Operational threshold and associated mechanisms in composting remediation of diesel-contaminated soils

Diesel hydrocarbons persist in soils and pose long-term ecological risks, yet the effectiveness of composting remediation at high contamination levels remains poorly understood. This study evaluated composting performance across a diesel contamination gradient under CPC-amended conditions. An operational transition range was identified at around 10-15% contamination, beyond which remediation performance declined markedly. Below this range, hydrocarbon concentrations decreased by more than 91.7% within 30 days and phytotoxicity was effectively eliminated, whereas contamination levels ≥15% led to substantially reduced remediation performance and persistent toxicity. Community differentiation was most strongly associated with the C₁₀-C₁₅ fraction, while electron-accepting capacity, dehydrogenase activity, and urease activity were identified as informative functional indicators distinguishing low- and high-contamination systems. Statistical analyses further suggested that shifts in electron transfer-related properties and enzyme activities were linked to the removal of different hydrocarbon fractions. These findings provide mechanistic insights into the threshold-dependent performance of CPC-amended composting systems and offer a basis for optimizing bioremediation strategies under varying petroleum contamination levels.