Abstract: Antibiotics are naturally occurring in soils as they are produced by both bacteria and fungi. These antibiotics are used to inhibit the growth of competitors as well as signaling compounds. Since the discovery of antibiotic properties from soil microbial excretions, millions of pounds of antibiotics a year are now used in livestock production. Ultimately, up to 90% of the antibiotics are excreted as un-metabolized active compounds into the environment far surpassing any naturally occurring antibiotics in the soil. They enter the soil from animals grazing on pasture, fertilizer, or as wind-borne particles. Antibiotics deposition may cause ecological consequences, such as the disruption of soil bacterial and fungal communities, affect soil extracellular enzyme activity, and altering microbial efficiency affecting ecosystem carbon/nitrogen cycling. Thus, antibiotic deposition is likely not insignificant and likely affect soil microbial communities and therefore, ecosystem function. We investigated the effects of antibiotics in a forest and prairie field experiment. We setup soil collars at both sites and administered common antibiotics that were either two types, natural or semisynthetic, and exhibited two functions, bactericidal or bacteriostatic. The antibiotic amendments mimic the concentrations in livestock production. We observed the effects on the soil carbon dynamics by monitoring soil respiration over time, quantifying soil organic carbon content and soil microbial biomass, and measuring soil extracellular enzyme activity. To assess soil microbial community composition, we conducted 16S and ITS Illumina sequencing analyses and used qPCR to determine the fungi-to-bacteria ratios.
Antibiotics affected both prairie and forest sites by increased mass-specific CO2 respiration (p < 0.01) and decreasing microbial biomass carbon (p < 0.05) relative to the control. This suggests microbial efficiency is reduced, which may lower soil carbon retention. In addition, antibiotics did not affect total soil enzyme activity, but did affect mass-specific enzyme activity (p < 0.05), with semisynthetic antibiotics increasing β-Glucosidase (BG), acid phosphatase (AP) and N-Acetyl glutamate synthase (NAG). However, antibiotics did not affect soil community composition. These results suggest that the increased antibiotic exposure stresses the microbial community and lowers their efficiency, ultimately affecting soil carbon dynamics and nutrient cycling. Under this study, semisynthetic antibiotics exhibited a greater effect than natural antibiotics. Bactericidal and bacteriostatic antibiotic did not significantly show differences in their affect. In all cases, antibiotic inputs had a greater effect on the prairie than the forest. These results show that antibiotic deposition on terrestrial ecosystems is potentially important, which leaves more to be studied.
