Impacts of Summer Afforestation and Multi-Stage Drip Irrigation on Soil and Vegetation in Coastal Saline Soils

Abstract

The improved multi-stage drip irrigation scheduling, combined with agronomic engineering, was successfully applied for spring re-vegetation in coastal saline soils. To date, few studies have addressed summer vegetation planting using this method. The aim of this study is to reveal the desalinization mechanism associated with summer afforestation and multi-stage drip irrigation. A three-year field experiment was conducted in the coastal saline land of southern China. The trial consisted of four irrigation stages, with the soil moisture potential (SMP) monitored directly beneath the drip emitter at a depth of 0.2 m, correspondingly controlled to be higher than −10 kPa (Stage I), −25 kPa (Stage II), and −45 kPa (Stage III), respectively. Results indicated that soil bulk density decreased by 14%, while soil moisture increased by 30% compared to initial conditions. The average electrical conductivity (EC) value across the entire soil layer decreased by 65.64% to 97.79%. Soil pH gradually increased during the first three irrigation stages, with the rate of increase accelerating during the rainfed stage, reaching values between 9.22 and 9.87. The concentrations of soil ions, including Ca²⁺, K⁺, Mg²⁺, Na⁺, and SO₄²⁻, decreased by 95.18%, 79.67%, 87.74%, 89.68%, and 57.19%, respectively, in the final irrigation stage. Throughout the entire soil profile, the average sodium adsorption ratio (SAR) decreased by 49.37%, while the average exchangeable sodium percentage (ESP) increased by 9.98%. This study demonstrated that multi-stage drip irrigation scheduling significantly influenced the soil physicochemical properties, soil salt ions, and vegetation growth, and thereby explained the efficient desalinization mechanism associated with this irrigation strategy. It is recommended to increase the amount of irrigation water and apply acidic regulators during the rainfed stage to reduce soil pH for vegetation establishment in coastal saline areas.

1. Introduction

The global area of salinized soils continues to expand, currently covering approximately 1.1 × 10⁹ hm², of which 14% is classified as unproductive wetlands, woodlands, and nature reserves [1]. Soil salinization poses a significant threat to global soil health [2], leading to decreased soil fertility and reduced crop growth. The effects of soil salinization on soil nutrients, water availability, microbial activity, and plant metabolism also impact biodiversity and ecosystem integrity, thereby severely limiting ecological and agricultural sustainability as well as soil utilization [3,4]. Severe soil salinization can result in water loss within plant cells, ultimately leading to plant mortality [5,6]. However, certain salt-tolerant halophytes can survive in highly saline coastal soils [7,8], including Suaeda (Suaeda glauca Bge.) and reed (Phragmites australis (Cav.) Trin. ex Steud) [9].

Artificial landscaping of severely saline soils presents significant challenges, underscoring the necessity of considering soil salinity when selecting greening methods for application to saline environments [10]. A prevalent approach involves importing less saline soils from external sources. Although this method provides soil with lower salinity levels, it necessitates large volumes of soil, leading to substantial drawbacks, including high costs and the disconnection of the planting layer from the original soil environment. Furthermore, the introduced soil is susceptible to gradual salinization [11,12]. Additionally, due to scheduling and time constraints, afforestation projects frequently encounter the issue of planting tree species that are not optimally suited for the season, adversely impacting the survival rates of afforestation trees and ultimately diminishing the effectiveness of the greening efforts.

The regulation of salt in coastal saline soils is commonly managed through water–salt regulation, necessitating the selection of an effective leaching method. Reducing the salinity and sodium adsorption ratio of moderately saline soils can enhance the movement of water and salt. Drip irrigation, recognized as the optimal method for cultivating plants in saline soils, is favored for its ability to irrigate areas evenly and accurately at high frequencies while maintaining a high soil water potential [13,14]. Previous studies have shown that multi-stage drip-irrigation scheduling is effective for the artificial afforestation of coastal saline–alkali soils, significantly enhancing the soil environment within 2–3 years [15,16]. Furthermore, drip irrigation combined with a sand–gravel barrier has been demonstrated to decrease soil salinity, increase soil hydraulic conductivity, and promote vegetation growth when planted in spring [17]. Other researchers have recommended soil matric potential thresholds of −5, −10, and −20 kPa for the first, second, and third years, respectively, as thresholds for initiating drip irrigation for Chinese rose cultivation in spring in coastal salinized soils [6,17].

Research has demonstrated that spring is an ideal season for planting due to the mild temperatures and expected rainfall, which create optimal conditions for tree establishment [18]. Furthermore, as roots begin to grow, the warming soil provides adequate time for strengthening before the onset of summer heat. Salix matsudana, a species that can tolerate moderate salinity, is both cost effective to cultivate and well suited for the establishment of riparian forests, windbreaks, and roadside vegetation [19,20,21]. Previous research has explored the effectiveness of integrating Salix matsudana, which is planted in spring, with drip irrigation systems to regulate in situ soil salinity and manage water in severely saline soils. Given the urgent demand for rapid and sustainable afforestation, it is crucial to assess whether summer is also a suitable period for plant growth. However, there is a notable scarcity of studies focusing on summer re-vegetation, particularly regarding the effects of soil physicochemical properties and plant responses.

This area is characterized by typical coastal saline land, marked by excessive salt accumulation, a shallow groundwater table, and challenges for vegetation growth. The objectives of the research were to (1) examine changes in soil physicochemical properties under drip irrigation; (2) investigate the spatial and temporal changes in soil salt ions resulting from drip irrigation; (3) assess the effects of summer afforestation on vegetation growth; and (4) offer recommendations for the improved implementation of summer afforestation and multi-stage drip irrigation.

2. Materials and Methods

2.1. Site of the Experiment

The experiments were conducted in the Xuwei New Area (119°17′–119°38′ E, 34°29′–34°40′ N), located in the southeastern city of Lianyungang, west of the Huanghai Sea (Figure 1). This area is classified as having a warm temperate humid monsoon marine climate, characterized by an annual average temperature of 14.10 °C, a minimum temperature of 18.10 °C, and a maximum temperature of 40.00 °C. The average annual rainfall is 900.90 mm, with over 70% occurring between June and September, while the annual evaporation is 855.10 mm, with 59% occurring between May and September.

The soils in this area have developed primarily from Quaternary sediments, with the parent material predominantly derived from the alluvial deposits of the Huanghuai River. Due to the sedimentary environment, these soils are characteristic of coastal saline soils, with the salt composition primarily consisting of chlorides. Chloride and sodium account for 46.07–56.26% and 30.20–36.91% of the anion and cation concentrations, respectively [22]. The soil profile within the 0–1 m depth of the experimental field comprises silt (0.002–0.05 mm; 75.44%) and sand (0.05–2 mm; 24.45%), while clay (<0.002 mm) constitutes only 0.11%. According to the United States Department of Agriculture classification [23], the soil texture is classified as silt. The average initial values of the electrical conductivity of saturated paste extracts (EC), pH, and organic matter content in the top 1 m of the soil profile were 22.44 dS m⁻¹, 7.92, and 5.46 g kg⁻¹, respectively (

Table 1. Basic physicochemical properties of soil in the study area.

Chemical Index	Mean	Standard Deviation	Minimum	Maximum	Median	Coefficient of Variation/(%)
Organic matter (g kg−1)	5.46	2.85	2.79	11.00	4.52	52.12%
Cation exchange capacity (cmol·kg−1)	18.07	5.44	6.70	25.40	19.45	30.11%
Available P (mg kg−1)	6.30	2.31	2.90	10.20	5.65	36.58%
Available K (mg kg−1)	184.10	183.94	43.00	586.00	101.50	99.91%
Alkali hydrolyzable nitrogen (mg kg−1)	52.19	10.85	37.00	71.70	49.85	20.78%
pH	7.92	0.15	7.36	8.57	7.90	2.00%
Electrical conductivity (dS m−1)	22.44	16.71	0.60	71.76	24.44	75.00%

). The groundwater is classified as sodium chloride water, with total dissolved solids exceeding 10 g L⁻¹ and a table depth of 1.00–1.50 m.

2.2. Field Experiments and Irrigation Management

In September 2021, Salix matsudana was cultivated under drip irrigation. A planting area of 3 × 3 m spacing was designed on a 200 m long and 30 m wide plot with 5 cm, 10 cm, and 1.5 m thick layers of gravel, gravel stones, and saline soil from top to bottom, respectively (Figure 2). The improved multi-stage drip-irrigation scheduling include four different irrigation stages: (1) high leaching (Stage I), from September to October in 2021; (2) normal leaching (Stage II), October to November in 2021; (3) regulation of salt and water (Stage III), March to November in 2022; and (4) the rainfed stage (Stage Ⅳ), March to September in 2023. The soil matric potential thresholds were maintained at −10 kPa, −25 kPa, and −45 kPa during the first three stages, respectively. To fulfill the requirements of sustainability and economic efficiency, drip irrigation tapes with a wall thickness of 0.6 mm, emitter spacing of 0.2 m, and an average flow rate of 0.62 L h⁻¹ were installed alongside Salix matsudana at intervals of 0.3 m for Stage I and 0.6 m for Stages II, III, and IV. The EC, pH, and SAR of irrigation water in the experiment are 0.35 dS m⁻¹, 7.58, and 0.97 (mmol L⁻¹)^0.5 (

Table 2. The ions content of irrigation water.

EC (dS m−1)	pH	Content of Ions (g L−1)					SAR (mmol L−1)0.5
		Ca2+	K+	Mg2+	Na+	SO42−
0.35	7.58	1.07	1.01	0.47	1.21	3.01	0.97

).

Tensiometers (WST-2B, Waterstar, Beijing, China) were utilized to monitor the soil matric potential threshold twice daily (8:00 and 17:00) at depths of 0.2 m and 0.5 m in all treatments. The amount of water applied during each irrigation event for all treatments was set at 6 mm when the soil moisture potential (SMP) reached the designated threshold value. This approach maintained the unsaturated state of the soil moisture treatment and optimized the daily transport of water to the plants in the vicinity. The irrigation depth, defined as the volume of irrigation water per unit area, was regulated by a water meter connected to the drip irrigation system.

2.3. Soil Physicochemical Properties

During the initial irrigation stage, soil samples were collected at horizontal distances of 0 cm, 5 cm, 10 cm, and 15 cm from the emitter, as well as at vertical depths of 0–10 cm, 10–20 cm, 20–40 cm, 40–60 cm, 60–100 cm, and 100–140 cm beneath the emitter. In the second and subsequent stages, the spacing of the drip tape was maintained at 60 cm, with sampling depths adjusted to 0 cm, 10 cm, 20 cm, and 30 cm from the emitter, ensuring three replicates for each treatment. Soil samples were collected using a spiral sampler (10 cm in diameter and 150 cm in height), and the soil water content and bulk density were measured using the ring-knife method. The collected soil samples were air-dried and filtered through a 2 mm sieve.

After amalgamating the three replicate soil samples into a single sample, the extraction of saturated soil paste was performed using a standardized method. The EC of the saturated soil extract was measured with a conductivity probe (DDS-11A, Yulong, Shanghai, China), while the pH of the soil was assessed using a pH meter (PHSJ-4F, Shanghai, China). Additionally, concentrations of Mg²⁺, Ca²⁺, Na⁺, SO₄²⁻, and K⁺ were determined using inductively coupled plasma atomic emission spectroscopy (ICP-OES) (Optima 5300DV, PerkinElmer, Waltham, MA, USA).

Calculation of the soil sodium adsorption ratio (SAR) was as follows [9]:

$$SAR={\displaystyle \frac{\left[{Na}^{+}\right]}{{(\left[{Ca}^{2+}\right]+[{Mg}^{2+}\left]\right)}^{0.5}}},$$

(1)

Soil exchangeable sodium percentage (ESP) was calculated as follows [24]:

$$ESP={\displaystyle \frac{\left[{Na}^{+}\right]}{\left[{Na}^{+}\right]+\left[K^{+}\right]+\left[{Ca}^{2+}\right]+\left[{Mg}^{2+}\right]}},$$

(2)

where cation concentrations are expressed in mmol/L.

2.4. Vegetation Observation and Analysis

The annual survival rates of trees and their growth indicators, including breast height (BH) and ground diameter (GD) were measured by a vernier caliper in the different growing seasons.

2.5. Statistics

ANOVA was conducted to assess for statistically significant differences in bulk density of soil and moisture content of the same layer of soil among different periods. A Pearson correlation analysis of soil physicochemical properties was conducted. Results plots were generated in Origin 2021 (OriginLab Inc., Northampton, MA, USA).

3. Results

3.1. Precipitation and Irrigation

Monthly precipitation during the experimental period is illustrated in Figure 3. The total precipitation during the irrigation seasons (September–November 2021, March–November 2022, and March–September 2023) was recorded as 156.7 mm, 685.2 mm, and 495.0 mm, respectively. The uneven distribution of precipitation across different seasons is characteristic of this region. The total irrigation amounts for Stages I to IV were 126 mm (30 days), 125 mm (60 days), 804 mm (306 days), and 216 mm (214 days), respectively. This indicates that the frequency of irrigation water application decreased progressively from Stage I to Stage IV, in alignment with the reduced soil matric potential threshold.

3.2. Density of Bulk Soil and Moisture Content of Soil

Figure 4 illustrates the bulk density (BD) and moisture content of soil across different periods. The bulk density in the 0–40 cm soil profile ranged from 1.55 to 1.57 g cm⁻³ prior to initial planting, whereas that in March and October 2022 and March 2023 were recorded values between 1.39 and 1.41 g cm⁻³, 1.29 to 1.31 g cm⁻³, and 1.33 to 1.35 g cm⁻³, respectively. These findings indicated that soil bulk density decreased compared to that of the pre-planting period (Figure 4A). The ranking of soil profiles in terms of change rate in the bulk density was 0–10 cm > 10–20 cm > 20–40 cm. Soil moisture content in the 0–40 cm soil profile over periods I–IV was 24.44 to 27.04%, 22.5 to 29.33%, 26.73 to 34.27%, and 27.63 to 35.10%, respectively (Figure 4B). The soil moisture content exhibited a gradual increased, positively correlated with soil depth.

3.3. Spatial Distributions of EC and pH

Figure 5 shows the spatial distribution of EC. The mean EC of the 0–140 cm soil profile prior to planting was 29.33 dS m⁻¹, representing severely saline soil, with notable salt accumulation at depths of 80–140 cm. After one month of leaching, the value in the 0–40 cm and 40–140 cm layers ranged from 0 to 8 dS m⁻¹ (mean of 4.48 dS m⁻¹) and 8 to 16 dS m⁻¹, respectively, representing moderately saline soil. By March 2022, the EC of the surface layer had leached to 0–2 dS m⁻¹ (mean of 1.86 dS m⁻¹), indicating non-saline soil, while the overall mean EC of soil of the entire profile was 3.77 dS m⁻¹, representing salinized soil.

In October 2022, the mean EC of the soil further decreased to 0.65 dS m⁻¹, remaining below 2 dS m⁻¹, which suggests a trend towards non-salinization conditions. However, in March 2023, the EC of the soil increased, with a mean EC of the upper layer at 1.01 dS m⁻¹ and an overall mean EC of 2.97 dS m⁻¹, indicating mildly saline soils over the entire layer. Compared to the pre-planting period, the mean EC of upper layer soil decreased by 86.92%, 93.77%, 98.32%, and 96.33% in overgrowth periods I–IV, while the entire soil profile exhibited reductions of 65.64%, 87.14%, 97.79%, and 89.87% during the same periods, respectively.

Soil pH is a critical factor influencing soil fertility and biogeochemical cycling [25]. Figure 6 illustrates the spatial distribution of soil pH recorded in September and October 2021, March and October 2022, and March 2023. The average pH value prior to planting was 7.76 in October 2021 for the 0–40 cm soil layer, which subsequently increased to an average of 8.01, with no significant changes observed in the deeper soil layers. In March 2022 and October 2022, the soil pH in the 0–140 cm layer measured 8.03 and 8.29, respectively. By March and September 2023, these values further increased to 9.87 and 9.22. These results indicate an increase in soil pH, accompanied by a decrease in electrical conductivity (EC), suggesting a transition from saline soil to highly alkaline soil.

3.4. Spatial Distributions of Soil Ions

The concentrations of Ca²⁺, K⁺, Mg²⁺, Na⁺, and SO₄²⁻ in the soil significantly declined in the second year following planting, attributed to frequent irrigation and effective salt leaching (Figure 7). The measured concentrations throughout the soil profile were 44.50, 9.88, 13.84, 135.27, and 263.99 mmol L⁻¹, respectively. Notably, the Na⁺ concentration surpassed those of Ca²⁺, K⁺, and Mg²⁺ by factors of 2.04, 12.69, and 8.77, respectively, while SO₄²⁻ exceeded Na⁺ by a factor of 1.95. In March 2023, the overall concentrations of Ca²⁺, K⁺, Mg²⁺, Na⁺, and SO₄²⁻ in the 0–140 cm soil profile layer were recorded at 2.14, 2.01, 1.70, 13.96, and 113.00 mmol L⁻¹, respectively. At this time, the Na⁺ concentration exceeded those of Ca²⁺, K⁺, and Mg²⁺ by factors of 6.52, 6.95, and 8.21, respectively, while SO₄²⁻ surpassed Na⁺ by a factor of 8.09. This trend was influenced by a yearly reduction in matric potential control and a decrease in the applied irrigation water, leading to only a minor increase in soil concentrations in 2023. Prior to planting, the Na⁺ concentration in the entire soil profile was 2.04 times greater than that of Ca²⁺, whereas SO₄²⁻ exceeded Na⁺ by a factor of 0.95. In contrast, by March 2023, the Na⁺ concentration was 5.52 times greater than that of Ca²⁺, and SO₄²⁻ exceeded Na+ by a factor of 9.09. These results clearly indicate an increase in the ratios of Ca²⁺ and SO₄²⁻ to Na⁺.

3.5. SAR and ESP

Table 3. Changes in soil SAR and ESP in different soil layers during different periods.

Index	Soil Depth (cm)	Sep. 2021	Oct. 2021	Mar. 2022	Oct. 2022	Mar. 2023
SAR (mmol L−1)0.5	0–10	15.00 ± 4.65	6.45 ± 1.81	3.96 ± 0.78	0.92 ± 0.32	2.06 ± 0.36
	10–20	18.21 ± 3.18	7.65 ± 1.74	5.27 ± 0.54	1.80 ± 0.41	2.26 ± 0.23
	20–40	18.44 ± 2.76	6.07 ± 1.65	8.34 ± 2.09	1.75 ± 0.65	7.46 ± 2.03
	40–60	18.36 ± 0.69	6.54 ± 1.19	4.39 ± 0.14	1.28 ± 0.04	10.34 ± 0.61
	60–140	19.26 ± 3.06	10.12 ± 1.79	5.30 ± 0.68	2.86 ± 0.44	11.52 ± 0.32
ESP	0–10	0.64 ± 0.09	0.64 ± 0.05	0.58 ± 0.03	0.31 ± 0.11	0.53 ± 0.04
	10–20	0.68 ± 0.04	0.72 ± 0.03	0.68 ± 0.01	0.48 ± 0.06	0.55 ± 0.02
	20–40	0.69 ± 0.02	0.56 ± 0.08	0.71 ± 0.04	0.48 ± 0.08	0.75 ± 0.04
	40–60	0.68 ± 0.02	0.50 ± 0.04	0.61 ± 0.10	0.41 ± 0.06	0.77 ± 0.05
	60–140	0.67 ± 0.02	0.59 ± 0.02	0.57 ± 0.12	0.56 ± 0.03	0.78 ± 0.05

presents the results for the soil SAR and ESP. In September 2021, the mean SAR in the 0–10 cm soil layer was measured at 15.00 (mmol L⁻¹)^0.5, while the SAR in the 10–140 cm layer ranged from 18.21 to 19.26 (mmol L⁻¹)^0.5, exhibiting minimal variation. After one month, the SAR values in both soil layers decreased, ranging from 6.45 to 10.12 (mmol L⁻¹)^0.5. By October 2022, the lowest SAR values were recorded due to leaching, with average SAR values across the different soil layers decreasing by 93.87%, 90.12%, 90.51%, 93.03%, and 85.15%, respectively, when compared to the initial SAR measured in September 2021. However, in March 2023, the mean SAR values for the various soil layers increased relative to those recorded in October 2022, ranging from 2.06 to 11.52 (mmol L⁻¹)^0.5.

The mean ESP values for September and October 2021, March and October 2022, and March 2023 ranged from 0.64 to 0.69, 0.50 to 0.72, 0.57 to 0.71, 0.31 to 0.56, and 0.53 to 0.78, respectively. A comparison of alkalinity across the five periods indicated a decreasing trend in ESP within the shallow 0–10 cm soil layer, whereas a fluctuating trend was observed in the 20–140 cm soil layer.

3.6. Correlations Between Soil Physicochemical Properties

As illustrated in Figure 8, the Pearson correlation analysis revealed no significant correlations between EC and pH, ESP, or bulk density. Conversely, EC exhibited highly significant positive correlations with Ca²⁺, Mg²⁺, Na⁺, SO₄²⁻, SAR, and moisture content, with correlation coefficients ranging from 0.69 to 0.96. Notably, Ca²⁺ displayed highly significant positive correlations with K⁺, Na⁺, Mg²⁺, and SO₄²⁻. Additionally, soil SAR demonstrated highly significant positive relationships with both moisture content and ESP. A correlation coefficient of 0.96 was observed between EC and SO₄²⁻. The correlations between SO₄²⁻ and K⁺, Ca²⁺, Na⁺, and Mg²⁺ yielded coefficients of 0.98, 0.92, 0.94, and 0.96, respectively, all exceeding 0.90.

3.7. Plant Responses

Table 4. Survival rate and increment of growth indexes of Salix matsudana.

Date of Determination	Growth Indicators of Salix matsudana
	Survival Rate (%)	BD (cm)	GD (cm)
Sep. 2021	100	9.05 ± 0.12	10.26 ± 0.91
Dec. 2021	97	9.12 ± 0.19	10.35 ± 0.91
Feb. 2022	96	9.27 ± 0.25	10.57 ± 0.92
May 2022	94	9.33 ± 0.23	10.67 ± 0.96
Jul. 2022	93	9.51 ± 0.23	10.85 ± 0.94
Oct. 2022	91	9.63 ± 0.21	11.00 ± 0.98
Dec. 2022	91	9.68 ± 0.23	11.17 ± 1.06
Mar. 2023	91	9.75 ± 0.22	11.26 ± 1.09

Note: BD means breast diameter; GD means ground diameter.

summarizes the growth and survival rates of Salix matsudana. The plant exhibited an increase in diameter, with breast diameter (BD) and ground diameter (GD) rising from 9.05 cm and 10.26 cm to 9.75 cm and 11.26 cm, respectively, after 18 months. This represents increases of 0.7 cm and 1.0 cm. Following one month of intensified leaching, the survival rate of Salix matsudana decreased from 100% to 97%. In October 2022, the survival rate further declined to 91%, after which it remained stable.

4. Discussion

4.1. Changes in Soil Salinity

Irrigation programming plays a crucial role in regulating soil salinity [26]. The first stage of the current experiment lasted approximately one month, while Stages II and III were conducted in 2022, and the rainfed stage took place in 2023. The results indicated that soil leaching effectively transformed severely salinized soil into moderately, mildly, and non-salinized soil between September 2021 and March 2023. Stage IV commenced in 2023, characterized by reduced access to fresh water for irrigation and an increased dependence on rainwater. During this period, the 0–40 cm soil layer remained non-salinized, while the 40–140 cm layer became lightly salinized (Figure 4). This increase can be attributed to the presence of salts in rainwater in coastal areas, combined with the processes of freezing, thawing, and evaporation of soil water that occurred during this period. The alternating wetting and drying cycles of the soil are likely to promote cation fixation, while cycles of freezing and thawing disrupt the mineral lattice structure, leading to the re-release of fixed cations and an overall increase in soil salinity [11,27].

As illustrated in Figure 7, the EC of the soil was significantly positively correlated with soil moisture content (Figure 7). Specifically, as soil moisture content increases, the soil water potential decreases, which facilitates the dissolution and migration of salts into the soil solution, thereby elevating soil salt concentration [28,29]. This finding confirms that water serves as the primary medium for salt movement through the soil and highlights the crucial role of soil moisture in the redistribution of salt cations [30]. In contrast to the pre-planting period, the rate of decline in soil EC in the shallow 0–40 cm layer exceeded that in the deeper 0–140 cm layer during the subsequent four time periods. This indicates that the upper soil layers desalted rapidly and were maintained in a mildly saline state, which is suitable for vegetation growth [9,11,31].

4.2. Changes in Soil Physicochemical Properties During the Desalination Process

The differences in bulk density among the soil layers of 0–10 cm, 10–20 cm, and 20–40 cm were minimal and exhibited a decreasing trend. However, in March 2023, these differences increased as the moisture content of the soil gradually rose, with moisture content positively correlating with increasing soil depth (Figure 3). Between September 2021 and October 2022, a decrease in SAR was observed, whereas minor increases in soil pH, bulk density, and SAR were noted in March 2023 compared to October 2022. This finding corroborates previous research [32]. The EC of the soil exhibited highly significant positive correlations with K⁺, Ca²⁺, Na⁺, Mg²⁺, and SO₄²⁻, indicating that fluctuations in these ions were consistent with changes in soil EC. Following the experiment, the abundance of soil cations was observed in the order of Na⁺ > K⁺ > Ca²⁺ > Mg²⁺, which aligns with prior findings reported by Li et al. [33]. These changes can likely be attributed to the adverse effects of highly exchangeable sodium on the hydraulic conductivity of the soil. Leaching processes led to the alkalinization of soil salts, enhanced the mobility of Na⁺, and resulted in an increase in pH during the leaching period [34,35]. The alkalinization process may lead to elevated soil alkalinity, which in turn promotes the dispersion of soil colloids and adversely affects soil physical properties, despite the low soil salt content.

4.3. Correlations Between Plant Growth and Soil Physicochemical Properties

Soil ions are the predominant environmental factors influencing vegetation properties [36,37]. In this study, the original soil exhibited severe salinization, characterized by high concentrations of Na⁺, which resulted in increased plant mortality rates [38]. A synergistic relationship exists between plants and their soil environment, where plant roots facilitate nutrient cycling and microbial reproduction during their development and growth [39].

In the first year following transplantation, the concentrations of SO₄²⁻, K⁺, and Na⁺ significantly decreased, and the improved plant survival observed (Table 3) indicates effective salt leaching. Furthermore, multi-stage drip irrigation created a low-salinity environment within the root zone, contributing to increases in both breast diameter and ground diameter of the plants. A tree survival rate of 91% was recorded in October 2022, confirming that the optimal silvicultural effect of cultivating willow in coastal saline lands is achieved. However, during this stage IV, limited precipitation, strong winds, high air temperatures, and elevated EC and pH levels were observed. Meanwhile, the proportions of Ca²⁺ and Mg²⁺ among all cation concentrations increased, while the concentration of Na⁺ decreased. These factors led to dieback of tree branches or even the death of entire plants, resulting in a decrease in the survival rate to 91%.

4.4. Recommendations for the Improved Implementation of Summer Afforestation and Multi-Stage Drip Irrigation

In this study, we present several recommendations for enhancing the implementation of summer afforestation and multi-stage drip irrigation, based on a comprehensive evaluation of field experiment results.

(1)

Maintaining the soil moisture potential (SMP) at a depth of 0.2 m directly beneath the drip emitters at −5 and −10 kPa during Stage I and II has proven effective in the restoration of saline–sodic soils, facilitating frequent irrigation and salt leaching. For subsequent phases, we recommend targeting SMP values of −20 and −45 kPa, considering the necessity for reduced irrigation frequency, which supports root system development, thereby enhancing soil water retention and improving the utilization of rainfall resources.

(2)

An irrigation quantity of 6 mm was found to be inadequate to meet plant demands during the rainfed phase (Stage IV). Therefore, it is advisable to implement a higher irrigation volume (e.g., 10 mm), which should be adjusted according to the SMP levels.

(3)

It is noteworthy that the pH value of coastal saline soils increased during the remediation process. During these processes, the addition of industrial phosphoric acid to the irrigation water is essential to create a favorable soil environment for plant growth. Furthermore, applying mulch during planting is recommended, as it helps minimize soil evaporation and reduces salt accumulation in the upper layers of the soil.

5. Conclusions

The vegetation, which was planted in summer, successfully survived in coastal saline soil due to the improved multi-stage drip-irrigation scheduling, which includes four stages (Stage I to Stage IV). The first three irrigation stages facilitated the transition of severely salinized soil to moderately, mildly, and non-salinized soil. Over time, there were decreases in the bulk density of the soil, as well as in concentrations of K⁺, Ca²⁺, Na⁺, Mg²⁺, and SO₄²⁻, while the moisture content of the soil increased. The rainfed stage (Stage IV) was associated with slight increases in the bulk density of the soil and ion concentrations compared to those in Stage III.

Specifically, soil concentrations of Ca²⁺, K⁺, Mg²⁺, Na⁺, and SO₄²⁻ decreased by 95.18%, 79.67%, 87.74%, 89.68%, and 57.19%, respectively, by Stage IV compared to Stage I. The percentage of ions in the soil solution changed after leaching, with a decrease in the percentage of Na⁺ and increases in the percentages of Ca²⁺ and SO₄²⁻. However, the soil pH gradually increased during the first three irrigation stages, with the rate of increase accelerating during the rainfed stage. These findings provide insights into summer re-vegetation using improved multi-stage drip-irrigation scheduling for landscaping on coastal saline land and offer guidance for strategies for subsequent irrigation. Long-term field monitoring studies are necessary to assess the impact of reclamation on saline wasteland ecosystems, particularly concerning alkalization that may result from leaching through drip irrigation.