Globally, the use of antibiotics in livestock production is projected to more than double between 2010 and 2030. A proportion of antibiotics administered to livestock are ultimately excreted into the surrounding ecosystems, typically starting with soils. Therefore, in settings either where antibiotics are administered to livestock or where manure from these livestock is applied, soil microbes are on the frontlines of residual antibiotic exposure. Exposure to these antibiotics can alter microbial community structure and physiology with implications for ecosystem function. To explore this further we focused on dairy operations and two commonly used dairy antibiotics – pirlimycin (bacteriostatic) and cephapirin (bactericidal). Manure was collected from three groups of cattle: those administered cephapirin, those administered pirlimycin, and control cattle not administered antibiotics. This manure was then applied monthly to separate grassland plots. Following 8-months of manure additions, the microbial communities were then assessed via fungal-to-bacterial ratios, and the cycling of carbon (C) and nitrogen (N) was assessed via a stable isotope pulse-chase experiment.
Results/Conclusions
Broadly, antibiotics were observed to i) alter microbial communities as evidenced by increased fungal:bacterial ratios, particularly in the bacteriostatic treatment; ii) alter ecosystem function as seen through alterations in the cycling of C and N in both the plant and soil pools. Specifically, the bacteriostatic manure treatment caused an approximate 2-fold increase in respiration of recently fixed-13C, and increased 15N retention across terrestrial ecosystem N pools compared to sites amended with control manure. These findings suggest that exposure to manure from livestock administered antibiotics can affect the terrestrial system as a whole, leading to alterations in N cycling and decreased ecosystem C use efficiency. This research provides insights into an overlooked way land use and land management decisions impact soil microbial communities and the ecosystem functions they regulate, with global scale implications.