The loss of soil carbon that results from removing corn field wastes to make biofuels could negate their value as a low-carbon fuel ingredient, a recent paper suggests.
Using corn residue -- corn stover and cobs -- to make cellulosic biofuels increases emissions from the burning of plant matter and other residue components in the biorefinery, instead of being mixed into the soil and remaining trapped as soil organic matter, researchers from the University of Nebraska, Lincoln, have found.
"If the residue had not been removed, some of it would be left in the field and be naturally added to maintain soil carbon," said Adam Liska, lead author of the paper and assistant professor of biological systems engineering at the University of Nebraska.
The researchers used a computer model to estimate emissions across the Corn Belt, 128 million acres, or about 96.8 million football fields. A removal rate of about 6 metric tons per hectare (2.68 tons per acre) per year over five to 10 years could decrease regional soil organic carbon by about 500 kilograms of carbon per hectare (446 pounds per acre) per year.
This can add an average of 50 to 70 grams of CO2 per megajoule of biofuel. At the high end, this makes these biofuels emit about 7 percent more carbon dioxide than gasoline, making them slightly worse from a climate perspective. Over 10 years, it would reduce gasoline emissions by about 15 percent.
Minnesota, Wisconsin and Iowa were found to carry the highest carbon losses. The cold weather in these states slows the release of soil organic carbon in the atmosphere. The high corn yields in these states add large amounts of carbon to the soil, representing a bigger loss if corn residue is collected, Liska said.
USDA expert disagrees
Under the renewable fuel standard (RFS), cellulosic biofuels must emit at least 60 percent less greenhouse gas emissions than fossil fuels in a life-cycle analysis, meaning that all of the carbon savings and expenses from the entire process must be accounted for.
2014 is slated to be a turning point in the development of biofuels from corn residues. Ethanol giant Poet LLC has partnered with Dutch biochemical company Royal DSM to build a 25-million-gallon plant in Emmetsburg, Iowa, that will open in around June. Spanish company Abengoa Bioenergy is expected to open its 25-million-gallon facility in Hugoton, Kan., in May. And DuPont will begin production at its 30-million-gallon plant by the end of the year. All the companies say they expect to produce ethanol that will achieve 80 to 90 percent less greenhouse gas emissions over fossil fuels.
Douglas Karlen, a research leader in the Agriculture Department's Agricultural Research Service, disagrees with the study's assumptions. The average corn stover yield for 10 corn-growing states is 3.61 tons per acre. Using the 2.68-tons-per-acre removal rate that is assumed in the study would mean that farmers would be removing 75 percent of their corn residue, Karlen said.
"You'd have corn residue stacks for miles if you tried to take that much off," said Karlen, who has advised corn stover ethanol companies on how to maintain soil quality. A more realistic collection rate is about 25 percent, the amount that Poet-DSM's Project Liberty says it collects, according to the company's website.
Mandate to move away from food-based fuel
The RFS, a mandate to increase biofuels production to 36 billion gallons per year by 2022, was designed to slowly taper off growth in first-generation biofuels like corn ethanol and promote the development of fuels that emit less carbon and don't compete with food crops.
However, the development of cellulosic biofuels -- fuels made from agricultural residues, forestry waste, grasses, wood and household trash, for example -- has been slow, and almost no cellulosic fuel has been blended into the country's gasoline supply to date. The RFS has been fiercely debated in Washington, with the oil industry, the livestock sector, and some environmental and food aid groups calling for a repeal or reform of the policy.
The Liska paper seems to be wading beyond the science of soil carbon, Karlen said.
"It's pushing the envelope into an area where, to me, there is more of an agenda than I would like to see in a paper," he said.
The paper's authors don't negate biofuels completely. Fuels made from perennial grasses or forestry resources "may provide feedstocks that could have less negative impacts on [soil organic carbon], [greenhouse gas] emissions, soil erosion, food security and biodiversity than from removal of corn residue," it states.
The facilities could also put carbon back into the soil by growing cover crops, or export electricity to save emissions elsewhere, Liska said.
The study was published in Nature Climate Change yesterday.