Assessment of Life Cycle Greenhouse Gas Emissions From Bioenergy Sorghum Production in Central Texas.

Estimating life cycle greenhouse gases (GHGs) from biofuel production scenarios are important for compliance with federally mandated reduction goals as well as quantifying the ‘carbon footprints’ of bioenergy cropping systems. Federal legislation has mandated increasing biofuel production to more than 136 billion liters of fuel by the year 2022 while minimizing overall carbon intensity. Both direct and indirect GHGs can have a significant impact on overall life cycle efficiency. Cellulosic biomass feedstocks, such as bioenergy sorghum, must reduce life cycle GHG emissions by 60% compared to a 2005 gasoline standard. This study utilized life cycle analysis to quantify well-to-wheel GHG emissions from eight different bioenergy sorghum production scenarios. The effects of crop rotation, N fertilization, and residue return on life cycle GHG emissions from bioenergy sorghum production in central Texas were examined in 2010 and 2011. Field measured values were combined with published and modeled GHG estimates to evaluate biofuel production efficiency. Nitrous oxide loss from crop production contributed the most CO₂-equivalent (CO₂-eq) emissions. Urea fertilizer production, ethanol production, and transportation and distribution were other major carbon-intensive activities. Net change in SOC to 90 cm was utilized to estimate net CO₂ emissions to the atmosphere. High annual SOC accrual by bioenergy sorghum contributed to more CO₂-eq sequestered per MJ ethanol produced than lost. Nitrogen fertilization significantly increased life cycle GHG emissions across both years of study and subsequently, fertilized treatments had lower production efficiency (less CO₂-eq sequestered) than unfertilized treatments. All treatments examined had net negative life cycle GHG emissions, and exceeded federally mandated reduction goals.