Dissolved Organic Matter Chemistry Modulates Biochar Effects on Phenanthrene Biodegradation in Agricultural Soil: Bioavailability and Microbial Responses

Abstract Biochar is increasingly applied to cropland, yet co-occurring dissolved organic matter (DOM) jointly regulates polycyclic aromatic hydrocarbons (PAHs) fate and food-safety risks. We amended phenanthrene-spiked agricultural soil with wheat-straw biochar and three DOM types (citric acid, oxalic acid and humic acid) and linked biodegradation to bioavailable pools and microbial responses. Citric and oxalic acids increased bioavailable fractions and raised microbial removal from 43.98 ± 0.72% (control) to 55.84 ± 0.87% and 54.74 ± 2.29%, whereas humic acid lowered effective bioavailability and reduced removal to 39.65 ± 0.73%. Biochar alone increased removal (48.20 ± 0.98%) despite decreasing initial bioavailability. Biochar-driven microhabitat improvement coupled with citric/oxalic-acid-enhanced bioavailability enriched key degraders and nidA copies (initiating phenanthrene oxidation), yielding the highest removal (64.20 ± 0.49% and 60.52 ± 0.09%). In contrast, biochar plus humic acid promoted sequestration and showed the lowest removal (36.55 ± 0.97%). DOM chemistry modulates whether biochar favors bioremediation or stabilization.