1 INTRODUCTION
Soil aggregates are the basic units of structure in soil. The breakdown
and dispersion of soil aggregates caused by raindrop splashing, initiate
soil erosion (Bronick and Lal, 2005; Legout et al., 2005; Xiao et al.,
2017). Soil aggregate stability plays a key role in affecting surface
runoff and soil erosion; furthermore, it has been widely used to
evaluate soil structural stability and soil quality (Six et al., 2000;
Ghadir et al., 2007; Anderson et al., 2019). Numerous studies have been
conducted on aggregates in soil erosion (Fu et al., 2017, 2019), and
these studies mostly analyzed the particle size distribution of split
aggregates after rainfall. During a rainfall event, the fragmentation
and separation of aggregates was mainly caused by large-diameter
raindrops; as the raindrop diameter increased, the degree of
fragmentation in the aggregates became stronger (Fu et al., 2017; Li et
al., 2018). Additionally, Fu et al. (2019) used a simulated rainfall
test to analyze the extent of soil splash erosion and the distribution
of particle sizes and determined the effects of different rainfall
energy levels on the breakdown of soil aggregates. Furthermore, Fu et
al. (2020) assessed the effects of secondary raindrop splash erosion on
aggregate fragmentation and fragment size distribution and found that
secondary raindrop splash erosion would cause soil aggregates to break
again, leading to a reduction in soil fertility and productivity.
Obviously, previous studies have ignored the impacts of raindrop splash
on the microstructure of aggregates.
In recent years, the development of X-ray computer scanning (CT)
technology has allowed for its application in the study of soil
structure and soil aggregates (Peth et al., 2008; Munkholm et al., 2012;
Garbout et al., 2013; Gao et al., 2019). Through synchrotron-based X-ray
microcomputed tomography (SR-μCT) scanning, high-resolution images of
soil can be obtained, and three-dimensional qualitative and quantitative
analysis of soil aggregates can be performed, thereby analyzing the
internal microstructure of aggregates in a nondestructive and
comprehensive manner (Taina et al., 2008; Ma et al., 2015; Han et al.,
2019). Some research has utilized CT scanning technology to quantify the
effects of different land use types, fertilization conditions, and types
of vegetation restoration and succession on soil pore structure and
aggregates (Luo et al., 2010; Zhou et al., 2012; Dal Ferro et al., 2013;
Zhao et al., 2017). Hu et al. (2016) and Li et al. (2019) applied CT
scanning technology to analyze soil macropores and root structure from
alpine vegetation and different ecosystems in the Qinghai Lake Basin of
China in an effort to explore the impact of roots on soil macropore
network characteristics. In addition, work from Ma et al. (2015) based
on SR-μCT scanning technology investigated the influences of pore
characteristics on water stability and the tensile strength of
aggregates under wetting and drying cycles. Additional research has used
high-resolution CT to characterize the development of crust over time,
and the results showed that the cumulative porosity varied according to
soil depth (Lee et al., 2008). With the help of simulation rainfall
experiments and SR-μCT scanning, Li et al. (2018) analyzed the effects
of raindrop splashing on the fragmentation mechanism of aggregate
microstructure from a two-dimensional perspective.
As we all know, the Loess Plateau is a region among those with the most
serious soil erosion in the world, and one of its main causes of erosion
is water erosion. The fragmentation and dispersion of topsoil aggregates
caused by raindrops is the first step of soil erosion, but it is unclear
how rainfall causes microstructural changes in soil aggregates. Although
SR-μCT scanning has been widely used in soil research, this does not
hold true in the field of soil erosion; to date, CT scanning has rarely
been employed to analyze the impact of rainfall on the microstructure of
aggregates. However, the microstructure of soil aggregates determines
the soil stability and quality characteristics (Zhou et al., 2012).
Therefore, utilizing SR-μCT and image analysis, the aims of this
research were (1) to compare the differences in the three-dimensional
microstructure of soil aggregates under raindrops and (2) to analyze the
relationship between the rainfall intensity and microscopic
characteristics of the soil aggregates. The results presented here may
be beneficial by revealing the mechanisms of aggregate fragmentation,
pore clogging and the formation of a surface crust during soil erosion.