Hydraulic functions and water repellency of forest floor horizons on sandy soils

For a better understanding of water flow phenomena in pine ecosystems, we characterized hydraulic functions as well as water repellency behaviour of forest floor horizons. The investigations covered 11 pine stands (Pinus sylvestris) on sandy soil in northeast Germany. All sites were classified as Haplic Arenosol. Two kinds of forest floor horizons were analysed: the slightly mineralised Of horizon under the soil surface and the more or less strongly humified Oh horizon which underlies directly the Of horizon. Data analysis shows a good correlation between soil organic matter, bulk density and water characteristics which can be easily used to predict soil available water. The soil available water (pF 1.8–4.2) of the forest floor horizons ranges from 22 vol.% (Of) up to 26 vol.% (Oh) with high spatial variability. However, the maximum of water storage capacity is available only in short time in spring after total rewetting. The unsaturated hydraulic conductivity of the Of horizon was slightly lower than that of the Oh horizon. Water repellency was studied using the water drop penetration time test (wdpt). Results show a strong dependency of water repellency on pressure head and soil water content. At pF 2.0 both organic horizons exhibited considerable portions of water repellency causing wdpt of up to 10 min. Extremely high water repellency of > 6 h occurred at pF > 3.6. Field transect measurements over a 2 year period show that water repellency was found throughout the year, being less developed at high water contents during winter time and being most pronounced at low water contents in summer. Even during winter time the soil profile rewetted only partly because of its high water repellent properties. Water repellency occurs whenever soil moisture reaches thresholds of a critical content of about 14–16 vol.% in the forest floor horizons, 5–6 vol.% in the humus enriched horizons and about 3 vol.% in the sandy mineral subsoil.