Biofuels from crop residue can reduce soil carbon and increase CO2 emissions

Removal of corn residue for biofuels can decrease soil organic carbon (SOC; refs 1,2) and increase CO2 emissions(3) because residue C in biofuels is oxidized to CO2 at a faster rate than when added to soil(4,5). Net CO2 emissions from residue removal are not adequately characterized in biofuel life cycle assessment (LCA; refs 6-8). Here we 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(9-11), we compared estimated daily CO2 emissions from corn residue and soil with CO2 emissions measured using eddy covariance(12-14), 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(15,16), life cycle emissions will probably exceed the US legislative mandate of 60% reduction in greenhouse gas (GHG) emissions compared with gasoline.