Emissions of Carbon Dioxide and Nitrous Oxide From Bioenergy Sorghum Production Under Integrated Agronomic Management Practices.

Biofuels represent a form of energy production which may serve to mitigate greenhouse gas (GHG) emissions relative to traditional petroleum-based fuels. However, if biofuels are not produced with minimal impact on overall GHG emissions, they may undermine part of their own fundamental objectives. Thus, agronomic management practices which produce minimal GHG emissions while simultaneously producing adequate biomass for fuel production must be identified.  Bioenergy sorghum is one source of biomass feedstock which is being examined in Texas and throughout the southern U.S. The aim of this study was to understand the effects of crop rotation (corn-sorghum vs. sorghum-sorghum), N fertilization (0 vs. 280 kg N ha^-1), and residue management (0 vs. 50% crop biomass return) on cumulative GHG (CO₂-C and N₂O-N) emissions and GHG-scaled ethanol production efficiency. Since June of 2010, field measurements of GHG fluxes and crop yield data have been collected from a bioenergy sorghum cropping system near College Station, Texas. The crop yield data was combined with data from the National Renewable Energy Laboratory’s ‘Theoretical Ethanol Yield Calculator’ to provide estimates of liters of ethanol produced per hectare for 2010 and 2011. Preliminary analysis on cumulative GHG emissions revealed returning 50% residue in 2010 and 2011 resulted in a 13% and 12% increase in CO₂-C emissions, respectively. Nitrogen fertilization increased cumulative CO₂-C emissions by 13% in 2011 and N₂O-N emissions by 48% in 2010. Estimates of GHG-scaled ethanol production efficiency ranged from 3 to 12 kg CO₂-eq L^-1 ethanol across all treatments. Unfertilized treatments reduced ethanol production efficiency by 56% across the 2-year average, largely due to reduced yields. The sorghum-sorghum rotations with no residue return represented some of the most efficient ethanol production treatments across both years, but soil data suggests nutrients in these treatments are being depleted over time and impacts on crop yields may be forthcoming. On-going research via life cycle analysis will examine the effects of indirect GHG emissions on overall ethanol production efficiency.