LTER Zöbelboden, Austria, Station Data

Description

Methods

The soil data stems from samples of two soil inventories conducted in the years 1992 and 2004. Addition samples were taken during campaigns. The bulk of the samples was taken at 64 first-order permanent plots (100 m mesh covering the entire catchment). During the second inventory, additional samples were collected from 6 second-order permanent plots (in the centre of the 100 m mesh) to warrant enough replications within the main soil types. The soil inventory plots are co-located with the forest and forest floor inventory plots. Where soil sampling was impossible in the north of the permanent plot for vegetation survey, the next side in clockwise direction was chosen. In 2004, the samples were taken 5 m apart from the samples in 1992 except when the soil type, the soil depth (more than one depth horizon) and the coarse fraction (>20% difference) differed significantly from 1992. The forest floor was sampled once with a 30x30cm frame. Large particles like branches protruding the frame were cut with a garden shear. The mineral soil was sampled with a metal pole with a 70-mm diameter at 3 points within a distance of two metres. Each undisturbed soil core was placed in polystyrene cases and divided into the following geometric horizons (0-5, 5-10, 10-20, 20-30,30-50 cm). In 1992 all geometric horizons were sampled triply, whereas in 2004 both uppermost horizons (0-5, 5-10 cm) were sampled five times and the subjacent horizons triply. Subsequently, the corresponding geometric horizons were pooled and stored in labelled plastic bags at +4° C until laboratory analysis was performed. The soil sampling procedure at the Intensive Plots comply to the same procedure.

In the laboratory samples were weighted. Green plant material was separated. Then roots were separated, weighted, dried at 30°C until constant weight was reached, re-weighted and stored separately. The remaining material was weighted, dried at approximately 30°C until constant weight was reached, whereas coarse aggregates were crushed and reweighted again. The dry soil was sieved through a 2.0 mm sieve and subsequently 10 g of the samples were dried at 105° C until constant weight was reached. In contrast to the mineral soil, the forest floor was grounded for 15 minutes before sieving and drying at 105°C.

The soil fractions were determined by a combination of wet sieving of the fraction >2µm and by a particle size analyzer (Micrometrics SEDIGRAPH 5000ET) for the fraction <2µm. The Sedigraph uses Stokes’ Law to determine the settling velocity of particles <2µm and uses an X-Ray adsorption to measure the concentration of these particles which fall under gravity through a liquid. Results are plotted on a cumulative curve of the sedigraph data and the sieving data as weight percent. Approximately 50 g of the air dried sample was weighted into a 100 ml jar. 200 ml 10% diluted hydrogen peroxid (H202) was carefully added (for proper dispersion and for removing organic matter). After ca. 3-4 days (until no more reaction is observed) reaction time and removal of the remaining hydrogen peroxid the sample was placed into the 950C water bath. The jars are removed from the water bath and cooled. Afterwards they are treated with ultrasound and pass through a series of sieves: 2 mm, 0.630 mm, 0.200 mm, 0.063mm and 0.020 mm. The <20 μm portion was suspended in water, a representative portion was taken out, treated with 0.05% calgon and ultrasound, and analyzed with sedigraph by X-Ray.

Saturated hydraulic conductivity was determined with volumetric soil samples and the falling head technique using the KSat Instrument (UMS GmbH, Munich, Germany), which can measure conductivities ranging between 0.01 to 5000 cm d-1. Following these measurements, the wet range of the Soil Water Retention Curve (SWRC) (0 to ~ -0.06 MPa) of the saturated samples were determined using Hyprop (UMS GmbH, Munich, Germany). The dry range of the SRWC was determined with a WP4C Dewpoint Potentiometer (Decagon Devics, Washington, USA). The results from the Hyprop and WP4C measurements were combined using non-linear least squares regression built into the Hyprop Data Evaluation software (UMS GmbH, Munich, Germany) fitting a number of SWRC and conductivity models and deriving the respective parameters. For hydraulic conductivity, the Mualem (1976) model (VARIABLE: KSat_Mualem_Tau, KSat_Mualem_Ks) resulted the best fit, for SWRC the van Genuchten (1980) model (VARIABLE: SWRC_vanGenuchten_Alpha, SWRC_vanGenuchten_n, SWRC_vanGenuchten_Theta.r, SWRC_vanGenuchten_Theta.s). References: Mualem, Y. (1976): A new model for predicting the hydraulic conductivity of unsaturated porous media. Water Resour. Res. 12 (3), 513–521; van Genuchten, M. T. (1980): A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892–898.