As part of the NOAA’s contribution to GEWEX a long term energy/carbon eddy-covariance tower system was established on a no-till corn and soybean field in central Illinois. Continuous measurements of CO₂ and water vapor fluxes, energy balance, and weather data have been collected from corn (1997, 1999, 2001) and soybean (1996, 1998, 2000) since August 1996. Plant growth analysis using destructive sampling techniques was also conducted during 1998, 1999, 2000, and 2001. The continuous eddy covariance measurements of CO₂ fluxes from agricultural canopies provide an estimate of the Net Ecosystem Exchange (NEE). However, destructive plant growth analysis from these canopies indicate the NEE significantly underestimates the productivity of corn (Zea mays) and soybean (Glycine max) canopies. This discrepancy is a result of the eddy co-variance measurements accounting only for the CO₂ that is exchanged between the canopy and the atmosphere at the height of the instruments. The atmosphere is only one source of CO₂ that a canopy uses in photosynthesis, other sources include inorganic and organic CO₂ from the soil (referred to as soil respiration). The CO₂ that is released from the soil during daylight hours can be easily recaptured by photosynthesis and may provide a significant contribution to total canopy productivity. Total canopy productivity includes both above and below ground plant biomass. The contribution of recaptured CO₂ is greater in a corn canopy than a soybean canopy. This paper will explore different methods to estimate the sources and quantity of recaptured CO₂ and propose methods to more accurately estimate this important source of carbon.