Seasonal Variations In Carbon Dioxide Exchange of Drought Stressed Switchgrass Ecosystem.

As switchgrass (Panicum virgatum L.) acreage is expanding for cellulosic biofuels, it is critical to improve understanding of carbon dynamics of the switchgrass ecosystem. This study takes advantage of net ecosystem CO₂ exchange (NEE) measurements during an exceptional drought experienced by switchgrass (2^nd year of establishment) in the southern Great Plains of the United States (Chickasha, OK). We used the eddy covariance system and examined seasonal changes in NEE and its components - gross ecosystem photosynthesis (GEP) and ecosystem respiration (ER) in response to environmental and physiological factors during a water deficit condition. Modified rectangular hyperbolic light-response curve to include vapor pressure deficit (VPD) limitation of photosynthesis yielded better estimates of GEP and daytime ER, and improved the model significantly. Interestingly, net ecosystem production (NEP = -NEE) and GEP showed negative correlation with leaf area index (LAI) due to drought. Ecosystem carbon use efficiency (CUE) was strongly correlated with NEP (R² = 0.84). Weekly averages of daily GEP and ER showed very high correspondence on seasonal scale (R² = 0.65). However, more rapid reduction in GEP than ER during drought reduced NEE. The cumulative ER and GEP during the growing season (late April – mid November, 2011) were 9.1 and 12.0 t C ha^-1, respectively. This result showed that the net gain by the ecosystem during the growing season was -2.9 t C ha^-1 and proves that switchgrass ecosystem can sequester significant amounts of carbon under marginal conditions.