Introduction

Hetao Irrigation District, as Asia's largest one-system irrigation district, serves as a crucial production base for commodity grains and oils for both the country and Inner Mongolia Autonomous Region1. The irrigated area of the district encompasses 5.7 × 105 hm22, with the primary source of irrigation water being the Yellow River diversion. In recent years, the implementation of national water conservation directives has resulted in a decline in the average annual volume of water diverted from the Yellow River for irrigation, decreasing from approximately 5 × 109 m3 year by year to the 4 × 109 m3 currently, which fails to meet the agricultural irrigation needs of the region3,4. The lower reaches of the river, in particular, do not receive adequate irrigation at critical times, necessitating the identification of alternative water sources. The Hetao irrigation area is rich in underground brackish water (2–5 g L−1), presenting significant development and utilization potential5. The scientific and reasonable exploitation of brackish water resources is not only conductive to crop growth, but also effectively mitigates the imbalance between water supply and demand6,7. However, inappropriate salinity associated with brackish water irrigation can adversely affect both crops and the soil. For instance, when irrigated with water having a mineralization greater than 4 g L−1, the high concentration of salt will inhibits the root activity of cotton, resulting in a decline in plant height, stem thickness, and leaf area content8. Wheat seed germination faces a critical salinity threshold of 3 g L−1, as salt concentration increases, the salt infiltrated into the soil, is absorbed by the root system and diffuses, leading to reduced of plant height and yield of wheat9,10. Similarly, irrigation with brackish water containing 2 g L−1 NaCl results in lower water utilization, causing salt accumulation in the soil and water deficit in cucumber, which leads to a 10.35% to 18.34% decrease in yield compared to freshwater irrigation11. Therefore, achieving the safe use of brackish water is of paramount importance for enhancing water conservation and irrigation efficiency in the region.

Lycium barbarum is notable for its has biological characteristics of drought resistance, salt tolerance, and significant economic value. It is extensively cultivated in regions like Ningxia and Inner Mongolia. The cultivating area in the Hetao irrigation area reaching as high as 1.33 × 104 hm212. As a homologous crop of medicine and food, Ningxia Lycium barbarum was included in the Pharmacopoeia of the People's Republic of China in 202013. Nonetheless, the growth response of Lycium barbarum to various types and concentrations of brackish water salinity varies. Guo Huan et al.14found that growth is adversely affected when salinity exceeds the salt tolerance threshold of 2.425 g L−1, with a notable reduction in yield observed at 3 g L−115. Salt stress hinders crop development across various stage, with significant growth inhibition occurring at concentrations greater than 200 mM16. Therefore, establishing a suitable typical salt concentration limit for brackish water irrigation that supports the growth of Lycium barbarum is crucial. Hetao Irrigation District is a typical salinized agricultural irrigation district8, the salt content of the soil-forming parent material is high, experiences exacerbated soil degradation under brackish water irrigation17. Liu Zhenyuan et al.10observed an increase in soil salinity with rising salt concentrations, and infiltration following irrigation with NaCl and CaCl2 at concentrations of 3 g L−1 and 5 g L−1 enhanced soil salt accumulation18. In summary, different concentrations of brackish water irrigation differently impact soil salinity, physicochemical properties, and crop growth. Due to its inherent salt resistance, Lycium barbarum is ideally suited for cultivation in brackish water-irrigated areas, representing an optimal cash crop to alleviate the scarcity of freshwater resources and the pressures of agricultural production in the Hetao irrigation area. The prevalent brackish water types in the Hetao irrigation area include HCO3-Na, SO4-Na, and Cl-Na19. Among these, the cations are predominantly Na+ and Ca2+, and Ca2+ is the dominant ion in the groundwater of the irrigation area; anions are dominated by Cl-, followed by SO42- and HCO3–20,21. Current research primarily focuses on the impact of individual typical salts22,23,24 and mixed salts25,26 on the fruit quality14, growth structure22, and photosynthetic activity27 of Lycium barbarum, as well as on seed germination and seedling growth. Studies on soil salinity generally concentrate on brackish water irrigation methods28, control of irrigation water quantity, and salinity29. However, comprehensive investigations into the effects of different salinity types on rhizosphere salinity distribution, growth conditions, and yield of Lycium barbarum during the high yielding growth period are less frequently reported, indicating a need for further research. Therefore, this study conducted a field cross test of 4 typical salts and 4 concentration levels for moderately salinized farmland in the Hetao irrigation area to investigate the effects of different typical exogenous salt concentrations on total soil salinity and the growth of Lycium barbarum under brackish water irrigation, and to determine the salinity threshold of irrigated brackish water that is suitable for the normal growth of Lycium barbarum and for alleviating of soil salinity accumulation. The results of the study can provide an important scientific basis for the safe and rational utilization of brackish water and the cultivation of Lycium barbarum in areas with varying saline water qualities of Hetao irrigation area.

Materials and methods

Overview of the study area

The experiment was conducted at the Hongwei Experimental Station, located in the downstream of the Hetao Irrigation District (108°45′-109°36′E, 40°30′-40°40′N) at an altitude of 1040 m. This area is characterized by scarce freshwater resources and higher availability of brackish water, with a mineralization degree of 3.84 g L−1 used for irrigation (Table 1). The climate is classified as a medium-temperate zone with semi-arid continental characteristics. The area experiences an annual average precipitation of 181.95 mm and evaporation of 2383 mm, with an average annual temperature of 7.9 ℃ (Fig. 1). The frost-free period lasts for 146 days. The soil in this region is moderately saline with total salt content ranging from 0.48 to 0.84% (Table 2).

Table 1 Irrigation of brackish water quality before the test.
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Fig. 1
figure 1

Daily rainfall and temperature during the fertility period of Lycium barbarum.

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Table 2 Soil texture before the test.
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Experimental design

The experiment employed the “Ningqi No. 1” variety of Lycium barbarum, now 12 years old, and utilized a randomized block design for a two-factor crossover test involving four typical salts (NaCl, CaCl2, NaHCO3 and Na2SO4) at four concentration gradients. Within the experimental area, 37 Lycium barbarum plants were planted per row, with a plant spacing of 1 m and a row spacing of 3 m. To ensure experimental integrity, 2 plants were positioned at the beginning and end of each plot, and every other row was planted to establish a buffer zone. The remaining 5 plants in each row were successively planted to form an experimental unit measuring 3.0 m × 13 m. The plots were isolated by a 1.5-m-high, seepage-proof plastic sheet. The study included 17 treatment groups, each replicated 3 times, making a total of 51 plots, each covering an area of 39 m2. Lycium barbarum was irrigated according to the local conventional volume, with 4 irrigation events during the growth period, each delivering 100 mm of water via border irrigation. Table 3 provides detailed information. For this experiment, the range of ion concentrations in the brackish water of the Hetao irrigation area downstream was used as a reference19,30, with ion concentrations of Na+, Ca2+, Cl−, SO42−, and HCO3− ranging from 13.8–100.4, 0.9–13.3, 11.5–108.8, 1.7–23.6, and 4.0–32.5 mmol L−1, respectively. Based on the water quality of the experimental site (with the ion concentrations of Na+, Ca2+, Cl−, SO42− and HCO3- being 18.6, 8.8, 63.0, 6.3 and 5.5 mmol L−1, respectively), salt concentrations were adjusted for different treatments. NaCl, CaCl2, NaHCO3, and Na2SO4 were added to the local brackish water (3.84 g L−1) at rates of 0.1, 0.5, 2.0, and 4.0 g L−1. The resulting ion concentration for each treatment fell within the upper limit of the salt ion concentration range found in the experimental and Hetao irrigation areas, providing a reference for the study’s significance.

Table 3 Experimental design.
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Measurement indices and methods

In this experiment, physiological indices were assessed during the spring shoot growth period (A1), early flowering stage (A2), fruit expansion stage (A3), and prime fruiting period (A4). (Table 4) For each treatment, three plants exhibiting uniform growth were selected for observation. New growth branches were tagged and left unpruned, while other non-essential branches were pruned approximately every 10 days throughout the growth period.

  1. (1)

    New branch growth: One new branch was selected from the east, south, west, north, and middle of each plant for measurement using a tape measure. Measurements were taken every 3 days from April 20 to early July, and thereafter every 10 days until leaf fall.

  2. (2)

    Crown width: measured east–west and north–south, was recorded with a tape measure prior to budding and subsequently in the middle of each month until the end of fruit harvesting.

  3. (3)

    Ground diameter: The ground diameter was measured using a vernier caliper before budding. Subsequent measurements were taken once in the middle of each month, continuing until the end of the fruit harvest.

  4. (4)

    Yield: 4 trees with similar growth potential were selected for each treatment for yield assessment. Harvested Lycium barbarum fruits were arranged by number on a fruit stack and uniformly dried in a heat pump dryer using a coal fire. The drying room was maintained at humidity of 25% and a temperature of 60–70℃. The fruits were weighed every 2 h, with the drying process lasting about 8 h or until the weight of the Lycium barbarum remained unchanged. The dry mass of Lycium barbarum from each district was then weighed to determine the yield of dried fruit per unit area.

  5. (5)

    Soil total salt content: Soil samples were taken before and after each irrigation during the crop’s growth period using a soil auger. Samples were collected from 0 to 0.4 m at every 0.1 m and from 0.4 to 1 m at every 0.2 m, resulting in a total of 7 layers. The content of eight ions (Na+, K+, Mg2+, Ca2+, Cl-, SO42-, and HCO3-) in the soil was analyzed according to LY/T 1251–1999, Analysis of Water-soluble salinity in Forest soil. Soil samples were air-dried, ground and sieved, and a clear liquid was extracted at a soil-to-water ratio of 1:5. The salt ions in the soil solution were analyzed using conventional methods: Cl− was measured through standard silver nitrate titration; water-soluble Na+ was assessed using NaOH melt-flame spectrophotometry; K+ was extracted via ultrasonic extraction, followed by solid phase extraction and filtrate volume determination; ion chromatography was employed for Ca2+ and Mg2+, which were determined using EDTA complexometric titration; HCO3- was quantified through dual indicator-neutralization titration; and SO42- was analyzed using indirect titration with EDTA. The sum of these eight ions represented the total salt content of the soil.

  6. (6)

    The local brackish water (3.84 g L−1) :Prior to sampling, the 500 mL sampling bottle was thoroughly rinsed with distilled water multiple times and dried for future use. The bottle was then rinsed with groundwater samples three to four times to eliminate potential contaminants before subpacking the samples. Following this, filled the bottle with water samples, sealed with a sealing film, taken 3 parallel samples, label them, and send them to Bayannur Institute of Water Conservancy Science for determination within 24 h. Strictly follow the standard determination method proposed by the Ministry of Health of the People's Republic of China (GB/T 5750–2006)31.

Table 4 Phenological period of Lycium barbarum in the experimental area.
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Data analysis

Each treatment was consisted of three replicates, and the data were presented as mean. Data were processed, plotted, and analyzed using Excel 2020 and Origin 2021 software, while SPSS software (version SPSS 26.0) was utilized for correlation analysis and variance analysis, the least significant difference (LSD) method was used for significance test, and the significance level was 0.05.

Path analysis, based on regression and correlation analysis, established interrelationships between multiple independent variables and the dependent variable, thereby revealing the direct and indirect effects of independent variables on the dependent variable and quantifying their influence.

Results and analysis

Response of physiological traits of Lycium barbarum to exogenous salt concentration

Growth rate of new branches

The growth rate of new branches of Lycium barbarum showed consistent trends across all four typical salts (Na2SO4, NaCl, CaCl2, and NaHCO3) added to brackish water (Fig. 2). For instance, using Na2SO4 (Fig. 2a) for detailed analysis, it was found that within a certain salt concentration, significant differences (P < 0.05) were observed in the growth rate of new branches across different fertility periods. The growth rate decreased over time and was 10.86%-15.67%, 26.1%-33.73%, and 61.33%-71.63% lower in the A2, A3, and A4 periods, respectively, compared to the A1 period. The growth rate in A1 increased and then decreased with the rising salt concentration, peaking at 5.25 mm d−1 with a 0.5 g L−1 concentration (NS2). This rate was 1.16%, 2.14%, 4.17%, and 1.74% higher than NS1, NS3, NS4, and CK treatments, respectively, with a significant difference noted with NS4 (P < 0.05). Similarly, for NaCl and CaCl2 during A1, the maximum growth rate was observed at a concentration of 0.5 g L−1, being 5.22 mm d−1 and 5.27 mm d−1, respectively. However, NaHCO3 showed an inverse relationship with salt concentration in A1, peaking at 0.1 g L−1 (NHC1), which rates higher by 0.19%, 0.78%, 1.37%, and 2.58% compared to CK, NHC2, NHC3, and NHC4, respectively. The remaining changes were consistent with the trend observed for Na2SO4.

Fig. 2
figure 2

Effects of different salt concentrations on the growth rate of new branches of Lycium barbarum. Different uppercase letters indicate significant differences at the 5% level between different concentrations in the same growth period, and different lowercase letters denote significant differences between different growth periods at the same concentration level, P < 0.05. This notation applies throughout the paper.

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Growth rate of ground diameter

The growth rates of ground diameter under the influence of four typical salts (Na2SO4, NaCl, CaCl2, and NaHCO3) in brackish water showed similar trends (Fig. 3). Taking NaCl as an example (Fig. 3b), a significant difference was observed in the growth rates across different fertility periods, demonstrating a single-peak distribution. Specifically, there was a 12.28% decrease in the A2 period compared to the A1 period, a 70% decrease in the A4 period compared to the A3 period, and a 44.31% increase in the A3 period compared to the A2 period (P < 0.05). The peak growth rate occurred in the A3 period, averaging 0.498 mm d−1, which initially increased and then decreased with rising salt concentrations. At concentrations of 2.0 and 4.0 g L−1 (NC3 and NC4), the growth rate significantly decreased by 3.52% and 7.63%, respectively, compared to CK (P < 0.5). In contrast, at lower concentrations of 0.1 and 0.5 g L−1 (NC1 and NC2), the differences from CK was not statistically significant (P > 0.5). When the salt concentration of A3 stage was 0.5 g/L and 0.1 g L−1, the growth rate of ground diameter varied with different salt types.At a 0.5 g L−1 concentration during the A3 period, the growth rates for Na2SO4, NaCl, and CaCl2 achieved their maxima (0.519, 0.513, and 0.524 mm d−1, respectively), which were 1.57%, 0.39%, and 2.54% higher than CK, respectively. Meanwhile, NaHCO3 reached its maximum of 0.512 mm d−1 at a concentration of 0.1 g L−1 (NHC1), showing an increase of 0.20% over CK.

Fig. 3
figure 3

Effects of different salt concentrations on the growth rate of ground diameter of Lycium barbarum.

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Growth rate of Crown width

The crown width was quantified as the mean of the maximum diameters measured in the east–west and north–south directions of a single Lycium barbarum plant. The growth rates of crown width under different concentrations of the four typical salts (Na2SO4, NaCl, CaCl2, and NaHCO3) followed a consistent pattern throughout the reproductive period (e.g., Fig. 4). For example, using CaCl2 (Fig. 4c), influenced by the salt concentration ,the crown growth rate varies across different fertility periods (1.28–4.09 mm d−1). Under the same concentration, the rate gradually decreased with the progression of the fertility period. The highest average rate of 4.00 mm d−1 was observed in the A1 period, which exhibited an initial increase followed by a decrease as CaCl2 concentration increased. The maximum rate of 4.09 mm d−1 was observed at a concentration of 0.5 g L−1 (CC2 treatment), which was NaHCO3 concentration in the A1 stage was significantly different compared to other salts. The crown growth rate was inversely related to the NaHCO3 concentration and peaked at 4.00 mm d−1 at a concentration of 0.1 g L−1 in the A1 period, which was 0.38%, 1.78%, 3.49% and 6.10% higher than that of the CK, NHC2, NHC3, and NHC4 treatments, respectively.

Fig. 4
figure 4

Effects of different salt concentrations on the growth rate of Lycium barbarum crown width.

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Effects of different typical exogenous salt concentrations on dry fruit yield of Lycium barbarum

As can be seen in Fig. 5, the dry fruit yield of Lycium barbarum decreased with increasing concentrations of NaCl, Na2SO4, CaCl2, and NaHCO3. The effect of different salt concentrations on dry fruit yield was statistically significant (P < 0.5). At a concentration of 0.1 g L−1 (CC1, NS1, NC1, and NHC1 treatments), the yields were 4698.41, 3878.44, 3086.42, and 2710.48 kg hm−2, respectively, representing the highest yields recorded. Notably, the CC1 treatment increased dry fruit yield by 11.90% compared to CK, conversely, the yields in NS1, NC1, and NHC1 treatments decreased by 7.63%, 26.49%, and 35.45%, respectively, demonstrating the superior effect of Ca2+ over Na+ in enhancing yield32.

Fig. 5
figure 5

Effects of different salt concentrations on the yield of dried Lycium barbarum fruit.

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Effects of different typical exogenous salt concentrations on total salt in farmland soil under Lycium barbarum cultivation

Soil total salt content is a critical indicator of soil salinization. Figure 6 illustrates the distribution of total salt content in the 0–1 m soil layer under different typical salt concentrations throughout the entire fertility period of Lycium barbarum under brackish water irrigation. The trend of total soil salinity was consistent across different salt concentrations treatments (NaCl, Na2SO4, CaCl2, and NaHCO3) irrigation Lycium barbarum. Analyzing NaHCO3 in detail, At the same concentration level during the whole growth period, the total salt content from the 0 to 1 m soil layer followed an order across different fertility periods: A3 > A4 > A2 > A1. During the A1 period, under different concentrations of NaHCO3, the salt content ranged from 4.382 to 7.348 g kg−1. The accumulation of salt increased with the concentration, and the soil total salt content for the NHC1 treatment (0.1 g L−1) had the lowest value, averaging 5.206 g kg−1. The following treatments-NHC2, NHC3, and NHC4-showed increments of 20.87%, 31.17%, 49.24%, and 56.34% higher than CK, respectively. The soil salt content in the NHC1 treatment during the A1 period showed fluctuations with increasing soil depth. The total soil (0–0.1 m) showed superficial accumulation, being 4.83%, 15.32%, and 31.54% higher than at depths of 0.2, 0.3, and 0.4 m, respectively. The 0.3–0.4 m soil layer presented a low peak, with the smallest salt content recorded at 5.254 g kg−1 in the NHC1 treatment. The 0.4–0.6 m layer, more sensitive to salt changes, showed a slight increase, averaging 6.567 g kg−1, which was 6.65% higher than the 0.3–0.4 m layer. Finally, the 0.6–1 m exhibited a decreasing trend, with salt content ranging from 4.382 to 5.222 g kg−1.

Fig. 6
figure 6

Effects of different salt concentrations on soil total salt content.

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Path analysis of growth indexes, soil total salt, and yield of Lycium barbarum

To further elucidate the direct and indirect effects of various indicators on the yield of Lycium barbarum, this study quantified the influence of each growth indicator and soil salinity on the yield and explored their interactions. Based on the results of field experiments, path analysis was conducted growth indicator, soil salt and yield of Lycium barbarum for the entire fertility period. The results, with a coefficient of determination R2 greater than 0.9, indicate a good fit, accurately reflecting the relationships among growth indices, total soil salinity, and the yield of Lycium barbarum. The relationships are illustrated in Fig. 7 and summarized in Table 5.

Fig. 7
figure 7

The results of path analysis for yield of Lycium barbarum yield and associated indices.

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Table 5 Path analysis between growth indices, soil total salt, and yield of Lycium barbarum.
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The analysis revealed that under the treatment of these 4 typical exogenous salts soil total salts exerted a negative impact on the yield of Lycium barbarum, acting as limiting factors, while indices such as crown width, new branches, and diameter positively influenced yield to varying extents.

As shown in Fig. 7, neutral salts(NaCl, Na2SO4, CaCl2) , as exemplified in Fig. a, the direct impacts of the indices on yield, in descending order of magnitude(in absolute value), were total soil salt (0.820), crown width (0.144), new branches (0.086), and ground diameter (0.019). Under the alkaline salt NaHCO3 treatment, the direct effects were 0.790 for total soil salt, 0.195 for crown width, 0.041 for new branches, and 0.022 for ground diameter, indicating that total soil salinity consistently had the most substantial direct effect on under both neutral and alkaline conditions. It can be seen that the direct determining effect of total soil salt on Lycium barbarum yield was greater in both neutral and alkaline salts, and the other indexes were Lycium barbarum crown width, new branches and ground diameter in that order. The interactions among the growth indices showed indirect effects on yield. For instance, under NaCl treatment, the correlation coefficients between new branches and crown width, new branches and ground diameter, and crown width and ground diameter were 0.778, 0.104, and 0.179, respectively. The negative correlations between soil total salt and ground diameter, crown width, and new branches were − 0.328, − 0.767, and − 0.321, respectively. Similarly, under NaHCO3 treatment, the correlation coefficients among growth indices were 0.599, 0.350, and 0.563, respectively, while the correlations between total salt and growth indices were − 0.420, − 0.756, and − 0.402, respectively, it can be seen that the indirect effects of growth indexes on yield were all mainly affected through the interactions between new branches and crown width, the indirect effect of total soil salt on yield was mainly through its interaction with crown width.

According to Table 3, the simple correlation coefficients of each index with yield, ranked by the magnitude of their absolute values, were X4, X3, X1, and X2. In terms of direct influence, the ranking was the same. However, from the perspective of indirect influence, the order was X3, X1, X2, X4. These results indicate that the direct impacts of soil total salinity and crown width on yield are substantial, with the direct effect of soil total salt being greater than the sum of its indirect effects. This suggests soil salinity predominantly affects yield directly, while the influences of other indicators are mainly manifested through indirect effects and are relatively lesser.

In summary, there is a mutual influence among all indices, with none acting in isolation. Each index affects the yield of Lycium barbarum through a combination of direct and indirect actions. To optimize the yield under brackish water irrigation, controlling the total soil salt content is imperative.

Discussion

Salt concentration is one of the main abiotic stress factors affecting plant growth and development, while the external morphology and growth status of plants are the most intuitive expression of the damage degree of saline-alkali stress33,34. Under high concentrations of saline and alkaline stress, plants experience osmotic stress and ionic toxicity, resulting in blocked metabolic functions and reduced intracellular enzyme activity, thereby affecting normal development35,36. However, in accordance with principle of “survival of the fittest,” plants can sustain their growth under adverse conditions by regulating inter-root osmotic pressure, changing the structure of cell membrane lipids, and adjusting growth patterns37. This study observed that under brackish water irrigation, the growth parameters of Lycium barbarum, including the number of new branches, ground diameter, and crown width, peaked when the combined concentration of exogenous NaCl, Na2SO4, and CaCl2 (neutral salts) was 0.5 g L−1. Conversely, the optimal growth of Lycium barbarum occurred at a concentration of 0.1 g L−1 of NaHCO3 (basic salt). It is evident that, compared to the other three neutral salts, alkaline salts inflict the most severe damage on various growth indices of Lycium barbarum, likely attributable to the elevated pH induced by alkaline salts38. This finding aligns with the notion that alkaline salt stress exerts a more pronounced inhibitory effect on plants compared to neutral salt stress, as proposed by Guo J et al.39in their study of crop responses to saline and alkaline stresses. Furthermore, our study revealed an inverse relationship between the growth indices of Lycium barbarum and the concentration of the four salts (NaCl, Na2SO4, CaCl2, and NaHCO3) when the exogenous salt concentration exceeded 0.5 g L−1. This phenomenon occurs because, as salt stress intensifies, a significant influx of Na+ ions hinders the absorption of other essential mineral elements, leading to ion imbalance and inducing ion toxicity. Concurrently, the increase in salt concentration will lead to the weakening of Ca2+ absorption by plants, which will cause the lack of Ca2+ in plants in the long run, thus inhibiting the growth and development of plants. Furthermore, the high concentration of Ca2+ itself is also a stress, and the calcium signal caused by adversity and the excessive reactive oxygen species produced by it are toxic to plants40,41,42,43. However, divergent opinions exist regarding the salt tolerance threshold range of Lycium barbarum. Yang Wanpeng et al.23 conducted potting salt control tests with 6 concentration gradients and found that the tolerated saline stress threshold value during the growth period of Lycium barbarum ranged from 50 to 150 mmol L−1, with various growth indicators decreasing with increasing salt concentration. Meanwhile, Hu J et al.44 through Lycium barbarum cultivation experiments with different NaCl concentrations in Minqin County, Gansu Province, concluded that a concentration of 50 mM NaCl is optimal for Lycium barbarum growth. Although the overall trend of brackish water salt concentration effects on Lycium barbarum stress is consistent, discrepancies in effect magnitude arise due to differences in testing locations and methods. Crop yield is influenced by different brackish water salt concentrations45,46. Li Jinguang et al.47 found that corn seed yield decreased when the salinity of brackish water exceeded 2 g L−1, as demonstrated by manually adjusting irrigation water salinity. Similarly, Ma Jiaying et al.48 observed tomato growth was significantly inhibited by brackish water irrigation at 6 g L−1, resulting in reduced energy yield compared to irrigation at 2 g L−1. In this study, it was observed that among the four exogenous typical salts, at a concentration of 0.1 g L−1, Na+ had a greater negative impact on dried Lycium barbarum yield than Ca2+, whereas Ca2+ significantly promoted yield compared to Na+32. Additionally, the results showed that the yields of NaCl, Na2SO4, and NaHCO3 were lower than CK at 0.1 g L−1, whereas the yields of CaCl2 exceeded that of CK.

In numerous studies, scholars have investigated different levels of mineralization in irrigation water to assess soil salinity. Overall, these studies have consistently shown that soil salt accumulation increases with higher levels of mineralization49,50. In this study, we found a positive correlation between total soil salinity from 0 to 1 m depth and the salt concentration from different typical salt sources in brackish water throughout the fertility period. Notably, the lowest total soil salinity was recorded when the exogenous salt concentration was 0.1 g L-1. Additionally, salinity levels fluctuated with soil depth, with surface soil exhibiting higher salinity levels. This phenomenon may be attributed to increased temperatures spring, leading to strong soil moisture evaporation, thereby exacerbating surface salt accumulation49. Furthermore, high concentrations of salt in brackish water may leach into the soil, elevating soil salt content15. Leaching of rhizosphere soil during the spring irrigation in the beginning of May resulted in declining salt in the 0 to 0.4 m soil layer, reaching a low peak in the 0.3 to 0.4 m soil layer. However, salinity increased in the 0.4–0.6 m layer, possibly due to salt leaching from deeper layers during fall watering in the previous year, which subsequently froze51. As soil depth increased, external factors gradually reduced, resulting in a minimal change in the total salt content in the 0.6–1 m soil layer, with an overall decreasing trend. It can be seen that the accumulation of salts in the soil and the leaching effect occur simultaneously in the process of brackish water irrigation. When the accumulation of soil salts is too high, it has a negative effect on the growth of Lycium barbarum, which in turn causes physiological water shortage of Lycium barbarum plants and affects its final yield. At this time, leaching plays a dominant role, and the low concentration (< 0.5 g L-1) of brackish water can be used for irrigation, which does not inhibit the growth of Lycium barbarum and even favours the growth of Lycium barbarum, which is probably due to the fact that a moderate amount of brackish water, which will bring the mineral elements to the root system to promote its growth41. When the brackish water concentration is higher than 0.5 g L−1, the rate of salt accumulation brought in by brackish water infiltration is higher than the rate of drenching, and at this time, in order to ensure that the soil salt accumulation is within the normal range, it is necessary to appropriately increase the measures such as freshwater irrigation, to drench the salt out of the root zone52.

In summary, although brackish water irrigation can mitigate water resource supply–demand imbalances in river-loop irrigation areas, irrational salt concentrations in brackish water irrigation can induce salt stress in Lycium barbarum, leading to physiological water shortages and decreased yields. Considering different brackish water salt concentration treatments, the suitable concentration range for optimal Lycium barbarum yield and its constituent factors is 0.1–0.5 g L−1.

Conclusions

  1. (1)

    The growth rate of new branches, ground diameter, and crown width of Lycium barbarum showed a trend of increasing and then decreasing with increasing exogenous salt concentrations of NaCl, Na2SO4 and CaCl2, with the optimal concentration being 0.5 g L−1. Conversely, growth decreased with increasing NaHCO3 concentration, with the optimal concentration being 0.1 g L−1.

  2. (2)

    Throughout the whole reproductive period, when the concentration of the exogenous typical salts in brackish water (NaCl, Na2SO4, CaCl2 and NaHCO3) was 0.1 g L−1, rhizosphere soil salt accumulation was lower, resulting in a higher dry fruit yield of Lycium barbarum.

  3. (3)

    Path analysis results showed that under the four exogenous typical salt treatments, soil total salt content was the primary direct factor influencing Lycium barbarum yield, while crown width of Lycium barbarum was the main indirect factor affecting yield.

  4. (4)

    Considering Lycium barbarum growth, stable yield, and soil salt control, the suitable concentration range under neutral salt (NaCl, Na2SO4 and CaCl2) treatments was 0.1–0.5 g L−1, whereas under alkaline salt (NaHCO3), the NHC1 (0.1 g L−1) treatment was the best choice.