Nitrous Oxide Emissions in Bioenergy Cropping Systems: Implications for Life Cycle Assessment.

The 2007 Energy Independence Security Act mandates the production of 80 billion liters per year of advanced biofuel, including cellulosic ethanol, by 2022. These biofuels will be required to have Life Cycle Assessment (LCA) emissions 60% below those of gasoline/diesel. Therefore, it is important to identify crops to be used as biofuel feedstock that provide maximum biomass with minimal greenhouse gas emissions. Nitrous oxide (N₂O) is a greenhouse gas that is currently the fourth largest radiative forcing agent of Earth’s climate, having a global warming potential 298 times that of carbon dioxide. The IPCC estimates agriculture contributes 65-80% of total anthropogenic N₂O emissions.  Much of this N₂O is produced by microorganisms in the soil as a function of N enrichment. Therefore, accurate estimates of N₂O emissions from soils under different biofuel crops are essential in evaluating the Greenhouse Gas (GHG) balance of biofuel produced from different feedstocks. Annual soil N₂O emissions were monitored from four biofuel crops during the 2011 growing season in Manhattan, KS. Using the GREET life cycle assessment model, the life cycle greenhouse gas emissions were simulated for ethanol in E10 blends using corn stover, photoperiod sensitive sorghum, switchgrass and miscanthus as feedstocks. Two GREET simulations were done for each feedstock, one using the IPCC Tier 1 protocol for direct N₂O emissions and one using measured N₂O flux. Using measured N₂O emissions as inputs increased the greenhouse gas emissions of ethanol compared to the IPCC Tier 1 protocol. Preliminary results indicate that cellulosic ethanol from corn stover, photoperiod sensitive sorghum, switchgrass and miscanthus all have a negative global warming potential relative to gasoline.