Si cycling in a forest biogeosystem – the importance of transient state biogenic Si pools

Abstract

Abstract. The relevance of biological Si cycling for dissolved silica (DSi) export from terrestrial biogeosystems is still in debate. Even in systems showing a high content of weatherable minerals, like Cambisols on volcanic tuff, biogenic Si (BSi) might contribute > 50% to DSi (Gerard et al., 2008). However, the number of biogeosystem studies is rather limited for generalized conclusions. To cover one end of controlling factors on DSi, i.e., weatherable minerals content, we studied a forested site with absolute quartz dominance (> 95%). Here we hypothesise minimal effects of chemical weathering of silicates on DSi. During a four year observation period (05/2007–04/2011), we quantified (i) internal and external Si fluxes of a temperate-humid biogeosystem (beech, 120 yr) by BIOME-BGC (version ZALF), (ii) related Si budgets, and (iii) Si pools in soil and beech, chemically as well as by SEM-EDX. For the first time two compartments of biogenic Si in soils were analysed, i.e., phytogenic and zoogenic Si pool (testate amoebae). We quantified an average Si plant uptake of 35 kg Si ha−1 yr−1 – most of which is recycled to the soil by litterfall – and calculated an annual biosilicification from idiosomic testate amoebae of 17 kg Si ha−1. The comparatively high DSi concentrations (6 mg L−1) and DSi exports (12 kg Si ha−1 yr−1) could not be explained by chemical weathering of feldspars or quartz dissolution. Instead, dissolution of a relictic, phytogenic Si pool seems to be the main process for the DSi observed. We identified canopy closure accompanied by a disappearance of grasses as well as the selective extraction of pine trees 30 yr ago as the most probable control for the phenomena observed. From our results we concluded the biogeosystem to be in a transient state in terms of Si cycling.

The contribution of tephra constituents during biogenic silica determination: implications for soil and palaeoecological studies

“…Melzer et al, 2012;Saccone et al, 2007;Sommer et al, 2013). The magnitude overlaps with Si Alk content attributed to tephra, the initial rapid dissolution of clay minerals and/or complete dissolution of nanocrystalline fractions in our experiments (Fig.…”

How big is the influence of biogenic silicon pools on short-term changes in water-soluble silicon in soils? Implications from a study of a 10-year-old soil–plant system

“…In forested biogeosystems Puppe et al (2015) found high individual numbers of SiO 2 -synthesizing testate amoebae at study sites with low amounts of Si(OH) 4 and vice versa. However, it is unlikely that testate amoebae depleted amounts of Si(OH) 4 at these sites, because corresponding protozoic Si pools are relatively small compared to phytogenic ones Sommer et al, 2013). Regarding vegetation and corresponding phytogenic Si pools, their influence on the amount of Si(OH) 4 in soils has been shown in several studies (e.g., Bartoli, 1983;Farmer et al, 2005;Sommer et al, 2013).…”

“…On the one hand, dust depositions (dry deposition) at Chicken Creek are very low (73-230 mg m −2 d −1 ) and only slightly above the annual average (70-90 mg m −2 d −1 ) measured in the state of Brandenburg . On the other hand, the total input of Si (as a lithogenic element) from precipitation (wet deposition) is negligible as well (< 1 kg Si ha −1 yr −1 , Sommer et al, 2013).…”

“…Furthermore, evidence arises that BSi pools are in disequilibrium at decadal timescales due to disturbances and perturbations by humans, e.g., by changes in forest management or farming practices (Barão et al, 2014;Keller et al, 2012;Vandevenne et al, 2015). As a consequence, BSi accumulation and BSi dissolution are not balanced, which influences Si cycling in terrestrial biogeosystems, not only on decadal but also on millennial scales (Clymans et al, 2011;Frings et al, 2014;Sommer et al, 2013;Struyf et al, 2010). Sommer et al (2013), for example, found the successive dissolving of a relict phytogenic Si pool to be the main source of dissolved Si in soils of a forested biogeosystem.…”

“…As a consequence, BSi accumulation and BSi dissolution are not balanced, which influences Si cycling in terrestrial biogeosystems, not only on decadal but also on millennial scales (Clymans et al, 2011;Frings et al, 2014;Sommer et al, 2013;Struyf et al, 2010). Sommer et al (2013), for example, found the successive dissolving of a relict phytogenic Si pool to be the main source of dissolved Si in soils of a forested biogeosystem. Due to the fact that the continuous decomposition of this relict phytogenic Si pool is not compensated by an equivalent buildup by recent vegetation the authors concluded that a BSi disequilibrium occurred on a decadal scale.…”