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\begin{document}

\title{Impact of raindrop sizes and intensities on the microcharacteristics of
soil aggregates}



\author[1]{Mingxi Yang}%
\author[2]{Gangan Ma}%
\author[3]{guangluli li}%
\author[4]{Yangyang Ren}%
\author[4]{Zefang Li}%
\affil[1]{Northwest Agriculture and Forestry University}%
\affil[2]{Northwest Agriculture and Forestry University College of Resources and Environment}%
\affil[3]{northwest A \& F University}%
\affil[4]{Northwest A\&F University}%


\vspace{-1em}



  
  \date{May 27, 2024}


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\selectlanguage{english}
\begin{abstract}
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-\selectlanguage{greek}μ\selectlanguage{english}CT) 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 \selectlanguage{greek}μ\selectlanguage{english}m range
and an increase in the \textless{} 500 \selectlanguage{greek}μ\selectlanguage{english}m 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.%
\end{abstract}%



\sloppy


\textbf{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-\selectlanguage{greek}μ\selectlanguage{english}CT) 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-\selectlanguage{greek}μ\selectlanguage{english}CT 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-\selectlanguage{greek}μ\selectlanguage{english}CT 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-\selectlanguage{greek}μ\selectlanguage{english}CT 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-\selectlanguage{greek}μ\selectlanguage{english}CT 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.

\textbf{2 MARERIALS AND METHODS}

\textbf{2.1 Soil samples}

Soil samples were collected from Meixian County (107\selectlanguage{ngerman}°45'36''E,
34°17'24''N), Shaanxi Province, which belongs to a warm and subhumid
continental climate. The average annual precipitation is 610 mm,
occurring mainly from July to September, with an average annual
temperature of 12.9\selectlanguage{english}. The study area was a traditional agricultural
cropland region located in the southern section of the Loess Plateau in
China. The soil was formed from loess parent material (according to the
USDA particle size classification criteria) (Fu et al., 2017). The
diagonal method was used to collect 30 undisturbed soil samples from the
surface (0-5 cm depth) with a ring knife (10 cm diameter x 5 cm height).
Bulk density (1.29 +- 0.07 g cm\textsuperscript{-3}) was determined with
oven drying at 105degC until a constant mass was obtained. The soil
organic matter content (1.33\% +- 0.04) was measured by the combustion
method. The pipette method was used to obtain the particle size
distribution (sand (0.02-2 mm) 45.96\% +- 0.12\%, silt (0.002-0.02 mm)
41.06\% +- 0.03\%, clay (\textless{}0.002 mm) 12.98\% +- 0.05\%).

\textbf{2.2 Rainfall test}

The simulated rainfall device was composed of a raindrop generator and a
splashing raindrop collection device (Figure 1) (Fu et al., 2017). The
raindrop generator was a cylinder (10 cm in diameter and 10 cm in
height) with a top opening, 21 syringe needles were installed at the
bottom of the cylinder, and raindrops of different diameters were
generated by changing the needle size. The collection device for
splashed raindrops was a stainless-steel pan (110 cm in diameter). To
prevent the raindrops from being disturbed by the horizontal airflow, a
shield was placed around the experimental device. The duration for
testing each raindrop diameter was 10 minutes, and each diameter test
was repeated 3 times.

Three rainfall intensity levels (5.76, 68.61, and 217.26 mm
h\textsuperscript{-1}) determined by three raindrop diameters were
selected in the experiment. The corresponding raindrop diameters were
2.67 mm, 3.39 mm and 4.05 mm, respectively. The rainfall intensity was
measured by the direct method. For all needles of each size, continuous
rainfall was performed for 10 minutes, and the precipitation volume was
determined. The rainfall intensity was calculated according to the
rainfall area and rainfall time. Each test was conducted 10 times, the
maximum and minimum values were removed, and the average of the
remaining 8 replicates was calculated to be the rainfall intensity. When
the raindrop diameter was greater than or equal to 1.9 mm, the modified
Newton formula (equation {[}1{]}) was used to calculate the final
velocity of the raindrops. The raindrop velocity under this test
condition was calculated using equation {[}2{]}. The raindrop energy was
calculated by equation {[}3{]}.

\emph{d} \textgreater{}1.9 \selectlanguage{english}(1\selectlanguage{english})

\selectlanguage{english}(2\selectlanguage{english})

\selectlanguage{english}(3\selectlanguage{english})

where V\textsubscript{i} is the terminal velocity (m
s\textsuperscript{-1}), d is the raindrop diameter (mm), V is the
raindrop velocity (m s\textsuperscript{-1}), g is the acceleration of
gravity (m s\textsuperscript{-2}), H is the height from which the
raindrop falls (m), E\textsubscript{rs} is the raindrop energy (J
m\textsuperscript{-2} s\textsuperscript{-1}), n is the number of
raindrops, i=0,\ldots{}, and m is the individual raindrop mass (g). The
main parameters of the simulated raindrops are shown in Table 1.

After the rainfall test, the soil in the cutting ring was air dried
naturally. Dry clods (2 cm cubes) from the surface of the impacted soil
(a depth of approximately 0.5 cm) were obtained by using a knife before
and after each rainfall event. The soil clods were placed in a container
with a sponge to keep the structure intact. A total of 54 dry clods were
selected for CT scanning.

\selectlanguage{greek}\textbf{2.3 SR-μCT~scanning and image processing}\selectlanguage{english}

The soil clods (5 mm in diameter) were scanned using synchrotron-based
X-ray microcomputed tomography (SR-\selectlanguage{greek}μ\selectlanguage{english}CT) at beamline BL13W1 in Shanghai,
China. The scan was performed at an energy level of 24 kV, with an
exposure time of 1.8 s, a detection distance of 11 cm, and a resolution
of 3.25 \selectlanguage{greek}μ\selectlanguage{english}m. The sample stage rotates at a constant speed from 0 to 360\selectlanguage{ngerman}°,
and the scan is continuously performed at a scan interval of 0.625 mm;
720 original images (tomo images) were obtained for each soil sample.
Then, PITRE (Phase-sensitive X-ray Image Processing and Tomography
Reconstruction) software was used for phase retrieval and slice
reconstruction based on the back-projection algorithm (Chen et al.,
2012). A total of 1500-2000 slice images (2048×2048 pixels) were
obtained for each soil clod and stored as 32-bit grayscale images in
tiff format.

The open-source software ImagePy was used to complete image processing,
visualization and quantification of the three-dimensional aggregate
structure (Wang et al., 2018). To avoid the influences of soil depth and
image boundary, a total of 512 images with serial numbers 512-1024 were
selected, and a nonedge section of 512×512×512 pixels (i.e., 1.664
mm×1.664 mm×1.664 mm) in size was selected as the target area for image
analysis. The process of dividing the grayscale image into the soil
matrix and pores by thresholding, which is called binary segmentation,
was key to the quantitative analysis of soil structure. This study used
the automatic Otsu algorithm of the global threshold method (Zhou et
al., 2012; Garbout et al., 2013) to perform binary segmentation which
was subsequently completed by visual observation and manual adjustment.
After image binarization, the three-dimensional visualization function
of the ImagePy software was used to visualize the three-dimensional
structure of soil aggregates (Wang et al., 2018) (Figure 2).

\textbf{2.4 Calculation and analysis}

ImagePy software (Wang et al., 2018) was used to analyze the segmented
binary image and obtain the quantity, diameter, surface area and volume
of soil aggregate particles; subsequently, the 3D structural
characteristics of the aggregate after splashing with different raindrop
sizes were obtained through calculation and analysis. Aggregate
fragments were divided into six classes according to their size:
500-1000, 250-500, 106-250, 53-106, 25-53 and \textless{}25 \selectlanguage{greek}μ\selectlanguage{english}m.

The quantity and volume parameters of soil aggregates were used to
measure the quantitative characteristics of aggregates. The
three-dimensional mass fractal dimension (FD) and specific surface area
(SSA) of each aggregate were used to characterize the extent of
aggregate fragmentation. The FD and SSA are parameters that can
appropriately reflect the geometry of soil structure. They represented
the size of aggregates, soil erosion resistance and soil permeability
(Pirmoradian et al., 2005; Wang et al., 2014). The FD was previously
used to describe the self-similarity and scale independence of objects
(Dal Ferro et al., 2013). The calculation of FD was based on the
principle of the box counting method: the image stack was covered with
cube boxes of different side lengths, and the number of boxes was
recorded (Kravchenko et al., 2011; Perret et al., 2003) to satisfy the
following relationship (equation {[}4{]}):

(4)

where N(\selectlanguage{greek}ε\selectlanguage{english}) is the number of boxes, \selectlanguage{greek}ε \selectlanguage{english}is the side length of the box, FD
is the value of fractal dimension.

FD was calculated by linear regression of log\{N(\selectlanguage{greek}ε\selectlanguage{english})\} vs log(1/\selectlanguage{greek}ε\selectlanguage{english}), and
the range was 2-3. Fractal dimension was computed using the open-source
software ImagePy (Wang et al., 2018). Specific surface area is
volume-specific surface area, expressed as the ratio of surface area to
volume of each aggregate fragment.

The data was analyzed using one-way analysis of variance
(ANOVA).~Duncan's multiple range method was used to compare mean values,
and significant differences between mean values were defined as
\emph{P}\textless{} 0.05.~The results were expressed as the mean
standard deviation, and all tables and figures were processed in
Microsoft Office Excel 2010.

\textbf{3 RESULTS}

\textbf{3.1 Soil aggregatequantitative~characteristics}

Raindrop splashing increased significantly with rainfall intensity
(\emph{P} \textless{} 0.05, Table 2) and resulted in an increase in the
number of soil aggregate fragments. Compared to the undisturbed soil,
the total number of soil aggregate particles increased by 38.71\%,
46.77\% and 76.77\% after the impacts of rainfall intensities of 5.76,
68.61 and 217.26 mm h\textsuperscript{-1}, respectively (Table 2).
Overall, the number of soil aggregate particles in the undisturbed and
disturbed soil showed an ''increasing-decreasing-increasing'' trend as
the aggregate particle size decreased (Fig. 3A). Following raindrop
splashing, the number of aggregate particles with different sizes
(except for larger aggregates of 500-1000 \selectlanguage{greek}μ\selectlanguage{english}m) was higher than that in
the undisturbed soil. After raindrop splashing, the quantity of soil
aggregate fragments between 250-500 \selectlanguage{greek}μ\selectlanguage{english}m increased by 34.88\%, 27.91\% and
44.19\%, respectively, compared to the undisturbed soil
(\emph{P}\textless{} 0.05, Figure 3A). After splashing with a rainfall
intensity of 217.26 mm h\textsuperscript{-1}, the largest quantity of
aggregate particles existed in the 106-250 \selectlanguage{greek}μ\selectlanguage{english}m range, accounting for
53.36\% of the total amount; this value was significantly increased by
98.65\% compared to that of the undisturbed soil (\emph{P} \textless{}
0.05, Figure 3A).

With the increase in rainfall intensity, the total volume~of aggregate
fragments raised gradually (Table 2). Compared to the undisturbed soil,
the difference was not significant after the impact with a rainfall
intensity of 5.76 mm h\textsuperscript{-1} (\emph{P} \textgreater{}
0.05, Table 2). Following impact with rainfall intensities of 68.61 and
217.26 mm h\textsuperscript{-1}, the total volume of aggregate fragments
increased 3.21\% and 3.73\%, respectively, (\emph{P} \textless{}0.05,
Table 2). The percentage of aggregate volume (\%) was the ratio of the
aggregate volume of a particles size aggregate to the total aggregate
volume in a soil clod, which reflected the distribution of aggregate
fragments in the soil. The percentage of aggregate volume under
different treatments showed a trend of increasing first and then
decreasing with the decrease of particle size, and reached the maximum
at 500-250 \selectlanguage{greek}μ\selectlanguage{english}m. Compared to the disturbed soil, the aggregate in 500-250
\selectlanguage{greek}μ\selectlanguage{english}m of the disturbed soil impacted by 5.76, 68.61 and 217.26
mm\selectlanguage{ngerman}·h\textsuperscript{-1} rainfall intensity were significantly
(\emph{P}\textless{} 0.05) increased by 39.49\%, 25.41\% and 24.76\%,
respectively (Fig. 3B). The percentage of aggregate fragments that were
\textgreater{}500 \selectlanguage{greek}μ\selectlanguage{english}m in disturbed soil was lower than that of
undisturbed soil (\emph{P} \textless{} 0.05), while it was higher for
fragments \textless{} 500 \selectlanguage{greek}μ\selectlanguage{english}m than that of undisturbed (Figure 3B). In
addition, for fragments \textgreater{}106 \selectlanguage{greek}μ\selectlanguage{english}m, the volume of soil
aggregate particles accounts for more than 99.00\% of the total
aggregate volume (Figure 3B).

\textbf{3.2 Soil aggregate fragments characteristic parameters}

As shown in Fig. 4A, the fractal dimension of the soil aggregates
increased significantly in comparison with the undisturbed soil
following different rainfall conditions (P \textless{} 0.05). After
being splashed by rainfall with an intensity of 5.76 or 68.61 mm
h\textsuperscript{-1}, the fractal dimension of the soil aggregates had
no significant change (P \textgreater{} 0.05). After being splashed by
rainfall at an intensity of 217.26 mm h\textsuperscript{-1}, the fractal
dimension was significantly higher than that of the other treatments (P
\textless{} 0.05). After being splashed by three rainfall intensities,
the fractal dimension of the aggregates increased by 2.26\%, 2.26\% and
3.67\% respectively, compared to that of the undisturbed soil.

After raindrop splash, the total specific surface area of the soil
aggregates increased significantly (P \textless{} 0.05, Fig. 4B). The
total specific surface area of the splashed soil was significantly
different from that of the unsplashed soil (P \textless{} 0.05), but
there was no difference between the rainfall intensities of 5.76 and
68.61 mm h\textsuperscript{-1} (P \textgreater{} 0.05, Fig. 4B). The
total specific surface areas of the soil aggregates splashed by rainfall
intensities of 5.76, 68.61 and 217.26 mm h\textsuperscript{-1} increased
by 18.82\%, 17.79\% and 30.24\% respectively (Figure 4B).

The specific surface area of the soil aggregate increased with
decreasing aggregate particle size, especially for the \textless{} 25 \selectlanguage{greek}μ\selectlanguage{english}m
aggregates (Figure 4C). For the 500-1000 \selectlanguage{greek}μ\selectlanguage{english}m aggregates, the specific
surface area of soil splashed with raindrops was significantly higher
than that of the unsplashed soil (P \textless{} 0.05), but there was no
significant difference between the different rainfall intensities (P
\textgreater{} 0.05, Fig. 4C). After raindrop splash, the specific
surface areas of the 250-500 \selectlanguage{greek}μ\selectlanguage{english}m aggregates were higher than those of the
undisturbed soil, whereas the specific surface areas of the 106-250 \selectlanguage{greek}μ\selectlanguage{english}m
and 53-106 \selectlanguage{greek}μ\selectlanguage{english}m aggregates fluctuated. After being splashed by rainfall at
an intensity of 68.61 and 217.26 mm h\textsuperscript{-1}, the specific
surface area of the 25-53 \selectlanguage{greek}μ\selectlanguage{english}m aggregates was significantly different from
that of the undisturbed soil, increasing by 31.99\% and 102.50\% (P
\textless{} 0.05), respectively, and the SSA was significantly higher
than that of other rainfall intensities (P \textless{} 0.05). The
specific surface areas of aggregates in the undisturbed soil and
splashed soil had a maximum value in the \textless{}25 \selectlanguage{greek}μ\selectlanguage{english}m range; the
specific surface areas of soil splashed by small, medium and large
rainfall intensities increased by 9.15\%, 10.92\% and 17.49\%,
respectively, compared with undisturbed soil.

\textbf{3.3 Microstructures of soil aggregates}

A significant difference was observed in the two-dimensional shape
between the splashed soil aggregates and the undisturbed soil (Figure
2D). The undisturbed soil aggregates primarily consisted of
macroaggregates with clear outlines and boundaries, containing more
macropores and a relatively loose soil structure (Figure 2A). Compared
with the undisturbed soil, the soil structure after raindrop splashing
was more compact, the number of macropores was reduced, the number of
small and medium aggregates was increased, and the phenomenon of
fragmentation was shown. Markedly, after being splashed by rainfall at
the intensity of 217.26 mm h\textsuperscript{-1}, the soil aggregates
were the most broken (Figure 2B-D). The 3D structural differences of
soil aggregates were more obvious than those in 2D (Figure 2 and 3D).
After splashing, the percentage of soil macroaggregates was reduced,
being replaced by an increased number of microaggregates; this result
indicates that the aggregates were obviously broken, and the soil
structure was relatively denser. Table 3 reports that the average
aggregate particle size in splashed soil was significantly smaller than
that of the undisturbed soil (P \textless{} 0.05), especially after a
rainfall intensity of 68.61 and 217.26 mm h\textsuperscript{-1}. The
average aggregate particle size decreased by 2.43\%, 3.25\% and 3.55\%
after splashed by three rainfall intensities, respectively. This result
is consistent with the micromorphology of aggregates.

\textbf{4 DISCUSSION}

\textbf{4.1 Effect of rainfall intensity on splashed soil aggregates
quantitative characteristics}

The number of aggregate fragments increased corresponding to the
increase in rainfall intensity, especially at 68.61 and 217.26 mm
h\textsuperscript{-1} (Table 1). In the process of water erosion, the
primary mechanisms of aggregate breakdown are fast wetting and
mechanical breakdown caused by raindrop impact (Legout et al., 2005; Shi
et al., 2010). The results here show that, with an increase of rainfall
intensity, the quantity of aggregates and the degree of fragmentation
gradually increase.

In regards to aggregate distribution, aggregates mainly consisted of
250-500 \selectlanguage{greek}μ\selectlanguage{english}m, 106-250 \selectlanguage{greek}μ\selectlanguage{english}m, and \textless{}25 \selectlanguage{greek}μ\selectlanguage{english}m particle sizes, whereas for
volume, the 106-1000 \selectlanguage{greek}μ\selectlanguage{english}m range made up the majority (Fig. 3A and 3B).
Raindrop impact on the surface could affect soil erosion and alter
aggregate structure in various ways (Kinnell et al., 2005). When the
erosion process was dominated by rainfall detachment, the particle size
distribution of eroded soil was different from that of the original soil
(Slattery and Burt, 1997). It was affected by the particle distribution
of the original soil and the destruction of aggregates during erosion
(Mahmoodabadi et al., 2014). Based on the number and volume of aggregate
fragments, we found that raindrop splashing dispersed larger aggregate
particles (500-1000 \selectlanguage{greek}μ\selectlanguage{english}m) and broke them into aggregates of smaller
particles (\textless{}500 \selectlanguage{greek}μ\selectlanguage{english}m). Raindrop splashing changed the
arrangement of soil particles, decreased the number of larger aggregate
particles, and increased the quantity of smaller aggregate particles. As
a result, the particles formed by dispersion and fragmentation were
deposited in the pores of the upper soil, clogging the pores of the
topsoil and forming a thin, dense surface crust with low permeability
(Assouline, 2004). In turn, the crust further reduced soil infiltration
and exacerbated soil erosion (Sajjadi and Mahmoodabadi, 2015).

\textbf{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\textsuperscript{-1} were
higher than those of the aggregates splashed by a rainfall intensity of
217.26 mm h\textsuperscript{-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
\selectlanguage{greek}μ\selectlanguage{english}m and \textless{}25\selectlanguage{greek}μ\selectlanguage{english}m aggregate particles, after being splashed by
68.61 and 217.26 mm h\textsuperscript{-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).

\textbf{4.3 Effect of raindrop diameters on microscopic characteristics
splashed soil aggregates}

The microstructure of soil aggregates was different in two and three
dimensions after raindrop splash (Figure 2). The splashed soil,
especially by large raindrops, displayed an obvious fragmentation of
aggregates (Figure 2D). This result indicated that the main cause of
aggregate fragmentation was large raindrops. It is easier to form a
structure consisting of large aggregates after drying. The formation of
surface crust depended on the degree of surface aggregate fragmentation
and the stability of the soil structure (Wick et al., 2016; Gelaw et
al., 2015). The splashed soil aggregate particle size was smaller than
that of the undisturbed soil (Table 2). This result is consistent with
that of Ramos et al. (2003), in which the fragmentation of aggregates
produced smaller particles than the original soil, allowing the soil to
be displaced and reoriented into a more continuous structure; this
consequently clogged pores and formed a surface crust. Sajjadi and
Mahmoodabadi (2015) found that soil containing finer particle aggregates
had a higher transportability of preseparated particles than larger
aggregates. Moreover, as particle size decreased and the texture became
finer, the pore structure of the soil became denser. The impact of
raindrops, which caused physicochemical compaction and dispersion of the
surface soil, was the main reason for crust formation and reduction of
the infiltration rate (Assouline et al., 2004; Fu et al., 2017). In this
study, larger raindrop diameters resulted in greater corresponding
rainfall intensity and energy. As a result, soil tends to form a surface
crust after heavy rains. The formation of a soil surface crust not only
further reduces porosity (Pagliai et al.,2004) but also reduces surface
roughness, thereby exacerbating surface runoff and soil loss (Robinson
and Phillips, 2001; Assouline and Ben-Hur, 2006).

\textbf{5 CONCLUSIONS}

This study found that the number of aggregates increased as the rainfall
intensity increased, especially for the rainfall intensities of 68.61
and 217.26 mm h\textsuperscript{-1} (\emph{P \textless{}} 0.05). The
distribution of aggregate particles was affected by the original soil
distribution and rainfall intensity, which decreased the number of
500-1000 \selectlanguage{greek}μ\selectlanguage{english}m aggregate particles and increased the number of \textless{}
500\selectlanguage{greek}μ\selectlanguage{english}m aggregate particles. The fractal dimension (FD) and specific
surface area (SSA) increased significantly after raindrop splash
(\emph{P \textless{}} 0.05); as rainfall intensity increased, the FD and
SSA values also increased. A greater degree of fragmentation led to a
higher proportion of fine particles in the soil and more unstable soil
aggregates. After raindrop splashing, especially from an intensity of
68.61 and 217.26 mm h\textsuperscript{-1}, the microstructure of
aggregates was denser than the undisturbed soil, and the average
particle diameter of the aggregate decreased by 2.43\%, 3.25\% and
3.55\%, respectively, compared with the undisturbed soil.

Aggregate breakdown was mainly caused by moderate to great rainfall
intensities. The degree of fragmentation increased with the increase of
raindrop diameter, rainfall intensity and rainfall energy, and more
aggregate particles converted to finer particles. Consequently, these
tiny particles intensified the pore clogging and infiltration weakening,
exacerbated surface runoff and soil loss and reduced soil fertility;
this process even accelerated the formation of surface crust and
destroyed the soil structure. Therefore, to improve our understanding of
the soil erosion process, synchrotron-based X-ray microcomputed
tomography should be more widely used in the field of soil erosion, in
combination with other methods, to study microstructure and soil
aggregate fragmentation mechanisms.

CONFLICT OF INTEREST

There is no conflict of interest to declare.

ORCID

Li Guanglu\emph{https://orcid.org/0000-0003-4107-8587}

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