Fractal features of soil particle-size distribution of different weathering profiles of the collapsing gullies in the hilly granitic region, south China

Collapsing gully erosion is one of the most remarkable natural hazards in hilly granitic region of southern China. However, there have been few studies examining the changes of soil particle-size distribution (PSD) associated with soil profiles of the collapsing gullies. In recent years, the fractal method has become a new one to estimate soil structure. It has been proved to be a useful tool to examine soil properties and the relationships with other eco-environmental factors. To study the fractal dimension (D) of soil PSD in soils within different weathering profiles (surface layer, red soil layer, sandy soil layer and detritus layer) and its relationships with PSD and soil properties, we conducted an experiment on four collapsing gullies in four counties (Tongcheng County, Gan County, Anxi County and Wuhua County, labeled as TC, GX, AX and WH, respectively) in the hilly granitic region of southern China. The results showed that different weathering profiles exhibited a significant effect on soil physical-chemical properties and PSD. With the fall of weathering degree (from surface layer to detritus layer), as a result, SOM indicated sharp decrease, bulk density first increased sharply and then declined slightly, the finer soil particles (silt and clay) decreased, and especially the clay contents declined noticeably, whereas the gravel and sand contents increased considerably. D values varied from 2.883 to 2.597 of TC, 2.880 to 2.727 of GX, 2.921 to 2.644 of AX and 2.845 to 2.708 of WH, respectively, finer textures giving greater D values. The D values in the sandy soil layers and detritus layers were lower than those in the surface layers and red soil layers. The linear regression analyses showed that D was remarkable negatively correlated with gravel and sand content and positively correlated with clay contents, SOM. The second-degree polynomial equation was more adequate to describe the correlations between D and soil PSD. The results of this study revealed that fractal dimension can be used to characterize the soil grain-size characteristics and was regarded as an informative indicator for reflecting the weathering degree of different weathering profiles of the collapsing gullies in the hilly granitic region.