Effects of soil transport processes on organic carbon storage in forested hillslope soils

Abstract

This study examines how coupled geomorphic and biologic processes influence the potential for hillslope soils to store organic carbon. A mass-balance model of soil organic carbon on hillslopes is developed by combining soil transport with biologic carbon production and respiration. The model is compared with soil carbon measurements on three hillslopes that represent various stages of hillslope evolution at Land-Between-the-Lakes National Recreation Area, northwest Tennessee. Both model results and field measurements suggest that locations of soil thickening are able to store more organic carbon than locations of soil thinning. In model simulations of a hillslope relaxing onto a fluvial terrace, locations of soil thickening, which propagate upslope, represent the greatest potential to increase soil organic carbon through time. The downslope increase in soil organic carbon seen in modeling and field measurements arises from the systematic variation in soil thickness associated with soil transport and hillslope evolution. Varying soil transport parameters (e.g. diffusivity, initial soil production) in model simulations reveals that physical soil transport processes exert greater control than biological carbon production and respiration on the change in soil organic carbon storage of a hillslope through time. At locations where thinned soils are converted to locations of soil thickening, the organic carbon storage of the soil increased in model simulations by 5-25%. This study suggests that physical soil processes are more important than biological carbon production and respiration processes in sequestering organic carbon in forested hillslope soils.