N2, CO2, and 1,2-dichloroethane as molecular probes of soil microstructure

For a representative measurement of soil water content and its changes at the scale of a forest stand, information about the spatial and temporal variability has to be taken into account. The scale on which single trees and tree species influence the variation in space and time was investigated in a mixed Norway spruce–European beech stand in Lower Austria. On a 0.5 ha plot the volumetric soil water content (VWC) was measured at 194 sample locations along square grids of different spacing, using a Trase1 TDR system with waveguides installed vertically over a soil depth of 0–30 cm. Discontinuous measurements were taken in approximately 14 day intervals during the growing season of 2000. Semivariogram analysis was used to summarize the spatial variation of VWC at selected dates. Kriging interpolation plots of soil water depletion (recharge) between these dates served to compare extraction (rewetting) patterns to tree (species) distribution. During a long drying cycle in spring, species specific transpiration behaviour turned out to be the main source of variation. Light rainfalls of medium intensity on moderately dry soil caused a rewetting pattern, which reflected the different interception efficiency of the contrasted species resulting from architectural properties. Rain events of high intensity on dry soil led to an erratic distribution pattern of water in the soil due to preferential flow in shrinkage cracks, which had opened up during drying. The effect of tree architecture was largely covered in this case. The extent to which a clear spatial correlation of VWC could be detected varied within a radius of 4 to 20 m, depending on VWC as well as on the drying and rewetting history.