Microbial population dynamics and mineralization rates in hydrocarbon contaminated Ultisol amended with organic wastes

Microbiological analysis and analytical techniques were used to determine the population dynamics and mineralization rates of a simulated hydrocarbon-contaminated garden ultisol treated with the bacterized organic wastes. The In-Vessel System of composting (IVC) was adopted to treat organic feedstocks (Sewage Sludge and Brewer Spent Grains; BSG), while derived amendments (biosolid and BSG) were seeded with an hydrocarbonoclastic strain of Bacillus subtilis before land application. A total of 22 species of microorganisms were isolated from the garden Ultisol at the beginning of this study. A 2log reduction (from 105 to 103 ) in abundance was observed for heterotrophic bacteria, while diazotrophs and phosphate solubilizers were also adversely affected after hydrocarbon treatment. Species richness was significantly reduced from 22 to 6, representing a reduction rate of 72.7%. As a consequence of amendment, microbial abundance significantly (p<0.05) increased (from 103 to 106 ) across microbial groups, while a boom in species richness (from 6 to 17 microbial species) was also observed. Mineralization rate constants were remarkably high in the garden soil amended with organic wastes with rates of 0.007 and 0.005 respectively when compared to contaminated but amended beds. This implies that the bacterized amendments generally improved the natural attenuating potential of the hydrocarbon contaminated soil. Organic wastes can therefore serve as nutrient rich bio-stimulants that can improve the nutritional status of hydrocarbon contaminated soil and also enhance biological activities vital in nutrient transformation and recycling in soil. The organic wastes used in this study successfully ameliorated the negative effects of hydrocarbon on soil microbial communities and dynamics. It may, therefore, be appropriate to recommend their use as a suitable biostimulant during the remediation of agricultural land previously contaminated with hydrocarbon.