Emerging understanding of the ecosystem silica filter

The annual fixation of dissolved Si (DSi) into terrestrial vegetation has been estimated to range from 60 to 200 Tmole, or 10-40 times more than the yearly export of DSi and biogenic Si (BSi) from the terrestrial geobiosphere to the coastal zone. Ecosystems form a large filter between primary mobilization of DSi from silicate weathering and its eventual export to the oceans, and a large reservoir of BSi accumulates in aquatic and terrestrial ecosystems. Although a number of synthesis activities within the last decade have discussed biological transformations in the terrestrial Si cycle, the timescales at which BSi is stored and recycled within ecosystems, BSi persistence and reactivity throughout soil profiles, the dependence of the BSi storage and recycling on ecological processes, the feedbacks to hydrology, the interaction with man's activities and ultimately the global relevance in Si budgets are poorly constrained. Here we discuss 5 key controls on the ability of ecosystems to filter and control the export of DSi: ecosystem biodiversity, BSi dissolution rates and reactivity, hydrology, interaction with the geosphere and anthropogenic impacts. These controls need to be further studied to better quantify the global and local importance of the terrestrial biogeochemical Si cycle and specifically the BSi reservoir in ecosystems.