Carbon and water dynamics in co-located winter wheat and canola fields in the U.S. Southern Great Plains

Abstract

The magnitudes and seasonal dynamics of net ecosystem exchange (NEE) of carbon dioxide (CO2) and evapotranspiration (ET), measured using the eddy covariance (EC) technique from co-located, paired (conventional till, CT and no-till, NT) winter wheat (Triticum aestivum L.) and canola (Brassica napus L.) fields, were compared during a presumably favorable growing season for both crops. The magnitudes (7-day average) of NEE, gross primary production (GPP), and ET reached approximately −8 g C m−2 d−1, 16 g C m−2 d−1, and 5 mm d−1, respectively, at both CT and NT wheat fields due to uniform canopy stands. The magnitudes (7-day average) of NEE reached −5.20 ± 0.49 and −4.66 ± 0.36 g C m−2 d−1, GPP reached 12.47 ± 1.16 and 10.86 ± 0.97 g C m−2 d−1, and ET reached 4.7 ± 0.42 and 4.28 ± 0.36 mm d−1 at CT and NT canola fields, respectively. Poor recovery of canola stand in the NT field after winter dormancy resulted in smaller magnitudes of fluxes in spring 2017. Wheat had a larger potential as a carbon sink during winter than canola. Larger magnitudes of CO2 fluxes and longer periods as carbon sinks for wheat caused large differences in carbon sequestration potential between wheat and canola. Fluxes showed similar responses to climatic conditions as optimum air temperature (Ta) was ∼22 °C for NEE and GPP, and ∼25 °C for ET, and the fluxes peaked at ∼1.7 kPa vapor pressure deficit (VPD) for both crops. However, the GPP-PPFD (photosynthetic photon flux density) relationship was weaker beyond optimum Ta and VPD in canola than in wheat. Ecosystem light use efficiency (ELUE) and ecosystem water use efficiency (EWUE), determined using different metrics, were higher in wheat than in canola. The results indicate higher adaptability, and water and light use efficiencies of wheat than canola. This study provides an initial baseline on the dynamics of CO2 fluxes, ET, EWUE, and ELUE for canola, and side-by-side comparison of eddy fluxes in two major winter crops grown in the Southern Great Plains.