Mixed prairie grasses have emerged as a leader in the quest to produce biofuels.

Photo Courtesy of Cedar Creek LTER Site

Diverse 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, according to a report by the National Science Foundation (NSF, www.nsf.org).

A new study led by David Tilman, an ecologist at the University of Minnesota (www.umn.edu), shows 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. The research was supported by the NSF and the University of Minnesota Initiative for Renewable Energy and the Environment (www1.umn.edu/iree/).

The findings are published in the Dec. 8, 2006, issue of the journal Science (www.sciencemag.org).

The study is based on 10 years of research at Minnesota”s Cedar Creek Natural History Area, one of 26 NSF long-term ecological research (LTER) 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.

NSF says fuels made from prairie biomass are “carbon negative,” which means that producing and using them actually reduces the amount of carbon dioxide (a greenhouse gas) in the atmosphere. Prairie plants store more carbon in their roots and soil than is released by the fossil fuels needed to grow and convert them into biofuels. 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, according to estimates by researchers working on the study. In contrast, the researches note, corn ethanol and soybean biodiesel are “carbon positive,” meaning they add carbon dioxide to the atmosphere, although less than 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.

According to Jason Hill, co-author of the paper and a member of the University of Minnesota research team, switchgrass is very productive when it”s grown like corn in fertile soil with lots of fertilizer, pesticide, and energy inputs, but it doesn”t yield as much energy gain as mixed species in poor soil, and it doesn’t provide the same environmental benefits.

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. Further, they say 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.