Carbon Dynamics of Biofuel Cropping Systems Compared to Conventional Cotton Cropping Systems.

To meet the growing renewable energy demand, increased biofuel production is required in the coming years. The USDA Biofuels Strategic production Report (USDA, 2010) recognized the Texas High Plains region as a potential cellulosic biofuel feedstock production region in the United States.  The dominant cropping system in the Texas High Plains region is cotton (Gossypium hirsutum L.).  A significant change in land use from conventional cotton cropping systems to biofuel production systems can impact carbon dynamics of this semi-arid region.  The objective of this study was to compare the seasonal changes in net CO₂ exchange of a conventional cotton cropping system to two potential biofuel feedstock systems. The biofuel feedstock systems studied are biomass sorghum (Sorghum bicolor L.) and Old World bluestem (Bothriochloa bladhii (Retz) S.T. Blake).  Data were collected in the 2013 growing season from three producer fields planted to cotton, biomass sorghum, and Old World bluestem (~250 acres) in the Texas High Plains region. Carbon dioxide flux between the vegetation and atmosphere was measured using eddy covariance flux method. Other environmental variables measured included air temperature and relative humidity, net radiation, global irradiance, precipitation, soil temperature, and soil moisture. Plant measurements collected include height, leaf area index, biomass and yield. Our results indicate that net ecosystem exchange, ecosystem respiration and gross primary production for these cropping systems were strongly affected by environmental variables. During the 2013 growing system, biomass sorghum accumulated more carbon compared to the other two systems. Ecosystem respiration was an exponential function of soil temperature.  Incorporating the results from our study with ecosystem models can improve our understanding of the impact of land use change associated with biofuel production to regional carbon balances.