Fractal features of soil particle-size distribution and total soil nitrogen distribution in a typical watershed in the source area of the middle Dan River, China

Fractal scaling theory was employed to analyze the fractal dimension of soil particle-size distribution (PSD) for different plant communities with similar soil types in a watershed in the middle Dan River Valley, China. A total of 296 soil samples were collected from 78 sites. PSD and total soil nitrogen (TSN) were determined in soil from depths of 0–60 cm in four soil horizons for different plant communities. Soils in this area typically comprise silt and fine sand. The fractal dimensions of the six selected plant communities ranged from 2.73–2.89, with fractal dimension (Dm) values of grassland and forestland being lower (2.73–2.78) than those of cropland (2.81 and 2.89). There was an obvious decreasing trend in TSN content with increasing depth under the various plant communities. Spatial patterns of TSN changed significantly with land-use types. Organic nitrogen was the main component of soil nitrogen. There was a strong positive correlation between the fractal dimension and the silt and clay content (n = 78, R2 = 0.96, P < 0.01), with increasing Dm values corresponding to higher silt and clay contents. The Dm value and TSN content both indicated positive correlations with silt and clay content at a depth of 20–60 cm. These results demonstrate that fractal dimension analysis offers a useful approach to quantify and assess the degree of soil degradation among similar soil types, but that anthropogenic disturbances can have a great impact on the fractal dimensions for different land-use types. Cropland was prone to soil degradation, especially on steep slopes. Consequently, improved conservation measures are needed to enhance and sustain soil and water quality, and to prevent further soil degradation in the middle Dan River.