Carbon Footprint of Four Bioethanol Cropping Systems in a Temperate Region

Abstract

The production of ethanol from biomass pursuant to the EU Renewable Energy Directive (2009/28/EC) requires an estimation of the levels of greenhouse gas (GHG) emissions from biofuels to assess the emissions savings in comparison to fossil fuels. Within this framework, the carbon footprint was estimated 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, considering the relevant inputs and processes for the emission of GHG from the crop management phases of soil preparation, planting, post-planting operations, harvesting, and transport. The carbon footprint varied between 0.04 and 3.68 kgCO2−eqL−1ethanol. Switchgrass had the smallest footprint and the highest ethanol yield per hectare (4,263 L [ha yr]−1). However, for annual systems, Ss had the highest emissions (3.68 kg CO2−eq L ethanol−1), 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, in Sw, 60% of the total GHG emissions came from post-planting and 46% from fertilizers. In Sw, soil erosion by water accounted for 35% 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 that corresponded to 94% of the overall emissions of this bioethanol cropping system.