The influence of organic amendments – poultry manure (PM), human sludge (HS), urea (UR) and metal toxicants (Cu, Pb, Zn) on total petroleum hydrocarbon (TPH) degradation in soil artificially contaminated with diesel was assessed in microcosms. A parent soil (sandy loam) was characterized and used for the rest of the study. TPH degradation in soil was monitored spectro photometrically (UV-visible) under the effect of diesel load, amendment dose and metal dose. TPH degradation rates decreased with increase in diesel load in soil treated with PM and HS, but remained somewhat constant under natural attenuation and UR amendment. Pseudo-firstorder rate constant, k1 (day-1) and half-life, τ½ (day) for TPH degradation at the lowest diesel load were: natural attenuation (k1 = 3.0 x 10-3, τ½ = 231.0); PM (k1 = 2.6 x 10-2, τ½ = 26.7); HS (k1 = 1.9 x10-2, τ½ = 36.4); UR (k1 = 1.7 x10-2, τ½ = 40.8). TPH degradation efficiencies (%) increased with amendment dose in the order: UR (35 – 61) < HS (39 – 78) < PM (44 – 88) with plateaux observed beyond 30 %w/w. TPH degradation was reduced in the presence of metals in the sequence Zn > Pb > Cu, implying the vitiation of biodegradation process by the cocontaminating metals. These findings may become useful whenever bioremediation is the option for cleanup of diesel-contaminated soils.

Keywords : Biodegradation, diesel, total petroleum hydrocarbon, natural attenuation, poultry manure, human sludge.