The widespread, routine use of antibiotics in livestock farming has generated fears of antibiotic-resistant “superbugs” that could threaten human health. Now, a new study suggests that threats of agricultural antibiotics extend beyond the realm of human health and into the environment, where they can alter microbial activity as they enter the soil through animal manure.

“The fact that there are so many ecosystem processes mediated by microbes makes this pretty interesting,” said Carl Wepking, a biologist at Colorado State University and lead author on the new paper.

Wepking and colleagues found that soil microbes consumed carbon less efficiently and released carbon dioxide more readily into the atmosphere when stressed by certain antibiotics. The scientists also saw shifts in the uptake of nitrogen, an important plant nutrient.

As the use of agricultural antibiotics continues to swell with human population growth, potentially increasing 70% from 2010 to 2070, the new findings could have implications when assessing long-term soil fertility and greenhouse gas emissions in agricultural fields. In the United States alone, livestock already receive an estimated 13 million kilograms of antibiotics every year.

And with genes associated with antibiotics able to travel widely through airborne particles, migrating birds, storm runoff, and other pathways, these environmental impacts may extend far beyond farm fields. “The fingerprint of human antibiotic usage is worldwide,” Wepking said.

Manure Matters

To study the ecological effects of animal antibiotics, Wepking’s team applied manure to a series of experimental plots on a pasture in Virginia. They collected manure from cows that had been treated with one of two different antibiotics: one that is “bactericidal,” which breaks bacteria apart, and another that is “bacteriostatic,” which halts bacterial protein growth. Both are routinely administered on dairy farms to prevent mastitis, an infection in or around a cow’s udders. The team also established a control plot with manure that didn’t contain any antibiotics.

After applying the manure to the plots at regular intervals over the course of 8 months, the team then conducted a series of experiments contained within plexiglass chambers to track the pathways of carbon and nitrogen entering and leaving the plants and soil, using isotopic tracers of those elements.

Researchers found an uptick in nitrogen accumulation in soil and plant material and a striking twofold spike in carbon released from soil treated with one of the antibiotics—a shift they attribute, in part, to a stress response within the bacteria. Bacteria “have to do their regular day-to-day activities plus deal with the threat of antibiotics,” said Wepking, explaining that microbes consume a certain amount of carbon to fuel their daily activities and that antibiotics add to those energy demands. “They suddenly have to work harder.”

Some bacteria use the extra fuel to produce an enzyme that can break down antibiotics, whereas other bacteria turn on an internal pump that expels antibiotics back out through the cell wall before they can do any damage.

The bactericidal antibiotic that broke apart bacteria also generated fresh organic material that other microbes could then break down and decompose, a process that may have also increased the release of carbon dioxide into the atmosphere.

Lost Potential

It’s common for farmers to spread manure on pastures as a fertilizer as a way to help build up soil carbon and promote plant growth. But these new findings suggest those benefits may become negated if the manure contains antibiotics.

“It’s pretty provocative,” said Serita Frey, an ecologist who studies microbial activity in soils at the University of New Hampshire in Durham but was not involved in this study. Frey hopes the new findings expand the conversation about agricultural antibiotic use beyond the realm of human health.

Frey was especially struck by the speed with which the effects arose. “For them to see these significant results in less than a year is striking,” she said, especially considering that many dairy operations add manure to fields for years or decades on end, she added.

Wepking’s team published their results in Ecology Letters and is now working to study the possible longer-term implications of their findings to determine whether microbial communities eventually adapt and bounce back to their status quo after continued exposure to antibiotics or the impacts become exacerbated over time. “We’re very interested to see what the long-term effect is going to be,” he said.

—Laura Poppick (@laurapoppick), Freelance Science Journalist