Multifractal parameters reveal the impacts of shrub encroachment on soil particle size distribution (PSD) in a coastal wetland of the Yellow River Delta

Shrub encroachment in coastal wetlands leads to heterogeneous distribution of soil resources, which may affect soil particle size distribution (PSD). Fractal theory was widely used to quantify the characteristics of soil PSD. However, exploration on the spatial changes in PSD of deep soil layers due to shrub-encroached vegetation succession is limited. Multifractal theory and GIS spatial analysis tools were used to spatially quantify the changes in soil PSD at different soil layers (0–80 cm) in a coastal wetland landscape (100 × 120 m) encroached by a native shrub Tamarix chinensis in the Yellow River Delta (YRD), China. We also discussed the potential mechanisms of soil PSD changes contributed by shrub encroachment. The results showed that higher content of fine sand, medium sand and coarse sand, while lower content of silt and very fine sand in 0–10 cm were observed in T.chinensis patches than in single T. chinensis and bare land (p < 0.05). In 30–40 cm, however, T.chinensis patches have higher soil clay content (p < 0.05). The generalized dimensional spectrum and multifractal parameters indicated that soil PSD in shrub patch area exhibited greater width and higher heterogeneity. Multifractal method could explain the changes of soil PSD better than single fractal dimension (D), and the Capacity Dimension (D₀) was the most effective one to quantify the soil PSD characteristics. The values of D₀ showed a clear order of shrub patch > single shrub > bare land in each layer. In addition, the D₀ suggested that influences of T.chinensis on soil PSD can remain 70 cm below the surface in our case. Considering the complex sedimentary process in the YRD, the processes of water and wind erosion mediated by shrub encroachment, the positive feedback based on plant preferences and the mixed effects of shrub and grass species might promote the redistribution of soil particle size. Encroachment in coastal wetland of the YRD caused changes in soil PSD, especially at the surface soil layer. Multifractal parameters could quantify subtle characteristics of soil PSD in shrub-encroached area. Our work indicates that shrub encroachment in coastal wetland plays a significant role in altering regional and even global soil environments.