Image: Corn harvest/US soybean board
The United States remains the largest corn producer in the world. A lot of it gets exported, another good chunk gets processed into ethanol to meet government biofuel mandates (and to snag government ethanol subsidies), and presumably some of it actually gets eaten in its native country. In 2013, all of that corn added up to 13.9 billion bushels, and wherever it winds up, harvested corn all leaves behind a large amount of bio-waste in the form of discarded stalks, leaves, and cobs left in the field. All of this stuff, making up about half of the total corn yield, is called corn stover and it's usually just left there in the field as nutrient-beneficial leftovers.
The stover can be used for animal feed, but it's also been targeted as a potential biofuel. The United States has spent $1 billion on research into cellulosic biofuel—fuel made from woods, grasses, and inedible plant material—and this includes stover. Despite flagging interest in ethanol as a fuel, there's still a small, government-backed industry devoted to its ascendancy as a climate change bandage. The catch, as described in a study out today in the journal Nature Climate Change (note: the study has yet to make it publicly online, as of Sunday afternoon), is that stover winds up being no better for the climate than regular gasoline and actually might be a bit worse. The study, which examined stover fuel models using a supercomputer, isn't fully damning but is enough to "cast doubt."
The problem comes in a trade-off: while the actual fuel burning is cleaner, the process of removing the stover from a field releases carbon dioxide from the soil to the tune of 50 to 70 grams per one megajoule (about a BTU) of energy produced. Looking at 128 million acres of corn across the American Midwest and averaging over five years of harvesting, that goes up to about 100 grams per megajoule. Crucially, it adds up to 7 percent more carbon dioxide released into the atmosphere than if we were just using plain old gasoline. It also pushes corn stover fuel below the 60 percent CO2 emissions reduction mandated by the ethanol-boosting 2007 Energy Independence and Security Act.
There's more to it than just carbon dioxide. Corn stover doesn't just go to waste if left on a field post-harvest. It helps protect against erosion and it keeps nutrients in the soil. Removing the stuff means soil loss (you know, Dust Bowl) and an increased need for fertilizer. Lead researcher Adam Liska, a professor at the University of Nebraska, notes that his team made numerous attempts to poke holes in their study, only to come up with the same disappointing results. There's no such thing as free energy.
For their study, Liska's team used carbon dioxide measurements taken between 2001 and 2010 to validate an existing soil carbon model developed from 36 field studies performed across the globe. They turned this modeled emission data into 580 million 30-meter by 30-meter geospatial cells spread across the Midwestern "corn belt" states. The northernmost of those states—Minnesota, Wisconsin, and Iowa—released the most carbon because their cooler climates help keep CO2 trapped in the soil.
The US is notoriously stubborn about its energy policies (or any policy, really), even when they're demonstrably wrong. Don't expect this study, or any forthcoming studies supporting today's results, to force an exception to that anytime soon. While there's some hope for stover in different soil carbon-fixing methods, like planting cover crops between corn rows, the study also goes on to suggest the obvious: more efficient vehicles.