Biofuel industry criticizes study on corn stover and soil carbon

By Erin Voegele | April 21, 2014

The biofuel industry is speaking out against a study completed by University of Nebraska-Lincoln researchers that claims cellulosic biofuels actually generate more greenhouse gases than gasoline.

The study, titled “Biofuels from crop residue can reduce soil carbon and increase CO2 emissions,” was published in the academic journal Nature Climate Change on April 20. According to information published on the study, removal of corn residue for biofuels can decrease soil organic carbon and increase carbon dioxide emissions because residue carbon in biofuels is oxidized to carbon dioxide at a faster rate than when added to soil.

The researchers, led by assistant professor Adam Liska, used a supercomputer model at UNL’s Holland Computing Center to estimate the effect of corn stover residue removal on 128 million acres across 12 states. The team’s model determined that removing this residue generated an additional 50 to 70 grams of carbon dioxide per magajoule of biofuel energy produced. The researchers claim that when averaged over five years, their analysis results in approximately 100 grams of carbon dioxide per magajoule, which is 7 percent higher than gasoline.

The Renewable Fuels Association has called the study deeply flawed and out of step with current science, noting the analysis shows a complete lack of understanding of current farming practices.

“The study’s methodology is fundamentally flawed and its conclusions are highly suspect,” said Bob Dinneen, president and CEO of the RFA. “The results are based on sweeping generalizations, questionable assumptions, and an opaque methodology. The authors offer no robust explanation for why their findings contradict other recent, highly regarded research. Ultimately, this paper should be seen for what it truly is—a modeling exercise of a hypothetical scenario that bears no resemblance to the real world.”

While the study assumes 60-70 percent stover removal, Dinneen stressed that is a level that nobody believes is sustainable. “Stover removal rates are currently in the 10-25 percent range, which well documented research demonstrates is sufficient to replenish soil,” he said.

“This study lacks sophistication and contradicts without explanation a larger highly–regarded, credible body of science,” Dinneen continued. “Other recent studies have examined the carbon impacts of using corn residue for bioenergy. For instance, an analysis conducted by the University of Illinois and Argonne National Laboratory showed 30 percent residue removal resulted in no additional direct or indirect carbon emissions. Furthermore, it showed certain levels of corn stover can be removed without decreasing SOC. Initial results from research at South Dakota State University showed that SOC levels remained constant from 2008-2012 in a harvest system with relatively high residue removal rates.”

The RFA has published a response to the study discussing five key points of contention. First, the RFA argues the study assumes unrealistic and inappropriate levels of residue removal, and makes no distinctions with regard to tillage practices. The authors of the study assume that all farmers would willingly and consistently remove approximately 60-75 percent of corn residue from their field, regardless of tillage practices, soil types, crop rotations, equipment capabilities and other factors. The RFA, however, notes that current best practices generally recommend removing no more than 40-50 percent of residue in no-till systems, no more than 20-30 percent in conservation tillage practices and 0-10 percent under conventional tillage.

Second, the RFA notes that the authors are incorrect in their argument that soil organic carbon fluxes related to stover removal have not been well characterized in previous studies. According to the RFA, several other models, including DAYCENT/CENTURY and EPIC have been used to estimate carbon fluxes in the atmosphere, vegetation and soil. “Not only does the Liska et al. study make absolutely no attempt to validate its results against those obtained from other established models, but it also fails to discuss model differences,” said the RFA, adding that the authors also completely failed to explain why their new model should be viewed as improvement over existing models and methodologies.

Third, the RFA stresses that the analysis absurdly suggests soil organic carbon loss I perfectly linear with stover removal rates, while previous studies have shown virtually no change in soil organic carbon levels at low residue removal rates and potentially larger fluxes at high removal rates under certain conditions.

Fourth, the RFA points out that the Liska et al. results are contrary to the findings of other studies, but reasons for these differences are not clearly presented. While several other recent studies have produced findings that starkly contrast the new UNL study, the authors make no attempt to dispute the results of those studies.

Finally, the RFA said that the authors assume no mitigation action is taken to prevent soil organic carbon losses or replace lost carbon. While the authors clearly admit that soil carbon emissions from residue removal can be mitigated, they don’t include any of those mitigation practices in their scenario analyses. “In fact, in its discussion of overall GHG impacts associated with residue-derived ethanol, the study apparently fails to include the emissions benefits related to the use of the lignin fraction for process energy production. Most, if not all, cellulosic biofuel projects under way or in planning stages will make use of lignin for power generation. While other mitigating factors (including no-till cover crops, improved management practices, animal manure application, rotation with forage crops, etc.) are mentioned, none figure into the authors’ calculations of overall GHG impacts,” said the RFA.

In a statement, Dinneen also pointed to a study published last week in the Proceedings of the National Academy of Sciences that determined the carbon impact of fracking may be up to 1,000 times greater than previously thought. “Curiously, that report was largely ignored by the media. Folks need to stop manufacturing scenarios to make biofuels look bad, and begin focusing on the true carbon menace—oil,” he said. 

Brooke Coleman, executive director of the Advanced Ethanol Council, has also weighed in on the study. “What we have here is an article trying to package itself as saying something completely new; that removing corn stover from the field has newly quantified impacts that would change our perception of making advanced ethanol out of corn stover,” he said. “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. The article says little about real world stover-to-ethanol fuel because it uses corn stover removal rates far exceeding those used in the field. The analysis also models a one-size-fits-all approach to managing soil carbon that, by definition, ignores how farmers manage their land. While it’s fair to model whatever scenario you want in the hypothetical, if it’s not happening in the real world then the modeling outcomes are something times zero. Our industry is more than willing to engage in important discussions about the climate impacts of using agricultural residues to make fuel, but the headline-chasing strategy of trying to sell extreme modeling assumptions as the norm does not facilitate that process. If you look at the full spectrum of peer-reviewed work, cellulosic biofuel is the lowest carbon fuel in the world.”

Information released by UNL notes that scientists have historically been unable to fully quantify how much soil carbon is lost to carbon dioxide emissions after removing crop residue. These efforts have been hampered by limited carbon dioxide measurements in cornfields, by the fact that annual carbon losses are comparatively small and difficult to measure, and the lack of a proven model to estimate carbon dioxide emissions that could be coupled with a geospatial analysis. The new study, funded by a three-year $500,000 grant from the U.S. Department of Energy, used carbon dioxide measurements taken from 2001-2010 to validate a soil carbon model that was built using data from 36 field studies across North America, Europe, Africa and Asia.

According to UNL, the research has been in progress since 2007, involving the coordinated effort of faculty, staff and students from four academic departments at UNL. Liska is an assistant professor of biological systems engineering and agronomy and horticulture. He worked with Haishun Yang, an associate professor of agronomy and horticulture, to adapt Yang's soil carbon model, and with Andrew Suyker, an associate professor in the School of Natural Resources, to validate the model findings with field research. Liska also drew upon research conducted by former graduate students Matthew Pelton and Xiao Xue Fang. Pelton's master's degree thesis reprogrammed the soil carbon model, while Fang developed a method to incorporate carbon dioxide emissions into life cycle assessments of cellulosic ethanol.