Individual and combined applications of biochar and pyroligneous acid mitigate dissemination of antibiotic resistance genes in agricultural soil

Highlights: • Biochar and pyroligneous acid reduced the level of soil ARGs. • Weakened HGT by pyroligneous acid mainly contributed to the ARG reduction. • Lowered heavy metal bioavailability by biochar was associated with decreased ARGs. • Shifted community and improved soil property by co-application reduced soil ARGs. Remediation of agricultural soils polluted with antibiotic resistance genes (ARGs) is important for protecting food safety and human health. However, the feasibility of co-application of biochar and pyroligneous acid, two multifunctional soil amendments, for mitigating dissemination of soil ARGs is unknown. Thus, a woody biochar (BC450) and its by-product, pyroligneous acid (PA450) simultaneously produced at 450 °C from blended wood wastes, were used to compare their individual and combined effects on soil ARG abundance using a 65-day pot experiment planted with leafy vegetable Brassica chinensis L. The individual and combined applications of PA450 and BC450 significantly reduced the absolute abundance of ARGs by 65.7–81.4% and 47.5–72.9% in the corresponding rhizosphere and bulk soil. However, the co-application showed little synergistic effect, probably due to the counteractive effect of BC450 on the PA450-mitigated soil ARG proliferation, resulted from the promoted soil bacterial growth and/or adsorption of antimicrobial components of PA450 by BC450. The decreased abundances of mobile genetic element intI1 and Tn916/1545 in the PA450 treatments demonstrated the potential of PA450 for weakening horizontal gene transfer (HGT). Furthermore, weakened HGT by individual PA450, lowered availability of heavy metals by individual BC450, and different bacterial community (e.g., reduced ARGs bacterial host) together with improved soil properties from co-application of PA450 and BC450 all contributed to the reduced ARG level. This study highlighted the feasibility of co-applications of biochar and pyroligneous acid amendment for mitigating soil ARG pollution. These findings provide important information for developing eco-friendly technologies using biochar and pyroligneous acid in remediating ARG-contaminated soils.