Impact of manure-derived dissolved organic matter on antibiotic transport in soil: A cross-scale study from molecular to column scale

• Binding to MDOM follows quinolone>tetracycline>macrolide>sulfonamide>chloramphenicol. • Smaller molecular energy gaps (Δ E ) facilitate stronger antibiotic-DOM bindings. • Enhanced leaching of weakly-bonded antibiotics results in increased spread risk. Land application of manure containing antibiotic residues poses significant environmental risks, primarily via leaching of these emerging contaminants into water resources. Despite extensive attention to antibiotic pollution, the mechanisms by which manure-derived dissolved organic matter (MDOM) influences their transport remain poorly understood. Using integrated techniques including fluorescence quenching and density functional theory (DFT) calculations, this study investigated molecular-scale interactions between eight typical antibiotics from five categories and four DOM species (humic acid, L-tryptophan, chicken MDOM and pig MDOM). These molecular interactions were linked to field-relevant conditions via soil column experiments and hydraulic modeling. Results demonstrated that the extent of antibiotic-DOM binding was determined by molecular HOMO-LUMO energy gaps (Δ E ), following the order: quinolones (QLs)>tetracyclines (TCs)>macrolides (MLs)>sulfonamides (SAs)>chloramphenicols (CAPs). Protein-like species exhibited stronger antibiotic affinity than humic acid (HA), with chicken MDOM showing higher reactivity than pig MDOM. MDOM impact on antibiotic leaching strongly depended on their molecular binding strength, governing soil-water interface behavior. MDOM acted as a mobile carrier for weakly-bound antibiotics, facilitating transport via competitive adsorption, whereas it enhanced the retention of strongly-bound antibiotics through co-adsorption. These findings bridge molecular interactions and macroscopic leaching behavior in soil, providing a scientific basis for improving risk assessment and sustainable manure management in agricultural systems.