Dynamics of some bioremediation techniques indices in calcareous oil-contaminated soil

Oil contamination from private refineries in the Kwashe industrial area, Iraqi Kurdistan, poses a severe threat to soil health. This study evaluated the efficacy of different bioremediation techniques—biostimulation via aeration/watering, and bio-composting using sheep manure, chicken manure, and compost—on soil contaminated with crude oil and its derivatives. The dynamics of key soil health parameters (Nitrogen, Organic Carbon, Organic Matter, and C/N ratio) were monitored over 60 days and compared to intrinsic (natural) bioremediation and a non-polluted control. Results showed that bio-composting significantly enhanced bioremediation efficiency compared to other methods. The compost amendment was most effective in conserving nitrogen (0.801–0.301%) and maintaining stable organic carbon (6.93–4.22%) and C/N ratio (13.41–22.60), indicating a balanced and sustained microbial degradation process. Chicken manure initially provided high nitrogen and organic matter but led to rapid nutrient loss and a high C/N ratio by day 60, suggesting less stable decomposition. Biostimulation (aeration/watering) and intrinsic bioremediation were the least effective, with the latter showing a critically high C/N ratio (> 100), indicating severe nitrogen limitation. The study concludes that bio-composting, particularly with compost, is the most effective strategy for rehabilitating oil-contaminated soils in the region, as it optimizes nutrient cycling, enhances microbial activity, and leads to a more stable and fertile soil condition.