A diverse mix of native grassland perennials produces biofuels more efficiently than corn and soybeans, even when grown on sub-par soil, according to a new study. In addition, the grassland species sequester more carbon than they produce, helping to fight global climate change. The findings suggest diverse prairie grasses grown on soils no longer suitable for agriculture can substitute for corn and soybean biofuels, keeping food acreage in production while sequestering greenhouse gases.
Carbon-based fuels, such as oil and coal, have taken a public-relations beating over the past several years because of their limited supply, climbing costs, and the pollution they cause. As an alternative, researchers have looked to plants to produce a more abundant and environmentally friendly source of fuel. Corn, for example, can be used to produce ethanol, a potential alternative to gasoline. And soybean oil can be converted to biodiesel, which can be burned in diesel engines or used as a heating fuel. But both crops are relatively expensive to maintain--they are competitive only when conventional fossil fuels prices reach about $1.50 per gallon--and growing them for fuel takes food acreage out of production.
To see if native grassland species, such as indiangrass and big bluestem, could provide a better alternative to these alternative fuel sources, a team of ecologists at the University of Minnesota in St. Paul grew 18 different species of prairie plants in 152 test plots, beginning in 1994. The researchers varied the species composition--sometimes plotting only a single species, sometimes plotting an array of species--on no-longer-productive agricultural lands in northern Minnesota. They then compared the biofuel energy yield when the plants were grown alone or in those different combinations. The most diverse plots produced 238 percent more bioenergy yield than the average plot containing a single species, says team leader David Tilman. Although the yield of biofuels from prairie grasses was about 33% lower per hectare than from cultured crops such as corn, the inputs (fertilizer, fuel for tractors, etc.) required to grow them are much lower. So the net energy output from native grasses is actually about five-fold compared to 1.25-fold for corn and 1.93-fold for soybeans. As a bonus, three-quarters of the prairie grasses' mass is in their root structure, allowing the plants to store a net 4.4 metric tons of carbon per hectare every year, a relatively large amount in the fight against climate change. The team reports its findings tomorrow in Science.
No one in the field would quibble with the basic message about the energy production potential of prairie grasslands, says Alan Knapp, a plant ecologist at Colorado State University. Still, he thinks the jury's out on whether polyculture is better than monoculture in terms of bioenergy yield, as experimental design can skew results.