Spatial Variation in Microbial Populations Across a Cattle Feedlot Transect From Barn to Retention Basin.

Run-off from livestock production can pose a risk to environmental quality particularly in karst terrains where geomorphic features like sinkholes provide rapid passage for contaminants to ground water resources.  In these environments, it is especially important to understand how variations in terrain influence contaminant distribution and concentration. In this study, spatial variation in microbial populations was analyzed across a cattle feedlot transect from barn to retention basin (located on a 4% slope). Soil samples (n= 28) were taken following a dry weather event and again following a wet weather event (n= 28). Microbial concentrations were assessed by DNA extraction and quantitative, real-time PCR analysis of targeted populations. Total bacterial cells, Escherichia coli and Bacteroides populations were quantified. Geographic information system (GIS) tools were used to model cell distributions from barn, grass, and retention basin soil samples. Concentrations of total cells varied little between sampling (2.7 to 6.3 X 10⁹ cells g^-1) or by terrain (average concentration 1.4±0.89 X 10¹⁰, 1.8±0.82 X 10⁹ and 2.4±1.0 X 10⁹ cells g^-1 in barn, grass and retention basin, respectively). Concentrations of the important indicator organism E. coli were below the limits of detection (10 copies) for all samples. Concentrations of Bacteroides sp. an increasingly important indictor organism were below detection in grass, but were higher and more variable in both barn (3.1 X 10⁵ to 2.2 X 10⁸ cells g^-1) and retention basin (1.4 X 10⁵ to 3.7 X 10⁷ cells g^-1) soil samples. Concentrations of cells were more consistent between sampling points (i.e., independent of wet or dry weather events) than between sample locations. Contaminant distribution and pollution risk potential within the feedlot varies with sample location and the type of microbial population targeted. Best management practices to control microbial contaminants must be specific for the site and the population of concern.