18 THE SCIENTIST | the-scientist.com
NOTEBOOKScientist, December 2017.) “It contains
more carbon than any other soil on Earth
and twice as much carbon as is available
in the atmosphere.” Because permafrost
tends to maintain temperatures close to
freezing, even in summer, microorgan-
isms inhabiting this soil tend to be rela-
tively inactive, so carbon is used up more
slowly here than it is in warmer soils. But
as global temperatures rise, microbes
thaw and start to eat more, researchers
hypothesize, transforming carbon stored
in the soil into greenhouse gases such
as carbon dioxide and methane that can
then diffuse into the atmosphere.
In previous attempts to test this
hypothesis, scientists took soil samples
from the Arctic, warmed them in the
lab, and found increases in emitted CO 2.
However, these lab studies were limited
in their ability to simulate the complex
interactions of microbial communities,
so Northern Arizona University ecosys-
tem ecologist Te d Schuur and colleaguesdecided to set up a field experiment. In
the winter of 2008, Schuur and his col-
leagues erected the six fences in Alaska,
where they have been insulating the
same patches of permafrost over the last
decade of winters.
The team collected soil samples from
around each fence—both from behind
the fence, where the snow had accu-
mulated during the winter, and in front
of it, where the soil was uninsulated—
about a year and a half after the fences
were installed, and then again three
years later. The researchers analyzed the
microbial DNA extracted from the soil to
reveal the community’s range of lifestyles
and metabolic activity. After 4.5 years,
the average temperature of the insulated
soil was roughly 1 ̊C higher than that of
the uninsulated soil, and the composition
and activity of the microbial communi-
ties living there had changed rapidly as
a result, the researchers reported in July
(PNAS, 116:15096–105, 2019).
Specifically, the team found that the
abundance of species and genes associated
with carbon dioxide and methane produc-
tion was greater in the insulated soil thanin the uninsulated soil, implying that tun-
dra microbes may indeed release greater
amounts of greenhouse gases from the
soil as the climate warms. The research-
ers also found that the scale of the differ-
ences in microbial composition between
the uninsulated and insulated soil seemed
to accelerate over time. After roughly one
year of warming, microbial communities
in the insulated soil still resembled those
of the uninsulated soil, but after 4.5 years,
the contrasts were substantial.
“We were surprised by how quickly the
microorganisms responded to warming,”
says study coauthor Eric Johnston, who
worked on the project as a graduate stu-
dent at Georgia Tech and is now a post-
doctoral researcher at Oak Ridge National
Laboratory (ORNL) in Tennessee. Partic-
ularly noticeable was the increase in abun-
dance of methane-producing microbes
deep in the insulated soil. For example,
at the end of the study period, the team
found that soil that was between 45 and 55
centimeters under the surface in the insu-
lated plots contained around three times
as many microbes in the order Metha-nosarcinales as the uninsulated soil did.
Methane warms the atmosphere 30 times
more than carbon dioxide does over the
same period of time, so it’s a “somewhat
alarming” finding from a climate perspec-
tive, Johnston says.
Dave Graham, a microbial ecologist at
ORNL who was not involved in the study,
says he finds the changes in microbial
composition in insulated tundra com-
pelling because the result aligns with
data that he and others have collected
on permafrost microbes in the lab. “Cer-If old carbon that has been
locked up for hundreds or
thousands of years is being
respired, this will cause a
long-term increase in the
concentration of carbon
dioxide in the atmosphere.
—Eric Johnston, ORNLSNUG BUGS: Snow fences in Alaska help keep
patches of soil insulated as part of an experiment
on microbes’ emissions of greenhouse gases.MEGHAN TAYLOR