Measurements of Terrestrial Silica Pools in the Mid-Continent of North America: A Test of the Biological Control Hypothesis

Silica (Si) cycling in terrestrial ecosystems has become a recent focus of research due to the linkages between C and Si biogeochemical cycles and its accompanying implications for climate change. Past research has demonstrated that vegetation is a primary driver of Si mobilization from stable to labile pools, and eventual export to watersheds. In essence, any Si exported to streams must first pass through the biogenic Si sub-cycle. As a consequence of this sub-cycle, modern soil A horizons should hold large, highly soluble biogenic Si pools from phytolith litterfall deposition. In contrast, C horizons should have small to nonexistent biogenic Si pools. In this study we present evidence that while vegetation is an important factor to consider, other variables may be more dominant. Working in the central Great Plains of Nebraska as well as southeastern Minnesota, certain soils have been found to hold large amounts of highly soluble Si in C horizons; occasionally more than the topsoil. These findings cannot be explained by vegetation Si sub-cycling. Nebraska soils were found to contain varying amounts of volcanic ash which, like biogenic Si, is highly soluble. On the other hand, some Minnesota soils contain siliceous shale sediments, which can also be highly soluble. These findings are problematic with respect to chemical Si extraction methods, since it is difficult to distinguish between highly soluble Si sources. Until pedogenic and weathering processes remove these large inorganic Si pools, vegetation Si sub-cycling will remain a secondary export process to watersheds. Therefore, the conclusion postulated by earlier studies that vegetation controls the rate of Si export to watersheds may not hold true in all cases.