Date Full Report Received12/08/2018
Date Abstract Report Received12/08/2018
Setback distance is a commonly used measure to minimize the impacts manure land application on the quality of nearby surface water. The goal of this study was to determine the effectiveness of varying setback distances on reducing the concentrations of antibiotics and antibiotic resistance genes (ARGs) in runoff and soil following the land application of swine manure slurry. To achieve the goal, field tests and laboratory measurements were conducted.
Longer setback distances helped reduce the concentrations of antibiotics and ARGs in runoff. The three antibiotics tests were chlortetracycline, lincomycin, and tiamulin. The ARGs tested included those conferring resistance to tetracycline, macrolide, and penicillin. The concentrations of all three antibiotics in runoff decreased exponentially with increased setback distance, with tiamulin decreasing the fastest and lincomycin the slowest. The concentrations of seven genes tested also decreased exponentially with increased setback distance. Also, for most of the antibiotics and genes tested, their concentrations in the runoff from the second rainfall event were much lower than those in the runoff from the first rainfall event. Taken together, a setback distance of 50 m is deemed necessary to reduce the levels of ARGs in runoff to levels similar to those in runoff from control plots.
As expected, the manure application had significant impacts on the antibiotic and ARG concentrations in the soil within the manured region. For the soil within the setback region, setback distance affected antibiotics and ARGs differently. The concentrations of antibiotics in the surface soil within the setback region reduced significantly with increased setback distance. Because several ARGs were not detected in the surface soil within the setback region, no statistical conclusion was drawn for these ARGs. One gene that is often involved in horizontal gene transfer reduced significantly in surface soil with increased setback distances.
In addition to the individual ARGs tested, we also examined the resistome of the runoff and soil samples. Resistome refers to all the ARGs in a sample. Both the diversity and abundance of the resistome in runoff decreased significantly with increased setback distances. Manure application significantly enriched the resistome of the soil within the manured region. However, manure application appeared to have very limited impacts on the resistome of the soil within the setback region.
This project presents a comprehensive study about the effects of setback distances on the quality of runoff, in terms of antibiotics and ARGs, from soils receiving swine manure slurry. This study was set up to simulate a worst-case scenario where rainfalls occurred shortly after manure application. Information generated from this project can help regulators determine their recommendation about the setback distance needed to minimize the impacts of manure application on the quality of nearby surface water.