Two forest soils rich in organic matter but differing in texture (sandy loam and silty loam) were heated under controlled laboratory conditions in order to examine the consequences of the heating effect that accompanies the passage of a fire on the physical properties of soil. Three samples of both soils were heated for 30 min in a muffle furnace at temperatures of 25, 170, 220, 380 and 460°C. At each temperature, the following parameters were determined: dry aggregate size distribution, water aggregate stability, total porosity, pore size distribution, water repellency and hydraulic conductivity. Heating the soils at 170 and 220°C caused no significant changes in aggregate size distribution or total porosity but increased water aggregate stability and the volume of pores 0.2–30 μm. Also, increased water repellency and strongly decreased the hydraulic conductivity. All parameters underwent much more dramatic changes at 380 and 460°C that can be ascribed to the combustion of organic matter. At such temperatures, water repellency was destroyed and the low hydraulic conductivity can be attributed to the aggregate breakdown observed under dry and wet conditions.

At three adjacent sites in steeply sloping woodland in Galicia (NW Spain), surface runoff and associated erosion under simulated rainfall (64 mm h 1 ) were measured on five occasions between June 1998 and July 1999. Two of the three sites had recently been deforested and topsoil added, and one of these two had been sown with grass, which was germinating at the onset of the study. Deforestation greatly increased runoff and erosion rates, and the recovery of plant cover reduced erosion. All three soils were very hydrophobic due to high levels of poorly humified organic matter, which led to higher runoff rates than expected, especially during dry periods. However, great structural stability prevented there being a significant correlation between runoff rate and soil erosion.

Abstract:Soil water repellency (hydrophobicity) is a naturally occurring phenomenon that can be intensified by soil heating during fires. Fire-induced water repellency, together with the loss of plant cover, is reportedly the principal source of increased surface runoff and accelerated erosion in burned soils.In this study, the surface water repellency of several soils affected by summer forest fires in northwest Spain was studied and compared with that of adjacent unburned soils. Soil water repellency was determined using the ethanol percentage test (MED).Most of the unburned soil samples exhibited water repellency that ranged from strong to very strong; only four of the unburned soil samples were non-repellent. Water repellency in the unburned soils was significantly correlated with the organic carbon content (r D 0Ð64, p < 0Ð05). Overall, fires increased the surface water repellency in soils with previously low degrees of water repellency and caused little change in that of originally strongly hydrophobic soils.In order to examine in detail the changes in water repellency with temperature, three unburned soil samples were subjected to a controlled heating program. Water repellency increased between 25 and 220°C, water repellency peaked between 220 and 240°C and disappeared above 260-280°C. Extrapolation of the results of the heating tests to field conditions suggested that the intensity of fire (temperature and time of residence) reached by most soils during fires is not too high. Based on the results, the determination of water repellency could be used as a simple test for the indirect estimation of the intensity levels reached on the soil surface during a fire.

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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