Crude oil is physically, chemically and biologically harmful to soil because it contains many toxic compounds in relatively high concentrations (Franco et al., 2004). Soil contamination can result in soil degradation, bring great loss to agricultural production and pose threat to human health (Lijuan, 2012). Crude oil spill is a common event in Nigeria especially in the Niger Delta areas of the country. Spills may arise from oil well drilling, production operations, transportation, refining, storage and marketing. It could also be from anthropogenic sources such as sabotage or accidental spills such as ruptured oil pipelines (Oberdorster and Cheek, 2000). Presently there are eleven oil corporations operating about 159 oil wells in Niger Delta region of Nigeria. Record indicates that the Niger Delta region of Nigeria experiences on the average, 273 oil spills resulting to about 115,000 barrels of crude oil annually from 1983-2005 making the region most vulnerable to oil spill than anywhere in the world (Agbogidi et al., 2009). Some recent oil spill in Nigeria include Assa-Rumekpe pipeline at Elele Alumini in 2014, Amukpe trunk line at Amukpe in 2011, Nkpoku Bomu pipeline at Ekporo in 2014, Nun-river Kolo creek at Oporoma in 2013 (SPDC, 2014). Oil spillage has immense impact on the ecosystem into which it is released. Contamination of soil arising from oil spills is one of the most limiting factors to soil fertility (Onwurah et al., 2007). It affects growth of plants (Nwadinigwe and Onwumere, 2003) and causes great negative impacts on food productivity (Onwurah et al., 2007). The work of Nwadinigwe and Uzodimma (2005) indicated the inhibitory effect of crude oil on the germination, growth and reproductive performance of Arachis hypogaea. Germination, growth and pod production of Glycine max have also been found to be inhibited by crude oil pollution (Nwadinigwe and Onwumere, 2003). Yellowing, dropping of leaves and complete shedding of leaves in areas of heavy pollution have been reported by Opeolu (2000). Environmental cleanup of crude oil can take several years (Short et al., 2002) and problems associated with the study and remediation of the polluted ecosystem can be very expensive. The legal problems related to compensation in terms of assigning monetary reward may bring serious controversy. Nigerian regulation of the oil industry is weak and rarely enforced, allowing in essence the industry to self-regulate. The ecosystem especially the agricultural sectors are very much prone to the adverse effects of oil pollution, since there has been no concerned and effective effort on the part of the government, let alone the oil operators, to control this environmental problem. Yet, there is still an extensive network of pipelines between the fields as well as numerous small networks of flow lines in the oil fields such as in the Niger Delta area, allowing more oil spill (Anderson, 2005).
1.1 RELEVANCE OF MICROORGANISMS IN THE SOIL
Microorganisms are important for soil quality and fertility. They play a major role in decomposition of organic matter, degradation of chemical pollutants and mineralization in the soil (Brussard, 1994). Amongst the different microorganisms inhabiting the soil, bacteria are the most abundant and predominant organisms (Hirsch, 1996). Bacteria and other microbes are chemical processors, tiny biotechnologists, capable of catalyzing thousands of chemical reactions that higher organisms are incapable of mediating. It has been reported that bacteria constitute the principal agents of hydrocarbon biodegradation (Jensen, 1975). It is well known that soil micro-flora and fauna can be manipulated indirectly through crop management practices. Similarly, cultivation of legumes leads to increased number of their compatible rhizobia (Hirsch, 1996).
1.2 DELONIX REGIA HOOK.
Delonix regia is a species of flowering plant in the class Magnoliophyta, order Fabales, family Fabaceae, sub-family Caesalpinioidae noted for its fern-like leaves and flamboyant display of flowers. It is known by different names such as Gulmohar, Flamboyant, Peacock, etc. (Francis and Liogier, 1991). D. regia is a tree of 10-18 m high. It has large smooth trunk which produces buttresses with aggressive surface roots. It has compound leaves and caesalpiniaceous flower. The fruit is green and flaccid when young, turning to dark brown, hard, woody pod when mature. There are many horizontally partitioned seed chambers containing hard oblong seeds. The tree is native of Madagascar and has been grown successfully in Burma, Jamaica, Nigeria, Borneo, South Africa, Egypt, Tanzania, Uganda etc. The species seems to tolerate many types of soils from clay to sandy soil. D. regia starts flowering in its 4th or 5th year. It is easily propagated from seeds. The seed is highly dormant which can be broken by hot water scarification. It is mainly valued as a decorative tree. The wood is used as fuel and to make wood pulp and furniture. The flowers produce bee forage. The seeds contain gum that may be used in textile and food industries (Don, 2005). The qualitative analysis with the extracts from the root bark of D. regia showed the presence of phytochemical constituents such as tannins, terpenoids, glycosides, alkaloids, carbohydrates, sterols which can be used as potential sources of herbal medicine (Kavith et al., 2012). It has been reported that the plant can grow in diesel oil-polluted soil with the help of poultry manure amendments. It has also been used for phytoremediation of lead and cadmium (Kamel et al., 2012).
1.3 OBJECTIVES OF THIS RESEARCH
The objectives of this research were:
- To ascertain the effect of crude oil contamination on the bacterial load of rhizosphere of D. regia.
- To ascertain the effect of crude oil contamination on mineral constituents of rhizosphere of D. regia. .
- To ascertain the impact of of Delonix regia on the bacterial load and mineral constituents of crude oil contaminated soil.
- To identify some of the bacteria associated with the crude oil contaminated rhizosphere of D. regia.
- To determine the effects of crude oil contamination on some vegetative parameters of D. regia.
This work was aimed at providing information for the improvement of bacterial activity and mineral constituents of polluted soils which will be useful in managing in-situ bioremediation in order to prevent ecological damage and promote soil fertility and plant growth in oil producing areas.