The Effects of dairy cattle antibiotics on soil microbial community cycling and antibiotic resistance
Antibiotic use in agricultural ecosystems has the potential to increase resistance to antibiotics in soil microbial communities since 40-95% of an antibiotic dose administered to livestock is excreted intact or as metabolites. Exposure to antibiotics is also known to alter microbial community composition, biomass, and physiology, but the potential influences of antibiotic residues on the essential ecosystem processes that microbes regulate, e.g., carbon and nitrogen cycling are not well understood. I investigated the effects of antibiotic residues associated with dairy cattle operations on soil microbial communities and the ecosystem processes they regulate. I examined the effects of antibiotic exposure on the biogeochemical functioning of soil microbial communities by measuring the activity of extracellular enzymes associated with organic matter processing and nutrient mineralization in soils collected from dairy cattle operations across the United States. At each experimental station paired sites were identified by local managers that represented sites with high and low stocking rates of dairy cows who had been treated prophylactically with antibiotics to prevent mastitis. Responses varied among individual enzymes, but I found an overall significant decrease in total hydrolytic enzyme activity under high cattle stocking rates indicating a change in the functioning of the microbial community in soils exposed to antibiotic laden manure. Principle components analysis suggest that while some of the variation in enzyme activities are associated with the abundance of antibiotic resistance genes, soil organic matter (total organic, mineralizable, and particulate organic carbon) was the most significant variable accounting for differences in enzyme activities. This reflects an inherent challenge in studies of antibiotic exposure in agricultural landscapes: the difficulty of distinguishing direct effects of antibiotic residues from the organic matter and nutrient subsidy associated with manure applications. To address this concern I conducted a series of incubation experiments manipulating soils to isolate the influences of antibiotics, manure resource subsidies, and bovine microbiome inoculants into soils. Specifically, I examined soil respiration and antibiotic resistance gene counts using qPCR following treatment with cephapirin, pirilimycin and a positive and negative control. I found that pre-exposure to antibiotics and manure is important in modulating the response of microbial communities (soil respiration, and gene copy numbers of AmpC and TetO) to further antibiotic exposure. I conclude that antibiotics themselves have a direct effect on soil communities and their functioning that is additive to the effect of manure (i.e., as a resource subsidy). This effect is mediated by the history of previous exposure to antibiotics, i.e., cattle stocking density. These results suggest that antibiotic residues from dairy cattle operation may have significant effects on microbial communities and the biogeochemical cycling they regulate in agricultural ecosystems.