PARENT SESSION
Oral Session #2: Riparian Ecology.
Presiding: M. Dixon
Monday, August 5. 8:00 AM to 11:30 AM. Greenlee Meeting Room, TCC.
Development of sodic patches in the riparian-upland boundary along the Phungwane River, South Africa.
Matoti, Ayanda*,1, Rogers, Kevin1, 1 Centre for Water in the Environment, School of Animal, Plant and Environmental Sciences, Private Bag 3, Wits University, Gauteng, South Africa
ABSTRACT- Sodic soils are characterized by accumulation of sodium ions, and the electrolyte concentration determines the sodium concentration at which the soil structure is modified. Landscape processes behind sodic soil formation are not well understood. Along the Phungwane River landscape, sodic soils form bare patches at the boundary between riparian and upland ecoystems. The aim of this study is to provide new insights into the distribution, and structure of sodic patches, and to infer the processes that control their development. The existing classical catena and evapotranspiration models have been integrated to explain the development of sodic patches. An integrated model is built on the premise that the flow of water and minerals from the riparian system outward, as well as input from the adjacent upland along the catena gradient, meet at the riparian-upland boundary. Transpiration from trees at the riparian-upland boundary brings soluble salts from deeper soil layers to upper soil layers. In the present study, soil pits were excavated across the riparian-upland boundary and soil samples were analyzed for cations (Na+, K+, Ca2+, Mg2+), pH, electrical conductivity and clay mineralogy. Electrical conductivity, and sodium absorption ratios were higher and increased with depth on bare sodic patches. This suggests that transpiration demands from trees at the riparian-upland boundary brings soluble salts from lower depths to upper soil horizons. Clay mineralogy was mainly dominated by quartz, mica and albite. The clay minerals on the surface horizons of upland profiles could not be traced on either sodic or riparian soil horizons. In sodic soils the argillic A-horizon which may have included the upland clays has been lost. The development of sodic soils along the Phungwane river landscape can be explained by a combination of the two existing models.
KEY WORDS: Sodic , sodium, riparian, mineralogy