| The importance of primary mineral dissolution and formation of secondary minerals has been recognized as a primary control on silica concentrations and fluxes in soil solutions and stream waters. Assessments of the controls on silica export from the terrestrial environment tend to ignore the role of plant silica cycling and biogenic silica storage in soils and vegetation, assuming that mineral weathering reactions alone controls this flux. To this end, we employed a mass balance study of Si pools and fluxes along a grassland climosequence in the Central Great Plains. Though biologically mediated Si accounts for only a few percent of the total Si in these systems, we believe that this Si is far more labile than mineral Si. In general, shortgrass systems tend to have greater pools of soil biogenic Si than tallgrass ones though the plants add less Si to the soil annually. As Ge behaves as a pseudo isotope of Si, differences in Ge/Si ratios among major pools in terrestrial systems allows for the potential use of Ge/Si as a tracer of silicate weathering and Si flux in these systems, and can provide a valuable tool for studying weathering processes and Si cycling in terrestrial and marine systems. Trends of Ge sequestration in secondary minerals, and depletion in both biologic and aquatic pools were similar in direction to those of tropical systems but less in terms of magnitude. Overlap in Ge/Si ratios among the pools examined (plant, soil, stream) in the less intensely weathered grassland ecosystems provide less conclusive of this particular isotopic tool in elucidating weathering relationships. To date, the biological fractionation of Ge during plant Si uptake has been deduced from studies of plants in the field, where isolation of source Si is difficult at best. We provide more solid evidence regarding the magnitude and direction of biologic Ge fractionation through a controlled greenhouse study, where source Ge/Si values are more easily isolated. Biologic Ge fractionation could be the result of differences in reactivity and speciation, as well as Ge toxicity and kinetically driven fractionation resulting from differences in molecular weight between Ge and Si. |
