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Micro-characteristics of soil aggregate breakdown under raindrop action
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Micro-characteristics of soil aggregate breakdown under raindrop action Guanglu Lia,b,⁎, Yu Fub, Baiqiao Lib, Tenghui Zhengb, Faqi Wua, Guanyun Pengc, Tiqiao Xiaoc a b c
College of Resources and Environment, Northwest A & F University, Yangling 712100, China Institute of Soil and Water Conservation, Northwest A & F University, Yangling 712100, China Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Raindrop splash Soil aggregate breakdown Micro-characteristics Surface seal Shape index of aggregate
Raindrop splash is considered to be the first step in soil erosion, and it is also a contribution to such erosion. Aggregate breakdown due to raindrop splash causes crusting and soil erosion. In this study, the micro-characteristics of soil aggregate breakdown under the action of three sizes of raindrops were examined using synchrotron-based X-ray micro-computed tomography (SR-μ CT). The results showed that soil aggregate breakdown was related to the size of raindrops, was mainly caused by large raindrops during a rainfall event, and formed an enrichment zone, transition zone, and dense airtight zone of aggregates in the surface soil. The shape index and the amount of aggregate in the splashed soil was higher than in the un-splashed soil, especially for microaggregate fragments (< 25 μm). We confirm that the existence of large amounts of micro-aggregate fragments surrounding large aggregates in a rainfall event is a principle factor in the formation of soil surface seals and the clogging of pores.
1. Introduction Raindrop splash is responsible for initiating water erosion because it is the first erosive force to occur during an erosive rainfall event (Sempere et al., 1994; Hudson, 1995; Morgan, 2005; Cuomo et al., 2016). Although the aggregate breakdown caused by raindrop splash can result in pore blockage (Falsone et al., 2012) and the formation of a crust (Abu-Hamdeh et al., 2006; Le Bissonnais et al., 1989; Warrington et al., 2009; Sajjadi and Mahmoodabadi, 2015) on the surface soil, the fundamental processes associated with the size of raindrops remain unresolved. Raindrop splash depends on the number, the size and velocity of raindrops impacting the soil surface, which determines the rainfall kinetic energy per unit surface (Ramos et al., 2003; Angulo-Martínez et al., 2012). The detachment of soil aggregates by splash depends not only on the energy of raindrops but also on soil erodibility, which relies on soil physico-chemical characteristics such as infiltration capacity (Fox et al., 2007; Legout et al., 2005a), the nature of soil aggregates (Ma et al., 2014; Huang et al., 2010; Poesen and Torri, 1988), organic matter content (Le Bissonais, 1990) and other factors (Bronick and Lal, 2005). Soil aggregate stability has been used to indicate the resistance of soil to erosive agents and soil quality (Ghadir et al., 2007; Nichols and Toro, 2011; Shainberg et al., 1992; Raine and So, 1993; Jasinska et al., 2006). Aggregate breakdown and dispersion, which are caused by raindrop
⁎
impact, can decrease the soil porosity and the infiltration capacity, and cause surface sealing (Salles et al., 2000; Li et al., 2008; Gregorich et al., 1994; Feller and Beare, 1997). In addition, soil aggregates physically protect organic matter (Field et al., 2006; Li and Pang, 2014; Feller and Beare, 1997), which is important for carbon sequestration. Some researchers have found that when rainfall detachment is the dominant erosion process, the particle size distribution in an eroded soil differs from the original soil (Li and Pang, 2014; Slattery and Burt, 1997). Many studies have also shown that aggregate breakdown duo to the raindrop impact is likely to be a main factor affecting particle distribution of sediments (Fu et al., 2016; Fu et al., 2017; Hairsine et al., 1999). Aggregate breakdown produces smaller particle than present on the original soil, which may be displaced and reoriented into a more continuous structure. They clog conducting pores and, consequently, a surface seal is developed (Ramos et al., 2003). The particle distribution of the eroded soil can be influenced by the particle distribution of the original soil, the aggregate breakdown during erosive events and the setting velocity of different size classes of particles (Mahmoodabadi et al., 2014). The particle size distribution of an eroded soil also seems to be dependent on the erosive agent of rainfall and or runoff, flow hydraulic characteristics and slope gradient. Several methods for measuring soil aggregate stability have been developed (Le Bissonnais et al., 1989; Le Bissonais, 1990; Pierson and Mulla, 1989; Beare and Bruce, 1993; Loch and Foley, 1994; Amézketa et al., 1996). However, studies on the micro characteristics of soil aggregate breakdown using
Corresponding author at: College of Resources and Environment, Northwest A & F University, Yangling 712100, China. E-mail address: [email protected] (G. Li).
http://dx.doi.org/10.1016/j.catena.2017.10.027 Received 18 February 2017; Received in revised form 10 October 2017; Accepted 23 October 2017 0341-8162/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Li, G., Catena (2017), http://dx.doi.org/10.1016/j.catena.2017.10.027
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the synchrotron based X-ray micro-computed tomography (SR-μ CT) have not been reported. In a rainfall event, raindrops occur in various sizes, ranging from large raindrops with a diameter of up to 5 mm to small raindrops with a diameter of < 0.2 mm. The objectives of this research were to evaluate the relationships between rainfall kinetic energies and soil aggregate breakdown by using the synchrotron based X-ray micro-computed tomography (SR-μ CT) and to analyze the morphological characteristic of aggregate breakdown in the splash detachment and transport of aggregate fragments. It may be extremely important for us to reveal decreased infiltration captivity and the formation of crust in the process of soil erosion. 2. Materials and methods 2.1. Soil samples Soil samples were collected from Yangling, Shanxi Province of China (108°03′30.03″E, 34°18′25.95″N), which is a traditional agricultural planting region. This region has a warm sub-humid continental climate with an annual average temperature of 13 °C and annual precipitation of 550–650 mm, which mainly occurs in July, August and September. The studied soils were formed from erosive loess and are relatively deep. They exhibit a loam or silt-loam texture (according to the USDA particle size classification criteria) (Fu et al., 2016). Major crops grown in this region include maize (Zea mays L.) and winter wheat (Triticum aestivum Linn.). Thirty undisturbed soil samples were obtained with a cutting ring (10 cm diameter, 5 cm height) from the top soil layer of croplands (0–5 cm) using the diagonal method at five points. Bulk density was determined with oven-dried at 105 °C until constant mass as described by Blake and Hartge (1986), organic matter by using combustion method, total nitrogen was measured by the semi-micro-Kjeldahl method, soil texture (clay, silt and sand contents) was obtained by the pipette method, and results are shown in Table 1. Aggregate size distribution was determined by wet and dry sieving (Kemper and Rosenau, 1986), and the mass percentage of aggregate in un-splashed soil is also shown in Fig. 1. The obtained results showed that the mean bulk density, soil organic matter, total nitrogen, and total phosphorous for this soil were 1.37 g cm− 3, 1.46%, 1.04 g kg− 1, and 0.62 g kg− 1, respectively. The investigated soil was a silty loam with a mean content of silt and clay of 44.07% and 22.67% (according to the USDA particle size classification criteria), respectively.
Fig. 1. The fraction percentage for soil aggregate obtained by wet and dry sieving procedure.
equal to 1.9 mm, the terminal velocity of raindrops was calculated by the modified Newton formula; For simulated rainfall, the velocity should be further corrected by Eq. (2) because the raindrops do not reach the terminal velocity (Yao and Chen, 1993). According the raindrop mass and velocity, the splash energy is calculated by Eq. (3).
vi = (17.20 − 0.844d) 0.1d
V = vi 1 − e Ers =
n
∑i =0
d ≥ 1.9
2g − 2H vi
(1)
(2)
1 mV 2 2
(3) −1
where V is raindrop velocity(m s ), d is the raindrop diameter (mm), vi is the terminal velocity (m s− 1), H is the height of falling raindrop (m), Ers is the splash energy of raindrops (J m− 2 s− 1),m is the individual raindrop mass (g), i = 0,…, n is the number of raindrops, and g is the gravity acceleration (m s− 2). The raindrop sizes, splash energies and the main parameters of the simulated raindrops are shown in Table 2. The dry clods (a diameter size of approximately 5 mm) for surface impacted soil (a depth of 0–0.5 cm) and spilled sediment were obtained before and after each rainfall event. A total of 90 dry clods were selected and subjected to CT scanning.
2.2. Rainfall test 2.3. SR-μ CT scanning Small raindrops (with a diameter of 2.67 mm), medium raindrops (with a diameter of 3.39 mm) and large raindrops (with a diameter of 5.45 mm) were produced using a raindrop generator (Fig. 2A). The raindrop generator consisted of a cylindrical box with an open top measuring 10 cm in diameter and 10 cm in height. In the floor of the box, 21 syringe needles (US needle of sizes 7 and 16) were installed in a grid of 2 cm. Raindrop size was controlled by changing the needle size and by adjusting the height of the hydraulic head (Fu et al., 2017). The collecting device for the impacted raindrops consisted of stainless steel pan (110 cm diameter). A rainfall duration of 10 min was evaluated for each raindrop diameter, with 5 replications. For natural rainfall, when the raindrop diameter was greater than or
Synchrotron-based X-ray micro-computed tomography (SR-μ CT) (in Shanghai, China) was used to scan the soil clods (with a diameter size of approximately 5 mm) at an energy level of 24 kV and a detection distance of 11 cm, with a pixel size of 3.25 μm. Scanning was performed in continuous scans, and the scanning interval was 0.625 mm (Fig. 2B). After scanning and reorientation, 720 original images (tomo images) were tested for every soil clod (Fig. 2C). The original images were transferred into PITRE (Phase-sensitive X-ray Image Processing and Tomography Reconstruction) software (V3.1) to conduct phase retrieval and slice reconstruction (Chen et al., 2012; Chen et al., 2013). A total of 1500–2000 slices were produced for every soil clod with a pixel
Table 1 Characteristics of the soil at the study site (mean value ± SD). Soil type
Bulk density g cm− 3
Soil organic matter %
Total nitrogen g kg− 1
Total phosphorus g kg− 1
Sand (%) > 0.02 mm
Silt (%) 0.02–0.002 mm
Clay (%) < 0.002 mm
Loess
1.37 ± 0.13
1.46 ± 0.03
1.04 ± 0.02
0.62 ± 0.02
33.36 ± 0.02
44.07 ± 0.03
22.67 ± 0.02
2
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Fig. 2. Photographs of the rainfall simulator (A), the synchrotron-based X-ray micro-computed tomography (SR-μ CT) (B), the tomo images (C), the slices (D), and the 3-D aggregate structure (E) used in the experiment.
Table 2 Raindrop sizes and main parameters for the simulated rainfall tests. Raindrop size mm
Drop height m
Raindrop velocity m s− 1
Rainfall intensity mm h− 1
Splash energy J m− 2 s− 1
Small Medium Large
2.00 2.00 2.00
5.36 5.48 5.61
5.76 68.61 169.72
2.41E − 05 5.15E − 05 2.24E − 04
2.67 3.39 5.45
size of 1052 × 1052 images (Fig. 2D), and a 3-D aggregate structure for every soil clod was produced using VG studio MAX2.1 (Fig. 2E). 2.4. Aggregate analysis The slice images were analyzed using Image Pro Plus 6.0 software to examine the characteristics of the aggregates. Approximately 15–20 slice images for each soil clod were obtained in 100 slices in a grid per interval and were used to analyze the sizes of aggregate fractions. The mean number, mean diameter, mean perimeter and area of the aggregate fragments for each soil clod in the slice image were calculated. Eight classes of aggregate fragments (> 2000, 2000–1000, 1000–500, 500–250, 250–106, 106–53, 53–25 and < 25 μm) were divided according to aggregate size. The technical route diagram and testing procedures are shown in Fig. 3.
Fig. 3. The technical route diagram and testing procedures performed in this study.
energy: 2.41E− 05 J m− 2 s− 1) becomes more disperse, and the small aggregate fragments are much more abundant and loosely arranged surrounding the large aggregate fragments (Fig. 4B). After the surface soil is impacted by medium raindrops (splash energy: 5.15E − 05 J m− 2 s− 1), small aggregate fragments increase surrounding the large aggregate fragments and a dense aggregate structure begins to appear in areas of the soil clod (Fig. 4C). However, the soil aggregate structure splashed by large raindrops (splash energy: 2.24E − 04 J m− 2 s− 1) becomes compacted and denser where the small aggregate fractions surrounding the large aggregate fractions are more abundant and show mutual adhesion around the large aggregates. Larger aggregate structures also developed (Fig. 4D). This results reveal micro-aggregates of breakdown are clustered around large aggregates, and form the enrichment zone, transition zone, and dense airtight zone of aggregates after the surface soil. The formation of dense airtight zone in the surface soil is closely related to surface seals and the clogging of pores. In general, aggregate breakdown occurs when soil strength is
2.5. Statistical analyses Differences between the measured parameters within groups were analyzed using one-way analysis of variance (ANOVA) and Fisher's protected least significant difference (LSD) test. 3. Results and discussion 3.1. Micro-characteristics of aggregate structure The characteristics of the aggregate structure under the impacts of different raindrops and rainfall kinetic energies are significantly different between un-splashed and splashed soils. Fig. 4A shows that the particle fragments of aggregate structure in un-splashed soil arrange loose and homogeneous, and the small aggregate fragments are less abundant surrounding the large aggregate fragments. The particle fragments of aggregate structure impacted by small raindrops (splash 3
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Fig. 4. 2-D microstructures of aggregate fractions in dry clods of surface soil (depth of 0–5 mm). A, The soil aggregate structure in un-splashed soil. B, The aggregate structure in soil impacted by small raindrops of 2.67 mm). C, The aggregate structure in soil impacted by medium raindrops of 3.39 mm). D, The aggregate structure in soil splashed by large raindrops of 5.45 mm).
reduced by wetting to a level in which the stress imposed by raindrops is sufficient to disrupt the aggregate (Assouline, 2004). The main mechanisms of aggregate breakdown during water erosion processes are slaking by fast wetting and mechanical breakdown due to raindrop impact (Legout et al., 2005b; Shi et al., 2010). Our results show that with an increase in the raindrop size, rainfall intensity and rainfall kinetic energy, the particle fragments of aggregate structure are smaller and denser in surface soil. Consequently, when aggregates are broken down by raindrop impact, the disaggregated particles are deposited within the upper soil pore spaces, forming a surface seal of a thin, dense and low-permeable layer (Assouline, 2004). Beuselinck et al. (1999) thought that in lower stream powers, finer particles were transported selectively and large particles remained on soil surface; however, with increasing stream power, larger particles were also transported. The finding in this study revealed that the redistribution of particles or aggregates on the surface of eroding soil depends on aggregate size distribution as well as raindrop size, rain intensity and rainfall energy; also, our finding implies the importance of raindrop size, rainfall intensity and splash energy on aggregate breakdown and seal formation, which can control infiltration rate and the consequent runoff and erosion rates.
SIagg =
A P2
(4)
where SI agg of > 0.5 indicates round aggregates, SI agg of < 0.5 and > 0.05 indicates irregular aggregates, and SI agg of < 0.05 indicates elongated aggregates. The shape index of aggregates is the characterization of morphology for soil aggregate. The larger the value is, the higher the degree of spherical aggregate will be. We found some interesting results from the analysis of > 100 thousand aggregate data obtained from nearly 100 slices. The shape index of aggregates gradually increases as the size of aggregate fractions decreases in un-splashed and splashed soils, and no significant differences between un-splashed and splashed soils except for aggregate fractions of < 25 μm (P < 0.05) (Fig. 5). The largest values are 0.56 for aggregate fractions of < 25 μm in un-splashed soil, 0.66 in splashed soil by small raindrops, 0.72 in splashed soil by medium raindrops, and 0.73 in splashed soil by large raindrops. This founding indicated that the aggregate fragments of < 25 μm belong to round aggregates. The shape indices of aggregate fragments of 53–25 μm are > 0.05 and < 0.5, and belong to irregular shapes, and in the aggregate fractions of < 53 μm are < 0.05 and are elongated shapes. Through the above analysis, we found an interesting phenomenon: the aggregate fragments of < 25 μm are more round than the aggregate fragments of > 25 μm, and aggregate fragments of 53–25 μm are more round than the aggregate fragments of > 53 μm (Fig. 5). This results showed that the smaller the soil aggregate fragment is, the more rounded the particle shape is, and the larger a soil aggregate fragment is, the more irregular or elongated the particle shape is. Raindrops hitting the soil surface influence soil erosion and change the structure of aggregate in various ways (Kinnell, 2005), although the size of the
3.2. Shape characteristics of the aggregate fragments To determine the shape characteristics of the aggregate fragments in splashed and un-splashed soil, the concept of a shape index for soil aggregates is introduced in this study. The shape index of the aggregates (SI agg) is calculated as the ratio of the aggregate area (A) to the squared aggregate perimeter (P): 4
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Fig. 5. The shape index of aggregate fractions in un-splashed soil and splashed soils. Different capital letters indicate difference of significance at p < 0.05 in shape index for the same particle size at different treatments and different small letters indicate difference of significance at p < 0.05 in shape index among the different particle size at the same treatment.
drops is a key factor (Cerdà, 1997). The shape indices of aggregates < 0.53 and > 500 μm in splashed soils were higher than in un-splashed soils, indicating that the action of raindrop splash can change the shape characteristics of soil aggregates, and cause soil aggregates to tend to be more round. These findings may also implied that as soil aggregate shape becomes more regular or more round, and it is easier to suspend and transport in the process of water erosion, especially for the micro-aggregate fractions.
3.3. Amount of aggregate fragments and percentage of aggregate area The amount of aggregate fragment (No. mm− 2) is the number of fragments of a size in a soil clod, and the percentage of aggregate area (%) is the ratio of the aggregate area of a particle size aggregate to the total aggregate area in a soil clod, which two indicators reflect the distribution of aggregate fragments in the soil. The amount of aggregate fragments increases with a decrease in the aggregate size, especially for the fragments of < 25 μm (Fig. 6A). The total amount of aggregate fragments per area is 432 No. mm− 2 in unsplashed soil, 570 No. mm− 2, 594 No. mm− 2 and 608 No. mm− 2 in splashed soils of small, medium and large raindrops, respectively. The maximum percentage of aggregate area is 38.09% in un-splashed soil for the 1000–500 μm aggregate fragments, 35.99% in soil splashed by small raindrops, 31.35% in soil splashed by medium raindrops, and 40.83% in soil splashed by large raindrops. The differences for percentage of aggregate area are not significant among the different raindrop sizes (P < 0.05) (Fig. 6B). In addition, the percentage of aggregate area for the aggregate fragments of > 250 μm accounted for 84.20% of the total aggregate area in un-splashed soil, 84.71% in soil splashed by small raindrops, 84.01% in soil splashed by medium raindrops, and 91.49% in soil splashed by large raindrops. The amount of aggregate fragments in < 25 μm is significantly different between un-splashed and splashed soils (P < 0.05) (Fig. 6A). The amount of aggregate fragments increases with an increase in the raindrop size and splash energy, especially for the micro-aggregates of < 25 μm. The number of micro-aggregates in the soil splashed by large raindrops is much larger than in un-splashed and other splashed soils, but the number in splashed soil by large raindrops is lower. Although the number of micro-aggregates is high, the total percentage of aggregate area for micro-aggregates is relatively lower (Fig. 6B). This result shows that the main body of the soil aggregates is still dominated by large aggregates. The aggregate fragments of > 250 μm account for more of the soil volume than aggregate fragments of < 250 μm. After the impact by large raindrops, the amount of aggregate fractions of > 1 mm is significantly higher than following the impact of small and medium raindrops(P > 0.05), which may mean that the greater the rainfall intensity and kinetic energy, and the higher the degree of
Fig. 6. Amount of aggregate fragments and percentage of aggregate area measured via CT in un-splashed soil and splashed soils. A, The amount of aggregate fractions. B, The percentage of aggregate area. Different small letters and capital letters indicate difference of significance among the groups and within groups (p < 0.05). Different capital letters indicate difference of significance at p < 0.05 for the same particle size at different treatments and different small letters indicate difference of significance at p < 0.05 in amount of aggregate fractions among the different particle size at the same treatment.
aggregate breakdown will be, and the easier it will be for a structure consisting of large aggregates to form after soil drying. A surface seal is formed by raindrop impact, which further leads to slaking and breakdown of soil aggregates (Assouline, 2004). The development of a surface seal depends on the extent of the breakdown of surface aggregates, which depends on soil structure stability (Wick et al., 2014; Gelaw et al., 2015). However, raindrop splash leads to an increase of fine and micro-aggregate fragments (clay and silt), which is increased with an increase in the size of raindrops or the intensity of rainfall and impacted kinetic energy. The increase of micro-aggregates caused by raindrops is the basis of a surface seal and the reduction of infiltration rate (Fu et al., 2017). Some studies have shown that seal formation is a key factor in soil erosion processes because it can reduce the surface roughness as well as infiltration rate (IR) and soil loss by splashing (Robinson and Phillips, 2001; Assouline and Ben-Hur, 2006). Comparing the results in Figs. 1 and 6B, we find that there is a significant difference in the trend of aggregate distribution. However, by careful comparison, we find that the aggregate distribution in the particle fragments of < 1 mm obtained by image analysis (Fig. 6B) is partially consistent with the results obtained by traditional dry sieving (Fig. 1), and the distribution in the fragments of > 1 mm is very different. The reason for this difference may be that the mechanical dry sieving method does not separate multiple aggregates that are bonded together, whereas the image analysis can well identify individual aggregates that are joined together.
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4. Conclusions We found that aggregate breakdown during a rainfall event depends mostly on large raindrops, and a greater size of falling raindrops corresponds to a higher fragmentation of the aggregate. Micro-aggregates are clustered around large aggregates, forming an enrichment zone, transition zone, and dense airtight zone of aggregates. The splash of raindrops can change the morphological features of aggregate fragments, where a smaller soil aggregate corresponds to a more regular shape. The shape indices of aggregates, especially micro-aggregates, under the raindrop action are more round, causing it to be easier to suspend and transport in the processes of water erosion. The amount of aggregate fragments in the surface soil increases with an increase in the size of raindrop splashes, especially for micro-aggregates. However, large aggregate fractions of > 250 μm account for more of the soil volume than small aggregate fractions of < 250 μm. This study shows that micro-aggregates are largely detached from large aggregates by large raindrops, which may be a main cause for the formation of a surface seal and reduction of infiltration rate in the process of soil erosion. Acknowledgments The authors thank the anonymous reviewers for insightful comments on the original manuscript. Thanks for Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences for the technical support and guidance. This work was supported by the National Natural Science Foundation of China (Grant. No. 41571262) and the Chinese Ministry of Water Resources Science and Technology Promotion Program (TG1308). References Abu-Hamdeh, N.H., Abo-Qudais, S.A., Othman, A.M., 2006. Effect of soil aggregate size on infiltration and erosion characteristics. European journal. Soil Sci. 57, 609–616. Amézketa, E., Singer, M.J., Le Bissonnais, Y., 1996. Testing a new procedure for measuring water-stable aggregation. Soil Sci. Soc. Am. J. 60, 888–894. Angulo-Martínez, M., Beguería, S., Navas, A., Machín, J., 2012. Splash erosion under natural rainfall on three soil types in NE Spain. Geomorphology 175, 38–44. Assouline, S., 2004. Rainfall-induced soil surface sealing:a critical review of observations, conceptual models, and solutions. Vadose Zone J. 3, 570–591. Assouline, S., Ben-Hur, M., 2006. Effect of rainfall intensity and slope gradient on the dynamics of interrill erosion during soil surface sealing. Catena 66, 211–220. Beare, M.H., Bruce, R.R., 1993. A comparison of methods for measuring water-stable aggregates: implications for determining environmental effects on soil structure. Geoderma 56, 87–104. Beuselinck, L., Govers, G., Steegen, A., Quine, T.A., 1999. Sediment transport by overland flow over an area of net deposition. Hydrol. Proc. 13, 2769–2782. Blake, G.R., Hartge, K.H., 1986. Bulk density. In: Klute, A. (Ed.), Methods of Soil Analysis. Part1. Physical and Mineralogical Methods, Second Ed. American Society of Agronomy, Madison, WI, pp. 363–375. Bronick, C.J., Lal, R., 2005. Soil structure and management: a review. Geoderma 124, 3–22. Cerdà, A., 1997. Rainfall drop size distribution in Western Mediterranean Basin, València, Spain. Catena 31, 23–38. Chen, R.C., Dreossi, D., Mancini, L., 2012. PITRE: software for phase-sensitive X-ray image processing and tomography reconstruction. J. Synchrotron Radiat. 19, 836–845. Chen, R.C., Rigon, L., Longo, R., 2013. Comparison of single distance phase retrieval algorithms by considering different object composition and the effect of statistical and structural noise. Opt. Express 21, 7384–7399. Cuomo, S., Chareyre, B., d'Arista, P., Sala, M.D., Cascini, L., 2016. Micromechanical modelling of rainsplash erosion in unsaturated soils by discrete element method. Catena 147, 146–152. Falsone, G., Bonifacio, E., Zanini, E., 2012. Structure development in aggregates of poorly developed soils through the analysis of the pore system. Catena 95, 169–176. Feller, C., Beare, M.H., 1997. Physical control of soil organic matter dynamics in the tropics. Geoderma 79, 69–116. Field, D.J., Minasny, B., Gaggin, M., 2006. Modelling aggregate liberation and dispersion of three soil types exposed to ultrasonic agitation. Aust. J. Soil Res. 44, 497–502. Fox, D.M., Darboux, F., Carrega, P., 2007. Effects of fire-induced water repellency on soil aggregate stability, splash erosion, and saturated hydraulic conductivity for different size fractions. Hydrol. Process. 21, 2377–2384. Fu, Y., Li, G.L., Zheng, T.H., Li, B.Q., Zhang, T., 2016. Impact of raindrop characteristics on the selective detachment and transport of aggregate fragments in the loess plateau
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Micro-characteristics of soil aggregate breakdown under raindrop action Guanglu Lia,b,⁎, Yu Fub, Baiqiao Lib, Tenghui Zhengb, Faqi Wua, Guanyun Pengc, Tiqiao Xiaoc a b c
College of Resources and Environment, Northwest A & F University, Yangling 712100, China Institute of Soil and Water Conservation, Northwest A & F University, Yangling 712100, China Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Raindrop splash Soil aggregate breakdown Micro-characteristics Surface seal Shape index of aggregate
Raindrop splash is considered to be the first step in soil erosion, and it is also a contribution to such erosion. Aggregate breakdown due to raindrop splash causes crusting and soil erosion. In this study, the micro-characteristics of soil aggregate breakdown under the action of three sizes of raindrops were examined using synchrotron-based X-ray micro-computed tomography (SR-μ CT). The results showed that soil aggregate breakdown was related to the size of raindrops, was mainly caused by large raindrops during a rainfall event, and formed an enrichment zone, transition zone, and dense airtight zone of aggregates in the surface soil. The shape index and the amount of aggregate in the splashed soil was higher than in the un-splashed soil, especially for microaggregate fragments (< 25 μm). We confirm that the existence of large amounts of micro-aggregate fragments surrounding large aggregates in a rainfall event is a principle factor in the formation of soil surface seals and the clogging of pores.
1. Introduction Raindrop splash is responsible for initiating water erosion because it is the first erosive force to occur during an erosive rainfall event (Sempere et al., 1994; Hudson, 1995; Morgan, 2005; Cuomo et al., 2016). Although the aggregate breakdown caused by raindrop splash can result in pore blockage (Falsone et al., 2012) and the formation of a crust (Abu-Hamdeh et al., 2006; Le Bissonnais et al., 1989; Warrington et al., 2009; Sajjadi and Mahmoodabadi, 2015) on the surface soil, the fundamental processes associated with the size of raindrops remain unresolved. Raindrop splash depends on the number, the size and velocity of raindrops impacting the soil surface, which determines the rainfall kinetic energy per unit surface (Ramos et al., 2003; Angulo-Martínez et al., 2012). The detachment of soil aggregates by splash depends not only on the energy of raindrops but also on soil erodibility, which relies on soil physico-chemical characteristics such as infiltration capacity (Fox et al., 2007; Legout et al., 2005a), the nature of soil aggregates (Ma et al., 2014; Huang et al., 2010; Poesen and Torri, 1988), organic matter content (Le Bissonais, 1990) and other factors (Bronick and Lal, 2005). Soil aggregate stability has been used to indicate the resistance of soil to erosive agents and soil quality (Ghadir et al., 2007; Nichols and Toro, 2011; Shainberg et al., 1992; Raine and So, 1993; Jasinska et al., 2006). Aggregate breakdown and dispersion, which are caused by raindrop
⁎
impact, can decrease the soil porosity and the infiltration capacity, and cause surface sealing (Salles et al., 2000; Li et al., 2008; Gregorich et al., 1994; Feller and Beare, 1997). In addition, soil aggregates physically protect organic matter (Field et al., 2006; Li and Pang, 2014; Feller and Beare, 1997), which is important for carbon sequestration. Some researchers have found that when rainfall detachment is the dominant erosion process, the particle size distribution in an eroded soil differs from the original soil (Li and Pang, 2014; Slattery and Burt, 1997). Many studies have also shown that aggregate breakdown duo to the raindrop impact is likely to be a main factor affecting particle distribution of sediments (Fu et al., 2016; Fu et al., 2017; Hairsine et al., 1999). Aggregate breakdown produces smaller particle than present on the original soil, which may be displaced and reoriented into a more continuous structure. They clog conducting pores and, consequently, a surface seal is developed (Ramos et al., 2003). The particle distribution of the eroded soil can be influenced by the particle distribution of the original soil, the aggregate breakdown during erosive events and the setting velocity of different size classes of particles (Mahmoodabadi et al., 2014). The particle size distribution of an eroded soil also seems to be dependent on the erosive agent of rainfall and or runoff, flow hydraulic characteristics and slope gradient. Several methods for measuring soil aggregate stability have been developed (Le Bissonnais et al., 1989; Le Bissonais, 1990; Pierson and Mulla, 1989; Beare and Bruce, 1993; Loch and Foley, 1994; Amézketa et al., 1996). However, studies on the micro characteristics of soil aggregate breakdown using
Corresponding author at: College of Resources and Environment, Northwest A & F University, Yangling 712100, China. E-mail address: [email protected] (G. Li).
http://dx.doi.org/10.1016/j.catena.2017.10.027 Received 18 February 2017; Received in revised form 10 October 2017; Accepted 23 October 2017 0341-8162/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: Li, G., Catena (2017), http://dx.doi.org/10.1016/j.catena.2017.10.027
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the synchrotron based X-ray micro-computed tomography (SR-μ CT) have not been reported. In a rainfall event, raindrops occur in various sizes, ranging from large raindrops with a diameter of up to 5 mm to small raindrops with a diameter of < 0.2 mm. The objectives of this research were to evaluate the relationships between rainfall kinetic energies and soil aggregate breakdown by using the synchrotron based X-ray micro-computed tomography (SR-μ CT) and to analyze the morphological characteristic of aggregate breakdown in the splash detachment and transport of aggregate fragments. It may be extremely important for us to reveal decreased infiltration captivity and the formation of crust in the process of soil erosion. 2. Materials and methods 2.1. Soil samples Soil samples were collected from Yangling, Shanxi Province of China (108°03′30.03″E, 34°18′25.95″N), which is a traditional agricultural planting region. This region has a warm sub-humid continental climate with an annual average temperature of 13 °C and annual precipitation of 550–650 mm, which mainly occurs in July, August and September. The studied soils were formed from erosive loess and are relatively deep. They exhibit a loam or silt-loam texture (according to the USDA particle size classification criteria) (Fu et al., 2016). Major crops grown in this region include maize (Zea mays L.) and winter wheat (Triticum aestivum Linn.). Thirty undisturbed soil samples were obtained with a cutting ring (10 cm diameter, 5 cm height) from the top soil layer of croplands (0–5 cm) using the diagonal method at five points. Bulk density was determined with oven-dried at 105 °C until constant mass as described by Blake and Hartge (1986), organic matter by using combustion method, total nitrogen was measured by the semi-micro-Kjeldahl method, soil texture (clay, silt and sand contents) was obtained by the pipette method, and results are shown in Table 1. Aggregate size distribution was determined by wet and dry sieving (Kemper and Rosenau, 1986), and the mass percentage of aggregate in un-splashed soil is also shown in Fig. 1. The obtained results showed that the mean bulk density, soil organic matter, total nitrogen, and total phosphorous for this soil were 1.37 g cm− 3, 1.46%, 1.04 g kg− 1, and 0.62 g kg− 1, respectively. The investigated soil was a silty loam with a mean content of silt and clay of 44.07% and 22.67% (according to the USDA particle size classification criteria), respectively.
Fig. 1. The fraction percentage for soil aggregate obtained by wet and dry sieving procedure.
equal to 1.9 mm, the terminal velocity of raindrops was calculated by the modified Newton formula; For simulated rainfall, the velocity should be further corrected by Eq. (2) because the raindrops do not reach the terminal velocity (Yao and Chen, 1993). According the raindrop mass and velocity, the splash energy is calculated by Eq. (3).
vi = (17.20 − 0.844d) 0.1d
V = vi 1 − e Ers =
n
∑i =0
d ≥ 1.9
2g − 2H vi
(1)
(2)
1 mV 2 2
(3) −1
where V is raindrop velocity(m s ), d is the raindrop diameter (mm), vi is the terminal velocity (m s− 1), H is the height of falling raindrop (m), Ers is the splash energy of raindrops (J m− 2 s− 1),m is the individual raindrop mass (g), i = 0,…, n is the number of raindrops, and g is the gravity acceleration (m s− 2). The raindrop sizes, splash energies and the main parameters of the simulated raindrops are shown in Table 2. The dry clods (a diameter size of approximately 5 mm) for surface impacted soil (a depth of 0–0.5 cm) and spilled sediment were obtained before and after each rainfall event. A total of 90 dry clods were selected and subjected to CT scanning.
2.2. Rainfall test 2.3. SR-μ CT scanning Small raindrops (with a diameter of 2.67 mm), medium raindrops (with a diameter of 3.39 mm) and large raindrops (with a diameter of 5.45 mm) were produced using a raindrop generator (Fig. 2A). The raindrop generator consisted of a cylindrical box with an open top measuring 10 cm in diameter and 10 cm in height. In the floor of the box, 21 syringe needles (US needle of sizes 7 and 16) were installed in a grid of 2 cm. Raindrop size was controlled by changing the needle size and by adjusting the height of the hydraulic head (Fu et al., 2017). The collecting device for the impacted raindrops consisted of stainless steel pan (110 cm diameter). A rainfall duration of 10 min was evaluated for each raindrop diameter, with 5 replications. For natural rainfall, when the raindrop diameter was greater than or
Synchrotron-based X-ray micro-computed tomography (SR-μ CT) (in Shanghai, China) was used to scan the soil clods (with a diameter size of approximately 5 mm) at an energy level of 24 kV and a detection distance of 11 cm, with a pixel size of 3.25 μm. Scanning was performed in continuous scans, and the scanning interval was 0.625 mm (Fig. 2B). After scanning and reorientation, 720 original images (tomo images) were tested for every soil clod (Fig. 2C). The original images were transferred into PITRE (Phase-sensitive X-ray Image Processing and Tomography Reconstruction) software (V3.1) to conduct phase retrieval and slice reconstruction (Chen et al., 2012; Chen et al., 2013). A total of 1500–2000 slices were produced for every soil clod with a pixel
Table 1 Characteristics of the soil at the study site (mean value ± SD). Soil type
Bulk density g cm− 3
Soil organic matter %
Total nitrogen g kg− 1
Total phosphorus g kg− 1
Sand (%) > 0.02 mm
Silt (%) 0.02–0.002 mm
Clay (%) < 0.002 mm
Loess
1.37 ± 0.13
1.46 ± 0.03
1.04 ± 0.02
0.62 ± 0.02
33.36 ± 0.02
44.07 ± 0.03
22.67 ± 0.02
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Fig. 2. Photographs of the rainfall simulator (A), the synchrotron-based X-ray micro-computed tomography (SR-μ CT) (B), the tomo images (C), the slices (D), and the 3-D aggregate structure (E) used in the experiment.
Table 2 Raindrop sizes and main parameters for the simulated rainfall tests. Raindrop size mm
Drop height m
Raindrop velocity m s− 1
Rainfall intensity mm h− 1
Splash energy J m− 2 s− 1
Small Medium Large
2.00 2.00 2.00
5.36 5.48 5.61
5.76 68.61 169.72
2.41E − 05 5.15E − 05 2.24E − 04
2.67 3.39 5.45
size of 1052 × 1052 images (Fig. 2D), and a 3-D aggregate structure for every soil clod was produced using VG studio MAX2.1 (Fig. 2E). 2.4. Aggregate analysis The slice images were analyzed using Image Pro Plus 6.0 software to examine the characteristics of the aggregates. Approximately 15–20 slice images for each soil clod were obtained in 100 slices in a grid per interval and were used to analyze the sizes of aggregate fractions. The mean number, mean diameter, mean perimeter and area of the aggregate fragments for each soil clod in the slice image were calculated. Eight classes of aggregate fragments (> 2000, 2000–1000, 1000–500, 500–250, 250–106, 106–53, 53–25 and < 25 μm) were divided according to aggregate size. The technical route diagram and testing procedures are shown in Fig. 3.
Fig. 3. The technical route diagram and testing procedures performed in this study.
energy: 2.41E− 05 J m− 2 s− 1) becomes more disperse, and the small aggregate fragments are much more abundant and loosely arranged surrounding the large aggregate fragments (Fig. 4B). After the surface soil is impacted by medium raindrops (splash energy: 5.15E − 05 J m− 2 s− 1), small aggregate fragments increase surrounding the large aggregate fragments and a dense aggregate structure begins to appear in areas of the soil clod (Fig. 4C). However, the soil aggregate structure splashed by large raindrops (splash energy: 2.24E − 04 J m− 2 s− 1) becomes compacted and denser where the small aggregate fractions surrounding the large aggregate fractions are more abundant and show mutual adhesion around the large aggregates. Larger aggregate structures also developed (Fig. 4D). This results reveal micro-aggregates of breakdown are clustered around large aggregates, and form the enrichment zone, transition zone, and dense airtight zone of aggregates after the surface soil. The formation of dense airtight zone in the surface soil is closely related to surface seals and the clogging of pores. In general, aggregate breakdown occurs when soil strength is
2.5. Statistical analyses Differences between the measured parameters within groups were analyzed using one-way analysis of variance (ANOVA) and Fisher's protected least significant difference (LSD) test. 3. Results and discussion 3.1. Micro-characteristics of aggregate structure The characteristics of the aggregate structure under the impacts of different raindrops and rainfall kinetic energies are significantly different between un-splashed and splashed soils. Fig. 4A shows that the particle fragments of aggregate structure in un-splashed soil arrange loose and homogeneous, and the small aggregate fragments are less abundant surrounding the large aggregate fragments. The particle fragments of aggregate structure impacted by small raindrops (splash 3
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Fig. 4. 2-D microstructures of aggregate fractions in dry clods of surface soil (depth of 0–5 mm). A, The soil aggregate structure in un-splashed soil. B, The aggregate structure in soil impacted by small raindrops of 2.67 mm). C, The aggregate structure in soil impacted by medium raindrops of 3.39 mm). D, The aggregate structure in soil splashed by large raindrops of 5.45 mm).
reduced by wetting to a level in which the stress imposed by raindrops is sufficient to disrupt the aggregate (Assouline, 2004). The main mechanisms of aggregate breakdown during water erosion processes are slaking by fast wetting and mechanical breakdown due to raindrop impact (Legout et al., 2005b; Shi et al., 2010). Our results show that with an increase in the raindrop size, rainfall intensity and rainfall kinetic energy, the particle fragments of aggregate structure are smaller and denser in surface soil. Consequently, when aggregates are broken down by raindrop impact, the disaggregated particles are deposited within the upper soil pore spaces, forming a surface seal of a thin, dense and low-permeable layer (Assouline, 2004). Beuselinck et al. (1999) thought that in lower stream powers, finer particles were transported selectively and large particles remained on soil surface; however, with increasing stream power, larger particles were also transported. The finding in this study revealed that the redistribution of particles or aggregates on the surface of eroding soil depends on aggregate size distribution as well as raindrop size, rain intensity and rainfall energy; also, our finding implies the importance of raindrop size, rainfall intensity and splash energy on aggregate breakdown and seal formation, which can control infiltration rate and the consequent runoff and erosion rates.
SIagg =
A P2
(4)
where SI agg of > 0.5 indicates round aggregates, SI agg of < 0.5 and > 0.05 indicates irregular aggregates, and SI agg of < 0.05 indicates elongated aggregates. The shape index of aggregates is the characterization of morphology for soil aggregate. The larger the value is, the higher the degree of spherical aggregate will be. We found some interesting results from the analysis of > 100 thousand aggregate data obtained from nearly 100 slices. The shape index of aggregates gradually increases as the size of aggregate fractions decreases in un-splashed and splashed soils, and no significant differences between un-splashed and splashed soils except for aggregate fractions of < 25 μm (P < 0.05) (Fig. 5). The largest values are 0.56 for aggregate fractions of < 25 μm in un-splashed soil, 0.66 in splashed soil by small raindrops, 0.72 in splashed soil by medium raindrops, and 0.73 in splashed soil by large raindrops. This founding indicated that the aggregate fragments of < 25 μm belong to round aggregates. The shape indices of aggregate fragments of 53–25 μm are > 0.05 and < 0.5, and belong to irregular shapes, and in the aggregate fractions of < 53 μm are < 0.05 and are elongated shapes. Through the above analysis, we found an interesting phenomenon: the aggregate fragments of < 25 μm are more round than the aggregate fragments of > 25 μm, and aggregate fragments of 53–25 μm are more round than the aggregate fragments of > 53 μm (Fig. 5). This results showed that the smaller the soil aggregate fragment is, the more rounded the particle shape is, and the larger a soil aggregate fragment is, the more irregular or elongated the particle shape is. Raindrops hitting the soil surface influence soil erosion and change the structure of aggregate in various ways (Kinnell, 2005), although the size of the
3.2. Shape characteristics of the aggregate fragments To determine the shape characteristics of the aggregate fragments in splashed and un-splashed soil, the concept of a shape index for soil aggregates is introduced in this study. The shape index of the aggregates (SI agg) is calculated as the ratio of the aggregate area (A) to the squared aggregate perimeter (P): 4
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Fig. 5. The shape index of aggregate fractions in un-splashed soil and splashed soils. Different capital letters indicate difference of significance at p < 0.05 in shape index for the same particle size at different treatments and different small letters indicate difference of significance at p < 0.05 in shape index among the different particle size at the same treatment.
drops is a key factor (Cerdà, 1997). The shape indices of aggregates < 0.53 and > 500 μm in splashed soils were higher than in un-splashed soils, indicating that the action of raindrop splash can change the shape characteristics of soil aggregates, and cause soil aggregates to tend to be more round. These findings may also implied that as soil aggregate shape becomes more regular or more round, and it is easier to suspend and transport in the process of water erosion, especially for the micro-aggregate fractions.
3.3. Amount of aggregate fragments and percentage of aggregate area The amount of aggregate fragment (No. mm− 2) is the number of fragments of a size in a soil clod, and the percentage of aggregate area (%) is the ratio of the aggregate area of a particle size aggregate to the total aggregate area in a soil clod, which two indicators reflect the distribution of aggregate fragments in the soil. The amount of aggregate fragments increases with a decrease in the aggregate size, especially for the fragments of < 25 μm (Fig. 6A). The total amount of aggregate fragments per area is 432 No. mm− 2 in unsplashed soil, 570 No. mm− 2, 594 No. mm− 2 and 608 No. mm− 2 in splashed soils of small, medium and large raindrops, respectively. The maximum percentage of aggregate area is 38.09% in un-splashed soil for the 1000–500 μm aggregate fragments, 35.99% in soil splashed by small raindrops, 31.35% in soil splashed by medium raindrops, and 40.83% in soil splashed by large raindrops. The differences for percentage of aggregate area are not significant among the different raindrop sizes (P < 0.05) (Fig. 6B). In addition, the percentage of aggregate area for the aggregate fragments of > 250 μm accounted for 84.20% of the total aggregate area in un-splashed soil, 84.71% in soil splashed by small raindrops, 84.01% in soil splashed by medium raindrops, and 91.49% in soil splashed by large raindrops. The amount of aggregate fragments in < 25 μm is significantly different between un-splashed and splashed soils (P < 0.05) (Fig. 6A). The amount of aggregate fragments increases with an increase in the raindrop size and splash energy, especially for the micro-aggregates of < 25 μm. The number of micro-aggregates in the soil splashed by large raindrops is much larger than in un-splashed and other splashed soils, but the number in splashed soil by large raindrops is lower. Although the number of micro-aggregates is high, the total percentage of aggregate area for micro-aggregates is relatively lower (Fig. 6B). This result shows that the main body of the soil aggregates is still dominated by large aggregates. The aggregate fragments of > 250 μm account for more of the soil volume than aggregate fragments of < 250 μm. After the impact by large raindrops, the amount of aggregate fractions of > 1 mm is significantly higher than following the impact of small and medium raindrops(P > 0.05), which may mean that the greater the rainfall intensity and kinetic energy, and the higher the degree of
Fig. 6. Amount of aggregate fragments and percentage of aggregate area measured via CT in un-splashed soil and splashed soils. A, The amount of aggregate fractions. B, The percentage of aggregate area. Different small letters and capital letters indicate difference of significance among the groups and within groups (p < 0.05). Different capital letters indicate difference of significance at p < 0.05 for the same particle size at different treatments and different small letters indicate difference of significance at p < 0.05 in amount of aggregate fractions among the different particle size at the same treatment.
aggregate breakdown will be, and the easier it will be for a structure consisting of large aggregates to form after soil drying. A surface seal is formed by raindrop impact, which further leads to slaking and breakdown of soil aggregates (Assouline, 2004). The development of a surface seal depends on the extent of the breakdown of surface aggregates, which depends on soil structure stability (Wick et al., 2014; Gelaw et al., 2015). However, raindrop splash leads to an increase of fine and micro-aggregate fragments (clay and silt), which is increased with an increase in the size of raindrops or the intensity of rainfall and impacted kinetic energy. The increase of micro-aggregates caused by raindrops is the basis of a surface seal and the reduction of infiltration rate (Fu et al., 2017). Some studies have shown that seal formation is a key factor in soil erosion processes because it can reduce the surface roughness as well as infiltration rate (IR) and soil loss by splashing (Robinson and Phillips, 2001; Assouline and Ben-Hur, 2006). Comparing the results in Figs. 1 and 6B, we find that there is a significant difference in the trend of aggregate distribution. However, by careful comparison, we find that the aggregate distribution in the particle fragments of < 1 mm obtained by image analysis (Fig. 6B) is partially consistent with the results obtained by traditional dry sieving (Fig. 1), and the distribution in the fragments of > 1 mm is very different. The reason for this difference may be that the mechanical dry sieving method does not separate multiple aggregates that are bonded together, whereas the image analysis can well identify individual aggregates that are joined together.
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4. Conclusions We found that aggregate breakdown during a rainfall event depends mostly on large raindrops, and a greater size of falling raindrops corresponds to a higher fragmentation of the aggregate. Micro-aggregates are clustered around large aggregates, forming an enrichment zone, transition zone, and dense airtight zone of aggregates. The splash of raindrops can change the morphological features of aggregate fragments, where a smaller soil aggregate corresponds to a more regular shape. The shape indices of aggregates, especially micro-aggregates, under the raindrop action are more round, causing it to be easier to suspend and transport in the processes of water erosion. The amount of aggregate fragments in the surface soil increases with an increase in the size of raindrop splashes, especially for micro-aggregates. However, large aggregate fractions of > 250 μm account for more of the soil volume than small aggregate fractions of < 250 μm. This study shows that micro-aggregates are largely detached from large aggregates by large raindrops, which may be a main cause for the formation of a surface seal and reduction of infiltration rate in the process of soil erosion. Acknowledgments The authors thank the anonymous reviewers for insightful comments on the original manuscript. Thanks for Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences for the technical support and guidance. This work was supported by the National Natural Science Foundation of China (Grant. No. 41571262) and the Chinese Ministry of Water Resources Science and Technology Promotion Program (TG1308). References Abu-Hamdeh, N.H., Abo-Qudais, S.A., Othman, A.M., 2006. Effect of soil aggregate size on infiltration and erosion characteristics. European journal. Soil Sci. 57, 609–616. Amézketa, E., Singer, M.J., Le Bissonnais, Y., 1996. Testing a new procedure for measuring water-stable aggregation. Soil Sci. Soc. Am. J. 60, 888–894. Angulo-Martínez, M., Beguería, S., Navas, A., Machín, J., 2012. Splash erosion under natural rainfall on three soil types in NE Spain. Geomorphology 175, 38–44. Assouline, S., 2004. Rainfall-induced soil surface sealing:a critical review of observations, conceptual models, and solutions. Vadose Zone J. 3, 570–591. Assouline, S., Ben-Hur, M., 2006. Effect of rainfall intensity and slope gradient on the dynamics of interrill erosion during soil surface sealing. Catena 66, 211–220. Beare, M.H., Bruce, R.R., 1993. A comparison of methods for measuring water-stable aggregates: implications for determining environmental effects on soil structure. Geoderma 56, 87–104. Beuselinck, L., Govers, G., Steegen, A., Quine, T.A., 1999. Sediment transport by overland flow over an area of net deposition. Hydrol. Proc. 13, 2769–2782. Blake, G.R., Hartge, K.H., 1986. Bulk density. In: Klute, A. (Ed.), Methods of Soil Analysis. Part1. Physical and Mineralogical Methods, Second Ed. American Society of Agronomy, Madison, WI, pp. 363–375. Bronick, C.J., Lal, R., 2005. Soil structure and management: a review. Geoderma 124, 3–22. Cerdà, A., 1997. Rainfall drop size distribution in Western Mediterranean Basin, València, Spain. Catena 31, 23–38. Chen, R.C., Dreossi, D., Mancini, L., 2012. PITRE: software for phase-sensitive X-ray image processing and tomography reconstruction. J. Synchrotron Radiat. 19, 836–845. Chen, R.C., Rigon, L., Longo, R., 2013. Comparison of single distance phase retrieval algorithms by considering different object composition and the effect of statistical and structural noise. Opt. Express 21, 7384–7399. Cuomo, S., Chareyre, B., d'Arista, P., Sala, M.D., Cascini, L., 2016. Micromechanical modelling of rainsplash erosion in unsaturated soils by discrete element method. Catena 147, 146–152. Falsone, G., Bonifacio, E., Zanini, E., 2012. Structure development in aggregates of poorly developed soils through the analysis of the pore system. Catena 95, 169–176. Feller, C., Beare, M.H., 1997. Physical control of soil organic matter dynamics in the tropics. Geoderma 79, 69–116. Field, D.J., Minasny, B., Gaggin, M., 2006. Modelling aggregate liberation and dispersion of three soil types exposed to ultrasonic agitation. Aust. J. Soil Res. 44, 497–502. Fox, D.M., Darboux, F., Carrega, P., 2007. Effects of fire-induced water repellency on soil aggregate stability, splash erosion, and saturated hydraulic conductivity for different size fractions. Hydrol. Process. 21, 2377–2384. Fu, Y., Li, G.L., Zheng, T.H., Li, B.Q., Zhang, T., 2016. Impact of raindrop characteristics on the selective detachment and transport of aggregate fragments in the loess plateau
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