Soil degradation factors along a Mediterranean pluviometric gradient in Southern Spain

The modifications to soil properties that resulted from changes in water availability along a pluviometric gradient from 240 to 1100 mm mean annual rainfall were investigated. The study was carried out at eight experimental sites in southern Spain and assessed the influence of a reduction in annual average rainfall along the pluviometric gradient on various physical, chemical and hydrological properties of the topsoil, and on the biotic characteristics of the environment. The aims were to assess the soil degradation status, and to determine if any of the soil variables studied could be used as soil degradation indicators. For each experimental site the relationships among a series of edaphic, hydrological and biological properties were investigated, and the principal factors affecting soil degradation were determined using principal component analysis. The properties included the clay, silt and sand content; organic matter; retained organic carbon; salinity; cation exchange capacity; structural stability; USLE K factor; bulk density; saturated hydraulic conductivity; soil moisture; the number of vegetal species; and vegetation cover. sults show that relationships among biotic and abiotic factors control soil degradation processes from wet to semiarid/arid conditions along the Mediterranean pluviometric gradient. A precipitation-based soil degradation threshold of approximately 500 mm year− 1 was established, as below this level the vegetation is no longer associated with the presence of greater soil moisture content, but adapts to degradation, as evidenced by the appearance of xerophytic species.