Carbon, Evapotranspiration and Energy Balance Dynamics of Potential Bioenergy Crops Compared to Cotton in the Southern Great Plains

The Southern Great Plains has potential to produce bioenergy crops on a large scale. However, the potential environmental impact of large scale production of bioenergy crops on carbon, evapotranspiration (ET) and energy dynamics of the region are not understood. This study focuses on land use change associated with bioenergy crops production in the Southern Great Plains and its implication on on-site carbon, energy and ET balances. The carbon and energy balance of two potential bioenergy crops, irrigated sorghum (Sorghum bicolor L.) and dryland old world bluestem (OWB) (Bothriochloa bladhii L.), were assessed and compared with conventional irrigated and dryland cotton (Gossypium hirsutum L.) cropping system of the region. Experiments were conducted on four large producer fields (irrigated sorghum, dryland OWB, irrigated and dryland cotton) in the Texas High plains region. An eddy covariance system was installed in the middle of each field. Continuous measurement of carbon dioxide, latent heat, and sensible heat exchange between plant canopy and atmosphere were made using eddy covariance systems. In addition, net radiation, soil heat flux, air temperature, soil temperature, relative humidity, vapor pressure deficit, soil moisture, photosynthetically active radiation (PAR), and total solar irradiance were measured at each site. Our results showed that mean seasonal net carbon uptake of the sorghum was -615.7 g C m^-2, OWB was -334.9 g C m^-2, irrigated cotton was -136.3 g C m^-2 and dryland cotton was -104.4 g C m^-2. Similarly, mean seasonal ET from sorghum was 480.4 mm, OWB was 384.4 mm, irrigated cotton was 462.9 mm and dryland cotton was 323.2 mm. At an annual scale both sorghum and OWB acted as robust sinks of carbon, whereas cotton cropping systems remained source of carbon. Mean annual net ecosystem exchange of carbon in was -248.1 g C m^-2 in sorghum, -284.9 g C m^-2 in OWB, 112.6 g C m^-2 in irrigated cotton, and 37.2 g C m^-2. At an annual scale irrigated crops recorded higher ET than total annual rainfall. It was also observed that bioenergy crops registered greater ecosystem water use efficiency (3.3 g C kg^-1 H2O in sorghum and 2.8 g C kg^-1 H2O in OWB) than cotton (2.0 g C kg^-1 H2O in irrigated cotton and 1.4 gC kg^-1 H2O in dryland cotton), indicating more sustainability in terms of water usage by bioenergy crops. Among bioenergy crops, sorghum due to irrigation performed better than OWB in terms of carbon assimilation. However, smaller annual ET from OWB than annual rainfall indicated greater groundwater recharge potential. Higher growth rates in C4 bioenergy crops were responsible for their high net carbon sinking capacities. Overall, land use shift from cotton to bioenergy crops was found to be more sustainable in terms of carbon sequestration and water usage