Crossing evolutionary and contemporary scales: the critical importance of grassland expansion to global silica cycling

Background/Question/Methods : The biogeochemical behavior of silica is closely linked to that of other important elements, including soil carbon and the carbon cycle.  While marine 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.”  The importance of biological controls on silica cycling in the terrestrial environment is much less well known, but this paper indicates that the formation and storage of biogenic silica (BSi) in plants and soils, particularly in grasslands, is of fundamental importance. We conducted intensive studies of the pedological and ecological controls on grasslands and for the first time we demonstrate the importance of grassland ecosystems on silica mobilization in the terrestrial environment. 

Results/Conclusions: Recent estimates suggest that the global uptake of biogenic silica by terrestrial biomass ranges from 60 to 200 Tmol yr^-1 and rivals quantities stored in the biologically active portions of the ocean systems. The largest reservoir of biogenic Si in terrestrial systems resides not in living biomass but in soils, as biogenic silica is deposited upon death and decomposition of the plant materials. Our estimates indicate that the global average BSi content for grassland aboveground biomass is ~8 Tmole Si yr^-1.  Global average BSi content for grassland soils is ~23,000 Tmole Si yr^-1 which, is approximately 100-400 times greater than BSi in all terrestrial aboveground biomass and approximately 3,000 times greater than global estimates for grasslands alone.  Nonwoody North American grasslands had a BSi content of ~ 1,476 (±164) Tmole Si yr^-1. Africa and Asia had the greatest stores of BSi in both plant and soil which, is reflective of their larger grassland area while Europe had the lowest stores of plant and soil BSi.