Mobilization of Silica in Terrestrial Grassland Ecosystems and the Potential Impact on the Global Silica Cycle

The biogeochemical behavior of silica is closely linked to that of other important elements and in particular si biogeochemistry may strongly impact the carbon cycle. This is well-documented in marine systems where diatoms are a major control on the distribution of silica in the oceans, and play a major role in controlling atmospheric pCO2 via the “biological pump”. However, the importance of biological and ecological controls on silica cycling in the terrestrial environment is much less well known, despite the fact that the source of marine Si is terrestrial. We quantified the biogeochemistry of biogenic silica along bioclimatic and geologic gradients in grass dominated ecosystems worldwide. Our results are based on intensive field investigations and modeled results from grassland production data. We have identified the possible effects of biogeographically diverse grassland ecosystems on the production, storage, and geochemical behavior of BSi, a requisite first step in forecasting global change impacts on the linkages between terrestrial and marine Si cycles. Our results demonstrate that the BSi stored in the soils of temperate grassland ecosystems alone rival the estimated global storage in biomass and is almost half of what is stored in the ocean. The importance of terrestrial Si and its export to marine systems is similar to Iron (Fe), which also has its major source on land and is considered a critical limiting element in marine systems. The literature reports that approximately 10 Tmol of Si is delivered to oceans via dust, and our first order assessment of North American grasslands suggests that dust export could be as high as 60 to 100 Tmol. This is a significant fraction of the BSi stored in the ocean - estimated to be 240 Tmol – and suggests that a strong link exists between global grassland productivity and marine C cycles.