Introduction

In recent years, with global warming, the freeze–thaw effect has intensified at high latitudes and high altitudes1,2,3. This trend of climate change has not only attracted widespread attention globally4,5 but also had far-reaching impacts on the western section of the agricultural and pastoral intertwined zone in northern China. The west section of the region has powerful seasonal freeze–thaw effects due to its unique geographical ___location and climatic conditions6. The increase in the frequency and intensity of freeze–thaw cycles will not only modify the soil structure, but also further aggravate soil erosion, causing a series of problems such as soil erosion, vegetation degradation, and land desertification7. While soil shear strength is an important indicator of soil resistance to erosion8, an in-depth study of its change rules and action mechanism under different freeze–thaw conditions is of great significance for coping with the impact of climate change on soil erosion and formulating effective disaster prevention strategies9.

At present, regarding the effect of freeze–thaw action on soil shear strength, scholars have mainly analyzed the soil types in different regions in terms of varying water contents10, temperatures11, salinity12, organic matter content13 and the number of freeze–thaw cycles14, and have achieved certain research results. Among them, the number of freeze–thaw cycles and water content had a significant effect on soil shear strength15. In terms of the number of freeze–thaw cycles, Lu et al.14 studied sandy loam, powdery loam, and clay loam soils in the Loess Plateau region and found that the shear strength of the three types of soils gradually decreased with the increase in the number of freeze–thaw cycles. Wang et al.16 studied coarse-grained saline soils in the Xinjiang region and found that cohesion decreased with the increase of freeze–thaw cycles. Xu et al.17 studied the soils in the loess in the Xi’an area and found that the cohesion of loess varied with freeze–thaw cycles until it reached the residual value. In terms of water content, Yan et al.10 studied silty clay in Northeast China and found that water content is linearly and negatively correlated with cohesion and angle of internal friction. Wang et al.18 studied permafrost in the Tibetan Plateau region and found that both cohesion and angle of internal friction showed a tendency to increase and then decrease with increasing water content. Gu et al.19 studied silt soil in Zhengzhou City, Henan Province, and found that the shear strength decreased with the increase in water content. These studies on different regions and types of soils have shown that freezing and thawing cycles hurt soil shear strength, which together emphasize the importance of freezing and thawing cycles and water content on soil mechanical properties and resistance to erosion. However, the effect of freeze–thaw cycles on soil shear strength may be more complex in the western part of the agro-pastoral zone in northern China due to its unique climatic conditions and soil types.

Based on the above analysis, this paper selects three representative soils, namely, loessial soil (World Reference Base for Soil Resources (WRB) classification: Luvisols), sierozem (WRB classification: Calcisols), and chernozem (WRB classification: Chernozems), in the western part of the agricultural and pastoral intertwined belt in northern China as the research object, and through the freezing and thawing cycling test and the straight shear test, the initial influence of freezing and thawing cycle on the shear strength, cohesion and internal friction angle of these soils is studied in depth and the mechanism of the role of water on the mechanical properties of soils during the process of freezing and thawing is also elucidated. At the same time, the synergistic effect of freeze–thaw in the process of wind and water erosion is further explored in light of the environmental characteristics of the soil sampling area. The innovation of this study is primarily reflected in the following aspects: Firstly, it thoroughly investigates the implications of differences in shear strength among three types of soil for soil stability in the western region, providing a theoretical foundation for regional soil erosion risk assessment and zonal management. Secondly, from the perspective of freeze–thaw and erosion interactions, it explores the synergistic effects of freeze–thaw processes in both wind and water erosion, clarifying the compound mechanisms by which freeze–thaw cycles influence soil erosion. These findings not only enhance the understanding of how soils in the western part of the agro-pastoral ecotone in northern China respond to climate change and environmental factors, but also hold significant importance for formulating targeted soil conservation measures.

Materials and methods

Overview of the study area and test soils

The ecological transition zone connecting the farming areas of Northeast and North China with the natural grassland animal husbandry areas is defined as the northern agro-pastoral zone20, and its western region (34°50′–38°10′ N, 100°50′–108°40′ E) is located in the transition zone from the Qinghai-Tibetan Plateau to the Loess Plateau(As shown in Fig. 1). The administrative area includes 13 cities (states) in three provinces (autonomous regions) of Qinghai, Gansu, and Ningxia and 57 counties (districts). The region has a predominantly temperate continental climate, with an altitude of 876−4881 m. According to the temperature and precipitation monitoring data in 2023, the average annual temperature of the region is − 10 to 12 °C, the average annual precipitation is 143−708 mm, and the soil type is dominated by loessial soil and sierozem. Due to the special geographic ___location and climatic conditions of this region, the freezing and thawing effects are frequent and strong, resulting in a particularly prominent soil erosion problem.

Fig. 1
figure 1

Soil types and sampling site locations in the western part of the agro-pastoral zone in northern China. The Fig. 1 was created by ArcGIS (version: 10.5, https://www.arcgis.com/). Data source of Fig. 1: National Earth System Science Data Center, National Science & Technology Infrastructure of China (https://www.geodata.cn/).

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According to the indices such as the area proportion of soil types, distribution of permafrost, length of freezing period, altitude, average annual temperature and average annual precipitation in the western region, loessial soil, sierozem and chernozem were selected as the test soils in this experiment (As shown in Fig. 1). The detailed indices are shown in supplementary material S2. Firstly, in terms of distribution area, loessial soil and sierozem accounted for the largest area (nearly 30%), which is highly representative of the soils in the western section of the region. Although the distribution area of chernozem is relatively small, only 5%, but it is mainly distributed in high altitude areas, which is more representative of the soil in high altitude areas. Specifically, the chernozem sampling site is located in Tongren City, Huangnan Tibetan Autonomous Prefecture, Qinghai Province (35°28′ N, 102°21′ E) with an elevation of 3472.5 m, an average annual temperature of 5.2 °C, an average annual precipitation of 125.1 mm, and a freezing number of days of 60–90 days, which is a typical distribution area of the chernozem, and it is able to represent the characteristics of this soil type under the conditions of high elevation and low precipitation; The sampling site of sierozem is located in Ping’an District, Haidong City, Qinghai Province (36°22′ N, 102°0′ E) , at an altitude of 2589.29 m, with an average annual temperature of 7.6 °C, an average annual precipitation of 310.1 mm, and a number of freezing days of 60–90 days; this area is in the transition zone between semi-arid grassland and desert grassland, and can represent the typical characteristics of sierozem under arid and semi-arid climatic conditions; the sampling site of loessial soil is located in Anding District, Dingxi City, Gansu Province (35°28′ N, 104°42′ E) , at an altitude of 1907.8 m, an average annual temperature of 7.2 °C, an average annual precipitation of 377 mm, and a number of freezing days of 30–60 days, which is located in a typical area of the Loess Plateau, and is able to represent the soil characteristics of loessial soil under similar climatic and topographic conditions. To collect the test soil samples, weeds and impurities (non-soil components such as pebbles, glass shards, plastic debris, etc.) were first removed from the surface near the sampling site, and then surface soil from 0 to 20 cm depth was taken randomly within the sampling area. The bulk density and water content of the test soil samples were determined by the drying method, and the particle composition was analyzed by the dry sieve method, which was replicated three times for each test (As shown in Table S1 of supplementary materials). In order to ensure the controllability of the test variables, all tests in this work were conducted using remolded soil. Detailed test methods are provided in the supplementary material.

Freeze–thaw cycle test

First, the soil samples collected in the field were air-dried, sieved through 2 mm holes, and filled in layers (that is, each layer was filled at 1 cm and compacted in three layers to the measured weight) into aluminum boxes. To ensure the homogeneity of the soil samples, the soil surface of each layer was scraped separately during the filling process. Then, the amount of water required to bring the sample soil to 10%, 15%, 20%, and 30% moisture content was calculated and uniformly sprayed on the test soil samples with a spray can. To spread the water in the soil well and evenly, the specimens in the aluminum box needed to be sealed with plastic wrap and then left indoors in a place protected from light for 24 h. Finally, the treated specimens were placed in a freezer and subjected to 0 (control group), 1, 3, 5, 7, and 10 freeze–thaw cycles. During the simulated freeze–thaw cycles, the freezing temperature was set at − 15 °C, the thawing temperature was set at room temperature, and both the freezing and thawing times were set at 12 h to ensure that the soil samples could be completely frozen and thawed.

The thousands of freeze–thaw cycles that soil undergoes during its formation is indeed a long and complex natural process, which is influenced by climate, geographical ___location, soil type, and many other factors. Studies have shown that the effects of freeze–thaw cycles on soil shear strength are particularly significant in the initial cycles, and these trends can be observed through a smaller number of freeze–thaw cycle experiments. In this paper, 10 freeze–thaw cycles were designed because, with the increase in the number of freeze–thaw cycles, the shear strengths of the three types of soils as a whole showed a tendency to decrease firstly and then tend to be stabilized. This phenomenon indicates that the soil structure will reach a new equilibrium state after a certain number of freeze–thaw cycles, at which time the shear strength of the soil is relatively stable and not easily affected by additional freeze–thaw cycles. Thus, 10 freeze–thaw cycles are sufficient to capture the initial and critical changes in the effects of freeze–thaw on soil shear strength and provide a basis for predicting long-term effects. With this design, the initial effects of freeze–thaw cycles on soil mechanical properties can be more accurately assessed, while providing an important reference for understanding the mechanical behavior of soils after long-term freeze–thaw cycles under natural conditions.

Straight shear test

The ZJ-type electric strain-controlled straight shear apparatus (produced by Nanjing Soil Instrument Factory) was used to carry out unconsolidated and undrained fast shear tests on the soil samples and calculate the shear strength of the soil according to the results of the fast shear test. The specimen preparation and straight shear test were performed in strict accordance with the geotechnical test method standard (GB/T50123-2019). Firstly, the soil samples that reached the preset number of freeze–thaw cycles and thawed were taken out with a ring knife and placed in the shear box. Then the initial position of the gauge ring and the percentile meter were adjusted, the shear speed was set to 0.8 mm/min, and the shear was performed at vertical pressures of 50, 100, 150, and 200 Kpa, respectively. During the shearing process, the handwheel was carefully observed and the meter readings were recorded for each revolution of the handwheel. When the meter readings no longer rise or have a significant decline, record the meter readings at this time, the shear displacement amount reaches 4 mm when the soil sample is recognized as shear; if the meter readings continue to rise, the shear displacement amount reaches 6 mm when the soil sample is recognized as shear.

The shear strength of the soil can be calculated according to Eq. (1):

$$\begin{array}{*{20}c} {\tau f = R \times C^{\prime } \times 10/{\rm A}_{0} } \\ \end{array}$$
(1)

In the above equation: \(\tau f\) represents the soil shear strength (Kpa); \(R\) is the reading of the gauge ring meter (0.01 mm); \(C^{\prime }\) is the gauge ring correction coefficient (1.479 Kpa/0.01 mm); and \(A_{0}\) is the area of the test sample subjected to force (cm2).

The cohesion \(C\) and the internal friction Angle \(\varphi\) of the soil can be calculated according to Eq. (2) of Coulomb’s law21.

$$\begin{array}{*{20}c} {\tau f = C + \sigma \tan \varphi } \\ \end{array}$$
(2)

In the above equation: \(\tau f\) represents the soil shear strength (Kpa); \(C\) is the cohesion of the soil (Kpa); \(\varphi\) is the internal friction angle of the soil (°); and \(\sigma\) is the vertical pressure (Kpa) on the soil sample.

Since each group of specimens was subjected to straight shear tests at four different vertical pressures, the soil shear strengths at different vertical pressures were weighted and averaged according to Eq. (3) to obtain the average shear strength.

$$\begin{array}{*{20}c} {\tau = \frac{{\mathop \sum \nolimits_{i = 1}^{4} \tau_{i} \sigma_{i} }}{{\mathop \sum \nolimits_{i = 1}^{4} \sigma_{i} }}} \\ \end{array}$$
(3)

It is worth noting that in the straight shear test in the laboratory, this work sampled at a depth of 0–20 cm in the surface layer and set a vertical pressure range of 50–200 kPa, which may differ from the actual stress state at the same depth in the natural soil profile. However, the choice of this pressure range is reasonable: first, 50–200 kPa can better simulate the loads that the surface soil may bear in actual engineering, such as shallow foundations or farmland soil scenarios; second, this experiment aims to study the impact of freeze–thaw cycles on soil stability, and setting a consistent vertical pressure range helps to more clearly reveal the mechanism of action of freeze–thaw cycles.

Results and analysis

Effect of freezing and thawing on soil shear strength

Freezing and thawing are some of the most important environmental factors affecting the mechanical properties of soils, especially in seasonally frozen areas. In these regions, the soil undergoes periodic freeze–thaw cycles, resulting in significant changes in soil structure and properties. Shear strength, which refers to the ability of soil to resist shear damage, is an important parameter to measure the mechanical stability of soil22. Therefore, this paper reveals the effect of freeze–thaw action on soil mechanical properties by comparing the changes in shear strength of different soil types after undergoing freeze–thaw cycles. The results of the study are shown in Fig. 2.

Fig. 2
figure 2

Effect of different number of freeze–thaw cycles and water content on soil shear strength. (a) Shows the changes in the shear strength of loessial soil under different numbers of freeze–thaw cycles and water content; (b) shows the changes in the shear strength of sierozem under different numbers of freeze–thaw cycles and water content; and (c) shows the changes in the shear strength of chernozem under different numbers of freeze–thaw cycles and water content.

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Freeze–thaw cycles reduce and then stabilize soil shear strength

With the increase in the number of freeze–thaw cycles, the shear strength of the three soils showed an overall trend of decreasing and then stabilizing, which is consistent with the previous study11. Compared with the unfrozen and thawed soils, the shear strength of loessial soil, sierozem, and chernozem decreased by 4.35%, 5.85%, and 8.64% on average, respectively, after 10 freezing and thawing cycles. It can be seen that the freeze–thaw effect had the strongest effect on the chernozem, followed by sierozem, and the loessial soil is relatively small.

During freeze–thaw cycles, significant changes occur in the microstructure and mechanical properties of the soil, leading to a reduction in its shear strength. First, during freezing, the water in the soil expands to form ice crystals, which exerts tremendous pressure on the soil particles, leading to the modification of the connections between the soil particles and the formation of tiny fissures. During thawing, these microfissures cannot be restored to their original state, making the structure of the soil looser and further reducing the shear strength of the soil. Secondly, the arrangement of soil particles changes during the freeze–thaw cycle, breaking the structure of the soil, reducing the large soil aggregates and increasing the small aggregates, and this structural modification makes the soil more prone to shear failure when subjected to external forces. Finally, the decrease of cohesion and internal friction angle will also lead to the decrease of soil shear strength. Through experimental studies, it was found that for 10% water content of loessial soil, sierozem, and chernozem, when the number of freeze–thaw cycles increased from 0 to 10 times, the soil cohesion decreased by an average of 33.33%, 31.25%, and 16.39%, respectively, and the angle of internal friction of soil decreased by an average of 1.74%, 4.17%, and 9.65%, respectively; the shear strength of soil decreased by an average of 7.92%, 8.23%, and 12.24%, respectively. In addition, when the moisture content increased from 10 to 30%, the cohesive strength of the loessial soil, sierozem, and chernozem, decreased by an average of 23.71%, 5.80%, and 27.62%, respectively, the angle of internal friction of the soil decreased by an average of 7.14%, 8.79% and 10.99%, respectively, and the shear strength of the soil decreased by an average of 10.64%, 9.70% and 15.52%, respectively. In summary, the phase change of moisture in the soil, the change of soil particle arrangement and structure, and the synergistic effect of soil cohesion and internal friction angle had a significant effect on shear strength, resulting in a decrease in the shear strength of the soil after freeze–thaw cycles. However, as the number of freeze–thaw cycles increases to a certain extent, the soil structure gradually adjusts, and the processes of water migration and ice crystal formation become more stable, leading to a stabilization of the soil’s shear strength.

Decreased soil shear strength due to increased water content

When the number of freeze–thaw cycles is the same, the shear strength of loessial soil, sierozem, and chernozem all showed a tendency to decrease with increasing water content. In particular, the shear strength of the three soils decreased by 13.41%, 13.53%, and 21.70% when the water content increased from 10 to 30% without freeze–thaw cycles. Whereas, after 10 freeze–thaw cycles, the reduction in shear strength of these three soils under the same change in water content is 9.54%, 10.26%, and 13.28%, respectively. This indicates that the sensitivity of soil shear strength to changes in water content increases with the increase in the number of freeze–thaw cycles, which is mainly due to the dual effect of the change in the phase state of moisture. On the one hand, the change of moisture from liquid to solid state when the soil freezes generate freezing expansion force, which reduces the cohesion between soil particles and destroys the soil structure, thus reducing the soil shear strength. On the other hand, the change of moisture from solid to liquid state during soil thawing leads to a reduction of contact between soil particles, which in turn reduces the frictional resistance between them23. Next, the mechanism of the freeze–thaw cycle’s effect on soil mechanical properties will be further revealed from the perspectives of cohesion and angle of internal friction.

Effect of freeze–thaw on soil cohesion

Soil cohesion and angle of internal friction are two important indices reflecting soil shear strength24, in which cohesion reflects the bonding force between soil particles. In the process of the freeze–thaw cycle, the increase or decrease of water and the repeated action of the freeze–thaw cycle will lead to a change of soil cohesion, which in turn affects the soil shear strength. Therefore, in this paper, freeze–thaw cycle tests are carried out on three kinds of soils under different moisture content conditions, and the results are shown in Fig. 3.

Fig. 3
figure 3

Effect of different number of freeze–thaw cycles and water content on soil cohesion. (a) Shows the change of cohesion of loessial soil under different numbers of freeze–thaw cycles and water content; (b) shows the change of cohesion of sierozem under different numbers of freeze–thaw cycles and water content; (c) shows the change of cohesion of chernozem under different numbers of freeze–thaw cycles and water content.

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Freeze–thaw cycles reduce and then stabilize soil cohesion

The magnitude of cohesion of the three soils as a whole showed a great deal of variation, with the sierozem being the smallest, the loessial soil the next largest, and the chernozem the largest. The sierozem had the weakest inter-particle bonding as compared to the loessial soil and the chernozem. The cohesive force of both loessial soil and chernozem showed a tendency to decrease with the increase in water content, which is consistent with the results of the previous study on the effect of different water content on soil shear strength. In addition, a careful comparative analysis also reveals that at a lower water content (10%), the cohesion of all three soils tends to first decrease and then stabilize with the increase in the number of freeze–thaw cycles. This is because the expansion and contraction of soil volume during the freeze–thaw process reduces the connectivity between soil particles, which leads to a decrease in cohesion. When the number of freeze–thaw cycles increases to a certain degree, the degree of soil modification reaches its maximum, the new freeze–thaw cycles have less influence on the structural state of the test soil samples, and the cohesion tends to stabilize.

Increase in water content weakens the tendency of cohesion to decrease and then stabilize

The tendency of cohesion to decrease and then stabilize decreases or even disappears with the increase in water content. For example, when the water content of loessial soil is increased to 20%, a clear tendency to decrease and then stabilize can still be observed. For sierozem and chernozem, when the water content increases (more than 10%), the change of cohesion does not show a strict dependence on the freeze–thaw cycle, but shows a fluctuating state. Related studies have shown that soil cohesion decreases nonlinearly with increasing water content, which may be due to the thickening of the hydration film between soil particles as a result of the increase in water content, weakening the attraction between particles25. This nonlinear relationship may be more complicated for sierozem and chernozem because they are not only affected by the hydration film but also by components such as salt and calcium carbonate. Salt can change the ion concentration in the soil, thereby affecting the electrostatic interaction between soil particles. At low salinity, the electrostatic repulsion generated by the negative charges on the surface of soil particles is strong, and the interaction between particles is weakened, resulting in reduced soil cohesion. At high salinity, the ions in the soil will compress the double layer of charges on the surface of the particles, weakening the electrostatic repulsion and enhancing the soil cohesion. In addition, calcium carbonate, as a binder, can not only increase soil cohesion, but also change the distribution of soil particles through its dissolution and precipitation processes, thereby affecting the soil structure. The combined effect of these factors makes the change in soil cohesion nonlinear and complex.

Effect of freezing and thawing on the angle of internal friction of soil

The internal friction angle of soil reflects the friction characteristics between soil particles and is related to the structural morphology and compactness of the soil body26. During freeze–thaw cycles, moisture can also affect the angle of internal friction by changing the structural morphology of soil particles and the interactions between particles, which in turn changes the shear strength of the soil. The effects of different numbers of freeze–thaw cycles and moisture content on soil internal friction angle are shown in Fig. 4.

Fig. 4
figure 4

Effect of different numbers of freeze–thaw cycles and water content on the angle of internal friction of soils. (a) Shows the variation of the angle of internal friction of loessial soil under different numbers of freeze–thaw cycles and water content; (b) shows the variation of the angle of internal friction of sierozem under different numbers of freeze–thaw cycles and water content; (c) shows the variation of the angle of internal friction of chernozem under different numbers of freeze–thaw cycles and water content.

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Freeze–thaw cycles vary the angle of internal friction of the three soils

With the increase in the number of freeze–thaw cycles, the internal friction angles of the three soils showed different trends. Among them, the angle of internal friction of the loessial soil showed a first decrease and then increase and then stabilized, which may be due to the initial freeze–thaw action led to the modification of the soil structure, which increased the sliding surface between the particles, thus reducing the frictional resistance. After a certain degree of destruction, the particles rearranged and combined to form a new and more stable structural form27. In contrast, sierozem and chernozem showed a decreasing trend with the increase in the number of freeze–thaw cycles, which may be due to the destruction of the cementing material between soil particles and the decrease in the contact area between the particles, thus reducing the frictional resistance18.

Decrease in soil internal friction angle due to increased water content

The angle of internal friction of the three soils showed an overall trend of decreasing with increasing water content. When experiencing 10 freeze–thaw cycles, the water content decreased from 10 to 30%, and the angle of internal friction decreased by 6.21%, 6.94%, and 10.84% for loessial soil, sierozem, and chernozem, respectively. The chernozem is most sensitive to the change in water content during the freeze–thaw cycle. A careful comparative analysis also reveals that the same soil exhibits very different behaviors under different water content conditions. For example, when the water content is 10%, the internal friction angle of the loessial soil decreased from 27.03° in the unfrozen soil to 26.56° after 10 freeze–thaw cycles. In contrast, when the moisture content is 30%, the angle of internal friction of the loessial soil increased from 24.84° in the unfrozen soil to 24.91° after 10 freeze–thaw cycles. At lower water content (such as 10%), moisture may act as a lubricant between soil particles, reducing the frictional resistance between particles, which leads to a decrease in the angle of internal friction. Whereas, at higher water content (such as 30%), moisture may play a cementing role between soil particles, which enhances the bonding force between the particles, thus leading to an increase in the angle of internal friction28.

Discussion

Exploration of the reasons for the differences in shear strength of the three soils

The test results showed that the shear strength of the three soils generally showed a decreasing trend under the influence of freezing and thawing. Among them, the shear strength of chernozem is most significantly affected by freezing and thawing, followed by sierozem, and relatively smaller by loessial soil. When the number of freeze–thaw cycles is the same, the soil shear strength decreases with the increase in water content. The differences in shear strength of these three soils may be related to soil properties, structure, texture, and water content29.

In terms of soil properties and structure, the chernozem has the highest measured soil bulk density in the field, indicating that it has a high degree of compaction in its natural state. This higher degree of compaction means that the soil particles are in closer contact with each other, thus providing higher shear strength when not affected by freeze–thaw. However, freeze–thaw cycles may disrupt the compact structure of the soil, especially in the presence of moisture, and the formation and expansion of ice crystals can weaken the contact between soil particles and cause a loosening of the soil structure30, which significantly reduces the shear strength of chernozem. In contrast, sierozem and loessial soil are rich in microagglomerates with diameters less than 0.25 mm, and these microagglomerates have weaker inter-particle cohesion, smaller soil bulk density, more pore space, and looser structure, which makes the effect of freezing and thawing on its structure not as significant as that on the chernozem; in terms of the soil texture, the accumulation of humus and the calcification process are obvious in the process of the formation of the chernozem, and the land has a good structure31, which improves the cohesion and shear strength of chernozem. While loessial soil is mainly composed of powder (particle size between 0.002 mm and 0.05 mm) and sand grains (particle size between 0.05 mm and 2 mm), and sierozem is dominated by sand grains with less clay content (particle size less than 0.002 mm). The voids between the powder and sand particles are large, which makes not easy to form a compact structure, and reduces the cohesion and shear strength of the loessial soil and sierozem. From the perspective of water content, the change of free and bound water content in the soil has a significant effect on the soil cohesion and shear strength in the process of the freeze–thaw cycle32, and this effect shows different regularities in different soil types.

It is worth noting that the remolded soil used during the test may not fully capture all the physical properties of natural soil, such as its inherent particle arrangement and structural characteristics. Therefore, the results of this experiment may differ somewhat from the soil under natural conditions. However, this method of controlling variables in remodeled soils provides a unified basis for studying the direct effects of freeze–thaw cycles on soil shear strength. The method allows for a clearer analysis of the freeze–thaw process itself, independent of potential confounding factors associated with natural soil variability (such as the amount of roots contained in the soil, etc.). The results of the study provide a more intuitive and more scientific picture of the behavior of the soil itself under freeze–thaw cycle conditions.

Exploration of the indicative significance of differences in shear strength of three soils

The differences in shear strength of the three soils not only indicate the diversity of soil properties in the western part of the northern agro-pastoral zone but also provide a scientific basis for the development of targeted soil protection measures. The shear strength of chernozem was greatest when the water content was low and there was no freeze–thaw cycle, while it was relatively small in sierozem and loessial soil. However, when the water content and freeze–thaw cycles increase, the shear strength of the chernozem area decreases most obviously. Specifically, the elevation of the sampling point of chernozem is 3472.5 m, the average annual temperature is 5.2 °C, and the average annual precipitation is 125.1 mm. This region belongs to the middle and deep seasonally frozen area, which has a long freezing period, and the soil stays frozen for a long time in winter. Under such conditions, the water in the soil is transformed into ice crystals, and the strong bonding between the ice crystals and the soil particles helps to maintain the stability of the soil structure, thus enhancing the shear strength of the chernozem. Moreover, the average annual precipitation in the area is low, and the soil moisture content is relatively low, which enhances the cohesion between soil particles, and these together improve the shear strength of chernozem33. However, in recent years, with global warming, the frequency of freezing and thawing as well as the amount of precipitation have increased at high altitudes. According to the results of this paper, these changes may affect the stability of chernozem which originally had high shear strength. Therefore, it is necessary to make corresponding adjustments in soil management and conservation measures in chernozem regions to meet the challenges posed by climate change.

The elevation of the sierozem sampling site is 2589.3 m, the mean annual temperature is 7.6 °C, and the mean annual precipitation is 310.1 mm. This region also belongs to the mid-deep seasonally frozen area, but the mean annual precipitation was significantly increased compared to the chernozem region. The increase in soil water content leads to a decrease in soil shear strength and an increase in freeze–thaw cycle effects34. Therefore, to maintain the stability of sierozem, vegetation cover should be increased in this area to mitigate the direct freeze–thaw effects on the soil, while the vegetation root system can also enhance the erosion resistance of the soil.

The elevation of the sampling point of the loessial soil is 1907.8 m, the mean annual temperature is 7.2 °C, and the mean annual precipitation is 377 mm. This area belongs to the shallow seasonally frozen area, which has a higher mean annual temperature, shorter freezing period, and relatively weaker effect of the freeze–thaw cycle. In addition, the higher average annual precipitation in the loessial soil area leads to an increase in soil water content and a weakening of the cohesion between soil particles, which further reduces the shear strength of the soil. To enhance the stability of the loessial soil, effective soil protection can be provided through measures such as increasing the organic matter content and optimizing the soil particle composition.

Exploration of the synergistic effect of freeze–thaw action in wind and water erosion

Freezing and thawing cycles have a significant effect on soil shear strength, which can affect the soil erosion process to some extent, but the manifestation of soil erosion is not limited to shear damage. Freeze–thaw cycles affect several physical properties of soil, including shear strength, bulk density, porosity, and water-stable aggregates. These changes can lead to a decrease in soil stability, which in turn affects the soil erosion process to some extent. In addition, soil erosion is also affected by a combination of factors such as wind and hydraulics. These factors together determine the specific form and degree of soil erosion. Soil erosion is the result of multiple forces rather than a single factor. During soil erosion, a single freeze–thaw cycle generally does not result in sand production on slopes but provides an effective source of material for other erosive forces35,36,37. Freezing and thawing are often compounded with wind, water, and other erosive forces to cause more severe soil erosion.

The effect of freezing and thawing on wind erosion mainly reduces the wind erosion resistance of soil by affecting soil physicochemical properties and structure35. In the western section of the region, due to the higher elevation and lower temperature, the strong freeze–thaw effect in winter leads to the reduction of soil shear strength, and the soil structure becomes loose, which provides a rich source of material for wind erosion during the spring thawing period. With the increase in the number of freeze–thaw cycles, the pressure of moisture on the surrounding soil increases, leading to the formation of cracks on the surface or within the soil, and these cracks increase the contact area between the soil and the wind, which becomes a channel for wind erosion and accelerates the wind erosion and transportation of soil particles36. The contribution of freezing and thawing to water erosion is mainly reflected in changing soil properties and blocking infiltration38. In the process of soil hydraulic erosion, shear strength, as a key indicator of water erosion, is a key factor influencing the critical mooring rate of soil hydraulic erosion39. The western section of the region belongs to the high-altitude cold zone, which is affected by extreme temperatures and precipitation, and seasonal freezing and thawing are prominent. During the freezing period, an impermeable layer will be formed on the soil surface, and when ice melting or precipitation occurs, the water can not infiltrate in time, forming a large amount of surface runoff, which increases the contact area between soil and water flow. This runoff will wash the soil surface layer and take away soil particles, thus accelerating the process of water erosion. At the same time, the soil moisture changes continuously between the frozen and thawed states throughout the process, resulting in a decrease in the cohesion and shear strength between the soils, and the soil structure becomes loose, and this loose structure makes the soil particles more likely to be scoured and carried by the flowing water, thus promoting the occurrence of water erosion.

Conclusion

In this paper, the effects of freezing and thawing on soil shear strength, cohesion, and internal friction angle are investigated by freeze–thaw cycling test and straight shear test on loessial soil, sierozem, and chernozem in the western part of the agricultural and pastoral intertwined zone in northern China. Meanwhile, the synergistic effect of freeze–thaw action in the process of wind and water erosion was further explored by combining the environmental characteristics of the soil sampling area. The conclusions of the study are as follows:

  1. (1)

    There is a significant difference in shear strength among the three soils when they are not freeze-thawed, with the chernozem being significantly larger than the sierozem and the loessial soil. After 10 freeze–thaw cycles, the shear strength of the three soils generally shows a decreasing trend, and an increase in water content further reduces the shear strength of the soil. For the 10% water content of loessial soil, sierozem, and chernozem, the shear strength of the three soils decreases by an average of 7.92%, 8.23% and 12.24%, respectively, after 10 freeze–thaw cycles. The results indicate that the shear strength of chernozem is most significantly affected by freeze–thaw cycles, followed by sierozem, while loessial soil is relatively less affected.

  2. (2)

    At low water content, the cohesion of all three soils shows a trend of decreasing and then stabilizing with the increase in the number of freeze–thaw cycles. For the 10% water content of the loessial soil, sierozem, and chernozem, the cohesion of the three soils decreases by an average of 33.33%, 31.25% and 16.39%, respectively, after 10 freeze–thaw cycles. However, this trend decreases or even disappears with the increase in water content. In addition, the internal friction angle of the three soils generally decreases with the increase in water content and shows different trends with the increase in the number of freeze–thaw cycles. The angle of internal friction decreases by an average of 7.14%, 8.79%, and 10.99% when the water content is increased from 10 to 30% for loessial soil, sierozem, and chernozem, respectively.

  3. (3)

    The differences in shear strength of the three soils indicate that the shear strength of the chernozem region in the western part of the agro-pastoral zone in northern China is the greatest when the water content is low and there is no freeze–thaw cycle, while the sierozem and loessial soil regions are relatively small. However, when the water content and freeze–thaw cycles increase, the shear strength of the chernozem area decreases most obviously. Soil erosion is the result of a combination of forces, and freeze-thawing is often compounded with wind, water, and other erosive forces, resulting in more severe soil erosion.