Using multi-fractal analysis to characterize the variability of soil physical properties in subsided land in coal-mined area

Underground coal-mining activities can result in severe land subsidence; the soils are severely damaged and the hydrological process is disturbed. Understanding the spatial variability of soil physical properties is helpful to analyze the effects of underground mining on these properties and select suitable land rehabilitation measures. In the present study, multi-fractal analysis was used to reveal the effects of land subsidence and rehabilitation on the spatial variability of soil physical properties. Three plots from Pingshuo coal mine area in Shanxi Province of China, including one unmined plot (UMP), one subsided plots (SUP), and one rehabilitated plot (RHP), were selected for the study. Sixteen sampling points were identified in each plot. Four typical soil physical properties, namely bulk density (BD), field capacity (FC), saturated hydraulic conductivity (K-s), and soil disintegration rate (SDR) at depths of 0-20, 20-40, 40-60 and 60-80 cm at each sampling point were measured. Generalized dimensions and singularity spectra of the distributions of different soil physical properties were calculated for each plot. Parameters D(1) and Delta D of the generalized dimensions, and Delta alpha and Delta f of singularity spectra, were extracted. Multi-fractal analysis is highly sensitive to the spatial variability of soil physical properties and provides valuable information for depicting spatial variability of these properties, including the concentration degree, dispersion degree, local variability degree, overall non-uniformity, and symmetry degree of spatial distribution. Land subsidence increased the spatial variability of soil physical properties, especially in terms of K-a and SDR, whereas land rehabilitation had the opposite effect. Compared to the UMP, the degree of local variability of K-s and SDR distributions increased by 191% and 132% in the SUP, respectively, whereas these factors decreased by 41% and 51% after land rehabilitation, respectively. The spatial variability of soil physical properties in the SUP was the highest compared to UMP and RHP. It is feasible to utilize multi-fractal techniques to assess the spatial variability of soil physical properties at the scale of a subsided plot in a coal-mined area, Information on the variability of soil physical properties at a small scale is required to more accurately restore soil environment and soil function through rehabilitation.