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Soil organic carbon (SOC) is a biophysical parameter, which is also directly linked to above ground land use and land cover (LULC). Currently changes in LULC and variations in SOC often are studied and modeled separately. However, both are conjoined and should be seen as part of a cascading ecosystem framework. This is not only true for SOC but also for other biophysical parameters which are governed by human activities. At a watershed scale, this relationship is exceptionally important and the focus should be towards studying the impact of LULC change on the levels of SOC in spatially explicit terms. To advance knowledge on this front, we studied transformations of LULC, erosion and SOC from the start of settled agriculture in a moderate size basin of 9340 ha in Union and Pulaski Counties of Southern Illinois. The primary objective of this research was to study the evolution of SOC at the regional scale, as a result of historical land use change and erosion from 1851 to 2005. To model SOC, we used CENTURY 4.0 whereas LULC changes in the area were derived by visually classifying aerial photographs. Long-term erosion associated with different LULC was computed through Revised Universal Soil Loss Equation (RUSLE). To simplify the task of carrying out numerous simulations (>5000), the study area was divided into cells of 100 m x 100 m. Since CENTURY is only vertically spatially explicit, each of these cells was designated as individual locations homogeneous with respect to different input and output parameters like SOC, erosion and LULC. Validation in the study was performed by aggregating and comparing CENTURY derived output with SOC estimates given in the Natural Resource Conservation Service (NRCS) Soil Survey Geographic Database for the year 2000. Our results show a correlation of 0.63 between the simulated and observed SOC estimates. In the study area, approximately 64% of SOC has been lost since the establishment of European-American settlements. Losses are considerable for the soil types which had higher initial levels of SOC. However, from the beginning of 1980, the simulations indicate rising carbon sequestration due to conservational management practices. This assessment is common across all LULC classes considered here. Though comparatively, rates of recovery of lost SOC are higher for areas which converted from agriculture to forest.
