Field-scale composting of solid fraction of swine slurry: Reducing veterinary antibiotics and resistance selection risk in agricultural soils

Antibiotic (AB) use in intensive livestock farming drives antibiotic resistance risks in agroecosystems, yet field-scale data on composting as mitigation strategy remain scarce. Here, we present a novel field-scale composting trial (24 m³ per pile) treating the solid fraction of pig slurry with different bulking agents (maize straw or tree prunings) over 280-day. Of 16 veterinary ABs monitored, 7 were consistently detected, spanning fluoroquinolones, tetracyclines, lincosamides, and sulfonamides, with concentrations up to 167 mg·kg⁻¹ dry weight. Over 90% AB removal was achieved, with faster degradation using tree pruning residues (half-life ~43 days) than maize straw (~55 days), highlighting the role of bulking agents in composting. Crucially, this is the first field-scale study to evaluate antibiotic resistance selection risk (RQRS) and leaching potential in real compost-amended soils. While raw pig slurry posed high resistance selection risk in soil (RQRS > 1), composting —especially with tree prunings— reduced this risk below 0.1. These findings provide strong field-based evidence that composting not only mitigates AB residues but also substantially reduces resistance risk under practical, operational conditions. • Field-scale composting reduced antibiotics in pig slurry by over 90%. • Tree prunings enhanced antibiotic degradation more than maize straw. • Thermophilic stage was key for antibiotic removal efficiency. • Composting lowered antibiotic resistance risk in soils below safety limits. • Final compost showed minimal leaching and improved soil safety.