Enhanced PAH Degradation in Freeze–Thaw Farmland Soil Using Composite Biochar-Immobilized Cold-Tolerant Microbial Consortium

This study focused on slightly to moderately PAH-contaminated farmland soils in freeze–thaw regions of Northeast China, aiming to fill the research gap in the in situ remediation mechanisms of PAHs under natural freeze–thaw conditions. A 12-month in situ experiment was conducted with four treatments—blank control (CK), biochar (BC), microbial agent (MA), and immobilized microorganisms (IM)—to verify that biochar-loaded IM alleviates temperature stress and sustains efficient PAH removal by regulating soil and microbial properties. PAH removal efficiency and soil chemical properties were monitored during both normal-temperature and freeze–thaw periods, and the soil bacterial community structure was analyzed at the end of the experiment. Results showed that IM achieved the optimal remediation performance with a total PAH removal rate of 72.53%, was least affected by temperature fluctuations, and maintained stable remediation during the freeze–thaw period. IM increased soil nutrient contents, with available potassium and nitrogen exerted positive effects on PAH degradation; it also enriched the functional genes K00626 and K00457 and comprehensively optimized the bacterial community. This study clarified the core remediation mechanism and provided scientific, technical, and theoretical support for related in situ remediation practices.