American Journal of Plant Sciences, 2013, 4, 1918-1923 Published Online October 2013 (
Soil and Leaf Micronutrient Composition in Contrasting
Habitats in Podzolized Sands of the Amazon Region
María Antonieta Sobrado
Laboratorio de Biología Ambiental de Plantas, Departamento de Biología de Organismos, Universidad Simón Bolívar, Caracas,
Received July 5th, 2013; revised August 5th, 2013; accepted September 1st, 2013
Copyright © 2013 María Antonieta Sobrado. This is an open access article distributed under the Creative Commons Attribution Li-
cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Plant macronutrient distribution in podzolized sands of the Amazon caatinga has received attention in several studies;
however, the distribution of micronutrients has not been assessed. Soil micronutrient availability has been hypothesized
to reflect contrasting habitat characteristics as well as fundamental differences in substrate, and leaf micronutrient
composition may reflect the macronutrient content needed to maintain balance for leaf cell functions. In this study, soil
and leaf samples were obtained in a toposequence (valley, slope, and mound). Available soil micro- and macronutrients
as well as total leaf content were measured by inductively coupled plasma-atomic emission spectrometer and mass
spectroscopy. Soil Zn (<1.41 mg·kg1) and B (<0.31 mg·kg1) as well as Cu (<1.33 mg·kg1) levels were very low. Soil
Mn was low in the valleys and slopes (0.62 - 0.87 mg·kg1), but higher in the mound (6.59 mg·kg1). Soil Fe (11.48 -
21.13 mg·kg1) was well above the critical level in all of the habitats. Leaf micronutrients Cu, B, Zn, and Fe were below
the critical levels for tropical crops of 3 - 7, 20 - 70, 15 - 20, and 72 mg·kg1, respectively. Leaf Mn (<188 mg·kg1) and
Al (<50 mg·kg1) were below the accumulators level. A strong relationship between leaf micro- and macronutrients
suggests the maintenance of a homeostatic elemental composition, which may favour photosynthetic function. There-
fore, the local distribution of species may be shaped by their abilities to maintain a balance of micronutrient collected
through roots under critically low levels of available Zn, B, and Cu whilst excluding potentially deleterious ions of Mn,
Fe, and Al.
Keywords: Acid Soils; Amazon Caatinga; Ionome; Leaf Nutrient Homeostasis; Micronutrients; Toxic Elements
1. Introduction
Micronutrient availability and cycles in tropical areas are
poorly understood, even though such information is im-
perative for a thorough understanding of the complexities
of major element cycles in these habitats [1]. Very low
micronutrient levels have been found in tropical natural
areas that have severely impoverished macronutrient le-
vels as well [2,3]. The micronutrient distribution is par-
ticularly relevant in a situation of deficiency, when alter-
native plant developmental processes and biochemical
pathways may be prioritized as a function of growth, de-
velopment, metabolic status, and environment [4]. Thus,
the interactions among elements within plant cells tend to
reflect the regulatory network involved in the homeosta-
sis of the ionome [5,6]. These interactions and the bal-
ance of ions in plant tissue may have an ecophysiological
significance as well, which requires further investigation
in order to gain insight into these processes.
The upper Rio Negro in the Amazon basin experiences
high yearly rainfall levels, as well as differential soil
types and topographical conditions. Consequently a mo-
saic of vegetation types is featured. The Amazon caatinga
complex is located on lowland areas with infertile, blea-
ched, sandy podzols surrounded by low rolling hills con-
taining oxisol soils [7]. A clear toposequence (valley-
slopes-domes) with gradual changes is found within the
caatinga, which forms a complex community continuum
[8]. The macronutrient concentration and fluxes in soil
organic matter as well as the organic matter turnover
rates have paramount importance for the maintenance of
fertility across the caatinga habitats [9,10]. Historically,
plant macronutrient distribution has received most atten-
tion for this vegetation type [9,11,12], and the distribu-
tion of micronutrients has not been assessed previously.
In this study, it was hypothesized that soil micronutrient
availability may reflect contrasting habitat characteristics,
Copyright © 2013 SciRes. AJPS
Soil and Leaf Micronutrient Composition in Contrasting Habitats in Podzolized Sands of the Amazon Region 1919
and that the leaf micronutrient composition of species
thriving in different habitats may reflect the macronutri-
ent content at particular sites in order to maintain the ion
balance for leaf cell functions. In this study, the micronu-
trient composition of soils and leaves of dominant tree
species within the toposequence of the caatinga vegeta-
tion was assessed. In particular, differences in the domi-
nant species thriving in each of these contrasting sites
were determined.
2. Material and Methods
2.1. Study Site and Species
The study site is located near the confluence of the Rio
Negro and the Casiquiare River in southern Venezuela
near the village of San Carlos de Rio Negro (1˚54’N,
67˚3˚W, 119 m ASL). The area features a mean annual
temperature of 26˚C and mean annual rainfall of 3600
mm. Podzol soils of the caatinga have developed proba-
bly from Precambrian sandstone, and the top soil con-
tains medium-textured sand mixed with humus [7]. In the
valleys, soils are water saturated and dominated by Epe-
rua leucantha Benth (Caesalpiniaceae; “yaguacana” for-
est). The slopes (ecotone) are dominated by Micranda
sprucei (Müll.Arg.) R. E. Schultes (Euphorbiaceae; “cu-
nuri” forest), which maintain a relatively stable water ta-
ble, and the slightly higher sandy mounds suffer droughts
during short rainless periods [7,8]. The species selected
for this study were E. leucantha and M. sprucei from the
close, high stature forests (18 - 25 m; valley and slopes).
The less fertile and drought-prone tops of the sandy
mounds contain tree species of low stature and open ar-
rangement (“bana” forest; 5 - 7 m). At this site, Pachira
sordida (R.E. Schult.) W.S. Alverson (syn. Rodognapha-
lopsis discol or A. Robyns), (Malvaceae) and Remijia mo-
rilloi Steyerm (Rubiaceae) are the dominant species. The
use of dominant species has proven to be ecological in-
dicators of typical habitats within the upper Rio Negro
vegetation [13]. From the valley towards the slopes and
sandy mounds, leaves become more scleromorphic, and
those on sandy mounds are relatively more drought-re-
sistant [14,15].
2.2. Leaf and Soil Sampling
For each species, three mature trees with a fully exposed
top canopy were selected and tagged for soil and plant
collection during August 2010 [13]. Four soil samples
consisting of sand and humus were collected from 0 - 5
cm depth under each tree (E. leucantha and M. sprucei)
or under a pair of trees (P. sordida and R. morilloi). The
four samples were pooled in the field after all visible
plant pieces were removed by hand. Each sample con-
sisting of sand and humus was dehydrated at room tem-
perature, ground, homogenized, and sieved though 2
mmmesh. A total of three pooled soil samples were ana-
lyzed in each of the three habitats. Three top canopy
branches were detached in each tagged tree and adult lea-
ves with a healthy appearance were collected and pooled.
Leaf blade samples, excluding major veins, were oven
dried to a constant weight at 60˚C and ground prior to
performing the analysis. A total of three pooled leaf blade
samples were analyzed for each plant species.
2.3. Soil Analysis
Soil subsamples were used for the following analyses:
soil pH was measured in a slurry of 5 g soil in distilled
water at a ratio of 1:1. Buffer pH or acidity was esti-
mated in subsamples of 5 g by using Mehlich 3 as a buf-
fer solution to estimate the acidity. Organic content (OM)
was determined by weight loss on ignition in subsamples
of approximately 5 - 7 g. Soil exchangeable micronutri-
ents (Fe, Mn, Zn, B, and Cu) and macronutrients (P, Ca,
K, and Mg) as well as Al were measured by using an In-
ductively Coupled Plasma-Atomic Emission Spectrome-
ter (ICP-AES, Varian, Model 730-ES, Palo Alto, Cali-
fornia, USA). The Mehlich 3 soil test extract was used in
subsamples for element extraction, except for B, which
was extracted in hot water. Soil subsamples weighing 10
g were used for B extraction and a subsample measuring
2.5 cm3 was used for the other elements. Soil cation ex-
change capacity (CEC) was estimated from the sum of
soil acidity (see above) and the Ca, Mg, and K content
and expressed as meq/100 g. The percentage of base sa-
turation (BS) was calculated as the percentage of soil
CEC occupied by Ca, Mg, and K. Detailed soil analysis
procedures have been previously described [16]. The soil
N and C composition analysis was performed at the Sta-
ble Isotope Research Facility for Ecological Research
(SIRFER), University of Utah (Salt Lake City, USA).
2.4. Leaf Analysis
Leaf samples were analyzed for the same elements mea-
sured in the soils using the ICP-AES as well. A total of
0.2 g per leaf sample was muffled and ashed, dissolved
in 2 ml of 5 N HCL, and heated at 200˚C for 2 min. Sub-
sequently, the samples were brought to a 10 ml volume
with de-ionized water and centrifuged at 2000 rpm for 4
minutes to precipitate carbon rests prior to performing
ICP measurements. The procedure followed is described
elsewhere [17]. The leaf N and C compositions of these
species have been previously reported [14,18].
2.5. Statistical Analysis
Measurements were performed on independent replicates
taken randomly, and data are presented as mean ± stan-
dard error. The normality of distributions was assessed
Copyright © 2013 SciRes. AJPS
Soil and Leaf Micronutrient Composition in Contrasting Habitats in Podzolized Sands of the Amazon Region
using the Kolmogorov-Smirnov test and equality of va-
riance using Levene’s test. When the data were normally
distributed with equal variance, one-way analysis of va-
riance (ANOVA) was used and multiple comparisons
were made with the Holm-Sidak test. Conversely, if data
normality and/or equal variance tests failed, then an
ANOVA-on-ranks test was used and statistical differ-
ences were determined with the Tukey test. Pearson cor-
relation coefficients were used to quantify the relation-
ships between micro- and macro-nutrients. Significance
level was set at P < 0.05 and the analyses were perform-
ed using SigmaStats 3.1 software for Windows (Systat
Software, Inc., Chicago, USA).
3. Results
3.1. Soil Analysis
The results of soils analysis are shown in Table 1. Soil
pH was significantly different across sites, with the low-
est (3.33 ± 0.06) and highest (4.22 ± 0.03) values found
in the waterlogged valley bottoms and sandy mounds,
respectively (p < 0.05). The pH was negatively related to
OM (r = 0.88; P < 0.001) and CEC (r = 0.87; P <
0.001). Additionally, pH was also negatively related to
Table 1. Soil characteristics and nutrient composition.
Toposequence sites
Valley Slope Mound
pH 3.33 ± 0.06 c 4.01 ± 0.04 b 4.22 ± 0.03 a
OM (%) 19.2 ± 4.4 a 7.5 ± 0.7 b 6.6 ± 1.2 b
CEC (meq/100 g) 20.6 ± 0.1 a 17.9 ± 0.4 b 14.4 ± 0.8 c
BS (%) 2.4 ± 0.2 b 2.9 ± 0.1 b 11.9 ± 0.3 a
Fe (mg·kg1) 14.36 ± 0.06 b 11.48 ± 0.79 b 21.13 ± 0.58 a
Mn (mg·kg1) 0.87 ± 0.08 b 0.62 ± 0.14 b 6.59 ± 1.83 a
Zn (mg·kg1) 1.41 ± 0.03 a 0.99 ± 0.07 b 1.35 ± 0.14 ab
B (mg·kg1) 0.20 ± 0.05 0.31 ± 0.05 0.17 ± 0.04
Cu (mg·kg1) 1.24 ± 0.06 a 1.33 ± 0.01 a 1.03 ± 0.01 b
Al (mg·kg1) 290 ± 43 a 159 ± 24 a 62 ± 4 b
C (g·kg1) 139 ± 32 a 49 ± 5 b 35 ± 14 b
N (g·kg1) 5.21 ± 1.30 a 1.92 ± 0.02 b 1.08 ± 0.04 c
K (mg·kg1) 107 ± 8.7 88.5 ± 6.1 107 ± 1.1
Ca (mg·kg1) 55.7 ± 2.9 b 77.1 ± 0.4 b 185 ± 12 a
Mg (mg·kg1) 33.0 ± 2.9 b 44.0 ± 4.0 ab 63 ± 9.2 a
P (mg·kg1) 29.5 ± 3.7 a 30.0 ± 3.8 a 18.1 ± 2.3 b
C/N (kg·kg1) 27.2 ± 0.5 b 25.5 ± 0.4 b 32.4 ± 1.4 a
N/P (kg·kg1) 171 ± 23 b 66.1 ± 6 a 63.6 ± 11 a
Values are mean ± SE of soil pH in water, organic matter (OM), cation
exchange capacity (CEC), base saturation (BS), micronutrients, and macro-
nutrients as well as C/N and N/P. Means followed by different letters are
statistically different at P < 0.05.
Al (r = 0.91; P < 0.001), C (r = 0.92; P < 0.001), N (r
= 0.92; P < 0.001), and the N/P ratio (r = 0.94, P <
0.001). In contrast, pH was positively correlated with Mn
(r = 0.60; P < 0.05) as well as with Mg (r = 0.73; P <
0.05), Ca (r = 0.77; P < 0.01), and consequently to BS (r
= 0.71; P < 0.05).
Soil K values were statistically indistinguishable among
sites, ranging from 88.5 to 107 mg·kg1. The slopes of
the toposequence had mineral concentrations (in mg·kg1)
that was statistically indistinguishable to that in the val-
leys: (Mn: 0.62 ± 0.14 vs. 0.87 ± 0.08; Cu: 1.33 ± 0.01 vs.
1.24 ± 0.06), macronutrients (P: 30 ± 3.8 vs. 29.5 ± 3.7;
Ca: 77.1 ± 0.4 vs. 55.7 ± 2.9), and Al (159 ± 24 vs. 290 ±
43), respectively. Slopes and domes had statistically com-
parable C content (49 ± 5 vs. 35 ± 14 g·kg1) but lowest
of that in the valley (139 ± 32 g·kg1), respectively.
Mounds had the statistically lowest concentration of Cu
(1.03 ± 0.01 mg·kg1) and Al (62 ± 2 mg·kg1), and the
highest Mn (6.59 ± 1.83 vs. 0.62 ± 0.14 and 0.87 ± 0.08
mg·kg1) compared to slopes and valley, respectively.
Similarly, mound had the lowest concentrations of the
macronutrients N (1.08 ± 0.04 vs. 1.92 ± 0.02 and 5.21 ±
1.3 g·kg1) and P (18.2 ± 2.3 vs. 29.5 ± 3.7 and 30 ± 3.8
mg·kg1) as well as the highest C/N ratio (32.4 ± 1.4 vs.
25.5 ± 0.4 and 27.0 ± 0.5 kg·kg1) as compared to those
in slopes and valley, respectively.
3.2. Plant Analysis
Plant analysis results are shown in Table 2. The values
of Fe (20.6 - 39.5 mg·kg1) and Zn (6.5 - 14.3 mg·kg1)
tended to decline significantly in species from the
mounds (P. sordida and R. morilloi) as compared to that
of the valley (E. leucantha). Values of Cu (1 - 2.5
mg·kg1) were species specific. Values of B were slightly
higher in E. leucantha (17.4 ± 1.2 mg·kg1) from the val-
ley and M. sprucei (15.3 ± 0.5 mg·kg1) from the slope,
as compared to P. sordida (11.2 ± 1.0 mg·kg1) and R.
morilloi (10.6 ± 0.3 mg·kg1) from the mound. In con-
trast, Mn (43.6 - 188 mg·kg1) was relatively high in all
species except P. sordida (mound; 12.8 ± 3.8 mg·kg1).
Differences in leaf Al concentrations (4.6 - 49.4 mg·kg1)
were species specific across habitats. Regarding the ma-
cronutrients, the trend of leaf N and P was to decline from
the valley towards the mounds. By contrast, leaf N/P, Ca,
Mg, and K were species specific and did not show a clear
trend along the toposequence.
Statistically significant correlations were found be-
tween micro- and macronutrients (Table 3). Thus, Fe is
correlated with N (r = 0.92; P < 0.001) and P (r = 0.62; P
< 0.05), Zn with N (r = 0.94; P < 0.001) and P (r = 0.68;
P < 0.01) and B with N (r = 0.77; P < 0.01) and P (r =
0.59; P < 0.05). Similarly, Mn was correlated with Ca (r
= 0.56; P < 0.05) and Mg (r = 0.63; P < 0.05). The set of
Copyright © 2013 SciRes. AJPS
Soil and Leaf Micronutrient Composition in Contrasting Habitats in Podzolized Sands of the Amazon Region 1921
Table 2. Leaf nutrient composition of the dominant species.
Toposequence sites
Valley Slope Mound
E. leucantha M. sprucei P. so r d i da R. morilloi
Fe 39.5 ± 1.0 a 26.9 ± 1.6 b20.6 ± 1.6 c 22.9 ± 0.3 c
Mn 88.5 ± 11.1 b 188 ± 43 a 12.8 ± 3.8 d 43.6 ± 6.1 c
Zn 14.3 ± 1.1 a 7.4 ± 04 b 6.9 ± 0.5 b 6.5 ± 0.4 b
B 17.4 ± 1.2 a 15.3 ± 0.5 a11.2 ± 1.0 b 10.6 ± 0.3 b
Cu 2.5 ± 0.1 a 1.0 ± 0.1 b 2.9 ± 0.2 a 2.2 ± 0.2 a
Al 14.9 ± 1.9 b 4.6 ± 0.6 c 5.8 ± 1.1 c 49.4 ± 5.6 a
N 16.7 ± 0.3 a 9.3 ± 0.3b 9.1 ± 0.5 b 7.4 ± 0.4 c
K 3.89 ± 0.39 c 6.42 ± 0.39 b10.0 ± 0.69 a 3.20 ± 0.29 c
Ca 1.57 ± 0.08 b 0.93 ± 0.14 b3.68 ± 0.89 a 1.64 ± 0.26 b
Mg 1.41 ± 0.03 b 1.28 ± 0.06 b3.05 ± 0.89 a 1.62 ± 0.09 b
P 1.15 ± 0.03 a 0.84 ± 0.01 b0.96 ± 0.08 b 0.45 ± 0.01 c
N/P 14.6 ± 1.4 a 11.1 ± 0.5 ab9.6 ± 0.5 b 16.4 ± 1.9 a
The values are in mg·kg1, except that of N, K, Ca, Mg and P (g·kg1) and
N/P (kg·kg1). Values are mean ± SE. Means followed by different letters
are statistically different at P < 0.05.
Table 3. Correlation coefficients between leaf micro- and
macronutrients across species.
Fe Mn Zn B Cu
N 0.92*** 0.11 0.94*** 0.77** 0.16
K 0.48 0.18 0.41 0.19 0.15
Ca 0.39 0.56* 0.25 0.34
Mg 0.58* 0.63* 0.35 0.57* 0.70**
P 0.62* 0.11 0.68** 0.59* 0.20
Significant correlations are in bold and P < 0.05 (*), P < 0.01 (**), and P <
0.001 (***).
Fe, Zn, and B as well as the set of Mn, Ca, and Mg are
involved in leaf chloroplast and mitochondrial function.
A negative correlation between Fe and Mg (r = 0.58; P
< 0.05) was consistent with its antagonic role in plants.
4. Discussion
Soil critical values for tropical crops of Fe (2.5 - 5.8
mg·kg1), Mn (1 - 4 mg·kg1), Zn (3.3 mg·kg1), B (1 -
2.4 mg·kg1), and Cu (1 - 2 mg·kg1) were previously
compiled by Oyendola and Chude [3]. Consistent with
these ranges, the Zn and B levels were low at all of the
sites, Cu was very low but above the critical value of 1
mg·kg1, Mn was limited in the valleys and slopes but
higher in the mounds, and Fe was well above the critical
level in all of the habitats. Values of Zn (0.99 - 1.41
mg·kg1) B (0.17 - 0.31 mg·kg1) and Cu (1.03 - 1.33
mg·kg1) were comparable to those found in other nutri-
ent poor environments [19,20]. However, in more fertile
soils the levels of Zn (2 - 4 mg·kg1), B (5 - 8 mg·kg1)
and Cu (12 - 20 mg·kg1) are considerable higher [21-23].
Micronutrient shortages greatly affect soil fertility and
consequently constrain plant productivity, inhibit growth,
and exacerbate leaf senescence [19,24]. Additionally, lit-
ter decomposition rates are also lower under limiting mi-
cronutrient availability, given that the process is depen-
dent on metallomic enzymes [25,26]. In leached and
strongly acid tropical soils, the pool of soluble Fe, Mn,
and Al are high, which may be potentially toxic for plants
[20,27,28]. Decreasing levels of soil N in the Amazon
caatinga habitats, from the valleys towards mounds, was
corroborated by the values of total N, available P, and
N/P ratio [9,10,14].
Mineral nutrient composition of leaves did not follow
the same pattern of availability found in the soils (Tables
1 and 2). The total level of micronutrients in the soil is
determined by the original geological substrate and sub-
sequent geological and pedogenic regimes [21], but the
accumulation of a given element within plants is control-
led by complex processes regulated by a network of gene
products critical for uptake, binding, transportation, and
sequestration [5]. The concentrations of leaf micronutri-
ents: Fe, Zn, B, and Cu were below the sufficiency levels
72, 15 - 20, 20 - 70, 3 - 7 mg·Kg1, respectively, as de-
fined by Marschner [29]. Furthermore, values of Fe, Zn,
and Cu were at the lower end of the range previously re-
ported in tropical acid soils: 10 - 2467, 8 - 139, and 1 -
53 mg·kg1, respectively [2,20,30]. Values of B in E.
leucantha (valley) and M. sprucei (slopes) were within a
range of marginal sufficiency (14 - 18 mg·kg1) [31].
Conversely, species from the mounds had the lowest B.
In contrast, Mn was above the sufficiency level (10-20
mg·kg1) in all species except P. sordida, but below the
accumulator levels of 1000 mg·kg1 [32]. Similarly, de-
spite across species differences in leaf Al concentrations;
values were much lower than 2300 - 3900 mg·kg1, which
is typical for Al accumulator species in tropical habitats
[33]. The species with the higher Al (49.4 mg·kg1) was
R. morilloi of the Rubiaceae family. This family has a
large number of Al accumulator species in tropical envi-
ronments [27]. Leaf macronutrient composition showed a
pattern similar to that previously found in the same spe-
cies [11,12]. In addition, the N/P ratio was comparable
across all sites and was less than 16.4 kg·kg1, which
confirmed the notion that N limitation overrides that of P
in the three zones of the Amazon caatinga toposequence
following the criteria outlined by Lambers [34].
Although the close relationships between macro- and
micronutrients were found in this study, they are not ful-
ly understood, Fe, Mn, Zn, N, P, and Mg all have essen-
tial roles in photosynthetic leaf function. We found posi-
tive and statistically significant correlations between these
micro- and macro-nutrients (Fe-P; Fe-N; Mn-Mg; Zn-N;
Zn-P). Consequently, the maintenance of a balanced ac-
Copyright © 2013 SciRes. AJPS
Soil and Leaf Micronutrient Composition in Contrasting Habitats in Podzolized Sands of the Amazon Region
cumulation of these micro- and macronutrients in leaves
may have an adaptive value for growth in habitats with
limited availability. Indeed, some interrelationships have
been previously identified between micro- and macronu-
trients. For example, a nucleic acid-protein set has been
correlated with concentrations of P, N, Cu, S, and Fe, and
structural, photosynthetic, and enzymatic sets have also
been correlated with concentrations of Mn, Mg, Ca, and
K [35]. However, further studies including a wide variety
of habitats and species are required for a complete under-
standing of this issue.
5. Conclusions
Based on the results of the present study, the following
conclusions were drawn:
1. The soil Zn, B, and Cu levels were very low in all of
the habitats of the Amazon caatinga toposequence.
2. The soil levels of Fe would be potentially toxic in
all habitats, Mn levels may be toxic in the mounds, and
Al may be toxic in valleys and slopes.
3. In all of the habitats, the leaf concentrations of Cu,
B, Zn, and Fe were below sufficiency levels, and con-
centrations of Mn and Al were within normal range found
in non-accumulators.
4. A strong relationship between leaf micro- and ma-
cronutrients suggested the maintenance of a homeostatic
nutrient composition, which would favor photosynthetic
function at the leaf level. Therefore, the local distribution
of species may be shaped for their abilities to maintain a
balanced collection of micronutrients through roots under
critical levels of available Zn, B, and Cu as well as the
ability to exclude Mn, Fe, and Al.
6. Acknowledgements
Financial support for soil and plant analysis was provided
partially by DID-USB-Fondo de Trabajo-2012. Helpful
discussions with Dr E. Olivares (IVIC, Caracas) during
the development of this study are sincerely appreciated,
as well the kind bibliographical materials provided. I
thank Pedro Maquirino for his invaluable help in the field
work, and to Editor and anonymous referees for sugges-
tions to improve the manuscript.
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