Contamination of soils and sediments with aliphatic hydrocarbons and polycyclic aromatic hydrocarbons(PAHs) is major concern because of their persistence and toxicity. Most of these hydrophobic organic contaminants are carcinogenic and exposure to these contaminants has been implicated in the aetiology of diseases, such as cancer. Although remediation of hydrocarbon contamination based on physical and chemical methods has been developed, these strategies are often expensive and technically challenging. Given the nature of petroleum hydrocarbon contamination of terrestrial environment and the importance of bioremediation strategies, understanding the fate and behaviour of both aliphatic and aromatic hydrocarbons is imperative, particularly microbe-contaminant interactions in soil. This thesis investigates a number of biostimulation strategies for enhance biodegradation of petroleum hydrocarbon by indigenous soils microflora; explore some of the issues surrounding the understanding of microbe-contaminant interactions in soil; and the effects of root-secreted chemicals on the development of PAHs mineralisation in soil. The 14C- radiolabeled techniques combined with standard microbiological methods were adopted for most of the experiments. Simple flask-based 14C-respirometer systems were used to assess mineralisation of 14C-labelled substrates under defined conditions. Some of the research findings showed that: (i) the indigenous microflora in the drill cuttings contaminated soil have the potential for biodegradation of petroleum hydrocarbons; (ii) soil amendment of drill cuttings in the ratios of 1: 1 and 1:2 drill cutting:soil, but less than 1:5 drill cutting:soil ratio enhances biodegradation of organic contaminants; (iii) organic manure amendments enhanced biodegradation of petroleum hydrocarbons, and cow manure (CM) and sheep manure (SM) amended treatments exhibited higher levels of biodegradation than poultry manure (PM) amended treatment (CM ≥ SM > PM); (iv) amendment with rhizosphere soil or root tissues of reed canary grass (Phalaris arundipacea), channel grass (Vallisneria spiralis), blackberry (Rubus fructicosus) and goat willow (Salix caprea) significantly (P < 0.001) enhanced 14C_ phenanthrene and 14C- naphthalene mineralisation, but did not stimulate mineralisation of 14C- hexadecane or 14C-octacosane in freshly contaminated soils; (v) there is evidence that amendment with plant- secreted chemicals, e.g. flavonoids (flavone and 3-hydroxyflavone) and hydroxycinnamic acids, at concentrations of ≤ 100 )lg kg-1 promote microbial activity and growth-linked mineralisation of P AHs in contaminated soils; and (vi) there is a possibility of using sterile soils as "the worst case scenario" in extrapolating bioavailability and ecotoxicity of organic contaminants. Biodegradation of petroleum hydrocarbons in soils is controlled by hydrocarbon physicochemistry, environmental conditions, nutrient availability, bioavailability, the presence of catabolically active microbes and exudate- microbe interactions in soil. The findings from this research can be further exploited to further the development of microbial strategies for bioremediation of petroleum hydrocarbon contaminated soils. Keywords: Biodegradation; Aliphatic hydrocarbons; P AHs; Drill cuttings; Soil; Rhizosphere; Flavonoids; Hydroxycinnamic acids