Chitosan mitigates the dissemination of antibiotic resistance genes caused by metalaxyl in the soil-earthworm system.

The widespread use of agricultural fungicides can lead to residual contamination and accelerate the emergence and spread of antibiotic resistance genes (ARGs) and antibiotic-resistant bacteria (ARBs) in soil ecosystems. Developing green remediation strategies that simultaneously reduce fungicide residues and mitigate antibiotic resistance risks is therefore beneficial for soil pollutant control. Here, we investigated the effects of chitosan (CHI) on the dissipation of metalaxyl (MET) enantiomers and their influence on the soil-earthworm resistome and microbiome through pot experiments and metagenomics analysis. The results showed that CHI significantly accelerated MET dissipation in soil and reduced its bioaccumulation in earthworms. MET enantiomers, particularly S-MET, promoted the dissemination of ARGs, including high-risk ARGs, in the soil-earthworm system, whereas CHI suppressed these effects, likely by limiting the potential for mobile genetic element (MGE)-mediated horizontal gene transfer (HGT). Compared with the corresponding treatments without CHI amendment, CHI amendment reduced the total relative abundance of ARGs by 16.8%-24.7% in soil and by 34.1%-58.3% in earthworm guts. Furthermore, CHI reshaped microbial community structure in both soil and earthworm gut samples by driving ecological niche differentiation of Actinomycetota and Pseudomonadota and reduced ARG host abundance. These findings offer practical insights for controlling contamination by fungicides and ARGs in agricultural soils.