4.3 Effect of raindrop diameters on microscopic characteristics splashed soil aggregates
The microstructure of soil aggregates was different in two and three dimensions after raindrop splash (Figure 2). The splashed soil, especially by large raindrops, displayed an obvious fragmentation of aggregates (Figure 2D). This result indicated that the main cause of aggregate fragmentation was large raindrops. It is easier to form a structure consisting of large aggregates after drying. The formation of surface crust depended on the degree of surface aggregate fragmentation and the stability of the soil structure (Wick et al., 2016; Gelaw et al., 2015). The splashed soil aggregate particle size was smaller than that of the undisturbed soil (Table 2). This result is consistent with that of Ramos et al. (2003), in which the fragmentation of aggregates produced smaller particles than the original soil, allowing the soil to be displaced and reoriented into a more continuous structure; this consequently clogged pores and formed a surface crust. Sajjadi and Mahmoodabadi (2015) found that soil containing finer particle aggregates had a higher transportability of preseparated particles than larger aggregates. Moreover, as particle size decreased and the texture became finer, the pore structure of the soil became denser. The impact of raindrops, which caused physicochemical compaction and dispersion of the surface soil, was the main reason for crust formation and reduction of the infiltration rate (Assouline et al., 2004; Fu et al., 2017). In this study, larger raindrop diameters resulted in greater corresponding rainfall intensity and energy. As a result, soil tends to form a surface crust after heavy rains. The formation of a soil surface crust not only further reduces porosity (Pagliai et al.,2004) but also reduces surface roughness, thereby exacerbating surface runoff and soil loss (Robinson and Phillips, 2001; Assouline and Ben-Hur, 2006).