A new study released Sunday questioning the environmental benefit from cellulosic fuels created using corn stover has renewables proponents concerned that its methodology may not be completely accurate.
The study suggests that using corn residue to make renewable fuels reduces soil carbon and generates more greenhouse gases than gasoline. It was prepared by a team of researchers at the University of Nebraska-Lincoln, led by assistant professor of biological systems engineering Adam Liska.
For the study, UNL said the researchers used a model to estimate the effect of residue removal on 128 million acres across 12 Corn Belt states. The team found that removing crop residue from cornfields generates an additional 50 to 70 grams of carbon dioxide per megajoule of biofuel energy produced. Averaged over five years, the study found emissions would equal about 100 grams of carbon dioxide per megajoule – 7% greater than gasoline emissions.
Research also found the rate of carbon emissions is constant whether a small amount of stover is removed or nearly all of it is stripped.
"If less residue is removed, there is less decrease in soil carbon, but it results in a smaller biofuel energy yield," Liska said in a UNL statement.
The research brings up concerns that production of biofuels from stover provides little environmental benefit – a claim the Renewable Fuels Association discredits.
“This study lacks sophistication and contradicts without explanation a larger highly–regarded, credible body of science," RFA President Bob Dinneen said Monday. He cited several other recent studies that resulted in different outcomes than the UNL study.
For example, he said, a University of Illinois and U.S. Department of Energy Argonne National Laboratory study showed that 30% residue removal that resulted in no additional or indirect carbon emissions. Another study from South Dakota State University showed soil organic carbon is not diminished when certain levels of stover is removed, Dinneen said.
The group also released a more complete rebuttal of the UNL study, which is posted on its website.
The Advanced Ethanol Council's Executive Director Brooke Coleman also refuted the research, suggesting that the study tries to say something new, but "in reality, the study confirms what we already know; that excessive agricultural residue removal is bad for the soil and has negative impacts on climate."
Coleman said the research, which uses corn stover removal rates of 60 to 70%, exceeds the 10% to 25% that is most commonly used in the field. Coleman said the research also uses a model that "ignores how farmers manage their land."
EPA spokeswoman Liz Purchia, like biofuels groups, suggested the study provides little value. Purchia told the Associated Press that it "does not provide useful information relevant to the life cycle greenhouse gas emissions from corn stover ethanol."
But Liska stands by the research. "If this research is accurate, and nearly all evidence suggests so, then it should be known sooner rather than later, as it will be shown by others to be true regardless," he said. "Many others have come close recently to accurately quantifying this emission."
According to the UNL statement, the research is based on carbon dioxide measurements taken from 2001 to 2010 that were then used to validate a soil carbon model built using data from 36 field studies across North America, Europe, Africa and Asia.
Previous studies, the statement said, have faced limited availability of carbon dioxide measurements in cornfields and difficult-to-measure annual carbon losses. Previous studies also did not have the benefit of a proven model to estimate carbon dioxide emissions that could be coupled with a geospatial analysis.
The research has been in progress since 2007. It was funded through a three-year, $500,000 grant from the U.S. Department of Energy, and is published in the journal Climate Nature Change. It is titled "Biofuels from crop residue can reduce soil carbon and increase CO2 emissions."
The study comes as the EPA considers final adjustments to the Renewable Fuel Standard, which stipulates production requirements for biofuels, including cellulosic fuels.