Reconstructing rangeland soil carbon erosion using sediment trapped behind small reservoir dams

Human-induced soil erosion significantly impacts soil organic carbon (SOC) dynamics, particularly in rangelands, where land degradation and overgrazing exacerbate SOC loss. This study investigates whether sediments from small farm reservoirs in South Africa's Karoo rangelands can serve as archives to reconstruct the temporal pattern of SOC changes during the 20th-century. Sediments from three dams, accumulated during different time periods, were analysed for radionuclide markers (e.g. 137Cs), organic carbon (OC) content, sediment yield and OC flux. The results illustrate the suitability of the farm dam sediments to reconstruct soil erosion history in the Karoo and reveal distinct temporal patterns of SOC erosion for each catchment. For all dams, increases in sediment input into the reservoirs did not correspond to a proportional increase in organic carbon accumulation, highlighting the influence of land degradation on SOC depletion over time. Dam-specific variations underscore the role of land use and local heterogeneity in shaping erosion and carbon fluxes. These findings suggest that lateral SOC transfer has been dynamic during the 20th century, challenging models that assume constant sediment OC concentrations over time. Therefore, this study illustrates the importance of reconstructing past SOC dynamics and gain a correct understanding of the carbon fluxes in degraded rangelands and their broader implications for the global carbon cycle during the 20th century.