2.4 Calculation and analysis
ImagePy software (Wang et al., 2018) was used to analyze the segmented binary image and obtain the quantity, diameter, surface area and volume of soil aggregate particles; subsequently, the 3D structural characteristics of the aggregate after splashing with different raindrop sizes were obtained through calculation and analysis. Aggregate fragments were divided into six classes according to their size: 500-1000, 250-500, 106-250, 53-106, 25-53 and <25 μm.
The quantity and volume parameters of soil aggregates were used to measure the quantitative characteristics of aggregates. The three-dimensional mass fractal dimension (FD) and specific surface area (SSA) of each aggregate were used to characterize the extent of aggregate fragmentation. The FD and SSA are parameters that can appropriately reflect the geometry of soil structure. They represented the size of aggregates, soil erosion resistance and soil permeability (Pirmoradian et al., 2005; Wang et al., 2014). The FD was previously used to describe the self-similarity and scale independence of objects (Dal Ferro et al., 2013). The calculation of FD was based on the principle of the box counting method: the image stack was covered with cube boxes of different side lengths, and the number of boxes was recorded (Kravchenko et al., 2011; Perret et al., 2003) to satisfy the following relationship (equation [4]):
(4)
where N(ε) is the number of boxes, ε is the side length of the box, FD is the value of fractal dimension.
FD was calculated by linear regression of log{N(ε)} vs log(1/ε), and the range was 2-3. Fractal dimension was computed using the open-source software ImagePy (Wang et al., 2018). Specific surface area is volume-specific surface area, expressed as the ratio of surface area to volume of each aggregate fragment.
The data was analyzed using one-way analysis of variance (ANOVA). Duncan’s multiple range method was used to compare mean values, and significant differences between mean values were defined as P< 0.05. The results were expressed as the mean standard deviation, and all tables and figures were processed in Microsoft Office Excel 2010.