Multi-year and short-term responses of soil ammonia-oxidizing prokaryotes to zinc bacitracin, monensin, and ivermectin, singly or in combination

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 0.1 mg/kg soil or 10 mg/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 100 mg/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 0.1 mg/kg had any effect on nitrification. Referenced to control soil, nitrification was accelerated in soil exposed to 100 mg/kg zinc bacitracin or 10 mg/kg of the pharmaceutical mixture, but none of the treatments inhibited nitrification. Neither AOB abundance nor AOA abundance was affected by the pharmaceuticals at 0.1 mg/kg. At 10 mg/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 10 mg/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. © 2014 Canadian Crown in right of Canada. Published 2014 SETAC