Physical and Thermal Properties of Coarse-Fragment Soil in the Moraine-Talus Zone of the Qilian Mountains

Abstract

Moraine-talus zone (hereafter referred to as MTZ) refers to the non-glacial area (including glacier mass supply area) above the upper boundary of alpine meadow in high mountains, where vegetation is sparsely distributed and coarse fragment (diameter > 2 mm) is widespread. The MTZ acts as the headwater region for many large river basins, and the freeze–thaw process of its coarse-fragment soil largely affects regional hydrological processes, carbon exchange, and ecosystem diversity. However, our knowledge of the physical and thermal properties of coarse-fragment soil in MTZs remains limited. Mainly distributed in the surroundings of Hala Lake, the area ratio of the MTZ in the Qilian Mountains is about 21%. On the basis of 170 samples collected from 22 soil profiles at a depth of 80 cm, coarse fragments dominated the compositions of soil textures in the MTZ, with relatively high volume proportion (about 63.3%) and mass proportion (about 75.0%). The mean volume and mass ratio of the coarse fragments tended to increase gradually from the surface to the deep soils and varied largely at different particle size ranges (i.e., 2–5, 5–10, 10–20, 20–40, 40–60, and >60 mm). Thermal conductivity measurements for the 24 samples collected from three soil profiles in dry (K_dry) and water-saturated (K_sat) conditions indicated that K_dry increased rapidly with temperatures from −20 °C to 25 °C (with a 5 °C interval), which appeared to be closely related with soil porosity. K_sat did not obviously change with temperature at the two designated temperature ranges (i.e., from −20 °C to −5 °C and from 5 °C to 25 °C), but fluctuated largely at 0 °C, possibly due to the drastic phase change. More detailed experimental designs combined with more influential elements should be considered in future research to fully understand the thermal properties of coarse-fragment soil in the MTZ.

1. Introduction

Moraine-talus zone (hereafter referred to as MTZ) refers to the non-glacial area (including glacier mass supply area) above the upper boundary of alpine meadow in high mountains, where relatively less-fine soil (diameter < 2 mm) and vegetation are present [1]. In MTZs, the soil layer is relatively thin, with low organic matter content and high coarse fragment content (diameter > 2 mm); the underlying surface is dominated mainly by exposed bedrock, talus, and moraine deposits; the vegetation is sporadic; and the terrain is steep [1]. In many large mountain ranges (e.g., the European Alps, the North American Cordillera, and the Himalayas), the characteristics of MTZs (such as steep slope, exposed bedrock, talus, and moraine) are significantly exhibited [2,3,4,5,6]. By estimating at a coarse resolution, the proportion of area of MTZ on the Qinghai–Tibet Plateau (QTP) and Tianshan Mountains of China is up to 20–30% [1].

Previous studies have reported that coarse geomorphic units in alpine regions (including talus slopes, debris fans, alluvium, and moraines) play an important role in storing and guiding groundwater flow [7,8,9,10,11], and evidently contribute to sustaining baseflow in streams [11,12,13]. Due to high precipitation resulting from orographic effects and low evapotranspiration resulting from sparse vegetative covers and low air temperature [11,14], the MTZ could generate a large amount of runoff [10,15,16,17] and act as the headwater region for many large river basins, such as the Yangtze River, the Yellow River, the Lancang River, the Tarim River, the Indus River, and the Ganges River. Under the conditions of rainfall and snow melt, the runoff generation in the MTZ is extremely rapid, and the river is mainly supplied by surface runoff and fast-moving soil flows [1,11]. In the MTZ of the middle section (northwest to southeast direction) of the Qilian Mountains in China, a runoff coefficient of more than 0.85 is reported at a small moraine-talus catchment [18]. Relatively large runoff coefficients are also detected at the source of the Urumqi River in the Tianshan Mountains (0.75) and the exposed hillside of the Binggou River in the Qilian Mountains (0.79) [19]. In western China, the spatial differences in the distribution of MTZs may cause different responses of the low flows to climate change [20]. However, research on MTZs has received less attention, and our knowledge of their spatial distribution characteristics remains limited.

Mainly controlled by altitude factor [21,22], frozen ground is widely distributed in MTZs, and its freeze–thaw process largely affects regional carbon exchange, ecosystem diversity, hydrological process, and engineering construction of cold regions [23,24,25,26,27]. However, as the key parameter to simulate the soil freeze–thaw process, existing observation and experimental research on the thermal conductivity of frozen ground is mainly concentrated on fine soil, and less on coarse-fragment soil [28,29,30,31,32], due to harsh natural environments and expensive observation costs. The majority of classical thermal conductivity estimation models commonly generalize or even ignore the effect of coarse fragments on soil thermal properties [30,31,33,34,35,36,37], leading to large uncertainties for the analysis and estimation of thermal conductivity in MTZs. An amplified signal of global warming is observed at high altitudes [38], where the MTZ is widely distributed. The performance of investigations on the physical and thermal properties of coarse-fragment soil in the MTZ is urgently needed and valuable to adequately simulate and analyze the soil freeze–thaw process of the MTZ under climate change.

Coarse-fragment soil is distributed widely on the QTP due to weak weathering and strong erosion [29,39,40,41,42,43]. Located on the northeast edge of the QTP, the Qilian Mountains are the birthplace of many inland river basins. Above 3600–3700 m, the MTZ is distributed widely [21], and its existence greatly affects the regional hydrological regime and other parameters. Selecting the Qilian Mountains as the study area, this study aims to (1) investigate the spatial distribution of the MTZ in the Qilian Mountains, (2) analyze the physical properties of coarse-fragment soil through field sampling and laboratory tests, and (3) test the thermal conductivity of coarse-fragment soil at designated temperatures in dry and water-saturated conditions.

2. Materials and Methods

Located on the northeastern edge of the QTP, the Qilian Mountains extend over 800 km from northwest to southeast and stretch over 200 km from south to north, with the geographical boundary of approximately 93.4°–103.4° E and 35.8°–40.0° N (Figure 1). The elevation varies largely in the Qilian Mountains, with most peaks exceeding 4000 m. Located at the juncture of three climate regions (the monsoon region, arid region and QTP climate region) [44], the climate of the Qilian Mountains is mainly influenced by the macroscopic atmospheric circulation and high elevation. The mean annual air temperature varies along an altitudinal gradient (0.58 °C/100 m), and annual precipitation ranges from less than 100 mm to more than 600 mm from northwest to southeast.

Twenty-two soil profiles were collected from northwest to southeast in the Qilian Mountains, with an average altitude of 4136 m ranging from 3492 m to 4676 m (Figure 1, Table S1). All sampling sites were located in a typical MTZ where vegetation was absent. The traditional cut ring sampling method is limited in the MTZ due to the relatively large soil particle size and loose soil texture in the MTZ. Here, we excavated soil profiles to take soil samples at depth ranges of 0–10, 10–20, 20–30, 30–40, 40–50, 50–60, 60–70, and 70–80 cm, placed them in large round containers (17 cm diameter and 1300 cm³), and kept the soil structure as raw as possible. Two replicates were sampled from the top of each depth range and sealed for analysis in the laboratory. During the sampling process, the samples placed in cloth bags were collected for particle separation in the laboratory. Not all eight samples were available in some soil profiles due to the presence of large bedrock or grave, which hindered excavation and sampling processes.

The quadrature method [45] was used to divide the sample into two parts after the soil samples in cloth bags were air-dried. For each sample, the coarse fragments (diameter > 2 mm, including gravel) were separated from the fine soils (diameter < 2 mm) by sieving. In accordance with the actual situation of soil texture in the MTZ of the Qilian Mountains, the coarse fragment was further classified by means of different sieves (5, 10, 20, 40, and 60 mm). In this study, soil porosity was measured by the difference between the saturated and dry soil weights of known volumes. The mass proportion of coarse fragment in the soil sample is the ratio between the weight of coarse fragment and the weight of the whole soil sample. Similarly, the volume proportion of coarse fragment is the ratio between the volume of coarse fragment and the volume of the whole soil sample.

The thermal conductivity of soil samples at designated temperatures under dry (K_dry) and water-saturated (K_sat) states was measured using a KD2 Thermal Properties Analyzer (Decagon Devices, Inc). This analyzer has high observational accuracy and has been widely used in scientific research and industry [29,34,46,47,48,49]. For the measurement of the thermal properties of each soil sample, we first (1) dried the soil samples in an oven and weighed them (0.001 g accuracy); (2) placed the dried soil samples in constant temperature and humidity chamber equipment (Model: DF-GDWJS-010; temperature range −30–100 °C; humidity range 30–98%; temperature resolution 0.01 °C), adjusted the equipment to the designated temperature (i.e., −20 °C, −15 °C, −10 °C, −5 °C, 0 °C, 5 °C, 10 °C, 15 °C, 20 °C, and 25 °C), and kept the samples at their designated temperature for about 4 h; (3) measured the thermal conductivity three times using the KD2 Pro with an SH-1 sensor and calculated the average; (4) injected a certain amount of water into the soil samples until the soil samples reached saturation; and repeated steps (2) and (3). In this study, we only showed the thermal conductivity measurements for the 24 samples collected from three soil profiles (i.e., S14, S16, and S21) in dry and water-saturated conditions at the designated temperatures.

3. Results

3.1. Spatial Distribution of the MTZ

As shown in Figure 1, the MTZ of the Qilian Mountains is mainly distributed in the surroundings of Hala Lake, with a narrow band on the northern and western slopes. Sporadic distribution of the MTZ is detected in the eastern part of the Qilian Mountains. The area ratio of the MTZ in the Qilian Mountains is about 21%, which is slightly lower than that on the QTP (29%). The average elevation of the MTZ in the Qilian Mountains is about 4045 m, ranging from 2246 m to 5814 m, and the average slope is 16, with the largest reaching up to 76.

3.2. Soil Physical Properties

The mean volume proportion of the coarse fragment for all 22 soil profiles was 63.3%, with the smallest value of 36.7% detected in soil profile S3 and the largest value of up to 90.5% in S16 (Figure 2). For all 170 samples collected from 22 soil profiles, the average volume proportion of the coarse fragment was about 63.3%, ranging from 13.0% to 92.3%. The average mass proportion of the coarse fragment was more than 75%, with a maximum of more than 90% in S16. Overall, the coarse fragment dominated the soil texture composition in the MTZ.

In terms of the vertical distributions, the volume and mass proportions of the coarse fragment all exceeded 50% (Figure 3). The highest volume (69.3%) and mass (79.5%) proportions were detected at a depth of 60–70 cm, whereas the lowest volume (55.3%) and mass (67.4%) ratios occurred at a depth of 0–10 cm. On the whole, the volume and mass proportion of the coarse fragment tended to increase gradually from the surface to the deep soils.

As shown in Figure 4, the mean volume and mass proportions of the coarse fragment varied at different particle size ranges (i.e., 2–5, 5–10, 10–20, 20–40, 40–60, and >60 mm). The largest volume proportion was detected at a particle size diameter of 20–40 mm (16.1%), followed by 10–20 mm (14.9%), and the least was at >60 mm (4.7%). Similar results were also detected for the mean mass proportion of the coarse fragment. The measured soil porosity of 24 coarse-fragment soil samples collected from three soil profiles (i.e., S14, S16, and S21) ranged from 10.4% to 21.2% with a mean of 15.2%.

3.3. Soil Thermal Properties

On the basis of the 24 samples collected from three soil profiles (i.e., S14, S16, and S21), the thermal conductivity of coarse-fragment soil under the dry state (K_dry) tended to increase gradually at the designated temperatures (from −20 °C to 25 °C), but the magnitude of K_dry varied greatly at different soil layers (Figure 5). A significant positive relationship of K_dry with the volume and mass proportion of the coarse fragment was detected for the 24 samples at the designated temperatures (Figure 6; only shown at 20 °C and −20 °C), indicating that the soil samples with a high volume or mass proportion of the coarse-fragment appeared to have a large value of thermal conductivity under the dry state. Figure 7 shows that the soil samples with large soil porosities tended to have a low value of thermal conductivity under the dry state.

For thermal conductivity under the saturated state, K_sat did not obviously change with temperature at the two designated temperature ranges, that is, from −20 °C to −5 °C (frozen state) and from 5 °C to 25 °C (unfrozen state, Figure 8). The difference in K_sat between frozen and unfrozen states was large, with relatively high values in the frozen state compared with the unfrozen state (Figure 8; Table 1). In addition, we also found that the thermal conductivity values at 0 °C were remarkably different from those at other designated temperatures, as shown in Table 1.

4. Discussion

In this study, the analysis of the spatial distribution characteristics of the MTZ in the Qilian Mountains relies on the distribution map of the MTZ across China, which is generated on the basis of the 1:4 million vegetation map of China [1]. In order to obtain the distribution map of the MTZ across China, Chen et al. (2010) extracted the land cover types such as alpine gravel, talus, alpine desert, Androsace tapete, and alpine sparse grass, based on the 1:4 million vegetation map of China [1]. The coarse resolution of the distribution map of the MTZ may not properly describe the distribution characteristics of the MTZ in the Qilian Mountains. Nevertheless, the results of this study may provide a preliminary perspective on the spatial distribution of the MTZ in the Qilian Mountains. The combination of high precision maps and extensive field surveys should be implemented in this study area in the future.

Previous studies reported that the mean mass proportion of the coarse fragment accounts for about 45% in alpine steppe soil and about 48% in alpine meadow soil at a depth of 0–20 cm [50]. At a depth of 1.5 m soil profile in the western QTP, the mean mass proportion of the coarse fragment ranges from 5% to 45% [51]. At a depth of 2.0 m soil profile in the central QTP, the mean mass proportion of the coarse fragment is up to 50% [28,29]. In accordance with the dataset of multilayer soil textures for permafrost modeling on the QTP [52], the coarse fragment widely exists on the western and central parts of the QTP. In this study, the physical properties of coarse-fragment soil in the MTZ of the Qilian Mountains were investigated on the basis of 170 samples collected from 22 soil profiles at a depth of 0.8 m. Compared with the reports at other regions [28,29,50,51], relatively high volume proportion (about 63.3%) and mass proportion (about 75%) of the coarse fragment were detected, which may suggest a more widespread distribution of coarse fragment in the MTZ of the Qilian Mountains. In terms of the vertical distributions of the coarse fragment, we found that the volume and mass proportion of the coarse fragment tended to increase gradually from the surface to the deep soils. Additionally, the distribution characteristics of the coarse-fragment soil at different particle size ranges were also analyzed in this study.

Coarse-fragment soil has distinctly different thermal and hydraulic properties compared with fine soil [29,33,46,53,54,55]. The possible influence of coarse fragment on the soil freeze/thaw process has been considered in some modeling studies, but the estimation of the thermal properties of coarse-fragment soil lacks validation [33,56], and the role of coarse fragment on the soil’s thermal properties is incompletely understood [29]. We tested the thermal conductivity of 24 samples in dry and water-saturated conditions at the designated temperatures by using the KD2 Thermal Properties Analyzer. The results showed that K_dry tended to increase gradually at the designated temperatures (from −20 °C to 25 °C), which appeared to be closely related with the volume or mass proportion of the coarse fragment, and the soil porosity (Figure 7 and Figure 8). This is because a reduction in soil porosity can lead to a large dry density and an increase in the number of mineral particle skeletons per unit volume of soil sample, and the thermal conductivity of mineral particle skeletons is much greater than that of gas-phase fillings [57,58]. Coarse fragment has a remarkable impact on the soil porosity by increasing the curvature of soil pores and soil macropore proportion [59], and then affects the soil thermal conductivity [60]. This phenomenon should be more pronounced in the MTZ, where coarse-fragment soils are widely distributed. K_sat did not obviously change with temperature during frozen and unfrozen states, but fluctuated largely at 0 °C. Relatively large K_sat fluctuations may be attributed to frequent ice–water phase variations occurring at 0 °C, when the soil sampling may be in the freezing/thawing transition period [61,62]. In addition, relatively large K_sat in the frozen state compared with the unfrozen state (Figure 8; Table 1) can be partially explained by the fact that the thermal conductivity of ice (2.29 W/mK) is more than four times that of water (0.57 W/mK at 5 °C). Similar results were also reported on the QTP [62,63], in Alaska [64], and in Canada [65].

In reality, the thermal conductivity of frozen ground is largely affected by soil texture, temperature, water (ice) content, porosity, organic matter content, and other factors [30,31,46,57,61,66,67]. In future studies, more detailed experimental designs combined with more influential elements should be considered. Although KD2 Pro has been applied in the thermal conductivity measurement of coarse-fragment soil [28,29], comparisons of thermal conductivity measurements using different equipment should be conducted in the future to obtain more accurate and reasonable measurement results.

5. Conclusions

The area ratio of the MTZ in the Qilian Mountains is about 21%, which is mainly distributed in the surroundings of Hala Lake, with a narrow band on the northern and western slopes and sporadic distribution in the eastern part of the Qilian Mountains. Coarse fragment dominated the compositions of soil texture in the MTZ, with relatively high volume proportion (about 63%) and mass proportion (about 75%). From the surface to the deep soils, the volume and mass ratio of the coarse fragment tended to increase gradually, and the mean volume and mass proportions of the coarse fragment varied at different particle size ranges. K_dry tended to increase gradually at the designated temperatures, which appeared to be closely related with the soil porosity. K_sat did not change obviously with temperature during frozen and unfrozen states, but fluctuated largely at 0 °C, possibly due to the drastic phase change.

The analysis of the spatial distribution characteristics of the MTZ in this study had some limitations due to heavy dependence on a distribution map with a coarse resolution. Thermal conductivity measurement is only conducted by KD2 Pro and lacks the comparisons of measurements using different devices. Further studies are required through extensive field surveys and more detailed experimental designs combined with more influential elements.

Author Contributions

Conceptualization, X.W. and R.C.; methodology, X.W. and R.C.; software, H.S.; validation, X.W. and R.C.; formal analysis, R.C.; investigation, H.S.; data curation, X.W. and H.S.; writing—original draft preparation, X.W. and H.S.; writing—review and editing, X.W. and R.C.; supervision, R.C.; funding acquisition, X.W. and R.C. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank all team members atin the Qilian Alpine Ecology and Hydrology Research Station, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Table

Figure 1. Location of the Qilian Mountains and 22 sampling sites (S1–S22). The boundaries of the moraine-talus zone in the Qilian Mountains were derived from the spatial distribution map of the moraine-talus zone across China [1]. Detailed information of sampling sites can be found in the Supplementary Materials (Table S1). QTP: Qinghai–Tibetan Plateau. DEM: digital elevation model.

Figure 2. Mean volume and mass proportion of the coarse fragment (diameter > 2 mm) for all 22 soil profiles in the Qilian Mountains. S1–S22 refer to the sampling sites (Figure 1 and Table S1). Error bar stands for standard deviation.

Figure 3. Mean volume and mass proportion of the coarse fragment (diameter > 2 mm) at different soil layers for the average of all 22 soil profiles in the Qilian Mountains. Error bar stands for standard deviation.

Figure 4. Mean volume (a) and mass (b) proportion of the coarse fragment (diameter > 2 mm) at different soil particle size ranges (mm) for the average of all 22 soil profiles in the Qilian Mountains. S1–S22 refer to the sampling sites (Figure 1 and Table S1).

Figure 5. Variations of Kdry (thermal conductivity under the dry state) of the coarse-fragment soil at the designated temperatures for soil profiles S14 (a), S16 (b), and S21 (c).

Figure 6. Relationship of Kdry (thermal conductivity under the dry state) of the coarse-fragment soil at 20 °C (a,c) and −20 °C (b,d) with the volume and mass proportion of the coarse fragment for 24 samples.

Figure 7. Relationship of Kdry (thermal conductivity under the dry state) of the coarse-fragment soil at 20 °C (a) and −20 °C (b) with soil porosity for 24 samples.

Figure 8. Variations of Ksat (thermal conductivity under the saturated state) of the coarse-fragment soil at the designated temperatures (excluding 0 °C) for soil profiles S14 (a), S16 (b), and S21 (c).

Table 1

Mean K_sat (thermal conductivity under the saturated state, W/mK) of the coarse-fragment soil at frozen (from −20 °C to −5 °C), unfrozen (from 5 °C to 25 °C), and 0 °C.

Soil Layer (cm)	S14			S16			S21
	Frozen	0 °C	Unfrozen	Frozen	0 °C	Unfrozen	Frozen	0 °C	Unfrozen
0–10	1.754	16.687	1.421	1.305	0	1.16	2.583	2.162	1.421
10–20	2.356	36.153	1.725	0.681	2.998	0.903	2.182	0	1.725
20–30	3.103	50.106	2.104	1.092	0.529	1.054	1.876	1.546	2.104
30–40	2.587	15.788	1.711	1.872	13.255	1.507	2.615	9.936	1.711
40–50	2.907	32.12	1.588	2.085	7.428	1.447	2.307	2.038	1.588
50–60	3.221	41.733	2.093	1.014	3.411	0.953	2.205	12.32	2.093
60–70	3.199	44.583	2.035	1.646	5.339	1.396	2.294	1.789	2.035
70–80	3.06	0	1.988	2.194	8.131	1.669	2.388	31.006	1.988

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su15021183/s1, Table S1: General information about the 22 sampling sites in the Qilian Mountains.

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