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.

Summary The composition of soil organic matter (SOM) is influenced by land use and fertilization. We studied changes in the SOM in a long‐term field experiment on a sandy Podzoluvisol. The control plot and four combinations of manurial treatments of the experiment were selected: one with mineral fertilizer only and three combinations of organic manure with mineral fertilizer: cattle manure + NPK, cattle manure + PK and straw + NPK. The SOM was extracted by sodium pyrophosphate solution (pH = 10) and hot water (100°C). The extracts were analysed by Fourier‐Transform Infrared (FT‐IR) spectroscopy and gel permeation chromatography (GPC). The FT‐IR spectra from sodium pyrophosphate extracts indicate that composition of SOM is indeed influenced by different fertilization. The C=O band at 1710 cm–1 in the samples of the plots fertilized with cattle manure has the highest absorption intensity, whereas the material from the plot fertilized with straw + NPK has the least intense. The GPC analyses of the extracts showed that adding cattle manure + NPK increased the molecular size of SOM in comparison with the control plot. The analysis of hot‐water extracts with FT‐IR showed no significant differences in functional groups, but GPC chromatograms distinguished features in molecular size distribution. Fertilization with cattle manure increased the molecular size of the SOM in comparison with the control, but the differences in content of carboxylic groups and molecular weight were detected in sodium pyrophosphate extracts only.

Abstract. The significance of biogenic silicon (BSi) pools as a key factor for the control of Si fluxes from terrestrial to aquatic ecosystems has been recognized for decades. However, while most research has been focused on phytogenic Si pools, knowledge of other BSi pools is still limited. We hypothesized that different BSi pools influence short-term changes in the water-soluble Si fraction in soils to different extents. To test our hypothesis we took plant (Calamagrostis epigejos, Phragmites australis) and soil samples in an artificial catchment in a post-mining landscape in the state of Brandenburg, Germany. We quantified phytogenic (phytoliths), protistic (diatom frustules and testate amoeba shells) and zoogenic (sponge spicules) Si pools as well as Tironextractable and water-soluble Si fractions in soils at the beginning (t 0 ) and after 10 years (t 10 ) of ecosystem development. As expected the results of Tiron extraction showed that there are no consistent changes in the amorphous Si pool at Chicken Creek (Hühnerwasser) as early as after 10 years. In contrast to t 0 we found increased water-soluble Si and BSi pools at t 10 ; thus we concluded that BSi pools are the main driver of short-term changes in water-soluble Si. However, because total BSi represents only small proportions of water-soluble Si at t 0 (< 2 %) and t 10 (2.8-4.3 %) we further concluded that smaller (< 5 µm) and/or fragile phytogenic Si structures have the biggest impact on short-term changes in water-soluble Si. In this context, extracted phytoliths (> 5 µm) only amounted to about 16 % of total Si contents of plant materials of C. epigejos and P. australis at t 10 ; thus about 84 % of small-scale and/or fragile phytogenic Si is not quantified by the used phytolith extraction method. Analyses of small-scale and fragile phytogenic Si structures are urgently needed in future work as they seem to represent the biggest and most reactive Si pool in soils. Thus they are the most important drivers of Si cycling in terrestrial biogeosystems.

The importance of silicon (Si) in nutrition is currently being recognized by its beneficial effects on many crops. Therefore, it is important to determine the soil Si status and to examine different extractants for testing plant-available Si. Little information is available about the Si status of Histosols and C org -rich Gleysols in temperate climate. This study was undertaken (1) to characterize different Si pools in C org -rich groundwater soils of an experimental site and (2) to study the influences of small-scale variability on element distribution. At the experimental site, the thickness of the C org -rich layer ranges between 4 and 5 dm overlying fine-sandy fluvial sediments. Four extractants were evaluated: 0.01 M CaCl 2 , 0.5 M acetic acid, 0.1 M sodium pyrophosphate, and 0.1 M Tiron (C 6 H 4 Na 2 O 8 S 2 · H 2 O). Further, total element content was determined following HNO 3 /HF digestion. Calcium chloride-soluble Si shows no significant relations to other parameters analyzed. On the basis of published data, the soils investigated could be classified as Sideficient. The Si fraction extracted with acetic acid displays relations to C org content of the soil and a weak correlation to CaCl 2 -soluble Si, indicating that both solutions extract overlapping but not the same fractions. Sodium pyrophosphate extracts mainly organo-mineral Fe and Al complexes in the soils studied, which is reflected in a highly positive correlation to C org . Pyrophosphate-soluble Si showed a negative relationship to C org , which means a closer relation of this Si fraction to mineral matter than to C org . The Tiron solution extracted most Si of all extractants, but this amounts only 1% of the total Si content. Taking into account the element-specific relationship between pyrophosphate and Tiron-extractable Fe, Al, and Si, it can be concluded that Tiron dissolves mainly the opaline silica present in Histosols and C org -rich Gleysols. The distribution of C org and ash content shows clear spatial trend at the experimental site, which is correlated to pyrophosphate-extractable as well as total Si. This small-scale variability of soil parameters itself is related to a distinct microrelief.

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