ECOTOXICOLOGICAL ASSESSMENT OF SOILS FROM AUTOMOBILE MECHANIC WORKSHOP (AMW) DUMPSITES IN LOKOJA USING PLANT SEED GERMINATION AND MICROBIAL INDICATORS

Abstract

Soils from automobile mechanic workshop (AMW) effluent dumpsites in Lokoja, Nigeria, are major sources of environmental contamination, yet their ecological impacts remain understudied. This study aimed to assess the effects of hydrocarbon and heavy metal pollution on soil health using plant seed germination and microbial indicators. Surface soil samples were collected from ten workshop sites (S1–S10) and a clean control site. Physicochemical properties, oil and grease and heavy metal concentrations (Pb, Cd, Cr, Ni) were determined using standard laboratory methods. Phytotoxicity was evaluated through germination and early growth of Vigna unguiculata (cowpea), while microbial ecotoxicity was assessed by enumerating total heterotrophic bacteria, fungi and hydrocarbon-utilizing bacteria. Data were analyzed using mean ± SD and Pearson’s correlation examined relationships between contaminants and biological endpoints. Results showed extreme contamination in all workshop soils, with oil and grease ranging from 4,400 ± 290 mg/kg (S9) to 12,100 ± 650 mg/kg (S8) and lead from 250 ± 18 mg/kg (S9) to 680 ± 45 mg/kg (S8), all exceeding national limits. Germination dropped from 97.5% in the control to 35.0% at S8, while root length decreased from 12.5 cm to 1.5 cm. Total heterotrophic bacteria and fungi declined from 7.54 and 5.40 Log CFU/g in the control to 5.30 and 4.20 Log CFU/g at S8, respectively, while hydrocarbon-utilizing bacteria increased to 5.30 Log CFU/g. Strong negative correlations (r = –0.91 to –0.97) were observed between contaminants and plant/microbial health. These findings suggest that mechanic workshop soils in Lokoja pose significant ecological risks and thus, necessary remediation and proper waste management to restore soil health and protect plant and microbial communities are essential.