Fractal features of soil and their relationship with soil nutrients under desert shrubs in the West Ordos region

Survival and adequate growth of plants require ideal soil mechanical composition and nutrient conditions. The fractal dimension of soil and the distribution characteristics of nutrients and their relationships play an important role for the stability of soil structure, resistance to wind erosion, and movement of water and fertilizer. Ammopiptanthus mongolicus, Tetraena mongolica Maxim, and Zygophyllum xanthoxylum Bunge are typical shrubs that are widely distributed in the West Ordos region. They are important natural resources and natural ecological barriers which are crucial for the stability of arid steppe ecosystems. In this study, we examined soils under three shrubs. The vertical distribution of soil particle mechanical composition, soil volume fractal dimension, and soil nutrients in 0– 60 cm soil depth were analyzed to explore the relationships between soil fractal characteristics and soil fertility and to reveal the mechanisms of survival and diffusion of shrub vegetation in an arid steppe. The results showed significant differences in the morphologies of the three typical shrubs, which resulted in different grain size characteristics of the soil underneath the shrubs after intercepting the sand flow. The volume of clay and silt content and fractal dimension differed significantly among shrub species (P < 0.05). The soil fractal dimension was significantly different (P < 0.05), i.e., T. mongolica (2.432 ± 0.041) > A. mongolicus (2.391 ± 0.046) > Z. xanthoxylum (2.276 ± 0.034) > bare sand (2.218 ± 0.059). The soil fractal dimension was correlated with < 0.5 mm particles such as clay and silt (< 0.05 mm), very fine sand (0.05～0.1 mm), fine sand (0.1～0.25 mm), and medium sand (0.25～0.5 mm). The 0.1 mm particle size was the critical size determining the soil fractal dimension under shrubs in the study area. The soil fractal dimension increased with increasing < 0.1 mm diameter content (such as content of clay, silt, and very fine sand). Soil rapidly available potassium content decreased with increasing soil depth, and soil organic matter, available nitrogen, and available phosphorus fluctuated over soil layers. Differences in soil nutrients among species were significant (P < 0.05). The soil structure and fertility of shrub-covered land were significantly improved compared with those of bare sand. A highly significant positive correlation of soil organic matter content and soil available nitrogen content with soil fractal dimension was observed (P < 0.01), and a significant positive correlation of soil available phosphorus content and soil rapidly available potassium content with soil fractal dimension was found (P < 0.05). The results indicate that soil fractal dimensions can be used to describe eolian sandy soil properties in the West Ordos arid steppe. Appropriate inclusion of T. mongolica and A. Mongolicus in the shrub community of this environment can improve soil particle status, promote soil quality in partial desert areas, and curb continuous coarsening and deterioration of surface sand. This study provides a scientific research basis for quantitative evaluation of desert shrub functions such as windbreaking, sand fixation, and soil conservation.