Biofuels may not be the clean substitute for gasoline that we’ve been looking for, as the release of soil carbon more than offsets other gains. Research from the University of Nebraska-Lincoln recently published in the journal Nature Climate Change found that when corn stover (stalks, leaves, and cobs) is removed from fields after the harvest to produce biofuel, the soil releases an additional 50 to 70 grams of carbon dioxide into the atmosphere per megajoule, bringing the total emissions to more than 100 grams per megajoule, 7% more than from gasoline.
Crop residue increases carbon capture
While the effect may be larger than expected, the idea that corn stover contributes to carbon capture in soil isn’t surprising – farmers typically leave crop residue behind because it increases soil quality. If it the practice helps retain nitrogen and other nutrients, it’s reasonable that it would help with carbon capture as well. The study suggested that planting cover crops might mitigate the harm, but the technique would need further research.
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The UNL study also found that the effect is constant regardless of how much corn stover is removed. “If less residue is removed, there is less decrease in soil carbon, but it results in a smaller biofuel energy yield,” said UNL assistant professor Adam Liska, the university reports, so neither economies of scale or skimming the part of the residue will push the numbers down per unit of energy produced.
Soil carbon release is hard to measure
The issue of soil carbon being released was already known, but it has been difficult to quantify in the past because of a lack of solid data. Liska and the rest of the research team got around this problem by creating a model of soil carbon emissions and then validating it against emissions data for 36 independent studies over a ten year period. Once the model was built, they broke the US Corn Belt into 580 million 30-square-meter chunks and ran a simulation to approximate the emissions if different amounts of residue were removed from the farm.
Even though this is just a simulation for now, Liska was confident in the result and expects confirmation from other research teams who “have come close recently to accurately quantifying this emission.”
The study doesn’t mean that biofuels have higher emissions across the board (when measured across the entire production process), but if Liska’s numbers hold up then policymakers may need to rethink their support for an alternative fuel that could be dirtier than gas.