Moisture, Temperature, and Salinity of a Typical Desert Plant (Haloxylon ammodendron) in an Arid Oasis of Northwest China

Abstract

The physical and chemical characteristics of soil and water sources affect desert plants’ growth, which is essential for the ecological protection in arid areas. The typical patch patterns of Haloxylon ammodendron in the oasis-desert ecotone in the southern margin of the Manas River Basin consists of bare patches (BP) and vegetation patches (VP). The water sources of H. ammodendron were studied using stable isotope technology, and the soil physical and chemical properties were monitored and analyzed. The results showed that the soil moisture presented a reversed “S” type curve, and the total salt content of the soil presented an “S” type curve. A “wet island” and “cold island” were formed in the low salt area with H. ammodendron at the center. NaCl was most abundant in the BP soil, and the milligram equivalent of Cl⁻ was 80–90%, while CaSO₄ was most abundant in the VP soil, in which the milligram equivalent of SO₄²⁻ was 80–100%. Before the rain, H. ammodendron mainly relied on the soil water from a deeper layer (≥60 cm) to maintain its growth. However, after the rain, H. ammodendron mainly relied on shallow soil water (<60 cm) to maintain its growth.

1. Introduction

Patch vegetation is composed of discontinuous bare patch (BP) and vegetation patch (VP) [1], which is a typical pattern of vegetation landscape in arid and semi-arid regions [2]. As an important part of the ecosystem in arid areas, natural vegetation is the most intuitive reflection of the natural environment [3]. It is of great significance in controlling the desertification process and protecting biodiversity [4]. Vegetation patch patterns result from the interactions between ecological and hydrological processes in arid areas and are common and relatively stable vegetation forms in arid areas [5]. Patch vegetation is distributed in arid areas worldwide [6]. The Junggar Basin is located in the arid inland area of Northwest China adjacent to the Gurbantunggut Desert, the second largest desert in China [7]. In this area, precipitation is scarce, evaporation is strong, and the ecological environment is fragile.There are many patches and strips of natural vegetation or coppice dune in the Oasis-Desert ecotone [8], forming a patch landscape pattern with H. ammodendron as the primary desert vegetation [9,10]. Growth and development of this vegetation mainly depend on atmospheric precipitation or local underground infiltration water [11], and the vegetation coverage is relatively low, generally at 10–30%. It is particularly essential to maintain the ecosystem’s stability in arid areas [12]. Since the 1990s, with climate warming and the aggravation of human activities [13], patch vegetation in arid and semi-arid areas of the world face severe degradation to varying degrees [14].

Water resources may be the leading ecological factors that affect vegetation growth and distribution patterns in arid desert areas. To a certain extent, they control patch vegetation’s composition and structure [15]. Also, soil moisture determines the hydrothermal balance of the entire ecosystem and the soil’s physical and chemical characteristics, which affect the distribution patterns of vegetation to some extent in arid and semi-arid regions [16,17]. The water sources for plants vary under different climatic and environmental conditions [18,19]. In arid areas, plants can only use surface soil water when there is adequate rainfall [20]. In the dry season with little rain, due to the low water content in the soil, the water is not easily absorbed by the plants, so the plants mainly absorb and utilize the deeper soil water through the deep primary root system [4]. In arid areas, limited by precipitation and a deficiency of available moisture in the soil, the feedback coupling system between water and vegetation closely connects ecological and hydrological processes [21,22]. The coupling of water cycles and patch patterns is the critical factor in maintaining the stable succession of arid desert ecosystems [23]. Quantitative studies on the capture, maintenance, transmission, and redistribution of water and other resources by different vegetation patterns can provide a critical research approach to evaluate the response mechanism of ecosystem changes to the water cycle [24]. Soil moisture determines the whole ecosystem’s water and heat balance [21]. In arid and semi-arid areas, soil moisture determines soil properties and the distribution of soil water, salt, and nutrients, which affect the distribution pattern of vegetation to a certain extent [25,26].

In three arid Mediterranean ecosystems in Spain, Greece and Morocco, Kéfi et al. found that the patch-size distribution of the vegetation follows a power law. They propose that patch-size distributions may be a warning signal for the onset of desertification [27]. For a variety of patchy semiarid vegetation types in Australia, Europe, and North America, Ludwig et al. found that patches significantly stored more soil water, produced more growth and had better infiltrability than interpatches [28]. Luo et al. described Spatio-temporal changes in population characteristics in two shrub populations in the transition zone between oasis and desert in the Heihe River Basin, northwestern China, and concluded that summer precipitation was a key factor that affected population characteristics and spatial patterns [29]. Xu et al. studied two typical desert shrub species, Tamarix ramosissima and H. ammodendron, co-occurring in the Gurbantonggut Desert (Central Asia). For T. ramosissima, the primary water resource was groundwater and vadose zone water. For H. ammodendron, the primary water resource was precipitation input [30]. In China, due to the high proportion of industrial water and agricultural water, and the low proportion of the ecological water, the carrying capacity of ecological environment in arid areas is reduced, the patch vegetation is seriously degraded, and the problem of land desertification is prominent, which makes the ecological environment construction one of the important problems to be solved urgently in arid areas of China [31]. H. ammodendron, as typical vegetation of patch pattern, is one of the most widely distributed desert plants in Central Asia desert area, with strong drought resistance, wind erosion resistance, sand burial resistance and other physiological characteristics [32]. The research on the adaptability of H. ammodendron in the arid environment can provide the basis for the restoration and reconstruction of artificial ecology and the sustainable development of oasis in the arid area. Plant stable isotopes are gradually becoming more widely used in ecological research in recent years. Hydrogen and oxygen stable isotopes can be used to study the water use sources of plants quantitatively and then reveal the selective absorption, water sources, and trends of plants exposed to different sources of water [33]. Stable isotope technology has been widely used in the quantitative study of plant water usage [34]. After the plant roots absorb the water, the hydrogen and oxygen isotopes in the stem xylem do not fractionate, and the components remain steady, which is consistent with the stable isotopes in the original water source. In a previous study, stable isotope mixing models were utilized to determine the proportional contributions of various water sources [35]. The overexploitation and utilization of water resources cause the groundwater depth in arid areas to drop sharply, which leads to local degradation of patch vegetation and aggravation of soil desertification. It is particularly important to study the formation, evolution, and spatial patterns of patch vegetation in arid regions. The survival and coverage rates of H. ammodendron directly determine the stability of the patch pattern; therefore, this study focused on the adaptive evaluation of H. ammodendron.

To assess the adaptability and water resources utilization for typical desert plant (H. ammodendron), a two-year experiment was conducted to compare the soil moisture, temperature, salinity and soil ions between the VP and BP in Manas River Basin, and the stable isotope technology was utilized to determine the water sources. In the experiment, the differences in soil physical and chemical properties between VP and BP were compared. The growth adaptability of H. ammodendron was obtained, and the results are significant can provide the basis for ecological conservation in arid areas. 2. Materials and Methods 2.1. Site Description

The experimental zone was located in the oasis-desert ecotone of the Manas River Basin (45°16′53″ N, 86°15′58″ E), where the annual rainfall is 100 mm, annual evaporation is 1000 mm–1500 mm, soil type is sandy loam, and the vegetation coverage rate is only approximately 30% (Figure 1). The annual average temperature is 10.0 °C, and the extreme maximum and minimum temperatures are 38.1 °C and −25.1 °C, respectively (Figure 2). There is a massive temperature difference between day and night in winter and summer. The annual sunshine duration is 2777 h, the accumulated temperature is 3594 °C, and wind primarily occurs in the spring and summer seasons, with a maximum wind speed of 20 m/s. The groundwater is at an approximate depth of 13 m. There are abundant sand plants and drought-resistant plants in this area. H. ammodendron is the dominant vegetation type.

2.2. Experimental Design

2.2.1. Soil Water Content and Soil Salinity

The soil profile was divided into five sampling intervals (0–30, 30–60, 60–90, 90–120, 120–150, and 150–180 cm) for the soil water content, temperature, and salinity measurements (Figure 3). Soil samples were collected from May to October in 2018 and 2019 at a depth of 0–180 cm from the VP of H. ammodendron and soil BP. An ENVI data-DT probe system (ENVI data-DT 1000 series; Beijing Yugen Technology Co., Ltd., Beijing, China) was used to monitor the soil water, temperature, and salinity, with a measurement accuracy of 0.01 m³/m³ for moisture, 0.1 °C for temperature, and 10 us/cm for salinity. Samples were taken every 30 cm to determine the soil pH, salinity, and ion concentrations. Meteorological factors are measured by a small automatic weather station (Product model: HOBO H21-USB; Origin: Cape Cod, Massachusetts, USA). The measurement indexes include rainfall (mm) and atmospheric temperature (°C). The measurement frequency is 30 min.

2.2.2. Hydrochemical Ions

The HCO₃⁻ and CO₃²⁻ contents were determined by a double indicator-neutralization titration [36], Cl^¯content was determined by silver nitrate titration [37], SO₄²⁻ content was determined by EDTA indirect complexometric titration [38], Ca²⁺ and Mg²⁺ contents were determined by EDTA titration [39], Na⁺ and K⁺ contents were determined by flame photometry [40], soil pH was determined by potentiometry using an acidity meter, a glass electrode, and a pH composite electrode [41], and soil moisture was determined by the oven drying method. The above measurements were repeated three times, and the average values were obtained.

2.2.3. Water Stable Isotope Measurement

The hydrogen and oxygen isotope compositions (δ¹⁸O and δD) in the soil water, groundwater, and plant water were measured using an LGR IWA-45EP water-vapor isotope analyzer (LGR IWA-45EP; LICA United Technology Limited, Beijing, China) in the Key Laboratory of Modern Water-Saving Irrigation of Xinjiang Production and Construction Group. The results were reported in conformity to the standard Vienna Standard Mean Ocean Water (VSMOW). The measurement precisions for δ¹⁸O and δD were 0.1% and 0.4%, respectively. The following equations gave δ¹⁸O and δD:

δ¹⁸O = [(¹⁸O/¹⁶O) _Sample − (¹⁸O/¹⁶O) _VSMOW]/(¹⁸O/¹⁶O) _VSMOW × 10³‰

δD = [(D/H) _Sample − (D/H) _VSMOW]/(D/H) _VSMOW × 10³‰

Deuterium excess (d), a second-order parameter which combines both oxygen and hydrogen isotopic species, is defined by the following expression:

d = δD − 8 × δ¹⁸O 10³‰

Water was extracted from the soil samples and branches of H. ammodendron using an LI-2000 liquid water extractor (LI-2000 liquid water extractor; LICA United Technology Limited, Beijing, China). The water samples were sealed in 50 mL glass bottles, capped with no headspace, sealed with parafilm, and stored in a refrigerator for future water isotope analyses.We used a two-tracer for the three-component model to calculate the mixing ratio based on the following equations:

F_A + F_B + F_C = 1

δ¹⁸O_A F_A + δ¹⁸O_B F_B + δ¹⁸O_C F_C = δ¹⁸O_Sample

d_A F_A + d_B F_B + d_C F_C = d_Sample

The water balance calculation utilized the iso-error dual-isotope three-source model described by Phillips and Gregg [42]. This mixing model calculates the estimates and confidence intervals of a source’s proportional contributions to a mixture using stable isotope analyses. For dual-isotope studies, the measurements of the two isotope signatures for the samples from the mixture population and the three source populations should be independent of each other.

3. Results 3.1. Soil Temperature

A decrease in the soil temperature with a corresponding increase in BP and VP soil depth was observed. It was most likely due to the high temperatures and sizeable solar radiation in August, which significantly increased the surface temperature (°C) (Figure 4 and Figure 5).

When the soil depth was 30–120 cm, the soil temperature decreased obviously with the increase in depth; the soil temperature decreased from 28 °C at a depth of 30 cm to 26 °C at a depth of 120 cm in August, from 28 °C to 25.5 °C in September, and from 24 °C to 20 °C in October. The most significant temperature difference was as high as 4 °C. When the soil depth was 120–180 cm, there was a slight change in the deeper layer’s temperature until it became stable. Vegetation covers weaken solar radiation and illumination, which causes VP to have lower-temperature centers than those of BP. The distribution characteristics of gradually increasing the temperature along with VP to BP form a “cold island effect” centered on VP. 3.2. Soil Moisture

The soil moisture content was obtained in the different soil layers with H. ammodendron as the center. The soil moisture content of both VP and BP presented a reversed “S” type curve with an increase in soil depth (Figure 6 and Figure 7).

The surface soil moisture content of VP was low at 2.8% at 30 cm. With the increase in soil depth to 120 cm, the soil moisture content gradually increased to 22.12%. The soil moisture content of patches in bare soil gradually increased with increased soil depth, from 2.8% at 30 cm and then gradually increasing to 13.83% at 180 cm. On VP, the growth and coverage of H. ammodendron reduced evaporation, and the root system of H. ammodendron was mainly concentrated in the 50–120 cm soil layer. The soil moisture content in VP was significantly higher than that in BP as the root system absorbed water for plant growth. The highest moisture content was detected in H. ammodendron, which gradually decreased to the periphery, forming a “wet island” centered on the H. ammodendron plant. 3.3. Soil Salinity

The soil salinity of VP and BP showed an “S” type curve (Figure 8 and Figure 9).

The high salinity at the soil surface was due to groundwater evaporation with the high desert temperature, which caused salinity accumulation on the soil surface. The soil salt content of VP was much lower than that of BP, with H. ammodendron as the lowest point. A low salinity area centered on H. ammodendron was formed; this phenomenon suggested that H. ammodendron plants’ root system has an individual storage capacity for groundwater. It is a salt-tolerant plant. White crystal precipitation occurred on the leaf surface. The root system of H. ammodendron absorbs water for its own growth while inhaling the soluble salt in the soil, and finally precipitates from the leaves, which can appropriately reduce the soil salt content, so that the salt content of the soil where H. ammodendron grows is lower than that of the surrounding BP. VP is the lowest point and gradually increase in the surrounding area, forming a “low-salt effect” centered on H. ammodendron. At the same time, due to strong solar radiation, the BP is not covered by vegetation, and the surface soil water is lost faster than VP. The soil salt in the BP moves from the deep layer to the surface of the soil in a large amount, resulting in a higher soil salt content in the BP. The content near the root system of H. ammodendron is lower than that in the surrounding BP. 3.4. Soil Major Ions

The main anions in the soil are SO₄²⁻ and Cl⁻, which was consistent with the anions observed in the VP and BP soil (Figure 10).

A decrease and then an increase in the SO₄²⁻ content with an increase in the soil depth was observed. However, Cl^- increased gradually with an increase in the soil depth. The distribution characteristics of SO₄²⁻ showed apparent shallow set surface aggregation, and Cl⁻ moved deeper into the soil. The main cations in the soil were Ca²⁺ and Na⁺. Ca²⁺ first decreased and then increased with an increase in soil depth, which showed distinct distribution characteristics of shallow set surface aggregation. The migration of Na⁺ gradually increased with the increase in soil depth due to the soil colloid’s effect on Na⁺. The contents of K⁺ in the cation and Mg²⁺ in the anion were the lowest, and the HCO₃⁻content remained the same. The soil salts observed in the oasis-desert ecotone in the Manas River Basin’s southern margin were mainly calcium sulfate, sodium sulfate, and sodium chloride.

According to the Piper map, the general hydrochemical types of BP were as follows: SO₄²⁻-Ca²⁺ was predominant at the surface soil, in which the milligram equivalent of SO₄²⁻ was 80–100%, and the milligram equivalent of Ca²⁺ was 60–90%; with the increase in soil depth, this gradually changed to SO₄²⁻-Na⁺·Ca²⁺, of which the milligram equivalent of Na⁺ was 60–90% (Figure 11). In deep soil, most exist in the form of SO₄²⁻·Cl⁻-Na⁺. The milligram equivalent of Cl^- was 80–90%. The general hydrochemical types of VP gradually changed from SO₄²⁻-Ca²⁺ to SO₄²⁻-Ca²⁺·Na⁺, in which the milligram equivalents of SO₄²⁻, Ca²⁺, and Na⁺ were 80–100%, 60–90%, and 30–60%, respectively. In deep soil, SO₄²⁻-Na⁺·Ca²⁺ was the most prevalent, and the milligram equivalent of Na⁺ was 60–90%.

3.5. Water Source of H. ammodendron

To measure the difference between the water sources of H. ammodendron under two different conditions, i.e., before and after rain, three representative rainy days; namely, 15 July, 23 August, and 16 September 2018, were selected, with rainfalls of 10.0, 12.3, and 8.6 mm, respectively (Figure 12). Before the rain, the values of ¹⁸O and D showed that the primary water use source of H. ammodendron was groundwater, with an average contribution rate of 37.13%. The second water use source was soil water of the 120–180 cm soil layer, with an average contribution rate of 29.73%, and the 60–120 cm soil layer, with an average contribution rate of 21.38%; and the water use rate of H. ammodendron for the shallow soil at 0–60 cm was relatively low, with an average contribution rate of 11.76%. The above data show that H. ammodendron utilised the deep soil water and groundwater to a greater extent.

As shown in Figure 12, the soil should be divided into two layers: 0–60 cm, which may be affected by evapotranspiration, and 60–120 cm, which is relatively stable. According to the isotope measurement from Section 2.2, each potential water source’s relative contributions are listed in Table 1, Table 2 and Table 3.

Groundwater was the primary water source for H. ammodendron before the rain, with contribution proportions of 34.14% on 15 July, 33.54% on 23 August, and 43.71% on 16 September. This was followed by the 120–180 cm soil layer, with average contribution proportions of 28.87% on 15 July, 30.51% on 23 August, and 29.80% on 16 September. The 0–60 cm soil layer was the primary water source for H. ammodendron after the rain, with contribution proportions of 39.93% on 15 July, 39.95% on 23 August, and 41.50% on 16 September. This was followed by 60–120 cm, with contribution proportions of 25.17% on 15 July, 25.47% on 23 August, and 24.67% on 16 September. In the case of extensive evaporation of soil water before the rain, H. ammodendron mainly relied on the central root system to absorb the groundwater and soil water from a deeper layer (≥60 cm) to maintain its growth. However, after the rain, the utilisation rate of H. ammodendron for shallow soil water (<60 cm) increased significantly because the small amount of precipitation could not infiltrate into the groundwater in a short time. 4. Discussion 4.1. Distribution Characteristics of Soil Temperature, Moisture, and Salinity

The soil temperature of bare land patches and vegetation patches decreased with an increase in soil depth. This was due to the high temperatures in August and the sizeable solar radiation; the surface temperature is greatly affected by the environment, so the surface temperature was on the high side [43]. With the increase in soil depth, the effect of environmental factors on soil becomes smaller and smaller, so the temperature of deeper soil changes slightly until it is stable (Figure 13). Vegetation coverage will weaken the solar radiation and light, making the vegetation patches the low-temperature center due to environmental factors, and the bare land patches the high-temperature center [6]. The distribution characteristics of gradually increasing temperature along the vegetation patches to the bare land patches form the “cold island effect” centered on the vegetation patches. The effect of soil temperature also affects the temperature of the air above the vegetation patch, which weakens turbulence development, inhibits the transpiration of plants and evaporation from the ground, and is conducive to vegetation growth [44]. Compared with the vegetation patches in the bare land patches, soil temperature change with soil depth was generally the same. However, the vegetation patches’ overall soil temperature was low due to vegetation coverage [45], forming a “cold island effect” centered on H. ammodendron vegetation.

With the increase in soil depth, the vegetation patches’ soil moisture content presented an apparent “s” type distribution, that is, the change rule of first increasing, then decreasing, and then increasing again. The vegetation patch’s soil water content was significantly higher than that of the bare land patch, which shows that water is a fundamental cause of patch patterns [21]. The surface soil moisture content was the smallest in the vegetation patch and bare land patch, caused by the increase in desert evaporation caused by the high temperature in September (Figure 13). With the increase in soil depth, the influence of temperature on water content gradually decreased, and the water content of VP and BP increased in deep soil [46] (Figure 13). The dry sand layer can block the loss of deep soil water, which is conducive to preserving infiltration water [4,32].

Furthermore, the deep soil has more clay particles, a compact structure, and a more effective water conservation effect [30]. Because of the growth and coverage of H. ammodendron, the soil moisture content of the vegetation patches was significantly higher than that of the bare land patches [47]. The soil water content of the vegetation patches decreased at a depth of 100–150 cm (Figure 13), which was because the roots of H. ammodendron were mainly concentrated in the 50–150 cm soil layer away from the surface, and the roots absorbed the water needed for plant growth, which reduced the soil water content near the roots [48]. The root system absorbs water for plant growth, so the vegetation patch’s soil moisture content was significantly higher than that of the bare land patch [47]. The H. ammodendron plant had the highest humidity, gradually reducing to the periphery and forming a “wet island” centered on H. ammodendron, which shows that water is an important cause of patch formation patterns. This phenomenon indicated that H. ammodendron has a substantial self-collecting property [49].

The total salt content of the vegetation and bare land patches showed a “positive s” curve. That is, the soil salt content decreased first and then increased with the increase in soil depth (Figure 13). The high salt content at the soil surface is due to groundwater evaporation caused by high desert temperatures, and the salt is enriched in the soil surface [50]. The soil salt content of the vegetation patch was smaller than that of the bare land patch, and the lowest point of the H. ammodendron plants gradually increased towards the surrounding area, forming a “low salt area” centered on H. ammodendron [25,51]. The soil salt content of the vegetation patch was smaller than that of the bare land patch. The sudden increase point of the vegetation patch’s soil salt content had a pre-phenomenon, which showed that H. ammodendron roots have an individual storage capacity for groundwater. The stem flow of salt-tolerant vegetation, such as H. ammodendron can take away the salt and reduce the salt content of the soil, which is also why the white crystals on the surface of the soil H. ammodendron [52]. Soil electrical conductivity as affected by soil moisture content and soil salt content. The vegetation and bare land patches’ soil electrical conductivity showed a noticeable “inverted s” curve. According to the theory of “salt comes with water, salt goes with water” combined with the soil moisture content, and it can be inferred that the soil electrical conductivity of the vegetation patch was greatly affected by the water content [50] (Figure 13). However, the soil water content in the bare land patch increased as the soil depth increased; therefore, we speculated that soil salt content’s influence on soil conductivity plays a significant role.

4.2. Changes of Soil Ions and Hydrochemical Types

The soil salt content of the vegetation patch increased obviously between May and October, but the salt secretion of H. ammodendron can reduce the soil salt content [53]. The bare land patches’ soil salt content tended to be stable, and there was no significant change over time. The relationship between the total salt and anion content in the vegetation and bare land patches was similar. The soil total salt content had a strong correlation with SO₄^2-, followed by Cl⁻, and had little correlation with HCO₃⁻. SO₄²⁻ and Cl^- are the main anions in the eight major ions of soil, and the content of SO₄²⁻ and Cl^- is higher in the deep layer of soil. This is due to the intense evaporation in arid areas, the low water content in the surface soil, and the difficulty of decomposing the salt compounds, which is also why the high salt content in the surface soil of arid areas [54]. The ion with the lowest content in the soil was HCO₃^-, because HCO₃⁻ is mainly derived from the hydrolysis reaction of CO₂ in the oasis-desert ecotone, which results from less soil moisture [55]. Therefore, sulfates and chlorides are the main salts, with sulfates being the primary salt, in the oasis-desert ecotone soil in the Manas River Basin [56]. The correlation between the total soil salt and Ca²⁺ was strong, followed by Na⁺, but there was no correlation with K⁺ and Mg²⁺. The main cations in the soil were Ca²⁺ and Na⁺. Ca²⁺ first decreased and then increased with an increase in soil depth, which showed distinct distribution characteristics of shallow set surface aggregation [57]. The migration of Na⁺ gradually increased with the increase in soil depth due to the soil colloid’s effect on Na⁺ [58]. The adsorption capacity of Na⁺ is greater than that of Ca²⁺, which caused the accumulation of Ca²⁺ at the soil surface as the water evaporated [59]. Ca²⁺ and Na⁺ showed opposite trends in the process of soil salt conversion [60]. As sodium was the primary replacement, calcium salt decreased, and sodium gradually increased.

Comparing the hydrochemical types of the bare land and vegetation patches, we found that the Na⁺ equivalent in VP was less than that in BP, which indicated that the root system of H. ammodendron could absorb sodium salt [61]. Some studies have pointed out white crystals on the surface of H. ammodendron. The roots of H. ammodendron absorb sodium salt for plant growth and development [57]. The sodium salt is extracted to the stem through the roots of H. ammodendron and then separated from the leaves of H. ammodendron [62]. Sodium chloride is an essential index of secondary salinization in soil. H. ammodendron vegetation can reduce the secondary soil salinization [63]. In conclusion, the stability of H. ammodendron patch patterns in the oasis-desert ecotone in the southern margin of Junggar Basin is the result of the interaction between H. ammodendron vegetation and the arid environment.

4.3. Analysis of Water Sources and Utilization of H. ammodendron

Generally speaking, the stable isotope values of groundwater barely fluctuate with time and are relatively stable, which is due to the weakening of seasonal variation characteristics of precipitation in groundwater recharge through soil pore infiltration [64]. The δ¹⁸O value of the soil water in the 0–100 cm shallow soil was significantly greater than that in the deep soil [65]. This was due to the fractionation of isotopes in water migration or phase transformation [66]. Under the influence of evaporation, water molecules composed of lighter isotopes in shallow soil water will preferentially evaporate into the air, which causes the heavy isotopes to become enriched in the shallow layer and increase the δ¹⁸O value [67]. Before the rain, the primary water use source of H. ammodendron was groundwater, followed by soil water in the 120–180 cm soil layer. The water use efficiency of H. ammodendron for shallow soil was low; after rain, the primary water use source of H. ammodendron was the 60–120 cm soil layer, followed by groundwater. The main reason for this is that soil moisture content is small before rain, evaporation is extensive, and the shallow soil water absorption capacity of H. ammodendron is small and mainly relies on the central root system to absorb groundwater and deeper (>1.2 m) soil water to maintain growth [68]. After the rain, because precipitation cannot penetrate groundwater in a short time, it is instead found in the shallow soil layer, so H. ammodendron soil water use occurs in the shallow soil layer after rain (≤1.2 m) [69].

H. ammodendron had a robust environmental adaptability for water uptake [70]. Due to the climate environment of high temperature and little rainfall in the desert, the water content of the soil surface was low, which inhibited the root activity of H. ammodendron [55]. The H. ammodendron root system tends to fishtail like a branch structure and has a robust spatial expansion ability [48]; the vertical root system is developed to obtain the groundwater over a larger space [71]. When there is rainfall supply, H. ammodendron can fully mobilise the shallow roots to absorb shallow soil water for growth. The adaptability of H. ammodendron to the environment was reflected in its soil water absorption and its consumption of water, and the liquid flow rate of H. ammodendron [5]. Specifically, the liquid flow rate of H. ammodendron appears to be in a temporary dormancy state when the solar radiation is the highest at noon, which is called a “noon break” [44,72]. This is because the solar radiation is too intense at noon, and the vegetation closes the stomata to reduce evaporation [47]. The difference between the daytime and night-time sap flow density of H. ammodendron increased with the decrease in soil salt content. The diurnal variation of the soil water content indicated that the root system of H. ammodendron had a reverse hydraulic lifting effect [53]. After precipitation, the utilisation ratio of H. ammodendron to the surface and deep groundwater increased. However, the ratio to the middle soil water decreased, and the “photosynthetic noon break” of the leaves was strong, which indicated that the root system of H. ammodendron had the function of hydraulic transmission [73]. The water redistribution of the H. ammodendron root system can maintain the balanced distribution of soil water, reduce the stress of the arid climate on Haloxylon vegetation, and improve the survivability of Haloxylon vegetation in the arid environment, which is also the fundamental reason for the formation and stability of Haloxylon vegetation patch patterns in the oasis-desert ecotone.

5. Conclusions A “cold island”, “wet island,” and “low-salt” area formed that were centered around the H. ammodendron in VP. Soil water content in VP presented a reversed “S” type curve, while BP’s soil salinity presented an “S” type curve. NaCl was the most abundant in the BP soil, and the milligram equivalent of Cl^- was 80–90%, while CaSO₄ was the most abundant in the VP soil, in which the milligram equivalent of SO₄²⁻ was 80–100%. H. ammodendron has a robust environmental adaptability for water absorption and consumption. Before the rainfall, the contribution rate of each potential water source to H. ammodendron was groundwater > soil water (120–180 cm) > soil water (60–120 cm). After the rainfall, the contribution rate of each potential water source to H. ammodendron was soil water (0–60 cm) > soil water (60–120 cm) > groundwater.

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Figure 1. Location of the experimental field (45°16' N, 86°15' E) in the Manas River Basin.

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Figure 2. Daily air temperature (red line) and daily precipitation (blue bars) in 2018 and 2019.

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Figure 3. Layout of the experimental plots to measure soil moisture, temperature, salinity, and hydrochemical ions.

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Figure 4. Soil temperature at different depths in VP and BP.

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Figure 5.'Cold island effect' map.

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Figure 6. Soil moisture content at different depths in VP and BP.

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Figure 7.'Wet island effect' map.

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Figure 8. Spatial and temporal distribution of soil salinity at different depths of VP and BP.

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Figure 9. Low salinity area centred on H. ammodendron.

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Figure 10. Major ion concentrations (0-180 cm soil depth) under BP and VP.

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Figure 11. Piper map of soil depth.

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Figure 12.δ18O and δD values of water at different soil depths.

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Figure 13. Relationship among the soil temperature, moisture content, and salinity at different soil depths (September) for (a) VP and (b) BP.

	Water Source	δ18O	δD	Relative Contribution Rate (%)
	Soil water (0–60 cm)	−4.62	0.05	13.52
Before the rain	Soil water (60–120 cm)	−7.68	0.12	23.47
	Soil water (120–180 cm)	−10.21	0.14	28.87
	Groundwater (Below 180 cm)	−11.67	0.15	34.14
	Soil water (0–60 cm)	−12.83	0.15	39.93
After the rain	Soil water (60–120 cm)	−9.64	0.08	25.17
	Soil water (120–180 cm)	−4.93	0.05	14.27
	Groundwater (Below 180 cm)	−7.12	0.06	20.63

	Water Source	δ18O	δD	Relative Contribution Rate (%)
	Soil water (0–60 cm)	−4.98	0.05	13.24
Before the rain	Soil water (60–120 cm)	−7.96	0.12	22.71
	Soil water (120–180 cm)	−10.19	0.12	30.51
	Groundwater (Below 180 cm)	−11.83	0.14	33.54
	Soil water (0–60 cm)	−11.96	0.12	39.95
After the rain	Soil water (60–120 cm)	−11.86	0.12	25.47
	Soil water (120–180 cm)	−3.42	0.05	13.25
	Groundwater (Below 180 cm)	−10.96	0.08	21.33

	Water Source	δ18O	δD	Relative Contribution Rate (%)
	Soil water (0–60 cm)	−4.57	0.05	8.53
Before the rain	Soil water (60–120 cm)	−9.74	0.06	17.96
	Soil water (120–180 cm)	−10.12	0.08	29.80
	Groundwater (Below 180 cm)	−11.59	0.12	43.71
	Soil water (0–60 cm)	−12.21	0.13	41.5
After the rain	Soil water (60–120 cm)	−11.62	0.07	24.67
	Soil water (120–180 cm)	−8.53	0.05	13.16
	Groundwater (Below 180 cm)	−10.01	0.08	20.67

Author Contributions

Conceptualisation, L.Z. and G.Y.; methodology, X.H.; software, W.L.; validation, L.Z., W.L. and G.Y.; formal analysis, L.T.; investigation, F.L.; resources, Y.G.; data curation, W.L.; writing-original draft preparation, L.Z. and W.L.; visualisation, K.Y.; supervision, X.H.; project administration, G.Y.; funding acquisition, X.H. All authors read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant number U1803244, 41761064); Key Technologies Research and Development Program (grant number 2017YFC0404303); Xinjiang Production and Construction Corps (grant numbers 2018CB023, CZ027204, 2018AB027, 2018BC007, 2019ZH13); and Shihezi University (grant number CXRC201801, RCZK2018C22).

Institutional Review Board Statement

Ethical review and approval were waived for this study, due to REASON. Not applicable" for studies not involving humans or animals.

Informed Consent Statement

Not applicable" for studies not involving humans.

Data Availability Statement

H. ammodendron plants were used in this study. H. ammodendron selected from 'Manas River Basin' (Wild plant, Shihezi, China) were kindly provided by Dr. Guang Yang (Xinjiang Production and Construction Group Key Laboratory of Modern Water-Saving Irrigation, Shihezi 832000, China).

Acknowledgments

The work was also supported by the Talent Program of Xinjiang Production and Construction Corps and Xinjiang Production and Construction Group Key Laboratory of Modern Water-Saving Irrigation.

Conflicts of Interest

The authors declare no conflict of interest.

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