Millions of pounds of antibiotics are produced annually in the U.S. alone. Over 80% are used solely for livestock production. Ultimately, up to 90% are excreted as un-metabolized active compounds into the environment far surpassing any naturally occurring antibiotics in the soil. These antibiotics are not just localized to their source but can be transported by wind infiltrating surrounding ecosystems. Antibiotic deposition may cause ecological consequences, such as the disruption of soil bacterial and fungal communities, increased antibiotic resistance, and altering microbial efficiency affecting ecosystem carbon/nitrogen cycling. Thus, wind-borne antibiotic deposition to ecosystems is likely not insignificant and likely affect soil microbial communities and therefore, ecosystem function. We investigated the effects of wind-borne antibiotics on forest and prairie soils using a laboratory microcosm experiment. Each microcosm received one of the four common antibiotics (penicillin, cephapirin, tetracycline, oxytetracycline) used in livestock production at dosages that mimic wind-borne concentrations. To observe the effects on the soil carbon dynamics, we tracked a 13C stable isotope in the soil respiration, soil organic carbon, and microbial biomass. To assess soil microbial community composition, we conducted 16S and ITS Illumina sequencing analyses and used qPCR to determine the abundance of antibiotic resistance genes and fungi-to-bacteria ratios.
Antibiotics affected soil microbial community function, but not their composition. There were no differences in fungi and bacteria community compositions between treatments and the control (respectively, p=0.248; p=0.67). However, antibiotics increased total CO2 respiration in both prairie and forest soils relative to the control (respectively, p=0.0001; p=0.002). Metabolic stress (qCO2), calculated as mass-specific respiration, increased in antibiotic treated prairie and forest soils relative to the control (respectively, p=0.0003; p=0.009). This is an indicator for microbial stress, and an increase in qCO2 suggests microbial efficiency is reduced, which may lower soil carbon retention. In accordance to increased respiration with antibiotics, there was an increase in antibiotic resistance gene abundance (tetW and bla-1 genes). Microbes that need to maintain an active resistance will have a higher metabolic cost that can hinder their efficiency to cycle carbon due to increased respiration. These results suggest that the increased antibiotic exposure stresses the microbial community and lowers their carbon use efficiency, ultimately lowering soil carbon retention. Under this study, wind-borne antibiotics did not impose community changes. The antibiotic concentrations transported by the wind may not be strong enough to affect the community composition. However, the low antibiotic concentrations was enough to affect their function.