Spatial Dispersion of Spent Engine Oil in Soil and Gradient-Dependent Effects on Growth and Physiological Responses of Spinach (Spinacia oleracea L.)

Abstract

Spent engine oil (SEO) is a persistent soil contaminant in many developing regions, yet its long-term spatial dispersion and indirect effects on surrounding vegetation remain poorly understood. This study investigated the spatial dispersion of SEO from a generator maintenance facility at Admiralty University of Nigeria and evaluated gradient-dependent effects on soil properties and the growth and physiological responses of spinach (Spinacia oleracea L.). Soil sampling followed a directional composite approach to capture spatial variability. At the disposal point (Site A), soil subsamples were collected from composite sample. The same procedure was applied at 50 m (Site B), 100 m (Site C), and 150 m (Site D) from the disposal point, while an uncontaminated composite soil (Site E) served as the control. Soil physicochemical properties and heavy metal concentrations were analyzed prior to transplanting and plant growth responses were monitored for 15 days after transplanting (DAT). Result showed that soil pH was strongly acidic (5.93) at the disposal point (Site A) and gradually increased with distance, while electrical conductivity was significantly (p = 0.02) higher at site A (80.8 µS/cm) and gradually reduced with distance. Concentrations of Zn, Pb, Cu, and Ca were highest at site A and declined progressively along the gradient (A > B > C > D > E). Spinach growth exhibited clear gradient-dependent inhibition, with 100% plant mortality at Site A within 7 DAT and reduction in leaf number (67%) and stem length (79.3%) between the (A-D) soils and the control. The findings demonstrate that chronic SEO disposal promotes spatial dispersion in soil, broadening phytotoxicity effects beyond the point of discharge and posing significant risks to soil fertility and crop productivity.