Alteration of the soil mantle by strip mining in the Namaqualand Strandveld

[摘要] The purpose of this study was to investigate and identify the occurrence of specificsoil properties that may be important for vegetation functioning and the possibleeffect of the loss of or changes in these properties on rehabilitation success on thesandy coastal plains of the West Coast, South Africa. The study area coveredapproximately 9 400 ha on the Namaqualand coast in the vicinity of Brand-se-Baai(31º18'S 17º54'E), approximately 350 km north of Cape Town and 70 km north-westof the nearest town, Lutzville.A soil survey was done to reveal the presence of important pedological features. The20 soil profiles surveyed are situated within six vegetation communities. Pedologicalfeatures such as surface water repellency, permeable apedal subsurface horizons,subsurface impediments such as cemented (calcrete or dorbank) hardpans andsignificantly more clayey (cutanic, luvic) horizons were identified.A comparative study between rehabilitated and natural soils indicates that miningoperations result in the formation of saline sand tailings, stripped of a large portion ofthe clay and organic matter fraction. The natural leaching of solutes, over a period of25 months, is sufficient to lower salinity of the tailings to levels comparable to naturalsoils. This leaching can also results in lowering of soil fertility. Removal of thedorbank and the dense neocutanic horizon in the western side of the mine, loss oftopographical features such as small dune systems and heuweltjies, destruction ofnatural soil profile morphology and the lowering of organic carbon and clay plus siltfraction can have detrimental effects on attempts at rehabilitation of this area to anatural condition similar to that which preceded the mining operation.Infiltration fingering and deep percolation results in the development of an aquiferbelow the reach of shallow-rooted desert shrubs. A method of water acquisition byvegetation through water distillation is investigated as a possible solution to theapparent discontinuum between the shallow root systems and deeper-lying aquifer.Volumetric water content measurements indicated that precipitation of 29.5 mm, overa period of 10 days, did not result in any variation at 235 mm, 360 mm and 900 mm depths. An average volumetric water content increase of 0.4 mm per night wasmeasured in the first 23.5 cm of soil surface. This amount is a significant source ofwater that can explain the shallow root distribution. Water vapour movement due totemperature gradients can explain the diurnal volumetric water content fluctuationsobserved. Further studies are necessary to determine to what extent the depth of waterinfiltration influences the capacity of subsurface dew to provide plants with anocturnal water source.Findings of this study can be summarised into two concepts namely:• Heuweltjies, small dune systems, and variation in depth of cemented hardpansare the main features that contribute to pedosphere variation and possibly tobiodiversity.• Pedogenic features such as topsoil hydrophobicity, and cemented dorbank anddense more clayey (cutanic, luvic) subsurface horizons are importantcomponents of a soil water distillation process that could be a driving forcebehind vegetation functioning in this region.Mine activities result in the loss of certain pedogenic features and soil properties thatthat could be key ingredients to ecosystem functioning. The inability to recognisetheir significance and ignorance thereof when planning rehabilitation methods mightprevent sustainable restoration of the environment.

[发布机构] Stellenbosch University