Variability of soil water repellency in sandy forest soils with different stand structure under Scots pine (Pinus sylvestris) and beech (Fagus sylvatica)

Soil water repellency has important consequences for hydrological and ecological properties of forest soils. It is a dynamic property, varying temporally throughout the course of the year. The objective of this study was to characterize the temporal variation of soil water repellency on sites at the Kahlenberg forest in northeast Germany, with comparable geological substrate, soil type, and climatic conditions, but with different stand ages and tree species in terms of the effects of forest transformation upon soil physical properties. The Kahlenberg forest area under investigation contains stands of Scots pine (Pinus sylvestris) and European beech (Fagus sylvatica) of different age structures forming a transformation chronosequence from pure Scots pine stands towards pure European beech stands. To measure water repellency, the water drop penetration time (WDPT) and the critical surface tension (CST) tests were carried out using soil samples from four forest plots at soil depths of 0-30 cm, and four sampling periods between autumn/winter 2000 and autumn 2001. Both WDPT and CST revealed a pronounced seasonal variability in repellency from being strongest in summer to weakest in autumn (2001), after a prolonged wet period. In general, plots in pine-only and beech-only stands exhibited relatively low water repellency values compared to those in mixed stands, which had higher proportions of mor-type humus forms and greater thicknesses of forest floor layer compared to the pure stands (as described previously-[Buczko, U., Bens, O., Fischer, H., Hüttl, R.F., 2002. Water repellency in sandy luvisols under different forest transformation stages in northeast Germany. Geoderma 109, 1-18]). The relatively small differences in the degree and persistence of hydrophobicity between the plots are largely overshadowed by the large differences between the sampling periods.