The Bio in Aluminum and Silicon Geochemistry

The translocation and transformation of Al and Si are of paramount importance in the processes of primary-mineral weathering, saprolite formation and soil formation. Geochemical mass balance studies of these processes have often not considered the important role of the biota in cycling of these omnipresent soil elements. In the Calhoun Experimental Forest, SC, we found a mean annual biological uptake of Al and Si of 2.28 and 15.8 kg ha-1 yr-1, respectively, with a mean annual accumulation in aboveground biomass of 0.48 and 2.32 kg ha-1 yr-1, respectively. In the case of Al, net soil leaching from 6 m depth is zero, thus biomass accumulation of Al accounts for the only removal from the soil system. There is an additional internal system mobilization of Al of 6.6 kg ha-1 yr-1, in response to biotic inputs of dissolved organic carbon. In the case of Si, net soil leaching to groundwater is 17.26 kg ha-1 yr-1. The accumulation of Si in aboveground biomass, 2.32 kg ha-1 yr-1, and in forest floor organic matter, 11.95 kg ha-1 yr-1, augments the annual weathering release estimate of Si by an additional 82%. The inclusion of biological cycling of both essential and non-essential mineral elements is important for properly evaluating the biogeochemistry of the earth's crust.