Effects of waste engine oil contamination on nodulation response in Vigna Unguiculata (cowpea)

Abstract

This study investigated the effects of spent engine oil contamination on the germination, growth, and nodulation of cowpea (Vigna unguiculata). A randomized experimental design was adopted using three soil treatments: Treatment A (control, 0 ml oil), Treatment B (moderate contamination, 2 ml oil), and Treatment C (severe contamination, 5 ml oil). The results showed that spent engine oil had little effect on seed germination, as all treatments recorded a similar average germination rate of about 55%. However, significant
differences emerged during the growth stage. After four weeks, plants grown in uncontaminated soil exhibited healthy growth, vigorous development, and abundant root nodules. In contrast, plants exposed to spent engine oil showed reduced growth and fewer nodules, with the most severe effects observed in Treatment C. Nodulation declined sharply as contamination levels increased, indicating that spent engine oil interfered with the symbiotic relationship between cowpea roots and nitrogen-fixing Rhizobium bacteria. The contaminated plants also displayed visible symptoms of stress, particularly yellowing of leaves (chlorosis), which was most pronounced in the severely polluted soil. This condition suggests nitrogen deficiency resulting from poor nodulation and reduced nutrient uptake. The findings indicate that while spent engine oil may not prevent seed germination, it significantly affects plant growth, nitrogen fixation, and overall plant health. The study highlights the negative consequences of petroleum waste on agricultural soils and emphasizes the importance of remediation measures in oil-contaminated environments.