Since antibiotic drugs were first used in farm animals in the mid-1940s, a debate has raged about the prudence of this practice. A study published last December in Ecology Letters adds a new wrinkle: Farmers often use manure to build up soil carbon and increase nutrient availability for plants, but the study showed that dung from dairy cows given two types of routine antibiotics also altered the composition of soil bacteria and fungi. These shifts affected how plants “fixed” carbon dioxide from the atmosphere to convert into organic matter—a process that figures into strategies for climate change mitigation.