Selenium concentrations in national inventory soils from Scotland and Sweden and their relationship with geochemical factors

Journal of Geochemical Exploration 121 (2012) 4–14

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Selenium concentrations in national inventory soils from Scotland and Sweden and their relationship with geochemical factors C.A. Shand a,⁎, J. Eriksson b, A.S. Dahlin b, D.G. Lumsdon a a b

Environmental and Biochemical Sciences, The James Hutton Institute, Craigiebuckler, Aberdeen, AB15 8QH, UK Department of Soil and Environment, Swedish University of Agricultural Sciences, SLU, P.O. Box 7014, SE-75007, Uppsala, Sweden

a r t i c l e

i n f o

Article history: Received 4 November 2011 Accepted 1 June 2012 Available online 9 June 2012 Keywords: Marine input Selenium Soil Soil organic matter

a b s t r a c t Soil is an important reservoir of trace elements that are essential for animal and human health. We report on the concentration of selenium (Se) in Scottish soils sampled during 2007 to 2009 and in Sweden for arable topsoils collected during 1988 to 2007 as part of national soil inventories. Soils in Scotland were sampled on the basis of their genetic horizons and included a range of land cover. In contrast, soils from Sweden were sampled from the plough layer of arable soils with an auger to a fixed depth from the surface. We found that the Se concentrations of soils from Scotland and Sweden, which are richer in organic matter than other more southerly latitudes in Europe, are more related to organic matter content than other geochemical factors. In the Swedish soils the Se concentrations were elevated in areas close to the sea. In areas away from the sea, concentrations were also higher in soil associated with alum shales. The Se concentrations in arable topsoils were lower in Sweden than in Scotland. The distribution of Se in Scottish topsoils showed a propensity for higher concentrations in the west of the country. For soils from Scotland there was a significant decrease (p = 0.05) in the mean Se concentration in soil from the A horizon (mean 0.64 mg kg− 1) compared to that of the C horizon (mean 0.26 mg kg− 1). The Se concentration of the plough layer soils from Sweden had a median value of 0.23 mg kg− 1 and a mean value of 0.30 mg kg− 1. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Selenium is an essential element for human and animal health (Fairweather-Tait et al., 2011; Foster and Sumar, 1997) and the element often occurs in low concentrations in soils from many parts of Europe including Scotland and Sweden (Berrow and Ure, 1989). Because of concerns about Se deficiency (Combs, 2001) there is a need for systematic information on the Se status of soils at national scales and elucidation of the factors influencing the concentrations. Selenium has chemical similarities to sulphur although Se is much less abundant in soils or in the Earth's crust (Sparks, 1995). The main features of the geochemical occurrence of Se are its association with sulphide deposits, accumulation under reducing conditions often associated with the presence of organic matter, and association with some types of phosphate deposits. Although Se minerals are known in nature, occurrences of substantial deposits of specific Se minerals do not exist apart from selenides found in low-temperature, hydrothermal deposits where sulphur is absent (Neal, 1995). The concentration of Se in igneous rocks is often low (b0.2 mg kg − 1), whereas the concentration of Se in metamorphic rocks varies widely but is greater when the rock is carbonaceous. The concentration of Se in

⁎ Corresponding author. E-mail address: [email protected] (C.A. Shand). 0375-6742/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2012.06.001

sedimentary rocks also varies but black shales and clay-rich sediments are Se rich, whereas sands and sandstones contain less. The concentration of Se in marine sediments is also positively related to the extent of reducing conditions and the presence of organic carbon (Berrow and Ure, 1989). The concentration of Se in soils depends to a large extent on the concentration of Se in the parent rock (Fordyce, 2007; Peterson et al., 1981; Spadoni et al., 2007; Ure and Berrow, 1982). On a global scale, the dynamic transport of Se from terrestrial, marine and anthropogenic sources through the atmosphere in particulate and gaseous phases is critical to the Se cycle (Haygarth, 1994). Although Se exists in several oxidation states the main forms of Se responsible for its activity in soils are selenite (SeO32 −) and selenate (SeO42 −). These ions, particularly selenite, are adsorbed onto clay and sesquioxide surfaces (Neal, 1995) and the accumulation of Se in soil may be related to the amounts of these reactive surfaces. In addition to its association with carbonaceous rocks, Se also has an association with soil organic matter. The nature of the bonding of Se with organic matter involves three possible scenarios: (1) direct complexation, (2) indirect complexation involving organo-metallic bonds, or (3) covalent bonding in organic compounds. The exact nature of the bonding of Se in soil organic matter is unclear but the details have been discussed (Coppin et al., 2009; Qin et al., 2012). In relation to the association of Se with soil organic matter, plant residues tend to enrich the soil with Se and the concentrations of Se in surface soils are expected

C.A. Shand et al. / Journal of Geochemical Exploration 121 (2012) 4–14

to be greater than in subsoils, which generally have lower organic matter concentrations (Adriano, 2001; Lévesque, 1974a). Although the chemistry of Se and sulphur are similar and the elements are geochemically related, differences in the behaviours of Se and sulphur have been observed in soils from Canada, with sulphur being more strongly held in organic soil surface layers (Lévesque, 1974b). Analysis of the Se concentration in 47 Scottish arable topsoils by Shand et al. (2010) showed they contained between 0.19 and 1.46 mg kg− 1 with a mean of 0.63 mg kg− 1. For 114 soils from 44 farms in Scotland sampled from a 5–20 cm depth, Fordyce et al. (2010a) found a narrower range (0.11 to 0.88 mg kg− 1) with a mean of 0.44 mg kg− 1. Shand et al. (2010) found no significant relationship between the Se concentration in the topsoils and the expected Se status of the parent rock, and that the Se concentration was more related to the loss on ignition or soil organic carbon concentration. Johnsson (1992) found that the variation in the Se concentration in Swedish arable topsoil was primarily determined by parent material while for mor layers in podzols, the atmospheric deposition was more important. Fordyce et al. (2010a) found that geological parent material from which the soil was derived only had a small but statistically significant influence on the Se concentration in soils, and that the loss on ignition was more strongly correlated with the total or water soluble Se. A summary of the Se concentrations in soils from Scotland, Sweden and other parts of the world is presented in Table 1. There are a few locations in the UK where soils contain relatively large concentrations (>300 mg kg− 1) probably as a result of the accumulation by soil organic matter (Walsh and Fleming, 1952). The first National Soil Inventory of Scotland (NSIS1), which commenced in 1978, was based on sampling soil profiles on a 10-km grid across the country (MISR, 1984). Because of analytical challenges at that time, the determination of Se was not included in the suite of analysis. The second National Soil Inventory of Scotland (NSIS2), which commenced in 2007, was based on sampling soil profiles on a 20-km grid and was designed to include additional elements such as Se. Physical samples of most of the NSIS1 soils and all of the NSIS2 soils have been archived to permit other analysis in the future. The Swedish monitoring programme started in 1995. In the first round, samples from more than 3000 sites were analysed. A re-sampling of around 2000 sites was made 2001–2007 (Eriksson et al., 2010). The aims of both the Scottish and Swedish programmes were to conduct a systematic mapping of soils in order to describe the current status and to produce base-line data that makes it possible to monitor temporal changes in soil properties. The objectives of our paper was to provide information about the spatial distribution of Se in topsoils from Scotland and Sweden, to further investigate the variation in Se concentrations in relation to geochemical properties, including soil parent material, clay content and organic matter content and to compare the two countries in these

Table 1 Selenium concentrations in soils. Se range (mg kg− 1)

Se mean (mg kg− 1)

Reference

Scotland (56) Scotland (2) Scotland (4) Scotland (10) Scotland (47) Scotland (114) Scotland (241) Sweden (24) Sweden (365) Sweden (297) Various (1623)

0.11–1.59 0.31–0.42 0.55–0.76 0.02–0.36 0.19–1.46 0.11–0.88 0.10–6.60b 0.16–0.98 0.14–2.04 0.06–1.70 0.03–2.0

0.69 0.37 0.66 0.17 0.63, 0.58a 0.44, 0.43a

Ure and Berrow (1982) Forbes et al. (1979) MacLeod et al. (1996) Ure et al. (1979) Shand et al. (2010) Fordyce et al. (2010a) Fordyce et al. (2010b) Lindberg and Bingefors (1970) Johnsson (1989) Johnsson (1992) See references in Ure and Berrow (1982)

0.39 0.60 0.28 0.41

Median value. Includes values associated with contaminated soils near Glasgow; the natural Se concentration range was estimated to be between 0.10 and 2.00 mg kg− 1. b

respects. Additionally, for Scotland we aimed to characterise the distribution of Se in the soil profiles according to genetic soil horizons and determine how the Se concentrations vary with land cover.

2. Materials and methods 2.1. Soils used to compare aqua regia and nitric acid extraction In connection with the last sampling of Swedish soils in 2007 when around 500 soils were sampled, there was a change in the Se extraction method from using aqua regia to using 7 M HNO3. To provide information about potential differences in the efficiency of extraction of Se between methods, we extracted a set of 42 soils (20 from Scotland and 22 from Sweden) by both reagents and analysed the extracts by inductively coupled plasma–mass spectrometry (ICP-MS). We used an Agilent 7500ce instrument (Agilent Technologies, Inc., CA, USA) fitted with an octopole reaction cell employing H2 to remove interferences, and 78Se + for measurement (Shand et al., 2010). The 42 soil samples had a wide range of properties, which might be expected to influence extraction efficiency and analysis. The properties and their ranges were: pH (water), 3.5–7.5; organic carbon, 0.5–32.6% by weight; clay, b0.5–69% by weight; acid ammonium oxalate extractable iron, 0.19–17.0 g kg − 1; acid ammonium oxalate extractable aluminium, 0.19–5.7 g kg − 1.

2.2. Soil inventory samples from Scotland Soil samples analysed for Se were from NSIS2, based on the aforementioned 20-km grid pattern across the country. Of the 195 possible sampling locations, 13 were non-soil or otherwise not available. At each available location (182 sites) a pit was excavated by professional soil surveyors using hand tools. The target depth of the pits was 1 m but reaching this depth was not always possible. Site and horizon descriptions were made by the surveyors in the field according to the Scottish system (MISR, 1984). Single soil samples were taken from the centre of genetic horizons. Thin soil horizons (generally b5-cm thick) were not sampled because of the difficulty in obtaining sufficient amounts of representative soil. For some of the very thick horizons, 2 or 3 replicate samples were taken at different positions within the main body of the horizon. There were a total of 688 soil samples analysed for Se. For the horizons where more than one sample was taken we used the uppermost sample yielding a total of 661 unique Se–horizon combinations (Table 2). Occasionally, the soil in the profile was formed from parent material that differed in some respect to that found lower in the profile. Where this phenomenon occurred we labelled the C horizon as “mixed”.

2.3. Soil inventory samples from Sweden

Country (number of samples)

a

5

Soil samples from Sweden were collected in the monitoring programme for arable soils (Eriksson et al., 2010). The programme started in 1995, but samples from older surveys (principally taken for cadmium analysis) in 1988, 1992 and 1994 were reanalysed for more elements, including Se, together with new samples from 1995 (60% of the samples were taken in 1995). A similar repeated sampling of 2035 new sites was made in 2001–2007. New soil sampling sites were selected because there were no exact coordinates available for the old sites. The new sites have exact coordinates and the soil will be sampled every ten years in the future. Soil samples were taken from the plough layer (0–20 cm) with an auger. Since there was no significant difference in the average Se concentration values between the two sampling rounds, the data were merged into a common set with 5170 observations.

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C.A. Shand et al. / Journal of Geochemical Exploration 121 (2012) 4–14

Table 2 Distribution of Se (mg kg− 1) in Scottish soils (n = 661) according to horizon and land cover. Horizona

Arable n

L F H O A E B C Mixed C Buried Horizona

0 0 0 0 19 0 22 14 3 1

Median

0.40

0.43

0.19 0.33 0.10 2.18

0.14 0.25 b 0.06 2.18

n 5 3 0 95 2 3 8 1 0 0

Semi-natural grassland n

L F H O A E B C Mixed C Buried

Bog Mean

0 0 3 21 16 12 36 19 0 0

Mean

2.73 2.95 0.68 1.15 0.67 0.28

Improved grassland Mean

Median

n

0.87 0.26

0.80 b 0.06

1.68 1.04 0.24 0.21 b 0.06

1.52 1.04 0.26 0.13 b 0.06

0 0 0 0 31 0 42 21 0 0

Mean

Moorland Median

0.58

0.42

0.30 0.21

0.28 0.24

Woodland

n

Mean

Median

2 2 7 55 8 20 46 8 1 2

0.73 1.19 1.22 2.61 1.52 0.48 0.58 0.36 1.26 2.35

0.73 1.19 1.00 2.01 1.26 0.19 0.42 0.36 1.26 2.35

Weighted mean, all types

Median

n

Mean

Median

nb

Meanc

Median

2.51 2.08 0.60 0.82 0.53 0.20

8 4 8 27 11 14 46 13 2 0

0.45 0.47 0.82 2.79 0.48 0.21 0.60 0.24 0.21

b0.06 0.46 0.59 1.67 0.4 0.19 0.44 0.10 0.21

15 9 18 198 87 49 200 76 6 3

0.63 ab 0.56 abc 1.29 cd 2.22 d 0.64 bc 0.55 ab 0.49 b 0.26 a 0.33 2.29

0.44 0.72 0.82 1.66 0.46 0.29 0.32 0.23 0.14 2.18

a

See MISR (1984) for explanation of symbols. In some instances there is a greater number of master horizons than sampling locations (e.g. 200 B-horizons) because the master horizons were sometimes subdivided (e.g. B into Bhs). c Mean values followed by a common letter are not significantly different (p = 0.05), mixed and buried horizons excluded. b

2.4. Chemical analysis of soils from Scotland The soil was air-dried at 30 °C, screened through a sieve (2-mm aperture), sub-sampled with a spinning riffler, and finely ground to a particle size b150 μm in an agate ball mill. To minimise any bias related to the order of soil sampling, the soils collected within a single year were analysed in a random order. The milled soil (1 g for organic and 2 g for mineral soils) was extracted with boiling aqua regia (21 ml of 12 M HCl followed by 7 ml of 15.8 M HNO3) in reflux systems designed to trap potentially volatile elements (ISO, 1995). The Se concentration of the diluted extract was measured by ICP-MS using the Agilent 7500ce instrument as described previously in this paper. Each digestion batch of 20 samples consisted of 17 soil samples, 1 reagent blank, and 2 in-house test soil samples to monitor performance. The test soil was a mineral soil regularly used as an in-house standard. The soil Se concentration of the in-house test soils (± the standard deviation) was 0.54 ± 0.21 mg kg− 1 verified by analysis of certified reference soil NCS DC 73387 (certified total Se concentration 1.34 ± 0.17 mg kg− 1, measured aqua-regia extractable value 1.21 ± 0.25 mg kg− 1). Where horizons were sufficiently thick, the bulk density for the Scottish soils was measured by drying intact soil cores at 105 °C but to provide a full set of data the values used here were calculated by partial least squares from their IR spectra (Moreira et al., 2009). 2.5. Chemical analysis of soils from Sweden The soil was air-dried at 30 °C and screened through a sieve (2-mm aperture). For the samples collected between 1988 and 2005, Se was extracted with aqua regia according to the method described in Application Note 015 from PS Analytical Ltd, Orpington, Kent, UK. Thus, 16 ml of aqua regia was added to 1 g of soil and the sample was heated to a gentle boil in a reflux system. After 10 min, 5 ml of de-ionised water was added and the extract refluxed for another 10 min. After cooling, the extract was diluted to 100 ml with de-ionised water and filtered into a polypropylene flask. The filtered extract was further

diluted with 50% (v/v) of 12 M hydrochloric acid and heated in a water bath at 70 °C for 30 min. Sodium tetrahydroborate stabilised with sodium hydroxide (1.2% NaBH4 in 0.1 M NaOH) was used for hydride generation. Selenium was measured by atomic fluorescence spectrometry. The soil samples collected during 2007 were not extracted with aqua regia but extracted with 7 M HNO3 in an autoclave at 200 kPa (120 °C) for 30 min (SIS, 1997). The Se concentration in the extract was measured by high resolution sector field mass spectrometry. To monitor performance each digestion batch of 50 samples contained 1 reagent blank, 1 in-house test soil sample and 1 of 10 regular samples with varying properties analysed in the first sampling round. The data for the samples indicated that the laboratory used had some problems in keeping a constant level of measurement in the analyses made over 2 or more years. The in-house standard had a median value of 0.36 mg Se kg− 1 and a relative standard deviation of 12% (n =107). See Eriksson et al. (2010) for details.

2.6. Prediction of the Se status of soils from Scotland Soils in Scotland are in part classified by their parent rock (soil association). For brevity the soil associations are described using Scottish place names (MISR, 1984). For each soil association at the 20-km national grid intersection positions, the relative Se concentration status of the soils was predictively assigned to high, low, mixed or indeterminate categories. Selenium was expected to have a relatively low concentration in soils derived from granites, sandstones, rhyolites, mica schist, and non-volcanic greywackes. On the other hand, soils derived from andesitic, basaltic, and argillaceous materials were expected to have relatively greater Se concentrations (Ure and Berrow, 1982). The link between parent material and Se composition of the soil was expected to be greater within the B rather than surface horizons since surface horizons are more likely influenced by anthropogenic processes, atmospheric deposition and biochemical cycling.

C.A. Shand et al. / Journal of Geochemical Exploration 121 (2012) 4–14

7

analysis of our data set of 42 test soils by extraction with 7 M nitric acid or aqua regia extraction are similar.

2.7. Statistical analysis The data were analysed using Excel 2007, Minitab v15, Gensat 13 or SAS 9.1. Where Se concentration values in the soil were below the limit of detection, the values for statistical purposes were assigned a value half the limit of detection. For comparison of arithmetic means the data were tested for normality using plots of residuals and transformed by taking logarithms or square roots as required.

3.2. Selenium in soils from Scotland The total Se concentration of the soils from Scotland (n= 661, all horizons) had a considerable spread of values (b0.06 to 19.2 mg kg − 1, Fig. 1), with 138 values less than the limit of detection (equivalent to 0.06 mg kg− 1 in the soil) and 15 values exceeding 6 mg kg− 1. The data were skewed and the overall mean and median Se concentration values were 1.04 and 0.51 mg kg− 1, respectively (Table 3). The soil with the highest Se concentration (19.2 mg kg− 1) was from a peat profile in south-west Scotland, which had variable Se concentrations throughout the profile, i.e. Of (0–9 cm), 3.65 mg Se kg− 1; Oa1 (9–28 cm), 1.15 mg kg− 1; Oa2 (28–60 cm), 19.2 mg kg− 1; and 2Ch (60–70 cm), 1.25 mg kg − 1: subscript “f” = fibrous, “a” = amorphous, and “h” = accumulation of organic matter; see MISR (1984) for further explanation on the notations. When the amount of Se per unit volume of soil was calculated (as the product of Se concentration and dry soil

3. Results 3.1. Comparison of Se extraction efficiency On average, nitric acid extracted 95% of the Se from the 42 test soils compared with the amount extracted by aqua regia. After log base 10 transformation the regression equation was: Se extracted with nitric acid = 0.807 × Se extracted by aqua regia − 0.120 (R2 = 0.89). Analysis of the data using the paired t-test showed that the mean difference between the two was not significant (p= 0.053), indicating that the

Scottish soils, all horizons (total of 661 samples)

Selenium (mg/kg)

20 15 10 5 0 00

A

2 n=

,

al

er

er

in

B

C

(m

,n al

vi

er

lu

in

(m

=4

=7

,n al

ri

9)

6)

n=

( ed

Bu

in

(m

3)

)

7)

=8

,n al

E

ed

p

om

( ed

(li

ty

ea

M

O

m

19

= ,n

ix

tte

L

8)

n=

=1

r,n

M

dO

e os

s po

6)

5)

=1

,n

li

(e

F

8)

=9

,n

r tte

(p

o ec

c de

(

(d

H

Scottish soils, all horizons (total of 661 samples) 6

Selenium (mg/litre)

5 4 3 2 1 0 00

er

A

in

B

(m

2 n=

,

al

a

er

in

(m

3)

)

7)

=8

l,n

ri

Bu

C

=4

=7

l,n

a

er

,n al

vi

lu

in

(m

9)

6)

n=

( ed

E

ed

p

om

m

tte

L

(li

8)

n=

=1

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r,n

ty

ix

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O

19

= ,n

ea

(p

o ec

c de

(

M

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e os

s po

6)

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=1

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li

(e

F

8)

=9

,n

r tte

H

(d

Fig. 1. Distribution of Se in Scottish soils. Top, concentration; bottom, per unit volume of soil. The boxplots show the first, second and third quartiles. Outliers are identified by asterisks and the vertical bar represents the lowest and highest values within the limits.

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C.A. Shand et al. / Journal of Geochemical Exploration 121 (2012) 4–14

Table 3 Selenium concentrations (mg kg− 1) in soil profiles from Scotland and in plough layers (Ap) of arable soils and mor layers of podzols in Scotland and Sweden. Percentile

5 10 25 50 75 90 95 Minimum Maximum Mean a

Scotland

Sweden

All samples in the profiles

Ap, organic C b 7%

(n = 661)

(n = 17)

b 0.06 b 0.06 0.13 0.51 1.35 2.51 3.93 b 0.06 19.2 1.04

– – – 0.43 – – – b0.06 0.69 0.37

0.51 – – – b 0.06 3.50 0.80

Mor layera

Peaty topsoil

Ap, all

Ap, organic C b 7%

(n = 17)

(n = 49)

(n = 5170)

(n = 4765)

(n = 365)

(n = 139)

– – –

– b0.06 0.74 1.48 2.34 3.67 – b0.06 8.39 1.80

0.11 0.13 0.17 0.23 0.33 0.48 0.73 b0.05 13.3 0.30

0.11 0.13 0.16 0.22 0.30 0.40 0.48 b0.05 1.48 0.25

– – – 0.54 – – – 0.014 2.02 0.60

– 0.53 0.88 1.15 1.68 2.31 – 0.28 13.3 1.44

Mor layer

Peaty topsoil

Data from Johnsson (1989).

bulk density, Fig. 1), the soil with the greatest amount of Se per unit volume (5.54 mg litre− 1) was from the B horizon of a mineral soil with an accumulation of sesquioxide material located on an island off the west coast of Scotland. The concentration of Se in Scottish topsoils at the grid intersections is presented in Fig. 2 along with a map of the major soil associations (parent materials) (MISR, 1984). There were 36 topsoils with Se concentration values below the limit of detection (0.06 mg kg − 1) and the maximum value of Se concentration in the topsoils was 8.86 mg kg − 1. Many of the soils with the greatest concentrations of Se (>the 90 percentile value of 2.93 mg Se kg − 1) are found in the west of Scotland, in areas associated with organic soils and peats. The distribution of Se in Scottish topsoils according to the land cover is shown in Table 4. The Se concentration of arable topsoils, which represented 17 out of the 182 sites sampled, had a median value of 0.43 mg kg − 1. The influence of pedological drainage on the Se concentration of the topsoils under different land covers is presented in Table 5. In most land covers, the topsoils with the greatest Se concentrations were associated with poorly drained soils, which generally contain more organic matter. The relative distribution of Se concentration values in the B horizons according to our prediction from parent material composition (Fig. 3) shows that the soils predicted to have the highest Se concentrations, i.e. Ettrick association (derived from greywackes and shales) and Foudland association (derived from slates, phyllites and other weakly metamorphosed argillaceous rocks), were within the group with values >2 mg Se kg − 1. The Tarves association soil, which also had a Se concentration value >2 mg kg − 1 was derived from intermediate rocks or mixed acid and basic rocks, both metamorphic and igneous, classified by us as indeterminate in relation to its relative Se status. Analysis of variance using the Se concentration data transformed by taking square roots, showed there was a significant difference (p =0.044) between the means of the low and the high categories. The correlation between the Se and organic carbon or clay concentrations for the Scottish topsoils, subdivided into land cover categories are shown in Fig. 4. The relationship between Se and organic carbon was significant (pb 0.001) but displays a bimodal distribution associated with the disparate distributions of C concentrations in the organic and mineral soils of Scotland (Paterson, 2012). There was no significant relationship (at p =0.05) between Se concentration and the percentage by weight of clay in the soil. 3.3. Selenium in soils from Sweden The Se concentration of the plough layer soils from Sweden (Table 3) had a median value of 0.23 mg kg − 1 and a mean value of 0.30 mg kg − 1. The 95% percentile Se concentration value was 0.73 mg kg − 1. Some of the plough layer soils are in peat or have their origin in peat. If the 405 soils with more than 7% organic carbon

are omitted the mean value decreases to 0.25 mg kg − 1. In general the highest soil Se concentrations in the plough layers were found along the coast of southwestern Sweden and in the farming district in Jämtland in the inland of Central Sweden (Fig. 5a). Soils with the lowest concentrations were found in the southeastern part of Sweden and towards the inland areas in Central and Northern Sweden. A similar trend for data on Se concentrations in mor layers of mixed coniferous broadleaf forests was found by Johnsson (1989), but the concentrations in Jämtland were only slightly higher than those in the surrounding areas with rather low concentrations. Since there was a strong indication that Se concentrations in soil tend to increase with soil organic carbon content (see correlation in Fig. 6) we mapped Se values normalised against organic carbon. The organic carbon-normalised map of Se (Fig. 5c) largely shows the same pattern as the map showing Se concentrations (Fig. 5a), but with differences in details. For example the high Se concentrations in the encircled areas of the non-normalised map disappeared on the organic carbonnormalised map, whereas the areas with high Se concentrations on the Swedish southwest coast also had high organic carbon-normalised Se concentrations. Stepwise regression analysis indicated that soil Se concentrations are most strongly correlated with organic carbon content, clay content and pH (Table 6). However, the latter factors explained a rather small proportion of the variation in Se concentrations and 61% of the variation was not explained by the factors included in the model.

3.4. Comparison of Se concentrations in soils from Scotland and Sweden The Se data sets presented in our paper cannot be directly compared since the Scottish data include many soil types and whole soil profiles, while the Swedish data only include the plough layer of arable soils and thus only represent about 6% of the total land area. However, it is possible to obtain an estimate of the differences in the general Se concentrations in the two countries by comparing subsets of the Scottish soils with similar data for Swedish soils. The 17 samples of plough layer soils from arable soils in the Scottish dataset (Ap horizons) have a median Se concentration of 0.43 and a mean of 0.37 to be compared with 0.22 and 0.25 mg kg − 1 (n = 4765) respectively in Swedish arable soils with less than 7.0% organic carbon (Table 3). The median concentrations of organic carbon were 2.4 and 2.3% in the Scottish and Swedish soils, respectively. Another data set on the plough layer Scottish arable soils is the one by Shand et al. (2010) mentioned in Table 1. The 37 samples with b7% organic carbon in this set had a median of 0.53 mg kg − 1and a mean of 0.54 mg kg − 1. However, the median concentration of organic carbon, 3.6% was also a little higher than in the data sets presented above. Seventeen Scottish organic topsoils from podzols formed under aerobic conditions had a median of 0.51 mg kg − 1 compared

C.A. Shand et al. / Journal of Geochemical Exploration 121 (2012) 4–14

Soil Association

Parent material

Arkaig

Drifts derived from schists, gneisses, granulites and quartzites principally of the Moine Series

Balrownie

Drifts derived mainly from sandstone of Lower Old Red Sandstone age, often water modified

Corby

Fluvioglacial and raised beach sands and gravels derived from acid rocks

Countesswells

Drifts derived from granites and granitic rocks

Darleith

Drifts derived from basaltic rocks

Ettrick

Drifts derived from Lower Paleozoic greywackes and shales

Foudland

Drifts derived from slates, phyllites and other weakly metamorphosed argillaceous rocks

Lochinver

Drifts derived from Lewisian gneisses

Rowanhill

Drifts derived from Carboniferous sandstones, shales and limestones

Sourhope

Drifts derived from Old Red Sandstone intermediate lavas

Strichen

Drifts derived from arenaceous schists and strongly metamorphosed argillaceous schists of the Dalradian Series

Tarves

Drifts derived from intermediate rocks or mixed acid and basic rocks, both metamorphic and igneous

Torridon

Drifts derived from Torridonian sandstones and grits

9

Fig. 2. Selenium concentrations in Scottish topsoils at the 20-km national grid intersection positions (n = 182) expressed as percentiles and map of the major soil associations (parent materials).

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C.A. Shand et al. / Journal of Geochemical Exploration 121 (2012) 4–14

Table 4 Distribution of Se (mg kg− 1) in Scottish topsoilsa (n = 182) according to land cover. Horizonb

Arable

Bog

n L F H O A E B

Mean

17

Horizonb

0.37

L F H O A E B b c

n

Improved grassland Mean

Median

5 3

0.87 0.26

0.80 0.03

27

1.43

1.45

n

0.43

28

Semi-natural grassland n

a

Median

Mean

Mean

0.52

Moorland Median

n

Mean

Median

0.25

2 2 5 29 3

0.74 1.19 0.96 2.06 2.76

0.74 1.19 1.01 1.90 2.35

Woodland Median

3 13 14

2.73 2.86 0.65

2.51 2.08 0.47

1

1.14

1.14

Weighted mean, all types

n

Mean

Median

n

Meanc

Median

8 4 3 9 5 1

0.45 0.47 0.49 2.68 0.43 0.59

0.03 0.46 0.44 2.00 0.40 0.59

15 9 11 78 67 1 1

0.63 0.56 1.31 2.05 0.60 0.59 1.13

0.44 0.73 0.86 1.81 0.42 0.59 1.13

a a ab b a

Topsoil refers to the uppermost soil horizon that was sufficiently thick to be sampled. See MISR (1984) for explanation of symbols. Values followed by a common letter are not significantly different (p = 0.05), mixed and buried horizons excluded.

to 0.54 mg kg − 1 in mor layers from Swedish iron podzols analysed by Johnsson (1989). Another way to assess differences in the soil Se status of the two countries is to compare the upper layer (up to ca 30 cm) in Scottish soils classified as peat with plough layers of Swedish soils with more than 20% organic carbon (the minimum value in both sets). The mean organic carbon content was slightly higher in Scottish soils (median = 46.5%, mean = 43.9%) than in the Swedish soils (median = 37.2%, mean =35.7%). The median Se concentration was slightly lower in the Swedish soil than in the Scottish organic topsoils while the mean was higher in the Scottish (Table 3). 4. Discussion The determination of the total amount of elements in soil by solution analysis techniques such as ICP-MS requires total dissolution of the sample. This is often impractical and a pseudo total is achieved by acid extraction with aqua regia or nitric acid. Our results show that aqua regia and nitric acid data can be compared. Thus it is also acceptable to include the data in the subset of samples extracted with nitric acid in the Swedish data set.

There is a growing amount of information highlighting the positive relationship between the concentration of Se and the organic matter in soils from temperate regions (Antanaitis et al., 2008; Fordyce et al., 2010a; Gustafsson and Johnsson, 1992; Låg and Steinnes, 1978; Shand et al., 2010). We also found such a correlation. Our data have further shown soil parent material to be less important than the amount of organic carbon in Scottish and Swedish soils, confirming the previous findings of Fordyce et al. (2010a) and Shand et al. (2010). A plausible reason for this is that soil parent materials in both countries are dominated by rocks low in Se, and any variation in the influence of rock type on soil Se concentrations is hidden by the larger effect of the varying organic matter concentrations. The organic carbon-normalised Se data indicate that a significant part of the correlation between Se concentration and soil organic carbon is just a co-variation, not a cause. In some areas, where the concentration of organic matter tends to be high, concentrations of Se also tend to be high, but due to factors other than the organic matter content itself. A good example is the high Se concentrations in the arable soils on the Swedish southwest coast. Also, organic carbonnormalised Se concentration values are elevated here indicating that the higher Se concentrations are not only due to the higher organic

Table 5 Distribution of Se (mg kg− 1) in Scottish topsoils (n = 182) according to drainage class and land cover. Drainage class Excessive Free Imperfect Moderate Poor Very poor Drainage class

Excessive Free Imperfect Moderate Poor Very poor a

Arable

Bog

n

Mean

Median

4 7 3 3

0.22 0.39 0.39 0.53

0.21 0.43 0.46 0.61

n

5 30

Semi-natural grassland

Improved grassland Mean

Median

1.69 1.18

1.04 1.28

Mean

Median

n

Mean

Median

8 12 5 3

0.50 0.37 0.20 1.64

0.66 0.24 0.12 0.97

6 4 4 19 8

1.12 2.06 1.49 2.21 1.73

1.02 2.08 1.34 1.89 1.64

Woodland

n

Mean

Median

1 5 1 5 15 4

0.03 0.99 1.20 0.88 2.49 1.89

0.03 0.68 1.20 1.14 1.59 1.65

n 12 4 2 7 5

Values followed by a common letter are not significantly different (p = 0.05).

Moorland

n

Weighted mean, all types Mean 0.26 0.32 0.61 1.60 3.41

Median

n

Meana

Median

0.10 0.17 0.61 0.80 2.51

1 35 28 19 52 47

0.03 0.56 0.64 0.73 2.03 1.57

0.03 0.40 0.41 0.59 1.46 1.43

a a a ab c bc

C.A. Shand et al. / Journal of Geochemical Exploration 121 (2012) 4–14

11

Scottish soils, B horizons (total of 162 samples) 6

Selenium (mg/kg)

5 4 3 2 1 0

) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) 28 =3 =3 =4 10 =6 =3 11 =3 17 =8 =3 =3 =7 =3 =3 =4 24 =3 =3 =7 =3 =3 n= al(n le(n ie(n (n= y(n ill(n (n= rd(n (n= d(n er(n ie(n ill(n g(n e(n n(n (n= o(n n(n s(n et(n w(n ( ig vi a n lls rb h ith fo ck an v u h lin op or n rs o ve n ro ka llu ied ow e Co urn rle ck ttri udl hin illb wan tir urh S iche hu rrid Tar Ty ar Y r T To Ar A err alr ssw D Da E E Fo oc M o S o L St S R B B nte u o C

Scottish soils, B horizons (total of 191 samples) 6

Selenium (mg/kg)

5 4 3 2 1 0

High (b)

Low (a)

No info (ab)

Predicted Se status (relative concentration values)

Fig. 3. Selenium concentrations in B horizons of Scottish soils. Top: restricted to those soil associations with ≥ 3 samples in the category. The boxplots show the first, second and third quartiles. Outliers are identified by asterisks and the vertical bar represents the lowest and highest values within the limits. The Se concentration predicted from parent rock properties are as follows: double hashed = predicted to be relatively high; plain grey = predicted to be relatively low; open = no information; Bottom: Boxplots for all B horizons. Values for groups followed by the same letter are not significantly different (p = 0.05).

matter concentrations of this humid area but also due to air-borne inputs of elements from the sea (Låg and Steinnes, 1978). Similarly, Johnsson (1989) found a positive correlation between the distribution of Se concentrations in the mor layer of Swedish forest soils and the distribution of annual mean precipitation. However, the encircled areas on our “normal” Se map (Fig. 5a) that cease to exist on the carbon-normalised map indicate that organic matter content itself also strongly influences Se concentrations, especially in areas away from marine influence. Another possible factor for higher Se concentration on the Swedish southwest coast could be that large parts of these areas were submerged under the sea after the last glaciation and that the sedimentary soils from this period are of marine origin. At the time when the glacial ice retreated, the east coast of Sweden around Stockholm as well as most of the land from there across to the west coast was covered by the sea. Thereafter, the eastern part was cut off from the sea and covered by fresh water while most of the land near the west coast rose out of the sea. Therefore, elevated Se concentrations in the

west coast soils of Sweden due to stronger marine influence can probably only be expected in the area within the immediate vicinity of the coast as only these have been under the sea from glacial up to recent time. Along the northern part of the west coast of Sweden, the carbon-normalised Se concentrations are indeed higher nearest to the sea. However, this could also be due to more air borne inputs from the sea. The map with organic carbon-normalised Se values also gives some indication of the influence of parent material on the Se concentrations in Swedish arable soils. The higher than average Se concentrations in the heavy clays in the region around Stockholm indicate that clay sediments may have higher Se concentrations than surrounding coarser textured soils. However, the regression data (Table 6) indicate that for the total arable land very little of the total variation is explained by clay content. The high Se concentration in the soils of the inland area of Jämtland in the central part of Sweden is presumably due to shale (McNeal and Balistrieri, 1989), which has been brought in from sedimentary rock from the

12

C.A. Shand et al. / Journal of Geochemical Exploration 121 (2012) 4–14

Scottish topsoils (total of 182 pairs of values)

Selenium (mg/kg)

9 8

Arable (n=17) Bog (n=35) I mproved Grassland (n= 28) Moorland (n= 41) Semi-natural Grassland (n= 31) Woodland (n= 30)

7 Selenium = (0.02615 × C) + 0.5641

6

5. Conclusion

5 4 3 2 1 0 0

10

20

30

40

50

Carbon(%weight)

Scottish topsoils (total of 64 pairs of values) Arable (n= 15) I mproved Grassland (n= 26) Moorland (n= 5) Semi-natural Grassland (n= 13) Woodland (n= 5)

2.0

Selenium (mg/kg)

in the soil as pedological drainage declines. Poor drainage is associated with anaerobic conditions and the accumulation of organic carbon. Unravelling the links between the accumulation of carbon and Se, in relation to climate, marine inputs, changes in soil bulk density, etc. is outside the scope of this paper.

1.5

Selenium concentration values obtained by extraction of a group of 42 test soils with 7 M nitric acid or aqua regia were not significantly different showing that data obtained by the two extraction procedures may be compared. The Se concentration of soils from Scotland and Sweden, which are richer in organic carbon than other more southerly latitudes in Europe, is more related to organic carbon concentration than other geochemical factors such as parent rock. For both countries a clear effect of parent material on Se concentration was mainly found in soils formed on parent material expected to be relatively rich in Se, i.e. drifts derived from slates, phyllites, greywackes, shales and other argillaceous rocks in Scotland for B-horizon soils and alum shales in Sweden for ploughed soils. In both Scotland and Sweden the topsoils with the highest Se concentrations tend to be from the west side of the country where the prevailing Atlantic weather from the west is expected to result in increased marine inputs. Without additional work we cannot categorically assume the link.

1.0

Acknowledgements 0.5

0.0 0

2

4

6

8

10

12

Clay (% weight) Fig. 4. Relationship between Se and organic carbon (top) or clay concentrations (bottom) in topsoils from Scotland coded according to land cover.

mountains north of this area by glacial ice. Also in other areas where soils are influenced by alum shales, soil Se concentrations are slightly elevated, i.e. south of lake Vänern in south Sweden, east of the nearby lake Vättern and in the southeastern tip of Sweden. Also for Scotland elevated Se concentrations were found in areas with rocks which, according to geochemical principles, are expected to have relatively high Se concentrations. A few soils with high Se concentration values (>3 mg kg− 1) in the B horizons were formed from drifts derived from slates, phyllites, greywackes, shales and other argillaceous rocks. It is difficult to compare the Se concentrations in the two countries from the available datasets since the sampling strategies are different. The stratification of the material to get more homogenous subsets is also uncertain. If we assume that the different averages are representative there is a tendency for higher Se concentrations in Scottish topsoils from arable land and peat than in corresponding Swedish topsoils. The mor layer concentrations on the other hand are more or less the same. For arable soils the data from Shand et al. (2010) also indicate that Se concentrations in plough layer soils are higher in Scotland than in Sweden. The soil sampling strategy and the range of properties recorded in the inventory of Scottish soils provide detailed information about the changes in Se concentration with depth from the soil surface. The data on the Scottish soils show a trend of decreasing Se concentrations down the profile from the A to the B to the C horizon and increasing from the L to the F to the H to the O horizon, although the differences are not always statistically significant at p = 0.05 (Table 2). Higher concentrations in the surface horizons compared to subsoil horizons in mineral soil are presumably due to accumulation of Se deposited from the air and biocycling from the subsoil via plants. The Scottish data show a trend of increasing Se concentration

The work is part of a collaborative project on micronutrient management strategies funded by the Scottish Government Rural and Environmental Science and Analytical Services (RESAS) Division and the Swedish Research Council, Formas. The Swedish monitoring programme is financed by the Swedish Environmental Protection Agency. We thank the staff of the Analytical Group of the James Hutton Institute for the analyses of the Scottish soils, Malcolm Coull for producing the Se topsoil map of Scotland, and Gordon Hudson for providing geological information and advice on matters related to the national soil inventories of Scotland. The analyses of the Swedish soils were made by ALS Scandinavia in Luleå. The Se soil maps of Sweden were produced by Mats Söderström, SLU. References Adriano, D.C., 2001. Trace Elements in Terrestrial Environments. Biogeochemistry, Bioavailability, and Risks of Metals, second ed. Springer, New York. Antanaitis, A., Lubyte, J., Antanaitis, S., Staugaitis, G., Viskelis, P., 2008. Selenium concentration dependence on soil properties. Journal of Food, Agriculture and Environment 6, 163–167. Berrow, M.L., Ure, A.M., 1989. Geological materials and soils. In: Ihnat, M. (Ed.), Occurrence and Distribution of Selenium. CRC Press, Boca Raton, Florida, pp. 213–242. Combs, G.F., 2001. Selenium in global food systems. British Journal of Nutrition 85, 517–547. Coppin, F., Chabroullet, C., Martin-Garin, A., 2009. Selenite interactions with some particulate organic and mineral fractions isolated from a natural grassland soil. European Journal of Soil Science 60, 369–376. Eriksson, J., Mattsson, L., Söderström, M., 2010. Tillståndet i svensk åkermark och gröda, data från 2001–2007 (Current status of Swedish arable soils and cereal crops. Data from the period 2001–2007). Naturvårdsverket. (in Swedish with summary in English). Fairweather-Tait, S.J., Bao, Y., Broadley, M.R., Collings, R., Ford, D., Hesketh, J.E., Hurst, R., 2011. Selenium in human health and disease. Antioxidants & Redox Signaling 14, 1337–1383. Forbes, S., Bounds, G.P., West, T.S., 1979. Determination of selenium in soils and plants by differential pulse cathodic-stripping voltammetry. Talanta 26, 473–477. Fordyce, F., 2007. Selenium geochemistry and health. Ambio 36, 94–97. Fordyce, F.M., Brereton, N., Hughes, J., Luo, W., Lewis, J., 2010a. An initial study to assess the use of geological parent materials to predict the Se concentration in overlying soils and in five staple foodstuffs produced on them in Scotland. Science of the Total Environment 408, 5295–5305. Fordyce, F.M., Nice, S.E., Lister, T.R., Ó. Dochartaigh, B.É., Cooper, R., Allen, M., Ingham, M., Gowing, C., Vickers, B.P., Scheib, A., 2010b. Urban soil geochemistry of Glasgow. British Geological Survey Open Report, OR/08/002. British Geological Survey, Edinburgh. Foster, L.H., Sumar, S., 1997. Selenium in health and disease: a review. Critical Reviews in Food Science and Nutrition 37, 211–228.

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Fig. 5. Selenium concentrations, organic matter concentrations and organic carbon-normalised Se values in Swedish, arable topsoils. Gustafsson, J.P., Johnsson, L., 1992. Selenium retention in the organic matter of Swedish forest soils. Journal of Soil Science 43, 461–472. Haygarth, P.M., 1994. Global importance and global cycling of selenium. In: Frankenberger, W.T., Benson, S. (Eds.), Selenium in the Environment. Marcel Dekker, New York, pp. 1–27. ISO (International Organization for Standardization), 1995. ISO 11466:1995, Soil Quality— Extraction of Trace Elements Soluble in Aqua Regia. BSI, London. Johnsson, L., 1989. Se-levels in the mor layer of Swedish forest soils. Swedish Journal of Agricultural Research 19, 21–28. Johnsson, L., 1992. Selenium in Swedish soils. Factors influencing soil content and plant uptake. PhD Thesis, Swedish University of Agricultural Sciences, Uppsala. Låg, J., Steinnes, E., 1978. Regional distribution of selenium and arsenic in humus layers of Norwegian forest soils. Geoderma 20, 3–14. Lévesque, M., 1974a. Selenium distribution in Canadian soil profiles. Canadian Journal of Soil Science 54, 63–68.

Relationship between Se and C concentrations in ploughed Swedish soils (n=5130) 6

Selenium (mg/kg)

Selenium = (0.03378x C) + 0.1734

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5 4 Table 6 Selenium concentration (mg kg− 1) as function of pH, organic C content and clay content in Swedish plough layer soils with b 7% organic C (n = s). Organic matter values have been log (base10) transformed and clay content values have been square root transformed to be more normally distributed.

3 2 1 0 0

10

20

30

40

50

60

Carbon (%weight) Fig. 6. Relationship between Se and organic carbon concentrations in Swedish topsoils.

Variable

Parameter estimate

Partial R2

Intercept pH Log organic C Square root of clay Model R2

− 1.32 0.05 0.65 0.03 –

– 0.02 0.30 0.07 0.39

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Sparks, D.L., 1995. Environmental Soil Chemistry. Academic Press, London. Ure, A.M., Berrow, M.L., 1982. The elemental constituents of soils. Environmental Chemistry, vol. 2. The Royal Society of Chemistry, London, pp. 94–204. Ure, A.M., Bacon, J.R., Berrow, M.L., Watt, J.J., 1979. The total trace element content of some Scottish soils by spark source mass spectrometry. Geoderma 22, 1–23. Walsh, T., Fleming, G.A., 1952. Selenium levels in rocks, soils and herbage from a high selenium locality in Ireland. Transactions of the Joint Meeting of Commission II, Soil Chemistry and Commission IV, Soil Fertility and Plant Nutrition, vol. 2, pp. 178–183. Dublin.