The broiler ( Gallus gallus domesticus ) industry in the United States and several other countries routinely includes subtherapeutic levels of antibiotics such as roxarsone, virginiamycin, and bacitracin in the feed to improve bird growth yields. Large fractions of the antibiotics fed to the birds are excreted in manure (litter), which is often applied to soils to improve fertility. Some concerns with this practice are antibiotic-induced alterations in microbially-mediated nutrient cycling, which could influence plant productivity and environmental quality. To investigate this possibility, a series of lab experiments were conducted to determine the effects of increasing levels of the three livestock antibiotics on nitrification, denitrification, and microbial community composition (fatty acid methyl ester profiles) of soils collected along a catena. Roxarsone and virginiamycin significantly influenced microbial community composition and inhibited nitrification in the soils, but only at levels that were several-fold higher than expected in poultry litter-applied soils. Bacitracin did not affect microbial growth, microbial community composition, or nitrification at any concentration tested (up to 500 mg · kg -1 ). None of the antibiotics influenced denitrification at environmentally-relevant concentrations. Amounts of antibiotics in soil solution were greatly reduced by sorption, which followed Freundlich models in the concentration range of 1 - 500 mg · L -1 . Results from this study indicated that addition of roxarsone, virginiamycin, or bacitracin to these soils at environmentally-relevant concentrations would not likely impact microbial community composition, nitrification or denitrification due to intrinsic resistance/insensitivity of microorganisms to these antibiotics and reductions in the bioavailable amounts due to sorption by soil surfaces.
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