Carbon dioxide exchange measured in a winter wheat field and a tallgrass prairie were analyzed for a complete annual cycle of growth and senescence. Both sites, located in north-central Oklahoma about 50 km apart, are one quarter section in size (160 acre or 64.75 ha). In the wheat field, traditional tillage/fertilizing practices were employed. The tallgrass prairie was ungrazed and burned in the spring. At both sites, soil moisture was not limiting during the period studied. In this presentation, we will discuss similarities and differences of carbon dioxide exchange from these two ecosystems under the same climatic conditions. The wheat carbon dioxide uptake occurs relatively quickly with larger magnitudes and greater leaf area than the prairie. The winter wheat canopy experiences its peak carbon dioxide uptake during early May. About this time (late April-early May), the prairie changes from a carbon source to sink. Also, the wheat ecosystem changes from a carbon sink to a source quickly as compared to the prairie. In both ecosystems the carbon loss during the “non-growing” periods (fallow period for wheat and the senescent period in the prairie) are significant as compared to the net carbon dioxide uptake during the respective growing seasons. Carbon lost due to crop harvest (or due to the prescribed prairie burn) is a significant portion of the annual ecosystem carbon budget.