A field experiment was initiated whereby a series of replicated plots received annual applications of ivermectin, monensin, and zinc bacitracin, either singly or in a mixture. Pharmaceuticals were added at concentrations of 0.1mg/kg soil or 10mg/kg soil. The authors collected soil samples in 2013, before and after the fourth annual application of pharmaceuticals. In addition, a 30-d laboratory experiment was undertaken with the same soil and same pharmaceuticals, but at concentrations of 100mg/kg soil. The impact of the pharmaceuticals on nitrification rates, on the abundance of ammonia-oxidizing bacteria (AOB), and on the abundance of ammonia-oxidizing archaea (AOA) was assessed. None of the pharmaceuticals at 0.1mg/kg had any effect on nitrification. Referenced to control soil, nitrification was accelerated in soil exposed to 100mg/kg zinc bacitracin or 10mg/kg of the pharmaceutical mixture, but none of the treatments inhibited nitrification. Neither AOB abundance nor AOA abundance was affected by the pharmaceuticals at 0.1mg/kg. At 10mg/kg, monensin, zinc bacitracin, and a mixture of all 3 pharmaceuticals suppressed the abundance of AOB, and zinc bacitracin and the mixture increased AOA abundance. The decrease in AOB abundance and increase in AOA abundance when exposed to 10mg/kg soil suggests that AOB are more sensitive to these chemicals and that AOA populations can expand to occupy the partially vacated niche. Environ Toxicol Chem 2015;34:618-625. (c) 2014 Canadian Crown in right of Canada. Published 2014 SETAC
