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Article

Changes in Soil Moisture, Temperature, and Salt in Rainfed Haloxylon ammodendron Forests of Different Ages across a Typical Desert–Oasis Ecotone

1
College of Geographical Sciences, Shanxi Normal University, Taiyuan 030000, China
2
Laboratory of Heihe River Eco-Hydrology and Basin Science, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, Linze Inland River Basin Research Station, Lanzhou 730000, China
3
Key Laboratory of Desert and Desertification, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou 730010, China
*
Author to whom correspondence should be addressed.
Water 2022, 14(17), 2653; https://doi.org/10.3390/w14172653
Received: 27 July 2022 / Revised: 24 August 2022 / Accepted: 26 August 2022 / Published: 28 August 2022
(This article belongs to the Special Issue Advances in Studies on Ecohydrological Processes in the Arid Area)

Abstract

:
Soil water and salt movement during the freeze–thaw period facilitate soil and water conservation and agroecological environment maintenance in the desert–oases transition zone of the Hexi Corridor; however, our understanding of soil salinization and the shifting water, heat, and salt states in soil ecosystems of Haloxylon ammodendron forests at different ages is poor. We analyzed the soil moisture, temperature, and salinity characteristics of Haloxylon ammodendron forests of different ages in the Hexi Corridor of Northwest China and determined their coupling. Our results indicated that shallow (0–120 cm) soil temperatures significantly correlated with air temperatures. With increased forest age, the soil freezing period shortened and the permafrost layer shallowed. Changes in soil temperature lagged those in air temperature, and this lag time increased with forest age and soil depth. With increases in forest age and planting years, the water in the shallow soil layer gradually declined, and the surface aggregation of salt increased. In deep soils (120–200 cm), both soil moisture and salinity increased with the number of planting years. Accordingly, the clay layer and deep root system of Haloxylon ammodendron greatly influenced the transport of soil water and salt; and temperature is a key driving force for their transport. Thus, water, temperature, and salt content dynamics were synergetic.

1. Introduction

The transition zone between desert and oasis ecosystems is typified by the frequent exchange of materials and energy and intense agricultural irrigation, which result in the excessive exploitation of groundwater and surface water, large-scale degradation of natural vegetation, and forward displacement of desert dunes. Meanwhile, water resources serve as the key limiting factor of social and economic development and impact ecological security in arid northwest China [1,2]. Planting artificial sand-fixing vegetation in the desert–oasis transition zone has become one of the primary measures to quickly and effectively control desertification in this region, thereby maintaining the stability of the oasis ecosystem [3]. However, in arid and semi-arid ecosystems, where precipitation is scarce and evaporation is strong, such large-scale planting of artificial vegetation results in extremely high soil salinity, especially in areas with high groundwater levels, where salinized soils are widely distributed. Moreover, declines in artificial vegetation [4] may eventually lead to further desertification and salinization [5,6].
Studies conducted worldwide have shown that soil salinization is closely related to the transport of soil water and salt [7,8]. Salt is produced from rock and mineral weathering [9] and is derived from the water via evaporation and transported by water—this is the general pattern of soil water and salt migration [10]. Seasonal freezing and thawing are the main factors driving soil salinization in arid regions. The changes in and interactions among soil moisture, temperature, and salinity over time and through space are important prerequisites for studying the characteristics and mechanisms of soil salinization. Jin et al. [11] and Hudan et al. [12] studied the properties of water and salt migration in cotton field soils within arid regions by performing numerical simulations and comparing the results with measured soil water and salt contents. Their results showed that the temperature change during the freeze–thaw process greatly impacted the behavior of soil water and salt transport. Based on field-measured data on the changes in salinity, moisture, and temperature during soil freezing and thawing over many years, Li et al. [13,14] determined that freezing and thawing is a key cause of soil salinization in the arid and cold regions of Northwest China. They also simulated the dynamic changes in soil moisture, temperature, and salt content during the freezing and thawing period using the coupled hydrothermal simultaneous heat and water (SHAW) model, which yielded a high degree of precision in their results. Li et al. [15] and Liu et al. [16] similarly studied the dynamic changes in soil water, temperature, and salt content. Gao et al. [17], Gong et al. [18], and Li et al. [19] focused on the spatial variation and distribution of water and salt. Nevertheless, relatively few studies have been conducted on the coupling between soil moisture, temperature, and salt in artificial, sand-fixing forests of different ages.
Haloxylon ammodendron (H. ammodendron), a C4 desert shrub in the Chenopodiaceae family, is widely distributed in arid and desert regions across China, including the Xinjiang Uygur and Inner Mongolia Autonomous Regions and Gansu and Qinghai Provinces [20]. Owing to its adaptability to wind, sand, salinity, drought, and other environments, it has become the main tree species employed for sand fixation and afforestation in the desert–oasis transition zone of the Hexi Corridor [21,22]. With the continuous expansion of the planting area of H. ammodendron forests in arid northwest China, some researchers have found that salt has accumulated in the surface soils of these forests approximately 30–40 years after planting [23]. However, the current research on artificial H. ammodendron forests has mainly focused on the physiological characteristics of H. ammodendron photosynthesis and water utilization [24], as well as the enrichment of soil nutrients in these forests [25,26], and the effects of long-term sand-fixing H. ammodendron forests on the soil environment [27,28]. Meanwhile, relatively few studies have been conducted on soil salinization after planting artificial H. ammodendron forests. In this study, we determined the migration characteristics of soil water, temperature, and salt during the freeze–thaw period of H. ammodendron forests of different forest ages in a typical desert–oasis transition zone of the central Hexi Corridor. We explored their respective distributions to provide a reference for controlling desertification.
In the present study, we selected 20-, 30-, and 50-year-old H. ammodendron plantations to investigate differences in soil moisture, heat, and salinity in the root zone of H. ammodendron across different stand ages. We hypothesized that soil salt content would increase with an increase in H. ammodendron plantation age, and seasonal freezing and thawing influence soil moisture, temperature, and salt content.

2. Materials and Methods

2.1. Study Area

Our study was conducted in the Linze Inland River Basin Station of the Chinese Academy of Sciences in the central Hexi Corridor of Gansu Province (38°57′–39°42′ N, 99°51′–100°30′ E), at an altitude of 1420 m (Figure 1). The region has a temperate steppe climate; the mean annual precipitation is 118.4 mm, the mean annual temperature is 7.7 ℃, the annual average wind speed is 3.2 m/s, and total evaporation is 1830.4 mm. Along the edge of the oasis is mainly a dune landscape, which is an extension of the Badain Jaran Desert and a passage for invading winds and sand. H. ammodendron shrubs have been artificially planted and coexist with the natural shrubs Calligonum mongolicum and Nitraria tangutorum, and annual herbs Bassia dasyphylla, Halogeton arachnoideus, and Suaeda glauca. Many artificial H. ammodendron were planted in multiple phases across the study area, gradually forming artificial H. ammodendron forests of different ages ranging from the edge of the oasis to the periphery (approximately 5 × 10 km). The plantation has aeolian sandy soil with a similar texture and organic carbon content as the surrounding area [23].

2.2. Sampling

In mid-July of 2018, we selected 20-, 30-, and 50-year-old H. ammodendron forests for sampling, and 5TE soil moisture, temperature, and conductivity sensors (Decagon Devices, Inc., Pullman, WA, USA) were installed along 0 to 200 cm soil profiles for long-term observation. The soil temperature, water content, and electrical conductivity of five soil layers (0–40, 40–80, 80–120, 120–160, and 160–200 cm) were logged every 30 min (CR1000, Campbell Scientific, Inc., Logan, UT, USA) (Figure 2). Soil salinity was represented by electrical conductivity. In addition, the soil samples (3 stand ages × 5 depth gradients) were collected across the five soil layers from the 20-, 30-, and 50-year-old H. ammodendron plantations site for soil physical property analyses. The bulk density of the soil from 0 to 200 cm was between 1.4 and 1.57 g/cm3 and it gradually increased with depth; soil porosity was between 43.75 and 49.81% and generally decreased with depth. These values indicated that shallow soils were better aerated than deep soils. Among forests of different ages, most soils were sandy, with sand contents >92%. Loam was found in the 160–200 cm soil layer of the 50-year-old H. ammodendron forest, with a silt content of up to 45.81%; a reddish-brown cohesive layer was also distributed across this depth, wherein the soil bulk density decreased and porosity increased (Table 1).
When the air temperature is ≤0 °C, water typically begins to freeze; therefore, when the soil temperature is ≤0 °C, soil water will begin to freeze [29], and when the soil temperature is ≥0 °C, it will begin to melt. On 5 November 2018, the daily mean temperature was <0 °C for the first time in the study period. Three temperature stages were subsequently identified: the freezing period (5 November 2018 to 6 January 2019), the melting period (6 January to 1 March 2019), and the initial evaporation period (1 March to 6 May 2019). According to the freezing characteristics of the soil during the freeze–thaw process and the root distribution characteristics of H. ammodendron in the study area [30], the soil profiles could be divided into two layers: a shallow layer (0–120 cm) and a deep layer (120–200 cm).
H. ammodendron is a super-xerophyte shrub with a well-developed root system. With aging, its root system continuously expands both laterally and vertically. While its lateral root system continues to expand, it is mainly distributed within the shallow soil (≤50 cm), while the main root system extends into the deep soil. After more than 20 years, the root system of H. ammodendron can be deep within the soil or in the saturated area close to the groundwater level, with many branch roots being present in deep soils. The proportion of biomass in the shallow root system also gradually decreases with the age of H. ammodendron, while the proportion in the deep root system increases continuously. The water supplied to the plant by its deep root system increases with age, as does the contributions of deep soil water and groundwater [31].

2.3. Data Analysis

This study used Excel (Microsoft Corp., Redmond, WA, USA) for preliminary data processing. We used SPSS v. 26 (IBM Corp., Chicago, IL, USA) to analyze the coefficients of variation (CV) and for regression analyses of soil temperature, water content, and electrical conductivity data. Origin 2018 was used for mapping (OriginLab Corp., Northampton, MA, USA), and the Corrplot package of R v. 4.1.3 [32] was used for correlation analysis and mapping of the soil layers of artificial H. ammodendron forests aged 20, 30, and 50 years [33].

3. Results

3.1. Variation in Soil Moisture, Temperature, and Salt by Forest Age

3.1.1. Soil Temperatures in H. ammodendron Forests

The variation in soil temperatures among the H. ammodendron forests from 0 to 200 cm was basically the same 20, 30, and 50 years after planting. However, with increases in stand age, the freezing period of the soil shortened and the permafrost layer shallowed (Figure 3a,c,e). The freezing periods ranged from 4 December 2018 to 1 March 2019, 4 December 2018 to 25 February 2019, and 4 December 2018 to 24 February 2019 at 20-, 30-, and 50-years post-planting, respectively. During the freezing period, with temperature decreases, the frozen layer gradually deepened, while with increases in forest age, the depth of the frozen layer shallowed. At the same time, soil temperatures changed significantly among different depths. In the shallow (0–120 cm) soils, changes in soil temperature were closely related to air temperature, while in the deep (120–200 cm) soils, the air temperature had less of an effect (Figure 3b,d,f). The amplitude of changes in soil temperature decreased with increasing soil depth. On 6 December 2018, the minimum temperature was −21.28 °C, and the difference between the minimum soil temperature and the minimum temperature at 40 cm in the 20-year-old H. ammodendron forest was 10.35 °C, with a lag of 24 days. The change in soil temperature lagged behind the change in air temperature, and the lag time increased with increases in stand age and soil depth (Table S1).

3.1.2. Soil Moisture and Salt in H. ammodendron Forests

Variations in the moisture and salinity of 0–200 cm soils varied with forest age. Soil moisture and salinity also exhibited different characteristics from 0 to 120 cm between the shallow and deep layers. With increases in the age of H. ammodendron forests, the soil moisture in the shallow layer (0–120 cm) gradually decreased, as did that in the deep layer of 30- and 50-year-old forests. Soil moisture showed an increasing trend with depth over time, which may be related to the uptake of groundwater by the root system (Figure 4a–c). At 20, 30, and 50 years, the changes in soil salinity from 0 to 200 cm significantly differed. The soil salinities in the 20- and 30-year-old forests were lower than that of the 50-year-old forest, and the upper soils’ average conductivities were 0.036 and 0.037 ds·m−1, respectively. Meanwhile, at 50 years, salt had accumulated in the shallow layer (Figure 4d–f).
During the study period, the moisture and conductivity of the upper layer first decreased and then increased (Figure 4); however, with increases in forest age, the range of variability decreased. When the soil was frozen, its moisture and conductivity were low. The water content in the shallow (0–120 cm) soil layer gradually decreased with increases in forest age, while salinity was high after 50 years and increased with forest age (Figure 5). During the freezing period (5 November 2018–6 January 2019), soil moisture decreased across the soil profiles in the H. ammodendron forests (Figure 6a,d,g), while in the thawing period (6 January–1 March 2019), soil moisture increased (Figure 6b,e,h). This trend continued during the initial evaporation period (1 March–6 May 2019) (Figure 6c,f,i). Moreover, the amount of variation in soil moisture significantly differed between different soil depths. In shallow soils (0–120 cm), the changes in soil moisture were large and consistent, while in deep soils (120–200 cm), the changes were small.
During the freezing period, the shallow soil salinity of H. ammodendron forests decreased, while the deep soil salinity remained stable (Figure 7a,d,g), while during the thawing period, the salinity of shallow soils (0–80 cm) increased and that of deep soils was stable (Figure 7b,e,h). In the initial evaporation period, soil salinity remained stable, with only a slight decline at 0–40 cm and a partial increase at 40–80 cm (Figure 7c,f,i). The CV of soil salinity was greater than that of soil moisture in artificial H. ammodendron forests with different planting years, especially in shallow soils (0–80 cm) during the freezing and thawing periods (Figure 8a,b,d,e,g,h). During the initial evaporation period, the variation in deep (120–200 cm) soil moisture and soil salinity increased (Figure 8c,f,i), especially in the 30-year-old forest.

3.2. Coupling between Soil Water, Temperature, and Salt with Forest Age

3.2.1. Correlation Analysis of Soil Water, Temperature, and Salt in Different Soil Layers

The changes in the soil moisture, temperature, and salinity of different soil layers were conspicuously different among the H. ammodendron forests of different ages. Comparing the correlation coefficients between adjacent soil layers, we found that in the 20-year-old forest, the temperature and moisture between different layers in the freezing period were significantly and positively correlated (r > 0.9), and the correlation between deep soils was better than in shallow ones. Surface soil electrical conductivity (EC) was also significantly and positively correlated, but deep soil conductivity was negatively correlated (Figure 9a,d,g). During the thawing period, soil moisture and salt content were significantly correlated between the topsoils. Temperature correlations between the deep soils weakened, soil moisture was negatively correlated between the layers, and the significant correlations in the EC of different layers weakened (Figure 9b,e,h). During the initial evaporation period, the significant and positive correlation between the temperatures of different layers increased while that of surface soil moisture decreased, and these correlations gradually strengthened with soil depth. A significant negative correlation was observed in the EC of the shallow soils, whereas a significant positive correlation appeared at depth (Figure 9c,f,i).
In the 30-year-old H. ammodendron forest, the temperature and moisture between different layers in the freezing period were significantly and positively correlated, with the correlations between soil temperatures being stronger than those of water. The EC of the surface and deep layers were also positively correlated, with deep soils exhibiting strong correlations (Figure 10a,d,g). During the thawing period, both soil temperature and moisture in the middle layer were negatively correlated, and the preexisting significant correlation in the surface EC was enhanced (Figure 10b,e,h). During the initial evaporation period, the temperature and moisture between different soil layers were significantly and positively correlated, with the correlation of deep soils being stronger; the ECs of shallow soils were significantly and negatively correlated (Figure 10c,f,i).
In the 50-year-old H. ammodendron forest, the soil temperature and moisture between different layers in the freezing period were significantly and positively correlated, and their correlations gradually increased with depth. The EC between the shallow layers was negatively correlated, while that between the deep layers was positively correlated (Figure 11a,d,g). During the thawing period, the soil temperature and moisture in the middle layer were negatively correlated, while the ECs between the shallow layers were significantly positively correlated, though this significance gradually weakened with depth (Figure 11b,e,h). In the initial evaporation period, the temperature and moisture between different soil layers were significantly and positively correlated, and this correlation was stronger at greater depths. The surface EC showed a significant positive correlation between layers, while the EC was negatively correlated at depth and the significance of this correlation gradually increased with depth (Figure 11c,f,i).

3.2.2. Regression Analysis of Soil Water, Temperature, and Salt in Different Soil Layers

The changes observed in the soil water, temperature, and salt content from 0 to 200 cm differed among H. ammodendron forests of different ages. In the 20-year-old forest, the change between the soil water and salt in the shallow (0–80 cm) layer was the most drastic (i.e., |k| was the highest). During the freezing period, as the temperature declined, the soil froze and the soil moisture, EC, and salinity decreased; slope k generally decreased with increasing soil depth. At 80–120 cm, the EC was negatively correlated with water and temperature (i.e., with temperature decreases, the water contents decreased, but the EC and salinity increased). During the thawing period, as air temperatures rose, so did soil temperatures, and the solid water (ice) in the soil became liquid (water). Compared to the freezing period, the absolute value of the slope of the regression equation (i.e., |k|) decreased, indicating that the changes in the soil water, temperature, and salt were more gradual. Salinity in the deep (80–160 cm) layer was sometimes negatively correlated with soil moisture and temperature. During the initial evaporation period, the relationship between surface water, temperature, and salt weakened, while the changes among them were the most significant at 80–120 cm (Table S2).
In the 30-year-old H. ammodendron forest, the changes in the salinity from 40 to 80 cm with water content and temperature were the most severe. During the freezing period, the relationship between soil temperature and moisture gradually weakened with increasing soil depth. The EC of the shallow layer was positively correlated with soil moisture and temperature and differed significantly from that of the deep layer. At soil depths of 120–160 cm, the EC was negatively correlated with soil temperature and water content. The relationship between soil temperature and moisture was greatly enhanced during the thawing period, especially in deep soils. The EC was positively correlated with moisture and temperature in both surface and deep soils. During the initial evaporation period, surface conductivity was negatively correlated with soil temperature and water content, indicating that as the temperature increased, the amount of surface water increased, and salinity declined. A positive correlation was observed in the salinity between deep soils, and the amount of soil water changed greatly with increases in temperature (Table S3).
In the 50-year-old H. ammodendron forest, the changes in the salinity from 120 to 160 cm with water content and temperature were also the most severe. During the freezing period, the relationships between soil salt, temperature, and moisture were significantly positive at 0–40 cm. However, at 40–160 cm, soil salt showed a negative relationship with soil temperature and moisture. The changes in soil salt decreased (i.e., |k|) with the increasing depth. During the thawing period, the relationships between soil salt, temperature, and moisture were significantly positive at 80–160 cm. During the initial evaporation period, the relationship between soil water and temperature was the most significant, especially at depths of 120–160 cm (Table S4).

4. Discussion

In this study, we found that changes in soil temperature directly affected soil water and salt transport, especially in shallow soils. Soil temperatures at depths of 0–40 and 40–80 cm in H. ammodendron forests of different ages significantly differed and were positively correlated. Since it takes time for heat to transfer downward from the surface, surface soils have high water content. Since the heat capacity of water is much greater than soil’s, a large amount of latent heat of condensation was released during the freezing process, which slowed the downward freezing of the soil [15,34]. Additionally, with increases in forest age, shallow soil moisture gradually decreased, and salinity gradually increased, such that the freezing temperature of the soil gradually declined with increasing salinity [35], and the freezing periods successively shortened. The freezing period of soil become shorter, and the permafrost layer became shallower with an increase in forest age; the change in soil temperature lagged the change in air temperature, and the lag time increased with an increase in forest age and soil depth. The findings are like the results reported by Jin et al. [11] and Li et al. [14].
The temperature of the frozen soil layer was 0 °C and its position moved continuously downward with decreases in soil temperature. During the freezing period, soil moisture between the two adjacent soil layers was significantly and positively correlated. Changes in the shallow soil temperature had a greater impact on soil moisture than those in deeper soils, and changes in soil moisture ultimately affected the salt content. This result was consistent with a previous simulation [36] of the migration of water and salt in soils under freeze–thaw conditions. Water in soils was not only a solvent but also a carrier for salt transport. The movement of water ultimately determined the movement of salt, and the movement of water under a given temperature gradient determined the general direction and trend of salt migration. Temperature changes during soil freezing and thawing led to the migration of water and salt. When the soil froze, the liquid water in the frozen layer turned into ice, and the liquid water content of the soil remained minimal as the ice content gradually increased. Salt was mainly concentrated in unfrozen water. When the deep soil moisture content was high, the salinity was diluted. In addition, the CV of salinity was greater than that of water, indicating that changes in salinity were more significant than those in soil moisture.
With increases in stand age, we found that the shallow soil moisture gradually decreased and salt accumulation on the soil surface increased. During the thawing period, the surface soil moisture of H. ammodendron forests of different ages increased with increasing temperature. When water was lost via evaporation, the remaining dissolved salts were concentrated, and the EC increased; furthermore, surface salinity increased with moisture, especially in the 50-year-old H. ammodendron forest, where the salinity of the surface layer (0–40 cm) increased most significantly. The patterns of water and salt migration observed in this study were consistent with the findings of a study by Jin et al. [11], who examined cotton fields in northern Xinjiang. During rapid temperature increases, the ice accumulated in the permafrost began to melt from the surface downward. Under the action of the resulting temperature gradient, meltwater migrated to the surface layer and rapidly evaporated. With the melting of the upper frozen layer, the soil water migrated to the surface and evaporated. The salt accumulated during the freezing period also quickly accumulated at the surface, causing the salt content of the topsoil to increase dramatically.
Salt accumulated on the soil surface in bursts, resulting in drastic changes in water and salinity in the shallow layers. Plant transpiration and soil evaporation were the primary routes of soil water loss and causes of salt accumulation at shallow soil depths in arid regions [37]. In the same study area, Chang et al. [38] found that the basal stem increased, and the daily water consumption increased with an increase in H. ammodendron age. When water was lost from evaporation and transpiration, the remaining dissolved salts were concentrated, and the salt content of the soil surface increased. During the initial evaporation period, with continuous increases in air temperature, the topsoil temperature increased most rapidly, evaporation was the most intense, and the water potential of the topsoil decreased. At the same time, with increased forest age and soil depth, the correlation of soil moisture between adjacent soil layers generally increased, reflecting the gradual upward movement of deep soil water. Salt also moved upward, and the surface aggregation of salt was enhanced. Due to the existence of a clay layer at 160–200 cm in the 50-year-old H. ammodendron forest, the migration of soil water and salt was more obvious. This clay layer increased soil moisture heterogeneity by changing the type and transport of soil water within the profile. Additionally, H. ammodendron is a dilute halophyte that absorbs a large amount of salt during its growth and development [39]. The absorbed salt entered the soil through litter, causing salt accumulation in the soil, which may be the reason for the observed increase in surface salinity over time.
In this study, the accumulation of salt on the soil surface was related to the uptake of groundwater by plant roots, as well as soil texture. The groundwater in the study area is shallow, mainly distributed at 3–5 m and has a high salt content (Table S5). With increases in forest age, the proportion of clay and silt in the topsoil continuously increased, yielding a greater water-holding capacity. In the 50-year-old H. ammodendron forest, with 160–200 cm of clay, the soil bulk density decreased, porosity increased, and the soil water content was the highest among the forest ages. Cui et al. [40] also showed that clay beds can significantly improve the soil’s water-holding capacity. We recorded an area with low water contents above the cohesive layer, which was consistent with the results of Sun et al. [41], who studied the water-holding characteristics of cohesive sand dunes at the edge of the oasis, where the soil water-holding properties are poor. When the clay and silt contents of a soil increase, the proportion of ventilation pores decreases and the soil developed a strong ability to hold water. At the same time, due to the capillary action of groundwater in the 160- to 200-cm soil layer, the soil moisture content was high in our study, and the existence of the cohesive layer increased the heterogeneity of soil moisture by changing the type and migration of soil water in the profile [42]. Zhou et al. [30] also found that there is a cohesive layer in the soil profiles of some sand dunes at the edge of the oasis.
The biomass of the coarse root system of H. ammodendron increased significantly at the interface of the clayey layer, and this root system has a strong ability to absorb groundwater (Figure 12). The consistent distribution of soil moisture in H. ammodendron forests and root systems highlight its adaption to a desert environment [43]. The clay layer greatly affects the distribution and movement of water and salt in the soil profile. With increased stand age, the water content in the upper soil gradually decreased and salinity increased, while the deep soil moisture and EC increased, and there was an upward movement of salt. This was mainly due to self-collection via trunk runoff [44,45,46], which formed a wet zone in the roots and enriched the soil water. Moreover, with increased stand age, the proportion of the deep root system of H. ammodendron increased continuously, and its main water source gradually changed from soil water to groundwater [31].
Changes in groundwater levels are well-correlated with soil freezing and thawing, as water levels fall during freezing and rise during thawing [47]. Zhu et al. [48] studied the temporal and spatial changes in soil moisture in the root zone of H. ammodendron stands of different ages and found that the soil moisture content was greater in mature forests than in middle-aged forests or bare land. The depth and distribution of the root system of H. ammodendron significantly change with age. Older plants have deeper root systems, and with the increasing proportion of deep roots, the contributions of deep soil water and groundwater increase [49]. Salt travels with water—this is the main way salt moves through the soil. Therefore, with increased stand age, water migrates to the roots, carrying salt ions, which accumulate there, and the water and salt content in the deep soil gradually increase [50].
The interactions between soil water, temperature, and salt in forests of different ages markedly differ over time and space. According to the changes we observed between the different soil layers, it was reasonable to divide the freeze–thaw period and structure of the soil layers. This was conducive to the in-depth exploration of the temporal and spatial variations in the coupling of soil moisture and salinity. Temperature indirectly affected soil salinity by directly affecting water migration, and freeze–thaw processes changed the distribution of salt and water in each layer in a directional manner. Driven by temperature gradients, water and salt migrated upward through the soil profile. Under the influence of evaporation, salt remained in the soil, and in the upper part of the soil, the salt content increased, resulting in salinization [51].

5. Conclusions

Seasonal freezing and thawing substantially impacted the transport of water, heat, and salt in shallow soils. Shallow soil temperature is closely related to air temperature. With increases in forest age, the soil freezing period shortened and the permafrost layer shallowed. However, changes in soil temperature lagged those in the air temperature, and this lag increases with an increase in forest age and soil depth. With increased forest age, the water content in the shallow soil layer (0–120 cm) gradually decreased, and the surface aggregation of salt increased. In deep (120–200 cm) soils, both soil moisture and salinity increased with planting years.
The existence of a clay layer and the deep root system of H. ammodendron greatly influenced the transport of soil water and salt. Temperature changes drove the migration of water, and the resultant temperature gradient played an important role in the direction of soil water and salt migration and their distribution. The freezing and thawing of soil also promoted the loss of soil moisture and salt accumulation at the surface.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/w14172653/s1, Table S1: Minimum temperature and date of soil profile of H. ammodendron forests of different ages; Table S2: Regression analyses of soil moisture, temperature, and salt in different soil layers of a 20-year-old H. ammodendron forest; Table S3: Regression analyses of soil moisture, temperature, and salt in different soil layers of a 30-year-old H. ammodendron forest; Table S4: Regression analyses of soil moisture, temperature, and salt in different soil layers of a 50-year-old H. ammodendron forest; Table S5: Groundwater quality parameters in the study area; Figure S1: Geological profile of the borehole at the research station.

Author Contributions

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

Funding

This research was financially supported by the National Natural Science Foundation of China (Grant No.42171033 and 41807518) and Opening Foundation of Key Laboratory of Desert and Desertification, Chinese Academy of Sciences (Grant No. KLDD-2020-05).

Institutional Review Board Statement

No applicable.

Informed Consent Statement

No applicable.

Data Availability Statement

Date cannot be made publicly available; readers should contact the corresponding author for details.

Acknowledgments

We would like to thank the Linze Inland River Basin Research Station Experimental for field experiment support. We also thank Wenzhi Zhao for providing useful suggestions.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location of the study area.
Figure 1. Location of the study area.
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Figure 2. H. ammodendron forest plots aged (a) 20, (b) 30, and (c) 50 years.
Figure 2. H. ammodendron forest plots aged (a) 20, (b) 30, and (c) 50 years.
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Figure 3. Variations in soil and air temperatures in H. ammodendron forests aged (a,b) 20, (c,d) 30, and (e,f) 50 years. The 0.00 °C curve represents the frozen layer, above which soil temperature ≤0 °C and below which soil temperature ≥0 ℃. DAT, daily average temperature.
Figure 3. Variations in soil and air temperatures in H. ammodendron forests aged (a,b) 20, (c,d) 30, and (e,f) 50 years. The 0.00 °C curve represents the frozen layer, above which soil temperature ≤0 °C and below which soil temperature ≥0 ℃. DAT, daily average temperature.
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Figure 4. Variation in (ac) volumetric water content (VWC) and (df) electrical conductivity (EC) in the soil profiles of H. ammodendron forests at (a,d) 20, (b,e) 30, and (c,f) 50 years and a schematic of the H. ammodendron root system.
Figure 4. Variation in (ac) volumetric water content (VWC) and (df) electrical conductivity (EC) in the soil profiles of H. ammodendron forests at (a,d) 20, (b,e) 30, and (c,f) 50 years and a schematic of the H. ammodendron root system.
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Figure 5. Variation in VWC and EC in the (a) shallow and (b) deep soils of H. ammodendron forests of different ages. The error bars in the figures represent standard errors.
Figure 5. Variation in VWC and EC in the (a) shallow and (b) deep soils of H. ammodendron forests of different ages. The error bars in the figures represent standard errors.
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Figure 6. Changes in VWC with soil depth during the (left) freezing (5 November 2018–6 January 2019), (middle) thawing (6 January 2019–1 March 2019), and (right) initial evaporation (1 March–6 May 2019) periods in H. ammodendron forests aged (ac) 20, (df) 30, and (gi) 50 years.
Figure 6. Changes in VWC with soil depth during the (left) freezing (5 November 2018–6 January 2019), (middle) thawing (6 January 2019–1 March 2019), and (right) initial evaporation (1 March–6 May 2019) periods in H. ammodendron forests aged (ac) 20, (df) 30, and (gi) 50 years.
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Figure 7. Variation in the soil EC of H. ammodendron forests aged (ac) 20, (df) 30, and (gi) 50 years during the (left) freezing, (middle) thawing, and (right) initial evaporation periods.
Figure 7. Variation in the soil EC of H. ammodendron forests aged (ac) 20, (df) 30, and (gi) 50 years during the (left) freezing, (middle) thawing, and (right) initial evaporation periods.
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Figure 8. Coefficient of variation of VWC and EC in the soil profiles of H. ammodendron forests aged (ac) 20, (df) 30, and (gi) 50 years during the (left) freezing, (middle) thawing, and (right) initial evaporation periods. The EC at 160–200 cm in the 20-year-old H. ammodendron soil was constant at zero, so the CV could not be calculated. In the freezing and initial evaporation periods, the EC at 120–160 cm was also constant at zero. Hence, the CV appeared abnormally high. Similarly, the CV of the EC at 80–120 cm in the 30-year-old H. ammodendron could not be calculated.
Figure 8. Coefficient of variation of VWC and EC in the soil profiles of H. ammodendron forests aged (ac) 20, (df) 30, and (gi) 50 years during the (left) freezing, (middle) thawing, and (right) initial evaporation periods. The EC at 160–200 cm in the 20-year-old H. ammodendron soil was constant at zero, so the CV could not be calculated. In the freezing and initial evaporation periods, the EC at 120–160 cm was also constant at zero. Hence, the CV appeared abnormally high. Similarly, the CV of the EC at 80–120 cm in the 30-year-old H. ammodendron could not be calculated.
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Figure 9. Correlations between temperature (ac), moisture (df), and salinity (gi) in different soil layers within a 20-year-old H. ammodendron forest over the (left) freezing, (middle) thawing, and (right) initial evaporation periods. Numbers 1 through 5 represent soil depths of 0–40, 40–80, 80–120, 120–160, and 160–200 cm, respectively. The EC at 160–200 cm was constant, and the standard deviation was zero; hence, the correlation could not be calculated. Temp.—temperature.
Figure 9. Correlations between temperature (ac), moisture (df), and salinity (gi) in different soil layers within a 20-year-old H. ammodendron forest over the (left) freezing, (middle) thawing, and (right) initial evaporation periods. Numbers 1 through 5 represent soil depths of 0–40, 40–80, 80–120, 120–160, and 160–200 cm, respectively. The EC at 160–200 cm was constant, and the standard deviation was zero; hence, the correlation could not be calculated. Temp.—temperature.
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Figure 10. Correlations between temperature (ac), moisture (df), and salinity (gi) in different soil layers within a 30-year-old H. ammodendron forest over the (left) freezing, (middle) thawing, and (right) initial evaporation periods. The EC at 80–120 cm was constant and the standard deviation was zero; hence, so the correlation could not be calculated.
Figure 10. Correlations between temperature (ac), moisture (df), and salinity (gi) in different soil layers within a 30-year-old H. ammodendron forest over the (left) freezing, (middle) thawing, and (right) initial evaporation periods. The EC at 80–120 cm was constant and the standard deviation was zero; hence, so the correlation could not be calculated.
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Figure 11. Correlations between temperature (ac), moisture (df), and salinity (gi) in different soil layers within a 50-year-old H. ammodendron forest over the (left) freezing, (middle) thawing, and (right) initial evaporation periods. The EC at 160–200 cm during the freezing period was constant and the standard deviation was zero; hence, so the correlation could not be calculated.
Figure 11. Correlations between temperature (ac), moisture (df), and salinity (gi) in different soil layers within a 50-year-old H. ammodendron forest over the (left) freezing, (middle) thawing, and (right) initial evaporation periods. The EC at 160–200 cm during the freezing period was constant and the standard deviation was zero; hence, so the correlation could not be calculated.
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Figure 12. Schematic of the root system and groundwater contribution of H. ammodendron for different forest ages: (a) 20, (b) 30, and (c) 50 years. Arrow sizes indicate relative groundwater contributions.
Figure 12. Schematic of the root system and groundwater contribution of H. ammodendron for different forest ages: (a) 20, (b) 30, and (c) 50 years. Arrow sizes indicate relative groundwater contributions.
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Table 1. Soil profile characteristics of H. ammodendron forests of different ages.
Table 1. Soil profile characteristics of H. ammodendron forests of different ages.
Age
(Years)
Profile
(cm)
Bulk Density
(g/cm3)
Porosity
(%)
Clay Content
(%)
Silt Content
(%)
Sand Content
(%)
Texture
200–401.4146.170.424.7294.86Sand
40–801.4545.280.645.8493.52Sand
80–1201.4645.270.895.3593.76Sand
120–1601.4843.770.915.3693.73Sand
160–2001.5443.750.976.1292.91Sand
300–401.4346.040.444.9294.64Sand
40–801.4445.290.745.3393.93Sand
80–1201.4844.150.915.3293.77Sand
120–1601.4943.791.026.1592.83Sand
160–2001.5744.110.945.4393.63Sand
500–401.447.150.475.3194.22Sand
40–801.4246.420.595.3694.05Sand
80–1201.4544.910.965.4293.62Sand
120–1601.4744.531.055.7493.21Sand
160–2001.3349.819.3245.8144.87Loam
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Gou, Q.; Shen, C.; Wang, G. Changes in Soil Moisture, Temperature, and Salt in Rainfed Haloxylon ammodendron Forests of Different Ages across a Typical Desert–Oasis Ecotone. Water 2022, 14, 2653. https://doi.org/10.3390/w14172653

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Gou Q, Shen C, Wang G. Changes in Soil Moisture, Temperature, and Salt in Rainfed Haloxylon ammodendron Forests of Different Ages across a Typical Desert–Oasis Ecotone. Water. 2022; 14(17):2653. https://doi.org/10.3390/w14172653

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Gou, Qianqian, Changsheng Shen, and Guohua Wang. 2022. "Changes in Soil Moisture, Temperature, and Salt in Rainfed Haloxylon ammodendron Forests of Different Ages across a Typical Desert–Oasis Ecotone" Water 14, no. 17: 2653. https://doi.org/10.3390/w14172653

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