The potential to reconstruct 20th century soil carbon erosion in rangelands from small reservoir sediments

Land degradation affects the productivity of the land, but is also associated with a flux of greenhouse gases (GHGs) from soil and vegetation into the atmosphere. The large area of rangelands (35 million km², cover about 40% of earth’s surface) can contribute significantly to changes of atmospheric CO₂ concentrations following even a minor alteration of the rangeland soil C pool (Wang et al. 2002). Degradation of South African rangelands has been a concern for more than 100 years (Rowntree, 2013). The Karoo drylands, covering 30% of the land surface of South Africa, have experienced particular intense soil erosion and thus loss of topsoil C. To sustain the large number of animals, many small farm dams have been constructed mainly in the first half of the twentieth century. As a consequence of the soil erosion, they are now often silted-up and have breached (Boardman, 2014). The sediment deposited behind such small dams offers the possibility to reconstruct the loss soil C for the time peirod between construction and breaching of dams. Five dams were chosen to explore the possibility to use their sediment as an environmental archive for 20^th century rangeland soil Carbon loss. The specific aims of our study are to 1) find out whether distinct C profiles can be discerned in dam sediments; 2) identify whether these changes potentially reflect erosion and soil C loss in the dam catchments; and 3) to discuss whether the dam sediments can serve as an environmental archive to reconstruct soil-atmosphere interaction during recent decades. The initial survey of the dams involved the sampling of individual sediment strata and the analysis of their organic and nitrogen content, as well as the ¹³⁷Cs activity of selected samples to gain an insight into the time of deposition. Two dams showed a profile that indicates a loss of soil C during the first decreased after their construction, while the other dams showed no clear signal or even an increase of sediment C in the younger sediment. One dam showed no ¹³⁷Cs activity, indicating that it was filled with sediment very quickly after construction. These results illustrate that soil degradation and associated loss of soil C stocks can potentially be reconstructed based on the small dam sediments. However, the source of the sediment C has to be tested, as well as the individual land use and erosion history of each dam catchment.