Abstract
Agricultural antibiotics are pervasive, anthropogenic global change drivers, yet their ecological impacts at diffuse, sub-inhibitory concentrations remain poorly understood. While clinical resistance is well-documented, the systemic influence of these compounds on terrestrial biogeochemistry and multi-stressor interactions is largely unresolved. This dissertation utilizes a multi-scale approach, spanning controlled microcosms to fully crossed field experiments across contrasting forest and prairie soils, to investigate how veterinary antibiotics disrupt microbial physiology, soil food webs, and ecosystem functions.Controlled microcosm experiments revealed that low-concentration antibiotic exposure forces a fundamental physiological trade-off. While broad taxonomic composition and total microbial biomass remained stable due to dormancy, the active microbial fraction suffered a severe collapse in efficiency. This physiological burden was driven by the severe metabolic costs required to maintain antibiotic resistance genes (ARGs) and defense mechanisms. Field experiments further demonstrated that this metabolic tax actively severs the terrestrial carbon cycle. Utilizing stable isotope tracing, results showed that antibiotic exposure suppressed microbial biosynthesis, severely limiting the formation of stable mineral-associated organic matter (MAOM). Furthermore, the chemical suppression of bacterial guilds triggered a massive trophic cascade, altering fungi-to-bacteria ratios and fundamentally restructuring microarthropod grazer communities, including an explosion of fungivorous Oribatid mites and a collapse of shredding Entomobryid collembola in vulnerable prairie systems.
Finally, a multiple-stressor field experiment utilizing passive warming and altered precipitation revealed that antibiotics interact dynamically with climate change. While climate filtered broad community composition, concurrent warming thermodynamically desorbed legacy antibiotics, exacerbating the physiological collapse of the active microbiome. To survive this interacting stress, the microbial community consolidated into tightly interconnected network modules and actively abandoned the production of energetically expensive nitrogen-mining enzymes, effectively uncoupling the soil carbon and nitrogen cycles. Ultimately, this research demonstrates a fundamental decoupling of microbial taxonomy from physiological function under chemical stress. Agricultural antibiotics do not act in isolation; their physical chemistry interacts with climate to impose a severe metabolic tax on the active microbiome, fundamentally reorganizing soil biodiversity and long-term biogeochemical cycling.