Carbon Dioxide Exchange In Switchgrass and High Biomass Sorghum Ecosystems In The Southern Great Plains Of The United States.

Switchgrass (Panicum virgatum L.) and high biomass sorghum (Sorghum bicolor L. Moench) are two major dedicated lignocellulosic feedstocks for biofuel production in the southern Great Plains of the United States. We analyzed eddy covariance carbon dioxide flux measurements from these two co-located contrasting ecosystems during the 2012 growing season. The objective of this study was to compare the magnitude and seasonality of net ecosystem CO₂ exchange (NEE) between ecosystems. The highest recorded biomass was about 17 t ha^-1 in switchgrass and 30 t ha^-1 in sorghum. The magnitudes of diurnal peak NEE (monthly average) were -33.02 ± 1.96 (May) and -35.86 ± 2.32 µmol m^-2 s^-1 (June) in switchgrass and sorghum, respectively. The weekly average of daily integrated NEE reached a maximum of -8.5 g C m^-2 day^-1 in switchgrass and -10.3 g C m^-2 day^-1 in sorghum during peak growth. The NEE rates in sorghum were larger during June and July than those in switchgrass, but they were smaller from August to October. Seasonal (March - October) cumulative gross ecosystem photosynthesis (GEP) and ecosystem respiration (ER) of switchgrass were 2017 and 1527 g C m^-2, respectively. Similarly, seasonal (May - October) cumulative GEP and ER of sorghum were 1164 and 903 g C m^-2, respectively. The result shows that the switchgrass ecosystem was a larger carbon sink with a cumulative seasonal carbon uptake of -490 g C m^-2 compared to -261 g C m^-2 by sorghum. The difference in sink strength between two ecosystems was driven mainly by the length of the growing season. Switchgrass ecosystem was a net sink of carbon during first 5 months (April - August) while the sorghum ecosystem was a net sink of carbon for only 3 months (June - August).