March 2020, ScientificAmerican.com 21GETTY IMAGESE C O L O G YManure
Problems
Antibiotic use in cows
alters carbon cyclingSince antibiotic drugs were first used in
farm animals in the mid-1940s, a debate
has raged about the prudence of this prac-
tice. A study published last December in
Ecology Letters adds a new wrinkle: Farm-
ers often use manure to build up soil car-
bon 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 affect-
ed how plants “fixed” carbon dioxide from
the atmosphere to convert into organic
matter—a process that figures into strate-
gies for climate change mitigation.
Carl Wepking, now at Colorado State
University, led the experiments as a gradu-
ate student at Virginia Tech. Every month
he hauled trash bags of cow manure to a
grassy field and sprinkled 648 grams per
square meter of three manure types onto
three plots. Several months into the exper-
iment, he covered the plots with Plexiglas
chambers for seven days and pumped in
carbon dioxide labeled with a specific car-
bon isotope for tracking. In the control
plot, Wepking says, the manure from
untreated cows had an overwhelmingly
positive effect, boosting plant growth and
retaining newly photosynthesized carbon
in the plants and soil microorganisms. Butin the plots with manure from antibiotic-
treated animals, more carbon was released
again as carbon dioxide—roughly twice as
much for one of the antibiotics. “Whether
or not you give cows an antibiotic changes
how carbon moves within the plants them-
selves,” he says, “which is wild.”
Soil stores about twice as much carbon
as the atmosphere does, and increasing that
storage could help address climate change.
Francesca Cotrufo, a soil ecologist at Colo-
rado State, who was not involved in the
study, says climate and carbon-sequestra-
tion models increasingly account for the role
plant compounds play in how efficiently
microbes store carbon in the soil. Although
the manure study does not account for car-
bon already stored, she adds, investigating
antibiotics’ effects on more recently fixed
carbon is a “novel and interesting angle”
that “definitely should receive attention.”
Wepking suggests that because two
different antibiotics (with different mecha-
nisms of action) both reduced carbon-use
efficiency, administering this category
of drug to cows could potentially negate
manure’s climate benefits. “What we’ve
shown so far is that the positive effects
of adding manure to the soil aren’t as
positive as they looked, if your manure is
coming from cattle that have been given
antibiotics,” he says—although “it’s still
kind of hard to tell” whether medicating
livestock neutralizes or negates any net
carbon- capture benefits of manure fertiliz-
ers. But it is critical to find out, he adds:
U.S. livestock may contribute up to 13 mil-
lion kilograms of antibiotics to the environ-
ment every year, and that figure is expect-
ed to increase. — Peter Andrey SmithAntibiotics from cows may
hinder plants’ carbon fixing.© 2020 Scientific AmericanGETTY IMAGESE C O L O G YManure
Problems
Antibiotic use in cows
alters carbon cyclingSince antibiotic drugs were first used in
farm animals in the mid-1940s, a debate
has raged about the prudence of this prac-
tice. A study published last December in
Ecology Letters adds a new wrinkle: Farm-
ers often use manure to build up soil car-
bon 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 affect-
ed how plants “fixed” carbon dioxide from
the atmosphere to convert into organic
matter—a process that figures into strate-
gies for climate change mitigation.
Carl Wepking, now at Colorado State
University, led the experiments as a gradu-
ate student at Virginia Tech. Every month
he hauled trash bags of cow manure to a
grassy field and sprinkled 648 grams per
square meter of three manure types onto
three plots. Several months into the exper-
iment, he covered the plots with Plexiglas
chambers for seven days and pumped in
carbon dioxide labeled with a specific car-
bon isotope for tracking. In the control
plot, Wepking says, the manure from
untreated cows had an overwhelmingly
positive effect, boosting plant growth and
retaining newly photosynthesized carbon
in the plants and soil microorganisms. Butin the plots with manure from antibiotic-
treated animals, more carbon was released
again as carbon dioxide—roughly twice as
much for one of the antibiotics. “Whether
or not you give cows an antibiotic changes
how carbon moves within the plants them-
selves,” he says, “which is wild.”
Soil stores about twice as much carbon
as the atmosphere does, and increasing that
storage could help address climate change.
Francesca Cotrufo, a soil ecologist at Colo-
rado State, who was not involved in the
study, says climate and carbon-sequestra-
tion models increasingly account for the role
plant compounds play in how efficiently
microbes store carbon in the soil. Although
the manure study does not account for car-
bon already stored, she adds, investigating
antibiotics’ effects on more recently fixed
carbon is a “novel and interesting angle”
that “definitely should receive attention.”
Wepking suggests that because two
different antibiotics (with different mecha-
nisms of action) both reduced carbon-use
efficiency, administering this category
of drug to cows could potentially negate
manure’s climate benefits. “What we’ve
shown so far is that the positive effects
of adding manure to the soil aren’t as
positive as they looked, if your manure is
coming from cattle that have been given
antibiotics,” he says—although “it’s still
kind of hard to tell” whether medicating
livestock neutralizes or negates any net
carbon- capture benefits of manure fertiliz-
ers. But it is critical to find out, he adds:
U.S. livestock may contribute up to 13 mil -
lion kilograms of antibiotics to the environ-
ment every year, and that figure is expect-
ed to increase. — Peter Andrey SmithAntibiotics from cows may
hinder plants’ carbon fixing.sad0320Adva3p.indd 21 1/22/20 4:57 PM
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