Using multi-fractal analysis to characterize the variability of the soil physical-chemical properties along deep soil profile through multipoint sampling

The root zone is one of the most important soil horizons through which plant obtains its needs of nutrient and water resources, especially for deep rooted plant. However, few studies exist on the multi-fractal of soil particle size distribution and its great influence on soil chemical properties and soil water status in the root zone for a deep soil profile, with most knowledge gained from shallow rooted plant. We obtained multiple soil profiles with the maximum rooting depth to 21 m, and applied single fractal and multifractal dimensions to characterize the soil particle size distribution, and explored their correlations with soil depth, soil chemistry properties and soil water. Our results found single fractal and multifractal dimensions of soil PSD and soil particle composition (clay, silt and sand content) varied with soil depth in a soil profile that can be categorized according to above and below the depth corresponding to 90% of the total root biomass as R-zone and D-zone, respectively. Soil fractal dimensions except capacity dimension (D₁) and correlation dimension (D₂), and clay and silt content differed significantly in the R-zone and the D-zone (p < 0.01). Correspondingly, the relationship between the soil PSD and soil chemical properties were higher in the R-zone than those in the D-zone. From the R-zone to the D-zone, the correlations between D₁ and D₂ and soil water content in dried soil layers changed from positive to negative. Based on these results, we concluded that more heterogeneity of soil physicochemical properties in the R-zone than the D-zone. Our findings highlight the importance and complexities of soil physicochemical properties in the root zone, some of which are valuable to characterize root function in the Critical zone and form integral components of vegetation models.