Soil nutrient content, above-ground biomass and litter in a semi-arid shrubland, South Africa

In semi-arid shrublands, nutrient focusing in vegetation clusters may lead to an increase in biomass production and higher rates of litter accumulation under shrub cover. This may provide a positive feedback mechanism for soil nutrient cycling. Fynbos biome vegetation growing on Late Pleistocene coastal dunes (Langebaan Dune Strandveld) exhibits a distinct pattern of thickets (clusters) surrounded by sparsely vegetated intergrowth areas. Plant species composition and structure in the thickets differ markedly from that found in the intergrowth areas with little overlap. The dominant plant genera in the thickets include Diospyna and Rhus whereas Restionaceae dominate the intergrowth areas. Thickets have an above-ground biomass of 1.81 kg/m(2) and litter biomass of 1.71 kg/m(2) which are significantly higher than the above-ground biomass of 0.47 kg/m(2) and litter biomass of 0.15 kg/m(2) in the adjacent intergrowth areas. The higher soluble soil phosphate content of the thicket (10.3 mg/kg) compared to the intergrowth (5.7 mg/kg) areas may contribute to the differences in biomass production. Micronutrients Mn, Zn, Mo, Ni and Cu are more abundant in bulk soil, bulk woody and leafy plant material and litter composed mostly of leaves than in soil saturated paste extracts. Micronutrients Fe, B, Mn, Zn, Cu, Mo, Ni, and Co and the macronutrients including K, Mg and Ca are higher in bulk soil from the thickets than soils in the surrounding intergrowth. Our data suggest the development of a soil nutrient gradient which we interpret as resulting from the transfer of moisture, nutrients and clay minerals through water runoff and wind transport of particulate materials from the sparsely vegetated intergrowth to the vegetation thickets, resulting in higher primary productivity and litter accumulation. The thickets represent resource areas in the ecosystem that are continually replenished through the cycling of nutrients from soil to plants and back to the soil through litter accumulation and decay, channeling from sparsely vegetated adjacent areas and possibly through occasional fires. Attraction of insects, birds and animals to broad-leafed and fruit-bearing plants may also contribute to the biofocusing of nutrients in the thickets. (C) 2011 Elsevier B.V. All rights reserved.