Carbon and Water Vapor Fluxes of Dedicated Bioenergy Feedstocks: Switchgrass and High Biomass Sorghum

Abstract:

We compared eddy covariance measurements of carbon and water vapor fluxes from co-located two major dedicated lignocellulosic feedstocks, Switchgrass (Panicum virgatum L.) and high biomass sorghum (Sorghum bicolor L. Moench), in Oklahoma during the 2012 and 2013 growing seasons. Monthly ensemble averaged net ecosystem CO₂ exchange (NEE) reached seasonal peak values of 36-37 µmol m^-2 s^-1 in both ecosystems. Similar magnitudes (weekly average of daily integrated values) of NEE (10-11 g C m^-2 d^-1), gross primary production (GPP, 19-20 g C m^-2 d^-1), ecosystem respiration (ER, 10-12 g C m^-2 d^-1), and evapotranspiration (ET, 6.2-6.7 mm d^-1) were observed in both ecosystems. Carbon and water vapor fluxes of both ecosystems had similar response to air temperature (T_a) and vapor pressure deficit (VPD). An optimum T_a was slightly over 30 °C for NEE and approximately 35 °C for ET, and an optimum VPD was approximately 3 kPa for NEE and ET in both ecosystems. The switchgrass field was a larger carbon sink, with a cumulative seasonal carbon uptake of 406-490 g C m^-2 compared to 261-330 g C m^-2 by the sorghum field. Despite similar water use patterns during the active growing period, seasonal cumulative ET was higher in switchgrass than in sorghum. The ratio of seasonal sums of GPP to ET yielded ecosystem water use efficiency (EWUE) of 9.41-11.32 and 8.98-9.17 g CO₂ mm^-1 ET in switchgrass and sorghum, respectively. The ratio of seasonal sums of net ecosystem production (NEP) to ET was 2.75-2.81 and 2.06-2.18 g CO₂ mm^-1 ET in switchgrass and sorghum, respectively. The switchgrass stand was a net carbon sink for four to five months (April/May-August), while sorghum was a net carbon sink only for three months (June-August). Our results imply that the difference in carbon sink strength and water use between two ecosystems was driven mainly by the length of the growing season.