Evaluation of the Carbon and Water Cycles and Climate Benefits of Mature Switchgrass (Panicum Virgatum L.)
With rising demand for bioenergy feedstock, interest in switchgrass (Panicum virgatum L.) has increased over the past decades. However, data on the carbon exchange dynamics and water budgets of mature switchgrass stands (>5 years) is limited. I conducted eddy covariance measurements of CO2 , H2O, and sensible energy fluxes over a 60 ha commercial switchgrass field in its sixth (2012), seventh (2013), and eighth (2014) year since establishment in Southern Ontario, Canada. Measurements from 2014 were compared to a corn field in the same region. Dry matter yield was highest in the dry and warm year of 2012 at 1090 g m-2 and lowest in 2014 at 568±93 g m-2 . Considering the carbon removed at harvest, the ecosystem was a carbon source in 2012 (net ecosystem carbon balance, NECB, of 106±45 g C m-2 ), but a carbon sink in 2013 and 2014 (NECB of −59±45 and −66±59 g C m-2 , respectively). These results show that mature switchgrass can vary between being a sink and a source of carbon on an annual basis. Water use efficiency results indicate that the switchgrass crop was able to reduce the water cost of aboveground biomass in 2012 by relocating carbon from belowground structures, but it fixed slightly more atmospheric carbon per unit water evapotranspired in 2013 compared to 2012 and 2014. Corn had less carbon uptake than switchgrass in 2014 making corn a net carbon source (NECB of 328±30 g C m-2 for a scenario where only corn grain is harvested and 634±34 g C m-2 if corn grain and stover are harvested). Switchgrass had, however, a lower yield in that year than either corn scenario (751±40 g m-2 for grain only and 1439±52 g m-2 for grain plus stover). The switchgrass field had a lower albedo than the corn field during spring and fall, resulting in an annual average negative radiative forcing of −4.5 W m-2. The results from this research project suggest that carbon and water cycling of mature switchgrass can differ substantially from young stands and that switchgrass has many potential climate benefits as a biofuel feedstock crop.