Response of Selected Soil Enzyme Activities to Pulverized Costus afer Stem and Cow Dung Augmentation in Crude Oil–contaminated Soil

Abstract

Crude oil contamination can alter soil biochemical functioning, and enzyme activities provide indicators of treatment-related microbial responses. This study investigated the combined effects of pulverised Costus afer stem and cow dung amendments on selected enzyme activities in crude oil-contaminated soil. Non-polluted soil samples were obtained from farmland at Mini Orlu Road by Ada George, Port Harcourt, Rivers State, Nigeria, and the soil was divided into nine (9) experimental groups of five kilograms (5 kg) each, placed in polypropylene bags (20 cm in diameter × 30 cm high). Group 1 served as the control (unpolluted group), while Groups 2 to 9 were contaminated with 1000 mL of crude oil each. Fresh whole stems of Costus afer were pulverised using an electric blender, measured, and inserted into the polluted soil samples, ensuring adequate depth for rooting. The polluted soil was mixed thoroughly with cow dung to introduce nutrients and hydrocarbon-degrading microbes, as described in the table containing the experimental design. Each bag was subsequently tied with masking tape. All bags were incubated at 28 ± 2°C for 60 days prior to laboratory analysis. The findings from the analysis of enzyme activities clearly show that laccase activity was significantly higher in treated soil samples than in the control. Oxygenase levels were generally low across the various groups, with Group 7 (Polluted soil (5 kg) + Costus afer (200 g)) showing the highest activity. Peroxidase activity fluctuated, with Group 3 (Polluted soil (5 kg) + Triton X-100) showing the highest activity compared with the control. The increase in treated soil samples indicates an enhanced oxidative capacity of the soil microbiome stimulated by the amendments.