Scaling properties of particle-size distributions of purple soils in a small agricultural watershed: A multifractal analysis

The particle-size distribution (PSD) of soils is a basic physical attribute that affects edaphic hydrological, chemical, and biological properties as well as soil quality and land productivity. Purple soil, as an early weathering product of purple mudstone, is a valuable resource for agricultural development in southwestern China. However, little is known regarding PSDs of purple soil, especially in small agricultural watersheds with diverse land-use types. The present study aimed to elucidate the scaling properties of purple soil PSDs by applying multifractal characterization. Results indicate that the PSDs of purple soils demonstrate clear multifractal behaviour, with generalised dimension and multifractal singularity spectra showing sigma and left-deviating unimodal convex-up curves, respectively. By comparing with soils under other land-use types, the present study found that forestland purple soil is characterised by coarser granularity and more complex PSD, and has the minimum volumetric fractal dimension and the maximum generalised dimensions. No statistically significant correlation was observed between multifractal parameters and soil texture separates, except for that between the Hölder exponent of order zero and the clay fraction (p < 0.05). In addition, most of the multifractal parameters do not exhibit statistically significant differences among soils under different land-use types (p < 0.05). The findings of the present study highlight that the effect of current land use on purple soil PSDs is limited and complex at a small watershed scale, and that the legacy effect of historical land use also plays a role in the evolution of purple soil PSDs. Overall, our findings can provide a reference for comprehensive understanding the influence of land use on purple soil quality.