Fate in soil of 14C-sulfadiazine residues contained in the manure of young pigs treated with a veterinary antibiotic

The fate of 14C-labeled sulfadiazine (14C-SDZ) residues was studied in time-course experiments for 218 days of incubation using two soils (Ap horizon of loamy sand, orthic luvisol; Ap horizon of silt loam, cambisol) amended with fresh and aged (6 months) 14C-manure [40 g kg−1 of soil; 6.36 mg of sulfadiazine (SDZ) equivalents per kg of soil], which was derived from two shoats treated with 14C-SDZ. Mineralization of 14C-SDZ residues was below 2% after 218 days depending little on soil type. Portions of extractable 14C (ethanol-water, 9:1, v/v) decreased with time to 4–13% after 218 days of incubation with fresh and aged 14C-manure and both soils. Non-extractable residues were the main route of the fate of the 14C-SDZ residues (above 90% of total recovered 14C after 218 days). These residues were high immediately after amendment depending on soil type and aging of the 14C-manure, and were stable and not remobilized throughout 218 days of incubation. Bioavailable portions (extraction using CaCl2 solution) also decreased with increasing incubation period (5–7% after 218 days). Due to thin-layer chromatography (TLC), 500 μg of 14C-SDZ per kg soil were found in the ethanol-water extracts immediately after amendment with fresh 14C-manure, and about 50 μg kg−1 after 218 days. Bioavailable 14C-SDZ portions present in the CaCl2 extracts were about 350 μg kg−1 with amendment. Higher concentrations were initially detected with aged 14C-manure (ethanol-water extracts: 1,920 μg kg−1; CaCl2 extracts: 1,020 μg kg−1), probably due to release of 14C-SDZ from bound forms during storage. Consistent results were obtained by extraction of the 14C-manure-soil samples with ethyl acetate; portions of N-acetylated SDZ were additionally determined. All soluble 14C-SDZ residues contained in 14C-manure contributed to the formation of non-extractable residues; a tendency for persistence or accumulation was not observed. SDZ's non-extractable soil residues were associated with the soluble HCl, fulvic acids and humic acids fractions, and the insoluble humin fraction. The majority of the non-extractable residues appeared to be due to stable covalent binding to soil organic matter.