Switchgrass shines as an energy crop
The prospect of producing bioenergy from marginal land got a big leg up when researchers recently released results of a five-year, on-farm study of switchgrass grown to produce ethanol.
“The research suggests that growing switchgrass for ethanol can be a highly sustainable system,” says Marty Schmer, agronomist, Northern Great Plains Research Laboratory, USDA Agricultural Research Service, Mandan, North Dakota.
“Given the levels of soil carbon sequestration we found, coupled with the high net energy balance and low emissions of greenhouse gases, switchgrass is a great energy crop for marginal cropland,” he says.
While research models showed that switchgrass, when managed for maximum production, can compete with corn in volume of ethanol produced per acre, it excelled in net energy output.
“For every unit of nonrenewable energy required to grow switchgrass, we got 6.4 units of energy back,” says Schmer. “With corn grain, for every unit of nonrenewable energy put into producing the crop, there are 1.2 to 1.8 units of energy that come out.”
Differences in the processing of the ethanol account for a large part of this difference in net energy outputs.
Energy efficiency aside, switchgrass shines, too, because of the soil organic carbon (SOC) the crop sequesters.
“We measured some significant carbon accrual rates in on-farm settings across three states,” says Mark Liebig, soil scientist at the Northern Great Plains Research Laboratory.
“This work impacts the carbon accrual rates researchers are using in models designed to calculate carbon offsets of various bioenergy feedstock crops,” he says. “Previous models evaluating switchgrass production for biomass energy have used carbon offset rates of fourfold to elevenfold lower than the carbon sequestration rates we found in this study.”
Scientists’ accurate calculation of carbon offsets is important because this information is used to draft formulas for life-cycle assessments (LCAs) of bio-energy production systems. The results of these assessments will ultimately determine which bioenergy production systems will gain political support.
“Federal law will require renewable biofuels to meet certain greenhouse gas emission reductions from conventional gasoline,” says Liebig. “Accrual rates of SOC observed in this study contribute significantly to the potential of switchgrass to provide a favorable net greenhouse gas balance.”