Soil structure variations under water and vegetation manipulations.

The responses of soil aggregate size and stability, and organic matter content to different water regimes, in different microenvironments (under shrub (US) and in no-shrubs area (NS)), were investigated for 2 years on an alluvial terrace in a mildly arid area in Israel. In the framework of an experimental project, dealing with combating desertification, three different water treatments were applied to selected plots by a water sprinkling system. In a specific area within each of these plots, Cistus salvifolius shrubs were planted, while the rest of the area was left unplanted. It was found that in both the US and NS microenvironments, aggregate size and stability increased with increasing amount of applied water. The improvement of soil structure is attributed to the effect of favorable soil moisture conditions that encouraged both micro-organism activity and root growth, and led to an increase of bonding agents in the soil. Changes in soil structure were also attributed to differences in the number of wet/dry cycles. The relatively weak soil structure and low soil organic matter of the US microenvironment compared with the NS microenvironment is attributed to the disturbance of the soil caused by planting the Cistus. However, the differences in these soil properties, between the microenvironments, decreased with time, especially in the wetter treatment, indicating a change towards a typical natural “steady state”, characterized by higher soil organic matter and stronger structure under shrubs.