Effects of freeze-thaw cycles on the size distribution and stability of soil aggregate in the permafrost regions of the Qinghai-Tibetan Plateau

As the basic units of soil structure, soil aggregate is essential for maintaining soil stability. Intensified freeze–thaw cycles have deeply affected the size distribution and stability of aggregate under global warming. To date, it is still lacking about the effects of freeze–thaw cycles on aggregate in the permafrost regions of the Qinghai-Tibetan Plateau (QTP). Therefore, we investigated the effects of diurnal and seasonal freeze–thaw processes on soil aggregate. Our results showed that the durations of thawing and freezing periods in the 0–10 cm layer were longer than in the 10–20 cm layer, while the opposite results were observed during completely thawed and frozen periods. Freeze–thaw strength was greater in the 0–10 cm layer than that in the 10–20 cm layer. The diurnal freeze–thaw cycles have no significant effect on the size distribution and stability of aggregate. However, < 0.25 mm fraction dominated wet sieving aggregate with the highest proportion during thawing period, while the < 1 mm fraction reached the highest during completely frozen period in the 10–20 cm layer (P < 0.05). Likewise, the mean weight diameter and water-stable aggregate were decreased during thawing period compared with the other periods, which were influenced by soil microbial biomass carbon and belowground biomass. Hence, the seasonal freeze–thaw processes destroyed macro-aggregate (> 0.25 mm) and reduced aggregate stability. Our study has scientific guidance for evaluating the effects of freeze–thaw cycles on soil steucture and provides a theoretical basis for further exploration on soil and water conservation in the permafrost regions of the QTP.


Introduction
Soil aggregate is the basic unit of soil structure (Shen et al 2022), and serve essential functions in regulating soil hydrothermal conditions and stability, as well as maintaining soil nutrients, facilitating plant growth, and influencing biological activities (Six et al 2004, Zhao andHu 2023).Some studies have shown that the physical protection in aggregate is one of the key mechanisms for conserving soil organic carbon (SOC) (Tang et al 2016, Xiao et al 2019).According to the aggregate size, which can be divided into macro-aggregate (> 0.25 mm) and micro-aggregate (< 0.25 mm) (Wang et al 2016).Macro-aggregate provides larger pore spaces and better structural stability, which were instrumental in reducing soil erosion, enhancing water-retention capacity (Kim et al 2023).Aggregate stability refers to the ability of aggregate to withstand external forces without breaking apart, which has important effects on soil erosion resistance, microbial activity, and nutrient cycling (An et al 2010, Lu et al 2022, Verchot et al 2011).In general, the mean weight diameter (MWD), geometric mean diameter (GMD), water-stable aggregate (WSA), and resistance to soil erosion (1/K) are used to characterize the aggregate stability, with the higher values indicating the greater stability (Jia et al 2023, Li et al 2020, Mao et al 2021, Zhou et al 2022).Some of the key factors affecting aggregate stability include soil organic matter, pH, and soil texture (Al-Kaisi et al 2014, Pihlap et al 2021, Pu et al 2022).
Permafrost is mainly distributed in high-latitude and high-altitude regions in the northern hemisphere (Obu et al 2019).As the highest altitude and largest permafrost distribution regions in the low-middle latitudes, the permafrost regions of the Qinghai-Tibetan Plateau (QTP) is 1.06 × 10 6 km 2 , accounting for 41.5% of the land area of the QTP (Gruber 2012).Under climate warming, the QTP permafrost is experiencing significant, rapid, and extensive changes, typically manifested by thickened active layer, increased ground temperature, and decreased permafrost area (Wu et al 2021).Freeze-thaw cycles of the active layer can be specifically categorized into diurnal and seasonal freeze-thaw processes, which are caused by diurnal or seasonal thermal changes in soils (Chen et al 2020, Lv et al 2022).Furthermore, soils in mid-to-high latitudes and high elevations suffer freeze-thaw cycles during thawing period in spring and freezing period in autumn (Lv et al 2022), which significantly influence the distribution and stability of soil aggregate (Feng et al 2020).For instance, some studies have indicated that the macro-aggregate were destroyed and the proportion of micro-aggregate was increased after freeze-thaw cycles (Henry 2007, van Bochove et al 2000).Specifically, freeze-thaw cycles change the internal structure of aggregate in the form of increased porosity and more asymmetrical pores or tubular pores (Kim et al 2023, Rooney et al 2022), which can further affect the aggregate stability.Freeze-thaw cycles also lead to reductions in soil nutrients and alterations in physicochemical properties such as electrical conductivity, and pH (Han et al 2018, Li et al 2022, Liu et al 2022, Shi et al 2023), which in turn can contribute to variations in aggregate.However, the veritable effects of freeze-thaw cycles on the aggregate stability are controversial so far.Relevant studies have focused on quantitative laboratory simulations of the effects of diurnal freeze-thaw cycles rather than seasonal freeze-thaw cycless on aggregate, and they are unable to absolutely replicate the freezing temperature, the number of freeze-thaw cycles and soil moisture in the realistic field conditions, and these differences may cause the varied final results.(Feng et al 2020, Han et al 2018, Li and Fan 2014, Li et al 2022, Li et al 2020).
To date, most of the studies of freeze-thaw cycles on aggregate in the permafrost regions have focused on the northeast regions in China, while in situ studies about the effects of freeze-thaw cycles on the size distribution and stability of soil aggregate in the permafrost regions on the QTP are still lacking.Hence, we selected the Shule River headwaters in the northeastern margin of the QTP as the study area, where permafrost develops widely, occupying about 97.98% of the total area.We collected soil samples from different depths and periods to explore the response characteristics of aggregate to freeze-thaw cycles.Our objectives were to: (1) analyze the effects of diurnal and seasonal freeze-thaw processes on aggregate size distribution, (2) investigate the changes in aggregate stability during the diurnal and seasonal freeze-thaw processes, and (3) explore the variables affecting the aggregate stability.Our results will provide scientific reference and data support for the influence of freezethaw cycles on soil aggregate in the permafrost regions of the QTP.

Study area and site description
The Shule River headwaters (96.2°∼ 99.0°E, 38.2°∼ 40.0°N) was located in the mid-western part of the Qilian Mountains on the northeastern margin of the QTP and has a continental arid desert climate.The study site was located in an observational field of the alpine meadow ecosystem (98°16′14″E, 38°21′17″N, Alt: 4014 m) and belonged to high-altitude permafrost (Chen et al 2012), which soil type is cold calcic soil (Liu et al 2012) with an active layer thickness of ∼ 2.3 m.According to the monitoring data in 2013, the mean annual air temperature was −3.49 °C, and the annual precipitation was 417 mm, which was concentrated from May to September.The vegetation type is alpine meadow with a community coverage of about 42%, and the dominant plants are Kobresia humilis, K pygmaea, Poa poophagorum, and Saussurea arenaria (Chen et al 2012).

Sample collection and measurement
The sampling time was set at April 28 and May 4, August 1, September 29 and October 5, and December 31 in 2013, representing thawing period, completely thawed period, freezing period, and completely frozen period, respectively, and the above four periods represent the seasonal freeze-thaw processes.Detailed information on the definition and delineation of the seasonal freeze-thaw processes can be found in Data Analysis.Based on the daily maximum and daily minimum soil temperature, and daily temperature recorded in 2013, the first sampling time of April 28 and September 29 was assumed to be the period before diurnal freeze-thaw cycles.If diurnal freeze-thaw cycles occurred until the second sampling time of May 4 and October 5, this time was defined as the period after diurnal freeze-thaw cycles.Therefore, the sampling time of April 28 and May 4 were defined as the period before and after diurnal freeze-thaw cycles in spring, and the September 29 and October 5 were defined as before and after diurnal freeze-thaw cycles in autumn.Specific sampling information was shown in table 1.
Three 50 cm × 50 cm quadrats were randomly set up in the observational field, and three soil samples were collected from 0-10 cm and 10-20 cm, respectively, and a total of six soil samples were collected in each period.It should be noted that samples collected on May 4 and October 5 were only from the 0-10 cm layer.Each soil sample was composed of five soil cores and mixed into three parts.One part was used to determine the aggregate size distribution using the dry and wet sieving methods.We have referred to the previous aggregate classification method and further improve it to categorize aggregate fractions into > 3 mm, 1 -3 mm, 0.5 -1 mm, 0.25 -0.5 mm and < 0.25 mm soil aggregate (Jia et al 2022).The second part was stored in a 4 °C refrigerator for determination of soil pH, redux potential (Eh), ammonium nitrogen (NH 4 + -N), nitrate nitrogen (NO 3 − -N), microbial biomass carbon (MBC), and microbial biomass nitrogen (MBN).The last part was air-dried under natural conditions for testing soil organic matter (SOM), belowground biomass (BGB) and inorganic carbon (IC).All these environmental variables were measured as described previously (Lv et al 2022, Wu et al 2021).The Hydra Probe II soil sensor (Stevens) was used to measure soil temperature at depths of 10 cm and 20 cm.The sensors were connected to the CR1000X datalogger (Campbell Scientific), which automatically recorded data at intervals of 10 min based on Beijing Standard Time.The soil physicochemical properties were shown in table 2.

Data Analysis
Based on the monitoring data of soil temperature and the previous method, the durations of freeze-thaw processes were defined into three categories: completely thawed days, where the daily minimum soil temperature was > 0.0 °C; completely frozen days, where the daily maximum soil temperature was 0.0 °C; and freeze-thaw days, where the daily minimum soil temperature was 0.0 °C and the maximum soil temperature was > 0.0 °C (Guo et al 2011).Then we divided the seasonal freeze-thaw processes into four periods: completely thawed and frozen, thawing and freezing periods (Chen et al 2020).Each period started with the first day of three consecutive freeze-thaw or completely frozen/thawed days, respectively.To evaluate the strength of soil freeze-thaw processes, we calculated the freeze-thaw strength.To begin, the frequency ranges of positive and negative soil temperature and the sum of positive and negative accumulated soil temperature were calculated in a day.Subsequently, the two were standardized to the value 0-1 and then multiplied to obtain the freeze-thaw strength (Jia et al 2023).
The stability of wet sieving aggregate was characterized by MWD and WSA, and the stability of dry sieving aggregate was characterized by GMD and 1/K.The calculation formulas were as follows (Dong et al 2022, Hu et al 2020): 7.954 0.0017 0.0494 exp 0.5 log 1.675 0.6986 4 2

( ) /
Where: X i is the mean diameter of size class i (mm), W i is the mass percentage of aggregate in size class i (%),n is the number of fractions, and GMD is geometric mean diameter.Statistical analysis and pictures were done using SPSS 26.0 and Origin 2023, respectively.One-way analysis of variance (ANOVA) and least significant difference (LSD) were used to compare the differences in soil physicochemical properties, and size distribution and stability of aggregate among the periods of seasonal freeze-thaw processes in the 0-20 cmlayers.The differences in the size distribution and stability of aggregate in the 0-10 cm layers during diurnal freeze-thaw cycles were analyzed using t-tests.The effects of the seasonal freeze-thaw processes, depths changes, and their interactions on the aggregate stability were examined by twoway ANOVA.Pearson correlation was used to calculate the correlation coefficient between the aggregate stability and environmental variables.Finally, stepwise regressions were used to explore the relationships between aggregate stability and environmental variables.

Characteristics of freeze-thaw cycles
Figures 1(a) and (c) showed the seasonal dynamics of soil freeze-thaw processes in the 0-10 cm and 10-20 cm layers, respectively.The durations of completely thawed and frozen periods in the 0-10 cm layer were shorter than that of the 10-20 cm layer, with the opposite results for thawing and freezing periods.Specifically, thawing period of the 0-10 cm and 10-20 cm layers began in early February and mid-April, respectively, with an interval of 76 days.Completely thawed period of both the 0-10 cm and 10-20 cm layers basically started in mid-May, but ended about two weeks earlier in the 0-10 cm layer than the 10-20 cm layer, with their durations essentially the same.Moreover, freezing period of the 0-10 cm layer lasted 15 days from October 3, while that of the 10-20 cm layer lasted only three days on October 15.For completely frozen period, the duration of the 0-10 cm layer was shorter by 76 days compared to the 10-20 cm layer.During most of the year, the value of freeze-thaw strength remained at zero (e.g., completely frozen and thawed periods), but increased above zero during thawing and freezing periods (figures 1(b) and (d)).Freezethaw strength varied greatly in different layers, as shown by the peak of freeze-thaw strength observed in the 0-10 cm layer was higher than that in the 10-20 cm layer, with 0.43 and 0.07 as their peaks, respectively.

Variations in soil aggregate size distribution
The wet and dry sieving aggregate before and after diurnal freeze-thaw cycles in the 0-10 cm layer were mainly comprised of < 0.25 mm and > 3 mm fractions, respectively (figure 2).After spring diurnal freeze-thaw cycles, the proportions of > 3 mm fraction of the wet and dry sieving aggregate were decreased by about 15% and 8%, while these of < 0.25 mm fraction were increased by about 15% and 5%, respectively.However, the diurnal freeze-thaw cycles had no obvious effects on the other aggregate size distribution.
In terms of seasonal freeze-thaw processes, the wet sieving aggregate from all soil layers were dominated by < 0.25 mm fraction, accounting for about 30% of all fractions, while that > 3 mm had the lowest proportion, representing about 14% of all fractions (figure 3(a)).The > 0.5-1 mm and 0.25-0.5 mm fractions in the 0-10 cm layer had lower proportion than that in the 10-20 cm layer.On the contrary, the proportion of > 3 mm fraction in the 0-10 cm layer was 9.66% higher than that in the 10-20 cm layer (figure 3(a)).Furthermore, the aggregate size distribution also showed significant variations during different periods in the same layer (P < 0.05).To be exact, the proportions of < 0.25 mm fraction during thawing period at the 0-10 cm and 10-20 cm layers were significantly higher than that in the other periods.For 0.25-0.5 mm fraction in the 10-20 cm layer, it had the  highest and lowest proportions in freezing and completely frozen periods, respectively.Meanwhile, both 0.5-1 mm and 1-3 mm fractions showed the highest proportions during completely frozen period.
The dry sieving aggregate was mainly composed of > 3 mm fraction (figure 3(b)).The proportion of > 3 mm fraction was increased and that of 0.5-1 mm and < 0.25 mm fractions were significantly decreased (P < 0.05) from the 0-10 cm layer to 10-20 cm layer.In the 10-20 cm layer, the proportion of > 3 mm fraction was the highest proportion during freezing period, while the 0.5-1 mm and 0.25-0.5 mm fractions were the highest proportions during completely frozen period among all the periods (P < 0.05).

Variations in soil aggregate stability
In general, the diurnal freeze-thaw cycles did not have significant effects on the aggregate stability, which was basically decreased during both spring and autumn after diurnal freeze-thaw cycles (table 3).The MWD and WSA values were decreased after the spring diurnal freeze-thaw cycles but the opposite results were observed during the autumn diurnal freeze-thaw cycles.For the GMD and 1/K, their values showed decreasing tendency after diurnal freeze-thaw cycles during any period.
The MWD and WSA of aggregate varied between different soil layers and were significantly affected by the freeze-thaw processes (figure 4).For the MWD, the highest values in the 0-10 cm and 0-20 cm layers were found during completely thawed period, while that in the 10-20 cm layer was observed during completely frozen period (figure 4(a)).Nevertheless, thawing period always had the lowest value regardless of the layers.The variations in WSA showed similar results in all layers, with significantly lower values during thawing period than that in the other periods (P < 0.05) (figure 4(b)).However, ranged from completely thawed period to completely Table 3. Results of the effects of before and after diurnal freeze-thaw cycles (DFTCs) on geometric mean diameter (GMD), resistance to soil erosion (1/K), mean mass diameter (MWD), and water-stable aggregate (WSA) (mean ± SE).

Pre-DFTCs
Post Pre-DFTCs: before diurnal freeze-thaw cycles in spring, Post-DFTCs: after diurnal freeze-thaw cycles in spring, Pre-DFTCa: before diurnal freeze-thaw cycles in autumn, Post-DFTCa: after diurnal freeze-thaw cycles in autumn, and P is the value that is judged to be significant.
frozen period, the values of the WSA tended to decrease in the 0-10 cm layer, but reversed in the 10-20 cm layer.
For the GMD and 1/K, there were no significant changes in the 0-10 cm and 10-20 cm layers.In the 10-20 cm layer, their values during completely frozen period were distinctly lower than that in the other periods (figure 5).

Effects of environmental variables on soil aggregate stability
The results of correlation analysis among environmental variables and soil aggregate stability were shown in figure 6.In the 0-10 cm layer, the MWD was significantly positive correlated with the MBC and sand content but significantly and negatively correlated with silt content (P < 0.05).The WSA was significantly positive correlated with the MBC, with the opposite results for freeze-thaw strength (P < 0.05).The GMD and 1/K were significantly negatively positive correlated with the MBN and IC, respectively (P < 0.05).In the 10-20 cm layer, the MWD was significantly and negatively correlated with freeze-thaw strength and similar results was observed for WSA, which was also significantly positive correlated with the BGB (P < 0.05).The GMD and 1/K were significantly positive correlated with NH 4 + -N, while significantly and negatively correlated with clay content (P < 0.05).In addition, we found that the seasonal freeze-thaw processes had significant effects on the MWD and WSA, and the soil layer had significant effects on the GMD and 1/K (P < 0.05).However, the interactions between the seasonal freeze-thaw processes and the soil layer had no remarkable influences on each index of stability (table 4).
We also found that the variables affecting the aggregate stability differed in different soil layers by using the stepwise regressions (table 5).In the 0-10 cm layer, the MBC drove the variations of MWD and WSA, with the explanations of 53.8% and 34.1%, respectively.The pH and IC together influenced the GMD and 1/K, explaining 84.1% and 83.2% of the variations, respectively.In the 10-20 cm layer, the BGB explained 40.3% of the variation in the MWD.The GMD and 1/K were affected by clay, which explained 35.3% and 39.5% of their variations, respectively.

Discussion
4.1.Effects of the freeze-thaw cycles on the soil aggregate size distribution Soil aggregate plays a crucial role in preserving essential aspects of soil properties and functionality, encompassing soil structure, stability, porosity, and nutrient availability.Freeze-thaw cycles cause alterations in aggregate fractions, resulting in changes in the size distribution of dry and wet sieving aggregate (Dagesse 2013, Table 4. Results of two-way ANOVA (analysis) of the effects of different periods of seasonal freeze-thaw processes and soil layer on mean mass diameter (MWD), water-stable aggregate (WSA), geometric mean diameter (GMD), and resistance to soil erosion (1/K).df is the degree of freedom, F is the statistic, and P is the value that is judged to be significant; SFTPs: seasonal freeze-thaw processes.Wang et al 2012), which was confirmed in our study.For instance, we found that the proportions of < 0.25 mm fraction and > 3 mm fractions during completely thawed period were generally lower and higher than that in the other periods, respectively (figure 3), which may be due to the growth of plant roots in summer.On the one hand, plant roots can provide exudates and soil organic compounds that act as adhesion agents for soil particles to help form healthy aggregate (Wang et al 2017).On the other hand, fresh organic matter from litter accumulates on the soil surface during completely thawed period, and soil organisms convert it into humus to promote the formation of macro-aggregate (Tisdall and OADES 1982).We also found the proportion of < 0.25 mm fraction was increased during the freeze-thaw period, especially during thawing period, which agreed with the previous study (Huang et al 2021).The study reported that the proportion of < 0.25 mm fraction was increased significantly after seasonal freeze-thaw cycles.Freeze-thaw cycles lead to the expansion and collapse of pores inside the aggregate, and obvious changes in the structure of the aggregate (Rooney et al 2022, Skvortsova et al 2018).Under the influence of repeated freeze-thaw cycles, soil aggregate undergoes notable changes in its pore structure.The intensification of freeze-thaw cycles leads to the formation of a distinct network structure characterized by finer and irregular pores within the aggregate (Ma et al 2021).The addition of slender pores further contributes to the modification of the aggregate's internal composition.These slender pores act as pathways, effectively separating soil particles within the aggregate (Gao et al 2021, Zhang et al 2016).

MWD
Past studies have provided compelling evidences that freeze-thaw cycles induce a continuous rearrangement and combination of soil particles, leading to the decomposition of macro-aggregates into micro-aggregates (Li et al 2020), as evidenced by an increase in the number of 0.25-1 mm fraction after the decompositions of a certain number of >1 mm fraction (Ma et al 2019).In all sampled depths, similar results were observed in our study, where 0.5-1 mm and 1-3 mm fractions of wet sieving aggregate were decreased in thawing period, while < 0.25 mm fraction was increased.This phenomenon may be caused by the change in aqueous phase, which leads to the direct destruction of the aggregate (Li et al 2020).The water in the soil pores undergoes a phase transition during freezing, turning into ice, which subsequently leads to the enlargement of pores inside the aggregates, during thawing, the ice converts back into water, the pores of aggregates could not return to the same as before freezing, and the repeated freeze-thaw cycles promote the increase of the aggregate pores.(Ma et al 2021).Moreover, > 3 mm fraction of wet sieving aggregate was higher in the 0-10 cm layer than that in the 10-20 cm layer (figure 3(a))., which may be due to decrease in the quantity and activity of plant residues and activity of soil microorganisms with the deepening of soil layer, which was not conducive to the formation of macro-aggregate (Wu et al 2015).In our study, the proportion of > 3 mm aggregate was decreased after freezethaw cycles.This phenomenon could be due to the difference in soil moisture content (Feng et al 2020, Kvaernø andØygarden 2006), with the impact of freeze-thaw cycling on aggregate intensifying with soil moisture content increases (Xiao et al 2020).Relatively high soil moisture was retained in macro-aggregate, so the proportion of >3mm aggregates decreased.In this study, there was no significant change in aggregate size distribution after diurnal freeze-thaw cycles, which could be attributed to the insufficient number of freeze-thaw cycles short freezing and thawing times.

Effects of the freeze-thaw cycles on the soil aggregate stability
In our study, from thawing period to completely frozen period the MWD and WSA were initially increased and then decreased, and the magnitude of the variations were greater in the 0-10 cm layer than that in the 10-20 cm layer (figure 4).This may be attributed to the more frequent freeze-thaw cycles and the greater freeze-thaw strength in the 0-10 cm layer compared with the 10-20 cm layer (figure 1).We also found that the MWD and WSA were significantly and negatively correlated with freeze-thaw strength (figure 6), that is, freeze-thaw cycles could decreased the aggregate stability, which further validated our results.In the study of permafrost in the high latitude area of northeast China, it was reported that the stability of aggregates decreased with the increase of freeze-thaw cycle frequency (Ma et al 2019, Ma et al 2021).In addition, aggregate stability reached its peak level during completely thawed period.It is possible that the warmer temperatures and increased biological activities promote the decomposition of organic matter, resulting in higher levels of litter and humus during summer completely thawed period (Ghazaryan et al 2016).Humus is an important cementing substance for the formation of aggregate and its increased content facilitates the formation and stability of aggregate (Erktan et al 2016).The MWD and WSA in the 10-20 cm layer were the highest during completely frozen period (figure 4), which could be the result of the incomplete decomposition of organic matter caused by low soil temperature during completely frozen period and poor oxidation in the 10-20 cm layer, where soil microorganisms and related enzymes can convert litter and dead roots into more stable humus (Six et al 2004, Tamura et al 2017).These substances are further combined with mineral particles such as clay to form macro-aggregate, and then improving the aggregate stability (Trivedi et al 2017).The aggregate stability of difference in spring and autumn freeze-thaw periods may be caused by the difference in freezing temperatures, rate and the number of freezethaw cycles.Although the freezing rate has little effect (Mostaghimi et al 1988), the freezing temperatures and the number of freeze-thaw cycles have important effects on the aggregate stability (Zhang et al 2021).
The relationship between aggregate stability and MBC is shown in figure 6 and table 5, which showed significant positive correlations (P < 0.05).Soil microbial biomass plays an important role in the soil aggregation process, and microorganisms produced soil cementing agents can combine micro-aggregate into macroaggregate, thereby promoting aggregate stability (Mao et al 2021), as also confirmed by our results.The IC mainly refers to carbon in the carbonate mineral state formed during soil weathering, while CaCO 3 is considered to be one of the most important stabilizers in aggregate formation (Pihlap et al 2021).In addition, pH affects the strength of the bonds between mineral surfaces and colloidal particles, as well as the solubility of multivalent cations in soils, which in turn affects the aggregate stability (Al-Kaisi et al 2014, Bronick and Lal 2005).Extreme pH can disrupt the electrochemical properties of soil particles, reducing the binding forces and leading to aggregate breakdown.We also came to the same result that the aggregate stability was influenced by pH and IC (table 5).The BGB provides plant-derived carbon, and its decomposition increases the carbon substrates and enhances aggregate stability by promoting the formation of macro-aggregate (Luo et al 2017), as confirmed in our results.The clay significantly affected the GMD and 1/K in the 10-20 cm layer, and the GMD and 1/K were significantly lower during completely frozen period than that in the other periods, which indicated that the decrease in aggregate stability could be owing to the oxidation of organic gelling agents caused by the increase of clay content during completely frozen period (table 2), thus reducing the aggregate stability (Oades 1984).

Conclusions
Our study investigated the size distribution and stability of soil aggregate in different soil layers and periods through the entire seasonal freeze-thaw processes.We found that the durations of completely thawed and frozen periods in the 0-10 cm layer were shorter than that in the 10-20 cm layer, while freeze-thaw strength was enhanced in 0-10 cm layer during thawing and freezing periods.The wet sieving aggregate mainly consisted of < 0.25 mm fraction but the dry sieving aggregate were dominated by > 3 mm fraction.The proportion of < 0.25 mm aggregate was increased but that of 0.5-1 mm and 1-3 mm aggregate were decreased during thawing period.The stability of wet sieving aggregate was the lowest during thawing period and the greatest during completely thawed period.The main variables influencing MWD and WSA were MBC and BGB, respectively, while the variables influencing both GMD and 1/K were clay, pH, and IC.Our study plays a pivotal role in enhancing our understanding of the impacts of freeze-thaw cycles on the size distribution and stability of aggregate in alpine permafrost regions.We aim to establish a scientific foundation for studying the mechanisms by which soil aggregate facilitate carbon sequestration and contribute to soil and water conservation.

Figure 2 .
Figure 2. Aggregate size distribution of in the 0-10 cm layer before and after diurnal freeze-thaw cycles, (a) and (b) were wet sieving aggregate size distribution; (c) and (d) were dry sieving aggregate size distribution.Pre-DFTCs and Post-DFTCs were before and after diurnal freeze-thaw cycles in spring, Pre-DFTCa and Post-DFTCa were before and after diurnal freeze-thaw cycles in autumn, respectively.

Figure 3 .
Figure 3. Aggregate size distribution in different soil layers of different periods of seasonal freeze-thaw processes, (a) and (b) were size distribution of wet and dry sieving aggregate, respectively.TP: thawing period; CTP: completely thawed period; FP: freezing period; CFP: completely frozen period.Different letters indicated significant differences among different periods in the same soil layer (P < 0.05).

Figure 5 .
Figure5.Dry sieving aggregate stability during different periods of seasonal freeze-thaw processes.Different letters indicate significant differences between different periods in the same soil layer (P < 0.05).GMD: geometric mean diameter; 1/K: resistance to soil erosion.TP: thawing period; CTP: completely thawed period; FP: freezing period; CFP: completely frozen period.

Figure 4 .
Figure 4. Wet sieving aggregate stability during different periods of seasonal freeze-thaw processes.Different letters indicate significant differences between different periods in the same soil layer (P < 0.05).MWD: mean weight diameter; WSA: water-stable aggregate.TP: thawing period; CTP: completely thawed period; FP: freezing period; CFP: completely frozen period.

Table 5 .
Results from stepwise regressions of soil aggregate stability and environmental variables.