Erosion and deposition history derived by depth-stratigraphy of 137Cs and soil organic carbon

There are a number of uncertainties in the use of 137Cs as a marker for deriving soil erosion rates. However, this should not limit other potential uses of this anthropogenic radionuclide in the study of soil landscape processes. This study outlines a sampling methodology which aids in the assessment of the history of erosion and depositional processes within a landscape unit. The depth distribution of 137Cs and soil organic carbon (SOC) was utilized as a means of determining the erosion and depositional history of a conventionally tilled agricultural field in southern Ontario, Canada. Three transects oriented along the slope of a large field had five soil profiles excavated at the summit, sideslope, shoulder slope, footslope and toeslope landscape positions. The soils were sampled in 5 cm increments, and 137Cs and SOC were determined on the samples. The results show that soil redistribution within landscape units of agricultural fields has been substantial both before and after fallout of 137Cs to the soil surface. Soils in depositional areas contained significant 137Cs and SOC at depths beyond which the plow can attain at present. This implies that a significant amount of carbon is being sequestered beneath the present plow layer, and the characterization of this pool must be considered in deriving the dynamics of SOC in agroecosystems.