Effect of Manure Application on Abundance of Antibiotic Resistance Genes and Their Attenuation Rates in Soil: Field-Scale Mass Balance Approach

The development of models for understanding antibiotic resistance gene (ARG) persistence and transport is a critical next step toward informing mitigation strategies to prevent the spread of antibiotic resistance in the environment. A field study was performed that used a mass balance approach to gain insight into the transport and dissipation of ARGs following land application of manure. Soil from a small drainage plot including a manure application site, an unmanured control site, and an adjacent stream and buffer zone were sampled for ARGs and metals before and after application of dairy manure slurry and a dry stack mixture of equine, bovine, and ovine manure. Results of mass balance suggest growth of bacterial hosts containing ARGs and/or horizontal gene transfer immediately following slurry application with respect to <i>ermF</i>, <i>sul1</i>, and <i>sul2</i> and following a lag (13 days) for dry-stack-amended soils. Generally no effects on <i>tet</i>(G), <i>tet</i>(O), or <i>tet</i>(W) soil concentrations were observed despite the presence of these genes in applied manure. Dissipation rates were fastest for <i>ermF</i> in slurry-treated soils (logarithmic decay coefficient of −3.5) and for <i>sul1</i> and <i>sul2</i> in dry-stack-amended soils (logarithmic decay coefficients of −0.54 and −0.48, respectively), and evidence for surface and subsurface transport was not observed. Results provide a mass balance approach for tracking ARG fate and insights to inform modeling and limiting the transport of manure-borne ARGs to neighboring surface water