Impact of Nitrogen Rate On Nitrous Oxide Emissions and Life Cycle Greenhouse Gas Emissions in Switchgrass-Based Cellulosic Ethanol.

The 2007 Energy Independence Security Act (EISA) mandates the production of 16 billion gallons per year of cellulosic biofuel by 2022. These biofuels will be required to have life cycle assessment (LCA) greenhouse gas (GHG) emissions 60% below those of gasoline/diesel. Switchgrass is one potential feedstock for the production of cellulosic ethanol in the US. It is important to quantify life cycle GHG emissions from cellulosic ethanol produced from switchgrass. Nitrogen fertilizer is a key parameter in LCAs of biofuel because of the high GHG emissions associated with fertilizer production and N fertilizer’s impact on N₂O emissions. The objectives of this study were to 1) measure the yield and N₂O emissions from switchgrass receiving different rates of N fertilizer and 2) determine the impact of these emissions on the LCA GHG emissions of cellulosic ethanol produced from switchgrass.Annual N₂O emissions were measured from switchgrass receiving 0, 50, 100 and 150 kg N / ha. Measured yields and N₂O emissions were used as inputs in the GREET LCA model to simulate the life cycle GHG emissions of cellulosic ethanol produced from switchgrass. As N rate increased, N₂O emissions increased exponentially. Nitrogen rate increased switchgrass biomass by an average of 20 kg / ha per kg N applied. Increasing nitrogen rate caused substantial increases in the LCA GHG emissions of switchgrass-based cellulosic ethanol. Much of the increase was due to increased N₂O emissions, which accounted for 58% of total LCA GHG emissions in switchgrass receiving 150 kg N / ha. LCA GHG emissions of ethanol were lower than emissions from gasoline at all N rates. However, emissions of ethanol from switchgrass receiving 150 kg N / ha did not have emissions 60% below those of gasoline, as is required by the EISA.