Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2018
Publication Date: 5/8/2018
Citation: Wagle, P., Gowda, P., Moorhead, J.E., Marek, G.W., Brauer, D.K. 2018. Net ecosystem exchange of CO2 and H2O fluxes from irrigated grain sorghum and maize in the Texas High Plains. Science of the Total Environment. 637-638:163-173. https://doi.org/10.1016/j.scitotenv.2018.05.018.
Interpretive Summary: Maize and grain sorghum are two most widely grown cereal crops in the world. Recently, utilization of maize and grain sorghum by the ethanol industry has been growing. Consequently, maize and grain sorghum rank first and second, respectively, for grain-based ethanol production in the U.S. Quantitative evaluation of carbon dynamics and evapotranspiration (ET) of major biofuel crops across different climatic regions is important to develop sustainable bioenergy crops and optimizing resource use efficiencies. Measurement of carbon and water dynamics is lacking for grain sorghum. Such measurement for maize is also lacking outside the U.S. Corn Belt, especially in the U.S. Southern Great Plains. We measured net ecosystem CO2 exchange (NEE – carbon uptake) and ET (water loss) of irrigated grain sorghum and maize using the eddy covariance technique during 2014-2016 growing seasons in the Texas High Plains. Peak growing season daily (weekly average) NEE and ET reached up to -12 g C m-2 and 6.5 mm, respectively, for sorghum and -14.78 g C m-2 and 7.3 mm, respectively, for maize. Both NEE and ET were not inhibited by climatic conditions during peak photosynthetic phases. The magnitudes of NEE and ET were larger in maize than in sorghum due to higher leaf area, biomass, and grain yield of maize. The magnitudes of CO2 fluxes for irrigated maize in the Texas High Plains (this study) were within the range or slightly smaller and larger than those for irrigated and rainfed maize, respectively, in the U. S. Corn Belt. Overall, our results suggest that both crops are well adapted to the Texas High Plains under irrigation.
Technical Abstract: Net ecosystem exchange (NEE) of carbon dioxide (CO2) and water vapor (H2O) fluxes from irrigated grain sorghum (Sorghum bicolor L. Moench) and maize (Zea mays L.) fields in the Texas High Plains were quantified using the eddy covariance (EC) technique during 2014-2016 growing seasons and examined in terms of relevant controlling climatic variables. Eddy covariance measured evapotranspiration (ETEC) was also compared against lysimter measured ET (ETLys). Sorghum grain yield averaged 8.1 and 9.4 t ha-1 in 2014 and 2015, respectively, and maize grain yield averaged 14.1 t ha-1 in 2016. Diurnal peak NEE reached -48.4 µmol m-2 s-1 for sorghum and -59 µmol m-2 s-1 for maize. Daily peak NEE reached approximately -12 g C m-2 in both years for sorghum and -14.78 ± 1.14 g C m-2 for maize. Daily peak ETEC reached up to 6.5 mm for sorghum and 7.3 mm for maize. Comparisons of 30-minute ETEC and ETLys showed a strong agreement (R2 = 0.93-0.96) while the EC system underestimated ET by 15-24% as compared to lysimeter without any corrections or energy balance closure adjustments. The NEE and ETEC were not inhibited by climatic variables during peak photosynthetic period even though diurnal peak values of photosynthetic photon flux density (PPFD), air temperature (Ta), and vapor pressure deficit (VPD) had reached > 2000 µmol m-2 s-1, 31-32º C, and > 2.5 kPa, respectively. Overall, our results suggest that both crops are well adapted to the Texas High Plains under irrigation. However, more sensitivity of NEE and H2O flux beyond threshold Ta and VPD for maize than for sorghum indicated higher adaptability of sorghum for the region. It is necessary to collect measurements over several years for assessing long-term carbon and water dynamics of these important agro-ecosystems in the Texas High Plains.