The jockeying for the alternative energy source of the future is like a horse race with a staggered starting order. Corn got out to a big lead on the back of government subsidies, but it's been brushed back by reports that it actually generates more fossil fuel emissions during its transformation to ethanol than it avoids as a fuel. This realization caused soybeans and switchgrass to catch up. Suddenly, technological advances allowed waste products such as straw and corn stover to zoom into the picture. Now, a new class of biomass is making a dash: Jockeyed by a team of ecologists and economists at the University of Minnesota, highly diverse mixtures of low-maintenance prairie grasses have officially entered the contest. The group, reporting in tomorrow's issue of Science, says these diverse grassland species constitute a carbon-negative source of energy that could alleviate 19 percent of global electricity consumption and 13 percent of the world's petroleum consumption.
Ecologist David Tilman notes that in five to seven years, the source of biofuels such as ethanol or biodiesel will expand from just corn grains and soybeans to cellulose, a plant sugar known to be an ethanol-rich source. "When we turn that corner," Tilman says, "we want to have available the most efficient way of producing cellulose to be a biomass feedstock." Based on his work with crops grown over 10 years at Minnesota's Cedar Creek Natural History Area and on estimates in other scientific papers, Tilman calculates that nitrogen-poor, degraded land planted with a mixture of perennial prairie grasses--such as goldenrod, Indian grass, big blue stem and switchgrass--can actually provide up to 238 percent more bioenergy than the same land planted with only one species. (Switchgrass, when grown alone in the poor soil, returned only one third of the energy of a diverse plot.) In addition, when compared with ethanol from corn grown in fertile soil, he claims his plots can return 51 percent more energy per acre.
The kicker to Tilman's finding: his process is reportedly 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.
Bruce Dale, a biomass conservation researcher at Michigan State University, believes that although Tilman "provides an interesting perspective on the use of degraded lands," he makes a critical "apples-to-oranges comparison": pitting the production of ethanol from corn, an actual working industry, to ethanol production from grasses--currently hypothetical, due to economic constraints. If grasses do become viable sources for ethanol, he notes that the technology will also be applicable to the unused parts of corn, such as stalks. Tilman counters that although breaking down the stover will result in more ethanol, it will further shift the carbon balance of corn. "If you take the stover off of the corn field and use it for bioenergy, you're going to decrease the carbon storage in the soil," Tilman explains. "We're trying to find something the gives us energy at the same time that it gives us a reduced greenhouse gas emission benefit."
Alex Farrell, a professor in the Energy and Resources Group at the University of California, Berkeley, says that the entry of prairie grass into the alt-fuel marathon is exciting, but it's likely to take several years to determine how best to use this new resource--be it for electricity or biofuels. "Even if we were to find an environmentally appropriate way to use this resource, it will still be only part of any solution," he cautions. "It's not a silver bullet."