US Mid-South Cotton Ecosystem Respiration Measurements

CO₂ flux or net ecosystem exchange was measured in US mid-south cotton under typical production practices of the region during the production season (May – October) in 2016-2018 growing seasons (planting to harvest). The average NEE was -0.40±0.29 g C m^-2 d^-1 ranging between -12 .7 to 10.0 g C m^-2 d^-1 during 2016 and -0.98±0.29 g  C m^-2 d^-1 ranging between -12.7 to 6.0 g C m^-2 d^-1 during 2017. Ecosystem respiration (R_eco) was modeled as a natural exponential function of nighttime NEE and soil temperature. R_eco was found to increase 50-90% at 55-65 DAP or first bloom. This increase in respiration could be related to higher base respiration and temperature sensitivity. Elevated respiration was followed by cooler soil temperatures.  Likely the increase was related to higher nighttime air temperatures which increased respiration and decreased carbohydrate content - meaning that more CO₂ was released into the atmosphere than turned into plant material. In this study, nighttime temperatures were 23.9 and 22.6°C before the R_eco increase, 26.9 and 25.7°C after the R_eco increase in 2016 and 2017, respectively. Mechanistically, the higher nighttime temperatures may have caused a decrease in soluble carbohydrates but not enough to mobilize starch into biomass. Concurrent and compounding flowering carbon cost may have induced respiration via increased maintenance of the plant which induced respiration amplified by higher nighttime temperature.  The explanations provided may not entirely describe all underlying processes. Understanding this respiration increase is important to understand plant physiology and how plants may function in current and changing environments.