Determining Antimicrobial Thresholds for Soil Microbial Processes.

There is a concern that antimicrobials administered to livestock may cause greater antibiotic resistance in the environment and are a risk for human health.  Manure and rainfall runoff from open animal production sites, both of which contains trace levels of antimicrobials, are generally applied to nearby fields.  At high enough concentrations, antimicrobials may affect important soil processes--N transformations and decomposition.  A soil slurry study was conducted in serum bottles with increasing concentrations of monensin, sulfamethazine, and lincomycin in order to assess the concentrations required to affect denitrification and organic matter decomposition. Soil that had historically recieved cattle feedlot runoff was used, and trace gas production was monitored.  At highest concentrations tested (>5 ppm), monensin and sulfamethazine inhibited denitrification (no N₂O production).  Monensin also affected methanogenesis (CH₄ production), but sulfamethazine had no effect.  Aerobic decomposition (CO₂ production) was unaffected by the three antimicrobials, but monensin at the highest level limited CO₂ production under anaerobic conditions.  Although the concentrations of antimicrobials tested in these incubations were several orders higher than what is observed in the environment, we identified threshold concentrations where important soil microbial processes were inhibited.  Further study under more natural soil conditions are underway to better understand how antimicrobials affect important soil functions.