Composting dairy cattle manure significantly reduces antibiotic-resistant bacteria with no biochar impact.

In two randomized controlled field studies, we investigated the effect of biochar-amended dairy manure composting on the concentrations and prevalence of generic and antibiotic-resistant IMPORTANCE: Composting is a microbially mediated decomposition of organic matter into a nutrient-rich product for use as fertilizer. Biochar, obtained from the pyrolysis of biomass including animal manure, has been used for soil amendment. Beyond its nutrient stabilizing potential, composting has been shown to reduce bacterial load in animal manure. However, its effect with or without a biochar addition was not evaluated under a randomized controlled field trial. Biochar was nearly bacteriologically "sterile," and the sawdust used as a bulking agent had low concentrations of innocuous bacteria indicating that dairy manure was the main source of bacteria to the pre-compost mixture. Biochar did not have an extra effect on the bacterial load beyond composting alone. Composting during spring/summer is more effective than fall/winter. Furthermore, it is more effective against gram-negatives than gram-positive bacteria. Importantly, composting effectively eliminated multidrug-resistant bacteria of critical importance to public health for safer use of dairy manure.