A new study led by David Tilman, Regents Professor of Ecology in the University of Minnesota's College of Biological Sciences, published a study showing that prairie grasses are more energy efficient that corn ethanol or soybean biodiesel and are better for the environment. The findings are published in the Dec. 8 issue of the journal Science. According to a University of Minnesota press release:
The study shows that mixtures of native perennial grasses and other flowering plants provide more usable energy per acre than corn grain ethanol or soybean biodiesel and are far better for the environment. Grass-based fuel can lead to a net decrease in atmospheric carbon dioxide, whereas ethanol and biodiesel increase it. Grass-based fuel can even lead to a net decrease in atmospheric carbon dioxide, whereas ethanol and biodiesel increase it.
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.
"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."
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 an earlier report, 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.
Tilman's team grew plots mixing 16 types of prairie grasses, including lupine, turkey foot, blazing star, and prairie clover. The plots with the most varieties produced the most biomass.
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.
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.
The process is carbon-negative--meaning the plants can store more carbon in their roots than they will create during their conversion to biofuels or electricity. Tilman says this negativity occurs because prairie grasses have complex root systems underground that typically make up two thirds of the plant total biomass, making them efficient carbon sinks: "When they grow, they have to [absorb] a lot of carbon to keep their roots alive and to make these very extensive root systems," Tilman speculates that these diverse systems may use more nitrate, "the limiting nutrient" in the soil, which starves bacteria that decomposes carbon, allowing the plants to better maintain their roots. All told, the Minnesota group estimates from its own soil sampling that, per acre, these grasslands could result in the sequestration of up to 1.8 tons of carbon dioxide per year.
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.