Temporal evolution of salts in Mediterranean soils transect under different climatic conditions.

摘要

The research was conducted in Israel at three sites along a south-north axis, characterized by increasing annual rainfall, from 310 mm at site LAV in the south through 600 mm at site MAT (600), to 800 mm at site EIN in the north. At each site soil samples were taken during several seasons (September 2001 through April 2003), in three dominant microenvironments at 0-2 cm and 5-10 cm. The following microenvironments were selected at LAV and MAT: "Under Shrub" (US), "Between Shrubs" (BS), and "Under Rock fragments" (UR). At EIN the selected microenvironments were US, BS, and "Under Tree" (UT). In each soil sample electrical conductivity (EC), pH, and concentrations of several ions were determined. The objective was to analyze the effects of soil microenvironments and climatic conditions on the temporal dynamics of salt concentrations. In all microenvironments at all sites the minimal values of EC were found in the rainy season (January or April), and the maximal values in the dry season (September). In the rainy season the temporal variability of EC in the topsoil was regulated by: (1) clay, which restricted the leaching of salts from the topsoil when EC was low; and (2) surface features (microenvironment), when EC was high. In the UT, US, and UR microenvironments the rainy season could be divided into two periods with respect to their effect on salt movement in the topsoil: at the beginning of the rainy season (September-January) the reduction in EC was relatively moderate, especially with regard to ions involved in biotic activity (Mg++ and K+), whereas, late in the rainy season (January-April) there was enhanced reduction in EC. In contrast, in BS the regulation of salt movement was weak at all sites. Hence, in this microenvironment the salts concentration (mainly Na+ and Cl-) responded rapidly to changes in rain amount and soil moisture and temperature. In the dry season (April-September) the temporal variation in EC varied not only between microenvironments but also between sites. In US, where local surface features were similar at all sites (the same shrub), the rise in EC was maximal at LAV (mainly Ca++ and Na+), and gradually diminished toward EIN. Thus, the contribution of regional sources to the salts added to the soil diminished toward the humid site, EIN, where the EC hardly changed in any microenvironment. In BS and UR microenvironments the rise in EC (mainly in Ca++, Na+, and K+) was greatest at site MAT, and decreased toward LAV and EIN. It seems that this pattern was affected also by changes in local biotic activity..