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 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. Therefore, it is important to quantify life cycle GHG emissions from cellulosic ethanol produced from switchgrass. Nitrogen fertilizer rate is a key parameter in LCAs of biofuel because of the high GHG emissions from the production of fertilizer and because of nitrogen fertilizer’s impact on nitrous oxide emissions, which can account for 5-80% of LCA GHG emissions. However, the yield response of switchgrass to nitrogen fertilizer is still unclear, as is the impact of different nitrogen rates on the GHG balance of cellulosic ethanol from switchgrass.

            The objectives of this study are to 1) measure the yield and N₂O emissions from switchgrass receiving different rates of nitrogen fertilizer and 2) determine the impact of these emissions on the LCA GHG emissions of cellulosic ethanol produced from switchgrass. The annual N₂O emissions were measured from switchgrass receiving 0, 50, 100 and 150 kg N / ha. The measured yields and N₂O emissions were then used in as inputs in the GREET LCA model to simulate the life cycle GHG emissions of cellulosic ethanol produced from switchgrass under different nitrogen rates. Significantly higher fluxes total N₂O fluxes and yield-scaled N₂O fluxes were observed with increasing nitrogen rate. It has also found that increasing nitrogen rate causes substantial increases in the LCA GHG emissions of switchgrass-based cellulosic ethanol.