Abstract:The aim of this work was to examine the distribution and persistence of water repellency in soils of different texture in the Spanish humid region as a function of land use and management. At 34 locations, samples of soil under different land uses (maize crop, grassland, Pinus pinaster forest and Eucalyptus globulus forest) and various geological materials were collected from the surface layer (0-5 cm). In addition, P. pinaster and E. globulus forest soil samples were collected at four different depths (0-5, 5-10, 10-20 and 20-40 cm) at 10 of the previous locations. Water repellency was determined by using the water drop penetration time (WDPT) test on field-moist samples (actual WDPT) collected during summer and was compared with the values for samples dried at 25 and 105°C (potential WDPT).Based on the results, the prevailing vegetation and land use dictate the development and persistence of surface water repellency in the studied soils. The E. globulus forest soil samples were found to be the most water repellent, followed by the P. pinaster forest samples (73 and 64%, respectively, with actual WDPT >6 h); on the other hand, 79% of the fieldmoist samples from soils under maize and 71% of those under grass were non-repellent. The influence of vegetation on water repellency was found to be related to the type and content of soil organic matter. Each type of vegetation resulted in significant differences in surface water repellency between soils with sandy-loam texture and also between soils with loam or silt-loam texture; repellency was higher in the samples with coarser textures. The persistence of water repellency decreased with increasing soil depth, the decrease being more marked in the finer-textured soils than in the coarser ones and also in the pine forest soils than in the eucalypt forest soils. The water repellency of most of the samples dried at 25 and 105°C was similar to that of the field-moist samples collected during the dry period (r D 0Ð90, p < 0Ð01).

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Abstract:This work was undertaken for two main purposes. One was to examine spatial and temporal variability in surface water repellency under field conditions in sandy loam forest soils of NW Spain, and its relationship to weather and soil moisture conditions. The other purpose was to get further inside in the dynamics of soil water repellency by studying a wetting-drying cycle under controlled laboratory conditions. Both for the field and laboratory study, water repellency was determined using the Water Drop Penetration Time test. Soil water repellency under field conditions was found to exhibit a seasonal pattern, i.e. it peaked during the summer and was absent between November and May. The time required for repellency to become re-established during the spring was shorter under eucalyptus than under pine. Spatial variability peaked at an early stage of soil drying and was minimal during the wet period when soils were hydrophilic as well as at the end of the summer, when repellency was strongest. Spatial and temporal variability in water repellency was found to be negatively correlated with soil moisture and, to a lesser extent, with antecedent rainfall. The moisture range of the so-called transition zone (below which the soil is hydrophobic and also above which it is hydrophobic) differed for the pine (21-50%) and eucalyptus plantations (17-36%). The lower and upper bounds of the transition zone agreed well with the soil moisture contents at the permanent wilting point and at field capacity, respectively. The laboratory results with samples in the wetting phase confirmed those of the field tests. Water repellency increased slightly during the drying phase, but not so much as in the field.

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The primary purpose of this work was to assess the persistence of water repellency in the surface horizon of coarse-textured soils under natural Quercus robur ecosystems, and Pinus pinaster and Eucalyptus globulus plantations, in the northwest of the Iberian Peninsula. Water repellency was determined by applying the water drop penetration test (WDPT) to soil samples collected from variable depths (0-40 cm). Measurements were made on field-moist samples obtained at the end of the dry period and on samples dried at 25ºC in the air. All soils exhibited very high (severe to extreme) water repellency in the topmost soil layer (0-5 cm) but no significant differences among the three plant species studied. Extreme persistence was observed down to 20 cm in the soils under eucalyptus and down to 10 cm in those under pine. The soils under oak were those exhibiting the highest variability in water repellency and the greatest decrease in it with increasing depth (especially in relation to soils under eucalyptus).Water repellency exhibited significant positive correlation with the C content and C/N ratio of the soils. Soil water repellency was similar in the air-dried samples and field-moist samples.