The Potential to Reconstruct 20th Century Soil Organic Carbon Erosion in Rangelands From Small Reservoir Sediments

ABSTRACT Soil erosion and soil organic carbon (SOC) loss are not always linked linearly because SOC‐rich topsoil is eroded at the initial stages of degradation, while horizons with lower SOC content are eroded later, but often at higher rates. Small, silted‐up farm reservoirs potentially document this change during the period of sediment accumulation. This study tests the specific potential of small farm reservoir sediments from the South African Karoo to reconstruct 20th century SOC and total nitrogen (TN) change in rangeland soils. Five reservoir sediment profiles were sampled and texture, total organic carbon (TOC), TN and 137 Cs of the samples were analyzed and compared. The results show that there clearly distinguishable flood couplets have been preserved in the sediment, illustrating their suitability for the chronological reconstruction of soil erosion and SOC. With one exception, the older sediments contain more TOC and TN than the younger ones. The TOC changed mostly in earlier than later stages of deposition, which is indicative of soil degradation early after the construction of the dams in the 1920s and 1930s. These distinct changes illustrate that the small reservoir sediments have the potential to reconstruct the impact of land‐use and associated soil erosion on SOC change in rangelands. Their analysis can therefore contribute to a better understanding of the land‐use associated changes of the global carbon cycle during the 20th century.