The southeastern United States is an economically important agricultural region, yet its role in the regional C budget is not fully understood. There is concern that climate change, particularly altered precipitation patterns, may induce a shift in how crops exchange CO2 with the atmosphere. This study examined the seasonal and interannual variation in net ecosystem exchange (NEE) of a winter wheat cover crop (Triticum aestivum L.) and soybean [Glycine max (L.) Merr.] using the eddy covariance (EC) method. This was conducted at Winfred Thomas Agricultural Research Station, Hazel Green, AL (2007-2009). Annual C balance ranged from a source in 2007 (NEE = 100 g C m(-2) yr(-1)) to a sink (-20 g C m(-2) yr(-1)) in 2009. Annual ecosystem respiration (Re) ranged between 750 and 1013 g C m(-2) yr(-1), while gross ecosystem productivity was between 650 and 1034 g C m(-2) yr(-1). Seasonal NEE for soybean ranged between 42 and -66 g C m(-2). The uptake rates from the cover crop (NEE = -80.0, -80.4, and -40.0 g C m(-2) for 2007, 2008, and 2009, respectively) suggested the importance of winter C uptake off setting C losses caused by summer droughts. The R-e varied between 286 and 542 g C m(-2) for soybean and between 160 and 313 g C m(-2) for the cover crop. Annual variations in NEE and R-e were primarily due to precipitation and air temperature, respectively, indicating a tight coupling between biophysical factors and C uptake. Our results were compared with those from other reported NEE crop estimates using EC.
