Soil particle size distribution characteristics of different land-use types in the Funiu mountainous region

Soil particle composition is one of the main physical properties of soil that affects soil fertility, and the fractal dimension of soil particle size distribution (PSD) can be used to quantitatively evaluate the particle composition of different soils. The single fractal dimension of soil PSD can quantitatively characterize the roughness of the soil particle, and the multifractal dimension of soil PSD can quantitatively characterize the inhomogeneous property of soil particle composition. Sloping farmland is widely distributed in hilly areas of China that are subject to the most serious soil erosion, and the loss of soil will lead to changes in soil particle composition and soil physical properties. Previous studies have not been used to evaluate the heterogeneity of soil PSD between sloping farmland and other land use types by the multifractal method. In order to quantitatively assess the differences in soil properties between the slope land and other types of land use, this paper takes four types of land use (sloping farmland, shrub-grass sloping land, terraced farmland, and oak forestland) in the warm temperate granite mountainous areas in eastern central China as research objects. A laser particle size analyzer and soil fractal model were applied to compare the soil PSD of different land-use types and their corresponding single fractal dimension (D-v) and generalized fractal dimension (D-q). The results showed that there were clear differences between the D-v and D-q of soil PSD in different land-use types. The D-v of the soil PSD increased as follows: sloping farmland < level terraced land < shrub-grass sloping land < oak forestland, indicating that the soil particle composition coarseness was highest in sloping farmland and lowest in the oak forestland. The sequence of characteristic parameters of D-q (capacity dimension, D-0; information dimension, D-1; and correlation dimension D-2) was as follows: sloping farmland < level terraced land < shrub-grass sloping land < oak forestland, indicating that the inhomogeneity of soil particle composition was the highest in sloping farmland, and lowest in oak forestland. The D-v, D-0, D-1, and D-2 of the soil PSD were significantly positively correlated to the content of clay and silt particles, and were significantly and negatively correlated to the content of sand particles, indicating that the fewer clay particles and silt in the soil or the higher the proportion of sand, the higher the coarseness and the inhomogeneity of soil particle composition. This study showed that the soil coarseness and the inhomogeneity of the sloping farmland soil particle composition are significantly greater than those of other land-use types. The mass loss of the soil fine particles (clay and silt particles) is an important cause of coarsening and increased inhomogeneity of the sloping farmland. This study provides a soil fractal index for quantitative assessment of soil texture and the degradation of soil properties in landscapes prone to soil erosion such as sloping farmland.