Factors that Control the Range and Variability of Amorphous Silica in Soils in the Hubbard Brook Experimental Forest

In terrestrial ecosystems, the largest pool of amorphous silica (ASi) is stored in soils and is an important reservoir of biologically active Si for the global biogeochemical cycling of Si. Only limited data are available that quantify the size of this reservoir and often these estimates are made from the physical separation of silt-sized phytoliths, which can underestimate the ASi pool. Soil samples from five watersheds in a temperate-zone continental ecosystem at the Hubbard Brook Experimental Forest, New Hampshire, were analyzed for ASi using alkaline digestion. Soils from two of the watersheds were analyzed after experimental forest removal. In undisturbed watersheds, ASi was concentrated at the surface of the soil profile, similar to organic matter, and then progressively decreased with depth. This investigation supports our hypothesis that forest disturbance leads to redistribution of Si in the soil. In fact, although deforestation led to significant decreases in ASi in the upper soil horizons, total profile ASi (similar to 17,400 kg SiO2 ha(-1)) remained essentially unchanged, implying translocation downward. Significant increases in the transport of dissolved silica (DSi) by rivers have been observed With deforestation, however, in which the ASi pool in soils may play an important role. Additional studies should target the potential role of ASi as a buffer for DSi losses from deforested watersheds.