Biofuel from a mix of native prairie grass would yield
more energy than ethanol from corn or switchgrass and
would do more to combat global warming. A recent study
in the journal Science found that a diverse mixture of
grasses can produce more raw material than a
single-species planting of, for example, switchgrass.
The University of Minnesota research compared several
such plantings on poor-quality soil without
If turned into ethanol, the grass mixture would
theoretically yield a higher ratio of energy output to
input than corn-based ethanol. With improved
technology, it also could slow global warming by
capturing carbon dioxide.
"This is what really surprised us," said ecologist
David Tilman, who led the study. "There is more carbon
stored year after year in the soil than the total
amount of fossil carbon that is produced into the
There still are numerous hurdles to bringing the
technology to market, and some experts say it might
not be practical.
But if successful, the authors say it could be part of
a solution to the nation's energy problems.
In last year's State of the Union address, President
Bush called for a program to make fuel from cellulose,
the fibrous structural material found in all plants.
One feedstock he mentioned by name was switchgrass.
But during the past couple decades, ecologists have
found convincing evidence that diverse mixtures of
species are usually more productive than one species
can be on its own. Tilman is a pioneer in
demonstrating this positive effect of biodiversity in
To study energy production, Tilman and his colleagues
used data from these biodiversity experiments -
performed at the Cedar Creek Natural History Area,
north of Minneapolis. They also gathered data on
carbon storage from old soil samples.
The results show that diverse mixtures average more
than triple the productivity of grass monocultures.
Growing these mixtures for several years also removes
about 1,750 pounds of carbon dioxide from the
atmosphere per acre.
Planting, harvesting and transporting the material
only releases about 400 pounds of carbon dioxide per
year, Tilman said.
Carbon dioxide is the main greenhouse gas implicated
in global climate change.
William Gibbons, a South Dakota State University
microbiologist, said diverse grass mixtures "are the
climax plant community of the Great Plains. Thus,
native grasses would be the most productive biomass
crop for us to produce."
But practical obstacles remain.
Stands of native grass are common in South Dakota,
often as part of the Conservation Reserve Program for
former cropland. Some of that is a mixture of species,
but some landowners say stands of pure switchgrass are
better pheasant habitat.
"No doubt, switchgrass, that's No. 1, and I think if
they started something with ethanol with this
switchgrass, it'd be a great deal," said Bob Farrell
of Mitchell, an avid hunter who has planted grass on
his land near White Lake.
Tilman says it probably would not make economic sense
to transport grass more than 50 miles from the field
to the processing plant.
And once it gets to the plant, the mix of grasses
could present problems, said Burton English, an
agricultural economist at the University of Tennessee.
"It depends on the conversion technology," he said.
"If you're doing a fermentation system, then you may
have some difficulties. You may have a less efficient
plant when you take multiple feedstocks versus a
single monoculture feedstock."