Ecosystem-Scale Carbon Fluxes and Evapotranspiration of Bioenergy Crops Compared to Conventional Cotton Cropping Systems.

We are facing an unprecedented challenge in securing America’s energy future. To address this challenge, increased biofuel crop production is needed. Although first-generation biofuels like corn ethanol are available, second-generation biofuels are gaining importance because they don't directly compete with food production. Second-generation biofuels are made from the by-products of intensive agriculture or from less-intensive agriculture on more marginal lands. The Southwestern U.S. Cotton Belt can play a significant role in this effort through a change from more conventional crops (like continuous cotton) to second-generation biofuel feedstocks (biomass sorghum and perennial grasses). While we believe there would be environmental benefits associated with this change in land use, their exact nature and magnitude have not been investigated for this region. We are conducting a multi-year project investigating the C flux, evapotranspiration and the components of the surface energy balance using eddy covariance flux measurement sites established at locations representing the major second-generation biofuel feedstock production systems (perennial grasses and biomass sorghum) in the Southwestern U.S. region. Eddy covariance flux measurements of CO₂, water and components of the energy balance shows considerable variation among the sites. The CO₂ uptake was highest in the biomass sorghum field and lowest in the perennial grass site. The lowest C uptake at the grass site was primarily due to limited irrigation compared to the other fields. Measured evapotranspiration was highest in the cotton field. In the biomass sorghum field, approximately 50–80% of available energy was used for evapotranspiration. In the perennial grass site, this ranged between 20–50%.