4.2 Effect of rainfall intensity on splashed soil aggregate fragment parameters
The impact of raindrops increased the FD of the soil aggregates (Figure 4A). A greater FD value corresponded to a greater degree of aggregate breakdown and detachment. The FD ratios of the aggregates splashed by rainfall intensities of 5.76 and 68.61 mm h-1 were higher than those of the aggregates splashed by a rainfall intensity of 217.26 mm h-1 because the higher rainfall intensity had higher energy, which strengthened the degree of fragmentation and caused more macroaggregates to decompose into microaggregates. The higher FD value indicates that the particle size distribution of aggregates is dominated by smaller particles, a lower proportion of stable aggregates is present, and the soil structure is inferior (Tyle et al., 1992, Huang et al., 2017). The results presented here show that the raindrop splash was able to break apart the aggregate and destroy the surface soil structure. This is consistent with the result of Fu et al. (2020).
Soil aggregate index, such as specific surface area (SSA), can also reflect the stability of soil aggregates (Pirmoradian et al., 2005). Raindrop splash led to an increase in the total specific surface area of soil aggregates, which increased as the intensity of rainfall increased (Figure 4B). As the particle size decreased, the corresponding specific surface area increased (Figure 4C). This result has proven that a higher specific surface area, leads to a finer soil texture and a stronger soil dispersibility (fragmentation) (Wang et al., 2014). The specific surface area of the aggregates increased significantly, especially in the 25-53 μm and <25μm aggregate particles, after being splashed by 68.61 and 217.26 mm h-1 (Figure 4B and 4C). This likely occurred because large raindrops had a stronger rainfall intensity and rainfall energy, thus, increasing the rainfall intensity strengthened the aggregate decomposition (Sajjadi and Mahmoodabadi, 2015). Therefore, the splashed soil aggregates contained more fine particles than before the splash (Figure 4C). Meanwhile, as the particle size of aggregates decreased, they exhibited a smaller mass and smoother shape; furthermore, migration and denudation were more likely to occur during runoff transportation and the underlying pores were more likely to be clogged (Assouline et al., 2004), causing compaction of the aggregate microstructures (Adesodun et al., 2007; Li et al., 2018). In addition, some studies have shown that a greater amount of organic matter content was present in surface soil macroaggregates (Jastrow, 1996) and that aggregates have physical protection for organic matter (Field et al., 2006), while the total specific surface area was negatively correlated with organic matter content. Compared with soil containing higher organic matter content, soil with lower organic matter content showed higher sealing and anti-crust properties (Ramos et al., 2003). Therefore, the aggregate breakdown caused by raindrop splash decreased the soil structure and fertility, reduced the land productivity, and even aggravated the formation of crust (Hu et al., 2018; Fu et al., 2020).