Carbon footprint of four bioethanol cropping systems in a temperate region

Under the EU Renewable Energy Directive (2009/28/EC), estimating biofuel GHG emissions is essential to gauge emissions reductions compared to fossil fuels. Within this framework, the carbon footprint (CF) was calculated for four bioethanol cropping systems: a maize-wheat-sorghum rotation without the harvest of crop residues (MWS), a maize-wheat-sorghum rotation with harvested crop residues (MWS-R), switchgrass (Sw), and continuous sweet sorghum (Ss). The estimation followed a life-cycle analysis strategy. The CF varied between 0.04 and 3.68 kg CO2-eq l - 1 ethanol. Switchgrass had the smallest CF and the highest ethanol yield per hectare (4,263 L [ha yr] - 1). However, for annual systems, Ss had the highest CF (3.68 kg CO2-eq l - 1 ethanol), 2 and 4 times larger than MWS-R and MWS systems. The soil preparation, planting, and post-planting emissions were 80% of the mean emissions in the annual cropping systems. By comparison, 60% of Sw's CF came from post-planting, and 46% from fertilizers. In Sw, soil erosion accounted for 11% of the soil organic carbon lost in the MWS-R and Ss systems. In addition, Sw was the system with the most significant carbon sequestration (-1,957 kg CO2-eq [ha yr - 1]), a value corresponding to 94% of the CF of this bioethanol cropping system.