4.1 Effect of rainfall intensity on splashed soil aggregates quantitative characteristics
The number of aggregate fragments increased corresponding to the increase in rainfall intensity, especially at 68.61 and 217.26 mm h-1 (Table 1). In the process of water erosion, the primary mechanisms of aggregate breakdown are fast wetting and mechanical breakdown caused by raindrop impact (Legout et al., 2005; Shi et al., 2010). The results here show that, with an increase of rainfall intensity, the quantity of aggregates and the degree of fragmentation gradually increase.
In regards to aggregate distribution, aggregates mainly consisted of 250-500 μm, 106-250 μm, and <25 μm particle sizes, whereas for volume, the 106-1000 μm range made up the majority (Fig. 3A and 3B). Raindrop impact on the surface could affect soil erosion and alter aggregate structure in various ways (Kinnell et al., 2005). When the erosion process was dominated by rainfall detachment, the particle size distribution of eroded soil was different from that of the original soil (Slattery and Burt, 1997). It was affected by the particle distribution of the original soil and the destruction of aggregates during erosion (Mahmoodabadi et al., 2014). Based on the number and volume of aggregate fragments, we found that raindrop splashing dispersed larger aggregate particles (500-1000 μm) and broke them into aggregates of smaller particles (<500 μm). Raindrop splashing changed the arrangement of soil particles, decreased the number of larger aggregate particles, and increased the quantity of smaller aggregate particles. As a result, the particles formed by dispersion and fragmentation were deposited in the pores of the upper soil, clogging the pores of the topsoil and forming a thin, dense surface crust with low permeability (Assouline, 2004). In turn, the crust further reduced soil infiltration and exacerbated soil erosion (Sajjadi and Mahmoodabadi, 2015).