Soils in the Mediterranean region: what makes them different?

Abstract

Mediterranean climates are characterized by winter rains — with some months of excess rainfall over evatranspiration, warm and dry summer months with moisutre deficits — drying out soils and their annual vegetation (xeric moisture regime). They are found on western parts of all continents, between the cooler temperature zone and the hot dry desert zone. The largest Mediterranean region, surrounding the Mediterranean Sea extends over 4,300,000 km 2 and exhibits a wide variety of soils and geo-ecosystems. Characteristics landscape attributes are the high proportion of mountains with sleep slopes, significant additions of Saharan desert dust to practically all soils of the region, and a large proportion of limestone and other calcareous rocks as soil parent materials. Characteristics soil behavior features are moderate weathering with pervection (leaching, lessivage) of mostly 2:1 clays into B horizons (Xeralfs:Luvisols), hematite-induced reddening of the clays due to summer dehydration of free iron oxyhydroxides, carbonate dissolution and reprecipitation with prevalence of calcic horizons (Xerolls; Calcisols) in semiarid regions, and development of Vertisols, mostly in lowlands, where deep layers of swelling/cracking clays have sedimented. Shallows soils on nearly bare slopes, mostly a result of erosion subsequent to deforestation, are frequent (Leptosols, Cambiosols; Inceptisols, Entisols). Red (or Brown) Mediterranean soils are no longer used as a separate classification group in modern, well defined, soil property-controlled taxonomies (Soil Taxonomy; FAO system), but were partially replaced by Duchaufour's term Fersiallitic soils in some classification systems. Terra Rossa continues to be used in some classification for hard limestone derived red soils, mostly shallow. The effects of man's past and current interference with the lanscape are pervasive in most regions, and predictions for possible future effects on the soils and ecosystems, due to greenhouse gas induced warming and rainfall change, are partly negative, partly still too uncertain.