Dust as a Nutrient Source for Fynbos Ecosystems, South Africa

Abstract

Fynbos is the main vegetation of the Cape Floristic Region, a biodiversity hotspot that occurs in southwestern South Africa. A major question concerning the fynbos ecosystem is how it supports abundant and diverse vegetation on soils derived from nutrient-poor bedrock. In addition to marine aerosols (recycled sea salts), geochemical analyses reported here suggest that dust (aeolian) deposition represents a significant source of nutrients (for example, K, Ca and Zn) to the fynbos ecosystem. Headwater portions of the Boontjies River sub-catchment near the Cederberg Mountains support mountain fynbos communities that are entirely underlain by the Peninsula Formation, a quartz arenite with greater than 98 wt% SiO2. Fynbos soils in these areas are composed of quartzose sand with 3–6 wt% kaolinitic clay and 1–2 wt% organic carbon. The minor amount of feldspar and mica minerals in the bedrock (0.5 wt% Al2O3) suggests an aeolian source for much of the clay minerals in the soil. The isotope composition of soluble Pb and Sr from fynbos vegetation and soils indicates a mixture of anthropogenic and terrigenous sources, most likely from washout of combusted petrol and dust from the arid interior particularly in association with Berg Wind events. Approximate mass balance calculations indicate that washout of aerosols provides an important source of nutrients such as Ca, K, P, Fe, Mn and Zn which the fynbos ecosystem is highly effective in retaining.