The scale of the problem in Kwashe is striking when placed in global context. Nigeria’s Niger Delta, one of the world’s most notorious oil-pollution hotspots, accumulated roughly 10 million tons of spilled crude over five decades, while China reports 1.5 to 2 million tons of petroleum-related soil contamination annually. A single industrial zone in Kurdistan releasing up to 20,000 tons per year represents an extraordinary local burden. Once oil enters soil, it follows three main pathways: it adsorbs onto organic matter and mineral surfaces, it dissolves only minimally into soil water because hydrocarbons are hydrophobic, and its lighter fractions volatilize into the air. In Kwashe, surface spills from refineries and unlined waste pits are the primary entry route, followed by downward migration through the vadose zone and lateral spreading via rain-driven runoff.

Oil contamination fundamentally rewires soil chemistry and biology. It reduces water infiltration, creates water repellency, lowers pH, strips out available nitrogen and phosphorus, and reshapes microbial communities toward hydrocarbon-degrading specialists. Conventional cleanup options such as incineration, landfilling, thermal desorption and chemical oxidation cost between US$200 and $1,500 per ton, consume enormous energy, generate secondary pollutants like dioxins and nitrogen oxides, and destroy the soil’s structure and living biota. For vast, resource-limited contamination zones like Kwashe, biological approaches that harness soil microbes to digest hydrocarbons are far more attractive, provided the microbes receive the nutrients they need to keep working.

The research team hauled two to three tons of polluted clay-loam soil, sampled from the top 30 centimeters at ten random points across a 500-square-meter contaminated area near the refineries, to the College of Agricultural Engineering Sciences in Duhok. There they built six experimental heaps, each one meter wide and 1.5 meters high, representing five treatments plus a non-polluted control soil collected from an uncontaminated agricultural field five kilometers upwind. The treatments were intrinsic bioremediation, in which polluted soil was simply left undisturbed; biostimulation through aeration and watering; and three bio-composting variants in which the soil was amended with sheep manure, chicken manure or mature compost. All treated heaps were moistened to 80 percent of water-holding capacity, covered with nylon sheeting to trap heat and odor, and turned at two-day intervals through the thermophilic phase, which began around day eight when temperatures climbed to 55 to 65 degrees Celsius, the window in which rapid hydrocarbon breakdown occurs.

Over 60 days, the researchers tracked four key soil health indices: total nitrogen, total organic carbon, total organic matter and the carbon-to-nitrogen ratio. The nitrogen story was dramatic. Chicken manure started with the highest nitrogen at 0.822 percent, followed closely by compost at 0.801 percent and sheep manure at 0.680 percent, while the unamended treatments languished below 0.07 percent. Yet by day 60, chicken manure had lost a staggering 76.5 percent of its nitrogen, sheep manure 62.4 percent, and compost only 37.6 percent, retaining a final 0.301 percent. The authors attribute the chicken manure collapse to its high uric acid and protein content, which microbes mineralize rapidly, followed by ammonia volatilization during the hot thermophilic phase. Compost’s lignin-rich, less labile nitrogen fraction releases slowly and resists volatilization, which is precisely what sustained bioremediation requires.

Organic carbon told a parallel tale. The compost treatment maintained the highest total organic carbon at every time point, declining just 39.2 percent from 6.931 to 4.216 percent, and its time-point differences were not all statistically significant, indicating genuine carbon stabilization. Sheep manure lost 36.7 percent, while chicken manure, which began with far less carbon, shed 54.6 percent of it. Biostimulation fared worst, losing 60.7 percent of its organic carbon, because aeration without amendments simply accelerates the oxidation of whatever carbon exists without replenishing the pool. Total organic matter followed the same pattern, with all three bio-composting treatments retaining 11 to 13.7 percent at day 60, versus a catastrophic 60.7 percent loss under intrinsic bioremediation. Interestingly, chicken manure retained the most absolute organic matter despite losing carbon fastest, because loss-on-ignition counts nitrogen, sulfur and microbial biomass alongside carbon, and its nitrogen-rich composition inflated that measure.

The carbon-to-nitrogen ratio proved to be the decisive indicator. Microbial hydrocarbon degradation works best between roughly 15:1 and 30:1, and only the compost treatment stayed within or near that optimal band throughout, drifting from 13.41 to 22.60 over 60 days. Sheep manure crossed into moderate nitrogen limitation by day 60 at 38.31, and chicken manure, despite its nitrogen-rich start, rocketed to 59.75, a paradoxical 293 percent increase driven by rapid carbon mineralization and nitrogen loss. Biostimulation peaked at 68.21 before easing to 34.56, but intrinsic bioremediation was the disaster the authors had hypothesized: its C/N ratio soared past 100, from 45.44 to 100.61, signaling extreme nitrogen starvation that effectively stalls microbial metabolism, because carbon-rich, nitrogen-poor conditions prevent the enzyme synthesis on which hydrocarbon digestion depends.

What makes these nutrient dynamics matter is how tightly they track actual oil removal. Supplementary total petroleum hydrocarbon data revealed a strong negative correlation between final C/N ratio and TPH reduction, with a correlation coefficient of minus 0.89. Compost achieved the highest TPH reduction at 75.5 percent, sheep manure a strong 67.9 percent, biostimulation a modest 31 percent, and intrinsic bioremediation a dismal 22.4 percent. A separate correlation of 0.82 between nitrogen retention and TPH reduction cements the conclusion that sustained nitrogen availability is the primary engine of hydrocarbon degradation in these calcareous soils. The authors caution that they did not directly enumerate microbes, so references to hydrocarbon-degrading genera such as Pseudomonas, Bacillus and Mycobacterium rest on established literature rather than direct measurement, and the 60-day window captures only the active composting phase rather than long-term recovery.

The practical implications for polluted regions are immediate. Chicken manure, often favored for its cheap, concentrated nutrients, emerges from this study as a short-term booster rather than a durable solution, better suited to an initial nutrient pulse than to lasting soil restoration. Sheep manure offers a respectable middle path with its balanced 21:1 starting ratio and fibrous structure. But mature compost, with its recalcitrant lignin, humic substances and slow-release nutrients, is the amendment the researchers recommend for large-scale deployment in Kurdistan and comparable calcareous environments. It optimizes nutrient cycling, keeps microbial degradation running within the optimal C/N window, and leaves the soil more stable and fertile than any alternative tested. As oil pollution continues to accumulate wherever refining outpaces regulation, this modest heap experiment in Duhok delivers a message with global resonance: the difference between a thriving microbial cleanup crew and a starved, stalled one can come down to the slow, steady chemistry of well-made compost.

Subject of Research: Bioremediation of crude-oil-contaminated calcareous soil using organic amendments

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

Article References: Hasan, S. Q., Umer, M. I., & Hussien, R. O. (2026). Dynamics of some bioremediation techniques indices in calcareous oil-contaminated soil. Discover Soil, 3(1), Article 119. https://doi.org/10.1007/s44378-026-00263-4

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00263-4

Keywords: bioremediation, oil-contaminated soil, calcareous soil, bio-composting, compost, chicken manure, sheep manure, C/N ratio, total nitrogen, organic carbon, total petroleum hydrocarbons, Iraqi Kurdistan