Lithologic controls on biogenic silica cycling in South African savanna ecosystems

The efficacy of higher plants at mining Si from primary and secondary minerals in terrestrial ecosystems is now recognized as an important weathering mechanism. Grassland ecosystems are a particularly large reservoir of biogenic silica and are thus likely to be a key regulator of Si mobilization. Herein, we examine the effects of parent material (basaltic and granitic rocks) on the range and variability of biogenic silica pools in grass-dominated ecosystems along two precipitation gradients of Kruger National Park, South Africa. Four soil pedons and adjacent dominant plant species were characterized for biogenic silica content. Our results indicate that although soils derived from basalt had less total Si and dissolved Si than soils derived from granite, a greater proportion of the total Si was made up of biogenically derived silica. In general, plants and soils overlying basaltic versus granitic parent material stored greater quantities of biogenic silica and had longer turnover times of the biogenic silica pool in soils. Additionally, the relative abundance of biogenic silica was greater at the drier sites along the precipitation gradient regardless of parent material. These results suggest that the biogeochemical cycling of Si is strongly influenced by parent material and the hydrologic controls parent material imparts on soils. While soils derived from both basalt and granite are strongly regulated by biologic uptake, the former is a tighter system with less loss of Si than the latter which, although more dependent on biogenic silica dissolution, has greater losses of total Si. Lithologic discontinuities span beyond grasslands and are predicted to also influence biogenic silica cycling in other ecosystems.