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Article

Impact of Freeze–Thaw Action on Soil Erodibility in the Permafrost Regions of the Sanjiangyuan Area Affected by Thermokarst Landslides

by
Bihui Wang
1,2,3,
Yidong Gu
1,2,3,
Kexin Zhou
1,2,3,
Shengnan Li
1,2,3,
Ce Zheng
1,2,3 and
Yudong Lu
1,2,3,*
1
School of Water and Environment, Chang’an University, 126 Yanta Road, Xi’an 710054, China
2
Key Laboratory of Subsurface Hydrology and Ecological Effect in Arid Region of the Ministry of Education, Chang’an University, Xi’an 710054, China
3
Key Laboratory of Eco-Hydrology and Water Security in Arid and Semi-Arid Regions of Ministry of Water Resources, Chang’an University, Xi’an 710054, China
*
Author to whom correspondence should be addressed.
Water 2025, 17(6), 818; https://doi.org/10.3390/w17060818
Submission received: 8 February 2025 / Revised: 28 February 2025 / Accepted: 11 March 2025 / Published: 12 March 2025
Browse Figures
Versions Notes

Abstract

:
The Sanjiangyuan region, known as the “Chinese Water Tower”, serves as a crucial ecological zone that is highly sensitive to climate change. In recent years, rising temperatures and increased precipitation have led to permafrost melt and frequent occurrences of thermokarst landslides, exacerbating soil erosion issues. Although studies have explored the impact of freeze–thaw action (FTA) on soil properties, research on this phenomenon within the unique geomorphological unit of thermokarst landslides, formed from degrading permafrost, remains sparse. This study, set against the backdrop of temperature-induced soil landslides, combines field investigations and controlled laboratory experiments on typical thermokarst landslide bodies within the permafrost region of Sanjiangyuan to systematically investigate the effects of FTA on the properties of soils within thermokarst landslides. Furthermore, this study employs the EPIC model to establish an empirical formula for the soil erodibility (SE) factor before and after freeze–thaw cycles (FTCs). The results indicate that: (1) FTCs significantly alter soil particle composition, reducing the content of clay particles in the surface soil while increasing the content of sand particles and the median particle size, thus compromising soil structure and enhancing erodibility. (2) FTA initially significantly increases soil organic matter content (OMC); however, as the number of FTCs increases, the magnitude of these changes diminishes. The initial moisture content of the soil significantly influences the effects of FTA, with more pronounced changes in particle composition and OMC in soils with higher moisture content. (3) With an increasing number of FTCs, the SE K-value first significantly increases and then tends to stabilize, showing significant differences across the cycles (1 to 15) (p < 0.05). This study reveals that FTCs, by altering the physicochemical properties of the soil, significantly increase SE, providing a scientific basis for soil erosion control and ecological environmental protection in the Sanjiangyuan area.

1. Introduction

The Sanjiangyuan region, often referred to as the “Chinese Water Tower”, serves as a critical ecological security barrier in China. It is also recognized as a sensitive and pivotal zone in global climate change dynamics [1,2,3]. Given its unique geographical and ecological significance, environmental issues within this region are of paramount importance for China’s ecological security and long-term development. Recent years have witnessed notable increases in temperature and precipitation in the Sanjiangyuan area, alongside enhanced climate instability. These changes have led to an elevation in ground temperatures, a reduction in permafrost area, an increase in the thickness of the active layer, and extended periods of permafrost thawing [4,5,6]. The intensification of freeze–thaw (FT) processes has resulted in frequent occurrences of phenomena such as thermokarst landslides, thermokarst ponds, and frost-induced debris flows [7,8]. The exacerbated freeze–thaw action (FTA) has induced alterations in soil properties [9,10] and diminished soil erosion resistance [11]. Moreover, freeze–thaw cycles (FTCs) alter soil moisture conductivity and capacity, thereby impacting surface runoff and soil permeability. This directly affects the hydrological processes of permafrost, alters surface parameters, accelerates soil degradation, and poses severe threats to the environment and land resources [12].
The essence of soil FT is the freezing and melting of soil water. Due to the different densities of water and ice, phase changes in soil water frequently cause frost heave and melt sink. This results in changes in the physical, chemical, and biological properties of the soil [13,14]. Studies have shown that FT changes soil water content, shear strength, organic matter content (OMC), and aggregate stability (AS) by destroying soil structure [15,16]. This increases soil erodibility (SE), making it more vulnerable to erosion.
Soil aggregates are an important part of soil. Their composition and stability are an important evaluation index of SE [17,18,19]. The stability of soil aggregates is affected by soil structure, OMC, initial soil water content, degree of FT, and other factors [20,21,22]. FT has both positive and negative effects on soil aggregates [15,21,23]. The influence of FT on AS is determined by the degree of FT of soil. The FTC count and soil initial water content are the main factors affecting the degree of FT [23,24].
Changes in soil organic matter under FT conditions are mediated through a combination of physical, chemical, and biological processes. FTCs lead to the fragmentation of large soil aggregates, exposing carbohydrates, fatty acids, and sterols, which subsequently increases the extractable amounts by two to three times the original levels, enhances microbial contact and utilization [22,25,26], and augments the adsorption capacity of the soil due to an increase in fine particles or clay that have larger specific surface areas. These changes facilitate the redistribution or leaching of organic matter [27,28]. Following FTCs, macromolecular organic matter bound to solid particles undergoes expansion and contraction, disrupting hydrogen bonds and releasing low-molecular-weight organic matter [29]. The movement of water during FT processes also transports organic matter [30], with increases in water content enhancing the organic matter at the freezing front [31]. Phase changes in water lead to contraction of organic matter and disruption of binding sites on soil particles, increasing the release of organic matter. The low temperatures result in the death of some microbial cells, releasing utilizable carbon sources such as sugars and amino acids, thereby increasing the release of dissolved organic matter [32,33]. With increasing numbers of FTCs, the content of soluble organic matter significantly increases [34].
In recent years, scholars both domestically and internationally have conducted extensive experiments on FTCs [9,21,35,36,37,38] to investigate their effects on soil moisture content, bulk density, organic matter, and soil particle composition. This research aims to provide evidence for soil erosion in permafrost regions, yet studies on the impact of FTA on thermokarst landslides formed due to permafrost degradation are scarce. Consequently, this study undertakes FT experiments on typical thermokarst landslide bodies in the source region of the Sanjiang permafrost area. It examines the effects of FTA on soil particle composition and organic matter, and it employs the EPIC model to establish empirical formulas for SE factors before and after FTCs in permafrost regions. The goal is to inform soil erosion prevention and control, as well as regional ecological environment protection and restoration efforts.

2. Materials and Methods

2.1. Overview of the Study Area

The Sanjiangyuan region, located in the hinterland of the Qinghai–Tibet Plateau (see Figure 1), is a critical ecological function zone in China. The area receives an average annual precipitation of approximately 400 mm. However, recent years have witnessed significant alterations in precipitation patterns, characterized by an uneven seasonal distribution. There has been an increase in rainfall during the summer and a decrease during the winter. These changes have had profound impacts on the regional climate and soil conditions, leading to increased soil moisture variability, which in turn affects vegetation growth and the stability of permafrost. The average temperature hovers around −5 °C, with a climate that is both cold and dry. The freezing period typically extends from October to April, while the thawing period occurs from May to September. Soil in the region primarily consists of alpine meadow soils and alpine desert soils, which are characterized by poor drainage and low fertility. Vegetation is predominantly composed of alpine meadows and shrubs, adapted to the high altitudes and cold climate conditions. Permafrost is widespread, especially above elevations of 4000 m, where it is more common, with thicknesses ranging from 20 to 80 m. The distribution of ground ice is closely linked to topography and geological conditions, usually concentrated in river valleys and low-lying areas. The active layer thickness varies seasonally; it increases during the summer due to higher temperatures, generally between 1.5 to 2 m, and experiences smaller changes during the winter, usually ranging from 0.5 to 1 m. The mean ground temperature varies between −1.5 °C and −3.5 °C, while the temperature at the base of the active layer remains between −0.5 °C and −1.5 °C. These natural characteristics collectively shape the unique ecological environment of the Sanjiangyuan area, exerting profound influences on regional water resources, biodiversity, and climate change.

2.2. Research Methods

2.2.1. FT Experiment

This study focuses on thermokarst landslides located between Wudaoliang and Fenghuoshan, adjacent to the Qinghai–Tibet Highway within the Sanjiangyuan Reserve (refer to Figure 1). In selected areas of typical thermokarst landslide activity, soil samples were collected using a stratified sampling method at various soil layer depths. During sampling, disturbance from plant roots was minimized by avoiding them. The collected soil samples were air-dried in the laboratory, ground, and sieved for subsequent analysis of soil properties.
The experiment employed a layered soil preparation approach, configuring the soil at initial moisture contents of 12%, 16%, and 20%. To ensure thorough mixing of soil and water, a sprayer was used to apply water in layers. Prior to watering, both the water temperature and room temperature were measured to maintain them within a range of 0 °C to 5 °C. The sprayer was kept at a distance of 20–30 cm from the soil surface to prevent damage from being too close or uneven moisture distribution from being too far. After soil preparation, the samples were sealed in an insulated foam box for 24 h to ensure uniform moisture distribution.
The FTC experiments were conducted in a temperature-controlled freezer, with temperatures set to −20 °C for freezing and 25–30 °C for thawing. Each FTC consisted of 12 h of freezing followed by 12 h of thawing, with a total of 0, 1, 3, 5, 7, 9, 12, and 15 cycles. During the freezing phase, the expansion of ice within the soil pores exerted compressive stress on the soil structure, leading to its degradation. During the thawing phase, the melting ice crystals altered the interparticle forces, and the redistribution of moisture also impacted the soil’s physical and chemical properties, thereby simulating the effects of natural freeze–thaw action on the soil. No drainage or irrigation was performed during the experiment to maintain natural moisture conditions.
Regarding sample quantity, for particle composition analysis, 24 samples were prepared across three initial moisture contents (12%, 16%, 20%) and eight FTC counts (0, 1, 3, 5, 7, 9, 12, 15) at a soil depth of 0–10 cm. Each sample was replicated three times for statistical analysis, totaling 72 replicates. For OMC, samples were prepared across four soil depths (0–10 cm, 10–20 cm, 20–30 cm, 30–40 cm), also considering three moisture contents and eight FTC counts, resulting in 96 samples, each replicated three times for statistical analysis, totaling 288 replicates.
The aim was to investigate, through laboratory simulation of the freeze–thaw process, the impact of FTA on the soil properties within the thermokarst landslide areas of the Sanjiangyuan region, providing a scientific basis for understanding the mechanisms of soil degradation in cold regions due to FTCs.

2.2.2. Soil Parameter Measurement and Erodibility Factor Estimation

For this study, the mechanical composition of the soil layers from 0 to 10 cm depth was measured using a Malvern 3000 (Malvern Panalytical, Malvern, UK) laser diffraction particle size analyzer before and after FTCs. The content of organic matter in the soil was determined via the dichromate oxidation method, using the following formula:
O r g a n i c   m a t t e r = C × 5 V 0 ( V 0 − V ) × 10 − 3 × 3.0 × 1.1 M × k × 1000
where C is the concentration of the standard solution at 0.80 mol·L−1 (1/6K2Cr2O7); 5 is the volume of the potassium dichromate standard solution (mL); V0 is the volume of FeSO4 used in the blank titration (mL); V is the volume of FeSO4 used in the sample titration (mL); 3.0 is the molar mass of a quarter of a carbon atom (g·mol−1); 1.1 is the oxidation correction factor; M is the mass of the air-dried soil sample (g); and k is the conversion coefficient from air-dried to oven-dried soil.
This study utilizes the K-factor from the Erosion Productivity Impact Calculator (EPIC) model, as proposed by Williams et al. [10], as an indicator of SE. The calculation formula is as follows:
K = 0.2 + 0.3 e x p 0.0256 × S A N × ( 1 − S I L 100 ) × S I L C L A + S I L 0.3 × 1.0 − 0.25 C C + e x p ( 4.22 − 2.95 C ) × 1.0 − 0.75 S N 1 S N 1 + e x p ( − 5.51 + 22.9 S N 1
where SAN represents the percentage of sand particles; SIL represents the percentage of silt particles; CLA represents the percentage of clay particles; C represents the organic carbon content (%); and SN1 = 1 − SAN/100.

2.2.3. Statistical Analysis

This study employed SPSS 26.0 software (SPSS, IBM Inc., New York, NY, USA) to carry out univariate analysis of variance (for comparing means across different groups) and correlation analysis (for assessing relationships between variables) on the experimental data. The significance level for all tests was set at p < 0.05.

3. Results

3.1. Impact of FTA on Soil Particle Composition

This research identified that FTA predominantly affects the surface soil layers (0 to 10 cm), significantly influencing the structure and physicochemical properties of the soil. Consequently, we examined changes in soil particle composition under FT conditions at varying moisture contents of the topsoil layer (0 to 10 cm) to explore the impact of FTA on the composition and size distribution of soil particles.
As illustrated in Figure 2, the composition of soil particles exhibited distinct patterns with the number of FTCs. During the FTCs, the content of clay particles (<0.002 mm), fine silt particles (0.002–0.01 mm), and coarse silt particles (0.01–0.05 mm) decreased, while the content of sand particles (0.05–1 mm) gradually increased. However, after 12 FTCs, the changes tended to stabilize. Overall, the analysis indicates that the FTCs significantly reduced the clay content and increased the sand content in the soil.
Table 1 presents the effects of various FTCs on the soil particle composition and the volumetric fractal dimension of the soil in the study area. The data analysis revealed that the FTCs were highly significantly correlated with both the median particle size (d50) and the volumetric fractal dimension of the soil particles (p < 0.01). During the FTCs, the volumetric fractal dimension of the soil particles showed a significant decreasing trend, whereas the median particle size (d50) exhibited a significant increasing trend. A larger d50 value indicates a higher proportion of larger soil particles, which correlates negatively with soil erosion rates; larger particles are more prone to being eroded, thus increasing SE. Consequently, as the median particle size d50 increases, SE also increases, suggesting that FTA enhances SE.
From Table 1, it is also evident that unfrozen soil primarily consists of silt particles, approximately 63.71%, with a lower content of sand particles, approximately 16.91%. Throughout the FTCs, the content of clay and silt particles significantly decreases (p < 0.01), while the content of sand particles increases with more FTCs. After 15 FTCs, the clay content decreases by about 5% and silt by about 12%, whereas sand content increases by about 18% compared to the unfrozen soil (0 cycles). The change in median particle size (d50) is significant during the 0 to 12 FTCs (p < 0.05), but the trend diminishes after 12 cycles. This is consistent with changes in soil particle sizes across different categories, as reflected in Figure 2 after 12 FTCs. The volumetric fractal dimension of soil particles tends to decrease gradually with increasing FTC counts, though not significantly (p > 0.05). This trend is primarily related to the cumulative content of soil particles from small to large sizes; during FTCs, changes occur across all particle size categories in the surface soil, resulting in minor variations in the volumetric fractal dimension.
From the analysis provided, it is evident that significant changes occurred in the composition of soil particles during the FTCs. Specifically, there was a reduction in the content of clay particles and silt particles, with a corresponding increase in the content of sand particles. This alteration primarily resulted from the migration of moisture and the physical movement of soil particles during the freeze–thaw process. During the freezing phase, the soil’s water content transitions from a liquid to a solid state, expanding in volume and consequently increasing the porosity between soil particles, which loosens the soil structure. At this time, larger soil particles, such as sand particles, due to their larger size and lower surface energy, move more easily within these pores and are pushed toward the soil surface. Conversely, smaller particles, such as clay particles and silt particles, with their greater specific surface area and higher surface energy, are more likely to adhere to the walls of the soil pores, thereby remaining fixed in place. This differential movement results in a relative increase in the content of sand particles and a decrease in the content of clay and silt particles. During the thawing phase, the frozen ice crystals melt, increasing the moisture content within the soil pores and further enlarging the porosity. At this stage, the water in the soil begins to move downward, transporting the finer clay and silt particles downward. Due to their smaller size, clay and silt particles are more readily carried by the water flow, whereas sand particles, with their larger size and higher density, are less likely to be displaced by the water movement. Thus, during the thawing process, the downward migration of clay and silt particles further decreases their content in the upper soil layers, while the content of sand particles relatively increases.
After 12 FTCs, the distribution of soil particles tends to stabilize. In the initial cycles, dynamic changes in soil particle composition were more pronounced, characterized by the migration of clay particles and silt particles to deeper soil layers and a relative increase in the content of sand particles. However, as the number of cycles increased, the distribution of soil particles gradually became more uniform. The migration capacity of clay particles and silt particles decreased, and the relative increase in sand particles also stabilized. Concurrently, the soil pore structure stabilized after multiple FTCs, and the migration pathways and velocities of moisture tended to stabilize as well, which limited further changes in particle composition. Additionally, although permeability and total fraction composition were not directly measured during the analysis, they significantly impact the migration of soil particles. Changes in permeability directly affect the velocity and pathways of moisture migration, thereby influencing the migration of clay particles and silt particles. In soils with higher permeability, moisture can move downward more quickly, thus facilitating the migration of clay particles; conversely, in soils with lower permeability, water flow is impeded, and the depth of migration of clay particles is also limited. Changes in the total fraction composition reflect the redistribution of soil particles, with a decrease in clay particles and silt particles and an increase in sand particles, which together lead to changes in the soil particle composition and consequently alter the physical properties of the soil.
Table 2 reveals that during the FTCs, the volumetric fractal dimension, median particle size (d50), and the content of clay (<0.002 mm), silt (0.002 mm–0.05 mm), and sand (0.05 mm–1 mm) particles are significantly influenced by the soil’s initial water content (p < 0.05). Notably, soils with a 20% moisture content exhibit significant differences in these parameters compared to those with 12% and 16% moisture contents (p < 0.01). From this analysis, it is evident that soil moisture content significantly impacts soil particle composition during FTCs, with higher moisture content leading to greater changes. Furthermore, the variation in clay content (<0.002 mm) during these cycles is highly correlated with the initial soil moisture content (p < 0.01), with significant differences observed between soils with high and low moisture contents, indicating that changes in particle composition during FTCs are predominantly driven by alterations in clay content.

3.2. The Impact of FTA on Soil Organic Matter

Experimental measurements were conducted to ascertain the pattern of variation in soil OMC under different initial soil moisture conditions and various soil strata across a range of FTC counts. As indicated in Table 3, there is a highly significant correlation between the changes in OMC and the number of FTCs (p < 0.01). Similarly, as shown in Figure 3, the content of organic matter in the soil exhibits a significant initial increase with the onset of the FTCs (p < 0.01). Further analysis of the variability in the soil OMC during the FTCs revealed that, during the first 0 to 12 cycles, the changes in OMC were highly significant as the number of FTCs increased (p < 0.01). However, once the FTC count reached 12, the increasing trend in OMC began to diminish, and although the changes remained statistically significant (p < 0.05), they did not reach the level of highly significant difference (p < 0.01). This suggests that the effects of FTCs on soil OMC also exhibit a threshold; beyond 12 cycles, the magnitude of change in OMC decreases, and the differences become less pronounced.
Variations in soil moisture content directly influence the degree of soil freezing and the severity of FT impacts on soil properties. Differences in soil moisture content result in distinct changes in soil OMC during FTCs. As demonstrated in Table 4, there is a highly significant relationship (p < 0.01) between the initial soil moisture content and the OMC of the soil. During the FTC process, soils with moisture contents of 12%, 16%, and 20% experienced increases in OMC of 18.2%, 21.3%, and 22.4%, respectively, after 15 FTCs compared to unfrozen soil. Notably, the soil with a 20% moisture content exhibited the largest increase in OMC. Changes in soil OMC across different initial moisture levels reached significant differences (p < 0.01).
The analysis indicates that soils with higher moisture contents show a more pronounced increase in OMC during FTCs compared to those with lower moisture contents. The primary reason for this is that soils with higher water content undergo more noticeable freeze expansion and thaw contraction during the freezing process, leading to faster changes in soil temperature. This accelerates soil aeration as well as the migration and transfer of water and nutrients. During the FTCs, microbial activity in the soil is enhanced, which increases the decomposition rates of soil organic carbon and plant residues, thereby promoting the migration and transformation of soil organic carbon. Additionally, soil aggregates are disrupted during the FTCs, exposing various forms of organic carbon contained within them, thus contributing to the notable increase in OMC in high-moisture soils. During the FTCs, soil organic matter that is typically difficult to detect becomes exposed, and larger organic materials such as straw and stubble within the soil are fragmented, reducing in volume. When determining the soil organic matter content, these fragmented organic materials are more easily ground and mixed into the soil, resulting in an increase in soil organic matter content after FTCs.
During the FTC, the soil at different depths freezes from the top downwards, leading to the redistribution of soil moisture and an increase in the heterogeneity of soil water content. This process, in turn, affects the degree of freezing and the intensity of FTAs. As indicated in Table 5, there is a highly significant correlation (p < 0.01) between the changes in soil OMC and the different soil depths during the FTCs. After 15 FTCs, the increases in OMC compared to unfrozen soil at depths of 0–10 cm, 10–20 cm, 20–30 cm, and 30–40 cm were 21.8%, 21.4%, 19.4%, and 19.1% respectively. The increases were more pronounced in the 0–10 cm and 10–20 cm layers compared to the 20–30 cm and 30–40 cm layers. Additionally, the changes in OMC at different soil depths were all statistically significant (p < 0.01).
From the analysis above, it is apparent that there is a noticeable increase in the OMC of surface soils during the FTCs. The underlying reason is that during the freezing process, soil temperature gradually decreases from top to bottom, creating a temperature gradient. Due to the lower temperature in the upper soil layers, the vapor pressure decreases, causing water vapor to migrate upwards from the lower layers, thereby increasing the water content in the surface soil. This redistribution of moisture not only alters the water content in the soil but also affects the migration pathways of soluble organic matter. As moisture migrates upward, soluble organic matter is also transported to the surface, leading to an increase in the OMC of surface soils. However, as the FTCs progress, changes also occur in the soil structure. The surface soil experiences more pronounced frost heave and thaw settlement effects, leading to increased porosity, enhanced aeration, and a faster rate of temperature change. These factors together facilitate the migration and transformation of moisture and nutrients. This intense physical disturbance initially causes a significant increase in the OMC of the surface soil, but as the cycles continue, the structure of the surface soil gradually stabilizes, and the magnitude of changes in OMC decreases. In contrast, deeper soils, due to their receipt of moisture from lower soil layers during the freezing process, maintain a relatively stable water content and are subjected to weaker frost heave and thaw settlement effects. Therefore, changes in the OMC of deeper soils are relatively minor. However, as the number of FTCs increases, the OMC in deeper soils also gradually increases. This is primarily because microbial activity in the deeper soils is activated during the FTCs, promoting the decomposition and transformation of organic matter, while the vertical migration of moisture also facilitates the redistribution of organic matter within the deeper soils.

3.3. The Impact of FTA on SE (K-Value)

SE is influenced by factors such as soil particle composition, OMC, and soil bulk density. Research indicates that FTA significantly affects the physicochemical properties of soil, thereby potentially altering the erodibility index (K-value) of the soil. Figure 4a illustrates the changes in the SE (K-value) throughout the FTCs. It is evident from the graph that SE generally exhibits an increasing trend during the FTCs. When the number of FTCs ranges from 0 to 7, there are significant differences in the erodibility K-values between the cycles (p < 0.01). After seven cycles, the SE increased by 0.0107 [t·ha·h/(ha·MJ·mm)], which constitutes a 19.55% increase compared to the soil that underwent no FTCs. Additionally, Figure 4a reveals that after seven cycles, as the number of FTCs continued to increase, the K-value of SE slightly decreased. After 9 cycles, up to 15 FTCs, the changes were relatively minor and not statistically significant (p > 0.05). The impact of FTCs on the surface soil moisture content also indicates that after five cycles, the soil moisture content decreased, reducing the intensity of FT effects and the extent of structural damage caused by freezing and thawing. Consequently, the fluctuation in the soil’s erodibility K-value decreased after seven cycles. Furthermore, a simple regression analysis showed a significant correlation between the SE K-value and the number of FTCs (1 to 15) (p < 0.05).
From the analysis above, it can be observed that with an increase in the number of FTCs, the erodibility (K-value) of the soil initially increases significantly, then stabilizes, and eventually reaches a state of dynamic equilibrium. The formation of this dynamic equilibrium can be explained from the perspectives of changes in the soil’s physical and chemical properties. In the early stages of the FTCs, the rearrangement of soil particles, loss of clay minerals, and an increase in sand particle content lead to a loosening of the soil structure, thereby significantly increasing SE. However, as the FTCs continue, the redistribution of moisture and migration of particles within the soil gradually reach a relatively stable state. At this point, the pore structure and particle composition of the soil no longer undergo significant changes, and the soil structure tends toward stability. Additionally, changes in the soil’s OMC also influence the stability of the soil structure to some extent. During the FTCs, an increase in soil OMC may contribute to the formation of soil aggregates, which can partially offset the destructive effects of FTA on the soil structure, resulting in no further significant increase in SE after multiple cycles.
From Figure 4b, it is observable that under varying levels of moisture content, SE tends to increase. However, with an increase in initial soil moisture content, the change in SE diminishes. Soils with lower moisture contents exhibit greater changes in erodibility, reaching statistical significance (p < 0.05). The changes are more pronounced during the initial zero to nine FTCs.
In permafrost regions undergoing FTCs, soils with varying initial moisture contents show highly significant differences in erodibility compared to soils that have not undergone any FTCs (p < 0.01). Specifically, soils with 12%, 16%, and 20% moisture contents exhibit increases in their erodibility K-values by 25.32%, 14.79%, and 12.18%, respectively, compared to the non-FT soil. This analysis confirms that FTA leads to an increase in the erodibility K-values across different initial moisture contents of soil.
As a kind of physical action, FT has a significant impact on soil physicochemical properties and soil structure, resulting in SE affected by FTA. Studying the change in SE (K) value before and after freezing and thawing in the permafrost region of China shows the changing situation of SE more directly. This study introduces an incremental coefficient, ∆, of the SE K-value to represent the change in SE K-value before and after freezing and thawing.
∆ = Kd/Kw
Here, Kd is the mean value of SE after the FTCs. Kw is the average erodibility of the unfrozen and thawed soil.
We calculated SE after FT and non-FT SE using the SE K-value increment coefficient ∆. The incremental coefficient of SE after FT and non-FT is 1.21; that is, Kfreeze–thaw = 1.21 Kunfrozen
From the above analysis, it can be seen that the FTC can lead to an increase in SE. The average change coefficient of SE before and after FT can be used as a simple evaluation and estimation of the effect of the increase in SE after the FTC in permafrost areas, namely Kfreeze–thaw = 1.21 Kunfrozen.
Although relationships between SE before and after FTCs have been established based on experimental data and statistical analysis within the Sanjiangyuan region, the applicability of the coefficient 1.21 in practical scenarios requires consideration of multiple factors. These factors include soil texture, OMC, climatic and moisture conditions, frequency of FTCs, and regional variations. Firstly, soil texture (such as the contents of sand particles, silt particles, and clay particles) and OMC are key determinants of the K-factor. Studies have shown that the K-values calculated using the EPIC model are more accurate when the organic matter content exceeds 12%. Therefore, in soils with high OMC, adjustments to the K-factor may be necessary depending on specific conditions. Secondly, meteorological conditions, such as precipitation and temperature, significantly influence SE. In humid regions with high rainfall intensity and frequency, higher SE may necessitate an increased K-factor; conversely, in arid regions, a lower K-factor might be more appropriate. Additionally, the frequency and intensity of FTCs directly affect the soil’s physical properties. As the number of FTCs increases, the content of dissolved organic carbon (DOC) in the soil increases, while the microbial biomass carbon (MBC) content decreases, indicating that FTCs significantly alter the soil’s physical and chemical properties. Therefore, in regions with frequent FTCs, adjustments to the K-factor may be needed based on the freeze–thaw frequency. Lastly, soil types and environmental conditions vary significantly across different regions, and adjustments to the K-factor should consider these regional characteristics. Thus, when applying the K-factor in different regions, it is necessary to calibrate it with local soil and meteorological data to ensure its accuracy and applicability.

4. Discussion

4.1. The Impact of FTA on Soil Particle Composition

Soil particle composition significantly affects SE. The size and distribution of soil particles determine the structure and stability of the soil. When the content of fine particles (such as clay and silt) is high, soil aggregation is enhanced, effectively reducing SE. Conversely, when the content of coarse particles (such as sand) increases, the soil structure becomes loose, porosity increases, and the soil is more susceptible to erosion by water, leading to a significant increase in SE. Under FTCs, the content of clay particles in this study decreased by about 5%, silt particles decreased by about 12%, and sand particles increased by about 18%. These changes indicate that FTCs significantly affect soil structure and increase SE.
During the freezing process, soil particles are not just fragments, but aggregates. This is because the fragmentation of coarse particles and the aggregation of fine particles are synchronized [39]. It can be seen from the results of FT experiments that before and after the FTC, the content of clay particles and silt particles in soil constantly changes and has a negative correlation. The results show that the transfer of water and salt causes the change in soil structure during the FTC. The initial stable state of soil particles changes through aggregation and fragmentation, resulting in changes in particle size distribution. At the same time, these changes may lead to further changes in the soil, including its composition, structure, and properties [40]. The research results of this study show that FTA will cause soil fragmentation and aggregation at different moisture contents and different FTC counts, thus affecting soil particle size distribution, which is obviously consistent with the research results of this study.
A study by Oztas and Fayetorbay [21] suggests that FTCs reduce the stability of soil aggregates, especially under conditions of high soil moisture, with these effects accumulating over successive cycles. The results of this study corroborate this, showing that with an increasing number of FTCs, the content of clay particles decreases while that of sand particles increases, indicating enhanced erodibility and reduced AS, as predicted by the EPIC model. After 15 FTCs, the change in sand particles is more pronounced than in silt particles, suggesting that FT has a more severe impact on larger soil aggregates than on smaller ones. However, findings from other researchers vary; Perfect et al. [41] noted that FTCs can enhance the stability of soil aggregates, though the extent of this increase is influenced by the initial water content of the soil. Lehrsch et al. [42] observed that the stability of surface soil aggregates increases with the number of FTCs, reaching maximum stability after two to three cycles, with clay content having a significant impact on AS. This is because during the FT process, moisture migrates within the soil, causing the upper soil layer to become moist due to an increase in water content, while the lower layers dry out due to a decrease in water content. In areas where water content increases, moisture freezes into ice crystals, expanding within the soil pores and disrupting the cohesive forces between soil particles, thus decreasing the stability of soil aggregates. Conversely, when the water content decreases and the soil dries, soil particles contract, increasing the cohesive forces between them, thereby enhancing AS.
The analysis above demonstrates that the particle size distribution of soil not only determines its texture but also affects its aggregability, stability, and erosion resistance. Soil particles are typically classified by size into clay particles (<0.002 mm), silt particles (0.002–0.05 mm), and sand particles (>0.05 mm). Soils with a higher content of clay particles generally exhibit stronger aggregability and stability because clay particles are smaller, have a larger specific surface area, and can promote soil particle aggregation through electrostatic and van der Waals forces. This aggregability enhances the soil’s resistance to erosion, as aggregates can reduce the direct impact of water on soil particles, thereby lowering the risk of erosion. Conversely, soils with a higher content of sand particles have lower aggregability and stability due to the larger size and smaller specific surface area of sand particles, which weakens their aggregative capacity and makes them more susceptible to being washed away by water flow. Therefore, the particle size distribution of soil has a significant impact on its physical properties and erosion resistance.

4.2. The Impact of FTA on Soil Organic Matter

The stability of soil organic carbon stores is crucial for their ability to mitigate greenhouse effects as carbon sinks. The dynamics of rapidly cycling organic carbon control the balance between inputs and outputs of soil organic carbon [43]. Soil organic matter is a key component of this rapid carbon cycling. Under FTCs, the content of organic matter in soil typically increases by an average of 20%. This suggests that FTCs significantly affect the stability of soil organic carbon stores.
After undergoing FTCs, soil OMC changes as moisture moves [44]. Research by Wang and others [45] indicates that the amount of soluble organic matter in soil tends to increase during the FT period. Some studies have revealed that the content of soil organic matter initially increases and reaches a peak early in the FT process, then begins to decline. This pattern shows an initial increase followed by a decrease as the number of FTCs grows [46]. From the results discussed in this study, it is clear that the first nine FTCs significantly increase OMC, but subsequent increases slow down and become less distinct. Furthermore, FTA alters the stability of soil aggregates, thereby affecting the physical protection of organic matter within these aggregates. FT processes disrupt aggregate structures, consequently releasing more soluble organic matter [30]. The content of soil organic matter also influences the post-FT stability of aggregates, with the stability of high-organic-matter soils typically increasing with successive FTCs [25], consistent with the findings of this study. Variations in OMC across different soil depths show significant differences (p < 0.01), with content generally increasing with soil depth. This pattern aligns with the findings of Wei et al. [47], primarily due to higher water content and poorer permeability in deeper soil layers, which are frozen during the cycle and whose thawing inhibits the downward movement of soil organic matter.
Research conducted by Wu et al. [48] demonstrates that FTCs accelerate the release and transformation of soil organic matter, thereby increasing the risk of its export via water flow and gaseous emissions. This process ultimately alters the distribution of soil organic matter. These findings are consistent with those discussed in this study, where FTA significantly increases the content of soil organic matter. This increase is attributed to enhanced microbial activity in the soil during FTCs, which accelerates the mineralization and decomposition rates of soil organic carbon and plant residues, thereby facilitating the migration and transformation of soil organic carbon. Additionally, soil aggregates are disrupted during FTCs, exposing various forms of organic carbon within the aggregates to varying degrees, leading to an increase in soil OMC throughout the cycles. Particularly in areas experiencing thermokarst landslides, the exposure of lower soil layers post-collapse reveals organic matter, which surges to the surface, further increasing the soil OMC. On the other hand, studies indicate that microbial communities undergo changes during FTCs [26], and between 6 and 40% of soil microbes may die off after such cycles [49]. This process enhances the potential release of dissolved nutrients from microbial cells, further increasing the levels of dissolved organic carbon and water-soluble organic carbon [50,51,52]. This mechanism aptly explains the observed increase in soil OMC with successive FTCs, as discussed in this study.

4.3. The Impact of FTA on SE

The SE factor (K-value) is an important indicator that measures the susceptibility of soil to erosion. The larger the K-value, the more easily the soil is eroded. It reflects the combined effects of soil particle composition, OMC, and soil structure on erosion. Under FTCs, the SE (K-value) in this study increased by an average of 17%. This indicates that FTCs significantly increased SE, making the soil more susceptible to erosion.
Researchers have studied how FTA affects SE, which is often measured by the K-value. According to studies like those by Wei Ning and others [53], both the soil’s water content and the number of FTCs significantly influence its erodibility. The experimental results in this study show that the initial moisture content of the soil, along with FTA, greatly impacts erodibility. Initially, as the number of FTCs increases, SE significantly increases. This is mainly because the content of clay particles decreases while the amounts of sand particles and organic matter increase, making the soil more prone to erosion. After many cycles, the soil’s erodibility tends to stabilize as the soil structure reaches a dynamic equilibrium, with the soil expansion and settling from the FT process nearly balancing out. Thus, as the experiments indicate, after multiple FTCs, changes in SE gradually become stable.
Kreyling has indicated that post-FTA, the onset of erosion is delayed compared to bare soil. However, for soils that have not undergone FTCs, an increase in water content reduces soil cohesion, which in turn increases erosion. Our findings corroborate this, showing that FT has a more pronounced impact on the erodibility of soils with high water content. This is due to the transformation of water into ice crystals during freezing, which expands in volume, fills the spaces between soil particles, and promotes the displacement of these particles. The greater the water content in the soil, the more significant the migration of water during freezing, leading to larger volume changes and increased soil porosity. Consequently, this process enhances the likelihood of soil being more susceptible to erosion.

5. Conclusions

This study, conducted on typical thermokarst landslide formations in the permafrost regions of the Sanjiangyuan area, employed FT experiments to investigate the impact of FTA on soil particle composition, OMC, and SE (expressed as the K-value). The principal findings are summarized below.
The FTCs markedly altered the soil particle composition. This was primarily manifested in the reduction of clay particles (<0.002 mm), fine silt particles (0.002–0.01 mm), and coarse silt particles (0.01–0.05 mm), coupled with an increase in the content of sand particles (0.05–1 mm). As the number of FTCs increased, there was a significant increase in the median particle size (d50) and a decrease in the volumetric fractal dimension, indicating a trend towards particle dispersion and the downward migration of finer particles, leading to soil structural damage and reduced AS. These changes enhanced soil susceptibility to erosion during rainfall events, significantly increasing SE.
The FTCs also had a significant impact on the soil’s OMC. In the initial stages of FT, the OMC notably increased, primarily due to the breakdown of soil aggregates, which led to the exposure of organic matter and enhanced microbial activity, thereby facilitating the release and migration of organic carbon. However, as the number of FTCs increased, the magnitude of change in OMC gradually diminished, suggesting the presence of a threshold. Moreover, soils with higher initial moisture contents exhibited more significant increases in OMC during the FTCs, a phenomenon closely associated with the frost heave and thaw settlement processes during soil freezing. Additionally, changes in the soil microbial community structure, including the death of certain microbes which then released dissolved organic carbon, further increased the soil OMC.
FTCs significantly increase the erodibility (K-value) of soil, making it more prone to erosion. During 0–7 FTCs, the soil’s erodibility K-value increased by 19.55% compared to unfrozen soil. However, as the number of FTCs increased, the changes in K-value became smaller, indicating that the soil structure tends to stabilize after multiple cycles. Additionally, the initial water content significantly affects the soil’s erodibility; soils with higher water content show a more noticeable increase in K-value. After calculating the increase coefficient (∆) of the soil’s erodibility K-value, it was found that the K-value of soil after FT is 1.21 times that of unfrozen soil.
In summary, FTA significantly increases the erodibility of soils in thermokarst landslides within the permafrost region of Sanjiangyuan by altering soil particle composition, OMC, and soil structure. These findings not only provide a scientific basis for understanding the impact of FTA on soil erosion but also offer theoretical support for soil erosion prevention and ecological protection in the region. Specifically, the proposed incremental coefficient for the K-value (∆ = 1.21) in this study more accurately quantifies the impact of FTA on soil erosion, providing a scientific basis for improvements to soil erosion models. The empirical formula established using the EPIC model aids in precisely identifying high-risk soil erosion areas, thereby optimizing land use and vegetation restoration strategies. From an ecological protection perspective, the results underscore the critical role of initial soil water content and OMC in moderating freeze–thaw effects, providing theoretical support for optimizing vegetation restoration measures in ecological protection practices. Moreover, by mitigating the damage FTA causes to soil structure, we can effectively enhance soil resistance to erosion, protect soil fertility and biodiversity, and thereby improve the overall stability of the ecosystem. Future research should further explore the long-term impacts of FTA on soil erosion under different land use and climate change scenarios to devise more effective ecological protection strategies.

Author Contributions

B.W.: writing—original draft, investigation; Y.G.: methodology, investigation; K.Z.: visualization, data curation; S.L.: data curation, formal analysis; C.Z.: data curation, investigation; Y.L.: supervision, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Qinghai Province Basic Research Project (No. 2024-ZJ-767).

Data Availability Statement

The data is contained within the article.

Conflicts of Interest

The authors declare that they have no known competing financial interest or personal relationships that could have appeared to influence the work reported in this paper.

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Water 17 00818 g001
Figure 1. Geographical location and distribution of thermokarst landslides in the Sanjiangyuan region (https://doi.org/10.11888/Cryos.tpdc.300333).
Figure 1. Geographical location and distribution of thermokarst landslides in the Sanjiangyuan region (https://doi.org/10.11888/Cryos.tpdc.300333).
Water 17 00818 g001
Water 17 00818 g002
Figure 2. Impact of FTA on soil particle composition at different moisture contents.
Figure 2. Impact of FTA on soil particle composition at different moisture contents.
Water 17 00818 g002
Water 17 00818 g003
Figure 3. Impact of FTCs on soil organic matter at different moisture contents and soil depths.
Figure 3. Impact of FTCs on soil organic matter at different moisture contents and soil depths.
Water 17 00818 g003
Water 17 00818 g004
Figure 4. The impact of FTCs (a) and initial moisture content (b) on the SE (K-value).
Figure 4. The impact of FTCs (a) and initial moisture content (b) on the SE (K-value).
Water 17 00818 g004
Table 1. Changes in soil particle composition during FTCs.
Table 1. Changes in soil particle composition during FTCs.
FTC Count
(Time)		Median Particle Size d50 (μm)Clay Particles
(<0.002 mm) (%)		Silt Particles
(0.002–0.05 mm) (%)		Sand Particles
(0.05–1 mm) (%)		Volumetric Fractal Dimension D
019.46719.37763.71316.9102.910
119.96719.21063.13017.6602.910
321.16718.53361.77019.6972.909
522.40017.46663.19719.3372.908
722.76716.87363.63419.4532.908
922.36719.15058.84022.0102.908
1236.16714.11049.80036.0902.901
1540.53314.30051.14034.5602.901
Table 2. Characteristics of soil particle composition changes under different initial moisture contents in FT conditions.
Table 2. Characteristics of soil particle composition changes under different initial moisture contents in FT conditions.
Soil Moisture Rate (%)Median Particle Size d50 (μm)Clay Particles
(<0.002 mm) (%)		Silt Particles
(0.002–0.05 mm) (%)		Sand Particles
(0.05–1 mm) (%)		Volumetric Fractal Dimension D
1234.66313.58556.30330.1190.098
1624.70017.27859.22523.4950.093
2017.45021.26962.69516.0360.089
Table 3. Impact of various factors on soil organic matter changes.
Table 3. Impact of various factors on soil organic matter changes.
FactorSum of SquaresDegrees of FreedomMean SquareF-ValueSignificance
Initial soil moisture content0.0420.0210p < 0.01
FTC count2.0170.29145p < 0.01
Soil layer depth4.5631.52760p < 0.01
Table 4. Changes in soil OMC among different moisture levels during FTCs.
Table 4. Changes in soil OMC among different moisture levels during FTCs.
Soil Moisture ContentFTC CountMeanStandard Deviation
0135791215
12%5.665.465.625.875.886.576.786.926.09 a0.54
16%5.335.625.725.925.996.186.626.766.02 a0.46
20%5.165.435.455.155.886.076.526.665.79 b0.55
Note: Different letters (a, b) indicate significant differences in soil OMC at different soil moisture contents during FTCs.
Table 5. Changes in soil OMC during FTCs.
Table 5. Changes in soil OMC during FTCs.
Soil DepthFTC CountMeanStandard Deviation
0135791215
0–10 cm6.086.296.736.616.987.407.677.786.94 a0.57
10–20 cm5.816.226.216.236.276.807.257.406.53 a0.52
20–30 cm4.914.804.935.025.695.606.026.105.38 b0.54
30–40 cm4.724.694.504.734.725.305.605.845.01 b0.52
Note: Different letters (a, b) indicate significant differences in soil OMC in different soil layers during FTCs.
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Wang, B.; Gu, Y.; Zhou, K.; Li, S.; Zheng, C.; Lu, Y. Impact of Freeze–Thaw Action on Soil Erodibility in the Permafrost Regions of the Sanjiangyuan Area Affected by Thermokarst Landslides. Water 2025, 17, 818. https://doi.org/10.3390/w17060818

AMA Style

Wang B, Gu Y, Zhou K, Li S, Zheng C, Lu Y. Impact of Freeze–Thaw Action on Soil Erodibility in the Permafrost Regions of the Sanjiangyuan Area Affected by Thermokarst Landslides. Water. 2025; 17(6):818. https://doi.org/10.3390/w17060818

Chicago/Turabian Style

Wang, Bihui, Yidong Gu, Kexin Zhou, Shengnan Li, Ce Zheng, and Yudong Lu. 2025. "Impact of Freeze–Thaw Action on Soil Erodibility in the Permafrost Regions of the Sanjiangyuan Area Affected by Thermokarst Landslides" Water 17, no. 6: 818. https://doi.org/10.3390/w17060818

APA Style

Wang, B., Gu, Y., Zhou, K., Li, S., Zheng, C., & Lu, Y. (2025). Impact of Freeze–Thaw Action on Soil Erodibility in the Permafrost Regions of the Sanjiangyuan Area Affected by Thermokarst Landslides. Water, 17(6), 818. https://doi.org/10.3390/w17060818

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