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The loss of organic carbon (OC) from soils because of agriculture is well established. Where that carbon goes, far less so. Accelerated oxidation could lead to a net source of CO2 to the atmosphere. However eroded soil OC sequestered in alluvia and reservoirs could create a net sink for atmospheric CO2. The Intensively Managed Landscape—Critical Zone Observatory (IML-CZO) has provided an opportunity to study the fate of the eroded soil OC. A preliminary inventory of post-settlement sediment and associated OC accumulation has been made in the IML-CZO site in the Sangamon River Basin of Illinois. Significant stores of OC were found in downslope depressions, floodplain sedimentary deposits and a reservoir at the terminus of the Upper Sangamon Basin, Lake Decatur. Approximately 90% of the OC was trapped by the landscape. Carbon isotopic (δ13C) measurements of bank exposures and Lake Decatur sediments indicate that row crop soils with corn (C4 plant) isotopic signatures contribute to the sequestered C pools but are not the sole sources of OC. C-isotope and biomarker measurements of Lake Decatur sediments reveal the episodic nature of row crop soil OC transport, which appears to be facilitated by sequences of storm events.