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Comparative Effectiveness of Nitrogen Sources for Nutrient
Amendment in Bioremediation of Petroleum-Contaminated Soils in
Nigeria’s Niger Delta Region
Egbebike, M. O.¹*, Moneke, A. N.², Ezeagu, C. A.³
¹,²Center for Environmental Management and Green Energy, University of Nigeria, Enugu Campus,
Nigeria
¹,³Department of Civil Engineering, Nnamdi Azikiwe University, Awka, Nigeria
*Corresponding author
DOI: https://doi.org/10.51244/IJRSI.2025.1208004127
Received: 22 Sep 2025; Accepted: 29 Sep 2025; Published: 24 October 2025
ABSTRACT
Oil spills are a recurrent environmental challenge in Nigeria’s Niger Delta, leading to significant ecological
and socio-economic impacts. Bioremediation, particularly nutrient amendment via biostimulation, has
emerged as a viable approach for enhancing the natural degradation of petroleum hydrocarbons by indigenous
microbes. This study investigates the comparative effectiveness of three nitrogen sources-ammonium (NH₄⁺),
nitrate (NO₃⁻), and organic nitrogen-on the degradation of petroleum hydrocarbons in oil-contaminated soils.
Using a mesocosm experimental setup with composite soil samples from three communities (Batan, Ajuju,
and Umusia), treatments were applied across varying oil concentrations. Results showed that nutrient
amendment generally increased total nitrogen (%), enhanced microbial population, and significantly reduced
both total petroleum hydrocarbon (TPH) and polyaromatic hydrocarbon (PAH) levels. Ammonium-nitrogen
was more effective in stimulating hydrocarbon degradation than nitrate, while organic nitrogen produced the
highest microbial proliferation. Regression analysis revealed a strong positive correlation between nitrogen
concentration and microbial population growth (r = 0.95). These findings support nitrogen-based
biostimulation as a practical, low-impact strategy for accelerating oil spill remediation in tropical
environments like the Niger Delta.
Keywords: Bioremediation, Hydrocarbon Contamination, Total Petroleum Hydrocarbon, Polyaromatic
Hydrocarbon, Nutrient Amendment, Niger Delta, Microbial Activity
INTRODUCTION
The Niger Delta region of Nigeria is among the most ecologically diverse and economically important areas in
West Africa, hosting over 30 million people and extensive oil and gas infrastructure. Despite its vast natural
wealth, the region has suffered chronic and severe environmental degradation due to decades of oil
exploration, pipeline leaks, sabotage, and spills. According to the United Nations Environment Programme
(UNEP, 2011), oil pollution in the region has compromised agricultural land, aquatic habitats, and drinking
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water sources, with many affected communities experiencing long-term exposure to toxic compounds such as
benzene, polycyclic aromatic hydrocarbons (PAHs), and heavy metals. Oil contamination has significant long-
term effects on soil structure, microbial dynamics, and ecosystem health (Renoux et al., 2000; Römbke et al.,
2005).
Oil spills in the Niger Delta are not only a local ecological crisis but also a major public health and socio-
economic concern. Contaminated soils and waters have led to declines in farm productivity, destruction of
fisheries, and disruption of livelihoods, especially in rural communities dependent on natural resources.
Despite global pressure and environmental justice campaigns, remediation efforts in the Niger Delta remain
limited, expensive, and often unsustainable. Conventional remediation methods-such as physical containment,
incineration, and chemical dispersants-are not always suitable due to their high costs, risk of secondary
pollution, and inability to restore soil health or support ecological recovery in tropical environments (Das &
Chandran, 2011; Obayori et al., 2009). This is particularly critical in tropical and wetland environments like
the Niger Delta, where bioavailability, ecological compatibility, and long-term soil health must be considered
(Renoux et al., 2000).”
As a result, bioremediation-using microbial communities to naturally degrade pollutants-has gained
considerable attention as an eco-friendly, cost-effective alternative. Bioremediation is particularly suitable for
large-scale, in-situ cleanups in regions like the Niger Delta, where widespread contamination persists across
vast and often remote terrains (Margesin & Schinner, 2001; Bento et al., 2005). This technique relies on the
metabolic capabilities of indigenous or introduced microorganisms to break down petroleum hydrocarbons
into less harmful substances, such as carbon dioxide and water. Płaza et al. (2005) emphasized the utility of
bioassays in tracking soil remediation success, complementing microbial population metrics used in this
study.
However, successful bioremediation depends on the availability of essential nutrients, particularly nitrogen
and phosphorus, which are often lacking in contaminated soils. The biodegradation of hydrocarbons is a
nitrogen-intensive process, as microbial metabolism requires nitrogen for the synthesis of proteins, nucleic
acids, and enzymes. Numerous studies have shown that hydrocarbon-degrading microbes thrive in nitrogen-
rich environments and that nitrogen amendment significantly accelerates the rate of degradation (Leahy &
Colwell, 1990; Seymour et al., 1996; Atlas & Hazen, 2011).
Nitrogen can be supplemented through various forms-such as ammonium (NH₄⁺), nitrate (NO₃⁻), or organic
nitrogen sources like urea or compost. Each nitrogen form presents different dynamics in terms of solubility,
bioavailability, soil retention, and microbial uptake. For example, ammonium is rapidly assimilable but may
acidify soils and be lost through volatilization or leaching. Nitrate, while also bioavailable, is more prone to
leaching and can pose risks to groundwater. Organic nitrogen releases slowly through microbial
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decomposition and may support long-term fertility but with delayed effects on hydrocarbon degradation
(Singh et al., 2007; Macaulay, 2015).
While international studies have explored nitrogen-enhanced bioremediation in diverse environments-from
alpine soils to marine shorelines-relatively few have addressed the specific context of the Niger Delta. The
region’s acidic, humid tropical soils, combined with frequent flooding and diverse microbial populations,
demand locally adapted remediation strategies. Moreover, the lack of comparative studies on the performance
of different nitrogen sources under field-representative conditions presents a significant research gap. Without
such insights, efforts to scale up bioremediation in the Niger Delta remain uncertain and inconsistent.
This study seeks to fill that gap by evaluating the relative effectiveness of ammonium, nitrate, and organic
nitrogen sources in promoting bioremediation of petroleum-contaminated soils from three communities in the
Niger Delta. Through a mesocosm simulation experiment, this research examines the impact of each nitrogen
source on microbial activity, nitrogen availability, and the degradation of total petroleum hydrocarbons (TPH)
and polyaromatic hydrocarbons (PAHs). The goal is to identify the most efficient and ecologically sound
nitrogen amendment approach suited to the environmental conditions of the Niger Delta.
By generating empirical data and insights on nitrogen-amended bioremediation, this study aims to inform
evidence-based environmental management practices and policy development. Ultimately, enhancing
remediation capacity can contribute to ecological recovery, food security, and health improvements for
communities grappling with decades of oil pollution.
LITERATURE REVIEW
2.1 Bioremediation: Principles and Relevance to the Niger Delta
Bioremediation is a natural attenuation process that employs microbial metabolism to degrade and detoxify
environmental pollutants, especially hydrocarbons. Microorganisms use hydrocarbons as a carbon and energy
source, converting complex organic compounds into simpler, non-toxic substances like carbon dioxide, water,
and biomass (Leahy & Colwell, 1990). As an environmentally sustainable and cost-effective alternative to
conventional remediation techniques, bioremediation has gained widespread application for managing soil and
groundwater pollution from petroleum products (Das & Chandran, 2011).
The relevance of bioremediation in the Niger Delta is particularly compelling due to the scale and persistence
of oil pollution in the region. Traditional clean-up methods, such as mechanical skimming or the use of
dispersants, are either ineffective in wetland environments or introduce additional toxicants into fragile
ecosystems (UNEP, 2011). Bioremediation, especially in-situ approaches, is better suited for large,
inaccessible areas and can enhance ecological recovery by restoring soil microbial communities and improving
fertility (Margesin et al., 2005).
2.2 Biostimulation and the Role of Nitrogen
Bioremediation can be achieved through two major strategies: bioaugmentation and biostimulation. While
bioaugmentation involves introducing exogenous microbial strains, biostimulation enhances the growth and
metabolic activity of indigenous microbial populations through the addition of nutrients, moisture, or oxygen
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(Atlas & Hazen, 2011). In nutrient-limited environments like oil-contaminated soils, biostimulation using
nitrogen and phosphorus amendments is often critical to accelerate hydrocarbon degradation. Salanitro et al.
(1997) observed that nitrogen-supplemented soils exhibited higher hydrocarbon breakdown and lower
ecotoxicity levels, especially when monitored over a sustained remediation period.”
Nitrogen is essential for microbial cell growth, enzyme production, and hydrocarbon catabolism. Microbial
metabolism of hydrocarbons requires a balanced carbon-to-nitrogen (C:N) ratio, ideally around 10:1 to 20:1
(Bossert & Bartha, 1984). However, oil-contaminated soils usually have an abundance of carbon (from
hydrocarbons) but are severely deficient in nitrogen and phosphorus, which limits microbial degradation
capacity. Numerous studies have demonstrated that nitrogen amendments significantly increase the rate of
hydrocarbon biodegradation in both temperate and tropical settings (Venosa et al., 2002; Bento et al., 2005). In
addition to nutrient augmentation, bioassays have been used to monitor microbial response and remediation
success under petroleum stress (Płaza et al., 2005).
2.3 Comparative Analysis of Nitrogen Sources
The effectiveness of biostimulation varies depending on the form of nitrogen applied. Common nitrogen
sources include:
Ammonium Nitrogen (NH₄⁺):
Ammonium sulfate [(NH₄)₂SO₄] or ammonium nitrate are frequently used due to their low cost and high
nitrogen content. Ammonium is readily available to microbes, promoting rapid growth and hydrocarbon
degradation. However, excessive ammonium can lower soil pH, affect microbial diversity, and contribute to
nitrogen loss through volatilization or leaching in sandy or acidic soils (Roling & van Bodegom, 2014).
Jackson and Pardue (1999) found that ammonium-nitrogen was more effective than nitrate in salt marshes
because it adsorbed more strongly to organic matter, reducing washout losses.
Nitrate Nitrogen (NO₃⁻):
Nitrate-based fertilizers, such as potassium nitrate (KNO₃), are also effective in aerobic conditions. Nitrate is
more stable in well-aerated soils and supports sustained microbial activity. However, it is more prone to
leaching, particularly in tropical regions with high rainfall like the Niger Delta. Additionally, nitrate
application may pose risks to groundwater contamination if not carefully managed (Singh et al., 2007).
Organic Nitrogen:
Organic sources, including compost, poultry manure, biosolids, and urea, decompose slowly, releasing
nitrogen over time. This gradual release helps maintain long-term microbial activity and improves soil
structure and fertility. Organic nitrogen also supplies additional micronutrients and organic carbon that may
benefit microbial consortia. However, the slow release limits their effectiveness in scenarios requiring rapid
remediation (Das & Chandran, 2011; Macaulay, 2015). Despite this limitation, organic nitrogen is often more
environmentally benign and suitable for sustainable land restoration.
This supports a dual-parameter approach combining TPH/PAH degradation and microbial assays, as
highlighted in ecological risk models by Saterbak et al. (1999).
2.4 Field Studies and Regional Research Gaps
Several studies have investigated nitrogen-enhanced bioremediation globally, but only a few have focused
specifically on the Niger Delta. Obayori et al. (2009) demonstrated the effectiveness of mixed inorganic
fertilizers in reducing TPH in Nigerian soils. Okoh (2006) emphasized the importance of understanding local
microbial communities and soil conditions for successful bioremediation. Similarly, Zabbey and Uyi (2014)
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stressed that regional variability in soil properties and microbial ecology necessitates site-specific nutrient
management strategies.
What remains lacking is a comparative evaluation of different nitrogen sources under controlled conditions
that simulate the Niger Delta’s acidic, waterlogged, and nutrient-poor soils. Without such evidence,
environmental managers risk applying inappropriate or suboptimal remediation strategies. Moreover, the long-
term effects of various amendments on microbial diversity, soil health, and pollutant transformation pathways
remain underexplored in the region.
2.5 Microbial Dynamics in Petroleum Degradation
Hydrocarbon degradation in soil typically involves a microbial succession process, beginning with rapidly
growing opportunistic species and followed by slower, more specialized degraders. Over 200 bacterial genera,
including Pseudomonas, Bacillus, Acinetobacter, and Rhodococcus, have been implicated in petroleum
biodegradation (Atlas, 1981; Zobel, 1973). Fungal species such as Aspergillus and Penicillium also contribute,
particularly in organic-rich or slightly acidic soils (Rahman et al., 2003).
The microbial response to nutrient amendment varies based on both the nitrogen source and the
physicochemical characteristics of the contaminated site. In soils with low cation-exchange capacities, such as
sandy loams common in the Niger Delta, ammonium may be lost more quickly than nitrate or organic
nitrogen, affecting nutrient availability (Jackson & Pardue, 1999). Therefore, assessing both microbial activity
and soil nutrient dynamics under different treatments is essential to determine the most effective
bioremediation strategy.
MATERIALS AND METHODS
3.1 Study Area Description
The study focused on three oil spill-impacted communities in the Niger Delta region of Nigeria: Batan and
Ajuju in Bayelsa State and Umusia in Oyigbo LGA of Rivers State. The Niger Delta is a low-lying, wetland-
rich area situated between latitudes 4°N and 6°N and longitudes 5°E and 8°E. The region experiences high
annual rainfall ranging from 2,000 mm to 3,800 mm, with relative humidity between 80% and 90% and mean
annual temperature around 25°C. These climatic conditions contribute to frequent flooding and high organic
matter accumulation, but also pose challenges for petroleum hydrocarbon degradation due to nutrient
depletion, poor aeration, and acidification.
The soils in the selected communities were predominantly sandy loam and sandy clay loam, with acidic pH
and low cation exchange capacities (CEC). These characteristics affect nutrient retention, microbial
proliferation, and hydrocarbon mobility, making them suitable for evaluating bioremediation interventions
under challenging field conditions.
3.2 Soil Sampling and Pre-treatment
A stratified random sampling technique was used to select representative plots from each of the three oil-
contaminated communities. Each site was divided into three sections to act as biological replicates. Composite
soil samples were collected at a depth of 0-15 cm using a soil auger and were air-dried, sieved, and stored in
sterile containers.
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Baseline analyses were conducted to determine:
Soil texture (via Bouyoucos hydrometer method)
Soil pH (1:1 soil-water ratio using a glass electrode pH meter)
Total organic carbon (Walkley-Black method)
Total nitrogen (Kjeldahl method)
Ammonium and nitrate nitrogen (colorimetric and ion-selective electrode methods)
Total phosphorus (persulfate digestion and photometric analysis)
Hydrocarbon content (TPH and PAH via GC-MS and gravimetry)
Microbial counts (total heterotrophic and hydrocarbon-degrading bacteria using colony forming units
[CFUs])
Meteorological and environmental data, including rainfall, temperature, and humidity, were obtained from the
Nigerian Meteorological Agency.
3.3 Experimental Design and Simulation Setup
A mesocosm experiment was conducted under controlled outdoor conditions in Enugu, Southeast Nigeria.
This location was chosen for logistical reasons and regional security concerns. The study simulated real-world
oil spill conditions using intentionally contaminated soils.
A 4 × 3 factorial randomized complete block design (RCBD) with three replicates was employed. The two
main factors were:
Nitrogen source (4 levels):
o N0 = No nutrient amendment (control)
o N1 = Potassium Nitrate (KNO₃)
o N2 = Ammonium Sulfate ((NH₄)₂SO₄)
o N3 = Organic fertilizer (biosolids)
Oil contamination level (3 levels):
o P0 = No oil (control)
o P1 = 20 g/kg of soil (moderate contamination)
o P2 = 80 g/kg of soil (severe contamination)
A total of 12 treatment combinations were established (table 1), with 3 replicates each, totaling 36
experimental plots.
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Table 1: Treatment Combinations
Concentration of oil
spill
Levels of Nutrients
N0 N1 N2 N3
P0 N0P0 N1P0 N2P0 N3P0
P1 N0P1 N1P1 N2P1 N3P1
P2 N0P2 N1P2 N2P2 N3P2
N0P0 = No nutrient amendment and no oil spilled/released
N0P1 = No nutrient amendment and 20 gm/kg of oil released
N0P2 = No nutrient amendment and 80 gm / kg oil released
N1P0 = Amendment with KNO3 and no oil spilled/released
N1P1 = Amendment with KNO3 and 20 gm/kg oil spilled/released
N1P2 = Amendment with KNO3 and 80 gm/kg oil spilled/released
N2P0 = Amendment with (NH4)2SO4 and no oil spilled/released
N2P1 = Amendment with (NH4)2SO4 and 20 gm/kg oil spilled/released
N2P2 = Amendment with (NH4)2SO4 and 80 gm/kg oil spilled/released
N3P0 = Amendment with Organic fertilizer and no oil spilled/released
N3P1 = Amendment with Organic fertilizer and 20 gm/kg oil spilled/released
N3P2 = Amendment with Organic fertilizer and 80 gm/kg oil spilled/released
3.4 Treatment Application and Monitoring
Each treatment plot was artificially contaminated with refined petroleum diesel (AGO) to simulate oil spills.
One week after contamination, nitrogen sources were applied based on their nitrogen equivalency.
Inorganic fertilizers (N1 and N2) were dissolved and applied via spraying.
Organic biosolids (N3) were analyzed for nitrogen content prior to application to ensure dosing
equivalence.
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Nutrient amendments were monitored to ensure uniform distribution and avoid nutrient losses from
leaching or volatilization.
Soil samples were collected on Days 0, 7, 49, and 108 post-treatment to assess changes in:
Total nitrogen (%)
Microbial population (log CFUs)
Total petroleum hydrocarbons (TPH, mg/kg)
Polyaromatic hydrocarbons (PAH, %)
3.5 Laboratory Analysis
Microbial Enumeration: Performed using plate count methods and expressed as logarithmic CFUs/g
soil. Selective media were used for total heterotrophs and hydrocarbon-degrading bacteria.
Hydrocarbon Analysis: TPH was determined via gravimetric techniques, while PAH fractions were
quantified using gas chromatography-mass spectrometry (GC-MS) in accordance with USEPA
protocols.
Nitrogen Concentration: Total N, NH₄⁺, and NO₃⁻ levels were measured using Kjeldahl digestion,
salicylate-hypochlorite colorimetry, and nitrate-specific electrodes respectively.
3.6 Statistical Analysis
Data were statistically analyzed using SPSS (v22) and ANOVA to determine significant differences among
treatment means. Fisher’s Least Significant Difference (FLSD) test at P ≤ 0.05 was used for post-hoc
comparisons.
Regression and correlation analyses were performed to determine the relationships between:
Nitrogen concentration (independent variable) and
o Microbial population
o TPH
o PAH
This analysis provided insight into the biostimulatory effectiveness of each nitrogen source.
RESULTS AND DISCUSSIONS
4.1 Baseline Soil Characteristics
The physicochemical analysis of soils from Batan, Ajuju, and Umusia (table 2) revealed that all three
locations had acidic soils (pH 4.5–4.7), low organic carbon (<2%), and low total nitrogen (<0.05%),
consistent with typical post-spill tropical soils. Soils were either sandy loam or sandy clay loam, indicating
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low cation exchange capacity and a high likelihood of nutrient leaching. These properties justify the need for
nutrient amendment to stimulate microbial degradation of petroleum hydrocarbons.
Table 2: Initial Physicochemical Properties of Soils from Selected Sites
Property Batan Ajuju Umusia
pH (1:1 soil:water) 4.61 4.55 4.70
Texture Sandy Loam Sandy Clay Loam Sandy Loam
Organic Carbon (%) 1.81 1.57 1.43
Total Nitrogen (%) 0.045 0.038 0.042
Available Phosphorus (mg/kg) 7.4 6.1 6.8
4.2 Effects of Nitrogen Amendment on Total Nitrogen Content
Across all oil concentrations (P0, P1, P2), total nitrogen content (%) increased in amended soils relative to the
unamended control (N0). The highest total nitrogen values were recorded in soils treated with organic
nitrogen (N3), especially after 49 and 108 days, suggesting gradual release and sustained nutrient availability
(table 3).
In P2 (80 g/kg) plots, total N in N3 rose from 0.062% on Day 0 to 0.110% by Day 108.
N2 (ammonium) and N1 (nitrate) treatments showed sharp early increases but began plateauing by Day
49.
Table 3: Total Nitrogen (%) Over Time in Soils Treated with Different Nitrogen Sources (P2 Level)
Treatment Day 0 Day 7 Day 49 Day 108
N0 (Control) 0.045 0.048 0.051 0.053
N1 (Nitrate) 0.045 0.072 0.084 0.089
N2 (Ammonium) 0.045 0.080 0.092 0.098
N3 (Organic) 0.045 0.077 0.101 0.110
This confirms literature assertions that organic sources provide long-term nutrient support (Singh et al., 2007;
Das & Chandran, 2011), while inorganic sources may be prone to leaching or volatilization in acidic soils.
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4.3 Microbial Population Dynamics
Microbial counts (expressed in log CFUs/g) showed significant increases in all amended plots compared to
control, with organic nitrogen (N3) again producing the highest microbial stimulation as illustrated in table 4
and figure 1 below.
Table 4: Microbial Population (log CFUs/g) Over Time at P2 Level
Treatment Day 0 Day 7 Day 49 Day 108
N0 (Control) 5.52 5.67 5.80 5.94
N1 (Nitrate) 5.81 5.96 6.09 6.22
N2 (Ammonium) 6.15 6.30 6.43 6.56
N3 (Organic) 6.33 6.48 6.61 6.74
Figure 1: Microbial Population Over Time at P2 Level
On Day 108, microbial population in N3 reached 6.74 log CFUs/g, compared to 5.94 log CFUs/g in
N0.
N2 (ammonium) followed closely with 6.56 log CFUs/g, while nitrate (N1) peaked at 6.22.
These results affirm that nitrogen availability, especially from organic sources, enhances indigenous microbial
proliferation, a key driver of hydrocarbon biodegradation (Atlas & Bartha, 1973; Rahman et al., 2003).
4.4 TPH Degradation Performance
Total Petroleum Hydrocarbon (TPH) concentration decreased steadily over the 108-day period in all amended
plots, particularly in soils treated with ammonium and organic nitrogen. This is illustrated in table 5 and figure
2 below.
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Table 5: Total Petroleum Hydrocarbon (TPH) Concentration (mg/kg) Over Time at P2 Level
Treatment Day 0 Day 7 Day 49 Day 108
N0 (Control) 18.81 18.00 17.19 17.11
N1 (Nitrate) 17.76 17.00 16.24 16.07
N2 (Ammonium) 16.77 16.05 15.33 15.08
N3 (Organic) 15.83 15.15 14.47 14.14
Figure 2: TPH Concentration Over Time at P2 Level
N3 reduced TPH from 15.83 mg/kg (Day 0) to 14.14 mg/kg (Day 108).
N2 followed with a final TPH of 15.08 mg/kg, while the control (N0) showed minimal reduction (17.11
mg/kg at Day 108).
The faster reduction in N2 (ammonium-treated) soils suggests rapid microbial response due to bioavailable
nitrogen, consistent with other tropical soil studies (Roling & van Bodegom, 2014; Bento et al., 2005).
4.5 PAH Degradation
PAH degradation data visualization is shown in figure 3 below. As illustrated in table 6 and figure 3, PAH
concentrations decreased steadily across all treatments. Organic nitrogen (N3) demonstrated the most
consistent reduction, reaching 1.00% by Day 108, compared to 1.06% for ammonium (N2), 1.12% for nitrate
(N1), and 1.19% for the control (N0). These trends reflect a strong response to nutrient-stimulated microbial
activity
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Table 6: PAH Concentration (%) Over Time at P2 Level
Treatment Day 0 Day 7 Day 49 Day 108
N0 (Control) 2.21 2.10 1.45 1.19
N1 (Nitrate) 2.14 2.03 1.38 1.12
N2 (Ammonium) 2.08 1.97 1.32 1.06
N3 (Organic) 2.02 1.91 1.26 1.00
Figure 3: PAH Concentration Over Time at P2 Level (80 g/kg oil)
These trends are in line with other findings where nitrogen supports microbial catabolism of PAHs,
particularly those with medium molecular weight fractions (Das & Chandran, 2011).
4.6 Regression and Correlation Analysis
Regression analysis demonstrated strong positive correlations between nitrogen concentration and microbial
proliferation (see table 7 below):
Table 7: Regression Models Between Nitrogen Concentration and Microbial Population at P2 Level
Treatment Regression Equation r-value Slope (b) Interpretation
N3P2 Y = 1.28x + 6.21 0.93 1.28 Strong effect of organic N
N2P2 Y = 0.61x + 5.86 0.96 0.61 Highest correlation
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N1P2 Y = 0.38x + 5.58 0.95 0.38 Moderate stimulation
N0P2 Y = 0.21x + 5.46 0.90 0.21 Weakest effect
These results support the hypothesis that organic nitrogen is most effective in enhancing microbial
populations, although ammonium accelerates hydrocarbon reduction more effectively. Therefore, the choice
of nitrogen source may depend on whether the remediation goal is long-term soil fertility recovery (organic)
or rapid hydrocarbon breakdown (ammonium).
INTEGRATED DISCUSSION
This study reveals that nutrient amendment is vital to effective bioremediation in oil-impacted soils of the
Niger Delta. Inorganic nitrogen (especially ammonium) enabled fast-acting hydrocarbon degradation, while
organic nitrogen supported superior microbial proliferation and more stable nitrogen profiles over time.
Our findings support the conclusions of Salanitro et al. (1997), who demonstrated that nitrogen availability
significantly enhances microbial degradation of hydrocarbons in contaminated soils. These findings also align
with those of Macaulay (2015), who concluded that a combination of fast-release and slow-release nitrogen
sources may provide the best outcomes in complex contaminated environments. Moreover, the use of
indigenous microbial populations without bioaugmentation further supports biostimulation as a sustainable
remediation option in rural and resource-limited settings.
CONCLUSION
This study comprehensively examined the comparative effectiveness of three nitrogen sources-ammonium
(NH₄⁺), nitrate (NO₃⁻), and organic nitrogen-in enhancing the bioremediation of petroleum-contaminated soils
in Nigeria’s Niger Delta region. Through a mesocosm simulation mimicking real-world oil spill conditions, it
was shown that nitrogen supplementation significantly improved microbial proliferation, total nitrogen
content, and the degradation of total petroleum hydrocarbons (TPH) and polyaromatic hydrocarbons (PAH).
Such outcomes reinforce the potential to incorporate microbial and hydrocarbon parameters into broader
ecological quality frameworks (Römbke et al., 2005).
Among the nitrogen treatments:
Ammonium nitrogen proved most effective in rapid hydrocarbon degradation, due to its immediate
bioavailability to microorganisms.
Organic nitrogen, while slower in action, supported the highest microbial population growth and long-
term nutrient sustainability, likely due to its gradual nutrient release and improved soil organic matter
content.
Nitrate nitrogen was moderately effective but less stable under the acidic, leaching-prone soil
conditions characteristic of the Niger Delta.
Regression and correlation analyses revealed a strong positive relationship between nitrogen concentration
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and microbial activity, especially in the organic nitrogen-amended plots (r = 0.93-0.96). These findings
reaffirm the critical role of nutrient limitation in controlling the efficacy of bioremediation and highlight the
importance of site-specific selection of nutrient sources.
Overall, the study demonstrates that biostimulation using nitrogen amendment is a viable, low-cost, and
ecologically sound strategy for oil spill remediation in tropical wetland environments. Moreover, the results
advocate for integrated remediation approaches that balance short-term contaminant reduction with long-term
ecological recovery.
RECOMMENDATIONS
Based on the findings of this study, the following recommendations are made:
1. Adopt site-specific nutrient amendment strategies:
Soil type, oil concentration, and environmental conditions should guide the selection of nitrogen
sources. Ammonium nitrogen may be suitable for rapid clean-up efforts, while organic nitrogen is
better for sustained soil health recovery.
2. Combine nitrogen sources for synergistic effects:
A blend of fast-acting (e.g., ammonium sulfate) and slow-release (e.g., compost or biosolids) nitrogen
may optimize both biodegradation efficiency and long-term microbial stability.
3. Promote the use of indigenous microbial populations:
Bioaugmentation may not be necessary in all cases, especially where indigenous degraders respond
positively to nutrient enrichment. Further profiling of native microbes is encouraged to enhance
biostimulation protocols.
4. Implement field-scale trials:
The mesocosm results should be validated through in-situ trials in various Niger Delta environments,
including swampy, flood-prone, and upland zones, to evaluate operational feasibility.
5. Integrate bioremediation into environmental policy and spill response plans:
National agencies and oil operators should consider biostimulation protocols as part of approved
remediation frameworks, especially for low-income rural communities affected by chronic oil
pollution.
6. Monitor long-term soil recovery:
In addition to contaminant removal, future studies should monitor changes in soil fertility, structure,
and microbial diversity post-remediation to assess ecological resilience.
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7. Evaluate nutrient interactions:
Subsequent research should investigate the role of phosphorus, potassium, and micronutrients in
combination with nitrogen to develop more holistic nutrient amendment formulations.
By addressing both scientific and implementation challenges, these recommendations aim to improve the
sustainability and effectiveness of bioremediation strategies for petroleum pollution in the Niger Delta and
similar tropical regions.
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