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

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The authors initiated a field experiment 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 following the fourth annual application of pharmaceuticals. In addition, a (30‐day) 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, and on the abundance of ammonia‐oxidizing bacteria (AOB) and 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. AOB and AOA abundance were not affected by the pharmaceuticals at 0.1 mg/kg, but at 10 mg/kg, monensin, zinc bacitracin and a mixture of all three 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 upon exposure 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. This article is protected by copyright. All rights reserved

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