Data on euglyphid testate amoeba densities, corresponding protozoic silicon pools, and selected soil parameters of initial and forested biogeosystems

The dataset in the present article provides information on protozoic silicon (Si) pools represented by euglyphid testate amoebae (TA) in soils of initial and forested biogeosystems. Protozoic Si pools were calculated from densities of euglyphid TA shells and corresponding Si contents. The article also includes data on potential annual biosilicification rates of euglyphid TA at the examined sites. Furthermore, data on selected soil parameters (e.g., readily-available Si, soil pH) and site characteristics (e.g., soil groups, climate data) can be found. The data might be interesting for researchers focusing on biological processes in Si cycling in general and euglyphid TA and corresponding protozoic Si pools in particular.


a b s t r a c t
The dataset in the present article provides information on protozoic silicon (Si) pools represented by euglyphid testate amoebae (TA) in soils of initial and forested biogeosystems. Protozoic Si pools were calculated from densities of euglyphid TA shells and corresponding Si contents. The article also includes data on potential annual biosilicification rates of euglyphid TA at the examined sites. Furthermore, data on selected soil parameters (e.g., readily-available Si, soil pH) and site characteristics (e.g., soil groups, climate data) can be found. The data might be interesting for researchers focusing on biological processes in Si cycling in general and euglyphid TA and corresponding protozoic Si pools in particular.
& Analyses of initial (chronosequence) and forest sites Data source location Germany, for latitudes & longitudes and further site details see Table 1.

Data accessibility
All data are presented within the paper.

Value of the data
The provided dataset is useful for comparison with the results of other authors regarding, e.g., ecological (euglyphid testate amoeba densities) and biogeochemical (protozoic silicon pools, annual biosilicification) issues in different global ecosystems.
Data on protozoic silicon pools and corresponding annual biosilicification rates might emphasize the need for detailed investigations of silicon (re-)cycling in unicellular organisms in general and testate amoeba in particular (e.g., qualitative characterization of biogenic silicon, isotope analysis).
Together with other datasets the presented data allow meta-analyses to examine significant controls (steady state soils) and dynamics (initial soils) of euglyphid testate amoeba densities and related amoebal biosilicification processes in soils in detail.
In combination with data of other authors the presented data can be used for modelling to assess the role of euglyphid testate amoebae (compared to other organisms that synthesize biogenic silicon, e.g., plants or diatoms) for silicon cycling in soils and corresponding silicon fluxes from terrestrial to aquatic ecosystems.
The presented data were the basis for analyses of protozoic Si pools in initial [12] and forested biogeosystems [3]. The dataset in the present article provides information on i) site characteristics, geographic positions, and climate data of the initial and forested biogeosystems (Table 1), ii) analyzed soil parameters (Table S1), iii) densities of euglyphid TA shells in soils (Table S2), and iv) corresponding protozoic Si pools as well as annual biosilicification rates of euglyphid TA at these sites (Table S3). Table 1 Site characteristics, geographic positions, and climate data of the examined initial and forested biogeosystems. Data on precipitation and temperature represent annual averages for the period 1981-2010 (German Meteorological Service).

Site Lithology
Soil    Table 1 and Section 2.1.
The construction of CC was completed in 2005 (time zero). In 2008 a small area in the west of the catchment was again restored to time zero (removal of the upper 20 cm of soil) for additional experimental plots. Construction of NL was finished in 2001 (time zero). Soils classify as Protic Arenosol (Calcaric, Transportic) or Haplic Arenosol (Hyperochric, Transportic) depending on site age [17]. Detailed information on site construction of CC and NL can be found in Gerwin et al. [18] and Kendzia et al. [19], respectively. All samples were taken from Quaternary substrate at 3-, 5-(CC), and 10-yearold (NL) spots representing a chronosequence (Fig. 1a-c). Samples (20 cm Â 20 cm Â 5 cm; subdivided in two compartments: 0-2.5 and 2.5-5 cm depth) were taken at randomly chosen spots within an area of approx. 25 m 2 . Vegetated (cov) and uncovered (unc) spots were sampled in four field replicates each to analyze possible impacts of vegetation (3 cov: Tussilago farfara and Trifolium arvense; 5 cov: Corynephorus canescens and T. arvense) on protozoic Si pools. At NL almost the whole surface was vegetated with biogenic crusts, Poales, and several shrubs, which is why only vegetated spots (10 cov) were sampled. Samples were taken in May 2010 (CC: 5 unc, 5 cov), May 2011 (NL: 10 cov), and August 2011 (CC: 3 unc, 3 cov).

Forest sites
Ten non-eroded forest sites showing huge differences in climate, parent material, and pedogenesis were selected. Mean annual precipitation rates range from 530 to 1600 mm, mean annual air temperatures from 8 to 11°C. Soils comprise (i) a sandy Arenosol developed from eolian sands (SL, dune), a Podzol and a Planosol from siliceous sandstones (HS, HK) very low in weatherable minerals (o 10% feldspars, mica), (ii) silty to loamy Luvisols and Stagnosols from calcareous, illitic loess (MR, PP) and sandy to loamy Luvisols and Stagnosols from glacial till (RO, AB), both parent materials with intermediate contents of weatherable minerals (feldspars, mica), (iii) a clayey Cambisol from dolomitic limestone (EG), (iv) a clayey Stagnosol from kaolinitic claystone (ZE), and finally (v) a clayey, smectitic Vertisol from basalt (HE) very high in weatherable minerals, like augite and plagioclase. The forest stands are old and are assumed to be in steady state in terms of TA dynamics at a decadal time scale (photographs of some selected forest sites can be found in Fig. 1d-f). Soil samples were taken in four field replicates (n ¼ 4) at all sample sites except HK (n ¼ 3). The field replicates (20 cm Â 20 cm each) were placed randomly within an area of approx. 100 m 2 . Per field replicate samples were taken in the upper 5 cm (incl. organic layers except for fresh litter) differentiating between two superimposed soil compartments about the same size (ideally 20 cm Â 20 cm Â 2.5 cm each) and transferred to plastic bags. Sampling took place in spring 2010 within six weeks (April 26th-June 6th).

Soil parameters
Bulk densities (BD, g cm À 3 ) were calculated by dividing weights of oven-dried (105°C) aliquots of soil samples by corresponding volumes. Remaining soil samples were air dried and sieved (2 mm) to separate fine-earth ( o2 mm) from skeleton content (42 mm). For soil analyses only fine-earth was used.

Soil pH, carbon, and nitrogen
Soil pH was measured using a glass electrode in a 0.01 M CaCl 2 solution with a soil-to-solution ratio of one-to-five. For total carbon as well as nitrogen analyses (C t and N t ) fine-earth samples were finely powdered in a disc mill. Subsequently, C t and N t were determined by dry combustion using an elemental analyzer (CNS TruSpec, Leco Instruments). Total inorganic carbon (TIC) was measured with a multiphase analyzer (RC 612, Leco Instruments). Soil organic carbon (SOC) concentrations were calculated by subtraction (C t -TIC) and C:N ratios were calculated by division (SOC:N t ). Soil C and N analyses were performed at the minimum of two lab repetitions per sample.

Readily-available silicon
For extraction of the calcium chloride (CaCl 2 ) soluble, so-called readily-or plant-available Si fraction (Si CaCl2 ), 2 g of soil was mixed with 20 ml of a 0.01 MCaCl 2 solution per sample and continuously shaken for 16 h using a lab roller mixer [20]. This Si fraction was extracted to characterize the Si supply for shell synthesis of euglyphid TA in soils. Subsequent to extraction, the extracts were centrifuged (4000 rpm, 30 min), filtered using 0.45 μm polyamide membrane filters, and Si concentrations were determined by ICP-OES (iCAP 6300 Duo, Thermo Scientific). Complete extraction work was done using plastic equipment only and results represent arithmetic means of three lab repetitions per sample.

Data conversion and calculation steps
All results except for pH were converted to an oven-dry basis (105°C). Fine-earth mass (FEM in kg m À 2 ) was calculated considering bulk density, thickness and skeleton content (wt%). Total FEM (FEM t ) of the upper 5 cm was calculated as the sum of FEM of superimposed compartments. For the upper 5 cm of soil pH was averaged as follows: Per compartment pH was multiplied with the corresponding FEM, divided by FEM t and subsequently these results were summed up. Mass densities (g m À 2 ) of SOC, N t , and Si CaCl2 were calculated compartment-wise by multiplying FEM with element concentrations (g kg À 1 ). Finally, the results of superimposed compartments were summed up for the upper 5 cm of soil.

Euglyphid testate amoeba densities, protozoic silicon pools, and annual biosilicification
Soil samples in the plastic bags were homogenized by gentle manual mixing and subsequently 2 g of fresh soil was taken per sample for TA analyses and stored in 8 ml of formalin (4%). Soil suspensions received from serial dilution (1000-31.25 mg soil in 8 ml of water each) were stained with aniline blue. TA were enumerated using an inverted microscope (OPTIKA XDS-2, magnifications of 200 Â and 400 Â ) differentiating between full (living incl. encysted individuals, stained) and empty shells (unstained) of the order Euglyphida. TA densities (shells cm À 2 ) were calculated considering TA shell numbers (g À 1 dry weight), bulk density (g cm À 3 ), and thickness (cm) per soil compartment. TA densities of the upper 5 cm were calculated by summing up the corresponding TA densities of superimposed soil compartments.
Protozoic Si pools (BSi TA ; mg m À 2 ) were calculated per soil compartment using the following formula: where N i is the number of euglyphid TA shells (g À 1 dry weight), Si i is the corresponding Si content (pg shell À 1 ; given in parentheses listed above), ρ b is the bulk density (g cm À 3 ), and t is the thickness (cm) of the corresponding soil compartment. In contrast, for estimation of annual biosilicification only living euglyphid TA (g À 1 dry weight) were considered for N i in Eq. (1) due to their ability of reproduction. After calculation steps as described in Eq. (1) results were multiplied with 13 and 90 (potential TA generations per year, see Foissner [21]) for minimal and maximal annual biosilicification rates, respectively. For calculation of protozoic Si pools and euglyphid TA biosilicification rates of the upper 5 cm of soil the results of superimposed soil compartments were added up.