Mechanisms and processes affecting aggregate stability and saturated hydraulic conductivity of top and sublayers in semi-arid soils

Soils are turned over during agronomic and environmental activities, such that the sublayer becomes the topsoil. Because subsoils have been subjected to high pressures over long periods in the field, their activities and functionality can be changed when they are shifted to the top layer. The present work objective was to investigated the mechanisms and processes affecting the structural stability, and consequently saturated hydraulic conductivity (K_s), of a semi-arid soils, which were taken from < 0.3 m depth (top soils) and > 0.3 m depth (sublayers soils). Disturbed soil samples, with similar aggregate-size distribution and bulk densities, were packed in columns, prewetted with saline solution (SS), and then their K_s values were determined during consecutive leaching with SS and deionized water (DI). The K_s values of the various soils under SS leaching differed due to slaking and swelling processes that changed the soil structure. The effect of the slaking process on K_s reduction was more significant in the top- than sublayer soils. Soil swelling under SS wetting and leaching caused mainly by penetration of water molecules into capillary pores in the soil that increases the pores volumes, and enlarges the aggregate swelling (matrix-type swelling). The average bulk density, and consequently the structural strength, of the aggregates in the sublayer soils were significantly higher than that of the top soils. This suggested that, under matrix-type swelling, less aggregates would be broken in the sublayer than in the top-layer soils. During leaching of the top and the sublayer soils with DI, the aggregates breakdown and K_s reduction were caused mainly by dispersion and osmotic swelling of the clay fractions in the soil. In this case, a negative relationship between the K_s and SAR values was obtained, regardless of the soil depths.