Implications of wind-borne agricultural antibiotics on soil microbial communities and ecosystem processes

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Dan V. Du and Michael S Strickland, Soil and Water Systems, University of Idaho, Moscow, ID, Jane Lucas, Department of Soil & Water Systems, University of Idaho, Moscow, ID

Background/Question/Methods
Antibiotics are naturally occurring in soils as they are produced by both bacteria and fungi. A plethora of antibiotics have been discovered from microbial excretions in the soil and many of these antibiotics, millions of pounds a year, are now used in livestock production. Antibiotics provides many great benefits to the livestock, but ultimately these antibiotics enter the environment as un-metabolized active compounds far surpassing naturally occurring antibiotics. These antibiotic residues are not just localized to their source but can be transported by wind via dust infiltrating surrounding ecosystems. The wind-borne antibiotic deposition have the potential to disrupt soil microbial communities, increase antibiotic resistance in these communities, and alter microbial efficiency leading to lower ecosystem carbon/nitrogen. To investigate the effects of wind-borne antibiotics, a laboratory microcosm experiment was conducted to examine their influence on prairie and forest soils that vary in soil community compositions. The microcosms were treated with four commonly used antibiotics in agriculture (penicillin, cephapirin, tetracycline, oxytetracycline) and a water as a control. These antibiotics are a mixture of semi-synthetic and natural antibiotics and have either bactericidal and bacteriostatic modes of action. A 13C stable isotope tracer was used to observe the influence of antibiotics on the soil carbon dynamics, where we tracked it in the soil respiration/13CO2, soil organic matter, and microbial biomass. To assess soil microbial community composition, we conducted 16S and ITS Illumina sequencing analyses. The abundance of antibiotic resistance genes (ARGs) was determine using qPCR.
Results/Conclusions
The antibiotic inputs did not significantly change the soil microbial community composition at the phylum level, which may indicate that wind-borne concentrations may not be strong enough affect it. However, there were differences in the soil respiration between treatments, where the soils that were treated with antibiotics exhibited a relative greater amount of CO2 being respired than the control. Furthermore, there were differences in the ARGs abundance between soil samples. These results suggest that the increased antibiotic exposure may stress the microbial community. This can lower their efficiency due to having a higher metabolic cost from maintaining ARGs and producing ARG products, ultimately reduced efficiency may decrease soil carbon retention. Under this study, windborne antibiotics did not impose community changes, but may likely affect their function instead. Antibiotic deposition on terrestrial ecosystems is potentially important, which may impact our ecosystems.