The Effect of Petroleum Oil Spill on the Sulphate, Nitrate, and Nitrite Level of the Soil
Chapter One
Objectives of the Study
The main objective of this study is to identify the effect of petroleum oil spills on the sulfate, nitrate, and nitrite levels of the soil.
CHAPTER TWO
LITERATURE REVIEW
Petroleum oil spills
Petroleum oil spills are catastrophic events that cause massive damage to ecosystems at all trophic levels. While most of the research has focused on carbon-degrading microorganisms, the potential impacts of hydrocarbons on microbes responsible for nitrification have received far less attention. Nitrifiers are sensitive to hydrocarbon toxicity: ammoniaoxidizing bacteria and archaea being 100 and 1000 times more sensitive than typical heterotrophs respectively. Field studies have demonstrated the response of nitrifiers to hydrocarbons is highly variable and the loss of nitrification activity in coastal ecosystems can be restored within 1–2 years, which is much shorter than the typical recovery time of whole ecosystems (e.g., up to 20 years). Since the denitrification process is mainly driven by heterotrophs, which are more resistant to hydrocarbon toxicity than nitrifiers, the inhibition of nitrification may slow down the nitrogen turnover and increase ammonia availability, which supports the growth of oil-degrading heterotrophs and possibly various phototrophs. A better understanding of the ecological response of nitrification is paramount in predicting impacts of oil spills on the nitrogen cycle under oil spill conditions, and in improving.
Average worldwide annual releases of petroleum are estimated to be around 1300 thousands of metric tons per year (National Academy of Sciences, 2003). Nearly half of the released petroleum originates from natural seeps of crude oil and the other half is associated with human activities (671 thousands of metric tons per year), which include spills from pipelines, tankers and coastal facilities. Oil spills that release large amounts of petroleum hydrocarbons into the ocean can disrupt marine ecosystems at various trophic levels (Atlas and Hazen, 2011). For example, nearly 800 thousands of metric tons of oil (4.9 million barrels) polluted the Gulf of Mexico during the Deepwater Horizon oil spill in 2010 (McNutt et al., 2012). Each marine organism has a different sensitivity to hydrocarbons and the degree of toxicity depends on the body size of the organism and mechanisms of toxicity (Echeveste et al., 2010). Microorganisms are essential components in marine ecosystems and certain marine bacteria actively contribute to oil biodegradation (Chakraborty et al., 2012; Head et al., 2006). Oil spills are known to significantly alter indigenous microbial composition, diversity and metabolic profiles at the contaminated sites (Gutierrez, 2013; Hazen et al., 2010; Joye et al., 2014; Ribeiro et al., 2013; Scott et al., 2014). Therefore, the majority of oil spill studies examining microbial responses to oil contamination have been focused on identifying dominant hydrocarbon-degrading bacteria and determining how these microorganisms metabolically respond to massive oil inputs in coastal areas (Allan et al., 2012; Kostka et al., 2011) and open oceans (Hazen et al., 2010; Joye et al., 2014; Valentine et al., 2012).
Since microorganisms play key roles in various biogeochemical processes, some functionally significant microbial groups could be particularly vulnerable to oil spills (Urakawa et al., 2012; Rodriguez-R et al., 2015). With most of the research focusing on hydrocarbon-degrading microorganisms, the potential impacts of hydrocarbons on microbes responsible for mediating other crucial biogeochemical functions, such as nitrification, have received far less attention. Microbial communities utilize the overflow of oil as a carbon source thereby increasing the demand for inorganic nutrients such as nitrogen within oil- impacted sites. As a result of the increased nutrient demand, nitrogen availability has been shown to be a potential limiting factor that controls biodegradation of hydrocarbons at contaminated terrestrial and marine sites (Vitousek and Howarth, 1991; Herbert, 1999). Previous studies on coastal ecosystems have demonstrated that the nitrogen cycle plays a critical role in primary production by phytoplankton, benthic algae and macrophytes (Howarth, 1988; Herbert, 1999; Howarth and Marino, 2006). Notably, it has been reported that nitrification is sensitive to environmental stressors and contaminants (Juliette et al., 1993; Suwa et al., 1994; Brandt et al., 2001; Stephen et al., 1999; Urakawa et al., 2008a, b; Radniecki et al., 2013). In the past, nitrifying microorganisms have been used as bioindicators to assess the health condition of environments and ecosystems (Urakawa and Bernhard, 2017). They are ubiquitously found in various natural and manmade environments (Urakawa et al., 2011) and play an important role in the marine nitrogen cycle (Bernhard and Bollmann, 2010). However, to our knowledge, very little is known about the sensitivity, environmental response and resilience of nitrifying bacteria and archaea to petroleum contamination (Roling et al., 2004; Urakawa et al., 2012; Radniecki et al., 2013). Hydrocarbon contamination is not limited to marine environments; it also happens in terrestrial environments (Xu et al., 2006; Wu et al., 2016). Microorganisms serve as a key component of bioremediation in oilcontaminated soil and even nitrifying bacteria are expected to play a role in the oil-degradation process (Deni and Penninckx, 1999, 2004; Wu et al., 2016).
CHAPTER THREE
MATERIALS AND METHODS
Source of Soil Samples
The soil samples used in the study were collected from wetland soil sites where there are no recorded cases of crude oil contamination. The soil was obtained within the vicinity of Ikot Obio Nko stream in Ibesikpo-Asutan Local Government Area of Akwa Ibom State. The prevalent species of legumes, Calopogonium muconoides and Centrosema pubescens, both members of the family Leguminosae found in the wetland sites were selected for the study.
Soil Analysis
Surface (0 – 10 cm depth) soil samples obtained from the wetland were air dried and passed through a 2 mm sieve. Particle size distribution of the soil samples were analyzed by combination of wet sieving and hydrometer techniques (Buurman , 1996) using calgon as the dispersing agent. The soil organic carbon content was determined by dichromate wet oxidation methods of Walkley and Black as modified by Dhyan Singh (1999). Total nitrogen was determined by Kjeldahl digestion methods of Brady and Weil (1999), Exchangeable bases were extracted with 1 mole of NH4OAC (pH 7). Potassium and sodium in the extracts were determined by Flame photometric according to AOAC, 2005, while calcium and magnesium were determined by methods of Brady and Weil(1999), Soil pH was determined in water using a Pye Unicam pH meter (AOAC, 2005) and electrical conductivity, determined as described by Brady and Weil(1999),
CHAPTER FOUR
RESULTS AND DISCUSSION
The results of the physicochemical parameters monitored during the study period are presented in Table 1. There was a continuous decrease in the soil pH, electrical conductivity, and total nitrogen and available phosphorous in the crude oil-polluted soils when compared to values obtained from the unpolluted soil (0%) (Table 1). The values ranged from 5.5 – 6.9, 5.9 – 6.7 µS/cm-1, 0.12 – 0.26 % and 10.66 – 49.99mg/kg respectively, with increase in pollution levels from 0.5% – 20%. v/w Conversely, organic matter and total hydrocarbon contents increased with increase in pollution level. Values of the exchangeable cations and acidity of soil at different pollution levels were not consistent. The results also revealed observable effects of oil pollution on the levels of nitrogen and its related compounds (nitrate, nitrite and ammonium) in wetland soil (Table 2). Ammonium and nitrate levels were high in unpolluted soil but nitrogen and nitrite were however limited in polluted soil probably due to the decrease in porosity and aeration that may affect the oxidizing activities of nitrifying bacteria.
CHAPTER FIVE
Conclusion
The present results imply that oil pollution enhances the growth of denitrifying bacteria but inhibits or reduces the proliferation of nitrifiers, Nitrosomonas and Nitrobacter species in wetland soil. The later is apparently the reason for nitrogen deficiency in crude oil polluted soils. However, the growth of the denitrifying species such as Pseudomonas, Bacillus and Thiobacillus were less affected. The denitrifiers effectively grew and utilize crude oil as the sole source of carbon and energy (John, 2007). This physiological group of free-living nitrogen fixers with hydrocarbon utilizing potential is recommended for effective remediation of tropical wetlands contaminated with crude oil.
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