Carbon Balance of No‐Till Soybean with Winter Wheat Cover Crop in the Southeastern United States

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 offsetting C losses caused by summer droughts. The Re 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 Re 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.