Shaping antibiotic resistance gene fate in soil-plant systems: Dual roles of biochar physicochemical traits mediated by pyrolysis conditions.

Antibiotic resistance genes (ARGs), emerging contaminants spreading via horizontal gene transfer, threaten ecosystems and human health. Biochar (BC) is a widely used agricultural soil amendment, yet its effects on ARG dissemination remain controversial, likely dependent on pyrolysis conditions. This study applied wheat straw BC prepared under three distinct pyrolysis conditions, including open-flame combustion (BC-ZJ), 500°C hypoxic pyrolysis, and 500°C anaerobic pyrolysis, to a Brassica rapa L.-soil system for exploring ARG transfer impacts and mechanisms. BC application increased plant stem/leaf total ARG relative abundance by 20.68-71.59% and selectively enriched specific subtypes. BC-ZJ enriched multidrug resistance ARGs, whereas BC-Y500 and BC-W500 dramatically elevated aminoglycoside and tetracycline ARGs, and vancomycin and sulfonamide ARGs became undetectable. BC-ZJ (rich in oxygen-containing functional groups) stimulated microbial co-metabolism and promoted ARG proliferation and translocation into plant tissues. In contrast, BC-Y500 and BC-W500 (with larger micropore volumes and stable aromatic structures) exerted dual effects: potential adsorption of partial ARGs but selective enrichment of key ARG-hosting taxa (e.g., Pseudonocardia), leading to the accumulation of aph(3')-I and tetC in plant tissues. Structural equation modeling revealed that BC exerted a direct negative effect on ARG abundance, but this was overwhelmed by positive indirect effects via enhanced soil properties and bacterial community restructuring, leading to a net increase in ARG abundance. The bacterial community emerged as the dominant driver integrating the influences of BC properties, soil conditions, and mobile genetic elements. These findings demonstrate that biochar-mediated ARG regulation balances adsorptive inhibition and microbial stimulation in a pyrolysis-dependent manner. This study provides a mechanistic basis for engineering pyrolysis-optimized BC to mitigate agricultural ARG dissemination.