Phytoremediation Potential of Acacia polyacantha in Soil Contaminated with Crude-oil Petroleum Hydrocarbons: A Controlled Pot Study

Abstract

Petroleum hydrocarbons can constrain plant establishment and alter soil biological processes, but tolerant woody legumes may support gradual contaminant dissipation through rhizosphere-associated mechanisms. This controlled pot study evaluated the growth response of Acacia polyacantha seedlings in clay soil amended with 0, 0.5, 1.0, 1.5, or 2.0% crude oil. Seedlings were maintained in a completely randomised design, with three independent pots destructively sampled at 60, 120, and 180 days for each concentration. Shoot length, root length, shoot fresh weight, and root fresh weight were measured. Extractable petroleum residue was determined by dichloromethane Soxhlet extraction, and selected hydrocarbon profiles were screened by gas chromatography-mass spectrometry. Culturable rhizospheric bacteria were enumerated on Starch Ammonium Agar. Seedlings survived all tested oil concentrations. Growth responses were non-linear across concentration and time: shoot length reached 37.63 cm in the 0.5% treatment at 120 days, whereas root length at 180 days was greatest in the 1.5% treatment (9.50 cm). Biomass varied markedly among harvests, with the highest shoot and root fresh weights recorded in the 1.0% treatment at 120 days. After 180 days, hydrocarbon reduction reached 76% in rhizosphere-associated soil compared with 49% in the comparison treatment. Qualitative chromatographic screening showed temporal changes in the number of detected compounds and peak profiles, with pentadecane frequently appearing as the first reported alkane peak. Heneicosane, tetracosane, and eicosane were detected in root extracts after 180 days. Bacterial abundance differed across sampling times (p = .001) but not among oil concentrations (p = .22). The results indicate that A. polyacantha can remain established under moderate crude-oil stress and is associated with substantial hydrocarbon dissipation. Because the analytical design did not fully resolve plant-mediated degradation from background losses, the findings support further controlled and field-scale evaluation rather than a definitive claim of remediation efficacy.