Essential and non-essential elements in natural vegetation in southern Norway: Contribution from different sources

Science of the Total Environment 502 (2015) 391–399

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Essential and non-essential elements in natural vegetation in southern Norway: Contribution from different sources Marit Nordløkken ⁎, Torunn Berg, Trond Peder Flaten, Eiliv Steinnes Department of Chemistry, Norwegian University of Science and Technology, N-7491 Trondheim, Norway

H I G H L I G H T S • • • •

Concentrations of elements in different plant species were studied. Changes in concentrations during a growing season were identified. PCA indicated a common source for many of the non-essential elements. Uptake by roots appeared to be the plant’s main source of nutrient elements.

a r t i c l e

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Article history: Received 22 May 2014 Received in revised form 11 September 2014 Accepted 12 September 2014 Available online xxxx Editor: Charlotte Poschenrieder Keywords: Forest plants Nutrient elements Non-essential elements Sources

a b s t r a c t Concentrations of essential and non-essential elements in five widespread species of natural boreal vegetation were studied with respect to seasonal variation and contribution from different sources. The plant species included in the study were Betula pubescens, Sorbus aucuparia, Vaccinium myrtillus, Vaccinium uliginosum, Calluna vulgaris and Deschampsia flexuosa. Concentrations of elements essential to plants remained essentially constant or decreased slightly throughout the growing season. Concentrations of most non-essential elements increased or tended to increase on a dry mass basis from June to July as well as from July to September. The increasing trend for these elements was observed for all species except C. vulgaris. Principal component analysis (PCA) of the material indicated a common source for many of the non-essential elements; Sc, Ti, V, Ga, As, Y, Sb, lanthanides, Pb, Bi, and U, i.e. both elements presumably of geogenic origin and elements associated with transboundary air pollution. Uptake by plant roots appeared to be the main source of nutrient elements as well as some non-essential elements. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Plant uptake of elements is affected by soil load and soil properties as well as by plant specific factors. The soil load of different elements and soil properties such as pH and cation exchange capacity affect bioavailability to the plants (Berggren et al., 1990; Davies, 1992; Folkeson et al., 1990; Gjengedal and Steinnes, 1994; Tyler and Olsson, 2001). Plant roots influence element uptake by altering their immediate solution environment; the rhizosphere (Aerts and Chapin, 2000; Chapin, 1980; Clarkson and Hanson, 1980). The roots of many plant families are also associated with mycorrhizae, which are important for the mineral nutrition of plants (Koide, 1991) and may also function as a defence mechanism towards heavy metal uptake in plant shoots (Bradley et al., 1981; Dehn and Schüepp, 1990). The plant's own influence on uptake of elements by the roots, and also on translocation within the plant, makes it possible for different ⁎ Corresponding author. Tel.: +47 73596184. E-mail address: [email protected] (M. Nordløkken).

http://dx.doi.org/10.1016/j.scitotenv.2014.09.038 0048-9697/© 2014 Elsevier B.V. All rights reserved.

plant species growing in the same soil to have different uptake of a given element, both essential and non-essential elements. Plant roots are to a variable degree able to respond to deficiencies and to excess availability of metals in the soil. Some species or ecotypes may also have developed tolerance mechanisms for toxic elements (Baker, 1981; Kahle, 1993; Punz and Sieghardt, 1993). Differences in metal uptake exist among different plant species. Studies of naturally growing plants in Norway have shown substantial species differences in concentrations of both essential and nonessential elements (Berthelsen et al., 1995; Gjengedal, 1992; Kålås, 2003; Sandvik et al., 2005). Some of the most common toxic metals such as Cd and Pb as well as some essential elements (Cu, Zn) have been studied quite extensively in Norway with respect to species differences (Berthelsen et al., 1995; Brekken and Steinnes, 2004; Gjengedal, 1992). Studies of a greater number of elements (Reimann et al., 2001) are more limited due to previous limitations in analytical techniques and less interest in elements considered neither essential nor particularly toxic to plants. Recent developments in multi-element analysis however have facilitated

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Table 1 Concentrations (in μg g−1 unless otherwise stated) of elements in leaves and twigs or shoots of the five plant species and in the parent soil. B. pubescens

S. aucuparia

Leaves

Li B Na (mg g−1) Mg (mg g−1) Al (mg g−1) Si P (mg g−1) S (mg g−1) K (mg g−1) Ca (mg g−1) Sc Ti (mg g−1) V Mn (mg g−1) Fe (mg g−1) Co Ni Cu Zn (mg g−1) Ga As Se Rb Sr (mg g−1) Y Mo Ag Cd Sb Cs Ba (mg g−1) La Ce Pr Sm Tb Er Yb Au Hg Tl Pb (mg g−1) Bi U

Twigs

V. myrtillus

Leaves

Twigs

Leaves

Twigs

Median

(MAD)

Median

(MAD)

Median

(MAD)

Median

(MAD)

Median

(MAD)

Median

(MAD)

0.02 33 0.15 2.9 0.02 b0.02 2.1 2.1 14 6.4 0.004 0.0008 0.11 0.52 0.060 0.21 2.3 7 0.37 0.009 0.02 0.1 34 22 0.013 0.02 0.009 0.29 0.016 0.11 14 0.023 0.038 0.0044 0.0032 0.0006 0.0012 0.0009 0.0012 0.023 0.008 0.90 0.0029 0.0015

(0.006) (8) (0.05) (0.2) (0.0006) (–) (0.8) (0.4) (2) (1.0) (0.001) (0.0003) (0.07) (0.17) (0.008) (0.06) (1.1) (1.2) (0.05) (0.003) (0.009) (0.04) (11) (0.003) (0.003) (0.01) (0.003) (0.09) (0.005) (0.03) (0.004) (0.005) (0.011) (0.0011) (0.0009) (0.0001) (0.0003) (0.0003) (0.0005) (0.006) (0.005) (0.0003) (0.0012) (0.0003)

0.01 16 0.16 1.1 0.01 b0.02 1.5 0.9 6.9 3.8 0.004 0.0005 0.19 0.20 0.038 0.12 1.8 10 0.31 0.010 0.02 0.1 19 0.023 0.011 0.03 0.017 0.45 0.013 0.11 0.025 0.018 0.035 0.0038 0.0031 0.0006 0.0013 0.0009 0.0009 0.018 0.016 0.0022 0.0033 0.0015

(0.003) (2) (0.05) (0.05) (0.003) (–) (0.3) (0.06) (2.3) (0.6) (0.001) (0.0001) (0.09) (0.07) (0.007) (0.03) (0.08) (2) (0.02) (0.003) (0.007) (0.05) (9) (0.004) (0.004) (0.008) (0.002) (0.13) (0.004) (0.02) (0.006) (0.006) (0.016) (0.0013) (0.0009) (0.0002) (0.0005) (0.0003) (0.0004) (0.002) (0.009) (0.0005) (0.0011) (0.0005)

0.05 30 0.06 3.7 0.02 b0.02 1.5 1.4 16 9.5 0.003 0.0005 0.08 0.27 0.061 0.02 0.4 6.3 0.016 0.005 0.02 b0.1 24 0.054 0.012 0.06 0.004 0.03 0.009 0.06 0.050 0.042 0.056 0.0052 0.0035 0.0006 0.0012 0.0007 0.0006 0.023 0.004 0.0007 0.0016 0.0014

(0.02) (2) (0.02) (0.3) (0.007) (–) (0.4) (0.2) (1) (1.4) (0.002) (0.0003) (0.05) (0.13) (0.004) (0.01) (0.1) (0.6) (0.003) (0.004) (0.01) (–) (4) (0.014) (0.005) (0.02) (0.001) (0.009) (0.007) (0.02) (0.008) (0.022) (0.032) (0.0030) (0.0015) (0.0004) (0.0007) (0.0004) (0.0002) (0.011) (0.002) (0.0004) (0.0010) (0.0008)

0.04 24 0.32 2.4 0.02 0.02 1.1 0.6 14 9.7 0.005 0.0008 0.15 0.14 0.042 0.05 0.4 6.7 0.16 0.012 0.03 0.1 28 0.11 0.019 0.03 0.006 0.13 0.015 0.08 0.18 0.046 0.071 0.0081 0.0069 0.0010 0.0021 0.0012 0.0007 0.013 0.013 0.0020 0.0032 0.0024

(0.009) (0.9) (0.10) (0.2) (0.005) (0.01) (0.2) (0.06) (2) (0.8) (0.001) (0.0001) (0.12) (0.06) (0.007) (0.02) (0.06) (0.8) (0.07) (0.004) (0.009) (0.05) (2) (0.007) (0.004) (0.005) (0.0007) (0.04) (0.006) (0.03) (0.02) (0.013) (0.010) (0.0007) (0.0013) (0.0002) (0.0006) (0.0005) (0.0003) (0.003) (0.007) (0.0013) (0.0016) (0.0004)

0.01 31 0.09 1.4 0.09 b0.02 1.1 1.7 7.3 5.6 0.002 0.0003 0.06 0.98 0.045 0.02 0.4 7.3 0.021 0.005 0.02 0.1 16 0.007 0.005 0.05 0.003 0.02 0.011 0.16 0.028 0.012 0.018 0.0021 0.0019 0.0003 0.0005 0.0003 0.0006 0.024 0.002 0.0003 0.0019 0.0007

(0.01) (6) (0.01) (0.2) (0.03) (–) (0.2) (0.1) (0.9) (1.6) (0.0009) 0.00008 (0.02) (0.37) (0.008) (0.01) (0.07) (1.2) (0.006) (0.003) (0.01) (0.04) (4) (0.001) (0.002) (0.01) (0.01) (0.01) (0.01) (0.05) (0.007) (0.004) (0.008) (0.0009) (0.0006) (0.0001) (0.0002) (0.0002) (0.0002) (0.005) (0.0005) (0.0001) (0.0008) (0.0002)

b0.01 12 0.17 0.8 0.12 b0.02 0.8 1.0 4.0 5.0 b0.002 0.0003 0.14 1.2 0.032 0.02 0.3 7.5 0.075 0.009 0.04 b0.1 10 0.011 0.008 0.03 0.005 0.13 0.011 0.15 0.090 0.018 0.025 0.0028 0.0032 0.0005 0.0009 0.0007 0.0014 0.015 0.013 0.0022 0.0023 0.0011

(–) (1) (0.02) (0.008) (0.02) (–) (0.06) (0.1) (0.5) (0.4) (–) (0.00003) (0.04) (0.3) (0.004) (0.006) (0.1) (0.7) (0.013) (0.002) (0.005) (–) (1) (0.002) (0.001) (0.005) (0.0006) (0.05) (0.002) (0.02) (0.011) (0.004) (0.004) (0.0004) (0.0006) (0.0001) (0.0002) (0.0001) (0.0003) (0.001) (0.007) (0.0007) (0.0004) (0.0003)

simultaneous studies of a large number of elements in plants (Tyler, 2005). The concentrations of elements in higher plants may change considerably during the growing season (Guha and Mitchell, 1966). Changes may be associated with uptake, leaching, and translocation within the plant. To obtain representative measurements of metal levels in plants it is therefore important to sample more than once during the growing season. The seasonal concentration changes of the elements are of principal interest in forest flux research. Tyler (2005) and Tyler and Olsson (2006) showed pronounced changes in concentrations and absolute amounts of many elements in developing, maturing, senescent and wilting leaves and in the wintering dead leaves attached to the branches of beech (Fagus sylvatica). In those studies the seasonal changes were related to chemical properties of the elements and to contributions from long-range air pollution. Other studies on the topic of seasonal changes (Brekken and Steinnes, 2004; Gjengedal, 1992; Guha and Mitchell, 1966) have focused on smaller groups of elements, mainly nutrients and toxic elements. The plant species included in the study are all typical of boreal forests, and therefore representative parts of the vegetation in large areas. The sample material is derived from an area of Norway

exposed to acid precipitation (Hanssen et al., 1980) mainly originating from other parts of Europe. These precipitation events also involve atmospheric deposition of many metals (Berg et al., 2008; Rühling and Tyler, 1973; Steinnes, 1980, 2001; Steinnes et al., 2011). This supply of metals causes increased exposure of these metals to various parts of the environment, including the soil and the vascular plants growing in this area. It is therefore of interest to document the concentrations of all elements influenced by this kind of pollution. The plant and soil material studied in the present work have previously been analysed for Cd and Zn only (Brekken and Steinnes, 2004). That study showed considerable concentration differences between some of the species. In the present study, the plant samples and associated surface soils were analysed for a large number of essential and non-essential elements, as it is likely that there might be considerable species differences in concentrations of elements that have been little studied in this context to date. The element concentration changes during a growing season were therefore studied with emphasis on intra-element similarities and inter-species variation. Simultaneous analysis of a large number of elements, both essential and non-essential, makes it

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393

Table 1 Concentrations (in μg g−1 unless otherwise stated) of elements in leaves and twigs or shoots of the five plant species and in the parent soil. V. uliginosum

D. flexuosa

C. vulgaris

Leaves

Organic topsoil

Twigs

0–3 cm

5–10(8)

Median

(MAD)

Median

(MAD)

Median

(MAD)

Median

(MAD)

Median

(MAD)

Median

(MAD)

b0.01 31 0.03 1.6 0.04 0.10 1.2 2.4 6.7 4.4 b0.002 b0.0002 0.02 0.23 0.063 0.01 0.4 5.8 0.12 b0.003 b0.01 0.2 9.3 0.008 0.003 0.02 0.002 0.72 b0.006 0.12 0.029 0.014 0.009 0.0010 0.0011 0.0002 0.0003 b0.0002 0.0004 0.032 0.005 0.0001 0.0009 0.0006

(–) (0.3) (0.01) (0.1) (0.002) (0.07) (0.4) (0.2) (0.3) (1.9) (–) (–) (0.009) (0.04) (0.0004) (0.001) (0.06) (1.2) (0.005) (–) (–) (0.002) (2.6) (0.003) (0.0005) (0.01) (0.0009) (0.06) (–) (0.04) (0.005) (0.009) (0.004) (0.0005) (0.0004) (0.00004) (0.0001) (–) (0.0001) (0.002) (0.004) (0.00006) (0.0001) (0.0004)

b0.01 15 0.03 0.8 0.03 b0.02 0.8 0.9 3.5 2.8 0.003 0.0004 0.22 0.33 0.036 0.02 1.0 8.7 0.091 0.009 0.02 0.1 6.1 0.009 0.009 0.04 0.005 0.91 0.015 0.06 0.061 0.017 0.031 0.0034 0.0033 0.0006 0.0011 0.0008 0.0003 0.018 0.012 0.0020 0.0029 0.0014

(–) (0.5) (0.01) (0.1) (0.02) (–) (0.03) (0.03) (0.5) (0.5) (0.002) (0.0002) (0.17) (0.03) (0.009) (0.007) (0.02) (0.05) (0.004) (0.007) (0.008) (0.07) (0.8) (0.0004) (0.006) (0.01) (0.003) (0.11) (0.007) (0.02) (0.007) (0.011) (0.021) (0.0021) (0.0014) (0.0004) (0.0008) (0.0006) (0.0001) (0.003) (0.002) (0.0014) (0.0020) (0.0010)

0.03 15 0.28 1.1 0.03 0.23 0.9 1.4 5.5 2.7 0.007 0.0012 0.37 0.32 0.056 0.05 1.5 8.2 0.026 0.021 0.05 0.1 15 0.003 0.015 0.12 0.021 0.02 0.021 0.72 0.016 0.030 0.060 0.0069 0.0050 0.0011 0.0021 0.0014 0.0007 0.021 0.98 0.0022 0.0059 0.0028

(0.002) (4) (0.06) (0.2) (0.005) (0.06) (0.2) (0.1) (0.7) (0.8) (0.002) (0.0004) (0.18) (0.007) (0.008) (0.01) (0.2) (0.9) (0.004) (0.007) (0.02) (0.07) (2) (0.0004) (0.003) (0.02) (0.001) (0.009) (0.007) (0.23) (0.005) (0.009) (0.017) (0.0019) (0.0013) (0.0002) (0.0006) (0.0004) (0.0003) (0.0016) (0.61) (0.0007) (0.0027) (0.0011)

0.02 6 0.05 0.8 0.004 0.66 1.4 1.9 23 1.3 b0.002 0.0003 0.04 0.20 0.037 0.02 1.8 3.8 0.042 b0.003 0.02 0.1 32 0.004 0.005 0.40 0.003 0.04 0.008 0.21 0.003 0.012 0.018 0.0019 0.0013 0.0003 0.0005 0.0003 0.0010 0.023 0.005 0.0005 0.0011 0.0006

(0.01) (0.6) (0.01) (0.2) (0.002) (0.17) (0.2) (0.2) (3) (0.2) (–) (0.0001) (0.01) (0.01) (0.007) (0.005) (0.3) (0.4) (0.005) (–) (0.003) (0.02) (2) (0.002) (0.001) (0.14) (0.001) (0.01) (0.003) (0.13) (0.001) (0.003) (0.005) (0.0007) (0.0003) (0.00007) (0.0002) (0.0001) (0.0002) (0.004) (0.003) (0.0002) (0.0008) (0.0001)

0.8 4.4 0.21 1.1 3.1 2.7 0.8 2.2 1.2 2.3 0.62 0.08 15 0.19 2.5 1.0 6.8 13 0.12 2.0 5.0 2.5 4.7 0.020 4.5 1.1 0.32 1.4 1.3 0.31 0.041 7 13 1.4 0.9 0.13 0.4 0.3 0.0008 0.19 0.38 0.16 0.60 0.15

(0.1) (0.2) (0.007) (0.2) (1.1) (0.7) (0.05) (0.2) (0.1) (0.2) (0.1) (0.03) (2) (0.13) (0.9) (0.1) (1,3) (2) (0.02) (0.5) (1.0) (0.4) (1.1) (0.002) (2.1) (0.3) (0.08) (0.5) (0.9) (0.07) (0.004) (3) (6) (0.8) (0.4) (0.06) (0.2) (0.1) (0.00004) (0.01) (0.20) (0.05) (0.16) (0.03)

1.0 4.9 0.19 0.6 6.1 2.8 0.6 1.3 1.0 1.0 2.1 0.14 10 0.05 4.3 1.2 4.7 16 0.062 4.1 7.0 2.4 4.3 0.014 7.9 0.8 0.29 1.7 0.6 0.29 0.035 10 22 2.6 1.6 0.21 1.0 1.2 0.0009 0.16 0.19 0.19 0.91 0.29

(0.09) (1.0) (0.02) (0.3) (1.0) (0.6) (0.08) (0.5) (0.2) (0.5) (0.4) (0.05) (2) (0.01) (1.9) (0.2) (0.2) (5) (0.026) (0.6) (1.3) (0.2) (1.2) (0.005) (3.4) (0.2) (0.06) (0.7) (0.4) (0.13) (0.005) (3) (8) (1.0) (0.5) (0.05) (0.4) (0.4) (0.0001) (0.05) (0.05) (0.05) (0.19) (0.12)

possible to compare and group the elements based on intraelement similarities e.g. according to different sources. A focus of the present study is to attempt to determine the relative contributions from long-range air pollution and natural sources.

2. Materials and method 2.1. Site description The study site is located at Lund in southwest Norway (58°33′N, 6°27′E). The climate in this area is humid oceanic with mean annual precipitation of about 2000 mm (Førland, 1993) and an average temperature over the year of 6.4 °C (Aune, 1993). The site is located about 25 km from the coastline and 300 m above sea level (Brekken and Steinnes, 2004). The soil (predominantly podzol) is shallow, acidic (pH 4.1–4.8), and has a high organic matter content (50–90% LOI) in the surface horizon (Brekken and Steinnes, 2004). The vegetation is dominated by little demanding species such as Vaccinium myrtillus and Betula pubescens. No significant local pollution sources are present in this area.

2.2. Sample material The plots selected for plant and soil sampling had a radius of approximately 10 m and were located at six plots along a 2.5 km long northwesterly transect in rather steep terrain (Brekken and Steinnes, 2004). The plant species selected for study were B. pubescens Ehrh. (downy birch/white birch), Sorbus aucuparia Poir. (rowan), V. myrtillus L. (bilberry), Vaccinium uliginosum L. (bog bilberry), Calluna vulgaris (L.) Hull (heather) and Deschampsia flexuosa (L.) Trin. (wavy hair-grass) (Lid and Gjærevoll, 1985). For D. flexuosa the straw was not included. For the rest of the species, leaves and twigs were sampled. In the case of B. pubescens, S. aucuparia and both Vaccinium species the twigs and leaves were analysed separately, while a composite sample of leaves/ twigs of the evergreen species C. vulgaris was analysed. This composite sample contained leaves and twigs at its natural ratio. Samples were from ca. 10 individuals at each sub-area. Samples from trees (birch, rowan) were sampled at ca. 1–1.5 m height. Samples of the above species except V. uliginosum were collected at most or all six plots. V. uliginosum was collected at only one plot. The plant samples were not washed prior to analysis, which means that substances adhering to the plant surface are included. The plant sampling was conducted

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in early June, mid-July, and early September. Samples of surface soil (0–3 cm and 5–10 cm) were collected at each plot in June only.

3. Results 3.1. Element concentrations in plants

2.3. Sample handling and chemical analysis Plant samples were dried in paper bags at 40 °C followed by a slight manual homogenisation. In order to minimise the risk of contamination, a more extended homogenisation was avoided. In the case of B. pubescens, S. aucuparia and both Vaccinium species the twigs and leaves were analysed separately. Weighed plant samples of about 0.2 g were digested in concentrated nitric acid (Scanpure) at high pressure (2 bar; 2 ∗ 105 Pa) and about 170 °C in a microwave oven (Multiwave 3000, Anton Paar) using closed digestion vessels, with gradual power rise according to a pre-set scheme. Soil samples of about 0.5 g were digested in an Ultraclave instrument (MLS/Milestone), with gradual temperature rise reaching a temperature of 250 °C and a pressure of 120 bar (1.2 ∗ 107 Pa). After digestion the samples were diluted with deionised water (Purelab Option-Q7, Elga) to a final HNO3 concentration of 0.6 M for analysis. Concentrations of elements were determined using high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS; Thermo Electron, Finnigan Element 2). For analysis of the soil samples 0.1% methane was added to the Argon plasma gas to minimise interferences from carbon, and to provide enhanced sensitivity (Rodushkin et al., 2005). The instrument was calibrated using 0.6 M HNO3 solutions of a multi-element standard at five different concentrations. One of these solutions was analysed for approximately every ten samples to check for instrumental drift. A certified multi-element aqueous standard solution (SPS-SW1; Spectrapure Standards, Norway) was analysed at the beginning and end of each analytical sequence as a quality control of the instrument. Elements determined (isotopes used are indicated) were 7Li, 11B, 23Na, 25Mg, 27Al, 30Si, 31P, 34S, 39K, 43Ca, 45Sc, 47Ti, 51V, 55 Mn, 57Fe, 59Co, 60Ni, 65Cu (63Cu for soil samples), 67Zn (66Zn for soil samples), 69Ga, 75As, 77Se, 85Rb, 88Sr, 89Y, 98Mo, 109Ag, 111Cd, 121Sb, 133 Cs, 137Ba, 139La, 140Ce, 141Pr, 147Sm, 159Tb, 166Er, 172Yb, 197Au, 202Hg, 205 Tl, 208Pb, 209Bi, and 238U. 2.4. Precision and accuracy Blank samples of deionised water and HNO3, and reference materials Moss M3, Moss M2 (Steinnes et al., 1997), and GBW 07408 (IAEA; NRCCRM, China) were treated as samples and distributed among the analytical series during the analysis. Twig samples and randomly chosen leaf samples were reanalysed. GBW 07408 was not used for evaluation of the accuracy as the certified values are based on a total digestion of the samples with hydrofluoric acid. The detection limits were determined from the detection capacity of the instrument and concentrations of elements in blanks, expressed as three times the standard deviation of element concentrations for replicate blank samples. The soil samples were analysed somewhat later than the plant samples, and analytical improvements made during the intermediate time improved detection limits for some elements. For reference materials Moss M2 and M3 recommended values of 26 elements are available. Concentrations of most elements (B, Na, Mg, K, Ca, V, Mn, Fe, Co, Ni, Cu, Se, Rb, Sr, Sb, Cs, Ba, La, and Hg) were in agreement with recommended values at 95% confidence level both in M2 and M3. 2.5. Statistical treatment Kruskal–Wallis and Mann–Whitney U tests were used for significance testing of differences between medians. Correlations between elements were examined by principal component analysis (PCA) and Spearman correlation coefficients. The rotation method used in the PCA was Varimax rotation. SPSS 14.0 was used to conduct these tests.

Concentrations of elements in leaves and twigs or shoots of the six plant species and in the parent soil are summarised in Table 1. The plant species exhibited considerable variation in the concentrations of many elements. Particularly high concentrations in one of the plant species (N 5 times higher than the rest of the species) were observed for Co and Zn in B. pubescens, for Cd in V. uliginosum, for Mo in D. flexuosa, and for Tl in C. vulgaris. Silicon was observed at about 5 times higher concentrations in leaves of V. uliginosum than in B. pubescens, S. aucuparia, V. myrtillus, and twigs of V. uliginosum, and in even higher concentrations in C. vulgaris and in D. flexuosa (Table 1). Generally, high concentrations of many elements (Na, Sc, Ti, V, Co, Ni, Zn, Y, Ag, Cd, most of the lanthanides, Au, Tl, Pb, Bi, and U) were observed in B. pubescens relative to the other species. Increased concentrations of several elements (Na, Sc, Ti, V, Y, Ag, Cs, the lanthanides, Au, Tl, Pb, Bi, and U) relative to the other species were also observed in C. vulgaris. In S. aucuparia elevated concentrations of Li, Mg, Ca, Sr, Y, Ba, and the lanthanides were observed. Al and Mn were observed in higher concentrations in V. myrtillus than in the other species and in V. uliginosum Si and Cd were observed in elevated concentrations relative to the rest of the species. Relatively high concentrations of Si, K, Ni, Mo, and Cs were observed in D. flexuosa. Generally low concentrations of many elements (Li, Na, K, Ti, V, Co, Ni, Rb, Sr, Y, Mo, Ag, lanthanides and Pb) were observed in V. uliginosum relative to other species. In S. aucuparia decreased concentrations of Co, Ni, Zn, Cd, and Cs were observed, and in V. myrtillus low concentrations of Co, Ni, Zn, Sr, Mo, and Tl were evident. Low concentrations of B, Mg, Ca, Zn, Sr, Cd, and Ba were observed in D. flexuosa, and in C. vulgaris low concentrations of P, K, Ca, Zn, and Sr were present. Lower concentrations of Mo than in the rest of the species were observed in B. pubescens. The ratio of metal concentration in plant shoots to metal concentration in soil (0–3 cm topsoil) was greater than unity in all plant species for B, P, K, and Rb and in most species for Ca, Mg, and Mn. The plant/ soil ratios were above or just below one in most species for Na, S, Cu, Zn, Sr, Cs, Ba, and Au. In C. vulgaris the ratio of Tl concentration in plant shoots to Tl concentration in soil was N 1 and about 100 times higher than in the rest of the species, in which the corresponding ratio was between 0.01 and 0.04.

3.2. Distribution between twigs and leaves Leaf/twig ratios of elements are given in Tables 2 and 3. Table 2 shows the mean of the leaf/twig ratios of B. pubescens, S. aucuparia, and V. myrtillus and Table 3 the corresponding values for the three Table 2 Leaf/twig concentration ratios (means of B. pubescens, S. aucuparia and V. myrtillus). Element

Ratio

Element

Ratio

Element

Ratio

S B Mg Mn K Hg Fe Rb Mo P Li Ca Ni

2.05 1.95 1.90 1.78 1.68 1.55 1.48 1.41 1.37 1.35 N1.31 1.26 1.26

Al Ti Co La Cs Sc Sb Cu Ce Au Y U Bi

1.01 1.01 0.99 0.95 0.94 0.90 0.89 0.88 0.87 0.87 0.81 0.74 0.74

Er Sr Yb As Ga Ag Na Zn V Ba Cd Tl Pb

0.71 0.70 0.69 0.64 0.62 0.60 0.56⁎ 0.53⁎

⁎ The ratio varies among plant species.

0.50 0.38 0.34 0.32 0.29

M. Nordløkken et al. / Science of the Total Environment 502 (2015) 391–399 Table 3 Leaf/twig ratios of V. myrtillus (VM), B. pubescens (BP) and S. aucuparia (SA), individually separated into six different groups. Ratio

N2.0

1.5–2.0

1.0–1.5

0.6–1.0

0.3–0.6

VM

B

Mg S K Rb Hg

B Mg S Mn K Ca Fe Co Th

Al Ti Mn Co As Y Ag La Yb Bi Th U V Cu Se Sr Yb Au Bi U

Na V Zn Sr Cd Tl Pb Ba Au

BP

SA

S Hg

P Ca Fe Ni Cu Se Sb Cs Na Al P Ti Ni Zn As Rb Y Sb Cs La Hg B Al P K Ti As Se Sb

Mg Mn Fe Ni

b0.3

Ag Cd Ba Tl Pb

Ca V Cu Rb Y Co Sr Ba Ag Cs La Yb Tl Pb Au Bi Th U

Na Zn Cd

3.3. Seasonal variations in element concentrations The concentrations of many elements increased in most species during the growing season. Increases throughout the season (up to 9 times compared to the concentration in June) were observed in As (9 times in leaves of S. aucuparia), V, Sc, Sb, Al, Ga, Y, La, Ce, Pr, Sm, Tb, Er, Yb, Bi, Pb, and Li. The significance levels of the concentration increase/decrease of As, Sb, Pb, and Bi and of La, Ce, Er, and Yb are shown in Tables 4 and 5. Significant concentration increases in most species were also observed for Ti, U, and Hg. Silicon was also observed in considerably higher concentrations in September than in June and July, but as many of the June and July samples were below the detection limit, the magnitude of this change is uncertain. This increasing tendency for all the above-mentioned elements applies to most species, but in twigs of S. aucuparia and V. myrtillus the concentrations did in most cases not change significantly. Moreover, in C. vulgaris the concentrations of the same elements decreased significantly (Tables 4 and 5). Table 4 Seasonal changes in the concentrations of As, Sb, Pb, and Bi in plant species: Increase or decrease with corresponding level of significance.

B. pubescens B. pubescens S. aucuparia S. aucuparia V. myrtillus V. myrtillus C. vulgaris D. flexuosa All species

Leaves Twigs Leaves Twigs Leaves Twigs Whole plant Straw

Table 5 Seasonal changes in the concentrations of La, Ce, Er, and Yb in plant species. Increase or decrease with corresponding level of significance. Species B. pubescens B. pubescens S. aucuparia S. aucuparia V. myrtillus V. myrtillus C. vulgaris D. flexuosa All species

Leaves Twigs Leaves Twigs Leaves Twigs Whole plant Straw

n

La

Ce

Er

Yb

6+6+6 0+6+4 5+5+5 0+3+4 5+6+6 4+6+6 3+3+3 3+0+4 26 + 35 + 38

(+)** /+** +*/0 /+* (+)** 0 −*/0 (+)* +*/+**

0/+** /+** +*/0 /0 +*/0 0 −*/0 (+)* +*/+**

0/+** /+*** +*/+** /0 +**/0 0 −*/0 (+)* +**/+**

0/+** /+** +*/+** /0 +*/0 0 −*/0 (+)* +**/+***

Explanations to this table are the same as for Table 4.

species (B. pubescens, S. aucuparia, and V. myrtillus) individually separated into six different groups. The highest leaf/twig ratios were observed for essential elements (S, B, Mg) and the lowest ratios for non-essential elements supplied to the natural environment in southern Norway by long range atmospheric transport (LRTP); Cd, Tl, and Pb. The leaf/twig ratios generally tend to be in the following order: “nutrient elements” N “soil derived nonessential elements” N “LRTP elements”. This order is however not totally clear. Some of the “LRTP” elements show higher leaf/twig ratios than some of those elements supposed to be of local crustal origin. Also, Hg differs quite extensively from other “LRTP” elements; with a higher leaf/twig ratio than all soil contamination derived elements as well as some of the nutrient elements. The low atomic number lanthanides (La, Ce) have higher leaf/twig ratios than the high atomic number lanthanides.

Species

395

n

As

Sb

Pb

Bi

6+6+6 0+6+4 5+5+5 0+3+4 5+6+6 4+6+6 3+3+3 3+0+4 26 + 35 + 38

+**/+* /+* +*/+** /0 +**/+* 0 −*/−* 0 +**/+***

+*/0 /+* +**/0 /0 (+)* 0 −*/0 (+)* +***/0

+**/+** /+* +**/0 /0 +*/0 0 0/−* (+)* +**/0

+*/+** /+** +**/+* /0 +*/0 0 −*/0 (+)* +***/0

n = number of samples analysed in June, July and September respectively. −: concentration decrease relative to the previous sampling. +: concentration increase relative to the previous sampling. 0: no significant concentration change. (+); (−): concentration increase/decrease throughout the growing season; not significant within each interval of growth (June–July/July–September). *; **; ***: p ≤ 0.1, p ≤ 0.01, p ≤ 0.001 (Mann–Whitney U test).

Though the changes were not pronounced, concentrations of P, S and Cu (Table 6) decreased significantly throughout the season in most species. A decreasing trend for K (Table 6) and Rb was also indicated in most species, however statistically significant only from June to July in leaves and twigs of V. myrtillus and in leaves of S. aucuparia (only Rb), and from July to September in twigs of B. pubescens. In this group of elements with mainly decreasing concentrations over the season C. vulgaris differed somewhat from the other species, with slightly increasing concentrations of alkali metals K and Rb and no significant change in concentrations of P, S, and Cu. In the case of S, a slightly increasing trend was also observed for D. flexuosa. The concentrations of earth alkaline metals increased in some species, mainly in leaves, but decreased in twigs of V. myrtillus (Mg, Ca; from June to July) and B. pubescens (Mg, Ca, Sr; from July to September, cf. Table 7). The concentration of Mg also decreased in leaves of B. pubescens. Concentrations of Tl and Ag tended to increase, though not significantly in most species, while there was no clear tendency mutual to most species of either increasing or decreasing concentrations of B, Na, Mn, Fe, Co, Ni, Zn, Mo, Cd, Cs, and Au. 3.4. Correlations between elements The rotated component matrices that resulted from PCA of all plant samples, twig samples, and leaf samples respectively are presented in Tables 8–10. Loadings with absolute values that is higher than 0.4 are underlined. According to Schaug et al. (1990) experience with environmental data has shown that loadings less than about 0.4 in absolute value may be dominated by random errors. In Table 8 the first component, which explains 35% of the variance, loadings are high for the following elements, with decreasing loadings from left to right; Yb, Er, Ga, Tb, U, Bi, Pr, Sm, V, Y, Sc, Pb, Sb, Ti, Ce, As, La, Ag. In the second component loadings are high for the following elements; Li, La, Mg, Ca, Hg, Ce. This component explains far less of the variance (9%) than component 1. The third component, also explaining 9% of the variance, shows intermediate to high loadings for Ni, Co, Zn, P, Ag, Rb, Cd, and Cu. In the fourth component, Si and Mo have high negative loadings, which might be due to particularly high concentrations of Si and Mo Table 6 Seasonal changes in the concentrations of P, S, K, and Cu in plant species. Increase or decrease with corresponding level of significance. Species B. pubescens B. pubescens S. aucuparia S. aucuparia V. myrtillus V. myrtillus C. vulgaris D. flexuosa All species

Leaves Twigs Leaves Twigs Leaves Twigs Whole plant Straw

n

P

S

K

Cu

6+6+6 0+6+4 5+5+5 0+3+4 5+6+6 4+6+6 3+3+3 3+0+4 26 + 35 + 38

−**/−** /−* −**/0 /0 −**/−* 0 0 (−)* −**/−**

−**/−* /−** −**/−* /0 −*/0 0 0 (+)* −***/0

0 /−** 0 /0 −**/0 −**/0 (+)* 0 −*/0

0/−** /0 −**/−* /0 −*/−** 0 0 0 −*/−***

Explanations to this table are the same as for Table 4.

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M. Nordløkken et al. / Science of the Total Environment 502 (2015) 391–399

Table 7 Seasonal changes in the concentrations of Mg, Ca, Sr, and Ba in plant species. Increase or decrease with corresponding level of significance. Species B. pubescens B. pubescens S. aucuparia S. aucuparia V. myrtillus V. myrtillus C. vulgaris D. flexuosa All species

Leaves Twigs Leaves Twigs Leaves Twigs Whole plant Straw

n

Mg

Ca

Sr

Ba

6+6+6 0+6+4 5+5+5 0+3+4 5+6+6 4+6+6 3+3+3 3+0+4 26 + 35 + 38

0/−* /−* 0 /+* +*/+* −**/0 0 (+)* 0

0 /−* (+)* /+* +*/+** −**/0 +*/0 (+)* 0

0 /−* (+)* /0 0/+** 0 0 (+)* +*/0

0 /0 0 /0 0/+* 0 +*/0 (+)* +*/0

PCA component explains 14% of the variance. In this component loadings are intermediate to high for K, Mg, B, Sr, Ca, Rb, Ba, Li, Na, and P. In the third component, which explains 13% of the variance, loadings are high for Cs, Tl, Si, Mo, and S, and in the fourth component, which explains 10% of the variance, loadings are intermediate to high for Co, Ni, Cu, P, Ag, Rb, Cd, and Zn. The rotated component matrix that resulted from PCA of the leaf samples (Table 10) was quite similar to that for all samples. However, the low atomic number lanthanides La (0.34) and Ce (0.61) had even lower loadings in the first component, which explained 31% of the variance. Yttrium, La, and Ce were positively correlated with Li and Hg (third component; cf. Table 10).

Explanations to this table are the same as for Table 4.

4. Discussion in D. flexuosa. The results for lanthanides indicate that the plants differ in their uptake of lighter and heavier members of the group. The rotated component matrix that resulted from PCA of the twig samples (Table 9) shows a strong correlation between many of the elements in the first component, which explains 44% of the variance. In order of decreasing loadings the following elements are represented in component 1 with intermediate to high loadings; Y, Er, Yb, Pr, U, Tb, Ce, Ga, Sb, Bi, V, Sm, Sc, Ti, Pb, La, Fe, Se, As, Hg, Li, and Ag. The second

Lithium, Al, Sc, Ti, V, Ga, As, Y, Sb, the lanthanides, Hg, Pb, Bi, and U showed very similar patterns of seasonal trends, with mainly increasing concentrations throughout the season of plant growth. Some of these non-essential elements are known to be constituents of atmospheric long-range transported pollutants (LRTPs) supplied to the surface soils and vegetation of this area (Berg et al., 2008; Steinnes, 2001); others are typical crustal elements that may be derived e.g. from dust aerosol

Table 8 Rotated component matrix, principal component analysis (PCA) of all plant samples. Rotation method: Varimax with Kaiser Normalization. Rotation converged in 23 iterations. Loadings with absolute values that is higher than 0.4 are underlined.

Table 9 Rotated component matrix, principal component analysis (PCA) of twig samples. Rotation method: Varimax with Kaiser Normalization. Rotation converged in 23 iterations. Loadings with absolute values that is higher than 0.4 are underlined.

Component

1

2

3

4

Component

1

2

3

% of variance explained

34.6

9.3

9.3

8.3

% variance explained

43.7

14.1

13.0

4 9.7

Li B Na Mg Al Si P S K Ca Sc Ti V Mn Fe Co Ni Cu Zn Ga As Se Rb Sr Y Mo Ag Cd Sb Cs Ba La Ce Pr Sm Tb Er Yb Au Hg Tl Pb Bi U

0.45 −0.10 0.41 −0.04 0.01 −0.07 −0.35 −0.34 −0.26 0.01 0.91 0.81 0.92 −0.14 0.43 0.06 0.04 −0.04 0.18 0.97 0.74 0.44 −0.26 0.18 0.91 −0.14 0.53 0.05 0.82 0.19 0.06 0.62 0.81 0.93 0.92 0.96 0.97 0.98 −0.13 0.34 0.33 0.84 0.93 0.94

0.74 0.24 −0.09 0.62 −0.20 −0.06 0.07 0.00 0.47 0.56 0.11 0.11 −0.18 −0.05 0.39 0.16 0.00 −0.16 −0.19 −0.06 −0.06 −0.46 0.36 0.42 0.34 −0.02 −0.11 −0.27 0.04 −0.14 0.16 0.70 0.53 0.33 0.19 0.19 0.16 0.05 −0.15 0.53 −0.05 −0.19 −0.07 0.04

−0.12 0.06 0.19 0.10 −0.35 −0.07 0.64 0.43 0.24 −0.08 0.09 0.11 −0.06 −0.07 0.28 0.86 0.89 0.53 0.68 0.00 −0.21 −0.01 0.60 −0.08 0.03 −0.06 0.62 0.57 −0.05 −0.03 −0.21 −0.11 −0.01 −0.01 −0.06 0.03 0.02 0.01 0.27 0.01 −0.05 −0.08 −0.01 0.05

−0.05 0.50 0.11 0.30 0.23 −0.90 0.09 −0.15 −0.49 0.49 0.07 0.07 −0.03 0.18 0.08 0.09 −0.21 0.41 0.24 0.02 0.00 −0.04 −0.20 0.20 0.08 −0.90 0.02 0.25 0.05 −0.19 0.24 0.06 0.06 0.05 0.10 0.06 0.06 0.03 0.07 −0.02 −0.08 −0.04 0.00 0.05

Li B Na Mg Al Si P S K Ca Sc Ti V Mn Fe Co Ni Cu Zn Ga As Se Rb Sr Y Mo Ag Cd Sb Cs Ba La Ce Pr Sm Tb Er Yb Au Hg Tl Pb Bi U

0.62 −0.03 0.38 0.08 −0.16 0.27 −0.21 0.04 −0.06 −0.12 0.92 0.90 0.93 −0.27 0.86 0.03 0.10 −0.06 0.07 0.95 0.73 0.84 −0.08 0.10 0.98 0.32 0.52 −0.13 0.94 0.11 −0.20 0.87 0.96 0.97 0.92 0.96 0.98 0.97 −0.28 0.70 0.19 0.88 0.93 0.97

0.53 0.89 0.46 0.91 −0.41 −0.04 0.40 −0.27 0.92 0.85 0.04 0.05 −0.17 −0.32 −0.08 0.15 −0.12 −0.04 0.14 −0.14 −0.24 −0.06 0.78 0.86 0.06 0.02 −0.16 −0.18 −0.13 −0.08 0.67 0.33 0.18 0.11 0.31 −0.02 0.01 −0.07 0.06 −0.41 −0.10 −0.05 −0.09 −0.03

0.28 0.13 0.36 −0.07 −0.19 0.90 −0.03 0.77 −0.01 −0.33 0.23 0.30 0.09 −0.16 0.31 −0.17 0.18 0.14 −0.57 0.20 0.20 −0.10 0.00 −0.35 0.05 0.88 0.38 −0.44 0.20 0.93 −0.38 0.02 0.06 0.11 0.03 0.16 0.09 0.13 −0.14 0.21 0.90 −0.20 0.19 0.16

−0.05 0.13 −0.02 0.01 −0.35 0.02 0.74 0.25 0.19 −0.20 0.14 0.05 −0.02 −0.22 0.12 0.89 0.87 0.87 0.45 −0.01 −0.13 −0.03 0.55 −0.13 −0.01 −0.03 0.61 0.54 0.02 0.04 −0.29 −0.13 0.05 −0.01 −0.12 0.04 0.03 0.03 0.09 0.21 0.03 −0.11 0.00 0.05

M. Nordløkken et al. / Science of the Total Environment 502 (2015) 391–399 Table 10 Rotated component matrix, principal component analysis (PCA) of leaf samples. Rotation method: Varimax with Kaiser Normalization. Rotation converged in 28 iterations. Loadings with absolute values that is higher than 0.4 are underlined. Component

1

2

3

4

% variance explained Li B Na Mg Al Si P S K Ca Sc Ti V Mn Fe Co Ni Cu Zn Ga As Se Rb Sr Y Mo Ag Cd Sb Cs Ba La Ce Pr Sm Tb Er Yb Au Hg Tl Pb Bi U

31.2

13.0

11.4

8.3

0.39 0.24 −0.05 0.20 0.02 0.29 −0.43 −0.30 0.05 0.32 0.90 0.79 0.90 −0.03 0.52 0.19 0.16 −0.52 0.33 0.98 0.71 −0.10 −0.24 0.26 0.71 −0.21 0.19 0.06 0.78 −0.19 0.12 0.34 0.61 0.80 0.86 0.87 0.89 0.93 0.05 0.58 0.44 0.88 0.91 0.76

−0.17 0.05 0.55 0.15 −0.38 −0.20 0.67 0.73 0.41 0.01 0.07 0.13 −0.14 −0.01 0.50 0.89 0.91 0.51 0.68 0.05 −0.19 −0.05 0.72 −0.05 0.09 −0.24 0.79 0.58 −0.07 0.13 −0.19 −0.08 −0.03 0.01 0.03 0.06 0.06 0.05 0.33 −0.07 0.03 −0.11 −0.01 0.12

0.75 −0.02 −0.13 0.34 −0.21 −0.05 −0.03 −0.19 0.48 0.33 0.16 0.12 0.11 0.00 0.26 0.02 −0.02 −0.20 −0.14 0.05 −0.04 −0.31 0.19 0.32 0.61 0.08 −0.02 −0.01 −0.10 −0.61 0.33 0.83 0.61 0.49 0.37 0.40 0.36 0.24 −0.03 0.58 −0.10 0.11 0.06 0.17

0.26 −0.32 0.05 0.63 −0.84 −0.29 0.31 0.01 0.58 0.16 0.07 0.22 −0.21 −0.80 0.25 −0.07 0.07 −0.08 0.25 0.03 −0.33 −0.07 0.28 0.55 0.03 0.14 0.08 0.10 −0.17 −0.20 −0.17 0.13 0.13 0.16 0.07 0.14 0.10 0.06 −0.12 −0.27 0.50 0.03 −0.23 0.20

particles partly of geogenic origin. The reason for the considerable increase of each element cannot be concluded from the present study, but may be generally related to passive uptake on the plant surface. In that case the concentration increase is caused by the longer time of exposure. The larger increase observed for leaves in most cases may be explained by the fact that twigs may accumulate elements all year long whereas the leaves inevitably start on zero every spring. Morphology and physiology may also influence differences between twigs and leaves. Barber et al. (2004) reported that many factors may affect airvegetation transfer including plant characteristics such as functional type, leaf surface area, cuticular structure and leaf longevity. The evergreen species C. vulgaris differs from the other species by showing a decreasing tendency in concentrations of these elements (Li, Al, Sc, Ti, V, Ga, As, Y, Sb, the lanthanides, Hg, Pb, Bi, and U) over the season. Like twigs, evergreen species are exposed all year long. Increasing concentrations of elements in some plant species during the first part of the season implies that this increase exceeds the dilution effect from the simultaneous mass increase (Guha and Mitchell, 1966). A concentration decrease during the time of foliation may therefore partly or fully be due to growth of the plant itself, as the present results are not adjusted according to the increase of plant mass. The major

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period of growth in aerial biomass of C. vulgaris occurs prior to the flowering period (Miller, 1979). Also during the flowering period growth occurs, although with lower rates (Miller, 1979). The slight decrease of some elements (P, S, Cu, K and Rb) over the growth season is largely consistent with prior findings in F. sylvatica by Tyler and Olsson (2006), although for S and Cu an early concentration increase was observed prior to a decrease similar to that of P. Other authors (Guha and Mitchell, 1966) have also observed high concentrations of P and K in recently developed leaves, and decreasing concentrations with the age of the leaves. This decrease may in general be related to uptake of large amounts of nutrients in the beginning of the leaf foliation period. A decrease during the last part of the season may be due to leaching or translocation within the plant, e.g. transport back to the roots. K is known to be a mobile element that can be easily translocated, and it is also subject to leaching. Rb, like other elements present in soils as the monovalent cation, is easily taken up by plants. The ionic radius of Rb is similar to the radius of K and therefore Rb can substitute at K-sites in plants, but not in metabolic processes (KabataPendias and Mukherjee, 2007). The decrease in Rb observed in the same species as for K is most probably due to the chemical similarity between the two. Cu has low mobility relative to most other elements in plants and most of this metal in leaf tissue appears to remain in the leaves until they senesce, only small amounts may move to young organs (KabataPendias and Pendias, 2001). The observed decreasing concentration of Cu in leaves during the last part of the season might therefore not be due to translocation. Different plant species may also have different abilities of nutrient re-translocation. Aerts (1989) found that above-ground nutrient turnover of C. vulgaris exceeded that of the perennial deciduous grass Molinia caerulea (Purple moor grass), because of a lower efficiency of nutrient retranslocation from senescing plant parts. Besides Rb, other elements that are chemically similar to nutrient elements may also follow the same mechanisms of root uptake as the nutrient elements do. Strontium and Ba have very similar patterns of seasonal variation to Mg and Ca, which indicates a common uptake mechanism for the earth alkaline metals. Cesium on the other hand does not show the same pattern of seasonal changes as K and Rb, although there are some similarities. White et al. (2003) observed that although Cs and K are chemically similar, different transport proteins control their uptake and delivery to the xylem. Thus, the Cs/K concentration ratio differs among plant species. Many elements are represented with high loadings in the first component of the rotated component matrices, which indicates a common source of these elements. Some of these elements (V, As, Sb, Pb, and Bi) are presumed to be associated with atmospheric long-range transported pollution, while others (Sc, Ti, Ga, Y, lanthanides, U) are more likely to be derived from contamination by soil dust. It is difficult to recognise a common source of all of these elements, as it seems to be a combination of geogenic dust and atmospheric long range transported pollutants. From the PCA it is not possible to distinguish between the two. Still, although not entirely consistent, the leaf/twig ratios of assumed soil contamination derived elements appear to be higher than for “LRTP”-elements. The components 2 and up of the principal component analyses seem to represent biological sources (root uptake). In these components both essential and non-essential elements are represented. Some elements seem to originate from multiple sources, as they have medium to high loadings in more than one component. This includes Fe, Se, Hg, and the lower atomic number lanthanides. Fe correlates stronger with elements of component 1 (geogenic elements such as Sc, Ti, Ga, and lanthanides and the LRTP elements V, As, Sb, Pb, and Bi) in twig samples, while in leaf samples Fe seems to originate from two different sources, where the second source probably is root uptake (component 2; S, Co, Ni, Ag). Se correlates strongly with geogenic and LRTP elements in twig samples, but not at all in leaf samples. Y, La, Ce,

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and Hg seem to originate from two different sources, where one is biological/root uptake. La is mostly associated with the biological source as far as leaf samples are considered. Several authors have pointed out a relationship between the REE content in plants and their occurrence in soils (Laul and Weimer, 1982; Robinson et al., 1958). Any essentiality of REEs has not yet been proved, but REEs have been used in agricultural fertilizers in China since the 1980s (Diatloff et al., 1995; Pang et al., 2001). In the present study the results for lanthanides indicate that the plants differ in their uptake of lighter and heavier members of the group. Differences in plant uptake of light and heavy REEs have also been previously observed. Xu et al. (2002) reported that the uptake of REEs of maize plants by both roots and shoots increased with increasing doses, and was higher for the light than for the heavy REEs. Wyttenbach et al. (1998) observed variable absorption of REEs by plants from a forest ecosystem and have related this phenomena to organic complexes or changes in the oxidation state of individual REEs. The alkali and earth alkaline metals correlate strongly in twig samples as shown by component 2 in the rotated component matrix (Table 9). This component represents a biological source, i.e. root uptake. A corresponding strong correlation between alkali and earth alkaline metals is not observed in leaf samples. Together with quite different leaf/twig ratios for these elements (Tables 2 and 3) this indicates similar features in root uptake but high selectivity in transport within the plant (from twigs to leaves). Cadmium and Tl seem to be related solely to root uptake and do not correlate strongly with other elements predominantly of anthropogenic origin. Cadmium and Zn are often closely correlated in nature, and Cd is easily available to plants. Cadmium and Zn are to some degree correlated in the present material too, but there are also distinct differences between these two chemically similar elements, in particular the relatively higher concentration of Cd in V. uliginosum than in the other species, as has been previously reported (Gjengedal, 1992). High concentrations in one of the species indicate selective uptake. Being an essential element, Zn is easily taken up by plants, and it is also easily translocated within the plant. B. pubescens has an effective uptake of Zn in shoots, as the concentrations in both twigs and leaves are about 3 times the concentration in soil (0–3 cm). High concentration of Zn in B. pubescens relative to other boreal plant species was also previously observed in Norway (Gjengedal, 1992). The Tl content in plants is suggested to be a function of the concentration of its mobile fraction in soils (Kabata-Pendias and Mukherjee, 2007). Monovalent Tl forms many species that are quite soluble and may be easily taken up by plants, especially by tree roots from acidified soils (Tyler, 2005). In the present material Tl correlates with Cs, this may indicate that Tl is present as the monovalent cation. High concentrations of both Cs and Tl are observed in C. vulgaris. A high transfer coefficient of 137Cs in C. vulgaris was previously observed in Norway (Thørring et al., 2012). 5. Conclusions Differences between essential and non-essential elements in the studied plants are distinct. The leaf/twig ratios are higher for essential elements, indicating selective transport from twigs to leaves. The principal component analyses also support root uptake as the predominant source for essential elements and elements chemically similar to essential elements (e.g. Rb, Sr, and Ba). Different sources of the non-essential elements not associated with root uptake are more difficult to distinguish from the present results, as both the PCA and the seasonal changes show similar features of elements assumed to be predominantly of geogenic origin as well as those known to be associated with atmospheric long range transported pollution. The leaf/twig ratios however, tend to be lower for most “LRTP” elements than for “geogenic” elements.

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