The U.S. Department of Energy has been counting on leftover residue from corn cultivation—such as stalks and cobs—as an abundant future source of renewable clean energy, and touted it as a potential goldmine for farmers as well.
But University of Nebraska-Lincoln (UNL) researchers may throw a damper on those plans, with a newly published study in the scientific journal Nature Climate Change. They calculated that harvesting the corn residue may actually result in the release more climate-altering carbon dioxide into the atmosphere than federal environmental regulations allow. (See related coverage: “Biofuels at a Crossroads.”)
Those findings, however, were challenged by both a renewable energy trade group and a prominent biofuels researcher, who found fault with the study’s methodology.
The UNL team, led by agronomy and horticulture assistant professor Adam Liska, used a supercomputer simulation to estimate the effect of removing the corn residue (also called corn stover) from 128 million acres across 12 states in the Midwest’s Corn Belt.
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If left undisturbed, most of the residue rots, releasing carbon dioxide into the atmosphere, but some of the material also becomes part of the soil, storing its carbon there. But when the corn residue is removed, converted to biofuel and burned in engines, it pumps out carbon dioxide more rapidly than a cornfield would release it, the researchers wrote. As a result, they calculated, biofuel made from corn residue would release so much carbon that it would probably exceed the limits set by federal regulations, which require biofuels to produce 60 percent fewer greenhouse gas emissions than gasoline. (Take the quiz: “What You Don’t Know About Biofuel.”)
The study’s lead author, UNL agronomy and horticulture assistant professor Adam Liska, declined to be interviewed by phone. In an email, he said that additional unpublished data shows that when both the actual corn and the corn residue are removed, “there is a net loss of soil carbon and there is also an absolute loss of soil carbon to the atmosphere.” In contrast, if only the corn itself was harvested for biofuel, on average the carbon going into the atmosphere would be canceled out by the amount going into the soil. (See related story: “Squeezing Gasoline from Plants.”)
The corn residue study, which was funded by a three-year, $500,000 grant from the U.S. Department of Energy, could signal a major problem for the nascent biofuels industry, which is heavily invested in using such waste material as a way to avoid competing for food supply with the use of actual corn.
Paul Winters, an official with the Biotechnology Industry Association, a biofuels trade group, said that corn residue is expected to provide three billion gallons of biofuel by 2022, or about 20 percent of the total production of cellulosic ethanol—that is, fuel produced from the inedible parts of plants. The study has no implications for materials such as switchgrass, a plant that is cultivated specifically for fuel. “You’re just harvesting the top, not disturbing the soil and the carbon in it,” Winters explained.
But Winters found fault with the UL study, saying that the computer model didn’t take into account the varying soil and other conditions in individual cornfields, or the careful efforts by biofuel producers to work with farmers to monitor how harvesting corn residue affected the carbon content of the soil. “The companies who are doing it now only take about 25 percent [of the corn residue],” he said. “They leave 75 percent on the ground.” (See related story: “U.S. Drought Fuels Debate on Ethanol.”)
John Sheehan, an engineer, biofuels entrepreneur and former researcher for the National Renewable Energy Laboratory who helped lead key research on the use of corn residue, said that he planned to contact Liska and raise questions about his methods. Sheehan said the study’s relatively short 10-year time scenario gave a misleading impression of the proportion of carbon that ended up in the atmosphere. The loss of soil carbon is highest in the first few years when corn residue is harvested, but it tends to decrease over a 20- to 30-year period, he said.
Liska “is using the most extreme response of the system as an indicator,” Sheehan said.
Sheehan also argued that the UNL researchers’ new model for calculating soil carbon loss failed to take into account variations in microbial activity and respiration rates in different corn fields, which are influenced by how farmers till the soil. “The respiration for aggressively tilling, versus conservation tillage, is very different,” he said. That, in turn, could produce big variations on how much carbon is stored or released. (Share your thoughts: “What Breakthroughs Do Biofuels Need Now?“)
In a second e-mail, Liska defended his team’s use of the shorter time frame. “Everyone really wants to know how these systems will perform in the first 10 years,” he said. He said that biofuels makers could switch to another source of material for fuel, or else plant cover crops in the fields to mitigate carbon loss. “The carbon lost in any residue that is removed must be either replaced or off-set somewhere else in the system,” he wrote.