Abstract: Amygdalus pedunculata, Caragana microphylla and Caragana stenophylla are common, widely distributed shrubs in Otindag sandy land. This research studied the soil properties beneath these three shrub species. The vertical distributions of soil particle mechanical composition, soil volume fractal dimension and soil nutrients in the 0-60 cm soil depth were analyzed to explore the relationship between soil fractal characteristics and soil fertility after, grassland colonization by shrubs, to assist understanding of the mechanisms of spread and survival and of shrub vegetation in desert steppe. It was found that there are significant differences in the morphology of the three common shrubs, which result in different particle size characteristics of the soil deposited under the shrub through interception of the sand flow. The proportions of clay and silt content and the fractal dimension differed significantly among the different plant species (P<0.05). Soil fractal dimension also differed significantly (P<0.05), as follows: C. microphylla (2.346)>C. stenophylla (2.259)>A. pedunculata (2.149)>bare sandy (2.057). Soil fractal dimension was correlated with particle diameter and proportions of <0.25 mm particles included clay and silt (<0.05 mm), very fine sand (0.05-0.1 mm) and fine sand (0.1-0.25 mm). The 0.1 mm particle size was the critical size that determined soil fractal dimension under shrubs in this study area. The greater the proportion of particles < 0.1 mm diameter (such as the clay and silt, very fine sand), the higher soil fractal dimension was. There was significant positive correlation between soil organic matter content, soil total nitrogen content and soil fractal dimension (P<0.01). Soil organic matter content decreased with increasing soil depth, and total nitrogen and total phosphorus fluctuated with soil layer. The differences in soil organic matter and total nitrogen content among species were significant (P<0.05), but the difference in total phosphorus content was not significant. The soil structure and fertility of shrub-covered land were significantly improved compared with values for bare sand sites. The soil improvement associated with the different species ranked: C. microphylla>C. stenophylla>A. pedunculata.

Key words: shrub encroachment, soil fractal dimension, fertile island effect, soil depth