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Feature

David Tilman with portrait of Ray Lindeman

A study led by Regents Professor of Ecology David Tilman shows how the diverse mixtures of prairie grasses that greeted the pioneers are an untapped source of renewable, greenhouse gas-lowering energy.

Prairie grasses emerge as rich energy source

Mixtures of grasses make best source of biofuel

By Deane Morrison

Dec. 8, 2006

With shrinking glaciers and other signs of global warming upon us, the search is on for alternative fuels to stem the release of greenhouse gases such as carbon dioxide into the atmosphere. This week a new contender burst on the scene: diverse mixtures of native prairie grasses. A University team led by David Tilman, Regents Professor of Ecology, found that these grasses yield more net energy than either ethanol from corn or "biodiesel" fuel from soybeans. Grass-based fuel can even lead to a net decrease in atmospheric carbon dioxide, whereas ethanol and biodiesel increase it. The study is based on 10 years of work at the University's Cedar Creek Natural History Area. Written by Tilman, postdoctoral researcher Jason Hill and research associate Clarence Lehman, it is the cover story in the Dec. 8 issue of the journal Science. For many years, renewable fuels from plants ("biofuels") have been seen as beacons of hope because the carbon dioxide released in burning them can be absorbed by the next year's crop. But in a report earlier this year, Tilman, Hill and others showed that corn grain ethanol and soy biodiesel do little to offset carbon dioxide emissions because it takes so much fossil fuel to produce them. The new work demonstrates that it's not monoculture crops like corn, soybeans or even switchgrass, but rather the "sea of grass" that fell to the plow in the 19th century that harbors a bright hope for the 21st. Mixtures of native perennial grasses and other flowering plants require little energy or fertilizer to turn into fuel, yield up to 238 percent more usable energy per acre than any single species and can even lower atmospheric carbon dioxide by storing it in their roots or in soil. "Biofuels made from high-diversity mixtures of prairie plants can reduce global warming by removing carbon dioxide from the atmosphere," says Tilman. "Even when grown on infertile soils, they can provide a substantial portion of global energy needs, and leave fertile land for food production." According to the research, biofuels from mixed prairie grasses could replace about 13 percent of global petroleum consumption for transportation and 19 percent of global electricity consumption. This could eliminate 15 percent of current global carbon dioxide emissions.

The beauty of mixed prairie grasses, say the researchers, is that, unlike corn, they can grow in old farmland or in marginal, degraded lands with little or no application of water or fertilizers. The challenge is finding enough such land.

A glance at the net energy yield from corn grain ethanol, biodiesel from soy oil and biomass from diverse prairie grasses shows why the grasses give a bigger bang for the buck than the other two renewable fuels and why petroleum is so hard to beat. In their previous study, Tilman and his colleagues calculated the energy outputs of ethanol and soy biodiesel and compared those numbers to the inputs of energy-mostly from fossil fuels-necessary to produce them. An input of 100 units of energy will yield 125 units from ethanol and 193 from biodiesel. In the new paper, Tilman, Hill and Lehman calculated that mixed prairie grasses, if converted to synthetic fuels by the right means, would yield 809 units. According to a Nov. 29, 2006 New York Times article, the yield for petroleum is now calculated at about 1,500.

Cleaner, more efficient

"Sixty percent of the energy input in making ethanol is fossil fuel-driven distillation," says Tilman. Although the increase in greenhouse gas emissions for ethanol is less than for gasoline, it's only 15 percent less. In contrast, the researchers estimate that mixed prairie grasses won't increase greenhouse gas emissions--mostly carbon dioxide--at all, and may even be "carbon negative" because they will store carbon. This storage could go on for as long as 100 years. Removal could range from 1.2 to 1.8 U.S. tons of carbon dioxide per acre per year.

The beauty of mixed prairie grasses, say the researchers, is that, unlike corn, they can grow in old farmland or in marginal, degraded lands with little or no application of water or fertilizers. The challenge is finding enough such land. "Minnesota has 1.5 million acres in the Conservation Reserve Program, and there are 30 million acres in the United States," says Tilman. Unfortunately, while CRP pays farmers to manage land to benefit the environment, it doesn't allow them to sell prairie grasses raised on this land for use as renewable energy. But the U.S. Farm Bill could be revised to permit this, Tilman says. Looking at the stiff competition from fossil fuels, Tilman sees a clear need for multiple strategies, including the optimal use of biomass, to stabilize or lower carbon dioxide emissions. Some scientists have proposed that eight strategies, if put into practice together, could do the job. "There are eight 'wedge' technologies that, if implemented, could stop further increases in carbon dioxide and prevent a runaway greenhouse effect," Tilman explains. Globally, about 1.25 billion acres planted with mixed prairie grasses or their equivalent in other ecosystems could supply about 15 percent of global energy needs for transportation and electricity, he says. But conservation of energy remains essential. "At the present time, we don't know of a mixture of technologies that would [stabilize greenhouse gas emissions] without a drastic cut in demand," Tilman says. "To decrease per capita consumption would mean walking a lot more and redesigning communities."