Polarity and fluorescent fractions of manure dissolved organic matter could affect differently the adsorption and desorption of antibiotics in soils.

The impacts of manure dissolved organic matter (DOM) on the adsorption and desorption of veterinary antibiotics in soils have not been fully explored. In this study, the effects of pig manure (PM) DOM and its hydrophilic and hydrophobic fractions on antibiotic adsorption and subsequent desorption in three topsoils of bamboo stands under different fertilization conditions (control, with heavy manure application, and with heavy manure application suspended) were examined by batch experiments, with the aid of excitation-emission matrix fluorescence spectroscopy, Fourier transformed infrared spectroscopy, and X-ray photoelectron spectroscopy. The addition of 100 mg-C L PM DOM was found to increase the adsorption (in terms of the Freundlich affinity coefficient (K)) of florfenicol by 71.27%-132.23% but decrease the adsorption of doxycycline, enrofloxacin, and sulfamethazine by 41.67%-94.78%, 98.09%-99.99%, and 22.93%-67.20%, respectively. These effects resulted mainly from the increases in solution pH by PM DOM addition and preferential adsorption of PM DOM's protein-like fraction to the soils. Effects of hydrophilic and hydrophobic fractions of PM DOM were antibiotic and soil specific, except the increases in adsorption K of florfenicol in all tested soils. Among the four antibiotics, sulfamethazine exhibited the lowest K values in the soils with PM DOM addition during both the adsorption and desorption processes, implying that sulfamethazine may be the most leachable antibiotic in the field. The results of this study indicate that it is essential to consider the varying effects of the aromaticity and hydrophobicity of manure DOM in assessing the leaching risk of veterinary antibiotics in manured soils.