The Role of Farm Management in Biofuel Life-Cycle Emissions: A Case Study from Minnesota.

The US Renewable Fuel Standard 2 specifies specific life-cycle greenhouse-gas (GHG) emissions reduction thresholds that must be met for different classes of biofuels to qualify as "renewable." However, these thresholds are based on broad-based, consensus estimates of the actual GHG emissions associated with different biofuel production pathways. This study explored the sensitivity of emissions associated with the feedstock production phase of the biofuel life-cycle to variations in farm management and site characteristics. Farmers in the vicinity of a proposed corn-grain-to-butanol biorefinery located near Luverne, MN were surveyed on a range of management practices, including fertilization levels, tillage, and manure application. Management practices and site characteristics were used to drive DayCent model simulations of corn-soybean crop rotations for 35 farms, and the GREET life-cycle assessment model was used to estimate emissions from on-farm materials and energy use. Simulated soil organic carbon (SOC) stocks increased on average, amounting to net emissions of -59 g CO2e m-2yr-1. Simulated soil nitrous oxide emissions averaged 108 g CO2e m-2yr-1 . Indirect nitrous oxide emissions due to nitrate leaching and ammonia volatilization contributed an additional 7.4 g CO2e m-2yr-1. Average emissions attributable to on-farm material and energy use accounted for an additional 138 g CO2e m-2yr-1. There was a large range in per-farm simulated emissions, with the lowest-emitting farm averaging -164 g CO2e m-2yr-1 from soil processes and 90 g CO2e m-2yr-1 from materials and energy use, for a total field-to-farm-gate emissions footprint of -74 g CO2e m-2yr-1. The corresponding values for the highest-emitting farm were 161, 178, and 339 g CO2e m-2yr-1, respectively. These results suggest that variability in actual farm management can lead to large differences in the field-to-farm-gate emissions footprints of corn grain-derived biofuels.