The fate of (14)C-labeled sulfadiazine ((14)C-SDZ) residues was studied in time-course experiments for 218 days of incubation using two soils (A(p) horizon of loamy sand, orthic luvisol; A(p) horizon of silt loam, cambisol) amended with fresh and aged (6 months) (14)C-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 (14)C-SDZ. Mineralization of (14)C-SDZ residues was below 2% after 218 days depending little on soil type. Portions of extractable (14)C (ethanol-water, 9:1, v/v) decreased with time to 4-13% after 218 days of incubation with fresh and aged (14)C-manure and both soils. Non-extractable residues were the main route of the fate of the (14)C-SDZ residues (above 90% of total recovered (14)C after 218 days). These residues were high immediately after amendment depending on soil type and aging of the (14)C-manure, and were stable and not remobilized throughout 218 days of incubation. Bioavailable portions (extraction using CaCl(2) solution) also decreased with increasing incubation period (5-7% after 218 days). Due to thin-layer chromatography (TLC), 500 microg of (14)C-SDZ per kg soil were found in the ethanol-water extracts immediately after amendment with fresh (14)C-manure, and about 50 microg kg(-1) after 218 days. Bioavailable (14)C-SDZ portions present in the CaCl(2) extracts were about 350 microg kg(-1) with amendment. Higher concentrations were initially detected with aged (14)C-manure (ethanol-water extracts: 1,920 microg kg(-1); CaCl(2) extracts: 1,020 microg kg(-1)), probably due to release of (14)C-SDZ from bound forms during storage. Consistent results were obtained by extraction of the (14)C-manure-soil samples with ethyl acetate; portions of N-acetylated SDZ were additionally determined. All soluble (14)C-SDZ residues contained in (14)C-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.