Fuels made from prairie biomass reduce carbon dioxide levels in the atmosphere, according to a new study. Mixtures of native prairie plant species have emerged as a leader in the quest to identify the best source of biomass for producing sustainable, bio-based fuel to replace petroleum.
The study led by David Tilman, an ecologist at the University of Minnesota, found that mixtures of native perennial grasses and other flowering plants provide more usable energy per acre than corn grain ethanol or soybean biodiesel. These grasses are also far better for the environment. The research was supported by the National Science Foundation and the University of Minnesota Initiative for Renewable Energy and the Environment.
"Biofuels made from high-diversity mixtures of prairie plants can reduce global warming by removing carbon dioxide from the atmosphere. Even when grown on infertile soils, they can provide a substantial portion of global energy needs, and leave fertile land for food production," Tilman said.
The study is based on 10 years of research at Minnesota's Cedar Creek Natural History Area, one of 26 National Science Foundation long-term ecological research sites.
It shows that degraded agricultural land planted with diverse mixtures of prairie grasses and other flowering plants produces 238 percent more bioenergy on average than the same land planted with various single prairie plant species, including switchgrass.
Tilman and his team estimate that fuel made from this prairie biomass would yield 51 percent more energy per acre than ethanol from corn grown on fertile land. Prairie plants require little energy to grow and all parts of the plant above ground are usable.
According to the study, fuels made from prairie biomass are "carbon negative," which means that producing and using them actually reduces the amount of carbon dioxide in the atmosphere. Using prairie biomass to make fuel would lead to the long-term removal and storage of from 1.2 to 1.8 U.S. tons of carbon dioxide per acre per year. This net removal of atmospheric carbon dioxide could continue for about 100 years, the researchers estimate.
In contrast, corn ethanol and soybean biodiesel are "carbon positive," meaning they add carbon dioxide to the atmosphere, although less than traditional fossil fuels.
Switchgrass, which is being developed as a perennial bioenergy crop, was one of 16 species in the study. When grown by itself in poor soil, it did not perform better than other single species and gave less than a third of the bioenergy of high-diversity plots.
"Switchgrass is very productive when it's grown like corn in fertile soil with lots of fertilizer, pesticide and energy inputs, but this approach doesn't yield as much energy gain as mixed species in poor soil nor does it have the same environmental benefits," said paper co-author Jason Hill, also of the University of Minnesota.
The researchers estimate that growing mixed prairie grasses on all of the world's degraded land could produce enough bioenergy to replace 13 percent of global petroleum consumption and 19 percent of global electricity consumption.
The practice of using degraded land to grow mixed prairie grasses for biofuels could provide stable production of energy and have additional benefits, such as renewed soil fertility, cleaner ground and surface waters, preservation of wildlife habitats, and recreational opportunities.