BIOCHEMICAL STUDIES ON SOIL HEALTH STATUS UNDER CRUDE OIL POLLUTION AND BIOREMEDIATION USING SELECTED CROPS AND STRESS INDICATORS

Abstract

Pollution of agricultural soil by crude oil is a wide spread phenomenon in petroleum producing countries worldwide. Therefore, there is need to study effects of crude oil on soil health and find effective and eco-friendly means of alleviating the toxic effects of crude oil on soil health and plant growth. This research investigated toxicity of different concentrations (0-2.5% per kg of soil) of crude oil on soil health and restoration of soil health under treatments with different concentrations of inorganic fertilizer (NPK: 0.05%, 0.075% and 0.1% per 5 kg of soil); organic fertilizer (cow dung: 4%, 8% and 12% per 5 kg of soil) and consortium of previously identified hydrocarbon degrading actinobacteria (10%). Soil enzyme activities and physicochemical parameters were determined using standard assay and evaluation procedures. Microbial distribution was also determined and analysed. Growth parameters, proximate composition and oxidative stress indicators of three indigenous commercial food crops: Maize (Zea mays L.), Soybean (Glycine max L.) and Amaranth (Amaranthus cruentus L.) grown on the polluted and bioremediated soils were also evaluated in comparison with controls. Results indicated that activities of all studied soil enzymes were inhibited by crude oil at week 2. Under bioremediation treatments of polluted soil, activities of the enzymes were significantly enhanced at week 2. Activity increase of 243%, 325% and 46% for dehydrogenase; 155%, 256% and 61% for α-amylase; 51%, 62% and 38% for β-glucosidase; 71%, 62% and 77% acid phosphatase; 68%, 64%, 65% for arylsulphatase, over polluted soil (control) was recorded during the growth of maize, soybean and amaranth, respectively, under bioremediation treatment with 4% cow dung. Some chemical properties (pH, available N, P, S and cation exchange capacity) of the polluted soil were adversely affected. Significant (p<0.05) improvements were recorded under bioremediation treatments of the polluted soil by week 6 of this study in comparison with polluted soil without bioremediation (control). Crude oil pollution increased bulk density with about 20% resulting in decline in porosity with 2.5% crude oil pollution. There were slight changes in bulk density and porosity of polluted soil under bioremediation treatments with inorganic and organic fertilizers, and consortium of bacteria. Microbial population of the polluted soil increased significantly (p<0.05) with increase in crude oil concentration at week 6 with 79%, 99%, 145% in comparison with soil polluted with 2.5% crude oil. Remarkably, treatment with 4% organic fertilizer (cow dung) resulted in an increase of 32%, 70% and 75% in microbial population for soil planted with maize, soybean and amaranth, respectively over microbial population of polluted soil. Improvement in height was recorded for the three plants in soils bioremediated with inorganic and organic fertilizers in comparison with plants grown on polluted soil without bioremediation. Reductions in levels of Superoxide dismutase, Catalase, Peroxidase activities and Malondialdehyde from stem of the three plants grown on polluted soil with bioremediation treatments were recorded compared with control. GC-MS analysis revealed more significant level of degradation in soil bioremediated with inorganic fertilizer. Research findings established the toxicity of crude oil to soil health and plants under study and efficacy of bioremediation treatments in alleviating toxicity of crude oil pollution, with NPK and cow dung as better bioremediating agents. The study provides deeper insights into the bioremediation process and effects on soil health and plant growth.