Organic Amendment–Driven Microbial Adaptation and Environmental Controls on Co-Metabolic PAH Degradation in Soil

Soil bioremediation offers a sustainable approach for reducing polycyclic aromatic hydrocarbons (PAHs), yet the effects of raw leaf litter and environmental conditions on co-metabolic degradation remain insufficiently characterized. This study evaluated the influence of mixed pine (Pinus sylvestris) and poplar (Populus nigra) leaf amendments on the degradation of phenanthrene (Phe) and pyrene (Pyr) and on microbial functional responses in silt-loam soil. Soil classified as a Spolic Technosol was spiked with 150 mg/kg Phe/Pyr and amended with 10, 30, and 70% leaf litter (w/w). PAH concentrations were monitored over 56 days, and decay constants (k) and half-lives (t1/2) were estimated using a one-phase exponential decay model. Soil pH, moisture, and temperature were recorded throughout incubation. Microbial functioning was examined using substrate-induced respiration (SIR) and nitrification assays. After incubation, microbial consortia were cultured in minimal salts medium (MSM) containing Phe/Pyr as the sole carbon source, and growth kinetics were determined from OD600. The soil exhibited favourable physicochemical properties, though a high C : N ratio indicated nitrogen limitation. The 10% amendment showed the highest overall removal (95.90% ± 4.13%), with k = 0.054 and t1/2 = 12.87 days. Higher amendment levels accelerated early losses but slowed later degradation. Variation in the decay constant (k) reflected treatment-level differences in soil moisture, temperature, and pH. SIR showed reduced biomass under PAH contamination, and nitrification remained low. PAH-adapted consortia from amended soils grew in PAH-only MSM, with the 10% inoculum reaching OD600 = 0.073 ± 0.004 and µ = 0.639 ± 0.127 d–1. Moderate leaf amendment (10%) optimally enhanced microbial activity, supported co-metabolic PAH degradation, and promoted PAH-adapted microbial consortia, indicating the suitability of raw leaf litter as a low-cost amendment for PAH-contaminated soils.