Fe/Mn oxide-loaded biochars mitigate antibiotic resistance gene pollution and dissemination potential in agricultural...

Spread of antibiotic resistance genes (ARGs) in agricultural soils threatens ecological and public health. Functionalized biochars hold promise for remediating biological contaminants, yet the performance of Fe/Mn oxide-loaded biochars (Fe/Mn-MBCs) in mitigating soil ARG pollution remains unclear. Herein, we investigated the effects and mechanisms of Fe/Mn-MBCs prepared via one-pot pyrolysis of pig manure impregnated with ferric nitrate and potassium permanganate on ARG abundance and risk using soil microcosms combined with bacterial conjugation and natural transformation assays. While pristine biochar increased total ARG relative abundance, Fe/Mn-MBCs bearing nano-sized oxides (e.g., Fe₃O₄, Mn₃O₄, MnO₂) decreased it by 23.17%-44.54%, and high-risk ARGs (aadE, floR) by 28.73%-50.19%, demonstrating the feasibility for mitigating soil ARG pollution and health risks. Mechanistically, Fe/Mn-MBCs reshaped the bacterial community by enriching Fe/Mn-redox bacteria (e.g., Terrimonas, Stenotrophobacter), while strengthening dispersal limitation of potential ARG hosts (e.g., Actinobacteria, Firmicutes). Fe/Mn-MBCs also suppressed horizontal gene transfer (HGT), as evidenced by the decreased abundances of mobile genetic elements (intI1, tn916, tnpA) by 10.60%-40.80%. This was further confirmed by the decreased frequencies of conjugation and transformation. Fe/Mn-MBCs suppressed intracellular ARG conjugations by reducing cell membrane permeability, ATP synthesis, and bacterial motility, while inhibiting extracellular ARGs (eARGs) transformation via plasmid adsorption and reduced eARG availability. These findings advance mechanistic understanding of Fe/Mn-MBCs in mitigating soil ARG pollution through potential ARG host reduction and HGT suppression, and guide the rational design of functionalized biochars as an in‑situ remediation strategy against antimicrobial resistance in agroecosystems.