Synergistic remediation of biochar-mediated petroleum-contaminated soil amended with rhamnolipid and nitrate: Underlying mechanism and microbial community

Rice husk biochar, a by-product of abundant agricultural waste, assumes a considerable position in the remediation of petroleum-contaminated soil. However, strong petroleum adsorption effect of biochar materials and depletion of nitrogen source critically limit bioremediation efficiency in biochar‑based system. Herein, this study systematically investigated variation of environmental factors, microbial community succession and remediation mechanism of biochar with rhamnolipid and nitrate through modulation of petroleum mobility and nitrogen availability. Mechanistic insights were gained through enzymatic assays, degradation kinetics, seed germination tests, microbial community, and functional gene. Five treatments were applied, including CK (control), T1 (biochar), T2 (biochar with rhamnolipid), T3 (biochar with nitrate) and T4 (biochar with rhamnolipid and nitrate), which achieved remediation efficiencies of 48.60 % (CK), 73.33 % (T1), 79.87 % (T2), 81.00 % (T3), and 87.47 % (T4). Concurrently, T4 treatment presented the highest degradation rate constant, the shortest half‑life and the best seed germination efficiency at 88.33 %, further corroborating the superior synergistic performance. During remediation process, functional petroleum‑degrading microorganisms were progressively enriched, and 15 genera with documented petroleum‑degrading capabilities were identified. More interestingly, remediation efficiency and associated indicators in T4 correlated strongly with ammonium‑N, nitrate‑N and urease activity, outperforming other treatments. Network analysis also highlighted central role of T4 and synergistic effect on nutrient cycling and enzyme activity. T4 consistently exhibited the highest ko00020 abundance in KEGG functional prediction. Mechanistically, nitrate served as a terminal electron acceptor, supplemented soil nitrogen, and activated key metabolic pathways, synergizing with rhamnolipid to drive efficient bioremediation. This study provides technical guidelines for optimizing petroleum biodegradation performance. • Mechanism of biochar co-amended with rhamnolipid and nitrate was demonstrated. • T4 treatment presented highest degradation rate constant and the shortest half‑life. • Functional degrading microorganisms were enriched during remediation process. • Correlation analysis revealed distinct roles and cooperation among microbial genera. • Nitrate served as nitrogen source, and stimulated electron transfer of microorganism.