Effects of grazing and cultivation on soil patterns and processes in the Paulshoek area of Namaqualand

Abstract

Soils were sampled at three different sites within the Paulshoek area of the Leliefontein District in Namaqualand, South Africa. At one site the effect of heavy grazing on soil characteristics under shrubs and from open positions was compared to a more lightly grazed area. At this site a diverse shrub community was mostly replaced by a single species, Galenia africana, when overgrazed. In another area the effects of cultivation on soil properties was investigated. At the third site the effect of Galenia shrub size on soil properties was measured. Soils from the Paulshoek area of Namaqualand are generally infertile and very low in arbuscular mycorrhizal propagules. Patterns of nitrogen distribution at the landscape level are a result of nutrient enrichment associated with perennial shrubs. Arbuscular mycorrhizal infectivity, soil moisture and pH are also affected by shrubs. Only species capable of forming arbuscular mycorrhizas support populations of mycorrhizal propagules in their rhizosphere. Differences in plant available phosphorus under some shrubs indicate that processes associated with nutrient cycling are also affected by shrubs. Galenia tended to deplete soil moisture and increase soil pH more than other shrubs. Small stock production results in a depletion of soil nutrients at the landscape scale since larger areas of low nutrient soil develop when shrub density decreases following heavy grazing. Heavy grazing also affects soil properties indirectly through a change in shrub composition. Cultivation resulted in a decrease in soil nitrogen and organic matter, a loss which was still detectable after 20 years of fallow. Zones of nutrient enrichment and higher pH were found where Galenia established in the old field. When soil was sampled under different size Galenia shrubs a positive correlation was found between shrub diameter and nitrogen, available phosphorus and pH indicating that Galenia was capable of establishing patches in the soil with different properties rather than selecting such areas for establishment. Thus, Galenia is able to maintain soil patterns typical of Succulent Karoo although it also establishes conditions which probably ensure its success. Individual shrub species also affect soil differently with respect to some components such as mycorrhizas.