Development of actual water repellency in a grass-covered dune sand during a dehydration experiment

Water repellency is a soil property which temporally changes in severity and is most pronounced when soils are dry. In the past, numerous researchers air- or oven-dried soil samples in the laboratory to determine the potential water repellency. However, measurement on air- and oven-dried samples can overestimate as well as underestimate the conditions occurring in the field under prolonged drought conditions. To reveal and determine realistic and potentially the highest persistence, we studied the influence of dehydration upon repellency in a dune sand grassland between 11 April and 5 September 2002, by artificially sheltering the soil. The shelter was built to protect the soil from getting wetted by precipitation during this period. The soil was sampled eight times in vertical transects over a distance of 75 cm to a depth of 33 cm during the dehydration period. On each sampling date soil water contents were measured and the persistence or stability of actual water repellency was determined in 120 field-moist samples collected at 8 depths. At the start of the dehydration experiment the mean volumetric soil water contents in the transect varied between 5 and 10.5%. At the end of the dehydration process the grass cover was wilted and yellow-brown and the soil profile dried-up to volumetric water contents between 1 and 2.5%. During the dehydration period, the average water storage in the upper 33 cm of the soil (total of 8 layers) decreased from 22 mm to less than 5 mm. Slightly water repellent and wettable dune sand layers at depths of 7 to 19 cm on 11 April were found to be altered into extremely water repellent soil within eight days of dehydration. The most extreme soil water repellency, with water drop penetration times of more than 6 h, was detected in large parts of the five soil layers sampled between depths from 9.5 to 33 cm on 4 June, 11 July and 5 September, 2002. The maximal water repellency was found to be evidently less in the organic rich surface layer in comparison with the organic poor deeper layers. For each soil layer the relation between soil water content and actual water repellency was determined. This resulted into three distinguished zones: a) a water repellent zone; b) a transition zone and c) a wettable zone. The threshold values of the volumetric water content that describe the transition zone varied per depth.