Persistence and availability of veterinary antibiotics in soil and soil-manure systems.

The availability and persistence of various antibiotics in soil and soil amended with composted poultry manure were investigated through laboratory incubation assays. Six veterinary antibiotics (one fluoroquinolone, two tetracyclines, two sulfonamides and one lincosamide) and one active metabolite (ciprofloxacin) were studied. The incubation assays were conducted at a controlled temperature of 25 °C with different water regimes, such as constant moisture content (80% of water holding capacity) and drying-rewetting cycles. The studied antibiotics were determined in soil and soil aqueous phase samples by LC-MS/MS using internal standards. The results indicated that the highest levels found in the soil aqueous phase were for sulfamethoxazole, followed by sulfamethazine and lincomycin, being very low the levels of chlortetracycline, doxycycline, ciprofloxacin and enrofloxacin (≤1.8%). A positive correlation was observed between the antibiotic concentrations and the content of the dissolved organic carbon in soil aqueous phase with the incubation time. An increase in the apparent sorption coefficients of these antibiotics, except chlortetracycline and lincomycin, was observed when the soil was amended with composted manure. Except for fluoroquinolones, with remaining residues around 70% after 90 days of incubation, a fast dissipation of antibiotics was observed during the assay, with half-lives ranging from 8 to 27 days. These values increased between 6% and 53% in manure amended soil; nevertheless, half-lives remained short (9 days and 27 days for lincomycin and sulfamethazine, respectively). Similar results were obtained with soil under drying-rewetting cycles showing somewhat lower values in soil aqueous phase and slightly shorter half-lives in some cases. The results obtained pointed out that the route of entry of antibiotics into the soil, through recycled water or manure, may have an important effect on their behavior, particularly regarding their availability in soil.