4.1 Differences in stable isotopes of precipitation
During the sampling period, δD and δ18O of precipitation showed a relatively consistent variation pattern (Fig. 3), with a fluctuating trend of "decrease - increase - decrease". The values of δD and δ18O reached their maximum on September 6 (6.12‰) and June 4 (0.02‰), respectively. The lowest values of δD (-99.48‰) and δ18O (-15.59‰) were found on October 6. In study area, the precipitation concentrates in July-August (Fig. 3), when the relative humidity of the atmospheric is higher and the kinetic fractionation due to sub-cloud evaporation is weak (Wu et al. 2021). In addition, the stable isotope is constantly depleted for long-distance transport of the westerly vapor (Ma et al. 2012; Ma et al. 2018; Zhang et al. 2022). This leaded to the values of δD and δ18O lower during precipitation period, which was consistent with the results in the Tarim River basin (Sun et al.2016). However, δD and δ18O exhibited high values on July 28 and August 20. This was because of the occurrence of small precipitation events during the week before July 8 and the absence of precipitation events during the week of August 20, which caused the stable isotopes were enriched by sub-cloud evaporation (Ma et al. 2012) and temperature effects (Zhang et al. 2006). The values of δD and δ18O drastically decreased on October 6. This was due to lower temperatures and increased precipitation, which leaded to weaker sub-cloud evaporation and lower isotope values. Overall, temperature and precipitation have important effects on stable isotope values, which the values of δD and δ18O are high during periods of high temperature and low precipitation, and it is vice versa.
D-excess values were mainly affected by evaporation, moisture sources and water vapor mixing (Bershaw 2018), and it was also affected by the strength of sub-cloud evaporation (Kong et al. 2013). In study area, the d-excess values of precipitation were mostly above 10‰, which was higher than those (< 8‰) in the arid area of northwest China. This may be because the study area is located in high altitude mountainous and mainly influenced by westerly water vapor and local recirculation water vapor (Yuan et al. 2020; Ma et al. 2012). From late May to early of July, the d-excess values fluctuate widely. This was related to precipitation sources that were more complex due to the influence of westerly moisture, local circulation water vapor and sub-cloud evaporation (Wu et al. 2011). Many scholars have proved the precipitation process in northwest China undergoes certain sub-cloud evaporation and local water vapor mixing (Liu et al. 2008; Ma et al. 2020; Wu et al. 2021). Except for July 30, the d-excess values showed an obvious decrease trend from early of July to August (from 22.34‰ to 3.91‰), which may be influenced by higher precipitation and monsoon moisture during this period. Previous results have shown that precipitation of the eastern Qilian Mountains in July and August has low d-excess values influenced by westerly winds, East Asian monsoon and South Asian monsoon (Jia et al. 2018). On July 30, temperature effects caused δD and δ18O with high values, but d-excess also showed high values (27.67‰), which was associated with low precipitation. It has been studied that smaller precipitation in the arid area of northwest China enriches stable isotopes and causes high d-excess values due to sub-cloud evaporation (Ma et al. 2012; Pang et al. 2011). From the beginning of September, the values of δD and δ18O in precipitation decreased, but the d-excess values gradually increased and then stabilized. This was because the temperature in study area was decreasing in autumn, the sub-cloud evaporation was weakening, and the water vapor source of precipitation was mainly transported by the westerly (Xiong et al. 2021). Some studies have also shown the d-excess of the northwest China reaches high values in October (Pang et al. 2011; Ma et al. 2018).
4.2 Differences in stable isotopes of soil water
Due to the combined effect of light, temperature, precipitation and soil texture, there were differences in the values of δD and δ18O of soil water in different aspects (Bale et al. 1998; Qin et al. 2017). On the semi-sunny aspect, the average values of δD and δ18O of soil water in study area were − 49.40 ± 5.70‰ and − 7.63 ± 0.99‰, with variations ranging from − 63.29 to -31.11‰ and from − 10.01 to -4.75‰, respectively. On the semi-shaded aspect, they were − 47.31 ± 7.75‰ and − 7.64 ± 1.18‰, with variation ranging from − 77.21 to -25.73‰ and from − 11.80 to -4.80‰, respectively. The variation range of stable isotope values of soil water was larger on the semi-shaded aspect than on the semi-sunny aspect. This indicated that the variation of soil water on the semi-shaded aspect was relatively unstable. This may be related to the soil texture of the study area, which the contents of clay and chalk of the semi-shaded aspect were lower than those of the semi-sunny aspect. This can also be confirmed that the variation rate of soil water content on the semi-shaded aspect was higher than that on the semi-sunny aspect (Fig. 4).
Based on values of δD and δ18O, SWL (semi−sunny aspect) was established: δD = 5.46 δ18O-7.79 (R2 = 0.91). Its slope and intercept were slightly smaller than SWL, indicating that soil water on the semi-sunny aspect was easy to evaporation and stable isotopes were significantly affected by evaporative fractionation (Qin et al. 2017). SWL (semi−shaded aspect) was also established: δD = 6.21δ18O+0.15 (R2 = 0.91). Its slope and intercept are higher than those of SWL and SWL (semi−shaded aspect), reflecting stable isotopes of soil water on the semi-shaded aspect were less affected by evaporation fractionation than those on the semi-sunny aspect. The semi-shaded aspect is located on the shady side of the mountain and soil water is not easily evaporation, thus stable isotopes of soil water are relatively less affected by evaporation fractionation.
As shown in Fig. 4, both the semi-sunny aspect and semi-shaded aspect showed that isotope values of soil water were higher in summer and autumn but lower in spring, which may be related to the seasonal variation of soil water sources (Pu et al. 2020). In spring, seasonal permafrost started to melt, which increased the water content of surface soil and caused stable isotopes of soil water depletion, and lower temperature resulted their evaporative fractionation to weaken (Yong et al. 2020). In summer, soil water was mainly recharged by precipitation which has high stable isotope values, and high temperature leads to isotope enrichment for evaporative fractionation. The δ18O values showed that the semi-sunny aspect (-7.26‰) > semi-shaded aspect (-7.43‰) in summer, while the semi-sunny aspect (-8.17‰) < semi-shaded aspect (-7.65‰) in spring. In addition, the variation of δ18O values on the semi-shaded aspect was smaller than on the semi-sunny aspect. The temperature was higher in the semi-sunny aspect than in the semi-shaded aspect, which caused the evaporative fractionation stronger in the former than in the latter. Compared to the semi-shaded aspect, the seasonal permafrost melted more in the semi-sunny aspect in spring, and its soil water content was also higher (69%>57%), which leaded to δ18O depletion.
The δ18O values of soil water showed a consistent variation trend with soil depth on different aspects (Fig. 4). Both semi-sunny aspect and semi-shaded aspect showed a decreasing trend with increasing soil depth (except for soil water of 0–10 cm in autumn), which was consistent with the results studied by Qiu et al.( 2019) and Zhang et al. (2017). In all seasons, the δ18O values of soil water on semi-shade and semi-sunny aspects showed inflection points at the soil layer of 30–40 cm, and showed an overall variation trend of decreasing, increasing to stabilizing. Since surface soil water is significantly affected by precipitation, evaporation and plant transpiration, isotope values of soil water gradually decrease with increasing of soil depth. With the increase of soil depth, the mixing of surface soil water and deep soil water caused the isotope values of soil water increasing and gradually stabilizing (Wang et al. 2021; Du et al. 2021; Zhao et al. 2018).
There were differences in the fluctuation of δ18O values of soil water of different aspects at different soil depths. The standard deviations of δ18O values of soil water on semi-sunny aspect at soil depth of 0–30 cm were larger (Fig. 5), while those at soil depth of 30–80 cm were relatively smaller. This was more consistent with the findings of many scholars. For example, the studies, by Du et al. (2021) in the Loess Plateau and Pu et al. (2020) in the Hani terraces, had shown that isotope variation of soil water was significant in shallow soil layers on the semi-sunny aspect and more stable in deeper soil layers (Yong et al. 2020). It has also been shown that shallow soil water of 0–30 cm was the main water source of subalpine shrubs and exhibits greater variability affected by precipitation, evaporation and transpiration (Szutu and Papuga 2019; Zhang et al. 2022). Different from the semi-sunny aspect, the variability of δ18O values of soil water with soil depth were relatively small in the semi-shaded aspect. That was due to the lower solar radiation and the weaker evaporation of soil water on the semi-shaded aspect, resulting in the fluctuations of isotope values of soil water smaller than on the semi-sunny aspect (Sun et al. 2019). However, the variation rate of δ18O values of the surface soil (0–30 cm) on the semi-shaded aspect were greater than on the semi-sunny aspect, which may be related to the higher contribution of soil water to plant water on the former than the latter (Zhang et al. 2022).
LC-excess is often used to reflect the evaporative fractionation of different water bodies relative to precipitation, and its value is relatively stable (Sprenger et al. 2017). Overall, the LC-excess in study area was rising with increasing of soil depth (Fig. 4), which was caused by the decrease of evaporative fractionation (Yong et al. 2020). LC-excess showed lower values on the semi-sunny aspect and in summer, reflecting that soil water existed the evaporation fractionation of stable isotope. Compared to the semi-sunny aspect, the seasonal differences in LC-excess of surface soil water were significantly larger, which may be related to the relative instability of soil water on the semi-shaded aspect.
4.3 Differences in stable isotopes of plants water
The values of δD and δ18O of plant water were significantly different on different aspects (Fig. 6). On the semi-sunny aspect, the average values of δD and δ18O were − 46.10 ± 10.78‰ and − 6.27 ± 2.15‰, with variation ranging from − 98.57 to 20.79‰ and from − 12.40 to 2.32‰, respectively. The mean (δD: -40.92‰; δ18O: -4.81‰) and standard deviation (δD: 11.27‰; δ18O: 3.70‰) of stable isotopes were the largest for Salix oritrepha Schneid., followed by Salix cupularis. The mean values of Rhododendron thymifolium Maxim., Potentilla fruticosa L., and Salix sclerophylla Anderss. were closer, reflecting the high similarity of their water sources. On the semi-shaded aspect, the average values of δD and δ18O were − 44.30 ± 8.11‰ and-6.14 ± 1.26‰, with variation ranging from − 86.71 to 25.95‰ and from − 9.67 to -1.34‰, respectively. In the same habitat (Fig. 6b), the mean values of stable isotopic were larger for the Caragana jubata (Pall.) Poir. (δD: -40.15‰; δ18O: -5.60‰), Rhododendron przewalskii Maxim. (δD: -42.29‰; δ18O: -5.79‰), and Salix oritrepha Schneid. (δD: -41.97‰; δ18O: -5.85‰), but they were the smallest for the Potentilla fruticosa L. (δD: -51.20‰; δ18O: -6.74‰). The standard deviations of δ18O values of Rhododendron anthopogonoides Maxim. (0.75) and Rhododendron przewalskii Maxim. (0.87) were relatively small, indicating that their water supplies were more stable during the growing period.
Compared to the δ18O values of plant water in study area, the mean was smaller (-6.27‰<-6.19‰) and the standard deviation was larger (2.15 > 1.87) on the semi-sunny aspect, but the maximum and minimum values of δD and δ18O of plant water appeared on the semi-sunny aspect. This indicated that the utilizations of soil water were different for plant species on the semi-sunny slope, and there were certain interspecific water competition relationships in the case of insufficient soil water (Yang et al. 2015). In general, compared to the semi-shaded aspect, soil water evaporation was stronger on semi-sunny aspect, resulting in higher isotope values of soil water and plant water. However, the stable isotopes of plant water were lower on the semi-sunny aspect than on the semi-shaded aspect in study area (Fig. 6), which may be related to the soil texture. The soil texture of the semi-sunny slope is fine, and the soil water is not easy to evaporate, which leads to stable isotopes of soil water lower and further leads to isotope values of plant water lower.
According to the δD and δ18O values of plant water, PWL (semi−sunny aspect) was established: δD = 4.40δ18O-20.14 (R2 = 0.67), and PWL (semi−shaded aspect) was also established: δD = 4.96δ18O-13.87 (R2 = 0.60). The slope of PWL (semi−sunny aspect) was smaller than the study area (4.40) and PWL (semi−shaded aspect) (4.96), which reflected the difference of plant transpiration in different slope aspects. Under the influence of light intensity, the plant transpiration on the semi-sunny aspect was stronger, resulting in a lower slope of PWL (semi−sunny aspect). However, the semi-shaded aspect was affected by the shady condition, which caused plant transpiration relatively weaker and the slope of the PWL (semi−shaded aspect) higher. These were consistent with the research results of SWL in study area.
Based on the growth characteristics of subalpine shrub plants, the growth period of shrub plants during sampling period was divided into three stages: germination and leaf development stage (May-June), flowering and fruiting period stage (June-September), leaf fall recession period stage (September-October). Judging from different growth periods, the δD and δ18O values of plant water in study area were as follows (Table 3): germination and leaf development stage (δD: -43.93‰, δ18O: -5.56‰) > flowering and fruiting stage (δD: − 44.49‰, δ18O: -6.15‰) > leaf fall recession stage (δD: -47.85‰, δ18O: -6.96‰). It could be seen that the δD and δ18O values of plant water showed a decreasing trend with the plant growth, reflecting that there were different characteristics of water absorption of subalpine shrubs in different growth periods. During the germination and leaf development stage, the shrubs had a strong absorption demand for water, but there was less precipitation, resulting in higher stable isotope values of plant water. During the flowering and fruiting stage, the shrubs also had a strong absorption demand for water, but there was more precipitation, resulting in a lower stable isotope value of plant water. During the leaf fall recession stage, the temperature and the precipitation were decreasing, resulting in a decrease in stable isotope value of plant water.
Table 3
Mean values of stable isotopes of subalpine shrubs at different growth stages
stable isotope | germination and leaf development stage ‰ | flowering and fruiting stage ‰ | leaf fall recession stage ‰ |
δD | -43.93 | -44.49 | -47.85 |
δ18O | -5.56 | -6.15 | -6.96 |
d-excess | 0.56 | 4.67 | 7.84 |
4.4 Differences in stable isotopes of river water
In study area, the average values of δD and δ18O in mainstream river were − 54.94 ± 3.62‰ and − 9.02 ± 0.41‰, and their variation ranging from − 65.36 to -45.66‰ and from − 9.63 to -7.68‰ (Fig. 7). Stable isotopes of mainstream river water reached two high peaks on August 5 (-7.96‰) and September 5 (-7.68‰), which may be related to the evaporative fractionation of stable isotopes of river water and recharged water sources. In early August, higher temperatures leaded to stronger evaporative fractionation of stable isotopes of river water. In the week before September 5, there was no precipitation and the temperature was relatively high, which leaded to δ18O values in precipitation higher (Fig. 3). Due to the influence of precipitation recharge, the δ18O values of mainstream river water were also higher. The average values of δD and δ18O in tributary river water were − 51.31 ± 4.33‰ and-8.74 ± 0.72‰, and their variation ranging from − 61.35 to -44.30‰ and from − 10.41 to-7.31‰, respectively. Compared with mainstream river water, the tributary river water had higher isotope values and its variation fluctuated largely (Fig. 6). This was consistent with the results of mainstream and tributary river water of the Oort River (Popescu et al. 2014). The higher isotope values and the larger fluctuations of tributary river water may be related to small water flow and its high evaporation rate. Overall, the δ18O values of tributary river water showed an upward trend during sample period, that the lowest and highest values appeared on the early of June (-10.41‰) and the mid-early of August (-7.31‰), respectively, and most high values mainly appeared after August.
There was obviously difference of the variation trend of δ18O values in mainstream and tributary river water. On seasonal scale, the δ18O values of the mainstream river water were autumn (-8.96‰) > spring (-8.98‰) > summer (-9.10‰), and its seasonal variation was not obvious, while that of the tributary river water was autumn (-8.35‰) > summer (-8.53‰) > spring (-9.55‰). In general, mainstream and tributary river water have similar seasonal differences (Li et al. 2016b). However, the isotopic values of the mainstream and tributary river water were different in study area, which may be related to the evaporative fractionation and the recharge source of river water (Chen et al. 2019). In spring, the main recharged sources of tributary river water were snow and permafrost meltwater, resulting in isotope values of river water lower. In summer and autumn, precipitation and soil water were the main recharge sources of tributary river water, which evaporative fractionation results in stable isotopic enrichment of river water. The mainstream river water had a large flow in summer, which the evaporative fractionation rate was relatively lower and the stable isotope value of the river water is lower also (Shi et al. 2019). In addition, the water source of the mainstream river water was relatively stable, so the seasonal variation of stable isotope values was smaller.