Dissipation and sequestration of the veterinary antibiotic sulfadiazine and its metabolites under field conditions.

Veterinary antibiotics introduced into the environment may change the composition and functioning of soil microbial communities and promote the spreading of antibiotic resistance. Actual risks depend on the antibiotic's persistence and (bio)accessibility, which may differ between laboratory and field conditions. We examined the dissipation and sequestration of sulfadiazine (SDZ) and its main metabolites in soil under field conditions and how it was influenced by temperature, soil moisture, plant roots, and soil aggregation compared to controlled laboratory experiments. A sequential extraction accounted for easily extractable (CaCl₂-extractable) and sequestered (microwave-extractable, residual) SDZ fractions. Dissipation from both fractions was largely temperature-dependent and could be well predicted from laboratory data recorded at different temperatures. Soil moisture additionally seemed to control sequestration, being accelerated in dry soil. Sequestration, as indicated by increasing apparent distribution coefficients and decreasing rates of kinetic release into CaCl₂, governed the antibiotic's long-term fate in soil. Besides, we observed spatial gradients of antibiotic concentrations across soil aggregates and in the vicinity of roots. The former were short-lived and equilibrated due to aggregate reorganization, while dissipation of the easily extractable fraction was accelerated near roots throughout the growth period. There was little if any impact of the plants on residual SDZ concentrations.