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Fragmentation of soil aggregates induced by secondary raindrop splash erosion
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Fragmentation of soil aggregates induced by secondary raindrop splash erosion ⁎
Yu Fua, Guanglu Lia,b, , Tenghui Zhenga, Yingsong Zhaob, Mingxi Yangb a b
Institute of Soil and Water Conservation, Northwest A&F University, Yangling 712100, Shaanxi, PR China College of Resources and Environment, Northwest A&F University, Yangling 712100, Shaanxi, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Soil aggregate Raindrop diameter Secondary raindrop splash Fractal dimension Loess Plateau
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.
1. Introduction Soil aggregates, the material basis of soil fertility, are the basic unit of soil structure. Raindrops splashing on topsoil can disintegrate and disperse soil aggregates, providing abundant loose particles for subsequent runoff transport (Fernández-Raga et al., 2017; Li et al., 2017a). Simultaneously, soil particle transport causes the reduction or blockage of soil surface pores and results in the compaction of soil and a decrease in soil permeability, providing favorable conditions for the subsequent formation of runoff (Hénin et al., 1958; Fu, 2017; Li et al., 2017b). Therefore, the fragmentation and transportation of soil aggregates caused by raindrop splash erosion is the root of reduced soil productivity and increased soil erosion.
Kubota and Mochizuki (2011) captured images of splash formation (single raindrop) using a high-speed CMOS camera. And found that the origin of the film flow as the primary splash is confirmed to be the water displaced by the head. The line connecting the head to the tail of the body affects the separation of the film flow from the body, and this is found to be the cause of the secondary splash. Thus, when a single raindrop impacts a thin layer of water film, it will partially disperse into smaller raindrops. However, natural rainfall is the continuous fall of multiple raindrops, which not only results in the dispersion and fragmentation of soil surface aggregates but also results in the dispersion and splashing of the raindrops themselves. Hence, we assume that dispersed raindrops will cause secondary raindrop splash erosion, causing soil aggregate fragments to break into smaller particles. To test
⁎ Corresponding author at: Institute of Soil and Water Conservation, Northwest A&F University, Yangling 712100, Shaanxi, PR China; College of Resources and Environment, Northwest A&F University, Yangling 712100, Shaanxi, PR China. E-mail address: [email protected] (G. Li).
https://doi.org/10.1016/j.catena.2019.104342 Received 14 January 2019; Received in revised form 11 August 2019; Accepted 28 October 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yu Fu, et al., Catena, https://doi.org/10.1016/j.catena.2019.104342
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Table 1 Basic physical and chemical properties of soil (mean value ± standard deviation). Soil type
Lou soil
Bulk density (g·cm3)
Soil organic carbon (%)
Total nitrogen (g·kg−1)
Total phosphorus (g·kg−1)
1.37 ± 0.13 1.46 ± 0.03 1.04 ± 0.02 The fragment size distribution of the undisturbed soil/% ＞2 mm 2–1 mm 1–0.5 mm 19.58 ± 7.66 14.04 ± 2.91 13.30 ± 3.48
Particle size composition/% Sand (2–0.02 mm)
Silt (0.02–0.002 mm)
Clay (< 0.002 mm)
0.62 ± 0.02
33.26 ± 0.05
44.07 ± 0.03
22.67 ± 0.02
0.5–0.25 mm 10.87 ± 3.07
0.25–0.106 mm 7.03 ± 2.48
0.106–0.053 mm 4.40 ± 1.64
< 0.053 mm 30.79 ± 5.69
Using a method for measuring secondary raindrop splash erosion, the objectives of this paper were (i) to clarify the characteristics of fragment size distribution under the conditions of the secondary raindrop splash test (SR) and the without secondary raindrop splash test (WSR) and (ii) to explore which rainfall conditions cause the greatest degree of aggregate fragmentation.
this hypothesis, the author performed a simulation rainfall experiment with a single raindrop (Fu et al., 2017a). The variation in the enrichment ratio with the fragment size was consistent under different raindrop diameters (2.67–3.79 mm), and the enrichment ratio of fragment size < 0.053 mm was less than 1. However, we found that the enrichment ratio of fragment size < 0.053 mm was greater than 1 for different raindrop diameters (2.67–3.79 mm) in a simulation rainfall experiment with multiple raindrops (Fu et al., 2017b). Sutherland et al. (1996) and Wan and Elswaify (1998) demonstrated that the enrichment ratio of fragment size < 0.063 mm was greater than 1. The above results demonstrate that the simulation rainfall experiment with multiple raindrops is more vulnerable to the breaking of aggregates into smaller particles than is the single raindrop experiment. There are two main reasons for this. First, the single raindrop impacts the topsoil, and then the soil aggregates are scattered and splashed, but few of the dispersed raindrops impact and destroy the aggregate fragments again. Second, the energy applied during single raindrop simulations (4.99 × 10−5–1.78 × 10–4 J m–2s−1) is small, so most of the aggregates with particle size > 1 mm break up into small size aggregates (0.053–1 mm), while those with less than 0.053 mm splash aggregates have less content (Fu et al., 2017a). The above studies showed that the larger number and higher energy of raindrops in the multiple raindrops simulations caused secondary raindrop splash erosion, which resulted in more fine particles (Fu et al., 2017b). Therefore, we assume that the impact of raindrops on the soil is a continuous process of destruction, including the primary raindrop splash erosion by the initial raindrops and the secondary raindrop splash erosion caused by the dispersed raindrops and the soil aggregate fragments again impacting the topsoil. In other words, the raindrops that splash the soil for the first time, it will cause the aggregates to be dispersed and broken, and in theory, the secondary raindrop splash erosion will disperse the aggregates into finer particles. It is not clear how the aggregate fragment size distribution of splash aggregates differs for secondary raindrop splash erosion and without secondary raindrop splash erosion. To further study the effect of secondary splash erosion on the fragmentation of aggregates, the authors proposed the definition of raindrop secondary splashing erosion, which is the phenomenon that when the raindrops impact the bare surface, the splashed soil particles are dispersed and broken again or multiple times. The scope of secondary raindrop splash erosion consists of three parts: (i) soil particles are broken because the dispersed raindrops and the splashed soil particles impact the surface; (ii) the raindrops impact the bare soil surface, dispersed into small raindrops which will splash the topsoil again or combine with other raindrops to splash the topsoil, and (iii) dispersed raindrops carried the fragments onto the ground and then scatter and splash. At present, many methods have been proposed to study splash erosion (Ellison, 1944a; Ellison, 1944b; Morgan, 1978; Legout et al., 2005; Hu et al., 2016). However, these methods all count the mass of aggregate fragments produced by secondary raindrop splash erosion into the total amount of erosion, which cannot distinguish the effect of secondary raindrop splash erosion on aggregate fragments. Therefore, we designed a method to distinguish secondary raindrop splash erosion. And the undisturbed soil was used in the study to obtain more reliable data.
2. Materials and methods The study site is located in Yangling, Shaanxi Province (108°03′29.18″E, 34°18′24.30″N), with a mean annual temperature of 13 °C and an annual rainfall of 550–650 mm. It is a semi-humid and semi-arid zone in a warm temperate region, with a mean altitude of 532 m. The studied soils were formed from loess parent material and are relatively deep with loam and silt-loam textures (according to the USDA fractions classification criteria). The major crops grown in this region include maize (Zea mays L.) and winter wheat (Triticum aestivum Linn.). Forty-three topsoil layer (0–5 cm) samples were collected randomly using a cutting ring (with a diameter of 10 cm × a height of 5 cm). Three samples were used to measure soil bulk density and moisture content, and the other 40 were used for raindrop splash tests. Additionally, 1 kg of undisturbed soil was collected and air dried for the determination of soil physical and chemical properties and aggregate fragment size distribution (wet sieve). The basic physical and chemical properties of the soil are shown in Table 1. 2.1. Experimental device The artificial rainfall device consisted of two parts (Fig. 1a): a raindrop generator and a collecting device. (i) The raindrop generator is a cuboid box with an open top (Length 10 cm × width 10 cm × height 10 cm). In the floor of the box, 21 syringe needles were installed at intervals of 2 cm, with different needle types yielding different raindrop diameters. Deionized water was used in the test. (ii) The collection
Fig. 1. Device schematic diagram (a: Experimental device; b: Splash pan; and c: Alcohol with a height of 5 mm in the splash pan). 2
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device is a 110-cm splash pan (Fig. 1b), which is surrounded by six concentric diaphragm circles (1 cm height, negligible thickness). From the centre of the splash pan, the centre of the inner circle has a 5-cm radius, which was used to place the cutting ring (diameter of 10 cm × height of 5 cm). Outside the cutting ring, there were fenced concentric diaphragm circles at distances of 10, 20, 30, 40 and 50 cm. The diaphragm was attached imperviously to the splash pan, and two drainage holes were symmetrically arranged between each concentric circle to collect splashed soil particles. The splash pan was used to collect soil particles for each of the five distances (0–10, 10–20, 20–30, 30–40 and 40–50 cm). A plastic film was placed around the outside of the entire test device to prevent the raindrops from being disturbed by transverse airflow during the falling process.
distances were washed respectively, collected from the drain holes using alcohol (concentration 98%; Le Bissonnais, 1996) and sieved to separate the aggregates into size fractions of > 2, 1–2, 0.5–1, 0.25–0.5, 0.106–0.25, 0.053–0.106 and < 0.053 mm, according to an aggregate analyser (HR-TTF-100, Shunlong Experiment Instrument Factory, Yuxi City, Zhejiang Province of China). All aggregate fragments were ovendried for 24 h at 105 °C and weighed.
2.2. Splash erosion test
W (δ > di ) d =1−⎛ i ⎞ W0 ⎝ d max ⎠
2.4. Data analysis The study used the fractal dimension, which was proposed by Yang et al. (1993) and used the quality of the fraction distribution, described as a soil fractal model: 3−D
⎜
The experiment was conducted in two parts: the secondary raindrop splash test (SR) and without secondary raindrop splash test (WSR). The devices and procedures of the two tests were identical, except that the WSR tests used alcohol (concentration 98%) at a depth of 5 mm within each splash distance interval of the splash pan (Fig. 1c); alcohol was used to remove the effect of dissipating physical clay on wetting expansion (Concaret, 1967; Le Bissonnais, 1996). Soaking soil aggregates in alcohol can greatly slow their disintegration, and aggregates soaked in the alcohol do not coalesce again (Le Bissonnais, 1996). Before the experiment, we tested the energy of splashing raindrops on different raindrop diameters and splash distances for WSR and SR experiments, and found that there was no significant difference between WSR and SR in raindrop energy under the same raindrop diameter and splash distances (P > 0.05). Then, aggregates of the same mass and particle size were put into glass containers containing alcohol (concentration 98%) and deionized water, respectively. It was found that the structure of aggregates in alcohol was more complete. Therefore, we believe that alcohol can protect aggregates and act as a solvent to distinguish whether there is secondary raindrop splash erosion or not. According to previous research on raindrop splashing on Lou soil (Fu, 2017), aggregate fragment size > 2 mm is almost non-existent (0–3.44%) for each splash distance. Therefore, an alcohol depth of 5 mm was enough to hold the splashed aggregate fractions. All of the undisturbed soil samples were placed in a water bath (deionized water). To maintain consistent moisture contents of each test sample, the porous cutting ring cap was placed on a piece of filter paper, and the height of water in the container was not allowed to exceed the top edge of the cutting ring. After soaking in water for 8 h, the undisturbed soil samples were placed on a layer of sand and allowed to drain for 12 h. Five raindrop diameters were tested in the experiment (2.67, 3.05, 3.39, 3.79 and 4.05 mm). The rainfall duration was 10 min, and each experiment was repeated 4 times. The simulated rainfall height was 2 m, and the corresponding rainfall intensity and rainfall kinetic energy are shown in Table 2.
and taking the logarithm of both sides:
lg
Raindrop energy (J m−2 s−1)
2.67 3.05 3.39 3.79 4.05
5.36 5.43 5.48 5.52 5.54
5.76 21 68.61 127.44 217.26
2.41 3.68 5.15 7.30 8.97
× × × × ×
⎟
3.1. Aggregate fragment size distribution for the WSR and SR tests The fragment size distribution of the average splash aggregates for the WSR and SR tests and the fragment size distribution for the undisturbed soils are shown in Fig. 2. Generally, the mass percentage of the splash aggregate fractions presented an up-down-up trend as the fragment size distribution decreased; there were two peaks in the fragment size 0.5–1 and < 0.053 mm, and these peaks were 18.24% and 44.67% for WSR test, 15.43% and 53.53% for SR test, respectively. The mass percentage of the undisturbed soil presented a down-up trend with decreasing fragment size distribution, with the maximum value for the mass percentage of fragments < 0.053 mm. For the WSR test, except for the fragment size 0.25–0.5 mm and the fragment size 0.106–0.25 mm, the mass percentage of the remaining fragment size was significantly different from the mass percentage of the undisturbed soil aggregate (P < 0.05); the mass percentage of fragment size < 1 mm in the WSR test was 37.12%, 6.42%, 8.75%, 50.49% and 45.09% greater than those in the undisturbed soil, which showed that the fragment size > 1 mm were broken into aggregate fragment size < 1 mm. For the SR test, the mass percentage of the fragment size > 1 and < 0.053 mm was significantly different from the mass percentage of undisturbed soil aggregate (P < 0.05); the mass percentages in SR test were greater than those of the undisturbed soil for all fragment sizes except > 1 and 0.106–0.5 mm. Comparing WSR test with SR test,
Table 2 Main raindrop parameters of simulated rainfall. Rainfall intensity (mm h−1)
⎜
3. Results
At the end of each test, all of the splashed aggregate fragments resulting from the five different raindrop diameters and the five splash
Raindrop velocity (m s−1)
W (δ > di ) d = (3 − D) lg ⎛ i ⎞ W0 d ⎝ max ⎠
where W(δ > di) is the cumulative mass of soil aggregate fractions with size δ greater than the comparative size di; W0 is the total mass of the fractions; di is the average of the sieve size range (dj, dj + 1); and dmax is the maximum fraction. Significant differences in the mass percentages of the aggregate fragment sizes for the different raindrop diameters or splash distances under the WSR and SR tests, and for the WSR, SR tests and the undisturbed soil were detected using a one-way analysis of variance (ANOVA) followed by LSD’s test (P < 0.05). Independent-Samples T–Test were used for significant differences in the mass percentages of the same aggregate fragment sizes under the WSR and SR tests for the same raindrop diameter or splash distance, and the values were statistically significant at the 95% confidence level. The relationship between the raindrop diameter and fractal dimension for the WSR and SR tests was analyzed using a simple regression method. All statistical analyses were performed using SPSS 16.0 (IBM SPSS Software, Armonk, NY, United States), and all figures were processed in Origin 8.5 (OriginLab Corporation, Northampton, MA, United States).
2.3. Aggregate fragment size distribution
Raindrop diameter (mm)
⎟
10–5 10–5 10–5 10–5 10–5
3
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fragment size < 0.053 mm for the WSR and SR tests was significantly different (P < 0.05), and the mass percentage of the fragment size < 0.053 mm for the SR test was 1.04 and 1.15 times of the WSR test. Generally, no significant differences were observed in the mass percentage of fragment size > 2 mm among the five raindrop diameters for the WSR and SR tests (P < 0.05). The mass percentages of fragment sizes 1–2, 0.5–1, 0.25–0.5, 0.106–0.25, and 0.053–0.106 mm presented a fluctuating variable trend with an increase in raindrop diameter (WSR and SR tests). The mass percentages of fragment size < 0.053 mm for the different raindrop diameters in WSR test were significantly different (P < 0.05). In SR test, no significant differences were observed in the mass percentages of fragment size < 0.053 mm for raindrop diameters 3.05–4.05 mm (P > 0.05), but the mass percentages for those diameters were significantly higher than that of fragment size < 0.053 mm for the 2.67 mm raindrop diameter (P < 0.05). Comparing the WSR and SR tests (Fig. 3), for the fragment size > 2, 1–2, and 0.5–1 mm, when the raindrop diameters were 2.67, 3.05 and 3.39 mm, there was no significant difference in the mass percentage of splash aggregates between WSR and SR tests (P > 0.05) in the case of 55.56%. But the mass percentage in WSR test was higher than that in SR test except for the raindrop diameter 3.05 mm for the fragment size 1–2 mm, which was 1.12–6.52 times that of SR test. These results indicate that alcohol has a good protective effect on fragment size > 0.5 mm under these rainfall conditions. However, for the raindrop diameters 3.79 and 4.05 mm, there was no significant difference between WSR test and SR test (P > 0.05), except for the fragment size 0.5–1 mm for the raindrop diameter 4.05 mm. For the fragment size 0.25–0.5, 0.106–0.25 and 0.053–0.106 mm, there was no significant difference in the mass percentage of splash aggregates between WSR and SR tests (P > 0.05) in the case of 66.67%. However, the mass percentage of splash aggregates in WSR test increased by 10.62–46.49%, 9.14–48.53% and 0.68–67.95% compared with SR test except for those of the 2.67 mm raindrop diameter and fragments 0.25–0.5 mm with a raindrop diameter of 3.39 mm. The mass percentage of fragment size < 0.053 mm was significantly greater for WSR test than for SR test for all raindrop diameters (P < 0.05).
Fig. 2. Fragment size distribution for WSR, SR and the undisturbed soil tests. Note: Different small letters indicate a significant difference (P < 0.05) in the mass percentage for the same fragment size and different tests.
the difference of the mass percentage of the fragment size > 0.053 mm was not significant (P > 0.05), but the mass percentage of the fragment size > 0.053 mm of WSR test was 1.13–1.39 times that of SR test, and the mass percentage of the fragment size < 0.053 mm was 0.83 times that of SR test.
3.2. Effect of raindrop diameter on aggregate fragment size distribution for the WSR and SR tests The distribution of fragment size for different raindrop diameters in the WSR and SR tests is shown in Fig. 3. In general, for WSR and SR tests, the mass percentage of the splash aggregate fractions presented an up-down-up trend as the fragment size distribution decreased; there were two peaks in the fragment size 0.5–1 and < 0.053 mm. Except for the 2.67 mm raindrop diameter in the WSR test, the mass percentages of fragment size < 0.053 mm for WSR and SR were significantly greater than for 0.5–1 mm fragments (P < 0.05), which were 1.75–6.44 and 1.62–5.98 times greater, respectively. Comparing the WSR and SR tests (Fig. 3), when the raindrop diameter was 2.67 mm, the WSR and SR tests were significantly different (P < 0.05) for the mass percentage of the fragment size > 2, 1–2, 0.5–1 and < 0.053 mm, and the mass percentages of fragment size > 2, 1–2 and 0.5–1 mm were greater for WSR test than for SR test, which was 6.52, 2.13 and 1.27 times that of the SR test; the mass percentage of fragment size < 0.5 mm was lower for WSR test than for the SR test, which was 1.44–43.20% lower than the SR test. For the 3.05 mm raindrop diameter, the difference between WSR and SR tests was not significant (P > 0.05) for the mass percentage of the fragment size > 2, 1–2 and 0.106–0.25 mm, but the mass percentages of fragment size > 2, 0.5–1, 0.25–0.5, 0.106–0.25 and 0.053–0.106 mm of WSR test were greater than that of SR test; the mass percentages of fragment size < 0.053 mm for WSR test was 24.36% lower than for SR test. For the 3.39 mm raindrop diameter, the difference of WSR and SR test was not significant except for the fragment size < 0.053 mm (P > 0.05). For the raindrop diameters of 3.79 and 4.05 mm, the difference between the WSR and SR test was not significant (P > 0.05) for the mass percentage of the fragment size > 1 mm; the difference between the WSR and SR test was significantly different in the case of 50% (P < 0.05) for the mass percentage of the fragment size 0.5–1, 0.25–0.5, 0.106–0.25 and 0.053–0.106 mm, and the mass percentages for WSR test increased by 17.93–27.72% and 10.62–37.24% compared with SR test under other conditions, except for the raindrop diameter 3.79 mm for the fragment size 0.5–1 mm. The mass percentage of the
3.3. Characteristics of fragment size distributions at different splash distances for the WSR and SR tests The fragment size distributions at different splash distances for the WSR and SR tests are shown in Fig. 4. On the whole, the mass percentage presented a fluctuating rising trend with decreasing fragment size for all splash distances (P < 0.05). For the splash distance 0–10 cm, there was no significant difference between WSR test and SR test (P > 0.05) except for the fragment size 0.5–1 and < 0.053 mm; but for > 0.053 mm particle size, the mass percentage in WSR test was higher than that in SR test, and it increased by 4.85–44.78% compared with SR test; for < 0.053 mm particle size, the mass percentage of splashed aggregates in WSR test was 17.18% lower than that in SR test. For the splash distance 10–20, 20–30 and 30–40 cm, when the fragment size > 0.053 mm, there was no significant difference in the mass percentage of splash aggregates between WSR and SR tests (P > 0.05) in the case of 50%; however, except for the splash distance 10–20 cm for the fragment size > 2 mm, the mass percentage of the WSR test was higher than that of the SR test, and increased by 0.65–35.67%, 2.59–75.10% and 1.07–46.76% compared with the SR test, respectively; when the fragment size < 0.053 mm, the mass percentage in WSR and SR test was significantly different (P < 0.05), and mass percentage in WSR test were 11.79%, 14.89% and 16.21% lower than that in SR test, respectively. For the splash distance 40–50 cm, there was no significant difference in the mass percentage of splash aggregates between WSR and SR tests (P > 0.05) except for fragment size 0.25–0.5 and < 0.053 mm; when the fragment size was < 0.053 4
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Fig. 3. Fragment size distribution for different raindrop diameters for the WSR and SR tests. Note: Different capital letters indicate a significant difference (P < 0.05) in mass percentage for the same raindrop diameter and different fragment sizes, and different small letters indicate a significant difference (P < 0.05) in mass percentage for the same fragment size and different raindrop diameters. Different symbols indicate a significant difference (P < 0.05) in mass percentage for the WSR and SR tests under the same raindrop diameters and fragment size.
4. Discussion
mm, the mass percentage in WSR test was significantly reduced by 13.18% compared with the SR test (P < 0.05). For the fragment size > 2 mm (Fig. 4), the splashed aggregate existed only in the splash distance 0–10 and 10–20 cm, and the difference of mass percentage between the WSR and SR tests was not significant (P > 0.05). The mass percentage of the splash distance 0–10 cm was significantly higher than other splash distances (P < 0.05). For the fragment size 1–2 mm, there was no significant difference in the mass percentage between WSR and SR tests (P > 0.05) in most cases (80%). For the fragment size 0.5–1, 0.25–0.5 and 0.106–0.25 mm, the WSR and SR tests were not significantly different in the case of 40% for all splash distances (P > 0.05), but the mass percentage in WSR test was higher than the SR test and increased by 14.46–46.46%, 4.85–46.76% and 13.70–32.98% compared with the SR test, respectively. For the fragment size 0.053–0.106 mm, there was no significant difference between WSR and SR tests for all splash distances (P > 0.05), but the mass percentage in WSR test was higher than that in SR test, and increases by 0.65–22.94% compared with SR test. For the fragment size < 0.053 mm, the difference of mass percentage between WSR and SR tests was significant (P < 0.05), and the mass percentage in WSR test were lower than that in SR test; the minimum values of WSR and SR tests were 40.06% and 48.36% at the splash distance 0–10 cm, respectively.
Although the mass percentage in WSR test was not significantly different from SR test for the fragment size > 0.053 mm (Fig. 2), the mass percentage in WSR test was higher than that in SR test, which indicated that alcohol had a good protective effect on the aggregate in the WSR test and can weaken the secondary raindrop splash action. Under all rainfall conditions (Fig. 3), the mass percentage for the fragment size < 0.053 mm between WSR and SR tests was significantly different (P < 0.05), and mass percentage in WSR test were 43.20%, 24.36%, 7.20%, 4.08% and 12.83% lower than that in SR test, respectively. This further indicated that alcohol can weaken the secondary raindrop splash action. For the WSR and SR tests (Fig. 4), when the fragment size was > 2 mm, the aggregate were only present at the splash distances 0–10 and 10–20 cm. This was mainly because the soil particles splashed were affected by their own size and quality. In theory, the closer the distance was, the smaller the energy required for the soil particles, and the greater the distance, the larger the energy required. Therefore, there were more aggregates with fragment size > 2 mm deposited in close range (Legout et al., 2005; Fu, 2017). The mass percentages of fragment size 1–2, 0.5–1, 0.25–0.5, 0.106–0.25 and 0.053–0.106 mm in WSR and SR tests presented a fluctuating variable trend with the increase of splash distances (Fig. 4). Although there was no significant difference in the mass percentage between WSR and SR tests (P > 0.05) in the case
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Fig. 4. Fragment size distribution at different splash distances for the WSR and SR tests. Note: Different capital letters indicate a significant difference (P < 0.05) in mass percentage for the same splash distance and different fragment sizes, and different small letters indicate a significant difference (P < 0.05) in the mass percentage for the same fragment size and different splash distances. Different symbols indicate a significant difference (P < 0.05) in mass percentage for the WSR and SR tests under the same splash distances and fragment size.
microaggregates or finer aggregates (Figs. 2, 3, 4), which are preferentially removed by erosion (Adesodun et al., 2007). Hence, The aggregate and nutrient loss caused by the secondary raindrop splash erosion will increase the soil fertility decline, reduce the land productivity, and even increase the surface crust and nutrient loss (Hu et al., 2018; Ma et al., 2014). Ferrandino and Elmer (1996) examined the spatial dispersal of conidia of Septoria lycopersici (causal agent of Septoria leaf spot) during and immediately following rain events. The observed transport at a long distance suggests that at least some conidia are carried in very small rain droplets or secondary splash droplets. This suggests that secondary raindrop splash can cause long-spreading of spores, which may have a major impact on the epidemiology of the disease. Therefore, to reduce the occurrence of these phenomena, effective measures can be taken to prevent raindrops from directly splashing the topsoil, for example, using wheat straw cover to protect the soil (Jin et al., 2009; Zhang et al., 2009), thereby reducing secondary raindrop splash erosion. The relationship between the fractal dimension (D) of aggregate fragments and the raindrop diameter in the WSR and SR tests is shown in Fig. 5. For the same raindrop diameter, the D values of the SR test were higher than those of the WSR test, which indicated that more fine particles were produced by the SR test and that the degree of aggregate fragmentation was greater. Photos of aggregate fragments for the WSR and SR tests are shown in Fig. 6. The aggregate fragments in the WSR test had almost no dispersion or fragmentation due to the effect of the
50%, the mass percentage in WSR test was higher than SR except for the splash distance 40–50 mm for the fragment size 1–2 mm. And with the decrease of particle size distribution, the mass percentage of WSR test were 1.01–1.75, 1.14–1.46, 1.05–1.47, 1.14–1.33 and 1.03–1.23 times that of SR test, respectively. This indicated that in the WSR experiment, alcohol can inhibit the breakdown of > 0.053 mm fragment size, that was, reduce the damage of the secondary raindrop splash erosion on the aggregate. Under the condition of fragment size < 0.053 mm (Fig. 4), with the increase of splash distance, the mass percentage in WSR and SR tests was significantly different (P < 0.05), and the mass percentage in WSR test were 17.18%, 11.79%, 14.89%, 16.21% and 13.18% lower than that in SR test, respectively. Aggregates and organic matter are the foundation of soil structure and fertility, and they interact and are inseparable (Dou et al., 2011). Some studies have shown that large aggregates contain more organic matter and that nearly 90% of the soil organic matter is located in the aggregate fraction of the topsoil (Jastrow, 1996); the content of soil organic matter is also a sign of the aggregate stability (Beguería et al., 2015; Besalatpoura et al., 2013; Canasveras et al., 2010). It can be seen from the above research that the impact of raindrops on the soil is a continuous process of destruction, including the primary raindrop splash erosion by the initial raindrops and the secondary raindrop splash erosion which caused by the dispersed raindrops and the soil aggregate fragments again impacting the topsoil. And secondary raindrop splash erosion causes macroaggregates to break into more 6
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5. Conclusions The mass percentage of fragment size > 0.053 mm of the WSR test was 1.13–1.39 times that of the SR test, but the mass percentage of fragment size < 0.053 mm of WSR test was 0.83 times that of SR test. For the five raindrop diameters (2.67, 3.05, 3.39, 3.79 and 4.05 mm), the mass percentages of fragment size < 0.053 mm in WSR test were 43.20%, 24.36%, 7.20%, 4.08% and 12.83% lower than those in SR test, respectively. These results indicated that alcohol could weaken the secondary splashing effect. For the five splash distances (0–10 cm, 10–20 cm, 20–30 cm, 30–40 cm, and 40–50 cm), the mass percentages of the splashed aggregates showed a fluctuating variable trend with decreasing fragment size (P < 0.05), and the mass percentages of fragment size < 0.053 mm were 17.18%, 11.79%, 14.89%, 16.21% and 13.18% lower in WSR test than in SR test, respectively. These results showed that alcohol can protect fragment size > 0.053 mm at each splash distance. The fractal dimension (D) of aggregates in the WSR and SR tests showed a downward opening parabolic relationship with raindrop diameter, and the values of the fractal dimension (D) of SR test were higher than those of 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, indicating the lowest protective effect of alcohol on the aggregates and the largest degree of fragmentation, respectively. This study proposes the definition and scope of the secondary raindrop splash erosion, and it is verified, but this is only a preliminary exploration. Future research can be further studied from the topography, vegetation, soil structure and other factors.
Fig. 5. Relationship between the fractal dimension (D) of aggregate fragments and the raindrop diameter in the WSR and SR tests.
Acknowledgments This work was supported by the National Natural Science Foundation of China (41571262) and the Chinese Ministry of Water Resources Science and Technology Promotion (TG1308). Fig. 6. Photos of aggregate fragments for the WSR and SR tests (Post-test photographs). Note: splash distance 10–20 cm for a raindrop diameter of 3.39 mm.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catena.2019.104342.
alcohol, which significantly reduced the effect of secondary raindrop splash erosion. However, the aggregate fragments in the SR test had a large degree of dispersion and fragmentation. Regression analysis of fractal dimension (D) and raindrop diameter (d) allows us to conclude that the fractal dimension of the soil splash aggregate (D) changed as a quadratic function following an up-down trend with increasing raindrop diameter (d). The equation is D = m (d − n)2 + p , where R2 ≥ 0.98, P < 0.05, and m, n and p are parameters. When the raindrop diameters are 3.64 and 3.74 mm, the values of D in WSR and SR tests are at their maximum, respectively, with the lowest protective effect of alcohol on aggregates and the highest degree of fragmentation of aggregates (Castrignano and Stelluti, 1999). With a raindrop diameter of 2.67 mm, the D values of the splash aggregate fragments for WSR and SR were at their minimum 2.61 and 2.74, respectively. This may be due to the smaller energy of the raindrop diameter 2.67 mm (Table 2), causing fewer total aggregate fragments to be splashed but with a higher proportion of the larger particles splashed because of their mass. The D values of the splash aggregate fragments for a raindrop diameter of 4.05 mm were 2.82 and 2.86 for the WSR and SR tests, respectively. The raindrop diameter of 4.05 mm had the largest energy in the study (Table 2), so that the fragmentation degree of the aggregates is larger, and the macroaggregates are broken into microaggregates (Legout et al., 2005). Thus, the fractal dimension for a raindrop diameter of 4.05 mm is relatively higher than that of a raindrop diameter of 2.67 mm.
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Li, G.L., Fu, Y., Li, B.Q., Zheng, T.H., Wu, F.Q., Peng, G.Y., Xiao, T.Q., 2017a. Micro–characteristics of soil aggregate breakdown under raindrop action. Catena 162, 354–359. Li, G.L., Zheng, T.H., Fu, Y., Li, B.Q., Zhang, T., 2017b. Soil detachment and transport under the combined action of rainfall and runoff energy on shallow overland flow. J. Mt. Sci Engl. 14, 1373–1383. Ma, R.M., Li, Z.X., Cai, C.F., Wang, J.G., 2014. The dynamic response of splash erosion to aggregate mechanical breakdown through rainfall simulation events in Ultisols (subtropical China). Catena 121, 279–287. Morgan, R.P.C., 1978. Field studies of rainsplash erosion. Earth Surf. Proc. Land. 3, 295–299. Sutherland, R.A., Wan, Y., Lee, C.T., Zieglera, A.D., 1996. Aggregate enrichment ratios for splash and wash transported sediment from an Oxisol. Catena 26, 187–208. Wan, Y., Elswaify, S.A., 1998. Characterizing interrill sediment size by partitioning splash and wash processes. Soil Sci. Soc. Am. J. 62, 430–437. Yang, P.L., Luo, Y.P., Shi, Y.C., 1993. Using weight distribution of soil particle size to express soil fractal features. Chinese Sci. Bull. 38, 1896–1899. Zhang, S.L., Lövdahl, L., Grip, H., Tong, Y.A., Yang, X.Y., Wang, Q.J., 2009. Effects of mulching and catch cropping on soil temperature: soil moisture and wheat yield on the Loess Plateau of China. Soil Till. Res. 102, 78–86.
Fu, Y., Li, G.L., Zheng, T.H., Li, B.Q., Zhang, T., 2017b. Splash detachment and transport of loess aggregate fractions by raindrop action. Catena 150, 154–160. Hénin, S., Monnier, G., Combeau, A., 1958. Méthode pour l’étude de la stabilité structurale des sols. Ann. Agron. 9, 73–92. Hu, F., Liu, J., Xu, C., Du, W., Yang, Z.H., Liu, X.M., Liu, G., Zhao, S.W., 2018. Soil internal forces contribute more than raindrop impact force to rainfall splash erosion. Geoderma 330, 91–98. Hu, W., Zheng, F., Bian, F., 2016. The directional components of splash erosion at different raindrop kinetic Energy in the Chinese Mollisol Region. Soil Sci. Soc. Am. J. 80, 1329–1340. Jastrow, J.D., 1996. Soil aggregate formatin and the accrual of particulate and mineral–associated organic matter. Soil Biol. Biochem. 28, 665–676. Jin, K., Cornelis, W.M., Gabriels, D., Baert, M., Wu, H.J., Schiettecatte, W., Cai, D.X., De Neve, S., Jin, J.Y., Hartmann, R., Hofman, D., 2009. Residue cover and rainfall intensity effects on runoff soil organic carbon losses. Catena 78, 81–86. Kubota, Y., Mochizuki, O., 2011. Influence of head shape of solid body plunging into water on splash formation. J. Visual. 14, 111–119. Le Bissonnais, Y., 1996. Aggregate stability and assessment of soil crustability and erodibility: I. Theory and methodology. Eur. J. Soil Sci. 47, 425–437. Legout, C., Leguédois, S., Le Bissonnais, Y., Issa, O.M., 2005. Splash distance and size distributions for various soils. Geoderma 124, 279–292.
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Fragmentation of soil aggregates induced by secondary raindrop splash erosion ⁎
Yu Fua, Guanglu Lia,b, , Tenghui Zhenga, Yingsong Zhaob, Mingxi Yangb a b
Institute of Soil and Water Conservation, Northwest A&F University, Yangling 712100, Shaanxi, PR China College of Resources and Environment, Northwest A&F University, Yangling 712100, Shaanxi, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Soil aggregate Raindrop diameter Secondary raindrop splash Fractal dimension Loess Plateau
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.
1. Introduction Soil aggregates, the material basis of soil fertility, are the basic unit of soil structure. Raindrops splashing on topsoil can disintegrate and disperse soil aggregates, providing abundant loose particles for subsequent runoff transport (Fernández-Raga et al., 2017; Li et al., 2017a). Simultaneously, soil particle transport causes the reduction or blockage of soil surface pores and results in the compaction of soil and a decrease in soil permeability, providing favorable conditions for the subsequent formation of runoff (Hénin et al., 1958; Fu, 2017; Li et al., 2017b). Therefore, the fragmentation and transportation of soil aggregates caused by raindrop splash erosion is the root of reduced soil productivity and increased soil erosion.
Kubota and Mochizuki (2011) captured images of splash formation (single raindrop) using a high-speed CMOS camera. And found that the origin of the film flow as the primary splash is confirmed to be the water displaced by the head. The line connecting the head to the tail of the body affects the separation of the film flow from the body, and this is found to be the cause of the secondary splash. Thus, when a single raindrop impacts a thin layer of water film, it will partially disperse into smaller raindrops. However, natural rainfall is the continuous fall of multiple raindrops, which not only results in the dispersion and fragmentation of soil surface aggregates but also results in the dispersion and splashing of the raindrops themselves. Hence, we assume that dispersed raindrops will cause secondary raindrop splash erosion, causing soil aggregate fragments to break into smaller particles. To test
⁎ Corresponding author at: Institute of Soil and Water Conservation, Northwest A&F University, Yangling 712100, Shaanxi, PR China; College of Resources and Environment, Northwest A&F University, Yangling 712100, Shaanxi, PR China. E-mail address: [email protected] (G. Li).
https://doi.org/10.1016/j.catena.2019.104342 Received 14 January 2019; Received in revised form 11 August 2019; Accepted 28 October 2019 0341-8162/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Yu Fu, et al., Catena, https://doi.org/10.1016/j.catena.2019.104342
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Table 1 Basic physical and chemical properties of soil (mean value ± standard deviation). Soil type
Lou soil
Bulk density (g·cm3)
Soil organic carbon (%)
Total nitrogen (g·kg−1)
Total phosphorus (g·kg−1)
1.37 ± 0.13 1.46 ± 0.03 1.04 ± 0.02 The fragment size distribution of the undisturbed soil/% ＞2 mm 2–1 mm 1–0.5 mm 19.58 ± 7.66 14.04 ± 2.91 13.30 ± 3.48
Particle size composition/% Sand (2–0.02 mm)
Silt (0.02–0.002 mm)
Clay (< 0.002 mm)
0.62 ± 0.02
33.26 ± 0.05
44.07 ± 0.03
22.67 ± 0.02
0.5–0.25 mm 10.87 ± 3.07
0.25–0.106 mm 7.03 ± 2.48
0.106–0.053 mm 4.40 ± 1.64
< 0.053 mm 30.79 ± 5.69
Using a method for measuring secondary raindrop splash erosion, the objectives of this paper were (i) to clarify the characteristics of fragment size distribution under the conditions of the secondary raindrop splash test (SR) and the without secondary raindrop splash test (WSR) and (ii) to explore which rainfall conditions cause the greatest degree of aggregate fragmentation.
this hypothesis, the author performed a simulation rainfall experiment with a single raindrop (Fu et al., 2017a). The variation in the enrichment ratio with the fragment size was consistent under different raindrop diameters (2.67–3.79 mm), and the enrichment ratio of fragment size < 0.053 mm was less than 1. However, we found that the enrichment ratio of fragment size < 0.053 mm was greater than 1 for different raindrop diameters (2.67–3.79 mm) in a simulation rainfall experiment with multiple raindrops (Fu et al., 2017b). Sutherland et al. (1996) and Wan and Elswaify (1998) demonstrated that the enrichment ratio of fragment size < 0.063 mm was greater than 1. The above results demonstrate that the simulation rainfall experiment with multiple raindrops is more vulnerable to the breaking of aggregates into smaller particles than is the single raindrop experiment. There are two main reasons for this. First, the single raindrop impacts the topsoil, and then the soil aggregates are scattered and splashed, but few of the dispersed raindrops impact and destroy the aggregate fragments again. Second, the energy applied during single raindrop simulations (4.99 × 10−5–1.78 × 10–4 J m–2s−1) is small, so most of the aggregates with particle size > 1 mm break up into small size aggregates (0.053–1 mm), while those with less than 0.053 mm splash aggregates have less content (Fu et al., 2017a). The above studies showed that the larger number and higher energy of raindrops in the multiple raindrops simulations caused secondary raindrop splash erosion, which resulted in more fine particles (Fu et al., 2017b). Therefore, we assume that the impact of raindrops on the soil is a continuous process of destruction, including the primary raindrop splash erosion by the initial raindrops and the secondary raindrop splash erosion caused by the dispersed raindrops and the soil aggregate fragments again impacting the topsoil. In other words, the raindrops that splash the soil for the first time, it will cause the aggregates to be dispersed and broken, and in theory, the secondary raindrop splash erosion will disperse the aggregates into finer particles. It is not clear how the aggregate fragment size distribution of splash aggregates differs for secondary raindrop splash erosion and without secondary raindrop splash erosion. To further study the effect of secondary splash erosion on the fragmentation of aggregates, the authors proposed the definition of raindrop secondary splashing erosion, which is the phenomenon that when the raindrops impact the bare surface, the splashed soil particles are dispersed and broken again or multiple times. The scope of secondary raindrop splash erosion consists of three parts: (i) soil particles are broken because the dispersed raindrops and the splashed soil particles impact the surface; (ii) the raindrops impact the bare soil surface, dispersed into small raindrops which will splash the topsoil again or combine with other raindrops to splash the topsoil, and (iii) dispersed raindrops carried the fragments onto the ground and then scatter and splash. At present, many methods have been proposed to study splash erosion (Ellison, 1944a; Ellison, 1944b; Morgan, 1978; Legout et al., 2005; Hu et al., 2016). However, these methods all count the mass of aggregate fragments produced by secondary raindrop splash erosion into the total amount of erosion, which cannot distinguish the effect of secondary raindrop splash erosion on aggregate fragments. Therefore, we designed a method to distinguish secondary raindrop splash erosion. And the undisturbed soil was used in the study to obtain more reliable data.
2. Materials and methods The study site is located in Yangling, Shaanxi Province (108°03′29.18″E, 34°18′24.30″N), with a mean annual temperature of 13 °C and an annual rainfall of 550–650 mm. It is a semi-humid and semi-arid zone in a warm temperate region, with a mean altitude of 532 m. The studied soils were formed from loess parent material and are relatively deep with loam and silt-loam textures (according to the USDA fractions classification criteria). The major crops grown in this region include maize (Zea mays L.) and winter wheat (Triticum aestivum Linn.). Forty-three topsoil layer (0–5 cm) samples were collected randomly using a cutting ring (with a diameter of 10 cm × a height of 5 cm). Three samples were used to measure soil bulk density and moisture content, and the other 40 were used for raindrop splash tests. Additionally, 1 kg of undisturbed soil was collected and air dried for the determination of soil physical and chemical properties and aggregate fragment size distribution (wet sieve). The basic physical and chemical properties of the soil are shown in Table 1. 2.1. Experimental device The artificial rainfall device consisted of two parts (Fig. 1a): a raindrop generator and a collecting device. (i) The raindrop generator is a cuboid box with an open top (Length 10 cm × width 10 cm × height 10 cm). In the floor of the box, 21 syringe needles were installed at intervals of 2 cm, with different needle types yielding different raindrop diameters. Deionized water was used in the test. (ii) The collection
Fig. 1. Device schematic diagram (a: Experimental device; b: Splash pan; and c: Alcohol with a height of 5 mm in the splash pan). 2
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device is a 110-cm splash pan (Fig. 1b), which is surrounded by six concentric diaphragm circles (1 cm height, negligible thickness). From the centre of the splash pan, the centre of the inner circle has a 5-cm radius, which was used to place the cutting ring (diameter of 10 cm × height of 5 cm). Outside the cutting ring, there were fenced concentric diaphragm circles at distances of 10, 20, 30, 40 and 50 cm. The diaphragm was attached imperviously to the splash pan, and two drainage holes were symmetrically arranged between each concentric circle to collect splashed soil particles. The splash pan was used to collect soil particles for each of the five distances (0–10, 10–20, 20–30, 30–40 and 40–50 cm). A plastic film was placed around the outside of the entire test device to prevent the raindrops from being disturbed by transverse airflow during the falling process.
distances were washed respectively, collected from the drain holes using alcohol (concentration 98%; Le Bissonnais, 1996) and sieved to separate the aggregates into size fractions of > 2, 1–2, 0.5–1, 0.25–0.5, 0.106–0.25, 0.053–0.106 and < 0.053 mm, according to an aggregate analyser (HR-TTF-100, Shunlong Experiment Instrument Factory, Yuxi City, Zhejiang Province of China). All aggregate fragments were ovendried for 24 h at 105 °C and weighed.
2.2. Splash erosion test
W (δ > di ) d =1−⎛ i ⎞ W0 ⎝ d max ⎠
2.4. Data analysis The study used the fractal dimension, which was proposed by Yang et al. (1993) and used the quality of the fraction distribution, described as a soil fractal model: 3−D
⎜
The experiment was conducted in two parts: the secondary raindrop splash test (SR) and without secondary raindrop splash test (WSR). The devices and procedures of the two tests were identical, except that the WSR tests used alcohol (concentration 98%) at a depth of 5 mm within each splash distance interval of the splash pan (Fig. 1c); alcohol was used to remove the effect of dissipating physical clay on wetting expansion (Concaret, 1967; Le Bissonnais, 1996). Soaking soil aggregates in alcohol can greatly slow their disintegration, and aggregates soaked in the alcohol do not coalesce again (Le Bissonnais, 1996). Before the experiment, we tested the energy of splashing raindrops on different raindrop diameters and splash distances for WSR and SR experiments, and found that there was no significant difference between WSR and SR in raindrop energy under the same raindrop diameter and splash distances (P > 0.05). Then, aggregates of the same mass and particle size were put into glass containers containing alcohol (concentration 98%) and deionized water, respectively. It was found that the structure of aggregates in alcohol was more complete. Therefore, we believe that alcohol can protect aggregates and act as a solvent to distinguish whether there is secondary raindrop splash erosion or not. According to previous research on raindrop splashing on Lou soil (Fu, 2017), aggregate fragment size > 2 mm is almost non-existent (0–3.44%) for each splash distance. Therefore, an alcohol depth of 5 mm was enough to hold the splashed aggregate fractions. All of the undisturbed soil samples were placed in a water bath (deionized water). To maintain consistent moisture contents of each test sample, the porous cutting ring cap was placed on a piece of filter paper, and the height of water in the container was not allowed to exceed the top edge of the cutting ring. After soaking in water for 8 h, the undisturbed soil samples were placed on a layer of sand and allowed to drain for 12 h. Five raindrop diameters were tested in the experiment (2.67, 3.05, 3.39, 3.79 and 4.05 mm). The rainfall duration was 10 min, and each experiment was repeated 4 times. The simulated rainfall height was 2 m, and the corresponding rainfall intensity and rainfall kinetic energy are shown in Table 2.
and taking the logarithm of both sides:
lg
Raindrop energy (J m−2 s−1)
2.67 3.05 3.39 3.79 4.05
5.36 5.43 5.48 5.52 5.54
5.76 21 68.61 127.44 217.26
2.41 3.68 5.15 7.30 8.97
× × × × ×
⎟
3.1. Aggregate fragment size distribution for the WSR and SR tests The fragment size distribution of the average splash aggregates for the WSR and SR tests and the fragment size distribution for the undisturbed soils are shown in Fig. 2. Generally, the mass percentage of the splash aggregate fractions presented an up-down-up trend as the fragment size distribution decreased; there were two peaks in the fragment size 0.5–1 and < 0.053 mm, and these peaks were 18.24% and 44.67% for WSR test, 15.43% and 53.53% for SR test, respectively. The mass percentage of the undisturbed soil presented a down-up trend with decreasing fragment size distribution, with the maximum value for the mass percentage of fragments < 0.053 mm. For the WSR test, except for the fragment size 0.25–0.5 mm and the fragment size 0.106–0.25 mm, the mass percentage of the remaining fragment size was significantly different from the mass percentage of the undisturbed soil aggregate (P < 0.05); the mass percentage of fragment size < 1 mm in the WSR test was 37.12%, 6.42%, 8.75%, 50.49% and 45.09% greater than those in the undisturbed soil, which showed that the fragment size > 1 mm were broken into aggregate fragment size < 1 mm. For the SR test, the mass percentage of the fragment size > 1 and < 0.053 mm was significantly different from the mass percentage of undisturbed soil aggregate (P < 0.05); the mass percentages in SR test were greater than those of the undisturbed soil for all fragment sizes except > 1 and 0.106–0.5 mm. Comparing WSR test with SR test,
Table 2 Main raindrop parameters of simulated rainfall. Rainfall intensity (mm h−1)
⎜
3. Results
At the end of each test, all of the splashed aggregate fragments resulting from the five different raindrop diameters and the five splash
Raindrop velocity (m s−1)
W (δ > di ) d = (3 − D) lg ⎛ i ⎞ W0 d ⎝ max ⎠
where W(δ > di) is the cumulative mass of soil aggregate fractions with size δ greater than the comparative size di; W0 is the total mass of the fractions; di is the average of the sieve size range (dj, dj + 1); and dmax is the maximum fraction. Significant differences in the mass percentages of the aggregate fragment sizes for the different raindrop diameters or splash distances under the WSR and SR tests, and for the WSR, SR tests and the undisturbed soil were detected using a one-way analysis of variance (ANOVA) followed by LSD’s test (P < 0.05). Independent-Samples T–Test were used for significant differences in the mass percentages of the same aggregate fragment sizes under the WSR and SR tests for the same raindrop diameter or splash distance, and the values were statistically significant at the 95% confidence level. The relationship between the raindrop diameter and fractal dimension for the WSR and SR tests was analyzed using a simple regression method. All statistical analyses were performed using SPSS 16.0 (IBM SPSS Software, Armonk, NY, United States), and all figures were processed in Origin 8.5 (OriginLab Corporation, Northampton, MA, United States).
2.3. Aggregate fragment size distribution
Raindrop diameter (mm)
⎟
10–5 10–5 10–5 10–5 10–5
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fragment size < 0.053 mm for the WSR and SR tests was significantly different (P < 0.05), and the mass percentage of the fragment size < 0.053 mm for the SR test was 1.04 and 1.15 times of the WSR test. Generally, no significant differences were observed in the mass percentage of fragment size > 2 mm among the five raindrop diameters for the WSR and SR tests (P < 0.05). The mass percentages of fragment sizes 1–2, 0.5–1, 0.25–0.5, 0.106–0.25, and 0.053–0.106 mm presented a fluctuating variable trend with an increase in raindrop diameter (WSR and SR tests). The mass percentages of fragment size < 0.053 mm for the different raindrop diameters in WSR test were significantly different (P < 0.05). In SR test, no significant differences were observed in the mass percentages of fragment size < 0.053 mm for raindrop diameters 3.05–4.05 mm (P > 0.05), but the mass percentages for those diameters were significantly higher than that of fragment size < 0.053 mm for the 2.67 mm raindrop diameter (P < 0.05). Comparing the WSR and SR tests (Fig. 3), for the fragment size > 2, 1–2, and 0.5–1 mm, when the raindrop diameters were 2.67, 3.05 and 3.39 mm, there was no significant difference in the mass percentage of splash aggregates between WSR and SR tests (P > 0.05) in the case of 55.56%. But the mass percentage in WSR test was higher than that in SR test except for the raindrop diameter 3.05 mm for the fragment size 1–2 mm, which was 1.12–6.52 times that of SR test. These results indicate that alcohol has a good protective effect on fragment size > 0.5 mm under these rainfall conditions. However, for the raindrop diameters 3.79 and 4.05 mm, there was no significant difference between WSR test and SR test (P > 0.05), except for the fragment size 0.5–1 mm for the raindrop diameter 4.05 mm. For the fragment size 0.25–0.5, 0.106–0.25 and 0.053–0.106 mm, there was no significant difference in the mass percentage of splash aggregates between WSR and SR tests (P > 0.05) in the case of 66.67%. However, the mass percentage of splash aggregates in WSR test increased by 10.62–46.49%, 9.14–48.53% and 0.68–67.95% compared with SR test except for those of the 2.67 mm raindrop diameter and fragments 0.25–0.5 mm with a raindrop diameter of 3.39 mm. The mass percentage of fragment size < 0.053 mm was significantly greater for WSR test than for SR test for all raindrop diameters (P < 0.05).
Fig. 2. Fragment size distribution for WSR, SR and the undisturbed soil tests. Note: Different small letters indicate a significant difference (P < 0.05) in the mass percentage for the same fragment size and different tests.
the difference of the mass percentage of the fragment size > 0.053 mm was not significant (P > 0.05), but the mass percentage of the fragment size > 0.053 mm of WSR test was 1.13–1.39 times that of SR test, and the mass percentage of the fragment size < 0.053 mm was 0.83 times that of SR test.
3.2. Effect of raindrop diameter on aggregate fragment size distribution for the WSR and SR tests The distribution of fragment size for different raindrop diameters in the WSR and SR tests is shown in Fig. 3. In general, for WSR and SR tests, the mass percentage of the splash aggregate fractions presented an up-down-up trend as the fragment size distribution decreased; there were two peaks in the fragment size 0.5–1 and < 0.053 mm. Except for the 2.67 mm raindrop diameter in the WSR test, the mass percentages of fragment size < 0.053 mm for WSR and SR were significantly greater than for 0.5–1 mm fragments (P < 0.05), which were 1.75–6.44 and 1.62–5.98 times greater, respectively. Comparing the WSR and SR tests (Fig. 3), when the raindrop diameter was 2.67 mm, the WSR and SR tests were significantly different (P < 0.05) for the mass percentage of the fragment size > 2, 1–2, 0.5–1 and < 0.053 mm, and the mass percentages of fragment size > 2, 1–2 and 0.5–1 mm were greater for WSR test than for SR test, which was 6.52, 2.13 and 1.27 times that of the SR test; the mass percentage of fragment size < 0.5 mm was lower for WSR test than for the SR test, which was 1.44–43.20% lower than the SR test. For the 3.05 mm raindrop diameter, the difference between WSR and SR tests was not significant (P > 0.05) for the mass percentage of the fragment size > 2, 1–2 and 0.106–0.25 mm, but the mass percentages of fragment size > 2, 0.5–1, 0.25–0.5, 0.106–0.25 and 0.053–0.106 mm of WSR test were greater than that of SR test; the mass percentages of fragment size < 0.053 mm for WSR test was 24.36% lower than for SR test. For the 3.39 mm raindrop diameter, the difference of WSR and SR test was not significant except for the fragment size < 0.053 mm (P > 0.05). For the raindrop diameters of 3.79 and 4.05 mm, the difference between the WSR and SR test was not significant (P > 0.05) for the mass percentage of the fragment size > 1 mm; the difference between the WSR and SR test was significantly different in the case of 50% (P < 0.05) for the mass percentage of the fragment size 0.5–1, 0.25–0.5, 0.106–0.25 and 0.053–0.106 mm, and the mass percentages for WSR test increased by 17.93–27.72% and 10.62–37.24% compared with SR test under other conditions, except for the raindrop diameter 3.79 mm for the fragment size 0.5–1 mm. The mass percentage of the
3.3. Characteristics of fragment size distributions at different splash distances for the WSR and SR tests The fragment size distributions at different splash distances for the WSR and SR tests are shown in Fig. 4. On the whole, the mass percentage presented a fluctuating rising trend with decreasing fragment size for all splash distances (P < 0.05). For the splash distance 0–10 cm, there was no significant difference between WSR test and SR test (P > 0.05) except for the fragment size 0.5–1 and < 0.053 mm; but for > 0.053 mm particle size, the mass percentage in WSR test was higher than that in SR test, and it increased by 4.85–44.78% compared with SR test; for < 0.053 mm particle size, the mass percentage of splashed aggregates in WSR test was 17.18% lower than that in SR test. For the splash distance 10–20, 20–30 and 30–40 cm, when the fragment size > 0.053 mm, there was no significant difference in the mass percentage of splash aggregates between WSR and SR tests (P > 0.05) in the case of 50%; however, except for the splash distance 10–20 cm for the fragment size > 2 mm, the mass percentage of the WSR test was higher than that of the SR test, and increased by 0.65–35.67%, 2.59–75.10% and 1.07–46.76% compared with the SR test, respectively; when the fragment size < 0.053 mm, the mass percentage in WSR and SR test was significantly different (P < 0.05), and mass percentage in WSR test were 11.79%, 14.89% and 16.21% lower than that in SR test, respectively. For the splash distance 40–50 cm, there was no significant difference in the mass percentage of splash aggregates between WSR and SR tests (P > 0.05) except for fragment size 0.25–0.5 and < 0.053 mm; when the fragment size was < 0.053 4
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Fig. 3. Fragment size distribution for different raindrop diameters for the WSR and SR tests. Note: Different capital letters indicate a significant difference (P < 0.05) in mass percentage for the same raindrop diameter and different fragment sizes, and different small letters indicate a significant difference (P < 0.05) in mass percentage for the same fragment size and different raindrop diameters. Different symbols indicate a significant difference (P < 0.05) in mass percentage for the WSR and SR tests under the same raindrop diameters and fragment size.
4. Discussion
mm, the mass percentage in WSR test was significantly reduced by 13.18% compared with the SR test (P < 0.05). For the fragment size > 2 mm (Fig. 4), the splashed aggregate existed only in the splash distance 0–10 and 10–20 cm, and the difference of mass percentage between the WSR and SR tests was not significant (P > 0.05). The mass percentage of the splash distance 0–10 cm was significantly higher than other splash distances (P < 0.05). For the fragment size 1–2 mm, there was no significant difference in the mass percentage between WSR and SR tests (P > 0.05) in most cases (80%). For the fragment size 0.5–1, 0.25–0.5 and 0.106–0.25 mm, the WSR and SR tests were not significantly different in the case of 40% for all splash distances (P > 0.05), but the mass percentage in WSR test was higher than the SR test and increased by 14.46–46.46%, 4.85–46.76% and 13.70–32.98% compared with the SR test, respectively. For the fragment size 0.053–0.106 mm, there was no significant difference between WSR and SR tests for all splash distances (P > 0.05), but the mass percentage in WSR test was higher than that in SR test, and increases by 0.65–22.94% compared with SR test. For the fragment size < 0.053 mm, the difference of mass percentage between WSR and SR tests was significant (P < 0.05), and the mass percentage in WSR test were lower than that in SR test; the minimum values of WSR and SR tests were 40.06% and 48.36% at the splash distance 0–10 cm, respectively.
Although the mass percentage in WSR test was not significantly different from SR test for the fragment size > 0.053 mm (Fig. 2), the mass percentage in WSR test was higher than that in SR test, which indicated that alcohol had a good protective effect on the aggregate in the WSR test and can weaken the secondary raindrop splash action. Under all rainfall conditions (Fig. 3), the mass percentage for the fragment size < 0.053 mm between WSR and SR tests was significantly different (P < 0.05), and mass percentage in WSR test were 43.20%, 24.36%, 7.20%, 4.08% and 12.83% lower than that in SR test, respectively. This further indicated that alcohol can weaken the secondary raindrop splash action. For the WSR and SR tests (Fig. 4), when the fragment size was > 2 mm, the aggregate were only present at the splash distances 0–10 and 10–20 cm. This was mainly because the soil particles splashed were affected by their own size and quality. In theory, the closer the distance was, the smaller the energy required for the soil particles, and the greater the distance, the larger the energy required. Therefore, there were more aggregates with fragment size > 2 mm deposited in close range (Legout et al., 2005; Fu, 2017). The mass percentages of fragment size 1–2, 0.5–1, 0.25–0.5, 0.106–0.25 and 0.053–0.106 mm in WSR and SR tests presented a fluctuating variable trend with the increase of splash distances (Fig. 4). Although there was no significant difference in the mass percentage between WSR and SR tests (P > 0.05) in the case
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Fig. 4. Fragment size distribution at different splash distances for the WSR and SR tests. Note: Different capital letters indicate a significant difference (P < 0.05) in mass percentage for the same splash distance and different fragment sizes, and different small letters indicate a significant difference (P < 0.05) in the mass percentage for the same fragment size and different splash distances. Different symbols indicate a significant difference (P < 0.05) in mass percentage for the WSR and SR tests under the same splash distances and fragment size.
microaggregates or finer aggregates (Figs. 2, 3, 4), which are preferentially removed by erosion (Adesodun et al., 2007). Hence, The aggregate and nutrient loss caused by the secondary raindrop splash erosion will increase the soil fertility decline, reduce the land productivity, and even increase the surface crust and nutrient loss (Hu et al., 2018; Ma et al., 2014). Ferrandino and Elmer (1996) examined the spatial dispersal of conidia of Septoria lycopersici (causal agent of Septoria leaf spot) during and immediately following rain events. The observed transport at a long distance suggests that at least some conidia are carried in very small rain droplets or secondary splash droplets. This suggests that secondary raindrop splash can cause long-spreading of spores, which may have a major impact on the epidemiology of the disease. Therefore, to reduce the occurrence of these phenomena, effective measures can be taken to prevent raindrops from directly splashing the topsoil, for example, using wheat straw cover to protect the soil (Jin et al., 2009; Zhang et al., 2009), thereby reducing secondary raindrop splash erosion. The relationship between the fractal dimension (D) of aggregate fragments and the raindrop diameter in the WSR and SR tests is shown in Fig. 5. For the same raindrop diameter, the D values of the SR test were higher than those of the WSR test, which indicated that more fine particles were produced by the SR test and that the degree of aggregate fragmentation was greater. Photos of aggregate fragments for the WSR and SR tests are shown in Fig. 6. The aggregate fragments in the WSR test had almost no dispersion or fragmentation due to the effect of the
50%, the mass percentage in WSR test was higher than SR except for the splash distance 40–50 mm for the fragment size 1–2 mm. And with the decrease of particle size distribution, the mass percentage of WSR test were 1.01–1.75, 1.14–1.46, 1.05–1.47, 1.14–1.33 and 1.03–1.23 times that of SR test, respectively. This indicated that in the WSR experiment, alcohol can inhibit the breakdown of > 0.053 mm fragment size, that was, reduce the damage of the secondary raindrop splash erosion on the aggregate. Under the condition of fragment size < 0.053 mm (Fig. 4), with the increase of splash distance, the mass percentage in WSR and SR tests was significantly different (P < 0.05), and the mass percentage in WSR test were 17.18%, 11.79%, 14.89%, 16.21% and 13.18% lower than that in SR test, respectively. Aggregates and organic matter are the foundation of soil structure and fertility, and they interact and are inseparable (Dou et al., 2011). Some studies have shown that large aggregates contain more organic matter and that nearly 90% of the soil organic matter is located in the aggregate fraction of the topsoil (Jastrow, 1996); the content of soil organic matter is also a sign of the aggregate stability (Beguería et al., 2015; Besalatpoura et al., 2013; Canasveras et al., 2010). It can be seen from the above research that the impact of raindrops on the soil is a continuous process of destruction, including the primary raindrop splash erosion by the initial raindrops and the secondary raindrop splash erosion which caused by the dispersed raindrops and the soil aggregate fragments again impacting the topsoil. And secondary raindrop splash erosion causes macroaggregates to break into more 6
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5. Conclusions The mass percentage of fragment size > 0.053 mm of the WSR test was 1.13–1.39 times that of the SR test, but the mass percentage of fragment size < 0.053 mm of WSR test was 0.83 times that of SR test. For the five raindrop diameters (2.67, 3.05, 3.39, 3.79 and 4.05 mm), the mass percentages of fragment size < 0.053 mm in WSR test were 43.20%, 24.36%, 7.20%, 4.08% and 12.83% lower than those in SR test, respectively. These results indicated that alcohol could weaken the secondary splashing effect. For the five splash distances (0–10 cm, 10–20 cm, 20–30 cm, 30–40 cm, and 40–50 cm), the mass percentages of the splashed aggregates showed a fluctuating variable trend with decreasing fragment size (P < 0.05), and the mass percentages of fragment size < 0.053 mm were 17.18%, 11.79%, 14.89%, 16.21% and 13.18% lower in WSR test than in SR test, respectively. These results showed that alcohol can protect fragment size > 0.053 mm at each splash distance. The fractal dimension (D) of aggregates in the WSR and SR tests showed a downward opening parabolic relationship with raindrop diameter, and the values of the fractal dimension (D) of SR test were higher than those of 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, indicating the lowest protective effect of alcohol on the aggregates and the largest degree of fragmentation, respectively. This study proposes the definition and scope of the secondary raindrop splash erosion, and it is verified, but this is only a preliminary exploration. Future research can be further studied from the topography, vegetation, soil structure and other factors.
Fig. 5. Relationship between the fractal dimension (D) of aggregate fragments and the raindrop diameter in the WSR and SR tests.
Acknowledgments This work was supported by the National Natural Science Foundation of China (41571262) and the Chinese Ministry of Water Resources Science and Technology Promotion (TG1308). Fig. 6. Photos of aggregate fragments for the WSR and SR tests (Post-test photographs). Note: splash distance 10–20 cm for a raindrop diameter of 3.39 mm.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catena.2019.104342.
alcohol, which significantly reduced the effect of secondary raindrop splash erosion. However, the aggregate fragments in the SR test had a large degree of dispersion and fragmentation. Regression analysis of fractal dimension (D) and raindrop diameter (d) allows us to conclude that the fractal dimension of the soil splash aggregate (D) changed as a quadratic function following an up-down trend with increasing raindrop diameter (d). The equation is D = m (d − n)2 + p , where R2 ≥ 0.98, P < 0.05, and m, n and p are parameters. When the raindrop diameters are 3.64 and 3.74 mm, the values of D in WSR and SR tests are at their maximum, respectively, with the lowest protective effect of alcohol on aggregates and the highest degree of fragmentation of aggregates (Castrignano and Stelluti, 1999). With a raindrop diameter of 2.67 mm, the D values of the splash aggregate fragments for WSR and SR were at their minimum 2.61 and 2.74, respectively. This may be due to the smaller energy of the raindrop diameter 2.67 mm (Table 2), causing fewer total aggregate fragments to be splashed but with a higher proportion of the larger particles splashed because of their mass. The D values of the splash aggregate fragments for a raindrop diameter of 4.05 mm were 2.82 and 2.86 for the WSR and SR tests, respectively. The raindrop diameter of 4.05 mm had the largest energy in the study (Table 2), so that the fragmentation degree of the aggregates is larger, and the macroaggregates are broken into microaggregates (Legout et al., 2005). Thus, the fractal dimension for a raindrop diameter of 4.05 mm is relatively higher than that of a raindrop diameter of 2.67 mm.
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