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The Great Karoo region of South Africa: a carbon source or sink?

Kuhn, N., Greenwood, P., Kuhn, B., Foster, I. D. L., Boardman, J. and Meadows, M. (2014) The Great Karoo region of South Africa: a carbon source or sink? Poster presented to: Triannual Conference of the International Association for Sediment Water Science (IASWS), Rhodes University, Grahamstown, South Africa, 15-18 July 2014. (Unpublished)

Item Type: Conference or Workshop Item (Poster)
Abstract: Work undertaken in the seasonally arid upland areas of the Great Karoo region of South Africa has established a link between land degradation and overgrazing that began approximately 200 years ago when European farmers first settled the area. In response to changing land use, coupled with shifting rainfall patterns, parts of the landscape are now characterised by badlands on footslopes of valley-sides and complex gully systems on valley floors. Limited precipitation and agricultural intensification, particularly from around the 1920s onwards, resulted in a growing demand for water, and led to the construction of many small reservoirs, most of which are now in-filled with sediment. Whilst the deposited material has provided a means of linking catchment-scale responses to land use changes over the last ca. 100 years, the influence of land degradation on erosion and deposition of soil-associated carbon (C) has received only limited attention. Despite a reversion to extensive agriculture and reduced livestock densities in certain areas, limited vegetation regrowth suggests that soil rehabilitation will be a long-term process. This communication presents preliminary results from an investigation to determine whether land degradation in the Karoo has resulted in a shift from a net sink of C to a net source of C. Sediment deposits from a silted-up reservoir in a small dry valley system was analysed for varying physicochemical parameters. Total Carbon (TC) content was recorded and the sharp decrease in total C content with decreasing depth suggests that land degradation during and after post-European settlement probably led to accelerated erosion of the relatively fertile surface soils, and this presumably resulted in the rapid in-filling of reservoirs with carbon-rich surface material. Overall, the results indicate a sharp decline in soil organic matter (SOM) of eroded material, presumably as a consequence of land degradation. This suggests that in landscapes such as the overgrazed drylands of the Karoo, the C sink effect caused by soil erosion is now much smaller than at the onset of overgrazing leading to accelerated erosion. Such a loss of C sinks on degraded rangeland soils raises the question whether past soil erosion may have had a greater attenuating effect on GHG emissions than modelled scenarios of present emissions suggest.
Subjects: Q Science > QE Geology > QE500 Dynamic and structural geology > QE570 Weathering. Erosion
G Geography. Anthropology. Recreation > GB Physical geography > GB400 Geomorphology. Landforms. Terrain > GB440.S6 Geomorphology of South Africa
Q Science > QE Geology > QE500 Dynamic and structural geology > QE571 Sedimentation and deposition
Creators: Kuhn, Nikolaus, Greenwood, Philip, Kuhn, Brigitte, Foster, Ian D L, Boardman, John and Meadows, Mike
Faculties, Divisions and Institutes: Faculties > Faculty of Arts, Science & Technology > Environmental Science
Date: 16 July 2014
Date Type: Presentation
Event Title: Triannual Conference of the International Association for Sediment Water Science (IASWS)
Event Dates: 15-18 July 2014
Event Location: Rhodes University, Grahamstown, South Africa
Event Type: Conference
Language: English
Status: Unpublished
Refereed: No
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