Effect of different oxytetracycline addition methods on its degradation behavior in soil.

The degradation behavior of veterinary antibiotics in soil is commonly studied using the following methods of adding antibiotics to the soil: (i) adding manure collected from animals fed with a diet containing antibiotics, (ii) adding antibiotic-free animal manure spiked with antibiotics and (iii) directly adding antibiotics. No research simultaneously comparing different antibiotic addition methods was found. Oxytetracycline (OTC) was used as a model antibiotic to compare the effect of the three commonly used antibiotic addition methods on OTC degradation behavior in soil. The three treatment methods have similar trends, though OTC degradation half-lives show the following significant differences (P<0.05): manure from swine fed OTC (treatment A)<antibiotic-free manure+OTC (treatment B)<OTC (treatment C). Differences could be caused by distinct chemical reaction equilibria due to dissimilar concentrations of 4-epi-OTC and α-apo-OTC. The pH could also have affected the concentration of 4-epi-OTC and α-apo-OTC, thus influencing OTC degradation. The treatments presenting manure (A and B) significantly enhanced EC, enzyme activity, microbial biomass carbon and nitrogen when compared to the treatment without manure (C), thus increasing degradation of OTC in the soil. Because the main entry route for veterinary antibiotics into soil is via the manure of animals given with antibiotics, the most appropriate method to study the degradation and ecotoxicity of antibiotic residues in soil may be to use manure from animals that are given a particular antibiotic, rather than by adding it directly to the soil.