Dynamics of CO₂ and H₂O fluxes in Johnson grass in the U.S. Southern Great Plains

Johnson grass (Sorghum halepense (L.) Pers.) is rapidly spreading throughout the continental United States (U.S.). Thus, determining magnitudes and seasonal dynamics of carbon dioxide (CO₂) and water vapor (H₂O) fluxes in Johnson grass is crucial to understand regional changes in hydrology and carbon balance. Using eddy covariance (EC), CO₂ and H₂O fluxes were measured from June 2017 to October 2019 over a rainfed Johnson grass field in central Oklahoma. Hay was harvested from late May to early July each year, with biomass yield ~7.5 t ha⁻¹. Weekly averaged daily integrated net ecosystem CO₂ exchange (NEE), gross primary production (GPP), and evapotranspiration (ET) reached −8.28 ± 0.76 g C m⁻², 20.02 ± 1.62 g C m⁻², and 5.42 ± 0.26 mm, respectively. Ecosystem water use efficiency (EWUE) and ecosystem light use efficiency (ELUE) ranged from 3.22 to 3.93 g C mm⁻¹ ET and 0.34 to 0.41 g C mol⁻¹ PAR (photosynthetically active radiation), respectively, during peak growths. Based on aggregated fluxes for each month over the three years (2017–2019), cumulative annual NEE was −434 ± 112 g C m⁻², indicating a carbon gain by the Johnson grass field. Cumulative annual ET (858 ± 72 mm) was ~86% of the average annual rainfall (996 ± 100 mm). Results showed Johnson grass could be a carbon sink from May to September in the U.S. Southern Great Plains. Both NEE and ET did not decline up to air temperature (T_a) of ~33 °C and vapor pressure deficit (VPD) of ~2 kPa, suggesting optimum T_a of ≥33 °C and VPD of ≥2 kPa for the fluxes. Results indicated that Johnson grass might be well suited for dryland production in the region. Additionally, these findings provide initial baseline information on CO₂ fluxes and ET for Johnson grass relative to other forage species in the region.