Purpose: In this study, concentrations of bacteria with ARG in soils with applied poultry litter (PL; 2 years) or from a beef cattle backgrounding operation (BB; 3 year) were determined.
Methods: Samples were taken (1) following PL application to soils under conventional or no till management and (2) from soils taken from the BB while livestock were on-site and following their removal. Microbial populations with genes conferring resistance to tetracycline (tetQ and tetW), erythromycin (ermB and ermF) or sulfonamides (sulI), were quantified using quantitative, real-time (qPCR) analysis.
Results: In soils with applied PL, concentrations of ARG for sulfonamide and tetracycline resistance increased up to 3.0 orders of magnitude (OM; mean concentrations 2.6 to 6.9 x 108 copies g-1) following PL application but were near background by the end of the season. Concentrations of bacteria with AR genes were highly variable across the BB landscape, but in general initial concentrations averaged between 2.5 and 3.3 OM higher in the dirt congregation areas than in grassy areas. The highest concentration of ARG remaining in those soils (2.1±3.2 x 109 copies g-1) and background in grass (4.8±3.5 x 106 copies g-1) were for sulfonamide resistance.
Significance: These results suggest that the concentration of bacteria with ARG significantly increase in soils where manures are deposited but levels are mitigated by time and landscape management. Future research should determine which AR populations remain in soils and the potential for transmission among microbial populations in the soil.