Soil-carbon losses could mean that maize-based biofuel production in the US will fail to meet a required 60% CO2 emissions reduction target, according to research published online this week in Nature Climate Change. Advanced biofuels, which use crop residues including stalks and leaves rather than harvested grains, remove the major trade-off between food and biofuel production. However, it is uncertain if they will deliver sufficient carbon savings if used instead of gasoline.

Carbon emissions from biofuel production are typically measured using methods of life-cycle assessment, but conventionally these do not take soil carbon into account. The removal of crop residues reduces carbon inputs into the soil and this might therefore be expected to reduce soil-carbon stocks. Estimates of CO2 emissions due to corn residue removal across the US Corn Belt indicate that soil-carbon loss adds an average of 50 to 70 grams of CO2 per megajoule of biofuel produced.

Adam Liska and colleagues argue that unless the lost soil carbon can be replaced through improvements to soil management, the full life-cycle emissions will not meet the mandatory 60% reduction laid out in the Energy Independence and Security Act of 2007.