Eddy Covariance Based Estimates of Net Ecosystem Exchanges in Bioenergy Cropping Ecosystems.

Growing concern on energy security, increasing impact of greenhouse gas emissions on climate, and escalating oil prices have led to the use of renewable energy sources. Biomass is the America’s largest domestic source of renewable energy and the only current renewable source of liquid transportation fuel. Expansion of bioenergy crop production in a larger scale may have considerable impact on the environment influencing CO₂, H₂O, and energy balances. Net ecosystem exchanges (NEE) of CO₂, H₂O, and heat are being monitored in eight hectares plot of each of the feedstock species [switchgrass (Panicum virgatum L.), sorghum (Sorghum bicolor (L.) Moench), and mixed-species grasses] at South Central Research Station, Chickasha, OK, using a micrometeorological technique-eddy covariance (EC) method. The objectives of the study are to determine NEE, to document the seasonal and inter-annual variability in NEE with respect to the major environmental variables and dynamic seasonal plant phenology, and to compare ecosystem-level water use efficiency among ecosystems. Preliminary results showed that during active growth (August) switchgrass was carbon sink with mean NEE of – 4 mg C m^-2 day^-1 while sorghum was small carbon source with mean NEE of 0.46 mg C m^-2 day^-1. In peak growth stage (October) both ecosystems behaved as carbon source with mean NEE of 0.43 and 2.05 mg C m^-2 day^-1 for switchgrass and sorghum, respectively. The differences in evapotranspiration (ET) rates between two ecosystems were observed during peak growing stage (1 mm day^-1 in switchgrass and 2.17 mm day^-1 in sorghum), and they were similar during active growing stage (2.35 mm day^-1 in switchgrass and 2.19 mm day^-1 in sorghum). The long term study will enhance our understanding of biophysical controls on land surface-atmosphere energy, H₂O, and gas exchanges and allow us to predict the responses of bioenergy cropping ecosystems to global environmental change in the future.