Enzyme–based Assay for Toxicological Evaluation of Soil Ecosystem Polluted with Spent Engine Oil

Aim: In this study, the experiment was designed to investigate the effect of contamination of soil ecosystem with spent engine oil at various concentrations.

Design: Soil samples were obtained from zoological garden University of Nigeria Nsukka while spent engine oil was obtained from the Mechanic Village, Nsukka. Test tubes labelled 1- 7 containing various percentages of spent engine oil 1.0, 1.5, 2.0, 2.5, 3.0, 3.5% w/w (oil-soil mixture); and into the 7th tube, the control contained only the soil sample. The study was designed for thirty-five-days (0, 14, 28 and 35 day) at various degrees of pollution by spent oil.

Results: The result showed that spent engine oil stimulated the activity of soil dehydrogenase in a concentration and time dependent manner: from (4.72 ± 0.015) mol/min at 1.0% contamination to (9.30 ± 0.021) mol/min at 3.5% contamination on day-zero; and from (5.29 ± 0.032) mol/min at 1.0% contamination to (9.78 ± 0.040) mol/min at 3.5% contamination on day-28; the activity of soil catalase was inhibited from (0.195 ± 0.005) mol/min at 1.0% contamination to (0.054 ± 0.004) mol/min at 3.5% contamination on day-zero; and from (0.18 ± 0.004) mol/min at 1.0% contamination to (0.042 ± 0.002) mol/min at 3.5% contamination on day-28. The moisture content increased from (6.4 ± 0.01) at 1.0% contamination to (24.24 ± 0.0) at 3.5% contamination on day-zero; and from (4.56 ± 0.056) at 1.0% contamination to (19.31 ± 0.0212) at 3.5% contamination on day-28. Similarly, there was an increase in total petroleum hydrocarbon (TPH) from (0.03 ± 0.0) to (0.86 ± 0.0) that cuts across days-zero to -28 at concentrations (1.0-3.5%) contamination. At increased concentrations (3.5% w/w) of contamination, hydrocarbons increased the abundance of hydrocarbon-degrading microorganisms from 3.26 x107 ± 0.02 cfu on day-zero to 6.55 x 10­­8 ± 0.04 cfu on day-28; but on the other hand, induced a limitation on microbial diversity.

Conclusion: The concentration of the hydrocarbonclastic bacteria in the spent engine oil-contaminated soil correlated with the enzyme induction activity. These effects which altered the entire soil biochemistry could disrupt ecosystem dynamics by slowing soil organic matter mineralization and associated nutrient re-mineralization.