Fractal Dimension of Soil Particle Size Distribution Along an Altitudinal Gradient in the Alxa Rangeland, Western Inner Mongolia

Abstract

The Alxa Plateau of Inner Mongolia exhibits vast differences in vertical distribution in vegetation and soil types. The plant communities range from the alpine meadows at the top of the Helan Mountain (3500 m) to the shrub desert at the foothills (1360 m). This type of ecosystem is common in the northwestern China, yet limited research has been conducted. We studied the fractal dimension (D) of particle size distribution (PSD) in the arid Alxa Plateau. Sixteen study sites were located along an altitudinal gradient of the Helan Mountain and the surrounding Alxa Desert rangeland. In the transformation from meadow, steppe to desert, the sand content in the 0 to 20-cm topsoil increased from 18% to 96%. Soil organic C (SOC) and total N decreased by 97% and 95%, respectively. Soil bulk density increased by 84%, and soil pH increased from 7.49 to 9.33. Vegetation cover decreased by 83% and biomass by 94%. The fractal dimension of PSD ranged from 2.66 to 2.06—the greater the sand content, the lower the D. Although the changes in D showed a narrow range from one range type to another, these changes were statistically significant (P < 0.0001). Vegetation cover, biomass, and soil fertility exhibited nonsignificant changes between some range types. Thus, D is a more sensitive parameter than vegetation cover, biomass, or soil fertility in monitoring the health and vigor of a range site or in estimating the degree of soil degradation. Once a trend in D is established, it could be used to monitor general directions of a particular ecosystem.