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

Authors: Wagle, Pradeep; Gowda, Prasanna; BILLESBACH, DAVID - University Of Nebraska; Northup, Brian; TORN, MARGARET - Lawrence Berkeley National Laboratory; Neel, James; BIRAUD, SEBASTIEN - Lawrence Berkeley National Laboratory

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2020
Publication Date: 6/9/2020
Citation: Wagle, P., Gowda, P.H., Billesbach, D., Northup, B.K., Torn, M., Neel, J.P., Biraud, S. 2020. Dynamics of CO2 and H2O fluxes in Johnson grass in the U.S. Southern Great Plains. Science of the Total Environment. 739:140077. https://doi.org/10.1016/j.scitotenv.2020.140077.
DOI: https://doi.org/10.1016/j.scitotenv.2020.140077

Interpretive Summary: Johnson grass has been intentionally cultivated as pasture and/or unintentionally disseminated as weeds in almost all states of the continental United States. Given the spread of Johnson grass as a weed and pasture crop across the United States, understanding carbon and water dynamics of Johnson grass ecosystems is crucial. This study analyzed eddy covariance measurements of CO2 and H2O fluxes over a Johnson grass field managed as a hay crop over multiple growing seasons. Biomass yield at early summer hay harvests was ~7.5 t ha-1. Eddy covariance measurements showed that the Johnson grass field could gain carbon for a longer period (from May to September) than other C4 species in this region. The field was a carbon sink (gaining more carbon than releasing) at the annual scale, with cumulative annual net ecosystem CO2 exchange (NEE) of -434 ± 112 g C m-2. Cumulative annual evapotranspiration (ET = 858 ± 72 mm) was ~86% of the average annual rainfall over the three years study period. Our results provide initial baseline information on the magnitudes and seasonal/annual budgets of NEE and ET for Johnson grass in the Southern Great Plains region of the United States.

Technical Abstract: Johnson grass (Sorghum halepense (L.) Pers.) is rapidly disseminating 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. Net ecosystem exchange (NEE) of CO2 and H2O fluxes was measured from June 2017 to October 2019 using the eddy covariance (EC) technique over a rainfed Johnson grass field (~8 ha) in central Oklahoma. The field was harvested for hay from late May to early July each year. Biomass yield during summer harvests was ~7.5 t ha-1, and ~4.5 t ha-1 was produced for the remainder of growing season. Weekly averaged daily integrated NEE and evapotranspiration (ET) reached -8.28 ± 0.76 g C m-2 and 5.42 ± 0.26 mm, respectively, which were slightly smaller than the magnitudes of daily NEE and ET for other C4 species in the U.S. Southern Great Plains (SGP) region. Ecosystem water use efficiency (EWUE) ranged from 3.22 to 3.93 g C mm-1 ET and ecosystem light use efficiency (ELUE) ranged from 0.34 to 0.41 g C mol-1 PAR 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 at annual scales. Similarly, cumulative annual ET was 858 ± 72 mm, which was ~86% of the average annual rainfall (996 ± 100 mm). Results showed Johnson grass could be a carbon sink in this region during May–September period. The NEE did not decrease up to air temperature of ~35 °C. Results indicated that Johnson grass might be well suited for dryland production in the region. Additionally, these findings provide initial baseline information on NEE and ET for Johnson grass in the SGP region.