Effects of slurry from sulfadiazine- (SDZ) and difloxacin- (DIF) medicated pigs on the structural diversity of microorganisms in bulk and rhizosphere soil

Conventional farming still consumes considerable amounts of antibiotics such as sulfadiazine (SDZ) or difloxacin (DIF) to protect livestock from infectious diseases. Consequently, slurries from medicated animals are applied to arable soils. Antibiotics, co-applied with pig slurry, are increasingly reported to change soil microbial community structures in un-rooted bulk soil. The effects in rhizosphere soil, as well as the medication-derived direct and indirect effects of an altered slurry composition are poorly investigated. We evaluated the response of microorganisms to slurry of SDZ- and DIF-medicated pigs in a 63-d mesocosm experiment, considering the natural complexity of a typical agricultural pig slurry amendment and developing Zea mays L. root systems. Slurry-derived fecal bacteria were still present in mesosocosm soil 14 days after amendment. Medication with DIF and SDZ further altered the molecular-chemical pattern of the pig slurry, confounding the precise antibiotic effect. This has to be considered when investigating antimicrobial effects under ecological relevant conditions. Effects on the microbial community in mesocosm bulk soil widely matched results from previous studies on directly spiked soil. Effects were also found in the mesocosm rhizosphere soil, but not more pronounced than in bulk soil. This was also verified under laboratory conditions after application of artificially SDZ-spiked control slurry.