Amanda J. Ashworth, USDA-ARS, Fayetteville, AR, Adam M Taylor, Department of Forestry Wildlife and Fisheries, Center for Renewable Carbon, University of Tennessee, Knoxville, TN, Daniel A Reed, Department of Forestry Wildlife and Fisheries, University of Tennessee, Knoxville, TN, Fred L. Allen, Plant Sciences, University of Tennessee - Knoxville, Knoxville, TN and Patrick D Keyser, Center for Native Grasslands Management, Department of Forestry Wildlife and Fisheries, University of Tennessee, University of Tennessee, Knoxville, TN
As the use of second-generation biofuel crops increases, so do questions about sustainability, particularly their potential to affect fossil energy consumption and greenhouse gas (GHG) emissions. The objective of this study was to use life cycle assessment (LCA) to compare environmental impacts associated with three switchgrass (Panicum virgatum L.) production scenarios: i) regional switchgrass production from a pool of Tennessee farmers based on in-field inputs and biomass yield; ii) varying nitrogen (N)-input levels i.e., a 100% and 9% decrease, and an 81% and 172%increase from ‘baseline levels’ of N inputs used under scenario i); and, iii) legume-intercrop system compared to baseline levels in order to determine effects of displacing synthetic-N with legumes. Comparing all agricultural inputs, nitrogen fertilizers resulted in the greatest environmental impacts for regional switchgrass production. Although fertilization increased crop yields, a 100% reduction in N-inputs from baseline levels resulted in the least negative impacts per unit of production (over a 10-yr period)across all mid-point categories [e.g., global warming potential (GWP), acidification, and ozone depletion]. These results indicate a ‘less is more’ scenario, as inputs beyond current recommended levels (67 kg N ha-1) are not environmentally remunerating. System inputs with lesser impacts included phosphorus, herbicides, pesticides, and diesel fuel. In addition, legume-intercropping may reduce GHG emissions and groundwater acidification compared with the current recommended N-rate (i.e., a 5% and 27% reduction in GWP and acidification, respectively). Results imply N-fertilizers have the highest impact on regional switchgrass feedstock sustainability; however, production can be sustainable under proper N-management.