Veterinary Antibiotics in Agroecosystems: Does the Removal of Veterinary Antibiotics Affect Soil Microbial Community Characteristics?.

Antibiotics administered to livestock to enhance growth rates typically pass through the animals’ G.I. tract unaltered and enter natural systems.  The presence of veterinary antibiotics (VAs) in soils may affect soil microbial community characteristics and subsequently nutrient cycling and soil fertility.  Also, VAs may contribute to the development of antibiotic resistance in microorganisms inhabiting soils and surface waters.  These concerns have led to proposed limitations for use of VAs on only sick animals treated by medically trained personnel.  We determined how soil microbial community characteristics may be affected by the removal of VAs from the system.  We considered two different livestock systems:  hogs from the Midwest (i.e., Missouri, USA) and cattle from the Northwest (i.e., Washington, USA).  We sampled current cattle management that either had or lacked the administration of sub-therapeutic levels of VAs.  We compared a current hog management with no VAs administered with two abandoned sites that previously housed hogs that were treated with VAs.  Surface soil samples (0-5 cm and 5-10 cm) were collected from areas adjacent to feeding barns; sample size varied (n=12-20) based on feedlot size.  Samples were tested for soil microbial community structure using phospholipid fatty acid (PLFA) analysis and for soil microbial community function using β-glucosidase and β-glucosaminidase enzyme assays.  In both systems, it appears that VAs have little effect on soil microbial community function.  No differences were observed for β-glucosidase activity among hog sites or for β-glucosaminidase activity in cattle sites.  However, the active hog site with no-VAs showed significantly higher β-glucosaminidase activity than either of the abandoned sites, which were not different from each other.  The cattle site under VA management had significantly greater β-glucosidase activity than the site lacking VA applications.  We speculate that a number of other factors including soil compaction and the inputs of feces and manure may be overriding VA effects on the soil microbial community.  We anticipate changes in the soil microbial community structure despite few changes in soil microbial community function, due to redundancy in microbial functioning.  These results suggest the need for additional studies under these and other systems to further clarify trends.