Kinetics and Microbial Population Dynamics of Leaf-Biostimulated Crude Oil Bioremediation in Contrasting Soils

Crude oil contamination remains a persistent environmental burden in petroleum-producing regions, and low-cost, locally sourced plant-leaf biostimulants offer a promising route to accelerate hydrocarbon-degrading microbial activity in impacted soils. This study evaluated the bacterial and fungal population dynamics accompanying biostimulated bioremediation of crude-oil-contaminated sandy, loamy, and clay soils amended with room-dried or sun-dried Dacryodes edulis, Canarium schweinfurthii, and Persea americana leaves at 50 g or 100 g doses, monitored over 42 days across 36 treatment bioreactors and three unamended controls. First- and second-order kinetic models were fitted to total petroleum hydrocarbon (TPH) depletion data, and total heterotrophic bacterial (THB) and fungal (THF) counts were enumerated in parallel. First-order kinetics best described the majority of treatments (R 2 generally > 0.98), with rate constants of 0.035–0.067 day⁻¹ and half-lives of 10.3–19.8 days. Soil type was the dominant determinant of TPH removal (ANOVA, F = 612.5, p < 0.001), with mean removal following sandy (93.2%) > loamy (88.6%) > clay (75.5%) soil, while biostimulant species, drying method, and dose showed no statistically significant effect. All biostimulated treatments substantially outperformed unamended controls (2.4- to 2.7-fold higher removal). THB counts peaked around Day 28 before declining, and this bacterial fold-increase correlated significantly with TPH removal (r = 0.49, p = 0.0026), whereas fungal population growth did not (r = 0.01, p = 0.96). These findings indicate that soil texture, more than biostimulant identity, governs degradation outcome, and that bacterial rather than fungal proliferation drives hydrocarbon removal under these leaf-based biostimulation regimes.