Soil Carbon Dioxide Equivalent Emissions and Theoretical Ethanol Yield from Midwestern Bioenergy Systems

Abstract

There has been a significant demand to increase biofuel production and usage in the USA. Bioenergy crops also have the potential to mitigate greenhouse gas (GHG) emissions from agricultural systems but crop type (annual vs. perennial) may impact mitigation potential. Objectives of this research were to (1) assess cumulative carbon dioxide equivalence (CO₂eq) of annual and perennial crops, (2) evaluate the theoretical ethanol yield (TTEY) from annual and perennial crops, and (3) assess the CO₂eq/TTEY ratio (CTR) of annual and perennial crops. This study was conducted from 2008 to 2016 at the Purdue University Water Quality Field Station near West Lafayette, IN. Replicated treatments (n = 4) included continuous corn (CC), continuous sorghum (CS), Miscanthus (MS), switchgrass (SG), and restored prairie (RP). This study calculated CO₂eq using CO₂ and N₂O emissions. To estimate TTEY, yield of stover carbohydrate pools, recovery efficiencies of sugars from cell wall polysaccharides, fermentation efficiency of sugars to ethanol, the theoretical ethanol yield from sugars, and grain (for corn and sorghum) were used. Averaging over the course of the study, the highest annual CO₂eq observed were 7.8 and 6.7 Mg CO₂-C ha⁻¹ year⁻¹ from RP and SG, respectively. Average CO₂eq for CC, CS, and MS were 5.8, 5.5, and 5.0 Mg CO₂-C ha⁻¹ year⁻¹, respectively. The highest and lowest TTEY were observed in MS and RP, yielding 189.4 and 26.7 GJ EtOH ha⁻¹ year⁻¹, respectively. Thus, MS had the lowest CTR out of these treatments. Averaging across plant type, CO₂eq, TTEY, and CTR were 6.5 Mg CO₂-C ha⁻¹ year⁻¹, 89.8 GJ EtOH ha⁻¹ year⁻¹, and 345 kg CO₂-C GJ⁻¹ EtOH, respectively, for perennials, and 5.7 Mg CO₂-C ha⁻¹ year⁻¹, 118 GJ EtOH ha⁻¹ year⁻¹, and 52.1 kg CO₂-C GJ⁻¹ EtOH, respectively, for annuals. Although these results suggest lower CO₂eq, higher TTEY, and lower CTR for annual crops compared to perennials, the mitigation potential of emitted GHGs from perennials in agricultural systems used for biofuel production is also highlighted.