Impacts of Antibiotics in Manure on Soil Nitrogen Cycling

Circular agriculture aims to close elemental cycles, which increases reliance on animal manure to provide nutrients. However, veterinary antibiotics are widely used in the livestock industry, and antibiotics may enter soil through manure application. Antibiotic effects on soil microbial functions related to nutrient cycling and climate regulation are still poorly understood. We hypothesized that antibiotics in manure affect soil nitrogen (N) cycling processes, such as nitrification, nitrous oxide (N2O) emissions, and plant N uptake. We tested these hypotheses with incubation experiments including different drug and soil types, and a greenhouse experiment with grass and clover. In the incubation experiments, we quantified soil N2O fluxes and mineral N pools over time after applying manure with and without antibiotics. In the greenhouse experiment, we harvested and analyzed the aboveground and belowground biomass of grass and clover after eight weeks of manure (with and without antibiotics) application. In the incubation experiments, antibiotic sulfadiazine significantly reduced cumulative N2O emissions compared to manure without antibiotics. In addition, sulfadiazine in manure substantially lowered the soil nitrification rate compared to antibiotic-free manure. In the greenhouse experiment, the belowground biomass of clover was significantly decreased by manure with sulfadiazine or oxytetracycline. Our studies suggest that antibiotics in manure alter soil N cycling by attenuating N transformations and impairing plant root growth. Therefore, antibiotic residues in livestock manure should be removed before fertilization to avoid its perturbation on soil nutrient cycling.