Biochar suppression of antibiotic resistance genes in the soil microbiome

Antimicrobial resistance (AMR) in soils frequently arises from low-level antibiotic presence in agriculture systems where livestock receive antibiotics for disease prevention. Here we assess the influence of oak ( Quercus robur ) wood biochar (pyrolysis at 760°C) on the microbiome of a parkland soil, after 300 days of incubation, in the presence and absence of the antibiotic sulphamethazine (SMZ). Stable isotope probing coupled to metagenomics was carried out to identify metabolically active microorganisms and quantify antimicrobial resistance gene (ARG) prevalence. Across all treatments, Pseudomonadota and Actinobacteriota dominated (63–78%), with Pseudomonadota enriched in the biologically active heavy H ₂ ¹⁸ O fraction. Gemmatimonadota increased, and Bacillota decreased in heavy fractions, while minor phyla displayed variable shifts. Biochar significantly reduced ARG prevalence in the active heavy H ₂ ¹⁸ O metagenome from 23,485 ARGs in control soils to 10,002 ARGs in biochar-amended soils, while SMZ alone increased to 26,193 ARGs found in their metagenome. Combined biochar and SMZ treatment resulted in lower ARGs (16,890 ARGs) than both control and SMZ-only soils, likely due to a combination of ARG adsorption, antibiotic immobilisation, and microbial community restructuring. These results highlight the potential of biochar as a soil amendment for mitigating the proliferation of AMR in agriculture environments.