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.

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Microsecond pulse radiolysis studies have been carried out on the leucocyanide of malachite green dye (MGCN) dissolved in either 1,2-dichloroethane, chloroform, carbon tetrachloride, acetone, cyclohexane, benzene, toluene, dimethyl sulfoxide, AyV-dimethylformamide, methanol, 2-propanol, tetrahydrofuran, dioxane, benzonitrile, or acetonitrile. The transient absorption spectra obtained in argon-saturated solutions, and with various added electron scavengers (N20, 02, or CC14), indicate that there are several intermediate species and radiolytic products. We suggest that one product is the carbonium ion (MG+) of malachite green dye (wavelength of the absorption maximum, = 620 nm) and the other 4,4'-bis(dimethylamino)triphenylmethyl radical (MG•, = 400 nm). There is evidence for the formation of an intermediate primary radical cation (MGCN+) and a triplet excited state of malachite green cyanide. In oxygen-depleted solutions the absorbing species with ™, at 620 nm is formed in a fast process completed within the time of the 5.5-ms pulse. However, only in 1,2-dichloroethane solution is this absorption stable during the observation time, whereas in all the other solvents it is unstable and decays within tenths of microseconds, the decay time depending of the solvent. In polar solvents such as Me2SO, DMF, alcohols, and acetonitrile, in the presence of oxygen, formation of the 620-nm absorption takes place in two kinetically distinct processes. The first is very fast (much shorter than the 5.5-ms pulse) and the second much slower (lasting tens of microseconds after the pulse). Possible mechanisms for the fast and slow components of radiolytic dye formation are postulated. 127 (1973), and references therein.(19) (a) Z.