The application of manure as a fertiliser to farmland is regarded as a major source of veterinary antibiotic (VA) contamination in the environment. The frequent detection of such emerging contaminants and their potential adverse impacts on the ecosystem and human health have provoked increasing concern for VA transport and fate. Extrinsic dissolved organic matter (DOM) may be introduced into farmland soil along with Vas, and thus exert significant effects on the transport of VAs via hydrological processes upon rainfall. The leaching of VAs can be either enhanced or reduced by DOM, depending on the nature, mobility, and interactions of VAs with DOM of different origins. From the aspect of the diversity and reactivity of DOM, the state-of-the-art knowledge of DOM−VA interactions and their resulting effects on the sorption−desorption and leaching of VAs in farmland soil was reviewed. Spectroscopic techniques for examining the extent of binding and reactive components of DOM with VAs are summarized and their usefulness is highlighted. Models for simulating VA transport under the effects of DOM were also reviewed. It is suggested that distinct impacts of DOM of various organic fertiliser/amendment origins should be considered for predicting the transport of VAs in farmland soil.

Antibiotic pollution from manured farmland soils is a major public concern, and their potential interaction with manure particles and/or manure–dissolved organic matter (DOM) often complicates their leaching behaviour. This study investigated the leaching of sulfadiazine (SDZ) and florfenicol (FFC) with manure-DOM in undisturbed field lysimeters and repacked soil columns under natural and simulated rainfall conditions. The results showed that manure-DOM reduced SDZ mass flux, but soil hydrological processes and heavy rainfall events led to accelerated leaching. FFC was more prone to leaching in a manured plot (0.48 µg m⁻² h⁻¹) compared to the control (0.12 µg m⁻² h⁻¹), suggesting DOM facilitated transport of FFC in the field lysimeter study via cotransport mechanisms favored by abundant macropores in the study site. In contrast, SDZ and FFC mobility were reduced in repacked soil columns under manure-DOM conditions, suggesting complexation and adsorption in matrix pores. Two kinetic site models and two-site nonequilibrium adsorption models revealed the existence of nonequilibrium conditions and kinetic sorption processes in the repacked column. FFC exhibited lower leaching potential compared to SDZ in both the repacked column and natural field conditions. Redundancy analyses revealed that FFC had a close relationship with humic-like components (C1 and C3), but SDZ was more related to the protein-like components (C2) of DOM. The presence of manure-DOM may decrease the ecological risks of highly mobile antibiotics under matrix flow through complexations and adsorption. However, a similar scenario in macroporous flow under heavy rainfalls may lead to accelerated leaching.

Field investigation on manure-sourced veterinary antibiotics migrating via runoff processes under natural rainfalls is quite limited due to hydrological complexity. The impact of manure application on antibiotics migration in soil is also scarcely discussed at the plotscale. This study examined the dynamic changes of concentration and mass flux of eight antibiotics migrating from chicken-raising orchards during rainstorms. Results showed instantaneous presence of antibiotics in the flow samples collected upon generation of surface runoff and leachate. Their concentrations responded strongly towards the flow rate, with concentration peaks mostly overlapping with flow peaks. Chicken-raising treatment resulted in significantly higher runoff generation than orchard without chickens. The total mass flux of antibiotics reached up to 5.46 and 9.41 μg/(m²·h) for surface runoff and leachate respectively, with sulfonamides (SAs) showing the highest migration (0.02-4.52 μg/(m²·h)) and tetracyclines (TCs) and quinolones (QLs) the lowest (10^-4-10^-1 μg/(m²·h)). Leaching performed as an important pathway for contaminant migration in the studied soil, with mass fluxes 1.5-11 times higher than those of surface runoff, and manure-derived colloids accelerated antibiotics leaching. pH, colloid concentration and instantaneous rainfall intensity were the primary influencing factors, all displaying positive correlations with antibiotics migration (p < 0.05). This study implied that raising chickens in orchard poses a risk of antibiotic contamination to surrounding ecosystem. It also highlighted the importance of exploring soil hydrological process in assessing their migration, particularly the previously underestimated contribution of subsurface flow via leaching.

As antibiotic pollution from manured farmland soils has aroused public concern, their potential interaction with manure matrix/colloids especially manure-derived dissolved organic matter (DOM) often complicated their leaching behavior. This study investigated the leaching of sulfadiazine (SDZ), and florfenicol (FFC) in undisturbed field lysimeters of chickens-integrated orchard farmland under continuous natural rainfall events. Repacked column experiments were also conducted with manure-DOM treatments under soil matrix flow. The results indicate that manure presence could reduce SDZ mass flux, but the real-time migration may increase under favorable soil hydrological processes and heavy rainfall events. Whereas FFC was more prone to leaching in manured plot (0.48 µg m^-2 h^-1) as compared to the control (0.12 µg m^-2 h^-1), suggesting manure-DOM facilitated FFC leaching in field via a cotransport mechanism favored by abundant macropore flow in the soil. In contrast, SDZ and FFC’s mobility was reduced in repacked columns under manure-DOM conditions, suggesting increased adsorption, complexation and straining effects in soil matrix pores. Two kinetic site model and two-site nonequilibrium adsorption model fitted the breakthrough curves of SDZ and FFC well (0.988> r² > 0.995 for SDZ and 0.973> r² > 0.989 for FFC), revealing non-equilibrium conditions and kinetic sorption process in the repacked column. FFC exhibits lower leaching potential compared to SDZ in both column and field trials. Redundancy analyses revealed close relationship of FFC with humic-like components (C1 and C3), but SDZ is more related to protein-like component (C2) of manure-DOM. Therefore, as manure-DOM has a potential of increasing adsorption of highly mobile antibiotics on soil surface, however, scenario of macroporous soil under heavy rainfalls still leads to accelerated leaching. These findings highlight the combined impacts of manure-DOM and soil hydrological processes in modulating the ecological risks of antibiotics under natural conditions.