John D. Toth1, Zhengxia Dou1, James D. Ferguson1, Charles F. Ramberg, Jr.1, Christine Wang1, Shelly Rankin1, Ququan Wang2, and Yucheng Feng3. (1) University of Pennsylvania School of Veterinary Medicine, 382 West Street Rd., Kennett Square, PA 19348, (2) Delaware State University, Dover, DE, (3) Auburn University, 202 Funchess Hall, Auburn University, 36849
Animal manures are generally considered in terms of their utility as a nutrient source for growing crops. However, they can also serve as a point of entry for manure-borne bacteria and veterinary antimicrobials into the agroecosystem. A variety of antimicrobials are used in US animal agriculture, and these may be excreted unmodified in manure and persist in the soil when manure is land-applied. We conducted a study in which the effect of three veterinary antimicrobials was tested on soil bacterial metabolic functions. The antimicrobials sulfadimethoxine or tetracycline were added to fresh layer poultry manure and homogenized, and the ionophore rumensin was mixed with fresh dairy manure. The manures amended with the antimicrobials were then mixed with a silt loam soil. There were four rates of each antimicrobial and three replications. The manure-soil mixtures were incubated at room temperature for 7 weeks. Soil subsamples were taken periodically and analyzed for soil respiration, nitrification and iron(III) reduction. Soil respiration and nitrification rates were relatively unaffected by presence of the antimicrobials, with most of the intertreatment differences a function of presence and type of manure applied. Iron(III) reduction was significantly reduced (P=0.05) in the poultry manure-amended treatements by all rates of sulfadimethoxine through day 50, and in the dairy manure treatments by rumensin, with the rumensin effect decreasing over time at lower rates of the antimicrobial. These results suggest that some veterinary antimicrobials in the soil system can suppress microbial community physiological functioning. Given the little that is currently understood about these effects, further research is desirable.
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