Yong Wang, Texas, Texas A&M AgriLife Research, Beaumont, TX, Fugen Dou, Texas A&M AgriLife Research & Extension Center, Beaumont, TX and Frank M. Hons, Department of Soil and Crop Sciences, Texas A&M University, College Station, TX
Life cycle analysis (LCA) was conducted for greenhouse gas (GHG) fluxes represented by carbon dioxide (CO2) to evaluate carbon (C) dynamics under different tillage practices in bioenergy sorghum production. Three years of sorghum biomass and soil organic carbon (SOC) data in conventional and reduced tillage practices were used for biogeochemical model DAYCENT calibration and the model performance was satisfactory with r2 values of 0.74 and 0.92, respectively. No-till practice was assumed to be conducted based on model parameterization in calibration process and associated simulation results were used for LCA. Overall, No-till had the highest GHG mitigation potential, with net GHG emission being -35.49 g m-2 yr-1, followed by reduced tillage with net GHG emission being -35.19 g m-2 yr-1. Conventional tillage in bioenergy sorghum production was also able to sequester C compared with fossil fuel combustion, with net GHG emission being -33.75 g m-2 yr-1, though not as capable as reduced tillage and no-till. For all the C sinks, displaced fossil fuel was the largest GHG sink, followed by SOC sequestration and CH4 oxidation. For all the C sources, feedstock conversion was the largest GHG source, followed by nitrous oxide (N2O) emissions and energy requirements for N manufacture and field machinery operations. Thirty years of simulation based on historical weather data (1982 - 2012) showed the same pattern for all three tillage practices, with higher total amounts and similar annual values, measuring -35.44, -35.11, and -32.73 g m-2 yr-1 of net GHG emissions for no-till, reduced tillage, and conventional tillage, respectively. However, higher yield variety may increase the C sequestration potential. On average, 15% of yield increase was able to sequester 2% more of GHG emissions.