The Effects of Veterinary Antibiotics On Soil Microbial Community Function In Vegetative Filter Strips and Cropland.

Veterinary antibiotics (VAs) administered to livestock for therapeutic or prophylactic uses have been shown to pass through the G.I. tract of animals unaltered.  These pharmaceuticals are often introduced to agroecosystems via land application of manure.  Subsequently, VAs enter soil, surface water and groundwater posing a human and environmental health risk.  Agroforestry and grass filter strips may act to mitigate the spread or may enhance the degradation of veterinary antibiotics.  However, shifts in soil microbial community characteristics are expected through suppression of antibiotic susceptible species.  Soils were collected from 0-10 cm depth from agroforestry and grass filter strips, and cropped areas located at the University of Missouri’s Greenly Memorial Research Center, Novelty MO (40°01’N, 92°11’W).  Bulk soil samples were moist sieved and added to incubation jars (total jars = 566).  For soils from each land management type, three treatments were applied:  i) an untreated control, ii) oxytetracycline at concentrations of 5, 50, and 200 mg kg^-1 soil, and iii) and lincomycin at concentrations of 5, 50, and 200 mg kg^-1 soil.  Soil samples were incubated for 0, 3, 7, 14, 21, 28, 35, 49 and 63 days.  At each time step, three sample jars per treatment and land management type were removed from the incubation experiment.  Samples were tested for soil microbial function using Biolog ECO microplates and dehydrogenase and fluorescein diacetate hydrolysis enzyme assays.  Microbial communities of these soils appear robust to effects of oxytetracycline and lincomycin at test concentrations.  While the assays show an initial inhibitory effect, this trend is generally reversed by day 7.  This shift in effect may be related to the inactivation of the VAs (e.g., sorption processes) and/or VA use as a carbon source.  Trends were similar in all treatments and land management types.  Further analyses will permit determination of microbial community changes that could account for the patterns observed.