Champaign, Illinois
December 2, 2008
Converting forests or fields to
biofuel crops can increase or decrease greenhouse gas emissions,
depending on where – and which – biofuel crops are used,
University of Illinois
researchers report this month.
The researchers analyzed data
from dozens of studies to determine how planting new biofuel
crops can influence the carbon content of the soil. Their
findings appear this month in the journal
Global Change
Biology Bioenergy.
Plants use the sun's energy to
convert carbon dioxide from the atmosphere into the organic
carbon that makes up leaves, stems and other plant parts. As
plants decay, this carbon goes into the soil. Organic carbon is
an important component of soil health and also influences
atmospheric carbon dioxide levels. Whenever the soil is
disturbed, as occurs when land is plowed or cleared of
vegetation, some of this carbon returns to the atmosphere in the
form of carbon dioxide.
"From the time that John Deere
invented the steel plow, which made it possible to break the
prairie sod and begin farming this part of the world, the
application of row crop agriculture to the Midwest has caused a
reduction of soil carbon of about 50 percent," said Evan
DeLucia, a professor of plant biology at Illinois and
corresponding author on the new study.
Any debate on the environmental
consequences of using plants to produce liquid fuels should also
consider how each option affects soil carbon, DeLucia said.
"The biggest terrestrial pool
of carbon is in the soil. The top meter of soil holds more than
three times the amount of carbon stored in either vegetation or
the atmosphere, so if you do little things to change the amount
of carbon in the soil it has a huge impact on the atmosphere and
thus global warming."
Unlike corn, which must be
replanted every year, perennial grasses such as switchgrass and
Miscanthus preserve and increase carbon stores in the soil.
These and other grasses have been proposed as high-energy
alternative feedstocks for biofuel production.
Currently, ethanol is produced
by fermenting the starch in corn kernels, but significantly more
liquid fuel energy can be harvested from the stems and leaves of
plants. The technology for producing this "cellulosic" ethanol
is still quite expensive, but many believe that it will displace
corn ethanol as the technology advances.
About 20 percent of the corn
crop currently goes into ethanol production in the U.S., DeLucia
said, "so we began with the hypothesis that it might be good for
soil carbon to put a perennial biofuel crop on the landscape
instead of corn."
The researchers analyzed
published estimates of changes in soil organic carbon in
landscapes converted from natural or agricultural land to
biofuel crops. They focused on corn, sugar cane, Miscanthus,
switchgrass and native prairie grasses. They also evaluated the
impact of harvesting and using corn stover (the plant debris
left over after corn is harvested) as a cellulosic biofuel
source.
Their analysis showed that
converting native land (grassland or forest) to sugarcane
dramatically reduced soil carbon, creating a carbon deficit that
would take decades to repay. While perennial grasses add carbon
to the soil each year, DeLucia said, it could take up to a
century for the sugar cane to rebuild soil carbon to former
levels on native land.
Harvesting the corn residue for
cellulosic ethanol production also reduced the carbon in the
soil. The more plant residue was removed, the more the soil
carbon declined.
Planting perennial grasses on
existing agricultural lands had the most beneficial effect on
soil carbon, the researchers found. Although there was an
initial drop in carbon as fields were converted from corn to
Miscanthus, switchgrass or native perennial grasses, the loss
was fairly quickly offset by yearly gains in soil carbon as the
grasses became established.
"Consistent with our
hypothesis, the perennial feedstocks like Miscanthus and
switchgrass start building soil carbon very, very early on,"
DeLucia said. "From a purely carbon perspective, our research
indicates that putting perennial biofuel crops on landscapes
that are dominated by annual row crops will have a positive
effect on soil carbon."
The finding "seems to walk you
right into the food for fuel debate," DeLucia said, referring to
the controversy over using agricultural land for fuel
production. But because the U.S. is already devoting about 20
percent of its corn crop to ethanol production, he said, it
would make sense to eventually use that land to produce a much
higher yielding biofuel feedstock that has the added benefit of
increasing organic carbon in the soil.
DeLucia and his colleagues
will present their findings this month at the 2008 Fall Meeting
of the American Geophysical Union.
Evan DeLucia is also an affiliate of the Institute for Genomic
Biology and the Energy Biosciences Institute at the University
of Illinois. |
