Ecological filtering enhanced by smaller PBS biodegradable microplastics constrains ARG dynamics in the soil plastisphere.

Microplastics (MPs) are increasingly recognized as hotspots for antibiotic resistance genes (ARGs), yet the combined effects of polymer type and particle size on ARG dynamics in the soil plastisphere remain unclear. Here, we employed metagenomic assembly and binning to explore how MP polymer type and particle size jointly modulate ARG carrying frequencies (ACFs), mobility, and microbial hosts with polyethylene (PE), polystyrene (PS), and biodegradable polybutylene succinate (PBS) MPs across a size gradient (1000, 500, and 106 μm). PBS, PS, and PE plastispheres exhibited different size-related trends in ARG association, with PBS showing the strongest and most consistent decline in ACFs. Only PBS showed a corresponding reduction in ARG-MGE co-localization, suggesting size-dependent constraints on horizontal gene transfer. Distinct ARG combinations in ARG-Carrying Contigs (ACCs) also showed plastic-type selectivity, with complex resistance clusters absent in 106 μm PBS samples, potentially due to environmental constraints that limit the assembly or persistence of multigene resistance structures. Potential pathogens Enterobacter bugandensis and Stutzerimonas urumqiensis were markedly reduced in 106 μm PBS samples, a pattern not observed in PS or PE. Bacterial community analysis revealed that smaller PBS particles were associated with reduced richness, increased evenness, and more competitive interactions within co-occurrence networks. These features, together with the decline in ARG abundance and mobility, suggest that enhanced ecological filtering may occur in smaller biodegradable plastispheres, jointly limiting the persistence of resistance genes and their bacterial hosts. Together, our findings highlight the importance of considering both MP type and particle size in assessing plastisphere-associated ARG risks.