Short-term microbial dynamics and changes in greenhouse gas emissions in cattle dung treated with ivermectin

The activity of dung beetles on herbivore dung enhances nutrient cycling in grassland ecosystems, mitigates greenhouse gas (GHG) emissions, and shapes the composition of soil microbiota. However, mounting evidence indicates a widespread decline in dung beetle populations, largely driven by the extensive use of veterinary antiparasitic agents, particularly ivermectin. We quantified GHG emissions (CO 2 and CH 4 ) and evaluated the effects of ivermectin on prokaryotic assemblages in Mediterranean pastures through field experiments conducted under controlled conditions. Treatments either allowed or excluded dung beetle colonisation of faeces treated with ivermectin or untreated controls, while also examining temporal changes in the richness and taxonomic composition of dung-associated prokaryotic assemblages during the first 48 h following dung deposition. Our findings show that GHG emissions at least double in the absence of dung beetles, reinforcing the causal link between ivermectin use and increased CO 2 and CH 4 emissions. The bacterial phyla Firmicutes , Bacteroidota , and Proteobacteria dominated the dung microbiota, with ivermectin potentially favouring certain anaerobic and facultative taxa. Temporal analysis of microbial succession revealed a gradual coalescence between gut-derived and soil-native prokaryotic assemblages. Anaerobic taxa predominated at initial time points, while aerobic, soil-associated families became more prominent after 24 to 48 h. The anthropogenic introduction of bioactive compounds such as ivermectin into grassland ecosystems may disrupt key functional taxa, with potential consequences for plant cover, soil fauna, greenhouse gas emissions, and the long-term sustainability of livestock production. These findings highlight the need to promote guidelines limiting the routine use of such compounds, advocating instead for their strategic application only when strictly necessary. • Dung beetles play a key role in nutrient cycling, the mitigation of greenhouse gas emissions, and the structuring of soil microbial communities in grassland ecosystems. • Greenhouse gas emissions at least double in the absence of dung beetles, and the long-term use of ivermectin further increases these emissions. • Temporal analysis of microbial succession during the first 48 hours after dung deposition shows that certain prokaryotic taxa are favoured by the presence of ivermectin. • A gradual coalescence between gut-derived and soil-native prokaryotic assemblages occurs, with anaerobic taxa dominating at early time points, and aerobic, soil-associated families becoming more prominent after 24-48 hours.