Fragmentation of soil aggregates induced by secondary raindrop splash erosion

The particles of soil splashed by raindrops are dispersed and broken again or more, which can be called secondary raindrop splash erosion. To determine the effect of secondary raindrop splash erosion on the fragment size distribution and fragmentation of soil aggregate, Lou soil, a typical cropland soil of the Loess Plateau, was selected to compare the characteristics of the fragment size distribution of soil aggregate between the secondary raindrop splash test (SR) and the without secondary raindrop splash test (WSR) under five different raindrop diameters (2.67-4.05 mm) and at five splash distances (0-50 cm). The results showed that on the whole, the mass percentages of the fragment size > 0.053 mm of the WSR test were 1.13-1.39 times that of the SR test, but the mass percentage of the fragment size < 0.053 mm was 0.83 times that of the SR test, which indicated that the secondary raindrop splash erosion can break the aggregate into finer particles. Under the condition of the five raindrop diameters or the five splash distances, when the fragment size was > 0.053 mm, the mass percentages of splashed aggregates showed a fluctuating variable trend with decreasing fragment size, and the mass percentages of fragment size < 0.053 mm were 4.08%-43.20% or 11.79%-17.18% significantly lower in the WSR test than in the SR test, respectively (P < 0.05). These results indicated that alcohol can protect fragment size > 0.053 mm at each raindrop diameter or splash distance. The fractal dimension (D) of the aggregates in the WSR and SR tests showed a downward opening parabolic relationship with raindrop diameters, and the values of the fractal dimension (D) of the SR test were higher than those of the WSR test for the same raindrop diameter. When the raindrop diameters were 3.64 and 3.74 mm, the values of D in WSR and SR tests were at their maximum, which meant the lowest protective effect of alcohol on the aggregates and the largest degree of fragmentation, respectively. The above research shows that the secondary raindrop splash erosion will cause the soil aggregates to break again, which will lead to the decline of soil fertility and reduce the productivity of the soil.