Aggregate breakdown and transportation caused by raindrops clogs soil pores and aggravates the formation of surface crusts. However, changes in soil pore microcharacteristics after raindrop impact are equivocal. In this research, we used synchrotron-based X-ray microcomputed tomography (SR-μCT) and image processing techniques to quantitatively analyze the pore microcharacteristics of undisturbed soil and disturbed soil impacted by raindrop diameters of 2.67 mm (small), 3.39 mm (medium), and 4.05 mm (large). The results showed that the changes in the soil pore microcharacteristics were mainly caused by medium and large raindrops. Compared with the undisturbed soil, the total imaged porosity decreased by 5.91% after splashing by small raindrops and significantly (P < .05) decreased by 15.83 and 18.28% after medium and large raindrop splashing, respectively. The total volume of pores and the porosity of the large pores had the same decreasing trend as the total porosity. The porosity of the elongated pores decreased by 11.51 and 23.41% after medium and large raindrop splashing, respectively. The raindrop action clogged the soil pores. The clogging ratios caused by small, medium, and large raindrops were 1.06, 2.83, and 3.29%, respectively, and increased with raindrop diameter, rain intensity, and rainfall energy. Thus the increase in pore clogging and the decrease in macroporosity are the main reasons for the formation of surface crusts.

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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.

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