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Communication
Morais Ultramafic Complex: A Survey towards
Nickel Phytomining
Ana R. A. Alves, Eduardo F. Silva and Luís A. B. Novo *
Laboratory of Experimental and Applied Phytotechnologies (LEAPH), GeoBioTec, Department of Geosciences,
University of Aveiro, 3810-193 Aveiro, Portugal
*Correspondence: novo@ua.pt
Received: 30 June 2019; Accepted: 8 August 2019; Published: 11 August 2019
Abstract:
Ultramafic areas are critical for nickel (Ni) phytomining due to the high concentration of this
element in their soils and the number of hyperaccumulators they harbor. The aim of the present study
was to evaluate the potential of the Morais massif, an ultramafic area in Portugal, for phytomining
using the hyperaccumulator species Alyssum serpyllifolium subsp. lusitanicum. Soil samples and
A. serpyllifolium specimens were collected in four locations of the Morais massif. After determination
of Ni concentrations in the samples, the results show that soil pseudo-total Ni concentrations in sites
number 1 and 2 are significantly higher than in the soil samples collected in the other two locations,
with 1918 and 2092 mg kg
−1
, respectively. Nickel accumulation is significantly greater in the aerial
parts of plants collected at sites 1, 2, and 4, presenting Ni harvestable amount means of 88.36, 93.80,
and 95.56 mg per plant, respectively. These results suggest that the sites with highest potential for
phytomining are sites 1, 2, and 4. A nickel agromining system in these locations could represent
an additional source of income to local farmers, since ultramafic soils have low productivity for
agriculture and crop production.
Keywords:
phytomining; nickel; agromining; Alyssum serpyllifolium; hyperaccumulators;
serpentine soils
1. Introduction
Serpentine (ultramafic) soils cover approximately 1% of the Earth’s terrestrial surface. Featuring an
uneven distribution, they can be found especially in tropical (e.g., New Caledonia, Cuba, and Indonesia)
and temperate regions (e.g., Italy, Spain, Turkey, and California) [
1
]. Serpentine soils result from the
weathering of ultramafic rocks that have a low silica (SiO
2
) content (<45%) and are rich in ferromagnesian
(ultramafic) minerals (at least 70% of minerals within olivine and pyroxene groups) [
2
]. Thus, serpentine
soils present high concentrations of magnesium (Mg) and iron (Fe), low calcium-magnesium (Ca:Mg)
ratio, and high concentrations of trace elements such as cobalt (Co), chromium (Cr), and nickel (Ni) [
1
].
These characteristics, particularly the elevated values of Mg and Ni and low Ca soil concentrations,
are considered the main cause of serpentine soil toxicity [
3
]. Whereas Ni concentrations in normal soils
may vary from 7 to 500 mg kg
−1
, serpentine soils present values that are typically in the range of 700
to 8000 mg kg
−1
[
4
,
5
]. Serpentine soils are also poor in important macronutrients [e.g., nitrogen (N),
potassium (K), and phosphorus (P)] and micronutrients [e.g., boron (B) and molybdenum (Mo)] and
have a low organic matter content and low water holding capacity [
6
]. These physicochemical properties
make these soils adverse environments for normal plant growth [
7
,
8
]. However, serpentine soils
endemic flora have developed specific resistance mechanisms under these environmental conditions
over thousands of years of natural selection [9].
The Morais massif is a serpentinized region from Portugal and therefore has a specialized flora
that thrives on metal rich substrates [
10
]. In Portugal, serpentine outcrops are restricted to Morais
Resources 2019,8, 144; doi:10.3390/resources8030144 www.mdpi.com/journal/resources
Resources 2019,8, 144 2 of 11
and Bragança massifs localized in the Tr
á
s-os-Montes region and harbor the nickel hyperaccumulator
species Alyssum serpyllifolium subsp. lusitanicum [
11
]. Hyperaccumulators are remarkable plants
that accumulate extreme amounts of metals (such as cadmium, cobalt, magnesium, and zinc) in
their tissues while remaining sufficiently healthy to maintain a self-sustaining population [
12
].
It has been hypothesized that high metal accumulation in these plants may serve various ecological
functions such as metal tolerance, resistance to environmental stress, competitive strategy (allelopathy),
and defense against pathogens and herbivores [
13
–
16
]. In the case of A. serpyllifolium, there is
evidence that the high concentrations of Ni protect the species against herbivores [
13
]. The first
discovery of a Ni hyperaccumulator was made in 1948 by Minguzzi and Vergnano with the species
Alyssum bertolonii [
17
]. In 1977, Brooks et al. defined a threshold for nickel hyperaccumulation of
1000 mg kg−1
in the dry matter of plant shoots [
18
]. Currently, there are approximately 500 plant
species identified that hyperaccumulate nickel, making up 70% of all known hyperaccumulators [
19
].
Nickel hyperaccumulators are widely dispersed, reflecting the distribution of serpentine soils from
which they absorb this element [
20
]. The plant A. serpyllifolium subsp. lusitanicum T. R. Dudley & P.
Silva (or Alyssum pintodasilvae) was described by Dudley in 1967 [
21
,
22
]. It belongs to the Brassicaceae
family, and it is classified as an herbaceous and perennial plant that presents a ramified stem that
can reach 10 to 30 cm and yellow flowers. It is endemic to Morais and Bragança massifs and can
accumulate up to 8000 mg kg−1(dry weight) of nickel [11].
Phytomining is a recent phytotechnology based on the use of hyperaccumulators to recover
economically relevant amounts of valuable metals from mineralized or contaminated soils [
23
,
24
].
The growing Ni demand and its market price together with the existence of extensive ultramafic
areas with high Ni concentrations and the number of hyperaccumulator species they shelter have
strengthened the importance of Ni phytomining [
23
]. In 2015, the concept of agromining (variant
of phytomining) was proposed as an agricultural strategy that would allow local communities to
farm for metals, providing them an economic profit [
25
]. Therefore, the two main approaches to
consider with this phytotechnology are (i) phytomining functioning jointly with phytoremediation
as part of a rehabilitation strategy (e.g., in degraded mine soils and quarries); (ii) agromining in soils
unsuitable for conventional agriculture and pasture (e.g., serpentine soils) [
23
,
25
]. The Ni phytomining
process consists of (i) the selection of an ultramafic area with high bioavailable nickel concentrations;
(ii) the cultivation of a hyperaccumulator species in the area (preferably native) that will be harvested
when maturity is reached (maximum biomass); (iii) the incineration of the harvested biomass and
the chemical processing that results in nickel recovery [
1
]. In this context, the survey of potential
areas for phytomining and hyperaccumulator species is crucial for future implementation of these
phytomining/agromining systems.
This study consisted primarily of the assessment of nickel concentrations in soils and
A. serpyllifolium plant species from four different sites in the Morais massif. To draw further conclusions,
other soil physicochemical properties and plant metal uptake parameters were also analyzed. The main
objective of this work is to provide support towards the implementation of nickel phytomining in the
Morais massif.
Morais Massif Location and Geology
The Morais massif is located in the eastern part of the Tr
á
s-os-Montes region in the north of
Portugal. It belongs to the Bragança district and is found mostly in the municipality of Macedo de
Cavaleiros. Geologically, the study area integrates the Galicia-Tr
á
s-os-Montes Zone (GTMZ) of the
Iberian Massif. The GTMZ is composed of four main sequences [
26
,
27
]: (i) parautochthonous complex
(schistose domain) with diversified metasedimentary and metavolcanic rocks; (ii) lower allochthonous
complex with gneiss, metasedimentary, and metavolcanic rocks; (iii) ophiolite complex (intermediate
allochthonous complex) composed by thrust sheets made of ophiolitic rocks such as amphibolites,
serpentinized peridotites, sheeted dike/gabbro complexes, flaser gabbros, and mafic cumulates;
(iv) upper allochthonous complex made of high-grade metamorphic rocks such as paragneisses,
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eclogites, mafic granulites, pyroxenites, and peridotites. Figure 1presents the main geological
composition of the Morais massif.
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paragneisses, eclogites, mafic granulites, pyroxenites, and peridotites. Figure 1 presents the main
geological composition of the Morais massif.
Figure 1. Simplified geological map of the Morais massif.
2. Materials and Methods
To evaluate the phytomining potential of the Morais massif, samples of soil and A. serpyllifolium
subsp. lusitanicum specimens were collected from 4 sites in the study area with the following
coordinates: (1) 41°31′10.54′′ N, 6°49′7.03′′ W; (2) 41°31′14.56′′ N, 6°48′50.58′′ W; (3) 41°29′38.47′′ N,
6°47′26.68′′ W; (4) 41°30′33.60′′ N, 6°45′23.74′′ W. Sites were randomly selected, with sites number 1
and 2 being located to the west and site number 4 further east within the ultramafic complex. Site
number 3 is located at a southern point, close to the urban area of Morais town. From each selected
site, 3 soil samples and 3 A. serpyllifolium specimens were collected, making a total of 12 soil samples
and 12 plant samples that were analyzed in this study. Sampling was performed in May 2018, and it
is worth noting that plants still had not reached full maturity (which would be normally expected
around 2–3 months later). At each site, soil samples were collected with a hand auger from surface
horizons covering a depth of approximately 15 cm, since it corresponded to the A. serpyllifolium
rhizosphere zone. Plant sampling followed a uniform criterion in terms of plant maturity and size.
The specimens were collected in an area of 1 m
2
, and the most mature plants were selected for
analysis. Shoot sizes varied between 12 and 15 cm and dry weights between 18 and 22 g.
Soil samples were air-dried and sieved through a 2 mm sieve. The plant samples were washed
once with tap water and twice with deionized water in order to remove any dust deposits and oven-
dried at 50 °C for 48 h. The dry weight of the different plant parts (roots, stems, leaves, and flowers)
was measured, and then the plants were ground into a homogenous powder using an electric grinder
(Selecline 851680/SHG269). Soil and plant samples were subjected to acid digestion with HNO
3
and
HCl (3:1, v/v), and elemental analysis was assessed through inductively coupled plasma mass
spectrometry (ICP-MS) [28]. Using this method, the pseudo total concentrations of Ca, Mg, P, Cr, and
Ni in the soil were determined, as well as Cr and Ni concentrations in the different plant tissues. The
former was analyzed since, along with Ni, it is the most abundant trace element in ultramafic soils,
and competition for the uptake of both elements could occur [29]. The bioavailable fractions of Cr
and Ni in the soil samples were determined through extraction with ammonium acetate (1 M NH
4
Ac,
pH 7) [30]. The levels of Cr and Ni in the filtrate were assessed by ICP-MS. The pH and the electrical
conductivity in the soil were determined using a suspension of 10 g of soil and 50 mL of deionized
water (1:5) [31]. The suspension was homogeneously mixed in an orbital shaker for 2 h at 150 rpm,
and the values were measured using a pH/conductivity meter (Peak Instruments S-620).
All results were based on the statistical analyses from three replicates. The Levene, the Brown–
Forsythe, and the Welch tests were used to evaluate the homogeneity of variances. One-way ANOVA
was conducted to assess the statistical significance of differences among values. Tukey and Dunnett’s
T3 tests were used in cases of equal and unequal variances, respectively. Pearson correlation
Figure 1. Simplified geological map of the Morais massif.
2. Materials and Methods
To evaluate the phytomining potential of the Morais massif, samples of soil and A. serpyllifolium
subsp. lusitanicum specimens were collected from 4 sites in the study area with the following coordinates:
(1) 41
◦
31
0
10.54
00
N, 6
◦
49
0
7.03
00
W; (2) 41
◦
31
0
14.56
00
N, 6
◦
48
0
50.58
00
W; (3) 41
◦
29
0
38.47
00
N, 6
◦
47
0
26.68
00
W; (4) 41
◦
30
0
33.60
00
N, 6
◦
45
0
23.74
00
W. Sites were randomly selected, with sites number 1 and 2 being
located to the west and site number 4 further east within the ultramafic complex. Site number 3 is
located at a southern point, close to the urban area of Morais town. From each selected site, 3 soil
samples and 3 A. serpyllifolium specimens were collected, making a total of 12 soil samples and 12
plant samples that were analyzed in this study. Sampling was performed in May 2018, and it is worth
noting that plants still had not reached full maturity (which would be normally expected around 2–3
months later). At each site, soil samples were collected with a hand auger from surface horizons
covering a depth of approximately 15 cm, since it corresponded to the A. serpyllifolium rhizosphere
zone. Plant sampling followed a uniform criterion in terms of plant maturity and size. The specimens
were collected in an area of 1 m
2
, and the most mature plants were selected for analysis. Shoot sizes
varied between 12 and 15 cm and dry weights between 18 and 22 g.
Soil samples were air-dried and sieved through a 2 mm sieve. The plant samples were washed
once with tap water and twice with deionized water in order to remove any dust deposits and
oven-dried at 50
◦
C for 48 h. The dry weight of the different plant parts (roots, stems, leaves, and
flowers) was measured, and then the plants were ground into a homogenous powder using an electric
grinder (Selecline 851680/SHG269). Soil and plant samples were subjected to acid digestion with
HNO
3
and HCl (3:1, v/v), and elemental analysis was assessed through inductively coupled plasma
mass spectrometry (ICP-MS) [
28
]. Using this method, the pseudo total concentrations of Ca, Mg, P, Cr,
and Ni in the soil were determined, as well as Cr and Ni concentrations in the different plant tissues.
The former was analyzed since, along with Ni, it is the most abundant trace element in ultramafic soils,
and competition for the uptake of both elements could occur [
29
]. The bioavailable fractions of Cr
and Ni in the soil samples were determined through extraction with ammonium acetate (1 M NH
4
Ac,
pH 7) [
30
]. The levels of Cr and Ni in the filtrate were assessed by ICP-MS. The pH and the electrical
conductivity in the soil were determined using a suspension of 10 g of soil and 50 mL of deionized
water (1:5) [
31
]. The suspension was homogeneously mixed in an orbital shaker for 2 h at 150 rpm,
and the values were measured using a pH/conductivity meter (Peak Instruments S-620).
All results were based on the statistical analyses from three replicates. The Levene,
the Brown–Forsythe, and the Welch tests were used to evaluate the homogeneity of variances.
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One-way ANOVA was conducted to assess the statistical significance of differences among values.
Tukey and Dunnett’s T3 tests were used in cases of equal and unequal variances, respectively. Pearson
correlation coefficients were used to examine the relationship between different parameters. Statistical
analyses were carried out using SPSS statistical program (ver. 25.0, IBM Corp., Armonk, NY, USA).
3. Results and Discussion
3.1. Soil
The physicochemical properties of the soil collected in different sampling sites are presented
in Table 1. All soil samples exhibit an approximately neutral to slightly acidic pH (6.8–6.4), typical
of serpentine soils that are characterized by a pH that ranges between 6 to 8 [
32
]. The electrical
conductivity values are low, meaning that there are no dissolved salts [
33
]. As expected for serpentinite
soils, the Morais massif presents high concentrations of Mg, Ni, and Cr (Figure 2), as well as a low
Ca:Mg ratio. The Ca concentration is significantly higher at sites 3 and 4 in comparison to the values
from sites 1 and 2. These results suggest that plants from sites 1 and 2 might be under more stress due
to the lower values of Ca, since this is considered one of the main causes of serpentine soil toxicity [
3
].
The concentration of P, which is one of the macronutrients essential to plants, is significantly lower at
site 3 in comparison to the other sampling sites.
Table 1. Selected physicochemical characteristics of the soils from each sampling site.
Site pH Conductivity
(µS/cm) Ca (mg kg−1) Mg (mg kg−1)Ca:Mg P (mg kg−1)
1 6.81 ±0.03a 84.70 ±51.42a 1359 ±460b 22,577 ±2647a 0.059 ±0.014b 295 ±21a
2 6.65 ±0.02b 101.30 ±14.66a 927 ±133b 26,028 ±2105a 0.036 ±0.003b 304 ±25a
3 6.50 ±0.02c 46.47 ±12.16a 3482 ±712a 23,606 ±2325a 0.147 ±0.018a 160 ±9b
4 6.39 ±0.03d 117.70 ±112a 3376 ±672a 21,117 ±2594a 0.159 ±0.013a 257 ±13a
Each value represents the mean of three replicate measurements
±
SD (standard deviation). Different letters indicate
significant differences between sites at p<0.05.
The results of the pseudo total concentrations of Cr and Ni in the soil from each sampling site are
presented in Figure 2. Chromium concentrations are significantly lower in site 3 in comparison with
the other sites, with a mean value of 753 mg kg
−1
. Sampling site 1 presents a significantly higher Cr
concentration in relation to site 4, with 1491 and 881 mg kg
−1
, respectively. The results from site 2,
with a mean of 1240 mg kg−1, do not show significant differences in comparison to sites 1 and 4.
Regarding Ni concentrations, sites 1 and 2, with 1918 and 2092 mg kg
−1
, respectively, present
significantly higher values in comparison with sites 3 and 4. Site 3 shows significantly lower
concentrations of Ni, with a mean of 721 mg kg
−1
, in relation to site 4, which presents a Ni concentration
of 1145 mg kg−1.
Table 2shows the results of some physicochemical characteristics of the ultramafic soil in the
Morais complex and the values obtained from studies in other ultramafic areas located in temperate
regions [
1
]. The values representing Morais correspond to the ones obtained at site number 2, since it is
the location analyzed with the highest Ni pseudo-total concentration in the soil. The Melide complex
is close to the village of Eidi
á
n in Galicia (Northwest Spain), and it corresponds to an ultramafic area
located very close to the Morais ultramafic complex [
1
]. As aforementioned, ultramafic soils present
low Ca:Mg ratios, scarcity of macronutrients, pH close to neutrality, and elevated levels of metals such
as Ni and Cr. Still, the ranges of these common features can vary at different locations, as the formation
of soils depends not only on the nature and the abundance of the weathered parent material (e.g.,
Cr-bearing minerals are essentially chromite and magnetite, and to a lower extent serpentinite and
pyroxene) but also on climate, topography, biota, and time [
34
]. Interestingly, data from Table 2show a
strong correlation between pH and Ni concentrations in soil (r =0.975, p<0.01).
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Figure 2. Pseudo total concentrations of Cr (A) and Ni (B) in the soils from each sampling site. Each
value represents the mean of three replicate measurements ± SD. Different letters indicate significant
differences between sites at p < 0.05.
Table 2 shows the results of some physicochemical characteristics of the ultramafic soil in the
Morais complex and the values obtained from studies in other ultramafic areas located in temperate
regions [1]. The values representing Morais correspond to the ones obtained at site number 2, since
it is the location analyzed with the highest Ni pseudo-total concentration in the soil. The Melide
complex is close to the village of Eidián in Galicia (Northwest Spain), and it corresponds to an
ultramafic area located very close to the Morais ultramafic complex [1]. As aforementioned,
ultramafic soils present low Ca:Mg ratios, scarcity of macronutrients, pH close to neutrality, and
elevated levels of metals such as Ni and Cr. Still, the ranges of these common features can vary at
different locations, as the formation of soils depends not only on the nature and the abundance of the
weathered parent material (e.g., Cr-bearing minerals are essentially chromite and magnetite, and to
a lower extent serpentinite and pyroxene) but also on climate, topography, biota, and time [34].
Interestingly, data from Table 2 show a strong correlation between pH and Ni concentrations in soil
(r = 0.975, p < 0.01).
Figure 2.
Pseudo total concentrations of Cr (
A
) and Ni (
B
) in the soils from each sampling site.
Each value represents the mean of three replicate measurements
±
SD. Different letters indicate
significant differences between sites at p<0.05.
Table 2.
Physicochemical characteristics of the soils from different ultramafic areas located in
temperate regions.
Site pH Ca:Mg Ni (mg kg−1) Cr (mg kg−1)
Portugal (Morais) 6.6 0.036 2092 1240
Spain (Melide) 5.8 0.169 967 1263
Austria 6.1 0.065 1450 1840
Greece 7.2 0.050 2347 -
Albania 7.5 0.065 3140 1600
Figure 3shows the bioavailable Ni concentrations in the soil from each sampling site. Site 2
presents a significantly higher bioavailable concentration in comparison to the other sampling sites.
Nickel bioavailable concentrations between sites 1 and 3 do not show significant differences and
are both significantly lower in relation to site 4. These results show that, despite the higher Ni
pseudo total concentrations in the soil from site 1, soil collected at site 4 has greater Ni bioavailability.
The bioavailability of metals in soil depends on various factors, including the metal pseudo-total
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concentrations in the soil and the pH. The soil pH affects the solubility of trace elements and therefore is
considered a major factor influencing Ni bioavailability (metal bioavailability dwindles with increasing
pH) [
32
]. According to the results presented in Table 1, site number 1 shows a significantly higher pH
value in comparison to the other sampling sites, meaning the soil is less acidic and consequently Ni
is less bioavailable. This is in conformity with the results presented in Figure 3. Hence, the greater
bioavailability of Ni in sites 2 and 4 may be explained by the lower pH values at both points (particularly
at the latter). The elevated bioavailability of Ni at site 2 in comparison to sites 1 and 3 is promoted by
its lower pH and higher Ni pseudo-total contents, respectively.
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Table 2. Physicochemical characteristics of the soils from different ultramafic areas located in
temperate regions.
Site pH Ca:Mg Ni (mg kg−1) Cr (mg kg−1)
Portugal (Morais) 6.6 0.036 2092 1240
Spain (Melide) 5.8 0.169 967 1263
Austria 6.1 0.065 1450 1840
Greece 7.2 0.050 2347 -
Albania 7.5 0.065 3140 1600
Figure 3 shows the bioavailable Ni concentrations in the soil from each sampling site. Site 2
presents a significantly higher bioavailable concentration in comparison to the other sampling sites.
Nickel bioavailable concentrations between sites 1 and 3 do not show significant differences and are
both significantly lower in relation to site 4. These results show that, despite the higher Ni pseudo
total concentrations in the soil from site 1, soil collected at site 4 has greater Ni bioavailability. The
bioavailability of metals in soil depends on various factors, including the metal pseudo-total
concentrations in the soil and the pH. The soil pH affects the solubility of trace elements and therefore
is considered a major factor influencing Ni bioavailability (metal bioavailability dwindles with
increasing pH) [32]. According to the results presented in Table 1, site number 1 shows a significantly
higher pH value in comparison to the other sampling sites, meaning the soil is less acidic and
consequently Ni is less bioavailable. This is in conformity with the results presented in Figure 3.
Hence, the greater bioavailability of Ni in sites 2 and 4 may be explained by the lower pH values at
both points (particularly at the latter). The elevated bioavailability of Ni at site 2 in comparison to
sites 1 and 3 is promoted by its lower pH and higher Ni pseudo-total contents, respectively.
The Cr bioavailability values from each sampling site cannot be discussed because the
corresponding results were under the detection limits. Hence, it stands to reason that the selected
extraction method (NH4Ac, pH 7.0) may not be reliable to assess Cr bioavailability in ultramafic soils
where its solubility can be influenced by high pH values. Further research should consider additional
extraction methods to determine Cr bioavailability, such as DTPA, NH4Ac (pH 4.5), CaCl2 and
Sr(NO3)2 [35].
Figure 3.
Bioavailable Ni concentrations in the soils from each sampling site. Each value represents the
mean of three replicate measurements
±
SD. Different letters indicate significant differences between
sites at p<0.05.
The Cr bioavailability values from each sampling site cannot be discussed because the
corresponding results were under the detection limits. Hence, it stands to reason that the selected
extraction method (NH
4
Ac, pH 7.0) may not be reliable to assess Cr bioavailability in ultramafic
soils where its solubility can be influenced by high pH values. Further research should consider
additional extraction methods to determine Cr bioavailability, such as DTPA, NH
4
Ac (pH 4.5), CaCl
2
and Sr(NO3)2[35].
3.2. Plants
The results of the concentrations of Cr and Ni in the different plant tissues from each sampling
site are presented in Figure 4. Regarding Cr concentrations in site 1, plants accumulate significantly
higher amounts of Cr in the roots (69 mg kg
−1
) in comparison to the other tissues (Cr concentrations
in stem, leaves, and flowers combined make a total of 20 mg kg
−1
). Likewise, site 2 accumulates
39 mg kg−1
in roots and a total concentration of 19 mg kg
−1
in the other tissues. Shanker et al. reported
that the maximum quantity of this element (Cr is a non-essential element for plants) is contained
in roots, the reason being its immobilization and its inability to translocate to leaves and flowers
for the plant to avoid toxicity [
29
]. However, this is not the case for plants collected in sites 3 and
4. Chromium concentrations in plant roots from site 3 (31 mg kg
−1
) show no significant differences
in comparison with concentrations found in leaves and flowers from the same site. Plants from
site 4 accumulate significantly higher Cr concentrations in leaves and flowers, 55 and 43 mg kg
−1
,
respectively, in comparison to the amounts of Cr in roots (17 mg kg
−1
). Studies show that mycorrhizae
and organic acids in the rhizosphere play important roles in increasing Cr translocation to shoots [
36
,
37
].
This may be the case for sites 3 and 4, since Ca values in these sites are significantly greater, and the
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general concentrations of heavy metals (Cr and Ni) are lower. Hence, the rhizosphere is under less
environmental stress and therefore is richer with greater amounts of microorganisms and consequently
a more adequate environment that allows Cr translocation in plants. It is also important to point out
that the molecules responsible for Ni uptake can be the same for Cr, thus extreme concentrations of
this elements in the soil may overload the molecules responsible for uptake and translocation [29].
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Figure 4. Cr (A) and Ni (B) concentrations in the different plant tissues from each sampling site. Each
value represents the mean of three replicate measurements ± SD. Different lowercase letters indicate
significant differences between different plant parts for each point; different uppercase letters indicate
significant differences between sampling sites for each plant part, p < 0.05.
Table 3 shows the harvestable amounts of nickel in plants from each site as well as the
translocation factor (TF) and the bioconcentration factor (BF). The BF indicates the capacity of a plant
to extract metals from the soil, and the TF shows the plant competence to translocate metals from the
root to the shoot [23]. The results confirm that the sites with most potential for phytomining are sites
Figure 4.
Cr (
A
) and Ni (
B
) concentrations in the different plant tissues from each sampling site.
Each value represents the mean of three replicate measurements
±
SD. Different lowercase letters
indicate significant differences between different plant parts for each point; different uppercase letters
indicate significant differences between sampling sites for each plant part, p<0.05.
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Regarding Ni concentrations in plant tissues, they are always significantly higher in leaves and
flowers in comparison to stems and roots. In sites 1, 3, and 4, the Ni concentrations in leaves are
significantly higher, with means of 8784, 5797, and 9545 mg kg
−1
, respectively, in comparison to the
values accumulated in flowers. The Ni concentration values in leaves are in agreement with other
results reported for the same species [
13
]. Flowers from sites 1, 3, and 4 show Ni concentrations of 5333,
3961, and 6758 mg kg
−1
, respectively. Leaves and flowers from site 2 present no significant differences
between them in terms of Ni accumulation, with 7723 and 7264 mg kg
−1
, respectively. These results are
expected, since other studies show that the leaves are the main storage organ where the majority of Ni
is accumulated [38]. Groeber et al. reported that flowers of hyperaccumulator plants can accumulate
Ni concentrations as high as that in leaves, which is in agreement with this study [
39
]. The plants from
sites 1, 2, and 4 show a greater Ni accumulation, which indicates that, despite site 4 having lower Ni
concentrations in the soil, the bioavailability of this metal plus other soil chemical characteristics (such
as higher Ca concentration and lower Cr values) makes this site equally suitable for Ni phytomining.
Table 3shows the harvestable amounts of nickel in plants from each site as well as the translocation
factor (TF) and the bioconcentration factor (BF). The BF indicates the capacity of a plant to extract
metals from the soil, and the TF shows the plant competence to translocate metals from the root to the
shoot [23]. The results confirm that the sites with most potential for phytomining are sites 1, 2, and 4,
since the HA of Ni is significantly higher. The TF is significantly greater in sites 1 and 2 in comparison
to the other sampling sites. It should be highlighted that the lower TF value observed in site 4 may be
ascribed to the competition between Ni and Cr uptake, since shoot Cr levels are significantly higher at
this site. The BF is greater at sites 3 and 4, suggesting that lower concentrations of metals in soils (such
as Cr) make the uptake of Ni from soil by the plant more efficient.
Table 3.
Nickel harvestable amount (HA), translocation factor (TF), and bioconcentration factor (BF)
in plants.
Site HA (mg) TF BF
1 88.36 ±4.62a 4.91 ±0.22a 2.05 ±0.34b
2 93.80 ±8.64a 4.53 ±0.35a 1.98 ±0.21b
3 67.44 ±1.05b 2.84 ±0.05b 4.18 ±0.20a
4 95.56 ±5.43a 2.21 ±0.11c 3.77 ±0.18a
Each value represents the mean of three replicate measurements
±
SD. Different letters indicate significant differences
between sites at p<0.05.
4. Conclusions
This study shows that the more appropriate sites for nickel phytomining are sites 1, 2, and 4,
because the plants present the highest nickel harvestable amount values, meaning that the process
could have a greater economic return in these sites. However, plants from site 3 also hyperaccumulate
considerable amounts of Ni. It is important to consider that biomass yield and metal concentrations of
plant shoots are crucial for phytomining, because they govern the quantity of metal to be harvested
from each plant (harvestable amount) [40].
Future research will encompass further laboratory and field trials in order to assess the viability
of the phytomining process at the Morais massif. More specifically, different agronomic aspects such as
plant density, cropping patterns, fertilization regimes, bioaugmentation with plant growth-promoting
bacteria, and application of phytohormones, to list a few, should be addressed [
1
,
41
]. If the upcoming
studies show promising results concerning potential nickel yield and profitability, the implementation
of a nickel agromining system in the Morais massif could represent an additional source of income to
local farmers, since this serpentinite area has low productivity for food production.
Author Contributions:
Conceptualization, L.A.B.N. and E.F.S.; field sampling, L.A.B.N. and E.F.S.; Laboratory
analyses, L.A.B.N. and A.R.A.A.; writing—original draft preparation, A.R.A.A.; writing—review and editing,
L.A.B.N. and E.F.S.; All authors have read and approved the final manuscript.
Resources 2019,8, 144 9 of 11
Funding:
This research was funded by the ERDF Interreg Sudoe Program (PhytoSUDOE-SOE1/P5/E0189) and the
Portuguese Foundation for Science and Technology (FCT; UID/GEO/04035/2019).
Acknowledgments:
The authors gratefully acknowledge the support provided by the representatives of the
Morais parish.
Conflicts of Interest: The authors declare no conflict of interest.
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