Rising global antibiotic resistance has caused concerns over sources and pathways for the spread of contributing factors. Majority of the antimicrobials used in the U.S. are involved in veterinary medicine, primarily with livestock rearing. Animal manure land application integrates livestock farming and agroecosystems. This manure contains antibiotic resistant elements (AREs) (resistant bacteria, resistance genes, and veterinary antibiotics) that contribute towards antimicrobial resistance. Altering manure application techniques can reduce surface runoff of other contaminants such as excess N and P, pesticides, and hormones, that can impact water quality. Conventional tillage practices in the U.S. has reduced or stopped, making subsurface injection of manure a promising option when compared to surface application. Our research compared manure application methods, manure application seasons, cropping system, and manure-rainfall time gaps to gauge the impact on AREs in the environment. Two field-scale rainfall simulation studies were conducted along with one laboratory study. Using the injection method lowered concentrations of manure associated AREs entering surface runoff. When manure was surface applied and rainfall occurred 7 d after application, 9-30 times less resistant fecal coliform bacteria (FCB) entered surface runoff when compared to 1 d time gap for that broadcast method. Within a day of manure application, antibiotic resistance gene (ARG) profiles in soil began to differ from each other based on manure application and soil ARG richness in all manure-amended soil increased compared to the background. Runoff from injection plots contained 52 ARGs with higher abundance compared to runoff from surface applied plots. ARGs in the former were more correlated to soil and more correlated to manure in the latter. The highest antibiotic concentrations were in the injection slit soil of those plots. Antibiotic concentrations in samples corresponded positively to concentrations of resistant FCB and ARGs, and there was a positive correlation between resistant FCB and their associated ARGs (Spearman's ρ = 0.43-0.63). A CRIISPR-Cas12a assay for quantification of ARGs in environmental samples was just as precise as conventional methods. There is also potential for in-situ detection. These combined results can hopefully help farmers improve manure management practices that mitigate spread of AREs to surrounding water, crops, and soil. / Doctor of Philosophy / Rising global antibiotic resistance cause concerns over sources and pathways for the spread of contributing factors. Most of the antimicrobials used in the U.S. are involved in veterinary medicine, especially with livestock rearing. Overuse of antibiotics that are medically important to human medicine compromises the effectiveness of our medicines. Animal manure contains antibiotic resistant elements (AREs) such as resistant bacteria, resistance genes, and antibiotics) that contribute towards resistance issues. Once these AREs enter the environment, they can be taken up by crops, runoff into surface water or leached into ground water, or even reside within the animal products we consume. Altering manure application techniques is beneficial for nutrient conservation but also potentially for reducing ARE spread. With our research, we compared manure application methods, manure application seasons, cropping systems, and manure-rainfall time gaps to find ways to balance the need for manure application and the spread of resistance. We used two field-scale rainfall simulation studies along with one laboratory study. Overall, using the injection method resulted in significantly lower concentrations of manure associated AREs entering surface runoff. When manure was surface applied and rainfall occurred 7 d after application, less resistant fecal coliform bacteria (FCB) entered surface runoff when compared to the 1 d time gap for broadcast methods. Within a day of manure application, antibiotic resistance gene (ARG) profiles in soil began to differ from each other and soil ARG totals in all manure applied soil increased compared to the background. Runoff from injection plots contained more soil ARGs and runoff from surface applied plots containing more manure associated ARGs. The subsurface injection method also caused highest antibiotic concentrations in the injection slit soil of those plots. High antibiotic concentrations in samples generally meant high concentrations of resistant FCB and ARGs, and resistant FCB were also found with their associated ARGs as well. A CRISPR-Cas12a assay for quantification of ARGs in environmental samples was just as precise as conventional methods. There is also potential for onsite detection. These combined results can hopefully help farmers improve manure management practices that mitigate spread of AREs to surrounding water, crops, and soil.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/102218 |
| Date | 03 February 2021 |
| Creators | Hiliare, Sheldon |
| Contributors | Crop and Soil Environmental Sciences, Xia, Kang, Gall, Heather, Williams, Mark A., Maguire, Rory O. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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