Introduction

Desert riparian forests serve as natural barriers for maintaining oasis ecological security, with water availability being a critical limiting factor for desert vegetation growth1,2. Plant water use is a vital component of the hydrological cycle in arid regions. Under the combined effects of global warming and human activities, the lower reaches of the Tarim River have experienced a shortage of surface water resources, a decline in groundwater levels, and drought stress on Populus euphratica in the downstream areas. However, since the implementation of the national ecological water conveyance project, the downstream P. euphratica communities have gradually recovered. Analyzing water sources of desert vegetation under ecological water conveyance is essential for providing theoretical foundations for the ecological restoration of P. euphratica in this region.

Hydrogen and oxygen stable isotope techniques, by comparing the isotopic composition of xylem water and potential water sources, enable precise identification of plant water uptake origins3,4. Compared to traditional methods such as whole-root excavation, sap flow measurement, and plant water potential analysis, stable isotope techniques minimize damage to plants and improve quantification accuracy5,6. These techniques have been widely applied in plant water source studies. For instance, isotopic tracing reveals that deep-rooted plants shift their water uptake from shallow to deep soil layers during different growth stages, a process closely linked to root distribution7,8. In arid regions, plants predominantly utilize shallow soil water during wet seasons and switch to deep soil water in dry seasons, exhibiting distinct seasonal patterns9. Under extreme drought conditions, clarifying water use strategies of desert vegetation has become a research focus. Shallow-rooted herbaceous plants primarily rely on shallow soil water despite their ability to exploit small rainfall events10. Shrubs can access water from soil layers at depths of 20 ~ 150 cm, with soil moisture serving as their primary water source11. Those with deeper vertical root systems exhibit greater water extraction depths, while demonstrating distinct variations in moisture utilization patterns from deep-layer soil and precipitation under different water conveyance contexts12,13. Trees with extensive root systems predominantly absorb deep soil water during droughts but shift to shallow and middle soil layers in rainy seasons14. Persistent rainfall reduces their reliance on deep soil water15, highlighting the influence of root distribution on water uptake. As a dominant tree species in desert riparian forests, P. euphratica can access groundwater through its roots. Its water uptake shifts from single soil layers to groundwater and deep soil water as groundwater depth increases16. Under drought stress, mature and overmature P. euphratica primarily rely on deep soil water and groundwater, while seedlings fail to survive17.

Studies have shown that plant water use strategies vary with growth stages, seasons, and vegetation types, with root distribution, groundwater depth, and growing seasons being key influencing factors18. Existing research on P. euphratica has focused on age and groundwater depth but lacks clarity on how ecological water conveyance alters its water use patterns in the lower Tarim River. The degradation of P. euphratica communities in this region has exacerbated sandstorm disasters and threatened oasis ecological security19,20. The ecological water conveyance project has promoted forest recovery, making it imperative to analyze its impact on water use strategies of P. euphratica across ages under varying groundwater depths.

This study integrates hydrogen and oxygen stable isotope techniques with the MixSIAR model to investigate differences in water uptake sources of P. euphratica (of varying ages) under ecological water conveyance and groundwater depth gradients in the lower Tarim River Basin. The objectives are threefold: (1) To elucidate variations in δ18O and δ2H signatures of potential water sources during water conveyance and non-conveyance periods. (2) To identify dominant water uptake layers and quantify their proportional contributions to P. euphratica of different ages under varying groundwater depths during conveyance and non-conveyance phases. (3) To assess the influence of groundwater depth, soil moisture, soil salinity, and soil texture on water uptake patterns of P. euphratica. This research contributes to promoting the ecological restoration of P. euphratica forests in the lower Tarim River basin and is of great significance for understanding regional eco-hydrological processes.

Materials and methods

Study area

The study area is located at the Kunaste (40°26′46.2″ N, 87°51′48.9″ E) and Yingsu (40°25′54.9″ N, 87°56′26.0″ E) sections in the lower Tarim River Basin (Fig. 1), characterized by a typical warm temperate continental arid climate. The mean annual precipitation is less than 15 mm, with an average temperature of 9–11 °C and a potential annual evaporation of 2500–3000 mm. The Daxihaizi Reservoir delivers ecological water to the lower Tarim River twice annually: from April to May in the first half of the year and from August to September in the second half.

Fig. 1
Fig. 1
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Overview of the study area. (Note: Generated by Xiaoqing Jiang using ArcMap 10.7 (Version: ArcMap 10.7, URL: https://desktop.arcgis.com.)

The plant community is dominated by Populus euphratica, Tamarix ramosissima, Haloxylon ammodendron, Nitraria tangutorum, Glycyrrhiza uralensis, and Karelinia caspia. P. euphratica forests are distributed along both riverbanks, but only middle-aged and old-growth trees survive in areas distant from the river channel.

Plot establishment and sample collection/processing

Fieldwork was conducted during the water conveyance period (26–28 September 2023) and non-conveyance period (5–7 July 2024). Based on the distribution of groundwater monitoring wells, six sampling plots were established at the Kunaste and Yingsu sections, with distances of 0–5 m, 200–210 m, 400–410 m (Kunaste) and 0–5 m, 800–810 m, 2990–3000 m (Yingsu) from the river channel.

Sample tree selection

A total of 48 Populus euphratica trees were selected across all plots, including 12 young trees, 18 intermediate-aged trees, and 18 mature trees. For each stand age class (young, intermediate-aged, mature) within a plot, three replicates were established. Basic information of the sample trees is summarized in Table 1.

Table 1 Basic information of sample tree.
Full size table

Soil sample collection

Adjacent to the sample trees, soil samples were collected using a 5 cm diameter auger. Sampling was conducted at 20 cm intervals from the surface to a depth of 200 cm, with two replicates per layer. In riverside plots, sampling extended to the phreatic layer. A total of 135 soil samples were collected. One portion was sealed in vials with Parafilm and stored at − 20 °C for soil water extraction and stable isotope analysis. The other portion was placed in aluminum boxes, immediately weighed for wet mass determination, and transported to the laboratory for soil moisture content measurement.

Plant sample collection

Plant samples were collected from the selected Populus euphratica trees. To minimize evaporation effects, sampling occurred between 08:00 and 10:00 (Beijing Time, UTC + 8). Non-green suberized branches (3–5 cm in length) free from pests and human disturbance were excised using pruning shears. After rapid removal of phloem tissues, samples were sealed in vials with Parafilm, stored in portable freezers, and transported to the laboratory. A total of 16 plant samples were collected, each with three replicates.

River and groundwater sample collection

River water samples (2 sets) were collected from the channel near the plots. Groundwater samples (12 sets) were obtained from monitoring wells surrounding the plots. Prior to collection, vials were rinsed 2–3 times with the target water. Samples were filled to capacity, sealed with Parafilm, and duplicated. In the laboratory, water samples were filtered through 0.45 μm cellulose acetate membranes, transferred to 1.5-ml vials, and stored at 2 °C for subsequent analysis.

Research methods

Water extraction and hydrogen-oxygen stable isotope analysis

Water extraction and stable isotope analysis of hydrogen (δ2H) and oxygen (δ18O) for all potential water sources were conducted at Xinjiang Laboratory of Lake Environment and Resources in Arid Zone. Prior to isotopic analysis, all water samples were filtered through 0.45 μm cellulose acetate membranes. Plant and soil water were extracted using a fully automated vacuum cryogenic distillation system (LI-2100, LICA, Beijing, China). The δ18O and δ2H compositions were measured with a liquid water isotope analyzer (LGR DLI-100, Los Gatos Research, Mountain View, USA). Isotopic values were calibrated using the correction curve established by Schultz21. The measured δ18O and δ2H values are reported as per mil (‰) deviations relative to the Vienna Standard Mean Ocean Water (V-SMOW) standard:

$$\:\delta \:\left({\permille} \right) = \frac{{R_{{{\text{sample}}}} - R_{{{\text{standard}}}} }}{{R_{{{\text{standard}}}} }} \times \:1000$$
(1)

where \(\:{R}_{\text{s}\text{a}\text{m}\text{p}\text{l}\text{e}}\) is the isotope ratio of the water sample, and \(\:{R}_{\text{s}\text{t}\text{a}\text{n}\text{d}\text{a}\text{r}\text{d}}\) is the isotope ratio of the reference standard (V-SMOW). The analytical precision (1σ) for δ18O and δ2H measurements is 0.15‰ and 0.5‰, respectively.

MixSIAR model and uncertainty analysis

The Bayesian mixing model MixSIAR, implemented as an R package, was used to quantify the proportional contributions of potential water sources to plant xylem water. Prior to accepting model outputs, convergence diagnostics (Gelman-Rubin and Geweke tests) were performed. Model convergence was confirmed when: Geweke diagnostics showed < 5% of iterations outside the 95% confidence interval (± 1.96), Gelman-Rubin potential scale reduction factors (PSRF) approached 1.0 (all < 1.01)22,23. In this study, Markov Chain Monte Carlo (MCMC) chains were set to “extreme” mode to ensure convergence.

Model performance was evaluated using the root mean square error (RMSE)24,25, calculated as:

$$\:{\text{p}}_{\text{i}}=\sum\:_{\text{i}=1}^{\text{n}}{\text{f}}_{\text{i}}{{\updelta\:}}_{\text{A}}$$
(2)
$$\:\:\text{R}\text{M}\text{S}\text{E}={\left[\frac{1}{\text{n}}\sum\:_{\text{i}=1}^{\text{n}}{\left({\text{p}}_{\text{i}}-{\text{o}}_{\text{i}}\right)}^{2}\right]}^{1/2}$$
(3)

where \(\:{\text{p}}_{\text{i}}\) is the predicted value of plant xylem water isotope ratios, \(\:\text{n}\) indicates the number of plant water sources, \(\:\text{i}\) is the plant water source, \(\:{\text{f}}_{\text{i}}\)is the contribution ratio of the ith source calculated using the model; \(\:{{\updelta\:}}_{\text{A}}\) is the isotope value of each potential water source; \(\:{\text{o}}_{\text{i}}\) and is the observed value of plant xylem water isotope ratios.

Soil texture, moisture content, and salinity measurements

Soil particle size distribution was determined using a Malvern Mastersizer 2000 laser diffraction analyzer (Malvern Panalytical, UK), with three replicates per sample. The measurement range spanned 0.02–2000 μm, and mean values were calculated. Particle size parameters were derived using the Folk-Ward equations in GRADISTAT software, and results were classified using the Wentworth scale26,27.

Soil moisture content was measured gravimetrically by oven-drying fresh samples at 105 °C for 24 h until constant weight. Soil salinity was quantified as electrical conductivity (EC) using a Multi 3420 Set B conductivity meter (WTW GmbH, Germany).

Results

Variation characteristics of groundwater depth and stable isotopes in different water bodies

Variation characteristics of groundwater depth during water conveyance and non-conveyance periods

As shown in Fig. 2, groundwater depth dynamics at the Kunaste and Yingsu sections exhibit pronounced responses to water conveyance operations. At Kunaste, depths range from 1.95 ~ 4.73 m during water conveyance periods but increase to 4.90 ~ 6.26 m in water non-conveyance periods, demonstrating progressive deepening with increasing distance from the riverbank. Similarly, the Yingsu section shows depths of 2.10 ~ 5.18 m under water conveyance and 4.13 ~ 5.67 m during water non-conveyance periods, likewise exhibiting distance-dependent amplification. Comparative analysis indicates that groundwater depth at the Yingsu section is moderately more responsive to ecological water conveyance than at Kunaste, with an overall variation amplitude of approximately 1.17 ± 0.95 m across the study area.

Fig. 2
Fig. 2
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Changes in groundwater depth during water conveyance and non-conveyance periods. (Note: Empty box means that no effective depth to groundwater was measured at that point)

Variation characteristics of soil water isotopic composition

As shown in Fig. 3, soil water δ18O and δ2H values generally decreased with increasing soil depth, with significant isotopic variability observed in the 0–120 cm soil layer. During the water conveyance period, soil water δ18O and δ2H ranged from − 12.09‰ to 0.46‰ and − 74.5‰ to −31.6‰, respectively, while during the non-conveyance period, the ranges shifted from − 10.93‰ to 1.87‰ (δ18O) and − 73.5‰ to −37.3‰ (δ2H).

Fig. 3
Fig. 3
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Temporal and spatial variations in soil water δ18O and δ2H values. (Note: Soil water extraction failed at Yingsu Plot 2 due to low soil moisture content)

Spatially, in the Kunaste section during the conveyance period, δ18O and δ2H values for Plots 1, 2, and 3 were − 14.19‰ to −1.01‰ and − 74.9‰ to −39.2‰, −11.51‰ to −1.55‰ and − 71.0‰ to −31.4‰, −6.42‰ to 0.46‰ and − 66.4‰ to −54.0‰, respectively. During the non-conveyance period, values shifted from − 9.67‰ to −0.93‰ (δ18O) and − 74.0‰ to −50.8‰ (δ2H) for Plot 1, −8.09‰ to 1.87‰ (δ18O) and − 62.9‰ to −37.3‰ (δ2H) for Plot 2, and − 8.27‰ to 0.81‰ (δ18O) and − 61.5‰ to −42.1‰ (δ2H) for Plot 3.

In the Yingsu section during the conveyance period, δ18O and δ2H ranges were − 13.20‰ to 5.25‰ and − 70.6‰ to −32.9‰ (Plot 1) and − 14.09‰ to −7.36‰ and − 75.8‰ to −32.1‰ (Plot 3). During the non-conveyance period, values were − 12.52‰ to 0.31‰ (δ18O) and − 78.4‰ to −40.8‰ (δ2H) for Plot 1 and − 12.73‰ to 1.60‰ (δ18O) and − 74.9‰ to −45.8‰ (δ2H) for Plot 3.

These results demonstrate that soil water δ18O and δ2H values in both the Kunaste and Yingsu sections increased with distance from the riverbank, and isotopic compositions were more enriched during the non-conveyance period compared to the conveyance period.

Variation characteristics of xylem water isotopic composition

As shown in Fig. 4, in the Kunaste section during the water conveyance period, δ18O and δ2H ranges for young trees, intermediate-aged, and mature P. euphratica were − 15.74‰ to −10.06‰ and − 35.3‰ to −10.1‰, −18.57‰ to −10.86‰ and − 46.2‰ to −20.7‰, and − 17.02‰ to −5.27‰ and − 38.1‰ to −24.6‰, respectively. During the non-conveyance period, these ranges shifted from − 15.91‰ to −8.99‰ (δ18O) and − 48.3‰ to −35.3‰ (δ2H) for young trees, −16.34‰ to −6.92‰ (δ18O) and − 58.8‰ to −39.5‰ (δ2H) for intermediate-aged trees, and − 17.79‰ to −8.66‰ (δ18O) and − 55.6‰ to −48.9‰ (δ2H) for mature trees.

Fig. 4
Fig. 4
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Variation of xylem water δ18O and δ2H values in Populus euphratica of different ages.

In the Yingsu section during the conveyance period, δ18O and δ2H values ranged from − 16.57‰ to −10.99‰ (δ18O) and − 36.3‰ to −28.1‰ (δ2H) for young trees, −14.87‰ to −8.12‰ (δ18O) and − 32.4‰ to −16.5‰ (δ2H) for intermediate-aged trees, and − 19.15‰ to −9.81‰ (δ18O) and − 57.0‰ to −27.1‰ (δ2H) for mature trees. During the non-conveyance period, values were − 15.71‰ to −10.47‰ (δ18O) and − 62.6‰ to −53.2‰ (δ2H) for young trees, −19.42‰ to −7.17‰ (δ18O) and − 55.9‰ to −48.8‰ (δ2H) for intermediate-aged trees, and − 22.47‰ to −8.80‰ (δ18O) and − 66.8‰ to −44.2‰ (δ2H) for mature trees.

These results indicate that xylem water isotopic compositions were more enriched during the conveyance period compared to the non-conveyance period. Spatially, δ18O and δ2H values generally decreased with increasing distance from the riverbank. Ecological water conveyance exerted strong effects on the isotopic signatures of xylem water in P. euphratica across different stand ages.

Variation characteristics of groundwater isotopic composition

As shown in Fig. 5, ecological water conveyance significantly altered groundwater isotopic composition. In the Kunaste section during the conveyance period, groundwater δ18O and δ2H ranged from − 9.81‰ to −9.23‰ and − 32.7‰ to −27.7‰, respectively, while in the non-conveyance period, these ranges shifted from − 9.98‰ to −8.58‰ (δ18O) and − 61.4‰ to −58.3‰ (δ2H), respectively, with magnitude of change in the range of 0.94 ± 0.25‰ and 29.8 ± 1.1‰ between the conveyance period and non-conveyance period. In the Yingsu section, groundwater δ18O and δ2H during the conveyance period ranged from − 9.80‰ to −9.00‰ and − 52.2‰ to −27.6‰, respectively, while non-conveyance period values expanded to −10.90‰ to −8.44‰ (δ18O) and − 67.8‰ to −53.3‰ (δ2H), respectively, with magnitude of change in the range of 0.91 ± 0.48‰ and 25.9 ± 12.7‰ between the conveyance period and non-conveyance period. The mean groundwater δ18O and δ2H values across the study area were − 9.47 ± 0.23‰ and − 33.2 ± 7.6‰ during the conveyance period, compared to −9.60 ± 0.92‰ and − 60.8 ± 3.8‰ during the non-conveyance period, indicating isotopic enrichment under water conveyance.

Fig. 5
Fig. 5
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Temporal changes in groundwater δ18O and δ2H values during water conveyance and non-conveyance periods. (Note: DW1–DW3: Kunaste section; Y-F1–Y-F14: Yingsu section. Y-F3, Y-F5, and Y-F10 were not sampled during the water non-conveyance period; Y-12, Y-F3, and Y-F4 were not sampled during the water conveyance period)

Spatially, groundwater δ18O and δ2H in both sections generally decreased with increasing distance from the riverbank. During the conveyance period, the maximum (−9.23‰ for δ18O, −27.7‰ for δ2H) and minimum (−9.81‰ for δ18O, −32.7‰ for δ2H) values in Kunaste occurred at DW3/DW1 and DW2/DW3, respectively, while in Yingsu, maxima (−9.00‰ for δ18O, −27.6‰ for δ2H) and minima (−9.80‰ for δ18O, −52.2‰ for δ2H) were observed at Y-F3/Y-F11 and Y-F1/Y-F6. During the non-conveyance period, Kunaste exhibited maxima (−8.58‰ for δ18O, −58.3‰ for δ2H) at DW2/DW1 and minima (−9.98‰ for δ18O, −61.4‰ for δ2H) at DW3, whereas Yingsu showed maxima (−8.44‰ for δ18O, −53.3‰ for δ2H) at Y-F6/Y-F4 and minima (−10.90‰ for δ18O, −67.8‰ for δ2H) at Y-F8, consistent with conveyance period trends.

Variation characteristics of water uptake sources in Populus euphratica

Water uptake sources under varying groundwater depths during the water conveyance period

As shown in Fig. 6, significant differences in water source utilization proportions were observed among Populus euphratica of different ages across the three plots in the Kunaste section during the water conveyance period. In Plot 1, young trees primarily absorbed groundwater (16.6%), followed by 120–160 cm soil water (14.9%) and river water (14.1%), while intermediate-aged and mature trees relied predominantly on groundwater (17.6% and 19.6%, respectively) and river water (15.2% and 16.9%, respectively). In Plot 2, groundwater was the dominant source for young trees, intermediate-aged, and mature trees, with utilization proportions of 19.9%, 21.4%, and 23.1%, respectively, showing an increasing dependency on tree age. In Plot 3, intermediate-aged and mature trees mainly utilized groundwater (21.6% and 21.9%) and 160–200 cm soil water (18.2% and 18.3%).

Fig. 6
Fig. 6
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Proportional contributions of Populus euphratica water sources in the Kunaste (a) and Yingsu (b) sections during the water conveyance period.

In the Yingsu section, water uptake patterns exhibited less variability among plots. Plot 1 (lacking young trees) showed intermediate-aged and mature trees depending primarily on groundwater (16.7% and 17.8%) and secondarily on river water (16.4% and 17.6%). In Plot 3, young trees absorbed groundwater (16.0%), river water (14.9%), and 80–120 cm soil water (14.4%); intermediate-aged trees prioritized river water (17.0%), followed by groundwater (16.7%) and 0–40 cm soil water (16.4%); mature trees relied on groundwater (18.4%), river water (15.8%), and 160–200 cm soil water (13.9%).

These findings indicate that during the ecological water conveyance period, groundwater constituted the primary water source for Populus euphratica across different age cohorts within riparian areas of both Kunaste and Yingsu sections, followed by river water. At off-channel sampling sites, the vegetation exhibited groundwater as the dominant water source, with deep soil water serving as the secondary supply.

Variation characteristics of water uptake sources under varying groundwater depths during the non-conveyance period

During the non-conveyance period, the lower Tarim River experienced complete flow cessation, leading to pronounced differences in water uptake sources among Populus euphratica of different ages (Fig. 7), with groundwater and deep soil water serving as the primary sources.

Fig. 7
Fig. 7
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Proportional contributions of Populus euphratica water sources in the Kunaste (a) and Yingsu (b) sections during the non-conveyance period.

In the Kunaste section, Plot 1 (near the riverbank) showed young trees primarily utilizing 120–160 cm soil water (17.9%) and 160–200 cm soil water (17.4%), while intermediate-aged and mature trees relied predominantly on groundwater with utilization proportions of 19.9% and 19.7%, respectively. In Plot 2, both young trees and intermediate-aged trees depended mainly on groundwater (12.4% and 23.2%), whereas mature trees shifted to 160–200 cm soil water (17.5%). Plot 3 exhibited intermediate-aged and mature trees predominantly absorbing groundwater (24.7% and 23.3%).

In the Yingsu section, Plot 1 revealed intermediate-aged trees prioritizing 0–40 cm soil water (19.4%) followed by groundwater (17.2%), while mature trees absorbed 160–200 cm soil water (19.4%), 120–160 cm soil water (19.1%), and groundwater (18.1%). Plot 3 demonstrated young trees, intermediate-aged, and mature trees all relying primarily on groundwater (18.4%, 18.4%, and 23.2%, respectively), supplemented by 160–200 cm soil water (16.8%, 16.7%, and 20.0%).

These results indicate that during the water conveyance period, the primary water sources for P. euphratica were groundwater, river water, and deep soil water, with a maximum groundwater utilization proportion of 23.1%. In contrast, during the non-conveyance period, groundwater and deep soil water became the dominant sources, with maximum groundwater utilization dropping to 19.9%. Groundwater dependency was significantly higher during the conveyance period compared to the non-conveyance period.

Discussion

Analysis of influencing factors on water uptake of Populus euphratica

As illustrated in Fig. 8, the influences of groundwater depth, soil moisture content, and soil electrical conductivity on water use proportions of Populus euphratica at varying stand ages differed between water conveyance and non-conveyance periods. During water conveyance periods, young trees exhibited strong positive correlations between water use proportion and groundwater depth (0.64) and EC (0.48), while displaying a weak negative correlation with soil moisture content. Intermediate-aged trees demonstrated positive correlations between water use proportion and both groundwater depth and soil moisture content, with soil moisture content and soil electrical conductivity exerting minimal influence. Mature trees showed significant correlations between water use proportion and groundwater depth, while soil moisture content and soil electrical conductivity had limited effects.

Fig. 8
Fig. 8
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Relationships between water sources of Populus euphratica at different stand ages and groundwater depth, soil moisture content (SWC), and soil electrical conductivity (EC) during water conveyance and non-conveyance periods.

During non-conveyance periods, young trees revealed a significant positive correlation between water use proportion and soil moisture content (0.74), and a strong negative correlation with soil electrical conductivity (−0.44), indicating potential salt stress that substantially inhibited water absorption and utilization, whereas groundwater depth exhibited weak influence. Intermediate-aged trees displayed a marked positive correlation between water use proportion and groundwater depth (0.62), a weak positive correlation with soil moisture content, and negligible effects from soil electrical conductivity. Mature trees maintained strong positive correlations between water use proportion and both groundwater and soil water, while simultaneously showing a strong negative correlation with soil salinity.

Consequently, during conveyance periods, water use proportions of young, intermediate-aged, and mature trees all positively correlated with groundwater depth, while soil moisture content and soil salinity exerted minor influences. During non-conveyance periods, soil moisture content and electrical conductivity significantly affected young trees but minimally impacted intermediate-aged and mature trees, with young trees experiencing pronounced salt stress. Groundwater depth substantially governed water use proportions of intermediate-aged and mature trees, demonstrating increased groundwater utilization with greater groundwater depth. Deep water sources held higher priority in their water use strategies, aligning with the moisture utilization patterns of deep-rooted phreatophytes28.

Influence of soil moisture content on water uptake sources

As shown in Figs. 9 and 10, soil moisture content in the Kunaste section generally increased with soil depth. The anomalous pattern in Plot 2 may be attributed to frequent groundwater irrigation in the experimental field, which elevated surface soil moisture. Across this section, the contribution proportions of soil layers to water uptake increased with higher soil moisture content.

Fig. 9
Fig. 9
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Relationships between soil water content (SWC), EC, and water source contribution proportions in the Kunaste section (non-conveyance period).

Fig. 10
Fig. 10
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Relationships between soil water content (SWC), EC, and water source contribution proportions in the Kunaste section (water conveyance period).

During the water conveyance period, the mean soil moisture contents in the 0–40 cm layer of Plots 1, 2, and 3 were 2.90%, 1.56%, and 0.20%, respectively, while during the non-conveyance period, these values decreased to 1.04%, 1.69%, and 0.12%. In the 40–120 cm layer, moisture contents during the conveyance period were 11.04%, 4.13%, and 1.71%, compared to 2.55%, 3.50%, and 1.58% during the non-conveyance period. The 120–200 cm layer exhibited the highest moisture during the conveyance period (20.56%, 3.06%, and 1.59%), declining to 4.04%, 3.60%, and 8.41% in the non-conveyance period. Maximum soil water utilization by P. euphratica (24.7%) occurred at 13.26% soil moisture, whereas the minimum (6.6%) was observed at 0.23% moisture.

In the Yingsu section, the 0–40 cm layer of Plot 1 showed mean soil moisture contents of 4.52% (conveyance period) and 3.49% (non-conveyance period), while Plot 3 exhibited 16.62% and 0.13% for the respective periods. The 40–120 cm layer in Plot 1 recorded 12.88% (conveyance) and 13.93% (non-conveyance), whereas Plot 3 values were 24.78% and 1.89%. During the non-conveyance period, the 120–200 cm layer moisture was 18.66% (Plot 1) and 8.33% (Plot 3).

As a deep-rooted species, P. euphratica adapts to drought stress by shifting water uptake from surface to deeper soil layers or groundwater when surface moisture declines29. This study confirms that deep soil water contributes most significantly to its water use, with minimal reliance on surface layers. Soil water utilization by P. euphratica across ages exhibited a positive correlation with soil moisture content.

Influence of soil texture on water uptake sources

Soil texture in the study area was dominated by sandy soil, transitioning to silt or clay with increasing depth, accompanied by higher soil moisture content. In the Kunaste and Yingsu sections, surface soil (0–40 cm) exhibited low moisture (approximately 0–5%) and high sand content (73.12 ± 0.30%), resulting in poor water retention. Consequently, P. euphratica of all ages showed minimal utilization of surface soil water, with the lowest proportion at 6.6%. In contrast, deeper soil layers (120–200 cm) contained up to 16.04% clay, enhancing water retention (maximum moisture: 22.11%) and supporting higher water uptake (maximum proportion: 19.4%). Thus, soil water utilization by P. euphratica increased with soil depth and showed a positive correlation with clay content.

Influence of soil salinity on water uptake sources

The lower Tarim River Basin is hyper-arid, with surface soil isotopic enrichment driven primarily by evaporation. When shallow soil (0–40 cm) moisture was low, P. euphratica relied on groundwater and deep soil water, but water uptake was also constrained by salt stress. As shown in Figs. 9 and 10, the mean electrical conductivity (EC) of the 0–40 cm layer in Kunaste Plots 1, 2, and 3 during the conveyance and non-conveyance periods was 754.33 µS·cm−1 and 139.22 µS·cm−1, 731.00 µS·cm−1 and 2880.00 µS·cm−1, and 249.25 µS·cm−1 and 3527.50 µS·cm−1, respectively. The EC values for Yingsu Plots 1 and 3 as 1393.25 µS·cm−1 and 5985.00 µS·cm−1, and 894.50 µS·cm−1 and 153.13 µS·cm−1, respectively, with surface salinity decreasing with distance from the river.

At a surface EC of 7150.00 µS·cm−1, the average utilization of shallow soil water by P. euphratica reached 19.4%, whereas at 362.50 µS·cm−1, it dropped to 6.6%. Young trees, with shallower roots, primarily absorbed surface soil water. However, even at the maximum surface EC (684.00 µS·cm−1), young trees utilized only 14.1% of shallow soil water, indicating that elevated salinity inhibits both water uptake and seedling recruitment30, posing challenges to ecological restoration of downstream P. euphratica communities.

Ecological strategies of Populus euphratica adaptation to groundwater depth variation

To investigate the impact of groundwater depth variations on the water uptake sources of Populus euphratica (Fig. 11), groundwater depth data from the Yingsu section between 2000 and 2020 were analyzed. As shown in Fig. 12, the groundwater depth in the Yingsu section generally exhibited an increasing trend with greater distance from the riverbank during 2000–202031, which aligns with the findings of this study. Under the context of ecological water conveyance, the groundwater depth in this section demonstrated a gradual decrease over time. Notably, significant reductions in groundwater depth occurred in 2005 and 2020, while an abrupt increase was observed in 2015. Within the temporal scope of this study (2023–2024), the groundwater depth displayed a decreasing trend compared to 2020, indicating that ecological water conveyance has effectively improved groundwater conditions in the Yingsu section, with a noticeable elevation of the groundwater table observed.

Fig. 11
Fig. 11
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Sources of water contribution proportions in Populus euphratica of different stand ages.

Fig. 12
Fig. 12
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The variations of groundwater depth from 2000 to 2024.

This study revealed that during water conveyance and non-conveyance periods, young trees primarily absorbed water from shallow soil layers, while intermediate-aged and mature trees relied on deep soil water and groundwater, consistent with findings by Rogers et al.32. Riverside P. euphratica directly utilized river water. Mature trees, equipped with well-developed root systems, dynamically adjusted water uptake from shallow/deep soil layers and groundwater based on environmental conditions, a critical survival strategy in arid environments32.

When groundwater depth exceeded 6 m (non-conveyance period), all age classes depended on deep soil water and groundwater, with no young trees surviving. Groundwater utilization increased by 3.85 ± 0.07% during the conveyance period compared to non-conveyance. Conversely, at shallower depths (< 4 m), young trees reduced groundwater use by 0.7% during non-conveyance, while intermediate-aged and mature trees increased utilization by 2.3% and 0.1%, respectively. These patterns reflect root architecture constraints: deeper groundwater impedes young tree access33, whereas mature trees exploit deep taproots to mitigate drought stress34.Under drought stress, P. euphratica employs physiological adaptations such as stomatal closure, reduced photosynthetic rates, and slowed sap flow to minimize water loss35,36,37.

Groundwater, as one of the relatively stable water sources in arid regions, serves as the primary water source for most deep-rooted desert plants. For instance, Tamarix taklamakanensis relies predominantly on deep soil water and groundwater38,39. However, shallow-rooted herbaceous plants and shrubs primarily utilize shallow soil water. As observed by Song et al.40, the root density of Stipa breviflora increases near the soil surface, enabling direct absorption of other near-surface water sources or minor precipitation. When shallow soil moisture decreases, the main water source for Artemisia ordosica shifts to deep soil water at 60–100 cm, while Leymus secalinus continues to depend on shallow soil water41. These variations are closely linked to the distribution patterns of plant root systems. Populus euphratica, as a deep-rooted plant, its survival heavily depends on groundwater accessibility. This study observed that groundwater depths > 6 m triggered drought stress, reducing groundwater utilization and eliminating young tree recruitment. Notably, Chen et al.42 reported that groundwater depths exceeding 8 m lead to P. euphratica mortality and stand degradation.

Conclusions

  1. (1)

    Soil water δ18O and δ2H values were more depleted during the water conveyance period compared to the non-conveyance period, increased with distance from the riverbank, and decreased with soil depth. Xylem water isotopic compositions were more enriched during the conveyance period, showing a hierarchy of young trees > mature trees > intermediate-aged trees, and decreased with increasing distance from the river. Groundwater isotopes were more depleted during the conveyance period and decreased with distance from the riverbank.

  2. (2)

    During the water conveyance period, the primary water sources for Populus euphratica across all age classes were groundwater, river water, and deep soil water (160–200 cm), with maximum utilization proportions of 23.1%, 17.6%, and 18.3%, respectively. In the non-conveyance period, groundwater and deep soil water became the dominant sources, with maximum utilization proportions of 19.9% and 20.3%.

  3. (3)

    Soil physicochemical properties and groundwater depth significantly influence the water utilization of P. euphratica. Soil layers with poor texture have low water retention capacity. The water source utilization proportion of P. euphratica increases with higher soil moisture content. During the non-conveyance period, groundwater depth strongly influences the water utilization proportion of intermediate-aged and mature trees (0.62 and 0.40). Young trees experience pronounced salinity stress (−0.44). Consequently, P. euphratica shifts its water uptake sources toward deeper soil water and groundwater.