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

[摘要] The relevance of biological Si cycling for dissolved silica (DSi) export fromterrestrial biogeosystems is still in debate. Even in systems showing a highcontent 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 generalizedconclusions. To cover one end of controlling factors on DSi, i.e., weatherableminerals content, we studied a forested site with absolute quartz dominance(> 95%). Here we hypothesise minimal effects of chemical weatheringof silicates on DSi. During a four year observation period(05/2007–04/2011),we quantified (i) internal and external Si fluxes of a temperate-humidbiogeosystem (beech, 120 yr) by BIOME-BGC (version ZALF), (ii) related Sibudgets, and (iii) Si pools in soil and beech, chemically as well as bySEM-EDX. For the first time two compartments of biogenic Si in soils wereanalysed, i.e., phytogenic and zoogenic Si pool (testate amoebae). We quantifiedan average Si plant uptake of 35 kg Si ha⁻¹ yr⁻¹ – most ofwhich is recycled to the soil by litterfall – and calculated an annualbiosilicification 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 chemicalweathering of feldspars or quartz dissolution. Instead, dissolution of arelictic, phytogenic Si pool seems to be the main process for the DSiobserved. We identified canopy closure accompanied by a disappearance ofgrasses as well as the selective extraction of pine trees 30 yr ago as themost probable control for the phenomena observed. From our results weconcluded the biogeosystem to be in a transient state in terms of Si cycling.

[学科分类] 地球化学与岩石