Nitrogen Form-Dependent Shifts in Microbial Functional Pathways Explaining Biochar-Mediated Bioremediation of Petroleum-Contaminated Soil

Petroleum leakage from agricultural machinery and storage facilities threatens soil health and crop safety. Pyrolysis of agricultural residues into biochar provides a sustainable soil remediation strategy. However, its efficacy is often limited by nitrogen availability. We applied biochar to petroleum-contaminated soil along with different N sources (nitrate, ammonium or organic N) and used multilevel analyses (soil chemistry, enzyme assays, 16S sequencing and PICRUSt/KEGG inference) to track degradation mechanisms. Co-amendment of biochar with N fertilizers enhanced petroleum removal (81.0-83.6% vs 50.4% in control). Integrated analysis revealed form-specific mechanisms: ammonium accelerated early degradation kinetics, nitrate was associated with nitrogen-transforming microbial taxa and nitrate reductase, while organic nitrogen sustained urease activity and long-chain alkane oxidation. Distinct microbial community shifts were associated with changes in predicted metabolic pathways. These findings elucidated the coupled mechanism between nitrogen forms and microbial functional profiles, providing a scientific basis for optimizing nutrient-assisted remediation strategies.