Effect of drying–wetting cycles on aggregate breakdown for yellow–brown earths in karst areas
© The Author(s). 2017
Received: 20 April 2017
Accepted: 7 September 2017
Published: 15 September 2017
Drying and rewetting process, frequently occurred during climatic changes, is an important process in soil aggregate slacking and dissolution. The severer interference of human activities on global climate makes the extreme climate scenarios like drought and rainstorm occur frequently. Therefore, there is necessity to further our understanding on the impact of the drying–wetting cycles and initial water content on the breakdown of soil aggregates. The typical yellow–brown earth composed of water–stable and water–unstable aggregates is selected. Variations of water-stable aggregate size distributions after drying-wetting cycles are measured by wet sieving, under variable initial water content and cycles respectively.
Drying-wetting cycles cause a significant aggregate slaking, especially within the first two cycles. After that, most aggregates show more slacking resistant. The variation curves of the proportion of water-stable aggregates with the size 1–5 mm shows a coexistence of slaking process and supplement. The critical initial water content (about 24%) and turning point (with the aggregate size of 0.3 mm) are proposed to describe the effects of initial water content on size distribution of water-stable aggregates. Overall, the increase of initial water content strengths the water stability. In addition, the mathematical model for the relative leakage ratio based on the drying–wetting cycle, initial water content and size distribution are established.
The findings reported in this paper may be capable of supporting the intensive study for the breakdown mechanism and assessing the leakage potential under the influence of climate change. However, there exists a certain mismatch between the drying-wetting cycles in the tests and in practice, mainly in the frequency and intensity, which should be paid more attention.
The karstification can induce land degradation, vegetation coverage loss, excess soil erosion, and the karst rocky desertification eventually (Wang et al., 2004). All these conditions lead to severe pressure on geo–disaster reduction and social resilience to disasters. As the most predominant process during the formation of karst rocky desertification, large amounts of studies have documented the aspects of soil erosion. Lal (2003) described the soil erosion process as a four-stage process involving detachment of particles, breakdown of aggregates, transport and redistribution of sediments over the landscape and deposition in depressional sites or aquatic ecosystems. Foster (1982), Hogarth et al. (2004), Rouhipour et al. (2006), Walker et al. (2007), An et al. (2012) and Parlange et al. (2012) have conducted model, mathematical and simulated methods to investigate the impacts of rainfall on soil erosion. However, literatures concerning the mechanisms of water-induced soil erosion are not expressive. Tabeda and Tomita (1972) studied the mechanism of soil erosion by correlating soil erosion with rainfall intensity based on the energy of raindrops. Fujiwara and Yamamoto (1981) searched the mechanism of soil erosion under simulated rainfall. Owoputi (1994) considered mechanism of sediment detachment in the soil erosion process. Relevant indicators of vast soil degradation and erosion include texture and porosity spoilage (Figueiredo et al., 1999; Jankauskas et al., 2008), water retention capacity decline (Ye et al., 2011) and soil leanness accompanied by chemical and biologic characteristics degradation (de Paz et al., 2006) have also been studied. Based on the analysis of the indicting parameters, several attempts at assessing and evaluating soil erosion have been made including those by Olson and Wischmeier (1963), Rose et al. (1983), Smith et al. (1999), Kheir et al. (2008), Febles-González et al. (2012), Tang et al. (2015), and by Nearing et al. (1990), Shelton and Wall (1998) and Feeser and O’Connell (2009) referring to some predictions.
The yellow–brown earths, forming on the Quaternary red clay or weathered material of limestone in hot, humid and rainy climate conditions in Guizhou karst areas, are viewed as the main body of soil erosion inherently and considered to be problematic in poor water-holding capacity, unstable structures, and even lean crop productivity (Gao and Li, 2007). As the basic unit of soil structure of the yellow–brown earth, the stability of aggregate is one of the most important factors controlling soil erodibility (Duiker et al., 2001). Soil aggregates are formed by the primary soil particles and soil cemented materials with the coagulation, cementation and cohesive actions, the stability of which is an important indicator for the soil quality evaluation. According to the different ways to destroy the soil aggregates, the stability of soil aggregates can be divided into mechanical stability, water stability, chemical stability, acidic and basic stability and biostability (Domżł et al., 1993). Water stability can be defined as the property and capacity of soil aggregates to resist hydraulic damage, and this stability may include wet aggregate stability (macro-aggregate stability >250 μm) and dispersible clay (micro-aggregate stability <250 μm) (Amezketa, 1999).
As the important indicator of soil erosion–resistance in the areas with rich runoff, the water stability of soil aggregates has been received considerable attention. Two main groups of factors affecting soil aggregate stability can be considered: soil primary characteristics or internal factors like clay mineralogy (Reichert et al., 2009) and organic matter (Jozefaciuk and Czachor, 2014), and external factors to the soils such as drying–wetting cycles, freezing-thawing cycles (Dagesse, 2012), biological factor (Jastrow and Miller, 1991), tillage (Bartlová et al., 2015) and so on. In nature, most surficial soils are subjected to drying–wetting cycles due to alternative rainfall and evapo-transpiration. During these cycles most of the properties of soils, especially their strength and hydraulic stability, are severely affected and as a result crack propagation and stability failure occur (Kemper et al., 1985; Denef et al., 2001a,b; Peng et al., 2007). However, the effects of drying-wetting cycles on water stability of soil aggregates have been considered to be variable. Pires et al. (2007) suggested that wetting-drying cycles can be used to repair some structurally damaged soils. Soulides and Allison (1961), Tisdall et al. (1978) found that drying-wetting cycles decreased the water-stability of soil aggregates. Hofman (1976) obtained a contrary result that the aggregate instability of aggregates sieved immediately after sampling was greater than that obtained when the soil was first air-dried and then rewetted to its original water content. In the research of Utomo and Dexter (1982), wetting and drying first increased the water stability of aggregates disturbed by tillage to a maximum value and then decreased it with the further wetting and drying.
Through the above literature review, the following potential problems arise. The effects of drying and rewetting process on soil aggregate stability are inconsistent, largely depending on soil type, percent of stabilizer, test methods and curing conditions, as forementioned. Thus, the effects on the erodible earths in karst areas, especially in the slacking process, the changes of the aggregates with different sizes and the water stability with further cycles, need to be studied. In addition, the role of the initial water content in the effects of drying–wetting cycles on the water stability has not been studied. For the quantitative assessment of soil leakage potential, Igwe et al. (1995) listed six aggregate indices to predict soil leakage potential. However no attempts have focus on the soil leakage potential considering the drying–wetting cycles and the initial water content.
Therefore, the objective of this study was (i) to describe the slaking behavior of a yellow-brown earth depending on variable drying and rewetting processes, (ii) to present the effect of the initial water content on size distribution of water-stable aggregates and (iii) to relate leakage potential to the drying–wetting cycles and initial water content.
Regional geological and hydrogeological conditions
The strata in the research area are mainly composed of post–Triassic limestone, dolomite and loose deposits. Soil parent material is Quaternary red clay. The soil type contains yellow–brown earth, red–brown earth and limestone soil with soil texture of silty clay, silty clay loam and clay loam. The surface soil is thin and poor in continuity, the average thickness of which is only 10–30 cm (Fig. 2d). It lacks intermediate transition layer between the lower parent rock and the surface soils. The geological structure shows the two–layer structure (carbonate rock and residual cohesive soil).
Due to the lithology and tectonics of carbonate rocks, the groundwater system in the research area is fully developed in limestone (T2g2), and is mainly along the northwest tectonic fissure. The thickness of the vertical seepage zone is small. The groundwater depth is generally 3–10 m and the average hydraulic gradient is about 7‰. The surface water system is not complete, and is mostly composed of closed depressions, sinkholes and karst springs. Generally, the lakes and springs are the seasonal ones.
Materials – Physical and mechanical properties of yellow–brown earths
Physical property indexes of intact yellow silty clay
w L (%)
w P (%)
Wet sieving method
In this paper, the water stability of soil aggregates was measured considering the effects of drying–wetting cycles and the initial water content. Therefore, the soil samples were divided into two series. Series I were the samples with the field–moist content affected by drying–wetting cycles of different cyclic numbers in five replicates. Series II were the samples with different initial water contents. In series I, 100 g of field–moist soil aggregates were subjected to drying–wetting cycles with the cyclic period of 192 h respectively. In series II, the equivalent of 100 g of soil aggregates with the initial water contents of 5.08%, 14.51%, 24.73%, 29.68%, 37.52% were prepared by the air–dried method.
A modification of the method of Yoder (1936) was used to measure the water stability of aggregates by wet sieving. 50 g of pre–prepared soil aggregates from series I or II were broken up to pass through a set of five sieves having 5.0, 2.0, 1.0, 0.5 and 0.25 mm aperture mesh respectively. The water level was adjusted so that the aggregates on the upper sieve were just submerged at the highest point of oscillation. The oscillation rate was 30 cycles per minute, the amplitude of the sieving action was 4.0 cm and the period of sieving was 10 min. The sieved materials remained in the five sieves and the part through all the sieves were selected and oven–dried respectively for at least 8 h. By weighting the six parts of dried materials, the mass percentage of soil aggregates in the range of >5, 5–2, 2–1, 1–0.5, 0.5–0.25, and <0.25 mm can be determined respectively.
Drying and wetting cyclic test
To describe the effects of the drying–wetting cycles on aggregate breakdown and water stability of aggregates, the yellow–brown earth samples were subjected to several drying–wetting cycles. In the present study, 10 drying–wetting cycles were applied. Each cycle was consisted of 96 h drying of the soil samples by air dry oven with 40 °C (Relative Humidity – 34%), and 96 h wetting by immersing the soil samples in water at room temperature of 20 °C. From the climatic changes in Fig. 3 we can know that the interval between two precipitations is approximately 7–10 d. Thus, we determine the duration of each drying-wetting cycle as 8 d. In drying period, the surface temperature under direct sunshine is about 40 °C and the air dry oven is used to make the air-drying occur sufficiently slowly and tenderly so as to decrease the influence at most. In rewetting process, we apply the immersing method, which is not as violent as in nature. So we reasonably extend the rewetting duration, which should be improved in the following research in order to restore the real field conditions as far as possible. The wet sieving was applied to the subsamples in the wetting state of the first, second, fifth and tenth cycles to obtain the mass contents of the water–stable aggregates.
Results and discussion
Water stability of soil aggregate
Impact of drying–wetting cycles
The distribution of grain size of soil aggregates under drying–wetting cycles
Mass percent content of soil aggregates in each size range (%)
Characteristic parameters of soil aggregates under drying–wetting cycles
WSA5 / %
WSA0.25 / %
SR5 / %
SR0.25 / %
MWD / mm
GMD / mm
The difference in the percent content of oven–dried aggregates in different soil samples may not be significant, but the contents of water–stable aggregates vary a lot. Therefore, the proportion of water–stable aggregates at each size range can better reflect the quality of soil aggregates. The relationships between the mass percent content of water–stable aggregates in different size range and the drying–wetting cycles are illustrated in Fig. 5b. With the increase of drying–wetting cycles, the percentage of the water–stable aggregates in the size range of 5–10 mm decreases obviously and the initial two cycles has the most violent effect on the disintegration of aggregates, which is consistent with the result by Denef et al. (2001a,b). After 10 times cycles, the aggregates in the size range of 5–10 mm remain almost no residue. Due to the supplement from the disintegration of the aggregates (5–10 mm) in the first cycle, the proportion of the aggregates (2–5 mm) increases slightly. After that period, the proportion of the aggregates (2–5 mm) decreases and remains almost no residue eventually. For the aggregates in the diameter of 1–2 mm, drying–wetting cycles first increase the proportion slightly; after this maximum, further drying–wetting cycles decrease the proportion. On the whole, with the further drying–wetting cycles, the proportion of the aggregates (1–2 mm) shows almostly no change. We speculate that there are two possibilities: one is that the supplement from slaking of greater aggregates and the slaking of aggregates with the size of 1–2 mm reach the equilibrium state; another is the coexistence of breakdown of larger water–stable aggregates and reunion of smaller water–stable aggregates (Degens and Sparling, 1995). Unfortunately, the results of this research are unable to distinguish.
The aggregates in the diameter of 1–2 mm are in the dynamic equilibrium state between the two reverse processes. It can also be seen from Fig. 5b that the proportions of water– stable aggregates in the size range of 0.5–1 mm, 0.25–0.5 mm and <0.25 mm increase with the increase of the drying–wetting cycles mainly due to the breakdown of the aggregates with the greater sizes. In the initial 1–2 cycles, the proportions increase sharply and then slow down to be stable. During the drying–wetting cycles, the supplement from the breakdown of larger aggregates holds the dominant position.
Figure 6b shows the proportion of water–stable aggregates with the diameter greater than 5 mm (WSA 5) and 0.25 mm (WSA 0.25) under the drying–wetting cycles. The variations of the corresponding slaking ratio of water–stable aggregates with the diameter greater than 5 mm (SR 5) and 0.25 mm (SR 0.25) are also illustrated. The figure directly shows that the drying–wetting cycles cause the breakdown of soil aggregates gradually. The proportions of WSA 5 and WSA 0.25 reduce in the logarithmic form with the increase of drying–wetting cycles, and SR 5 and SR 0.25 increase in the logarithmic form with the increase of drying–wetting cycles accordingly. After 10 times of cycles, there are almostly no aggregates larger than 5 mm remained. The results show that the water stability of soil aggregates has been weakened with the increase of drying–wetting cycles. The more the drying–wetting cycles are, the higher the slaking ratio of water–stable aggregates is and the less the larger water–stable aggregates are.
The breakdown degree of water–stable aggregates by exogenic actions has a significant effect on corrosion resistance of earths. Generally, the breakdown mechanism of water–stable aggregates are as follows: the degrees of water intruding into the pores or cracks are unbalanced, making the thickening speed of diffusion layer among the particles and the condition of repulsive force exceeding the suction to be unbalanced. These unbalance stress relationships will furtherly cause the stress concentration. The breakdown will occur in the surface with the maximum difference between the repulsive force and van der Waals attractive force (Chinchalikar et al., 2012; McBride and Baveye, 2002). Hu et al. (2015) suggested two steps in aggregate breakdown when dried aggregates were re-wetted: (1) separating soil particles in aggregates to a distance of 1.2–1.4 nm between two adjacent particle surfaces by the surface hydration forces (the repulsive force); (2) breaking soil aggregates in a way of explosion or dispersion when the repulsive force was greater than the attractive force.
Impact of initial water content
The initial water content of soil, affected by climate, temperature and humidity, has important influence on the stability of aggregates and slope runoff. The initial water content correlates to the thickness of hydrated films surrounding the soil particles, namely the diffusion layer. The diffusion layer has a weakening effect on the suction among the soil particles, which means the increase of initial water content will weaken the matrix suction among the soil particles and reduce the infiltration of water. In addition, the diffusion layer will weaken the structural links among soil particles or micro–aggregates, causing the micro–aggregates to be more susceptible to external force. In addition, the increase of initial water content will decrease the effective void ratio, thereby narrowing the seepage space and weakening the intrusion effects.
The distribution of grain size of soil aggregates under different initial water contents
Initial water content w/%
Mass percent content of soil aggregates in each size range (%)
Characteristic parameters of soil aggregates under different initial water contents
Initial water content w/%
WSA5 / %
WSA0.25 / %
SR5 / %
SR0.25 / %
MWD / mm
GMD / mm
After wet sieving, the proportions of water–stable aggregates with the grain size greater than 5 mm (WSA 5) and 0.25 mm (WSA 0.25) with the initial water content are illustrated in Fig. 8b. The variations of the corresponding slaking ratio of water–stable aggregates with the diameter greater than 5 mm (SR 5) and 0.25 mm (SR 0.25) are also illustrated. The proportions of WSA 5 and WSA 0.25 increase linear with the increase of initial water content, and SR 5 and SR 0.25 increase linearly with the increase of initial water content accordingly. The results show that the water stability of soil aggregates enhances with the increase of initial water content. The increase of initial water content can obviously strengthen the resistance ability of soil aggregates on rainfall and runoff erosion, reducing the degree of disintegration of aggregates.
Mathematical model to estimate leakage ratio
In general, our results suggest that the drying-wetting cycles cause a significant aggregate slaking, especially within the first two cycles. However, the extent of the slaking process is significantly reduced with repeated drying-wetting cycles, showing that most aggregates become more slacking resistant. The variation curves of the proportion of water-stable aggregates with the size 1–5 mm firstly increase by drying and wetting to a maximum value and then decrease, showing a coexistence of slaking process and supplement.
We have found that there exists the critical water content (about 24%), with the initial water content less than the critical value soils are predominantly composed by smaller aggregates. A turning point has also been obtained that the contents of the water–stable aggregates larger than 0.3 mm increase with the increase of initial water content. Overall, the increase of initial water content strengths the water stability.
The mathematical model for the relative leakage ratio based on the drying–wetting cycles, the initial water content and the size distribution of water–stable aggregates may help propose more precise leakage model of loams in karst areas to assess the leakage potential. Further investigations are required to consider more reasonable match between the drying-wetting cycles in the tests and in practice, especially in the frequency and intensity.
The work presented in this paper was supported by the National Natural Science Foundation of China (Grant No. 41572285).
The work presented here was carried out in collaboration between all authors. TYQ, ZJ and XJ defined the research theme. XJ and ZJ designed methods and experiments, performed the statistical analysis, analyzed the data and interpreted the results. XJ and ZJ co–worked on conceived of the study, and participated in its design and coordination and helped to draft the manuscript, associated data collection and their interpretation. All authors revised and approved the final version of the manuscript.
The authors declare that they have no competing interests.
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