“Removal of corn residue for biofuels can decrease soil organic carbon and increase CO2 emissions because residue C in biofuels is oxidized to CO2 at a faster rate than when added to soil. Net CO2 emissions from residue removal are not adequately characterized in biofuel life cycle assessment”. A $500,000 study – paid for by the U.S. federal government and released last Sunday in the peer-reviewed journal Nature Climate Change – concludes that biofuels made with corn residue release 7% more greenhouse gases in the early years compared with conventional gasoline.
The researchers, led by Adam Liska, assistant professor at the University of Lincoln-Nebraska, used a model to estimate CO2 emissions from corn residue removal across the US Corn Belt at 580 million geospatial cells. To test the SOC model, they compared estimated daily CO2 emissions from corn residue and soil with CO2 emissions measured using eddy covariance, with 12% average error over nine years. The model estimated residue removal of 6 Mg per ha−1 yr−1 over five to ten years could decrease regional net SOC by an average of 0.47–0.66 Mg C ha−1 yr−1. These emissions add an average of 50–70 g CO2 per megajoule of biofuel (range 30–90) and are insensitive to the fraction of residue removed. Unless lost C is replaced, life cycle emissions will probably exceed the US legislative mandate of 60% reduction in greenhouse gas (GHG) emissions compared with gasoline.
In conclusion, biofuels made from the leftovers of harvested corn plants are worse than gasoline for global warming in the short term. While biofuels are better in the long run, the study says they won’t meet a standard set in a 2007 energy law to qualify as renewable fuel. The conclusions deal a blow to proponents of cellulosic biofuels, which have received more than a billion dollars in federal support but have struggled to meet volume targets mandated by law. About half of the initial market in cellulosics is expected to be derived from corn residue.
The biofuel industry and administration officials immediately criticised the research as flawed. They said it was too simplistic in its analysis of carbon loss from soil, which can vary over a single field, and vastly overestimated how much residue farmers actually would remove once the market gets underway.
“The core analysis depicts an extreme scenario that no responsible farmer or business would ever employ because it would ruin both the land and the long-term supply of feedstock. It makes no agronomic or business sense,” said Jan Koninckx, global business director for biorefineries at DuPont, to the Associated Press.
Later this year the company is scheduled to finish a $200m-plus facility in Nevada, Iowa, that will produce 30 million gallons of cellulosic ethanol using corn residue from nearby farms. An assessment paid for by DuPont said that the ethanol it will produce there could be more than 100% better than gasoline in terms of greenhouse gas emissions.
The research published in Nature Climate Change is among the first to attempt to quantify, over 12 corn belt states, how much carbon is lost to the atmosphere when the stalks, leaves and cobs that make up residue are removed and used to make biofuel, instead of left to naturally replenish the soil with carbon. The study found that regardless of how much corn residue is taken off the field, the process contributes to global warming.