4.2 Effect of rainfall intensity on splashed soil aggregate
fragment parameters
The impact of raindrops increased the FD of the soil aggregates (Figure
4A). A greater FD value corresponded to a greater degree of aggregate
breakdown and detachment. The FD ratios of the aggregates splashed by
rainfall intensities of 5.76 and 68.61 mm h-1 were
higher than those of the aggregates splashed by a rainfall intensity of
217.26 mm h-1 because the higher rainfall intensity
had higher energy, which strengthened the degree of fragmentation and
caused more macroaggregates to decompose into microaggregates. The
higher FD value indicates that the particle size distribution of
aggregates is dominated by smaller particles, a lower proportion of
stable aggregates is present, and the soil structure is inferior (Tyle
et al., 1992, Huang et al., 2017). The results presented here show that
the raindrop splash was able to break apart the aggregate and destroy
the surface soil structure. This is consistent with the result of Fu et
al. (2020).
Soil aggregate index, such as specific surface area (SSA), can also
reflect the stability of soil aggregates (Pirmoradian et al., 2005).
Raindrop splash led to an increase in the total specific surface area of
soil aggregates, which increased as the intensity of rainfall increased
(Figure 4B). As the particle size decreased, the corresponding specific
surface area increased (Figure 4C). This result has proven that a higher
specific surface area, leads to a finer soil texture and a stronger soil
dispersibility (fragmentation) (Wang et al., 2014). The specific surface
area of the aggregates increased significantly, especially in the 25-53
μm and <25μm aggregate particles, after being splashed by
68.61 and 217.26 mm h-1 (Figure 4B and 4C). This
likely occurred because large raindrops had a stronger rainfall
intensity and rainfall energy, thus, increasing the rainfall intensity
strengthened the aggregate decomposition (Sajjadi and Mahmoodabadi,
2015). Therefore, the splashed soil aggregates contained more fine
particles than before the splash (Figure 4C). Meanwhile, as the particle
size of aggregates decreased, they exhibited a smaller mass and smoother
shape; furthermore, migration and denudation were more likely to occur
during runoff transportation and the underlying pores were more likely
to be clogged (Assouline et al., 2004), causing compaction of the
aggregate microstructures (Adesodun et al., 2007; Li et al., 2018). In
addition, some studies have shown that a greater amount of organic
matter content was present in surface soil macroaggregates (Jastrow,
1996) and that aggregates have physical protection for organic matter
(Field et al., 2006), while the total specific surface area was
negatively correlated with organic matter content. Compared with soil
containing higher organic matter content, soil with lower organic matter
content showed higher sealing and anti-crust properties (Ramos et al.,
2003). Therefore, the aggregate breakdown caused by raindrop splash
decreased the soil structure and fertility, reduced the land
productivity, and even aggravated the formation of crust (Hu et al.,
2018; Fu et al., 2020).