Dust or Crust?: Surface Soil Nutrients in the Kalahari

The Kalahari covers 2.5 million km2 of southern Africa and consists of 95% fine sand-sized, aeolian-deposited sediment. Soils are predominantly deep, structureless and lacking in N, P and organic matter. Plant available nutrients are concentrated in the top centimetre and thus vulnerable to degradation and transportation by wind erosion. The fertility of Kalahari soils is important for nutritious grass production as livestock grazing remains the predominant livelihood throughout the region. The aim of the research was to investigate the interrelationships between dust, biological crusts, vegetation and disturbance across the Kalahari. Crust, vegetation and surface nutrient characteristics were determined at five locations of different land uses. The spatial and temporal variability of rainfall results in an incomplete vegetation cover and spatially heterogeneous soil nutrients. Our studies show that nutrient heterogeneity is controlled by various factors operating at different scales and is important for ecosystem functioning and degradation vulnerability. At a landscape scale, variability is minimal due to limited topography and a relatively uniform Kalahari Sand cover. At a smaller scale, spatial heterogeneity of soil N and P is also low (11 to 28%) compared to shrublands in the SW United States. Wind erosion is not the principal cause of nutrient heterogeneity because the amount of dust movement and associated nutrient fluxes are low. This is because of an extensive vegetation cover that reduces erodibility and coarse grain sizes, aggregation and biological crusts which reduce surface erosivity. Enrichment occurs under bush canopies due to organic inputs from the vegetation as well as fixation by biological soil crusts that develop preferentially in protected sub-canopy niches. Biological soil crusts are widespread and are able to persist in landscapes where there is a high level of disturbance. However, crust composition is largely restricted to simple Microcoleus spp. Crustal succession is limited by both breakage of crusts and burial by wind blown sediment. In a typical communal grazing area, 48% of the surface was unconsolidated, 44% was crusted and 8% buried crusts compared to up to 95% crust cover in National Parks. Bush sub-canopies are sites of preferential crust development and thus nutrient enrichment, but are also subjected to periodic burial by wind blown dust and plant litter. The overall impact on surface nutrients will depend upon the bush species composition, canopy structure and degree of disturbance.