total and extractable nickel and cadmium contents in natural soils
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This article was downloaded by: [Dicle University]On: 09 November 2014, At: 18:07Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,UK
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Total and Extractable Nickeland Cadmium Contents inNatural SoilsRaquel Caridad‐Cancela a , Antonio Paz‐González a &
Cleide Aparecida de Abreu ba Facultad de Ciencias , Universidad de La Coruña ,A Coruña, Spainb Centro de Solos e Recursos Agroambientais ,Instituto Agronómico , Campinas (SP), BrazilPublished online: 05 Feb 2007.
To cite this article: Raquel Caridad‐Cancela , Antonio Paz‐González & CleideAparecida de Abreu (2005) Total and Extractable Nickel and Cadmium Contents inNatural Soils, Communications in Soil Science and Plant Analysis, 36:1-3, 241-252,DOI: 10.1081/CSS-200043057
To link to this article: http://dx.doi.org/10.1081/CSS-200043057
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Total and Extractable Nickel and CadmiumContents in Natural Soils
Raquel Caridad-Cancela and Antonio Paz-Gonzalez
Facultad de Ciencias, Universidad de La Coruna, A Coruna, Spain
Cleide Aparecida de Abreu
Centro de Solos e Recursos Agroambientais, Instituto Agronomico,
Campinas (SP), Brazil
Abstract: Trace element analysis in natural soil provides information on background
levels, which may be also useful to detect anthropogenic inputs. The main objective of
this study was to provide background levels of Cd and Ni for natural soils in Galicia
(Spain). Ten natural soil profiles, representative of different parent material with a
wide range of elemental composition, including ultramafic rocks such as serpentine,
were selected in this region. All samples were digested with nitric acid in a
microwave oven (U.S. EPA-SW 846 305 1 method) to assess “total” Cd and Ni
contents. Trace element extractions were carried out with diethylenetriaminepentaace-
tic acid (DTPA) and Mehlich-3. All analyses were performed by ICP-AES. Soil Cd
concentrations obtained by the U.S. EPA method ranged from ,0.01 to
4.42 mg kg21, with an average of 2.03 mg kg21, and Ni concentrations ranged from
12.66 to 2066 mg kg21 with an average of 156 mg kg21. The mean Ni content was
higher, because the used sample included a soil that was developed over serpentine.
The DTPA-and Mehlich-3-extractable Cd and Ni average levels were 0.06 and
8.78 mg kg21 and 0.16 and 3.57 mg kg21, respectively. Nickel levels obtained by
both extractants were highly correlated (r2 ¼ 0.91). A correlation analysis between
total and extractable Cd and Ni form, and soil general properties showed that the
highest significant dependence was for CdDTPA vs. organic matter content and CEC.
Address correspondence to Raquel Caridad-Cancela, Facultad de Ciencias,
Universidad de La Coruna, Campus de la Zapateira, s/n, A Coruna 15071, Spain;
E-mail: [email protected]
Communications in Soil Science and Plant Analysis, 36: 241–252, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0010-3624 print/1532-2416 online
DOI: 10.1081/CSS-200043057
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INTRODUCTION
Soil composition is extremely diverse and governed by many factors,
but climatic conditions and parent material are most predominant (1). The
element contents vary widely within soils, depending largely on the
geology. According to Oertel (2), there is a close relationship between the con-
centration of trace elements in a soil and its parent material. So, Cd is likely to
be concentrated in argillaceous and shale deposits, whereas Ni contents are
highest in ultramafic rocks. These concentrations decrease with increasing
acidity of rocks, so that, for example, in sedimentary rocks the highest
range is found in argillaceous rocks and the lowest in sandstones (1).
A review of the literature has revealed that the average content of total Cd
in soils is between 0.06 and 1.1 mg kg21, and the amount of total Ni ranges
from 20 to more than 2000 mg kg21 (1, 3).
The “normal concentrations” of trace elements in soils are of great
interest because background values are needed to assess the degree of soil con-
tamination (1). Furthermore, heavy metals in soils can have negative influence
on the food chain because of their potential toxic effects. Thus, it is necessary,
in natural soils, to provide a quantitative orientation of these elements through
background levels. The problem is to select the best analytical method, both
for “total contents” and “available contents” to obtain these values.
As trace elements, Cd and Ni are found in small concentrations in most of
the natural and agricultural soils. Moreover, both elements are not needed for
plant growth. However, when high soil concentrations of Cd and Ni occur, the
content of these element in the plant also increases (1, 3).
There are different methods of soil analysis that could be suitable to
obtain background levels. In our study, the U.S. EPA method (SW 846 305 1)
was applied (4) by using closed vessels and a microwave as a heating
source. This method has a fast digestion, therefore, eliminating the risk of
external contamination and loss of volatile elements (5). Nevertheless,
some extractants have been suggested to estimate availability for plants of
trace elements, the list of methods being extensive. For Cd or Ni, an officially
recommended extractant does not exist.
The complexity of soil chemistry and soil-plant relationships is probably
the main reason for the existence of so many soil-testing methods, which also
clearly demonstrate the lack of agreement on the best alternatives (6).
The DTPA and Mehlich-3, both multielement extractants, are commonly
used in the extraction of trace elements such as Fe, Cu, Mn, or Zn. Although
these methods are applied in numerous works (7–11), the effectiveness of
DTPA and Mehlich-3 for other elements such as Cd and Ni has not been
demonstrated.
The DTPA soil test has a sound theoretical basis, is inexpensive, reprodu-
cible, and easily adapted to routine laboratory procedures (12). Mehlich-3 (7),
considered to be an “universal extractant,” is being widely used. Numerous
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authors have recommended the use of Mehlich-3 because it not only extracts
simultaneously various nutrients but also it shows good correlations between
the amount extracted from the soil, the amount absorbed by the plant, and
crop’s response (9).
The main purpose of this article is to provide background levels of Ni and
Cd from natural soils in Galicia (Spain) and to determine the effect of parent
material on the abundance of these two metals in the soil. The effectiveness of
DTPA and Mehlich-3 in the evaluation of Ni and Cd availability was also
studied. The importance of soil properties in the behavior of these elements
has also been discussed.
MATERIALS AND METHODS
Ten natural soil profiles developed over different parent material were selected
for this study. A total of 38 samples from 10 soil profiles were taken from
several regions of the provinces of La Coruna and Lugo (Galicia, NW
Spain). These soils were chosen as representative of the geology in Galicia,
including granite, basic schist, granulite, serpentine, tertiary-quaternary
clays, amphibolite, coastal sediment, phyllite, and limestone (Table 1).
According to the taxonomic categories, the studied soils belong to
Umbrisols, Phaeozems, Gleysols, Fluvisols, and Leptosols (13).
After the collection, mixed samples were air-dried, and passed through a
2-mm sieve. Routine analysis of soil properties was determined, conforming
to accepted methods described in detail in Guitian and Carballas (14) and
MAPA (15). Soil pH ranged mainly between 4.47 and 8.67; however, pH
values .7.0 were only found in the surface horizon of profile No. 8
developed over coastal sediments and in profile No. 10 developed over
limestone (Table 1). As expected, profiles over granite showed the most
acidic conditions, but profiles over materials containing mafic minerals, and
even over serpentine, and ultramafic rock, were also acidic. Soil organic
matter content range for A horizons (including both A1 and A2 layers) was
between 1.5 and 26.5%, but for most samples it was .4%; for A/B, B, and
C horizons, this figure was between 0.38 and 5%. The minimum and
maximum clay contents were 9.10 and 32.66%, respectively. Higher
CEC values were found in surface A horizons (ranged from 6.77 to
46.58 cmolþkg21) with regard to subsoil A/B, B, and C horizons (ranged
from 3.88 to 18.88 cmol kg21) (16, 17).
The method adopted to assess “total” metal content was digestion by
nitric acid (U.S. EPA-SW 846-305 1). For analyses, 500 mg of each sample
were placed in a Teflon PFA digestion vessel, and 10 mL of concentrated
nitric acid were added. The vessels were capped and placed in a microwave
(CEM model MDS-2000) oven carousel in groups of six samples each
time (4, 18).
Total and Extractable Nickel and Cadmium Contents in Natural Soils 243
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Table 1. Total and available Cd and Ni contents in studied soils (in mg kg21)
No. Parent material Depth (cm) CdEPA NiEPA CdDTPA NiDTPA CdM-3 NiM-3
1 Granite (0–12) 3.01 21.74 0.18 0.30 0.31 0.97
(12–50) 2.81 20.68 0.11 0.22 0.28 0.88
2 Basic schist (0–8) 1.77 55.81 0.11 4.09 0.15 1.92
(8–52) 1.65 40.07 0.02 0.29 0.09 0.58
(52–90) 1.55 44.56 ,0.01 ,0.01 0.16 0.26
(90–110) 1.76 46.16 ,0.01 0.04 0.09 0.28
(þ110) 1.91 48.58 ,0.01 0.06 0.03 1.90
3 Granulite (0–12) 1.67 105.26 0.05 3.79 0.15 2.31
(12–45) 1.04 176.04 0.04 1.35 0.09 1.18
(45–60) 1.15 203.02 ,0.01 0.40 0.04 0.67
(60–90) 1.41 210.01 ,0.01 0.12 ,0.01 0.59
(þ90) 1.35 155.49 ,0.01 ,0.01 0.07 0.60
4 Serpentine (0–8) 2.79 838.95 0.07 81.08 0.31 42.65
(8–35) 0.73 1101.1 ,0.01 143.31 0.20 39.90
(35–40) ,0.01 2066.1 ,0.01 61.43 0.09 22.99
5 Sediment (0–6) 2.00 20.34 0.08 0.90 0.18 1.23
(6–11) 2.14 12.66 0.08 0.33 0.19 0.62
(11–40) 2.18 13.53 0.03 0.06 0.11 0.54
(þ40) 1.75 13.74 0.03 0.11 0.06 0.51
6 Granite (0–4) 2.42 22.97 0.14 0.31 0.20 0.95
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(4–29) 2.22 20.86 0.02 0.12 0.07 0.36
(29–44) 2.39 24.38 ,0.01 ,0.01 0.09 0.43
(44–84) 1.43 25.93 ,0.01 0.05 0.09 0.22
(þ84) 2.14 22.10 ,0.01 0.06 ,0.01 0.46
7 Amphibolite (0–15) 1.60 48.27 0.05 0.43 0.15 0.75
(15–22) 2.07 53.98 ,0.01 0.16 0.13 0.74
(22–45) 1.61 52.59 ,0.01 0.01 0.04 0.50
(45–65) 1.89 38.77 0.02 0.02 0.03 0.43
8 Sediment (0–6) 2.06 40.18 0.13 0.99 0.41 1.47
(6–35) 2.19 41.12 0.03 0.86 0.31 1.14
(35–95) 2.09 39.89 0.05 0.81 0.23 0.87
(þ95) 1.72 39.41 0.12 2.06 0.23 1.19
9 Phyllite (0–4) 2.82 40.13 0.03 1.37 0.26 1.13
(4–23) 2.77 43.05 0.02 0.74 0.17 0.68
(23–45) 2.11 43.85 ,0.01 0.25 0.09 0.66
10 Limestone (0–18) 4.42 49.04 0.04 0.22 0.25 0.98
(18–40) 2.61 50.29 0.08 0.45 0.26 1.19
(þ40) 1.82 47.95 0.02 0.36 0.15 0.91
Mean 2.03 156.28 0.06 8.78 0.16 3.57
Minimum ,0.01 12.66 ,0.01 ,0.01 ,0.01 0.22
Maximum 4.42 2066.15 0.18 143.31 0.41 42.65
CV (%) 32.51 245.65 83.33 328.25 56.25 271.99
TotalandExtra
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The available Ni and Cd contents were determined both after extraction
with DTPA (19) and with Mehlich-3 (7) using the following procedures:
1. DTPA: 0.005 mol L21 DTPA, 0.1 mol L21 TEA (triethanolamine) and
0.01 mol L21 CaCl2 at pH 7.3. Soil volumes of 10 cm3 and 20 mL
DTPA solution were used for the extraction. The suspensions were
placed in polyethylene flasks covered with a plastic stopper and shaken
by horizontal-circular movements at 240 oscillations per minute for 2 hr.
The suspensions were then filtered, and nutrient concentrations were
measured.
2. Mehlich-3: 0.2 mol L21 CH3COOH, 0.25 mol L21 NH4NO3,
0.015 mol L21 NH4F, 0.013 mol L21 HNO3, and 0.001 mol L21 EDTA
adjusted to pH 2.5. Soil volumes of 5 cm3 with 50 mL of Mehlich-3
extractant were taken. The suspensions were placed in polyethylene
flasks covered with plastic stoppers and shaken by horizontal-circular
movements at 240 oscillations per minute for 5 min. After filtration, the
nutrient contents were determined.
All determinations were replicated two times. Both total and available
forms of Ni and Cd were analyzed by inductively coupled plasma emission
spectrometry (ICP-AES), Jobin Yvon JY 50-P model (18).
The resulting data were statistically treated. Linear correlation was used
to guarantee the dependence of Ni and Cd concentrations from general soil
properties and relationships between the different methods. The statistical
significance levels considered were p , 0.01 and p , 0.05 (20).
RESULTS AND DISCUSSION
Total Cd and Ni Content
The Cd and Ni total concentrations obtained by using the U.S. EPA method
are shown in Table 1. It is observed that the Cd concentrations in the
soils studied ranged from ,0.01 to 4.42 mg kg21, with an average of
2.03 mg kg21. The highest total Cd content was found in profile No. 10
(over limestone), where profile No. 4 (over serpentine) contained the lowest
total Cd. These results are in accordance with those in the literature (21),
where it is reported that ultramafic materials present lower Cd amounts than
limestone rocks.
To assess background levels of Cd in the soils studied, the values obtained
were compared with those reported by other authors (1, 3). Comparison with
other references may produce ambiguities in the evaluation because there are
differences in soil, parent material, and climatology among the various
countries and geological materials (22). Nearly all samples studied here
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showed high total Cd values, higher than the range of 0.06–1.1 mg kg21
quoted in the literature for the natural soils (1). These results suggest the
existence of problems during the analysis in determining Cd content by the
U.S. EPA 3051 method, or that this method is not suitable to evaluate total
Cd content. This last statement is supported by data obtained for the same
samples using another method (acid extraction by HF). In this case, the
mean Cd concentration was 0.14 mg kg21 with a range of 0.07–
0.86 mg kg21, and these results were closer to those found in the literature.
For Ni, the range was 12.7–2066 mg kg21, with a mean of 156.3 mg kg21.
Taking into account that one profile (profile No. 4) in the set of samples was
developed over serpentine, Ni-rich parent material, it is not surprising that the
mean concentration is considered excessive. In this particular case, it is
assumed that high natural Ni levels could have phytotoxic effects, causing
unfavorable growth of vegetation. Nevertheless, the rest of the samples
showed Ni contents similar to those found in the literature (1, 3). The
results obtained by the U.S. EPA method for Ni, in general, are consistent
with those in the literature, including those for profile No. 4. This method
could be adequate for the determination of this element.
As expected, the results showed an important influence of parent material
on Cd and Ni content in the soils studied. Perhaps this factor should be con-
sidered when the objective is to provide background levels. Examination of
the coefficient of variation (CV) revealed a clear indication of the wide geo-
logical variability of the soils studied, mainly for Ni. Clearly, parent
material influences the amount of Ni present in the given soils.
In general, the patterns of distribution followed by total Cd decreased
with depth and howed an irregular distribution (considered when the
element neither accumulates at the surface or at a specific depth). The total
Ni content in the profile tends to increase with depth or remain relatively
stable, with the exception of the soil developed with limestone. Nevertheless,
the soil composition and parent material could have a significant role in the
distribution of Cd and Ni in the profile.
When the study was conducted according to soil groups (Table 2), few
significant differences were observed between them for total Cd, but for
Table 2. Total and available Cd and Ni contents by soil groups studied (in mg kg21)
Element Umbrisols Phaeozems Gleysols Fluvisols Leptosols
CdEPA 1.85 2.47 2.02 2.02 2.57
NiEPA 68.44 692.24 15.07 40.15 42.34
CdDTPA 0.07 0.05 0.06 0.08 0.03
NiDTPA 0.66 47.81 0.35 1.18 0.79
CdM-3 0.12 0.21 0.24 0.30 0.17
NiM-3 0.81 18.10 0.73 1.17 0.82
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total Ni, the content in the samples analyzed was inherent to the geology. So,
the highest levels were found in profile No. 4, developed over ultramafic
material (serpentine), and classified as Phaeozems, whereas the lowest total
Ni contents were for Gleysols, developed over tertiary-quaternary clays,
followed by Fluvisols over coastal sediments.
DTPA and Mehlich-3 Extractable Cd and Ni Content
A summary of the DTPA and Mehlich-3 (M-3) extraction results for all
samples is shown in Table 1. The CdDTPA ranged form 0.01 to
0.18 mg kg21, with a mean value of 0.06 mg kg21. The highest CdDTPA
content was found in the more superficial sample of profile No. 1, which
was also richer in organic matter than the rest of the samples. Organic
matter content could have an important effect on the content of this form of
available Cd, taking into account that DTPA will extract preferentially
complexed metals for this soil property. The range in Cd extracted with
M-3 was ,0.01–0.41 mg kg21, with a mean value of 0.16 mg kg21. In all
the samples analyzed, M-3 generally showed a greater extraction capacity
than DTPA, and considering the mean values, differences between extractants
were translated as 2.66 times more Cd with M-3 than with DTPA. This result
was expected because the presence of acid reagents and chelates such as
EDTA im M-3 is thought to extract higher amounts of elements than DTPA.
When the analysis was conducted for Ni, the range obtained with DTPA
was ,0.01–143.31 mg kg21 with a mean value of 8.78 mg kg21, and
0.22–42.65 mg kg21 with a mean value of 3.57 mg kg21 for M-3. The
highest Ni concentrations were found in profile No. 4, with both DTPA and
M-3, and the lowest contents were observed in profiles No. 2, 3, and 6 with
DTPA, and in profile No. 6 with M-3.
Comparing the extraction capacity of DTPA vs. M-3, in most samples
M-3 extracted more Ni than did DTPA, which may be due to its acid
condition (pH 2.5). However, exceptions were seen in some samples
(e.g., in profile No. 4) over serpentine, where DTPA extracted more Ni
than M-3. Consequently, large differences are found for mean NiDTPA and
NiM-3values, DTPA extracting about 2.46 times the amount obtained by
M-3. Presumably, soil pH is the main factor that explains the Ni soil
content, because in samples with a more elevated pH, DTPA showed the
highest extractability. This finding could be related to the higher effectiveness
of this method at neutral or alkaline conditions (16, 17). If profile No. 4 was
considered in the calculation of the average, the extraction capacity of the M-3
method (0.86 mg kg21) resulted much better with regard to those of DTPA
(0.67 mg kg21), thus M-3 extracted 1.28 times more than DTPA.
With reference to CV for Cd and Ni extracted with DTPA and M-3, it is
noteworthy the high values found for Ni, CV giving values in the order of
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328% (Table 1). The notable differences seen among the parent material of the
soils studied elevate the variability, especially in the case of Ni.
The results when evaluated according to soil groups (Table 2) showed that
elevated Cd concentrations were observed, both with DTPA and M-3, in
Fluvisols. High contents of available Cd in Fluvisols can be due to several
soil properties. According to the correlation coefficients values obtained in
Fluvisols, M-3 extracted Cd associated with organic matter (r ¼ 0.96) and
CEC with (r ¼ 0.98). With both extractants Phaeozems showed the highest
available Ni concentrations. Again, it is evident that parent material exerts a
strong effect on the available content of Ni, as quoted earlier for total Ni
content.
Available Cd and Ni contents followed the expected distribution in soil
profile, because, in general, in the 10 profiles studied, the highest amounts
were observed in the top layer, enriched with organic matter. These results
indicate that the same distribution trend observed for organic matter can be
shown for available Cd and Ni contents in the soils studied. Thus, an
important role of organic matter content on Cd and Ni dynamics between
the soil and plant is suggested.
The correlation of DTPA and M-3 was used to predict the relationship
between both extractants for the determination of available Cd and Ni in
the soils studied (Fig. 1). Both extractants behave differently for Cd, each
one extracting the Cd associated to different pools, as is reflected by the
low correlation coefficient obtained (r ¼ 0.33), despite it being significant
(p , 0.01). Nickel extracted by DTPA was highly correlated with that
extracted by M-3 (r ¼ 0.91). This good correlation suggests that both DTPA
and M-3 extract Ni from the same pool and that the two extractants can be
used to predict the available Ni contents in a given soil. The large
magnitude of variation for Cd and Ni extracted by DTPA or M-3 seems to
depend on the nature of soil parent material and in some cases on the soil pH.
Correlation Analysis Between Total and Available Cd and Ni
Content and Soil Properties
A correlation analysis was carried out between Cd or Ni contents, extracted by
the U.S. EPA method and by DTPA or M-3 and soil properties (Table 3). In the
case of total contents, both CdEPA and NiEPA were significantly correlated with
clay content. This relationship could be explained because the clay fraction has
exchangeable transition metal cations (mainly Cu, Fe, and Co) (1), and in
particular Ni has an ionic radii similar to Cu, Fe, or Co, suggesting that
isomorphic substitution can be produced in the clays.
When the relationship between available forms of Cd and Ni contents and
soil properties was studied, it was seen that CdDTPA correlated significantly
with organic matter content (r ¼ 0.70) and CEC (r ¼ 0.66), whereas NiDTPA
Total and Extractable Nickel and Cadmium Contents in Natural Soils 249
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showed no significant correlation with selected soil properties. Using the
Mehlich-3 extractant, CdM-3 was significantly correlated with pH (r ¼ 0.47),
OM (r ¼ 0.59), and CEC (r ¼ 0.60), and NiM-3 was significantly correlated
with clay content (r ¼ 0.35) and CEC (r ¼ 0.33). This likely indicated that
Figure 1. Relationship between Cd and Ni extracted by DTPA and M-3.
Table 3. Correlation between total (U.S. EPA) and available (DTPA and M-3) Cd and
Ni content and selected soil properties (pH, clay content, organic matter content, and
exchange cationic capacity)
n pH Clay OM CEC
CdEPA 37 ns 0.32� ns ns
NiEPA 38 ns 0.34� ns ns
CdDTPA 24 ns ns 0.70�� 0.66��
NiDTPA 35 ns ns ns ns
CdM-3 36 0.47�� ns 0.59�� 0.60��
NiM-3 38 ns 0.35� ns 0.33�
ns, not significant.�p , 0.05; ��p , 0.01.
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available Cd is mainly associated with the organic matter fraction, whereas
available Ni is controlled by clay content. The correlation of Cd and Ni
with CEC is a function of soil organic matter and clay content, so that
finding probably reflects the effect of both properties on Cd and Ni adsorption.
Let us summarize the above results. In the natural soils studied, it was
observed that total Cd and Ni contents were highly dependent on parent
material. The U.S. EPA method, specifically in the case of Cd, was not
suitable for obtaining background levels, but it was suitable for determining
Ni content.
In general, M-3 had a higher extraction capacity than DTPA, except for
samples with pH in the neutral or alkaline range. The extractant M-3 corre-
lated significantly with DTPA specially for Ni content.
Organic matter and clay content together with CEC are correlated with
different forms of Cd and Ni, which suggests an important role in the
dynamics of available Cd and Ni.
ACKNOWLEDGMENTS
This work was funded by Xunta de Galicia (Spain) in the frame of a soil
quality assessment project. Corrections and suggestions of two anonymous
referees are acknowledged with thanks.
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