The veterinary parasiticide ivermectin increased the activity of an enzyme that mediates soil chitin degradation on a prairie grassland

Ivermectin is a common parasiticide used to control nematodes and arthropods affecting livestock. Treated animals faecally excrete at least 80 % of the drug with residues known to adversely affect dung-breeding insects. However, its effect on soil organisms beneath the dung, particularly soil microorganisms, has been little studied. We examined the effect of ivermectin on the soil microbiome and enzyme activities by comparing changes in the soil microbial community composition, diversity and enzyme activities in bare soil (Soil) to that in soil covered with dung of untreated cattle (DungC) or with dung to which ivermectin had been added (DungI). The study was conducted in two 16-week periods in 2016 and 2017. Treatment differences in soil microbial biomass C (MBC) and the activities of enzymes that mediate C, N, P and S cycling were evaluated in both years, and the prokaryotic and fungal microbiomes were characterized in 2017. Relative to Soil, DungC increased soil MBC and the relative abundances of the prokaryotic classes gamma-Proteobacteria and Bacteroidia. DungC also increased the relative abundance of the most predominant fungal class Sordariomycetes (phylum Ascomycota) while decreasing those of five other fungal classes. Relative to DungC, DungI did not affect MBC or the composition and diversity of the soil microbiome, but it did increase the activity of N-acetyl-8-glucosaminidase, an enzyme that mediates chitin degradation. Penicillium (phylum Ascomycota) was the only identifiable core fungal genus observed in at least 80 % of all the soil samples, and its relative abundance was positively correlated with the activity of acid phosphomonoesterase. Of the core prokaryotic genera, the relative abundances of the bacterium Candidatus_Udaeobacter and archaea Nitrososphaeraceae were also positively correlated with acid phosphomonoesterase activities. Therefore, ivermectin residues in dung probably boosted the nitrogen mineralization potential of the soil because the hydrolysis products of N-acetyl-8-glucosaminidase are amino sugars that are a major source of mineralizable nitrogen in soils.