Water repellence assessment in humid Mediterranean carbonated environments: influence of shrubland species

Abstract

It is stated the importance of the natural or induced (by fire) water repellence in terms of water redistribution in the soil profile, reduction in soil infiltration capacity and, thus, in runoff magnification. Hydrophobicity has been established around the world in different weather conditions, land uses, soil and vegetation types. Regarding soil and vegetation, many studies are based on coarse acidic soils with vegetation type of pine forest, eucalyptus, deciduous trees, grassland, cropland, chaparral, and lately in shrubland, but there is a scarce number of studies concerning Mediterranean carbonated environments. In this work, we studied the natural occurrence of soil water repellence in a carbonated Mediterranean area by means of the WDPT. The study was carried out in Podentes (Coimbra), central Portugal, where 4 has. of shrubland composed by Quercus coccifera, Pistacia lentiscus and Arbutus unedo, and developed on an Umbric leptosol (WRB) and a Calcaric cambisol (WRB) soil type, were delimited in order to study the natural occurrence of the water repellence. This parameter was assessed depending on the shrubland type, slope orientation, soil depth and on different soil fractions (<2 mm, 2-1 mm, 1-0.25 mm and <0.25 mm). The largest hydrophobicity (56% strong persistence and 44% slight persistence) was measured on the soil fraction <0.25 mm of the soil surface layer. Soil water repellence has been demonstrated that decreased with depth also in a carbonated Mediterranean environment. The studied shrubland species showed an increasing trend on the soil hydrophobicity persistence: A. unedo > Q. coccifera ¿ P. lentiscus; and depending on the orientation: NE > SW. Direct relationships were obtained between the soil organic matter content and the log WDPT, on almost all the surface soil samples. The soil pH and carbonate content did not display correlation with soil water repellence. The hydrophobic compounds generated by the different tissues, waxes and resins produced by the different shrubland types, and incorporated to the soil as a binding agents into the soil micro-aggregates, seems to be the explanation for the differences on the WDPT obtained. The patchy distribution of the vegetation governs the persistence of the natural soil water repellence, impeding water infiltration mainly by micropore flow, being then the soil hydrology managed by the macropore flow, crack and root system. In this way, a further research into soil organic matter quality in the finer soil aggregates could be necessary to confirm this link between the components of the different shrub species and the trends on soil water repellence.