Ammonium Oxidizing Bacteria Activity and Nitrification Rate in Oil Contaminated Wetland Soil under Remediation with Nutrient Supplements and Leguminous Plants

Abstract

Previous investigation on the impact of crude oil on the growth of tropical legumes and its effect on nitrogen dynamics in wetland ultisol showed that oil contamination reduced N uptake by plants but increased N accumulation in soil microbial biomass. Moreover, the presence of hydrocarbons widened the C/N ratio in soil and led to more available N being immobilized by soil microorganisms. The present study was carried out to evaluate the activity of ammonium oxidizing bacteria (AOB) and their nitrification potential rate (NPR) in wetland soil under a remediation course. Mineralization studies showed that ammonium-N levels decreased while nitrate-N increased progressively in the uncontaminated soil (control) cultivated with leguminous plants (cover crops) during the 12 weeks remediation period. However, the remediated soils were affected in different ways. The experimented soil cultivated with <i>Centrosema pubescens</i> had higher mineral nitrogen (NH₄-N, NO₃-N, NO₂-N, Total N and P) than soil cultivated with <i>Calopogonium mucunoides</i> and <i>Pueraria phaseoloides</i>. AOB counts recorded were in the ranged, 2.25 × 10² - 2.66 × 10⁵, 2.31 × 10² - 2.11 × 10⁴ and 4.25 × 10² - 2.98 × 104 respectively. The highest NPR was found in uncontaminated soil (11.68 - 60.92 nmol N/g dry weight soil (DWS)) followed by soil treated with poultry manure (9.65 - 24.86 nmol N/g DWS/h), NPK (7.88 - 39.45 nmol N/g DWS/h) and in the oil-contaminated soil (0.11 - 1.87 nmol N/g DWS/h). The relations between NH4-N concentration and NPR in soil cultivated with <i>Centrosema</i> (r = 0.852), <i>Calopogonium</i> (r = 0.745) and <i>Pueraria</i> (r = 0.722) were positively significant at 95% confidence limit. Similarly the relations between AOB density and NPR for <i>Centrosema</i> (r = 0.654; P = 0.05), <i>Calopogonium</i> (r = 0.588; P = 0.05) and <i>Pueraria</i> (r = 0.518; P = 0.05) were significant. The findings imply that nitrification potential of crude oil- contaminated soil differs significantly with the nutrient amendment/treatment technique adopted for remediation. Our research has shown that treatment of uncontaminated soil with cover crops increased AOB and nitrification rate. More so, contaminated soil treated with poultry manure and NPK-fertilizer, cultivated with covers crops resulted in remarkable reduction in hydrocarbons content and increased population of nitrifiers and nitrification potential rates of wetland soil over time. However, contaminated soil treated with poultry manure and cultivated with <i>Centrosema pubescens</i> is more effective in bioremediation of crude oil-contaminated soil.