Hindawi
Applied and Environmental Soil Science
Volume 2020, Article ID 4614286, 8 pages
https://doi.org/10.1155/2020/4614286
Research Article
Enhancing the Phytoremediation of Hydrocarbon-Contaminated
Soils in the Sudd Wetlands, South Sudan, Using Organic Manure
J. A. Ruley ,1,2 A. Amoding,1 J. B. Tumuhairwe,1 T. A. Basamba,1 E. Opolot,1
and H. Oryem-Origa3
1
Department of Agricultural Production, Makerere University, P.O. Box 7062, Kampala, Uganda
Department of Agricultural Sciences, University of Juba, P.O. Box 82, Juba, South Sudan
3
Department of Natural Sciences, Makerere University, P.O. Box 7062, Kampala, Uganda
2
Correspondence should be addressed to J. A. Ruley;
[email protected]
Received 25 August 2019; Revised 18 December 2019; Accepted 6 February 2020; Published 11 March 2020
Academic Editor: Claudio Cocozza
Copyright © 2020 J. A. Ruley et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Phytoremediation of hydrocarbon-contaminated soils is a challenging process. In an effort to enhance phytoremediation, soil was
artificially contaminated with known concentration of light crude oil containing Total petroleum hydrocarbon (TPH) at a
concentration of 75 gkg−1 soil. The contaminated soil was subjected to phytoremediation trial using four plant species (Oryza
longistaminata, Sorghum arundinaceum, Tithonia diversifolia, and Hyparrhenia rufa) plus no plant used as control for natural
attenuation. These phytoremediators were amended with concentrations (0, 5 and 10 gkg−1 soil) of organic manure (cow dung).
Results at 120 days after planting, showed that application of manure at concentrations of 5 and 10 gkg−1 soil combined with an
efficient phytoremediator can significantly enhance reduction of TPH compared to natural attenuation or use of either manure or
a phytoremediator alone (p < 0.05). The study also showed that a treatment combination of manure 5 gkg−1 soil, with a phytoremediator gives a similar mean percentage reduction of TPH as manure 10 gkg−1 soil (p > 0.05). Therefore, the study concludes
that use of phytoremediators and manure 5 gkg−1 soil could promote the restoration of TPH contaminated-soils in the Sudd
region of South Sudan.
1. Introduction
Crude oil activities often lead to changes in the functioning
of the soil ecosystem [1, 2]. The crude oil products contaminate soil leading to deficiency of the much needed
nutrients for normal functioning of plants [1, 3]. Studies
such as [3, 4] have provided proof that crude oil contaminated soils have less content of nitrogen and phosphorus.
Besides, the water repellant properties interrupt water infiltration into the soil [5], leading to water and nutrient
deficiencies. These adversely affect plant growth and microbial populations such that where oil toxicity persists, and
the soil becomes unsuitable for plant growth [4].
Over the years, in oil rich and exploiting countries, efforts
have increasingly been taken to remediate contaminated sites
[6]. Different approaches; physical, chemical, and biological
have been undertaken. However, some of these are expensive
while others have harmed the environment, particularly soil
health and human livelihoods [3, 7, 8]. For example, excavation (physical approach) has logistics and transport constraints [9–12] while incineration (chemical approach) adds
greenhouse gases in the atmosphere leading to global
warming. This leaves use of biological approaches such as
phytoremediation as the safest, feasible, and desirable [13].
Phytoremediation uses plants and microbes [13]. However, as
indicated earlier, contaminated soils have deficient nutrients.
To correct this defect, addition of supplementary nutrients
such as organic manure is necessary [14, 15].
The use of organic manure is an environmentally safer
option because it releases nutrients at a slower rate and as
well act as a soil conditioner [4, 8, 16]. Also, organic manure
contains nitrogen, magnesium, sulphur, phosphorus, and
potassium that support plant growth [17–20]. Organic
manure improves soil physical and chemical conditions and
2
further maintains an adequate supply of soil organic matter
with high microbial loads [4]. This enables faster degradation of hydrocarbon contaminants [8, 16]. Evidence is
provided by Kaimi et al. [21] in their study of rye grass, that
addition of compost manure to the soil increases the rate of
removal of Petroleum Hydrocarbons (PHCs) while Obasi
et al. [22] reported removal of 60–65% of hydrocarbon from
soils treated with manure and municipal biowaste compost.
Different from these, this study was on biostimulation of
phytoremediators using cow dung due to its prevalence in
different environments making it almost cost free [23]. In
South Sudan, cow dung is locally available owing to large
numbers of cattle. According to Catley [24]; an average
household in South Sudan owns four cows, Sudd region
inclusive.
South Sudan is the third largest oil producing country in
Africa after Nigeria and Angola [25]. Crude oil activities in
the Sudd region of South Sudan have affected underground
herbs and shrubs and as well, destroyed microbial organisms
[26–29]. These environmental hazards are expected to
worsen with continued crude oil drilling activities in the
region [28]. Hence, biostimulation of crude oil contaminated soil in the Sudd with cow dung is necessary, given its
availability. The usefulness of cow dung and other biostimulants have been previously reported. In Nigeria, Essien
et al. [30] using Eleusine indica established that augmenting
crude oil polluted soils with cow dung enhances its remediation potential leading to restoration of polluted soils.
Njoku et al. [31] found the same effect using Glycine max;
Isaac [32] used Panicum maximum and Talinum triangulare;
and Oyedele and Amoo [33] confirmed similar results using
Maize plant while Omara et al. [34] used Sorghum Bicolor L.
(Moench) in petroleum adulterated soils from an automobile repair workshop in Kampala city, Uganda.
The Sudd Region, South Sudan, has a double advantage
for benefiting from this innovation, given the prevalence of
cow dung [24], and abundance of excellent phytoremediators. Ruley et al. [35] established Oryza longistaminata, Sorghum arundinaceum, Tithonia diversifolia,
and Hyparrhenia rufa as very important phytoremediators
in the Sudd wetland. Also, an earlier study by Ruley et al. [28]
established their abundance in region. In the 2019 study,
Ruley and colleagues observed that these plant species reduced TPH in the contaminated soil by over 50% in the
concentration TPH 75 gkg−1 soil. Despite the excellence of
these phytoremediators (such as H. rufa) confirmed by Ruley
et al. [35] and the availability of cow dung in the region, no
studies have assessed the potential of augmenting crude oil
contaminated soils planted with the abovementioned phytoremediators with cow dung. Thus, the objective of this
study was to determine the optimal concentration of cow
dung capable of enhancing the phytoremediation of hydrocarbon contaminated soils by these phytoremediators in
the Sudd region, South Sudan.
2. Material and Methods
2.1. Experimental Site and Design. The study was carried out
in a greenhouse subjected to the following treatments;
Applied and Environmental Soil Science
concentrations of cow dung (0, 5, and 10 gkg−1 soil) and
TPH concentrations of 0 and 75 gkg−1 soil. The crude oil
(light) used in the experiment was obtained from Dar Petroleum Operating Company Ltd., Operation Base Camp in
Paloch, South Sudan. Seeds of four efficient phytoremediators were obtained from the Sudd region of South
Sudan (Table 1). To remove TPH under the natural attenuation process, no plants were used (i.e., control). The four
selected phytoremediators have high potential for removal of
over 50% TPH in contaminated soils assessed in the Sudd
region (Table 1).
The trial was conducted in a Completely Randomized
Design (CRD) with 30 treatments and replicated three times
using Genstat. This gave 90 treatment pots (5 plants × 2 TPH
concentrations × 3 concentrations of cow dung × 3
replicates).
Partially decomposed cow dung with a nutrient composition ratio of 1.7 : 0.6 : 0.8 (NPK) was used for the experiment while the soil was collected from uncontaminated
natural land in the Sudd region as composite top soil
samples at a depth of 0–30 cm. The Sudd region is located
within latitudes 60 30′–90 30′ N and longitudes 300 10′–310
45′ E, with an elevation of 320 m above sea level. Table 2
shows the characteristics of the soil sample used.
The soil samples were air-dried and sieved to remove
debris and then apportioned into 5 kg per pot for the
subsequent experiments. The soil, cow dung, and crude oil
were thoroughly mixed on a metallic sheet and then
returned into 8-litre pots. The polypropylene pots were
perforated at the base to allow drainage and aeration. They
were labeled with respective treatments and left to stand for
one week before planting. To cater for TPH drainage from
the pots, a lid was placed under each perforated pot to collect
the leached water. The water was reused to irrigate the pots
which controlled TPH loss. Also, periodically, the pot lids
were rinsed with deionized distilled water and the resulting
wash solution was poured back into the respective pots,
further minimizing any losses of TPH.
The seed viability was determined through floatation
technique with those remaining at the bottom of the water
considered potentially viable. Ten seeds of each plant were
sown in each pot and on establishment; the seedlings were
thinned to three plants per pot and irrigated with deionized
water up to field capacity at two-day intervals up to the end
of the experiment (four months after planting). Any weeds
that emerged from some of the pots were hand-pulled. Also,
the whiteflies were controlled by foliar sprays of Dimethoate
(0.05%).
2.2. Data Collection. Data on plant height, total dry matter,
and percentage reduction of TPH were collected once at
120 days after planting (DAP). For plant height measurements, the abovementioned ground parts (shoots) of
the plants were cut off at the soil surface, followed by
destruction of the pots. The carefully crushed pots were
shaken into a vat to carefully collect the roots which were
washed under running tap water and air-dried to remove
surface water. The fresh weights of the partitioned plants
Applied and Environmental Soil Science
3
Table 1: Sudd region phytoremediators of petroleum hydrocarbon.
Phytoremediator
Common/local name
Thatching grass
Wild rice
False sunflower
Sudan grass
No plant (control)
Scientific name
Hyparrhenia rufa
Oryza longistaminata
Tithonia diversifolia
Sorghum arundinaceum
No plant (control)
Percentage reduction of TPH from concentration of 75 gkg−1 soil
74.40
56.17
55.92
50.12
14.80
Source: Ruley et al. [35].
Table 2: Physical and chemical characteristics of the soil used for the greenhouse experiment.
Parameters
Units
Test value
Critical valuea
Sand
Clay
Silt
STC
(%)
24.2
—
61.3
—
14.5
—
Clay
—
pH
(H2O)
6.71
5.5
P
mg/kg
15.64
15
TN
SOC
CEC
5.01
3
26.6
25
(%)
0.27
0.2
K
Na
Mg
(cmol(+)/kg soil)
1.69
0.94
1.25
0.5
<1.0
0.6
Ca
9.92
10
Source: Ruley et al. [35]. STC: soil texture class; SOC: soil organic carbon; CEC: cation-exchange capacity; acritical value according to Okalebo et al. [36] for
most crops in East Africa.
were measured. Each of the plant parts was oven-dried at
65°C to constant weight in order to determine the plant
total dry matter yield. After removing the roots, the soil
from each pot (planted and unplanted) was first homogenized before being sampled and stored at −4°C until
further processing and analysis of TPH. The TPH concentrations were determined by extracting 5 g of soil
samples with 10 mL dichloromethane (DCM). The extract
was then filtered, evaporated, and passed through silica gel
before injection to gas chromatograph. The Varian CP3800 gas chromatograph used was equipped with a flame
ionization detector (GC-FID) and split less injector in DB5 capillary column of 100% polydimethylsiloxane
(30 m × 0.25 mm I.D × 0.25 µm film thickness). The carrier
gas was helium at a flowing rate of 1.5 mL/min and the
injector and detector temperature of 300°C and 320°C. The
column head pressure was175 kPa. Oven temperature was
programmed at an initial temperature of 35°C, initial hold
time of 8°C/min, temperature rate of 27°C/min up to
100°C, temperature rate of 35°C/min up to 300°C, and a
final hold time of 5 min. The calibration of GC-FID was
performed with the following standards: 2, 10, 25, and
1000 mg/L. TPH was calculated using a programmed integration event timetable. The United States Environmental Protection Agency (USEPA) SW-846 series,
method 9071Bd5 was used to compute TPH concentrations. The percentage of TPH degradation was subtracted
from the final gas chromatograph results of the soil sample
after harvesting and the output was multiplied by 100%.
The quality of the batch of soil extracts used was ensured by
including blank samples to improve the evaluation of data.
The blank samples were prepared using hydrocarbon free
soil and then processed using the same extraction technique with known amounts of normal alkanes to check
percent recoveries. The hydrocarbon components were
analyzed by gas chromatography techniques. The data
were displayed as Total Ion Chromatogram (TIC). The
information obtained from TIC was used to identify or
classify individual components contained in the sample.
The reference material was obtained from Petroleum
Laboratories, Research and Studies of Sudanese Petroleum
Corporation (SPC).
2.3. Statistical Analyses. The data for plant height, total dry
weight, and TPH percentage reduction in the soil were
analyzed using Genstat to generate treatment means using
Fisher’s Least Significant Difference (LSD) test at 5% level of
significance.
3. Results
3.1. Plant Height and Dry Weight. The mean plant heights for
the control with cow dung manure treatments were higher
than that of all treatment combinations with TPH concentration 75 gkg−1 soil. Generally, and as would be expected, the
shortest plants were observed at treatments where there was
no manure applied yet the soil was contaminated with TPH
concentration 75 gkg−1 soil (Table 3). Plant growth (in terms
of height) was noticeably improved in treatments containing
combinations of TPH concentration 75 gkg−1 soil with either
application of manure 5 or 10 gkg−1 soil. For plant height, no
significant difference was observed between treatment combinations of manure 5 and 10 gkg−1 soil with TPH concentration 75 gkg−1 soil (p > 0.05) (Table 3).
The plants in the control yielded more dry weight
content than in all treatment combinations with TPH
concentration 75 gkg−1 soil. Generally, and as would be
expected, the light weight plants were observed in treatments
where there was no manure applied yet the soil was contaminated with TPH concentration 75 gkg−1 soil (Table 4).
There were more pronounced reductions in the total plant
dry weight in all the four phytoremediators in treatment
combination of 75 gkg−1 soil manure and TPH concentrations, respectively. This improved with the addition of
manure 5 and 10 gkg−1 soil to TPH-contaminated soils
(75 gkg−1 soil) leading to increased biomass yield for all the
plant species studied. However, a least significant difference
4
Applied and Environmental Soil Science
Table 3: Treatments effect on the height of four phytoremediators.
Phytoremediators (plant
species)
−1
M, 0 gkg
cm
160
97.1
116.1
130.1
H. rufa
O. longistaminata
T. diversifolia
S. arundinaceum
CV %
LSD (5%)
TPH, 0 gkg−1 soil
soil
M, 5 gkg−1
soil
163
102.0
121.2
134.3
M, 10 gkg
soil
164.4
102.2
123.6
135.9
−1
−1
M, 0 gkg
cm
129
40.1
90.3
70.5
TPH, 75 gkg−1 soil
soil
M, 5 gkg−1
M, 10 gkg−1
soil
soil
141
142
60.0
62.7
107.2
109.4
100.5
103.3
1.9
3.08
TPH � total petroleum hydrocarbon; M � manure.
Table 4: Treatments effect on the total plant dry weight of four phytoremediators.
Phytoremediators
(plant species)
H. rufa
O. longistaminata
T. diversifolia
S. arundinaceum
CV %
LSD (5%)
−1
M, 0 gkg
soil (g)
13.5
7.8
12.1
15.5
TPH, 0 gkg−1 soil
M, 5 gkg−1
soil
15.1
8.2
15.2
20.2
TPH, 75 gkg−1soil
M, 10 gkg
soil
15.9
9.2
16.5
20.9
−1
−1
M, 0 gkg soil
(g)
7.1
4.2
9.0
10.1
2.8
0.9
M, 5 gkg−1 soil
10.1
6.1
11.3
12.1
M, 10 gkg−1
soil
10.9
6.7
12.0
13.2
TPH � total petroleum hydrocarbon; M � manure.
between treatment combinations of manure 5 and manure
10 gkg−1 soil with TPH concentration 75 gkg−1 soil was
observed in S. arundinaceum (p < 0.05) while no significant
differences were observed in H. rufa, O. Longistaminata, and
T. diversifolia (p > 0.05) (Table 4).
3.2. Effect of Manure on Phytoremediation of Hydrocarbon
Contaminated Soil. The mean percentage reduction of TPH
in the four phytoremediators with manure treatments was
measured at 120 days after planting (Figure 1). The reductions in the control (i.e., soils with no phytoremediator
planted) were lower than those in treatments with 0, 5, and
10 gkg−1 soil manure concentrations. Thus, it is evident that
the presence of treatments of manure 5 and 10 gkg−1 soil
improved the percentage reduction of TPH compared to
treatments of phytoremediators without manure. No significant differences (p > 0.05) in mean percentage reduction
of TPH were observed between the manure 5 and 10 gkg−1
soil treatments for all the four plant species (Figure 1).
The total ion chromatograms (TICs) of the hydrocarbon
fractions from the control and treatments are shown in
Figure 2. The chromatograms gave qualitative and semiquantitative information on the changes in the composition
of hydrocarbons in the samples. The compounds in the
hydrocarbon fractions of the control ranged from n-C13 to
n-C40 and maximizing at n-C26. In the treatment with plant
alone, it ranged from n-C20 to n-C40. In both treatments of
plant, manure 5 and 10 gkg−1 soil, the compounds ranged
from n-C29 to n-C32.
4. Discussion
Biostimulation of hydrocarbon contaminated soils with cow
dung in the Sudd wetland holds potential of restoring the
hydrocarbon contaminated soils through bioremediation.
This is boosted by the abundance of O. longistaminata, S.
arundinaceum, T. diversifolia, and H. rufa plant species as
naturally existing phytoremediators [28], although their
growth is inhibited by high concentration of TPH in the
soils. In this study, in the treatments without manure, plant
species grew short and recorded light dry weight content
compared to the control and treatments with manure. The
inhibition is attributed to the unconducive conditions that
are caused by crude oil contamination such as water repellency which causes reduced access to water and oxygen by
the plants. This partly explains the plant shortness and
lightness of the plant dry weight. On the other hand, the
crude oil contaminants altered the soil physical properties
such as permeability which affected the growth of plant
species. The finding is supported by Akinwumi et al. [37] and
Nazir [38] who unanimously observed that contamination of
soil by crude oil alters the soil physical properties which
affects the free flow of total organic carbon and soil mineral
nutrients such as potassium, sulfate, phosphate, and nitrate
of soil. More light is shed by Akubugwo et al. [39] and Wang
et al. [40] that nutrient deficiencies inhibit the growth of
plant species in hydrocarbon contaminated soils.
Though Ruley et al. [35] ascertained four plant species
(O. longistaminata, S. arundinaceum, T. diversifolia, and H.
rufa) as prominent phytoremediators, this study confirmed
that augmentation with cow dung led to a marked improvement in the plant growth characteristics. There were
least significant differences between treatments without
manure and those with manure in terms of both plant height
and dry weight (p < 0.05). The improvement in plant height
and dry weight content is attributed to the restoration of lost
nutrients by addition of cow dung since it contains high
nutrient composition hence providing polluted soil with
H. rufa
Applied and Environmental Soil Science
0
–20
–40
–60
Plant species
O. longistaminata T. diversifolia
S. arundinaceum
No plant
120.00
n-Pentacosane
n-Heptacosane
n-Octacosane
n-Nonacosane
n-Tricotane
n-Untricotane
n-Dotricotane
n-Tritricotane
n-Tetratricotane
n-Pentatricotane
n-Hexatricotane
n-Heptatricotane
n-Octatricotane
n-Nonatricotane
n-Tetracotane
–80
n-Docosane
n-Tricosane
n-Tetracosane
–100
100.00
120.00
n-Tetracotane
(a)
100.00
n-Tricosane
n-Tetracosane
n-Pentacosane
n-Heptacosane
n-Octacosane
n-Nonacosane
n-Tricotane
n-Untricotane
n-Dotricotane
n-Tritricotane
n-Tetratricotane
n-Pentatricotane
n-Hexatricotane
n-Heptatricotane
n-Octatricotane
n-Nonatricotane
60.00
80.00
Retention time (min)
60.00
80.00
Retention time (min)
n-Docosane
Manure (g/kg soil)
40.00
40.00
(b)
Figure 2: Continued.
n-Eicosane
n-Unacosane
n-Hexadecane
n-Heptadecane
Prytane
n-Octadecane
Phytane
n-Nonadecane
n-Eicosane
n-Unacosane
0
5
10
20.00
20.00
n-Tetradecane
n-Pentadecane
% reduction of total petroleum hydrocarbon
–120
10000000
8000000
6000000
4000000
2000000
0.00
10000000
8000000
6000000
4000000
2000000
0.00
n-Tridecane
5
Figure 1: Effect of manure application on phytoremediation of hydrocarbon-contaminated soil; bars show the standard errors (SE) for
mean percentage reduction of TPH, n � 3.
Abundance (counts)
Abundance (counts)
6
Applied and Environmental Soil Science
Abundance (counts)
10000000
8000000
6000000
4000000
2000000
0.00
20.00
40.00
60.00
80.00
Retention time (min)
100.00
120.00
100.00
120.00
(c)
Abundance (counts)
10000000
8000000
6000000
4000000
2000000
0.00
20.00
40.00
60.00
80.00
Retention time (min)
(d)
Figure 2: Total ion chromatograms (TIC) of the TPH extracts of the control and treatments during the bioremediation experiment after 120
days. (a) No treatment. (b) Treated with plants. (c) Treated with plant and manure 5 gkg−1 soil. (d) Treated with plant and manure 10 gkg−1 soil.
nutrient elements. Also, cow dung is effective, economic,
and ecofriendly and leads to complete mineralization of
hydrocarbons [41] which enhances phytoremediation. Evidence of this effect is provided by the analyses based on TIC
which showed a gradual decrease in the hydrocarbon
compounds after 120 days implying that some of the
compounds had been completely biodegraded and could not
be observed in the chromatograms. Basing on this finding, O.
longistaminata, S. arundinaceum, T. diversifolia, and H. rufa
become more viable phytoremediators when augmented
with cow dung. This assertion rhymes Oyedele and Amoo
[33] that addition of cow dung manure improves on the
calcium, magnesium, phosphorus, potassium, and nitrogenous contents which are vital elements for better growth of
plant species. Essien et al. [30] also observed that the
ubiquitous nature of cow dung reduces the cost of using
inorganic fertilizers which further reduces the cost of
cleaning up crude oil contaminated soils.
In this study, there is a nonsignificant difference between
use of manure 5 gkg−1 soil and 10 gkg−1 soil. The study
recommends that any plans should settle for 5 gkg−1 soil as the
optimal amount. There is a high possibility that using 10 gkg−1
soil could divert the attention of the any existing microbes
from feeding on crude oil to feeding on nutrients in cow dung.
This diversion slows down the process of phytoremediation as
was observed in some studies such as Essien et al. [30] that
excess application of cow dung has the potential of causing the
existing microbes to abandon crude oil and turn to feeding on
the nutrients provided by the cow dung.
5. General Conclusion
In different environments, petroleum and petroleum-derived products inhibit plant growth and development. TPH
inhibits the normal functioning of plants by interfering with
the processes of intake of water and minerals from the
substrate. Also, it slows down and impedes a number of
metabolic processes from taking place. Case in point, when
oil penetrates seed coats, it causes death of the seed embryo.
Although remediation of TPH contaminated soils is a
challenging task, results of this study have revealed that
biostimulating crude oil polluted soils with cow dung significantly enhances plant growth parameters. Consequently,
this increases their efficiency as phytoremediators. This
study has demonstrated that application of cow dung at
concentrations of 5 and 10 gkg−1 soil, combined with efficient phytoremediators can significantly enhance the reduction of TPH compared to natural attenuation or use of
either manure or phytoremediators alone. Furthermore, this
study has elucidated that a combination of organic manure
5 gkg−1 soil with a phytoremediator yields the same mean
percentage reduction of TPH as 10 gkg−1 soil. This study
concludes that cow dung manure improves the phytoremediation potential of plant species in the Sudd wetland,
South Sudan, and recommends the use of a combination of
phytoremediators and 5 gkg−1 soil of cow dung as the best
combination for enhancing the remediation. Use of cow
dung proves cost-effective compared to other remediation
techniques and provides an ideal solution to the government
Applied and Environmental Soil Science
of South Sudan and its development partners for phytoremediation of TPH contaminated soils in and around the
Sudd region. Facilitating conditions to support this strategy
exist in the region such as the availability of high numbers of
cattle, with an average of 4 heads per household. This
presents potential opportunities for restoration of crude oil
contaminated soils in the region at a low cost.
7
[8]
[9]
Data Availability
The data used to support the findings of this study are
available on request from Jane Alexander Ruley (janenajeb@
yahoo.com; +256756352256)
[10]
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors would like to thank NORAD for funding this
study through the Sudd project (NORHED Project no. SSD13/0021) implemented by University of Juba, Makerere
University, and the Norwegian University of Life Sciences.
The authors also acknowledge the support of the Ministry of
Petroleum and Gas, Dar Petroleum Company Ltd, Sudanese
Petroleum Laboratories, for analysis of the soil samples.
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