Abstract
Abstract. Silicon (Si) released as H4SiO4 by weathering of Si-containing solid phases is partly recycled through vegetation before its land-to-rivers transfer. By accumulating in terrestrial plants to a similar extent as some major macronutrients (0.1–10% Si dry weight), Si becomes largely mobile in the soil-plant system. Litter-fall leads to a substantial reactive biogenic silica pool in soil, which contributes to the release of dissolved Si (DSi) in soil solution. Understanding the biogeochemical cycle of silicon in surface environments and the DSi export from soils into rivers is crucial given that the marine primary bio-productivity depends on the availability of H4SiO4 for phytoplankton that requires Si. Continental fluxes of DSi seem to be deeply influenced by climate (temperature and runoff) as well as soil-vegetation systems. Therefore, continental areas can be characterized by various abilities to transfer DSi from soil-plant systems towards rivers. Here we pay special attention to those processes taking place in soil-plant systems and controlling the Si transfer towards rivers. We aim at identifying relevant geochemical tracers of Si pathways within the soil-plant system to obtain a better understanding of the origin of DSi exported towards rivers. In this review, we compare different soil-plant systems (weathering-unlimited and weathering-limited environments) and the variations of the geochemical tracers (Ge/Si ratios and δ30Si) in DSi outputs. We recommend the use of biogeochemical tracers in combination with Si mass-balances and detailed physico-chemical characterization of soil-plant systems to allow better insight in the sources and fate of Si in these biogeochemical systems.
Highlights
Silicon (Si) is the second most abundant element of the Earth’s crust with a mean content of 28.8 wt% (Wedepohl, 1995) and it occurs in a large range of minerals at the earth surface, ranging from
Litter-fall leads to a substantial reactive biogenic silica pool in soil, which contributes to the release of dissolved Si (DSi) in soil solution
The geochemical signatures will be strongly influenced by secondary precipitates: the Ge/Si ratio will increase in soil solution if secondary clay-sized mineral dissolution is larger than the biogenic silica (BSi) dissolution, and δ30Si in soil solution will decrease with both secondary clay-sized mineral and phytolith dissolution since they are both enriched in light Si isotopes (Fig. 3b)
Summary
Silicon (Si) is the second most abundant element of the Earth’s crust with a mean content of 28.8 wt% (Wedepohl, 1995) and it occurs in a large range of minerals at the earth surface, ranging from 106 years), weathering of silicate minerals plays an integral role in soil development promoting the decrease in atmospheric CO2 (Rai and Kittrick, 1989; Chadwick et al, 1994; White and Blum, 1995; Berrner, 1997; Hartmann et al, 2009). The interaction between the Si cycle and C cycle regulates the atmospheric CO2 through silicate mineral weathering (Berner, 1997), soil organic carbon sequestration in stable organo-mineral complexes (Torn et al, 1997) and the DSi nutrition of phytoplankton CO2consumers in oceans (Conley et al, 1993; Smetacek, 1999; Treguer and Pondaven, 2000). The continental Si cycle can be represented by five components (Fig. 1): soil solution, groundwater, rivers, biomass and soils, which are linked to the global Si cycle by two other major reservoirs (oceans and atmosphere)
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