Carbon dioxide and water vapor fluxes of multi-purpose winter wheat cropping systems in the U.S. Southern Great Plains

Authors: Wagle, Pradeep; Gowda, Prasanna; Northup, Brian; Neel, James; Starks, Patrick; Turner, Kenneth; Moriasi, Daniel

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/29/2021
Publication Date: 9/8/2021
Citation: Wagle, P., Gowda, P.H., Northup, B.K., Neel, J.P., Starks, P.J., Turner, K.E., Moriasi, D.N. 2021. Carbon dioxide and water vapor fluxes of multi-purpose winter wheat cropping systems in the U.S. Southern Great Plains. Agricultural and Forest Meteorology. 310:108631. https://doi.org/10.1016/j.agrformet.2021.108631.
DOI: https://doi.org/10.1016/j.agrformet.2021.108631

Interpretive Summary: Daily magnitudes, and seasonal and annual budgets of carbon dioxide (CO2) fluxes and evapotranspiration (ET) were determined for multi-purpose winter wheat (grain-only, graze-grain, and graze-out) fields managed under conventional tillage and no-till systems in the Southern Great Plains of the United States. Additionally, quantitative relationships of CO2 fluxes and ET against satellite-derived enhanced vegetation index (EVI) and ground-measured biometric variables (leaf area index - LAI, dry biomass, and canopy coverage) were determined to investigate variability in fluxes across sites (spatial) and years (temporal). Large variations in fluxes were observed among seasons even for the same purpose wheat (e.g., grain-only) in different fields due to differences in crop growth and development, and meteorological conditions. The magnitudes of fluxes (daily net ecosystem CO2 exchange (NEE), gross primary production (GPP), and ecosystem respiration (ER) of -11, 19, and 12 g C m-2, respectively, and daily ET of ~7 mm) observed in this study were comparable to the reported values for winter wheat in the literature. Wheat fields, including graze-out fields, were sinks (ranged from -149 to -564 g C m-2) of carbon at seasonal scales, but they ranged from near carbon neutral to large sinks of carbon at annual (calendar year) scales. Regression analyses showed biometric variables and EVI were key to explain spatio-temporal variations of CO2 fluxes and ET in grain-only wheat fields. In comparison, fluxes showed poor relationships with biometric variables and EVI in grazed wheat fields. The findings of this study will aid in understanding and providing refinement options for development of more sustainable wheat production systems that enhance carbon sequestration and decrease greenhouse gas emissions.

Technical Abstract: Eddy covariance measurements from eight production-scale multi-purpose winter wheat fields (grain only, graze-grain, and graze-out) managed under conventional till (CT) and no-till (NT) systems in the Southern Great Plains of the United States were synthesized to determine seasonality, daily magnitudes, seasonal, and annual budgets of carbon dioxide (CO2) fluxes and evapotranspiration (ET), and to investigate spatio-temporal variability of the fluxes. Maximum daily net ecosystem CO2 exchange (NEE), gross primary production (GPP), and ecosystem respiration (ER) approximated -11, 19, and 12 g C m-2, respectively, and daily ET approximated 7 mm. All wheat fields, including graze-out, were large sinks (ranged from -149 to -564 g C m-2) of carbon at seasonal scales. Wheat fields, which were kept fallow during summer, were from near carbon neutral to large sinks of carbon at annual (calendar year: January–December) scales. Cumulative annual NEE was up to -242 g C m-2 in a NT and -183 g C m-2 in a CT field, which had grain-only wheat in spring followed by graze-grain wheat in fall. Cumulative seasonal ET ranged from 260 mm to 521 mm, and maximum annual (calendar year) ET approximated 800 mm. In general, ET was smaller under NT than CT. Eddy fluxes showed stronger relationships with remotely-sensed enhanced vegetation index and in-situ biometric variables in grain-only fields than grazed fields. Across-site analysis for grain-only wheat showed biomass and leaf area index alone explained >80% of variations in NEE, GPP, and ET, and ~70% of variations in ER. Similarly, Canopy coverage explained >80% of variations in NEE and GPP, and ~60% of variations in ER and ET. A strong correspondence of biometric observations with the fluxes demonstrated their potential to model and explain spatio-temporal variability of CO2 fluxes and ET. Results also indicated huge implications of management practices on carbon and water budgets by altering the vegetative properties of winter wheat.