Raindrop splash engenders the dispersion and transport of the soil particles, that is the primary stage in the process of soil erosion. Raindrops with different diameters may have different influences on different soil structures. The research objective was to quantitatively and visually analyze the change in surface aggregates and pore microstructure of five soils (Eum-Orthic Anthrosol, Ustalf, Cumulic Haplustoll, Ustochnept and Quartisamment) in the Loess Plateau caused by various raindrop diameters (2.67, 3.39 and 4.05 mm) using rainfall tests, synchrotron-based X-ray micro-computed tomography (SR-μCT) and digital picture processing. Surface aggregate fragmentation and pore plugging rose as growing raindrop diameter. Under raindrop splash, the increase in raindrop diameter increased the number of microaggregates ({less than or equal to} 250 μm) of Cumulic Haplustoll, Ustalf and Eum-Orthic Anthrosol; the irregular pore-shape factor of Quartisamment and Ustochnept; and the total number of aggregates and pores. Moreover, the soil physicochemical properties also had a significant impact on surface aggregate breakdown and pore plugging (P < 0.01). Higher sand contents made the soil structure of Quartisamment and Ustochnept more susceptible to splashing. The FD of Eum-Orthic Anthrosol, Ustalf and Cumulic Haplustoll were lower than those of Quartisamment and Ustochnept. The results showed that during rainfall, both raindrop diameter and soil properties affect surface aggregate stability and pore connectivity, which creates the material basis for forming surface crust, clogging pores and reducing the infiltration rate.

Aggregate breakdown caused by the impact of raindrops clogs soil pores, reduces soil infiltration and aggravates the formation of soil crusts. To determine the influence of raindrop splash on the microstructure of soil aggregates, the typical loess was studied. We used synchrotron-based X-ray microcomputed tomography (SR-μCT) to analyze the microcharacteristics of soils impacted by rainfall intensities. The results showed that raindrop splash increases the number of surface soil aggregates, especially when the rainfall intensity is 68.61 or 217.26 mm h-1. Compared to the undisturbed soil, the number of soil aggregates increased by 38.71%, 46.77% and 76.77%, and the volume increased by 1.09%, 3.21% and 3.73%, after the impact of rainfall intensities of 5.76, 68.61 and 217.26 mm•h-1, respectively. Raindrop impact on the surface affects the distribution of aggregate particles, causing a decrease in the number of aggregate particles in the 500-1000 μm range and an increase in the < 500 μm range. Compared with unsplashed soil, there is a significant increase in the fractal dimension (FD) and total specific surface area (SSA) of surface soil aggregates in splashed soil. Particularly, the rainfall intensity of 217.26 mm h-1 resulted in an increase of the FD and SSA by 30.24% and 17.49%, respectively. Under the rainfall intensities, the average particle diameter of the soil aggregates decreased by 2.43%, 3.25% and 3.55%, respectively, compared with that of the undisturbed soil. These results indicated that raindrop splash decreased the number of macroaggregates and increased the number of microaggregates in the surface layer of soil.