Nighttime CO2 exchange in a tallgrass prairie and a winter wheat field

Two Ameriflux sites were established in mid to late 1996 to study the exchange of CO2 in a native tallgrass prairie and cultivated winter wheat field located in north-central Oklahoma in the DOE ARM-CART (Atmospheric Radiation Measurement - Cloud and Radiation Testbed) region. The prairie, dominated by warm season C4 grasses, is typical of the central Kansas/northern Oklahoma region. The prairie was burned in the spring of 1997. The wheat crop was planted mid October, emerged about 2 weeks later and reached maturity in late May. The crop was harvested early July. For most of the measurement period the soil was well watered. In this paper, we will discuss nighttime carbon exchange during the first year of measurements using eddy covariance. We will examine the magnitudes and seasonal trends of nighttime carbon exchange in the two ecosystems. We will evaluate the role of the storage term, especially on nights with light wind speeds. We will seek to minimize the impact of problems related to nocturnal flux measurements. Biophysical controls such as soil temperature, green leaf area (as indicator of root activity) and growth stage will be examined. For both sites, we will examine the y-intercept (for photosynthetically active radiation=0) of the daytime CO2 exchange-light response relationship as a potential method to estimate night CO2 emissions when eddy covariance data are not reliable or unavailable.