–1945) and Goldschmidt (1888–1947) first recognised that to understand the geochemical processes between lithosphere, pedosphere, biosphere, and hydrosphere a holistic approach is needed in terms of sample materials, elements covered, size of area, geology, topography, climate and vegetation. Since their time the holistic approach was rarely realised (notable exceptions being the Kola Ecogeochemistry Project — Reimann et al., 1998 — and the Eastern

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Barents Region Project — Salminen et al., 2004), mostly because collecting the required samples from a sizeable area and analysing them is expensive and needs tight quality control. Rather, progress was achieved in different disciplines of science most often on local scales, lacking the much needed multi-disciplinary approach at the continental to global scale. In 1937, V.M. Goldschmidt stated that both the B and the O horizon natural soils are important geochemical barriers where a fractionation especially of the trace elements according to their physicochemical characteristics takes place during soil forming processes. According to Goldschmidt (1937), many elements (Be, Co, Ni, Zn, Ge, As, Cd, Sn, Pb, Tl, Ag and Au are shown in his original graphic) tend to become enriched in the O horizon due to plant uptake and litterfall. The amount of enrichment at the Earth's surface depends on binding characteristics and the kinetics of build up and decay of the organic layer as well as on bioproductivity (e.g., Reimann et al., 2000, 2007). This fact is widely neglected in today's environmental sciences and enrichments of these elements in the O horizon or topsoils are usually described as contamination (i.e. man-made — see discussion in Reimann and Caritat, 2000, 2005). The fact that plants can take up and enrich many trace elements has been used successfully by exploration geochemists in the search for mineral deposits (e.g., Dunn, 2007). Pioneering work in biogeochemical exploration was carried out by Kovalevsky (1974, 1987) in Russia. In general, Russian scientists were the first to recognise the impact of the biosphere on element cycling at the Earth's surface (Vernadsky, 1926) — a topic that is still (or again?) at the very forefront of science (e.g., Amundson et al., 2007). The observation that climate and vegetation, in combination with the geological setting (rock types present), influence element concentrations as measured at the Earth's surface led to the development of the concept of landscape geochemistry (Perel'man, 1961,

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1966). Fortescue (1980, 1992) introduced this concept to western literature. In plant ecology a comparable concept has been recently discussed under the term “biogeochemical niche” (Sardans and Peñuelas, 2014). Human impact on the environment at the continental and global scale has been a much discussed topic during the last 30 years. Longrange transport (LRT), especially of the so-called “heavy metals” via the atmosphere (e.g., Lantzy and MacKenzie, 1979) has been demonstrated using a variety of different sample materials, e.g., snow and ice cores (e.g., Boutron et al., 1994), and terrestrial moss (e.g., Rühling and Steinnes, 1998). A consensus over the relative impact of anthropogenic versus natural element sources of metals in soils in remote settings, however, was and is still missing (see, e.g., the discussion in Rasmussen, 1998; Reimann et al., 2009a,b; Steinnes, 2009). When comparing the geochemistry of a soil O horizon sample with the geochemistry of its mineral soil layers, analytical results usually differ significantly (see, e.g., Reimann et al., 1998, or Salminen et al., 2004, for data on a large regional scale). It is intensely debated whether such differences, especially for potentially toxic elements (PTEs), are primarily due to human impact (contamination — e.g., Steinnes et al., 1989; Steinnes and Njåstad, 1995; Shotyk, 2008), differences in the geological substrate or due to biogenic processes as originally suggested by Goldschmidt (1937) and more recently repeatedly demonstrated by Reimann et al. (2001, 2007, 2009a). Many authors (e.g., Facchinelli et al., 2001; Massas et al., 2009; Yang et al., 2009) have suggested that the calculation of the ratio of element concentrations in a topsoil (e.g., the A or O horizons) over a deeper (i.e. less contaminated) soil layer such as the soil B- or C-horizon from the same location may provide a better indication of surface contamination than the raw data alone. This approach has been criticised by Reimann and Caritat (2000, 2005). In order to obtain the sought after proof for a major anthropogenic impact on the surface environment,

Fig. 1. Location and bedrock geology of the studied area. The thin dashed violet lines mark county borders, the fat violet line the border to Sweden. Locations named in the text are shown in black. GOC = Grong Olden Culmination.

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the use of several more complex ratios such as the “Igeo-Index” (Müller, 1979), the “contamination factor CF” (Hakanson, 1980) and “enrichment factors (EFs)” (e.g. Atteia, 1994; Loska et al., 1997; Blaser et al., 2000; Manta et al., 2002; Singh et al., 2010) have been promoted. All these approaches require direct comparison between data on topsoils and on deeper mineral soil layers or the continental crust and neglect any impact of the biosphere or soil formation processes on the geochemistry of the Earth's surface. Many of these papers focus on a very limited range of PTEs and the a priori hypothesis that all enrichments of these elements at the surface must be due to LRT instead of studying as many as possible different elements in various climate and vegetation zones in order to unravel processes and likely sources from a careful comparison of the behaviour of different elements, including the major nutrients, as suggested in this contribution.

As part of a reconnaissance geochemical mapping programme for potential ore deposits in the Nord-Trøndelag and Fosen regions of central Norway, O and C horizon soil samples were collected and analysed for 53 elements (Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Pd, Pt, Rb, Re, S, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn, and Zr) in an aqua regia extraction. Sampling was based on a 6 × 6 km grid, i.e., 1 sample site/36 km2, and about 25,000 km2 were covered with 752 samples of each material. The study area, as shown in Fig. 1, covers the whole of Nord-Trøndelag county and the northernmost part of Sør-Trøndelag to the south. However, for purposes of simplification hereafter the investigated area is referred to as ‘Nord-Trøndelag’. During the early 1980s a stream sediment survey of the same area was carried out at a sample density of approximately 1 site per 4 km2 (Sæther, 1987;

Table 1 Analytical results of the soil survey Nord-Trøndelag 2013/14: the analytical programme, detection limits (DL), number of samples below detection (N b DL) and the results for both sample materials are summarised (minimum (MIN)–median (MED, in bold)–maximum (MAX) values) in the soil O and C horizons. O horizon

C horizon

Element

Unit

DL

N b DL

MIN

MED

MAX

DL

N b DL

MIN

MED

MAX

Ag Al As Au B Ba Be Bi Ca Cd Ce Co Cr Cs Cu Fe Ga Ge Hf Hg In K La Li Mg Mn Mo Na Nb Ni P Pb Pd Pt Rb Re S Sb Sc Se Sn Sr Ta Te Th Ti Tl U V W Y Zn Zr

mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg

0.002 100 0.3 0.0005 0.5 0.1 1 0.02 100 0.05 0.01 0.01 0.1 0.005 0.01 10 0.1 0.02 0.006 0.001 0.02 100 0.01 0.1 10 1 0.01 10 0.01 0.1 10 0.01 0.01 0.002 0.1 0.001 50 0.02 0.15 0.15 0.15 0.5 0.001 0.2 0.005 1 0.01 0.01 0.2 0.01 0.001 0.1 0.01

0 0 110 445 12 0 747 0 0 0 2 0 0 0 0 0 0 54 40 0 671 0 0 138 0 0 0 0 0 0 0 0 748 739 0 605 13 0 9 16 0 0 16 753 4 0 0 0 28 0 0 0 0

0.0288 304 b0.3 b0.0005 b0.5 6.54 b1 0.0229 403 0.055 0.483 0.206 1.09 0.0262 2.74 376 0.119 b0.02 b0.006 0.0587 b0.02 282 0.213 b0.1 302 7.45 0.0885 28 0.0323 0.917 200 3.10 b0.01 b0.002 0.839 b0.001 b50 0.0542 b0.15 b0.15 b0.15 4.44 b0.001 b0.2 b0.005 21 0.0193 0.0158 b0.2 0.0134 0.102 3.38 0.0704

0.198 2030 0.810 b0.0005 1.82 36 b1 0.131 2681 0.512 5.11 1.48 3.03 0.266 7.86 3003 0.792 0.137 0.0232 0.214 b0.02 851 2.38 0.232 1401 58 0.393 153 0.419 3.23 775 27 b0.01 b0.002 4.18 b0.001 1441 0.330 0.813 0.897 0.749 30 0.0108 b0.2 0.197 161 0.0795 0.200 2.91 0.0790 1.09 38 0.767

5.87 43,010 112 0.0240 6.50 286 4.71 0.470 21,802 1.91 404 93 1112 4.31 305 183,300 11 0.635 0.211 1.000 0.0512 2527 174 47 35,750 5254 11 789 6.78 361 2141 3511 0.0183 0.0237 30 0.0107 4008 123 11 5.27 10 97 0.0801 0.211 7.64 4437 0.553 24 80 2.98 65 247 11

0.002 100 0.1 0.0002 1.0 0.5 0.1 0.02 100 0.01 0.1 0.1 0.5 0.02 0.10 300 0.1 0.1 0.02 0.01 0.02 50 0.5 0.1 50 1 0.01 10 0.02 0.1 10 0.01 0.03 0.004 0.1 0.001 20 0.02 0.1 0.5 0.1 0.5 0.05 0.20 0.1 10 0.02 0.05 2 0.05 0.01 0.1 0.1

58 0 73 192 529 0 191 25 27 118 2 24 5 1 5 10 0 650 94 67 501 3 4 22 10 0 1 26 1 14 0 0 750 715 0 647 28 89 4 505 4 2 751 750 5 0 89 3 8 444 0 0 1

b0.002 321 b0.1 b0.0002 b1 b0.5 b0.1 b0.02 b100 b0.01 b0.1 b0.1 b0.5 b0.02 b0.1 b300 b0.1 b0.1 b0.02 b0.01 b0.02 b50 b0.5 b0.1 b50 1.28 b0.01 b10 b0.02 b0.1 20 0.43 b0.03 b0.004 0.175 b0.001 b20 b0.02 b0.1 b0.5 b0.1 b0.5 b0.05 b0.2 b0.1 39 b0.02 b0.05 b2 b0.05 0.205 0.173 b0.1

0.0134 11,180 1.48 0.0007 b1 15 0.206 0.092 1200 0.031 23 6.14 26 1.03 13 19,420 4.28 b0.1 0.0558 0.0206 b0.02 696 9.52 8.49 4370 167 0.416 66 1.35 14 276 6.61 b0.03 b0.004 8.13 b0.001 137 0.0421 2.17 b0.5 0.443 6.87 b0.05 b0.2 2.98 1032 0.0706 0.682 31 b0.05 4.80 27 2.50

0.4938 51,013 622 0.0711 8.7 477 3.46 5.03 13,721 0.569 150 144 1162 15 346 91,830 20 0.286 0.365 0.263 0.186 13,810 150 51 37,700 3241 24 534 8.39 752 2109 80 0.0831 0.0124 99 0.0066 3888 0.905 26 3.54 4.28 52 0.0613 1.47 25 9311 1.28 52 259 2.25 65 249 19

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Sæther et al., 2005). An additional stream water survey of this area detected only minor and local-scale signals of possible anthropogenic sources such as agriculture and mining, while the main processes determining the stream water composition were geology and, along the coast, the input of elements through sea spray (Banks et al., 2001). In a recent paper, Reimann et al. (2015b) discussed this soil composition dataset with respect to its indications of mineral resource potential and its ability to reflect the nature of the underlying geology. In the present contribution, the two sample materials are compared in terms of element concentrations and variation. The dataset is used to characterise geochemical differences between the O and C soil horizons and to understand geochemical, climatic and biological processes that govern the distribution of the elements in the two sample materials. It will be demonstrated that the elemental input via LRT plays such a subordinate contribution to the composition of the O horizon of Nord Trøndelag forest soils that it remains undetectable. 1.1. Topography, climate, vegetation, geology and mineralisation A geochemical survey of Nord-Trøndelag faces a number of challenges: over fifty percent of the area of the county is covered by lakes, bogs and fjords, twenty percent is developed or cultivated, and the rest consists of forest and mountainous areas above the treeline with limited vegetation and soil development. Though the area has been glaciated, there is no contiguous layer of till covering the county; till rather occurs in interspersed pockets. When choosing the local soil C horizon (mineral soil) as a sample material, it has to be accepted that the mineral soil will be developed from till in some areas and elsewhere from very shallow soil profiles developed directly on bedrock. Furthermore, at the end of the last glaciation in this region, i.e., at about 9000 years ago, the sea level was 120–190 m higher than it is today (Sveian and Solli, 1997, p. 51) and large deposits of marine clays occur locally up to 200 m above present-day sea level. To find suitable sample material for county-scale geochemical mapping and mineral exploration is no trivial task. Fig. 1 provides an overview of the geology of the survey area. NordTrøndelag has a long coastline facing the North Atlantic Ocean which is characterised by many islands, skerries, inlets and fjords, the most prominent and largest fjord being Trondheimsfjord in the southern half of the survey area. The border with Sweden is located ca. 150 km from the coast, also being the area with the maximum altitude in the survey area, 1200–1500 m above sea level (a.s.l.). About 38% of the area has an altitude below 300 m a.s.l., 22% an altitude above 600 m a.s.l., but only 2% lies above 900 m a.s.l. The main cities or towns within the survey area are Trondheim (c. 200,000 inhabitants) and Steinkjer (c. 20,000 inhabitants). Norway's main highway, E6, runs approximately north–south through the centre of the map area, connecting Trondheim and Steinkjer (Fig. 1). There is currently little industrial activity and no active mining (see below). The average population density in the county is low with 6 inhabitants per km2. The climate of Nord-Trøndelag is mostly cold and temperate with prominent differences between coastal and inland areas. Along the coastline the climate is humid with an average yearly precipitation of 700–1000 mm, reaching 1500–2000 mm in the mountainous parts of Fosen Peninsula and decreasing to 500–750 mm at the Swedish border. The average yearly temperature is 4–6 °C at the coast, decreasing inland towards the Swedish border to between 0 and − 2 °C. Temperature differences between summer and winter increase, on a general basis, towards the eastern, inland areas of Nord-Trøndelag. The county is part of the boreal vegetation zone. Along the coast, birch and willow dominate sparse woodland, whereas farther inland there are extensive conifer forests. Open mountainous landscapes prevail above the treeline towards Sweden. Almost half of the survey area (49%) lies above the altitude limit for commercial forestry. For detailed maps of the vegetation and climate, see Moen (1998).

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Geologically, the region forms part of the Scandinavian Caledonides, an Early- to Mid-Palaeozoic orogenic belt comprising a broad diversity of rock types ranging in age from Palaeoproterozoic to Devonian. In the project area, two major volcanogenic massive sulphide deposits were mined in former times. The Joma deposit was mined from 1972 to 1998 and 11.453 Mt of Cu–Zn ore was extracted (Sandstad et al., 2012). At the Skorovas Cu–Zn deposit, 5.6 Mt of ore was mined between 1952 and 1984. In addition, the Fosdalen Fe mine yielded 35 Mt of ore in the period from 1906 to 1997 (Sandstad et al., 2012). As shown on Fig. 1, Precambrian granitic to granodioritic orthogneisses occupies much of the area along the coast and in an E–W-trending antiformal belt, the Grong-Olden Culmination (GOC). Cambro-Ordovician ophiolites, volcanic arc complexes and associated volcanosedimentary lithologies occur towards the Swedish border. On the Fosen peninsula, the situation is complicated by intense Caledonian deformation producing what has been termed a ‘Banded Gneiss Complex’ (Möller, 1988), and also by a NE–SW-trending infolding of higher-level nappes in tight synforms comprising garnet-mica schists and amphibolites. In addition, the major, ENE–WSW-trending, Møre-Trøndelag Fault Complex crosses the peninsula and extends farther northeast across the GOC (Grønlie and Roberts, 1989; Grønlie et al., 1991; Sæther et al., 2005). A great variety of Cambro-Silurian phyllites, mica schists, calcareous schists, limestones and both felsic and mafic volcaniclastic rocks characterise the allochthons elsewhere in the study area (green colours in Fig. 1). 2. Methods A 6 × 6 km grid covering the county of Nord-Trøndelag and northern parts of Sør-Trøndelag, was used for the selection of sample sites. The exact location of each sample site within a grid cell was determined in the field based largely on accessibility, trying to come as close as practically possible to the centre of the cell. Though most sites could be reached by car, some of the sites required either lengthy walking trips or helicopter access. A Quaternary geology base map was always on hand in order to avoid sampling on top of glaciofluvial and marine deposits. All samples were taken on undeveloped land such as forested areas. The size of the survey area is approximately 25,000 km2. 2.1. Sampling and sample preparation O-horizon soil samples were collected as composite samples of 5 sub-samples, either taken with a special steel tool or cut out with a steel spade over an area of approximately 100 m2. The steel tool was used to cut out a cylindrical sample with a diameter of 10 cm and a

Table 2 Relative enrichment/depletion of elements in the soil O (O/C) horizon from NordTrøndelag (NT) compared to the Oslo transect (Reimann et al., 2007) and the South Norway (SNOR) transect (Reimann et al., 2007). na: not analysed. Elements from Table 1 that are not named here show a ratio of around 1. Higher in O horizon (O/C)

Higher in C horizon O/C

NT

Oslo

SNOR

NT

Oslo

SNOR

N=

752

40

40

N=

752.00

40.00

40.00

Cd Ag S Hg Sb Sr Pb P Ba Na Ca Sn Bi Zn Au

17 15 11 10 8 4.4 4.1 2.8 2.3 2.3 2.2 1.7 1.4 1.4 DL

8 12 10 8 8 2.4 7.6 1.9 1.8 1.2 1.7 na 4.3 1.8 7.5

31 18 10 9 21 5.1 8.7 2.5 3.1 2.5 1.3 2 5.8 2.5 4.2

Li Th V Cr Fe Ti Al Ga Ce Y Ni Co La U Sc

0.03 0.07 0.09 0.12 0.15 0.16 0.17 0.19 0.22 0.23 0.24 0.24 0.25 0.29 0.37

na 0.40 0.77 0.40 0.50 0.40 0.50 na na na 0.30 0.40 0.50 0.50 0.30

0.03 0.06 0.30 0.45 0.17 0.06 0.42 0.22 0.42 0.56 0.71 0.42 0.71 0.34 0.26

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depth of 14 cm at each sub-site; when the spade was used a square piece of soil of about 15 × 15 × 10 cm was cut out. Live plant material was removed from the top of the sample and any non-organic material was removed from the bottom so that only the uppermost 2–5 cm of the humus and litter layer was retained. The sub-samples were composited and stored in contamination-free RILSAN bags. To collect the soil C-horizon samples, a soil profile was dug at one of the O-horizon subsample sites. Whenever possible a location where soil was developed on a till pocket was chosen and here Podzol was the dominant soil type. The profile was dug down to the C horizon (median depth to top of C horizon: 30 cm, range 10–80 cm). A C horizon sample was collected at the maximum depth of each profile. At each sample site the soil pit, the vegetation and the general landscape were documented in a number of photos. The sampling tools were thoroughly cleaned (washed in a nearby stream or lake) before moving on to the next location. In total, 752 sites were sampled. All soil samples were air dried within a few days after sampling. They were subsequently sieved using a b2 mm nylon mesh (lumps were disaggregated by hand) and the passing fraction was retained for analysis. Before submission to the laboratory, all samples were randomised and a

project standard and sample duplicates were inserted such that they were not recognisable by the laboratory. 2.2. Analysis and quality control Splits of all samples were shipped to AcmeLabs (now doing business as Bureau Veritas Minerals) in Vancouver, Canada, by courier. The soil C horizon samples underwent a modified aqua regia digestion prior to analysis which consists of equal parts of concentrated ACS grade HCl and HNO3 and de-mineralised H2O. 15 g of the sieved mineral soil samples (C horizon) were digested in 90 mL of aqua regia and leached for 1 h in a hot (95 °C) water bath. After cooling, the solution was made up to a final volume of 300 mL with 5% HCl. The sample weight to solution volume ratio is 1 g per 20 mL. The solutions were analysed using a Spectro Ciros Vision emission spectrometer (ICP-AES) and a Perkin Elmer Elan 6000/9000 inductively coupled plasma mass spectrometer (ICP-MS) for 53 elements (Ag, Al, As, Au, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Fe, Ga, Ge, Hf, Hg, In, K, La, Li, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Pd, Pt, Rb, Re, S, Sb, Sc, Se, Sn, Sr, Ta, Te, Th, Ti, Tl, U, V, W, Y, Zn, and Zr). The organic O-horizon samples were treated somewhat differently. A 5 g aliquot of sample material was first leached with

Fig. 2. Notched Tukey boxplots (Tukey, 1977) showing element concentration and variation in the O (OHO) and C (CHO) soil horizons for selected elements. The notches represent a graphical test of comparability — much like the formal t-test — of the medians.

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concentrated HNO3 for 1 h and then digested in a hot (95 °C) water bath for an additional hour. After cooling, a modified aqua regia solution of equal parts of concentrated ACS grade HCl and HNO3 and de-mineralised H2O was added to each sample (6 mL/g) to leach in a hot (95 °C) water bath for 2 h. After cooling, the solution was made up to a final volume with 5% HCl and then filtered. The sample weight to solution volume ratio is 1 g per 20 mL. The solutions were analysed using a Perkin Elmer Elan 6000 inductively coupled plasma mass spectrometer (ICP-MS) for the same suite of 53 elements as above. Quality control (QC) was based on (1) random plots of sample number against analytical results for all elements, (2) X-Charts for the standard results (N = 39 for the C horizon and N = 38 for the O horizon) for all elements and (3) an evaluation of the analytical results for the duplicate pairs (N = 37 for the C horizon and N = 38 for the O horizon). Results of QC are documented in two NGU reports (Finne and Eggen, 2014; Finne et al., 2014) which can be downloaded from NGUs internet site (http://www.ngu.no/NordTrondelagFosen/). A remaining issue it that there is a large difference in the bulk density of the two sample materials. While the bulk density of C horizon soils is around 1.5 kg/dm3, the density of the O horizon is lower by at least a factor of 4–5, that will matter when deposition estimates are to be calculated. Here, however, element concentrations in dry matter are compared between the two materials. The complete datasets can also be downloaded from NGU's internet site: (http://www.ngu.no/NordTrondelagFosen/). 2.3. Data analysis The analysis of geochemical data has to take into account the closure effect (Aitchison, 1986). Significant problems with closure occur when studying bivariate plots for element concentrations within one sample material, because each data point — representing one sample — could be shifted along a straight line y = a x through this point by changing the abundance of other elements within this sample. The effect can be substantial (Filzmoser et al., 2010; Reimann et al., 2012). Calculated bivariate relations therefore have to be interpreted as being exploratory and qualitative. They cannot be quantitatively compared if they relate element concentrations within the same sample. To compare two

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independent sample materials (here O and C soil horizons) bivariate plots can be used as usual. For geochemical mapping, plotting diagrams and statistical data analysis, the software package DAS® of the Technical University of Vienna (Dutter et al., 1992) was used. This package is based on exploratory data analysis (EDA) techniques (Tukey, 1977), which are better suited for geochemical data than classical statistics. For more details see Reimann et al. (2008). 3. Results & discussion The analytical programme, detection limits, number of samples below detection and the results for both sample materials (minimum– median–maximum values) are summarised in Table 1. For a number of elements more than 50% of the analytical results were below the detection limit; in both materials this was the case for Pd, Pt, Re and Te. For the following elements more than 50% of all results were below their detection limits in the O horizon: Au, Be and In; and in the C horizon, B, Se and Ge (see Table 1). Major elements with a median concentration above 1000 mg/kg are (in sequence of decreasing median value) Fe, Ca, Al, S and Mg in the O horizon and Fe, Al, Mg, Ca and Ti in the C horizon. A cursory glance at the table reveals that for some elements, the median values between the two sample materials vary by one order of magnitude (e.g., S, which is a major element in the O horizon but not in the C horizon of the soil). 3.1. Comparison between O and C horizon soils — differences in element concentrations Table 2 compares element concentrations as measured in the O and C soil horizons via the O/C ratio. For comparison, the same ratio has been calculated for soil samples collected for two other projects, a 120 kmlong transect running through the city of Oslo (Reimann et al., 2007) and a 200 km-long transect at the southern tip of Norway (Reimann et al., 2009a,b). In Nord-Trøndelag, Cd is most enriched in the soil O horizon relative to the C horizon (on average ×17). Cadmium is also extremely enriched in the O horizon samples collected along the South Norway transect (on average ×31, Table 2). Silver, S and Hg show a more than 10-fold elevated median for the O horizon relative to the C

Fig. 3. Scattergrams for ROI (100-LOI) as a measure of organic material (the most organic soils plot towards 1) versus selected elements as analysed in the soil O horizon.

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horizon in Nord Trøndelag. The latter elements are comparably enriched in the O over the C horizon samples in the two transects in southern Norway. Lithium, Th and V, in contrast, are 10-fold enriched in the soil C over the O horizon. In the South Norway transect, Li and Th are enriched by a factor of more than 10 in the C over the O horizon. For the Oslo transect, the differences between the C and the O horizons are less pronounced. Elements enriched in the O horizon are those that either are incorporated into the plants easily and are needed as nutrients (e.g., Ca, P, S), or elements for which a mechanism exists to fix them strongly to organic material (e.g., Hg, Pb, Sn). Elements with considerably higher concentrations in the C horizon are either much more abundant in the mineral soil than required by plants (e.g., Fe) or elements the uptake of which is avoided (e.g., Li, Th, U), or are present in minerals that weather sparingly and are not transferred to the O horizon, e.g., Cr, Ti and Sc. The boxplots depicted in Fig. 2 compare directly the analytical results for the O and C soil horizons. They do not only show relative enrichment or depletion in one of the two soil horizons relative to the other, but also quantify the compositional differences of the analysed media. These results in themselves, as shown by previous work (e.g., Reimann and Caritat, 2000, 2005), imply that the soil C horizon does not directly reflect the O horizon's composition. The concentrations of numerous elements appear to be strongly regulated in the O-horizon samples, and these often show a much narrower variation in concentrations in the O horizon relative to the C horizon. Most major plant nutrients show such a regulation to an assumed optimal concentration for plant growth, resulting in little variation in the O horizon: Ca is enriched relative to the C horizon, K is limited to a narrow concentration range, Mg is depleted in the O relative to the C horizon, whereas P and S have higher concentrations in the O than in the C horizon. Of the minor nutrients, Cu is slightly depleted and Zn is slightly enriched in the O over the C horizon, while Fe and Mn show no such effect at all. Other elements that show a limited variation in concentration in the soil O horizon when compared to the C horizon include As (depleted), Bi (enriched), Cd (strongly enriched), Hg (strongly enriched), Ni (depleted), Sn (enriched) and Tl (enriched). Many similarities between the behaviour of PTEs with the major nutrients thus become apparent here. Table 3 Relative enrichment/depletion of elements in the soil O horizon expressed as O/C ratio using data from the Barents Project (Reimann et al., 2001; Salminen et al., 2004). Catchment 6 (C6) represents a highly contaminated area less than 10 km from the nickel smelter in Monchegorsk, Kola Peninsula, Russia. C7 is the northernmost and most remote catchment in Norway (arctic tundra) directly on the coast near Berlevåg, Varanger Peninsula. Catchment 2 also lies on the coast in the southern tundra in Russia, C1 farther inland in the tundra, C5 in the southern taiga, C4 in the middle taiga, and C8 and C9 are located in Finland in the boreal forest zone. Bold: enrichment in O horizon N1.

Table 4 Soil survey Nord-Trøndelag: coast/inland ratio for soil O and C horizons (elements sorted according to declining ratio) based on two subsets of 200 samples each, along the coast and along the Swedish border. O/C ratio for the coastal and inland samples sorted according to declining ratio. The number of elements differs depending on the availability of elements with MEDIAN N detection limit. Coast/inland ratio

Na U La Ce Y Nb Se Al Ga Fe Ti Ge Mo B W V Sn Ta Sc Mg Sr As Sb Hg Cr Li Th Pb Bi Cu Hf K Zr S Co P Cd Cs Tl Ca Ni Mn Zn Ag Ba Rb

O/C ratio-sorted

Horizon

Horizon

O

C

O/C coast

O/C inland

1.6 1.6 1.6 1.4 1.3 1.2 1.2 1.2 1.1 1.0 1.0 0.9 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.3 0.3

Cd Hg Ag Sb S Sr Pb P Na Ca Zn Ba Bi Sn As Cu Tl K Mo Y La Mg Ce U Sc Hf Ni Co Mn Zr Nb Al Rb Fe Cs Ga Cr Ti V Th Li

Ag Cd S Hg Sb Pb Sr Ba P Ca Sn Zn Na Bi K Tl Mo Rb As Cu Mn Hf Sc Cs Nb Zr Mg Co Ni U Ga Y La Ti Ce Al Fe Cr V Th Li

3.5 3.4 3.0 2.9 2.8 2.5 2.3 2.2 2.1 2.1 1.9 1.8 1.7 1.7 1.7 1.7 1.5 1.4 1.4 1.4 1.4 1.3 1.3 1.3 1.3 1.3 1.3 1.2 1.2 1.1 1.0 1.0 1.0 1.0 0.9 0.9 0.9 0.9 0.8 0.8 0.8 0.7 0.7 0.6 0.6 0.5

Nb S Sn Ga Ti Mo Na Hg V Sb Rb Pb Cs U Ag Bi Ba K Sr Fe Sc Hf Al Cr Zr Mn La Th Li Cd Tl Y Ce Zn Be Ca Mg Co P As Ni Cu

Ratio

26 11 10 9.5 8.0 5.9 5.0 4.2 4.1 3.2 2.1 1.8 1.7 1.7 1.6 1.5 1.5 1.3 1.2 0.66 0.64 0.62 0.62 0.59 0.53 0.49 0.44 0.42 0.41 0.40 0.37 0.36 0.32 0.28 0.23 0.21 0.19 0.17 0.11 0.10 0.05

16 15 13 10 7.3 3.9 3.4 2.6 1.9 1.8 1.7 1.4 1.4 1.2 1.0 0.9 0.85 0.58 0.47 0.44 0.32 0.31 0.27 0.24 0.23 0.22 0.20 0.19 0.19 0.16 0.14 0.12 0.12 0.12 0.10 0.10 0.10 0.09 0.07 0.04 0.02

O/C horizon ratios, Barents Project catchment study El

C1

C2

C3

C4

C5

C6

C7

C8

C9

Median

Ag Al As Bi Cd Co Cr Cu Fe Hg Mn Mo Ni P Pb S Sb V Zn

3.8 0.2 0.7 1.2 4.3 0.3 0.3 0.7 0.2 19 0.6 3.0 0.4 2.1 3.3 17 1.4 0.3 1.0

4.7 0.9 0.9 3.6 7.0 0.7 1.8 2.5 0.8 6.0 1.7 0.6 0.9 3.2 13 31 1.3 1.1 4.3

25 0.2 0.4 1.4 7.5 0.2 0.2 0.4 0.2 20 1.3 2.3 0.5 1.8 8.3 58 3.1 0.4 1.6

36 2.5 9.8 12 19 3.2 2.7 5.8 4.8 24 6.2 4.8 2.8 15 27 117 18 3.4 8.3

3.2 0.7 1.3 1.5 4.2 0.4 0.8 0.7 0.3 11 0.9 2.5 1.0 1.7 4.7 42 1.5 0.9 1.3

176 0.3 35 64 44 9.1 1.0 29 0.8 21 1.0 22 49 2.5 30 24 32 0.9 3.1

5.3 0.2 0.2 0.6 3.4 0.1 0.2 0.3 0.1 21 0.1 0.8 0.2 1.4 1.3 27 0.2 0.3 0.3

24 0.3 1.5 19 12 0.3 0.2 1.4 0.2 17 5.0 1.5 0.6 2.4 18 102 28 0.4 5.9

6.3 0.3 0.6 4.3 7.7 0.3 0.3 0.6 0.3 26 2.6 0.7 0.7 2.6 13 5 6.5 0.5 2.5

6 0.3 0.9 4 7 0.3 0.3 0.7 0.3 20 1.3 2 0.7 2 13 31 3 0.5 3

The elements Al, Ce, Cs (weak), Fe, Ga, Hf, La, Li, Nb, Ti, U, V and Y show the opposite behaviour. Their concentrations have a higher variation in the O horizon than in the C horizon, which can be explained by variable but small amounts of mineral soil particles in the soil O horizon samples. This can be demonstrated by plotting the residue on ignition (ROI = 100%-LOI) versus element concentration for the soil O horizon samples. Fig. 3 shows some example plots of ROI versus element concentration. A logarithmic scale is used to better resolve the effect for O horizons with high content of organic matter. The majority of the soil O horizon samples have a LOI (480 °C) N 85 wt.% (median: 89 wt.%, minimum: 15 wt.%, maximum: 98 wt.%). Aluminium, Ce, Co, Cr, Cs, Fe, Ga, La, Li, Nb, Sc, Th, Ti, U, V and Y all display a linear dependence on ROI which reflects an admixing of mineral grains into the organic sample. Additional effects contribute to the results determined for the O and C horizon samples such as the input of marine aerosols, which results in a considerable variation in the O horizon. This effect can be traced best using B, Na, Se and Sr, which are known to be redistributed by sea

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Fig. 4. Notched Tukey boxplots (Tukey, 1977) showing element concentrations in subsets of the soil O (left two boxplots) and C horizon (right two boxplots) samples (N = 200 each) collected near the coast (Cos) or farther inland (Inl) close to the Swedish border, c. 100 km from coast. The notches represent a graphical test of comparability — much like the formal t-test — of the medians.

spray (see below). Some further elements that show a comparable variation in the O horizon but are not expected to be redistributed via sea spray include Ag, Ba and Ta.

Arsenic, Ca, Cd, Cu, Hg, K, Mg, Ni, P, Pb, S, Sb, Sn, Tl and Zn are characterised by concentration levels in the soil O horizon that are strongly adjusted (see Fig. 3 for examples). These elements in some

Fig. 5. Scattergrams comparing concentrations of some major and minor nutrients (Ca, Fe, K, Mg, Mn, P and S) and a number of potentially toxic elements (Ag, Pb, Hg and Cd) in the soil O (+) and C (o) horizons. Open circles: C horizon, crosses: O horizon samples.

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cases show effects (higher variation and/or lower element concentration) at the lowest LOI (i.e. the least organic samples). An additional effect of mineral weathering on samples diluted by minerogenic material becomes visible in an overall trend towards higher concentrations and a very high variation over the whole concentration range at the same time. Typical examples are the plots for Al, Ce, Co, Cs, Fe, Hf, La, Li, Rb, Th, U, W and Y (see Fig. 3 for Al). The observation that certain PTEs appear to be strongly enriched in the soil O horizon when directly compared to the C horizon, independent of location, is supported by the Barents Ecogeochemistry Project data (Reimann et al., 2001; Salminen et al., 2004; Table 3). For the Barents Ecogeochemistry Project, O and C horizon soil samples were collected from 9 catchments spread over a 1,500,000 km2 area in northernmost Europe (Reimann et al., 2001). The data demonstrate that some PTEs (and two nutrients, S and P) are invariably enriched in the soil O horizon in all catchments. The most contaminated catchment (C6) is marked by particularly high values for a long list of elements (Ag 176×, Bi 64 ×, Ni 49 ×, Cd 44 ×, As 35 ×, Sb 32 ×, Pb 30 ×, Cu 29 ×, S 24 ×, Mo 22 ×, Co 9 × and Zn 3 ×). These values are often more than 10-fold higher than those observed in the other catchments. These extreme values thus truly reflect contamination. On average S, an important and commonly deficient plant nutrient in the far north (Kashulina and Reimann, 2001, 2002; Reimann et al., 2003), is most enriched in the O horizon from the Barents catchments, followed by Hg, Pb, Cd, Ag, Bi and Sb — a list very similar to the elements that are

detected as enriched in the O horizon in the Nord-Trøndelag dataset and independent of the presence of any likely contamination source. The Nord-Trøndelag dataset is well suited to study the differences between the O and C soil horizons not only in terms of the difference in median (Table 2) and variation (Fig. 2) but also in terms of location, i.e. a possible coastal versus inland effect. For this purpose the dataset was subdivided into two subsets, (1) about 200 samples collected all along the coast and (2) another 200 samples collected in the mountains along the Swedish border. Both the coast/inland ratio for the O and C horizons and the O/C ratio for coastal and inland sites are shown in Table 4. Some of the most interesting differences are shown in the form of boxplot comparisons in Fig. 4. Among the elements showing a clear influence from the input of marine aerosols along the coast are Na, Se and B (the Na map for the O horizon looks strikingly similar to the Cl− map in stream water presented in Banks et al., 2001). The other typical sea-spray elements, Mg and Sr, show only a very minor effect, while the coast/inland ratio for S is 1. Here, effects other than the steady input via marine aerosols must play a dominant role in the observed concentration of these elements in the O horizon. Uranium, La, Y and Nb show a strong enrichment in the O horizon samples collected at coastal sites (Table 4, Fig. 4). For U (and La, Ce), which shows one of the strongest coastal enrichments in the O horizon, this is not reflected in the C horizon and thus cannot be explained by contamination of the O horizon with mineral grains. At present, their enrichment along the coast cannot be explained better

Fig. 6. Maps of the Ag and Hg concentrations determined in the soil O (top) and C horizons (bottom).

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than by assuming a weathering effect that results in a higher availability of these elements in aqua regia along the coastline. Silver, Cd and S are invariably enriched in the O over the C horizon (Fig. 5). Silver is slightly more concentrated in O horizon samples at inland sites than at coastal sites, while Cd is slightly higher in coastal areas than in the inland sites (Table 4). Cadmium is especially enriched in deciduous trees like willow and birch, whereas Ag is usually more enriched in the needles of coniferous trees (see e.g., Reimann et al., 2001, 2015a). These differences thus most likely reflect the change in vegetation from coast to inland. 3.2. Compositional differences between O and C horizon soils Bivariate plots can be used to decipher compositional differences between the two sample materials and specific processes changing element concentrations on the way from the geosphere to the biosphere via plant uptake and decay of organic material in the O horizon. Fig. 5 implies that vegetation co-controls the O horizon's composition as plants assimilate and steadily re-use nutritional elements from the soil O horizon. At the same time they “strive” to avoid the re-uptake of PTEs from the O horizon. Different plant species follow different strategies, evergreen species have usually a much stronger tendency to avoid the uptake of PTEs (excluders, for a discussion see Baker, 1981). Depending on chemical form and binding characteristics, many of the PTEs originating from litterfall remain strongly fixed in the O horizon. Consequently, many, but by far not all, PTEs become strongly enriched and over time adjusted to a certain level (Figs. 2 & 5; see also

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Reimann et al., 2007) in the O horizon (Goldschmidt's plant pump — Goldschmidt, 1937). 3.3. Spatial distribution differences between O and C soil horizons Separate maps (for mapping techniques, see Reimann et al., 2008) displaying the regional distribution of selected elements in the O and C horizons (for maps of more elements see Reimann et al., 2015b) imply that the set of sample materials differ in terms of composition and thus source of elements. This means that element anomalies in the C horizon are not necessarily reflected in the O horizon and vice versa. Fig. 6 depicts element maps for Ag and Hg, which are both strongly enriched in the O horizon relative to the C horizon. Neither Ag nor Hg in the O horizon maps allow for identifying any potential contamination source such as the city of Trondheim at the southern border of the survey area or the E6, which is Norway's busiest road, crossing the survey area. Contamination was identified by mapping on a more local scale (several samples per km2), which revealed, for example, Hg anomalies to be related to the proximity of hospitals and crematoria in the city of Trondheim (Anderson et al., 2010, and references therein). The Joma and Skorovas mining areas are marked by Ag anomalies in the O horizon but our data do not allow for distinguishing between contamination from, e.g., dust deposition and the natural soil signal from the presence of the underlying mineralisation. The map for Hg shows a certain enrichment along the coast and one could discuss whether this is due to the input of marine aerosols (the highest precipitation rates are reached a bit farther inland) or whether this reflects the build-up of

Fig. 7. Maps of the O/C (TOP/BOT) ratio for Ag, As, Cd and S.

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more organic soils in the wet climate at the coast with Hg binding strongly to organic material, independent of source. The input of the radioactive isotopes 137Cs and 134Cs to NordTrøndelag forest soil via long range atmospheric transport following the Chernobyl accident has been well documented (e.g., Smethurst et al., 1995). It is sometimes used as an illustrating example of the impact of LRT on the chemistry of surface soil (e.g., Steinnes, 2009). This impact is not visible in our total Cs data. Both radioisotopes do not occur in nature, thus even a small input is sufficient to produce a clear signal on a map when measuring the isotopes. The total Cs concentration in the soil O horizon (only the isotope 133Cs occurs in nature) does not appear to be affected at all. Mapping of the O/C element ratios (see Fig. 7 for examples) allows studying the regional trends that should explain the enrichment of some elements in the O horizon. Elevated Ag O/C ratios are spatially associated with former mining centres and areas of potential future exploration interest such as Joma, Skorovas and Meråker. There are additional O/C horizon element anomalies in areas without any known mineralisation and the overall regional NE–SW trend of these anomalies is interesting in terms of the mineral potential of the area. The O/C ratio for As, an element that is not generally enriched in the O horizon (see Table 2), shows in contrast a very strong relationship to geology. Arsenic is enriched by a factor of 2 (or locally higher) in the O horizon in those areas underlain by Precambrian orthogneisses (compare Fig. 1) and is otherwise characterised by lower concentrations in the O than in the C horizon. This example highlights the importance of local conditions on the relative enrichment of an element in the O horizon, the observed regional scale difference can clearly not be blamed to anthropogenic activities. Each of the selected elements in Fig. 7 shows a quite different regional distribution, which is not the signal one would expect from anthropogenic contamination sources via LRT. As discussed above, element anomalies do not coincide spatially with obvious anthropogenic contamination sources. This is Mother Nature at work — in most cases the enrichment is due to natural processes at the sample sites, the landscape position and the binding characteristics of the different elements to organic material and the kinetics of humus build-up/decay (details discussed in Reimann et al., 2009a,b). Lithosphere and biosphere are two different entities. The effects as such have long been recognised in landscape geochemistry (e.g., Perel'man, 1961, 1966; Fortescue, 1980, 1992) or are discussed in terms of the “biogeochemical niche” in plant sciences (e.g., Sardans and Peñuelas, 2014). Unfortunately, currently, these important processes and changes happening at the interface geosphere-biosphere are widely ignored by many environmental scientists though there are noteworthy exceptions (e.g., Clarholm and Skyllberg, 2013). 4. Conclusions The O and C soil horizons are found to be geochemically distinct entities in the interaction field of the lithosphere, the biosphere and the atmosphere, and discrepancies in terms of element concentrations relate to soil formation and disparate physico-chemical processes in mineral and organic soil. - Investigation of elements transported as marine aerosols reveals that sea-spray related transportation of Na, Se and B has resulted in elevated concentrations in the O horizon in coastal areas over substantial distances from the coastline. - Weathering effects and/or analytical artefacts play a subordinate though visible role when comparing the O and C horizons for some elements such as U, La and Ce. These are considered to be less soluble in weathering C horizon than in the O horizon and/or more easily soluble in the matrix of the organic O horizon, leading to higher concentrations in O horizon samples than in the C horizon. - Comparison of the compositions of the soil C and O horizons in Nord-

Trøndelag reveals that vegetation is one of the major factors in controlling the soil's composition with respect to a number of elements including nutrient elements and PTEs. In this northern European climate, the vegetation (i.e., the full complement of all plants present in the sample sites) has the ability to adjust concentrations of nutrient elements and PTEs to narrow compositional ranges over time. Adjustment of the respective elements occurs above or below the concentration level determined in the underlying C horizon. Nutrient elements and PTEs are buffered to variable concentration arrays through different element cycles: nutrient elements are incorporated into plants and recycled to the O horizon during the course of seasonal vegetation cycles, while PTEs remain fixed in the organic soil and are not repeatedly incorporated into plants. Clearly, selective element mobility through plant physiology leads to an adjustment of the concentration levels of nutrients and PTEs in the organic matter-rich soil of the O horizon. - The dataset does not reveal contamination from obvious sources of anthropogenic output of PTEs such as larger cities and industrial zones. Serious contamination becomes only visible when mapping at a much more local scale. Based on the irregular distribution of the PTEs input is neither indicated by means of short-distance transport nor can it be explained by long-range transport. The major natural processes determining the chemical composition of a soil sample and anthropogenic input operate at different scales. In conclusion, it is inferred that enrichments of PTEs in the organic matter-rich O horizon compartment of the ecosystem, as observed here, are the result of natural processes and are controlled by the interaction of the weathering lithosphere, the overlying biosphere and, the atmosphere (such as the input of marine aerosols along the coast). Hence, a number of overlapping and interfering processes such as natural conditions at the sample sites, the landscape position, the binding characteristics of the different elements to organic material and the kinetics of humus build-up/decay control the composition of the O horizon at any one site. It is suggested to avoid the use of the term contamination (toxic though the element may be) in the context of natural processes since contamination implies a measure of anthropogenic involvement. Potentially toxic elements are, however, a naturally occurring feature, even in an undisturbed ecosystem. Plants have developed quite different strategies for dealing with these elements. It has been shown here that the strong enrichment of several PTEs in Nord-Trøndelag forest soils is a natural process and, thus, has nothing to do with “contamination”. Acknowledgements Special funds from Nærings-og handelsdepartementet allowed us to carry out this project. Malin Andersson, Ola Eggen, Tor Erik Finne, Belinda Flem, Guri Venvik Ganerød, Henning K.B. Jensen, Jostein Jæger, Øystein Jæger, Iselin Pettersen, Agnes M. Raaness, Anna Seither and Ola Vikhammer participated in fieldwork and/or sample preparation and are thanked for their contribution to the project. Fylkesmannen in Nord-Trøndelag, several municipalities, Verdalsbruket AS, BlåfjellaSkjærfjella national park board and a number of local land owners, as well as the people of Nord-Trøndelag county, all contributed to the success of the project. We are grateful to Rognvald Boyd, NGU, and Robert Garrett, Geological Survey of Canada, who proofread the whole manuscript and suggested numerous improvements. The comments of three anonymous reviewers were appreciated and helped to further improve the manuscript considerably. References Aitchison, J., 1986. The Statistical Analysis of Compositional Data. Chapman & Hall, London (416 pp.). Amundson, R., Richter, D.D., Humphreys, G.S., Jobbágy, E.G., Gaillardet, J., 2007. Coupling between biota and earth materials in the critical zone. Elements 3, 327–332.

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