Impacts of Biomass Sorghum Feedstock Production On Carbon Sequestration and Greenhouse Gas Emissions in the South Central Region.

The Energy Independence & Security Act of 2007 mandates cellulosic biofuels to decrease life cycle greenhouse gas (GHG) emissions by 60% compared to a 2005 petroleum standard. Bioenergy crop production accounts for a significant portion of overall life cycle GHGs. Quantifying the efficiency of bioenergy crop production requires data on soil properties, plant tissue composition, crop yield, and direct emissions of greenhouse gases in addition to indirect emissions associated with management practices (fertilization, irrigation, tillage, etc.). This study attempts to identify and quantify various sinks and sources of life cycle GHGs from bioenergy sorghum production in central Texas. The goals of this study are to identify sustainable agronomic management practices able to reduce, both direct and indirect, GHG emissions associated with bioenergy sorghum production. The effect of crop rotation, N fertilization, residue management, and their interactions on cumulative emissions of GHGs (CO₂-C and N₂O-N, converted to CO₂-equivalents) were examined in 2010 and 2011. Nitrogen fertilization and 50% biomass residue return significantly increased the two-year average of direct GHG emissions by 16% and 9%, respectively. Crop yield and plant tissue composition data from 2010 and 2011 were used along with National Renewable Energy Laboratory ethanol-conversion values to determine estimated liters of ethanol produced per hectare. The sum of direct and indirect GHG emission values associated with each plot were divided by estimated ethanol production values to provide an ethanol production efficiency value in kg CO₂-equivalents L^-1 ha^-1. Regardless of N fertilization, continuous sorghum with no residue return had the lowest ethanol production efficiency. In general, treatments with 50% biomass residue returned were not as efficient, largely because of the large yield decreases associated with the high biomass return rate. However, soil data suggests treatments receiving no biomass returned diminished nutrients and soil microbial biomass over time, which will impact crop yields and soil health in the future. Additional ongoing research will determine the impact of soil organic carbon sequestration on reducing life cycle GHGs.