Impacts of Reclaimed Water Irrigation on Soil Salinity, Nutrient Cycling, and Landscape Plant Growth in a Coastal Monsoon Environment

Abstract

This study investigated the impacts of reclaimed water (RW) irrigation on soil properties and landscape plant growth in a coastal monsoon city over a 13-month period. Soil properties in plots irrigated with RW and tap water (TW) were monitored monthly, including electrical conductivity, total nitrogen, total phosphorus, soil organic matter, and overall variations of soil enzyme activities. The results show that RW irrigation led to increased fluctuations in soil salinity indicators, with higher peaks during periods of low rainfall. Rainfall can efficiently mitigate the salinity increase associated with RW irrigation, highlighting the influence of monsoon climate variability on salinity dynamics. RW application increased soil total nitrogen and organic matter and decreased soil total phosphorus. This suggests that RW irrigation induces complex nutrient interactions within the soil–plant system. Furthermore, RW irrigation promoted the activities of soil enzymes related to carbon, nitrogen, and phosphorus cycling, indicating potential alterations in nutrient bioavailability. Plant growth responses varied among species, with Nephrolepis cordifolia and Cordyline fruticose exhibiting signs of salt stress, especially in the initial months of planting in RW plot. Other species demonstrated greater tolerance to RW irrigation, suggesting the importance of species selection for sustainable landscape management with RW. This study demonstrates the challenges and opportunities associated with RW utilization for urban greening.

1. Introduction

The escalating global freshwater crisis, driven by a confluence of factors, including population growth, rapid industrialization and urbanization, and climate change, demands approaches to water resource management [1]. With irrigation accounting for a substantial portion of global freshwater consumption [2,3], the utilization of reclaimed water (RW) for irrigation, particularly in urban green spaces, presents a promising strategy for enhancing water security. While RW offers potential benefits such as reduced reliance on potable water sources and providing supplemental nutrient input [4], its application necessitates careful consideration of potential environmental impacts.

One of the primary concerns associated with RW irrigation, especially in coastal regions, is elevated salinity. Salt accumulation disrupts soil pore structure, hindering water infiltration, aeration, and root development, thereby limiting access to essential nutrients and water [5]. Specific ions, like sodium and chloride, can lead to soil degradation and negatively affect plant growth [6,7]. While irrigation with lower salinity water could mitigate these negative impacts, the effectiveness of such strategies is highly dependent on local climatic conditions [8,9]. For example, in monsoon climates, the highly variable rainfall patterns, including intense precipitation events associated with typhoons, can significantly influence soil salinity dynamics [10]. Heavy rainfall can leach salts effectively from the topsoil within a depth of 100 cm or more [11,12], potentially affecting soil salinity dynamics and reducing salt stress on plants. However, the magnitude and duration of this effect, and the subsequent response of soil salinity during drier periods, remain critical knowledge gaps. Understanding these temporal variations in soil salinity under RW irrigation, as well as the interplay between the rainfalls, is crucial for developing effective management strategies.

RW typically contains nitrogen, phosphorus, and potassium derived from various sources within the wastewater stream. These nutrients, when applied appropriately, can enhance soil fertility and promote plant growth, offering environmental benefits [13,14,15]. However, the nutrient composition of RW is highly variable, and may not always align with the specific needs of landscape plants. Imbalanced nutrient ratios, particularly excessive nitrogen and micronutrients (e.g., iron, zinc, and manganese) [16], result in groundwater pollution [17,18]. Careful monitoring of soil nutrient levels, together with the soil enzymes related to the nutrient dynamics, is crucial for optimizing the benefits of RW irrigation while mitigating potential risks.

While RW presents a promising solution, traditional studies often overlook the complex interplay between RW quality, climate variability, and soil health [14,19]. This study addresses this gap by investigating the effects of RW irrigation on landscape plants and soil physicochemical properties within the context of a monsoon climate. Employing monthly monitoring of soil properties, RW quality, and rainfall over a 13-month period, along with a comparative control group irrigated with tap water (TW) and the incorporation of guest soil mulch, this research aims (i) to assess the impacts of RW irrigation on selected soil properties and the influence of rainfall in a monsoon climate; (ii) to evaluate the impacts of RW on the soil nutrients and the related enzymes; and (iii) to understand the complex interactions between RW irrigation, climate, soil properties, and plant health to evaluate the potential of this RW irrigation for urban greening in the coastal city. Ultimately, this study provides a comprehensive evaluation of soil dynamics under RW irrigation in monsoon climates, analyzes the underlying mechanisms driving these changes, and offers preliminary insights into the long-term implications of this practice.

2. Materials and Methods

2.1. Experimental Design

Two adjacent experimental plots were established, one irrigated with RW and the other with TW, covering areas of 264 m² and 258 m², respectively. The plots were established on 8 December 2022, with both filled with a uniform clay loam soil (Table S1). Eight common landscape plant species, representing typical roadside planting in Xiamen City, were transplanted into each plot following a defined layout (Figure S1). The species included Zoysia japonica, Nephrolepis cordifolia, Duranta erecta, Cordyline fruticose, Camphora officinarum, Bischofia javanica, Plumeria rubra, and Bougainvillea spectabilis.

Irrigation was carried out twice weekly using a sprinkler system, delivering an average volume of 2 tons of water per irrigation event. Irrigation volumes were adjusted based on rainfall levels and temperature to prevent overwatering. Equal amounts of fertilizer were applied to both plots on 6 April 2023. The experiment ran for 13 months, concluding on 18 January 2024. Throughout the study period, the average temperature was 22.0 °C, with a maximum of 36.8 °C and a minimum of 4.9 °C (Figure S2). Total annual precipitation measured at 1256 mm with heavy rainfall event during Typhoon Doksuri on 28 July 2023, Typhoon Saola on 2 September 2023, and Typhoon Haikui from 3 to 5 September 2023 (Figure S2).

2.2. Soil Sampling and Analysis

Soil samples were collected monthly on 5 January, 16 February, 16 March, 18 April, 16 May, 19 June, 18 July, 16 August, 17 September, 16 October, 17 November, 18 December in 2023, and 18 January 2024. Each plot was divided into three sections (e.g., RS1, RS2, RS3, or TS1, TS2, TS3). Within each section, a five-point sampling strategy was employed at depths of 0–20 cm as topsoil. Additional samples were collected at a depth of 20–40 cm in January 2024. Soil samples were air-dried, cleaned of debris, and then grounded for analyses.

A total of 23 soil properties were analyzed, encompassing (1) physical properties (pH, bulk density and porosity); (2) nutrient contents (soil organic matter, total nitrogen, total phosphorus, total potassium, available phosphorus, and cation exchange capacity); (3) salinity indicators (electrical conductivity, and water-soluble salt); (4) heavy metal concentrations (Ni, Cu, Zn, As, Pb); and (5) enzyme activity (soil urease, acid phosphatase, and saccharase).

A 40-mesh sieve was used for soil enzyme activity, which was determined using Solarbio^® assay kit (Beijing, China) for soil urease (BC0120), soil acid phosphatase (BC0140), and soil saccharase (BC0240). Samples were further sieved through a 100-mesh (0.15 mm) sieve for soil organic matter and heavy metal analyses. Soil organic matter was measured using an assay kit (ml092786) from mlbio Co., Ltd. (Shanghai, China). Total Ni, Cu, Zn, As, and Pb concentrations were determined by strong acid digestion (8.0 mL HNO₃ and 2.0 mL HClO₄), following the method described by Lee et al. [20], and quantified using inductively coupled plasma mass spectrometry (7700 ICP-MS, Agilent, Santa Clara, CA, USA. The detailed procedures are documented in Table S5 in the Supplementary Material [19,21,22,23,24,25,26,27,28,29].

2.3. RW Samples Analysis

The water sources used in the study comprised locally supplied RW. pH, total phosphorus, total nitrogen, and ammonia nitrogen were analyzed daily according to China AQSIQ technical specifications [30]. Chloride and total dissolved solids were measured strictly in accordance with the analytical methods specified in “The reuse of urban recycling water—Water quality standard for green space irrigation” [31], while chlorine was measured daily, and total dissolved solids was measured weekly. Detailed analytical methods used are provided in Table S6 [32,33,34,35,36,37].

2.4. Plant Growth Analysis

The heights of N. cordifolia and C. fruticose were measured on 8 December 2022, 19 June 2023, and 16 August 2023. Density assessments for C. fruticose were conducted on 21 April 2023. Within each treatment plot, three 1 m² quadrats was randomly placed at least 0.5 m from the plot edge. The total number of C. fruticose individuals, as well as the number exhibiting healthy growth (defined as the absence of withered leaves) and the number exhibiting poor growth (defined as the presence of at least one withered leaf), were recorded within each quadrat. Density (plant/m²) and the percentage of thriving individuals were then calculated for each section. For height measurement, ten individuals of each target species (N. cordifolia and C. fruticose) were randomly selected within each section. The height (cm) of each selected individual was measured, and the mean height for each species within each section was calculated.

2.5. Statistical Analysis

The independent samples t-test and the paired samples t-test were used to analyze monthly and overall differences in soil parameters between RW and TW treatments, respectively. Normality was assessed using the Shapiro–Wilk test. For non-normally distributed data, the Wilcoxon signed-rank test was applied. Pearson correlation analysis was used to assess the relationship between rainfall and electrical conductivity. Redundancy analysis (RDA) correlation analysis using CANOCO 5 software was applied to analyze the effect of environmental factors on soil indicators. Outliers in RW quality parameters (pH, total dissolved solids, chloride, total nitrogen, total phosphorus, ammonia nitrogen) are excluded using the interquartile range (IQR) method, where values within (Q₁ − 1.5IQR, Q₃ + 1.5IQR) are normal, while the others are outliers. The formula for IQR is shown as follows:

IQR = Q₃ − Q₁

Q₁ represents the 25th percentile; Q₃ represents the 75th percentile.

3. Results

3.1. Characteristics of RW for Irrigation

Table 1. Reclaimed water (RW) chemical properties.

Parameters	Unit	RW	n
pH	-	6.96 ± 0.29	414
Total dissolved solids	mg·L−1	1788.0 ± 755.5	52
Chloride	mg·L−1	909.3 ± 354.3	402
Total nitrogen	mg·L−1	8.81 ± 1.61	414
Total phosphorus	mg·L−1	0.16 ± 0.07	414
Ammonia nitrogen	mg·L−1	0.22 ± 0.31	414

Note(s): Values present means ± standard deviations (n) for entire experimental period. Total dissolved solids were measured weekly, while all other indicators were measured daily (except for chlorine, which was measured twice a week in January 2024).

summarizes the characteristics of the RW used for irrigation. Measurements included pH and concentrations of total dissolved solids, chloride, total nitrogen, total phosphorus, and ammonia nitrogen, all of which can influence soil quality and plant growth [6,9,19]. The quality of RW used in this study exhibited significant temporal variability. After removing outliers using the IQR method to avoid the effects of extreme values, the pH values of RW were found to range from 6.3 to 7.9. The concentrations of total phosphorus, total nitrogen and ammonia nitrogen were found to be within the range of 0.03–0.35, 5.84–12.00, 0.03–0.40 mg/L, respectively, with median values of 0.16, 8.86, 0.15 mg/L (Figure S3). Chloride and total dissolved solids concentrations exhibited significant variability, ranging from 144 to 1770 and 321 to 3240 mg/L, respectively, with median values of 849 mg/L and 1695 mg/L. From April to September 2023, these parameters remained relatively low, with median chloride concentrations ranging from 646.0 to 755.5 mg/L and total dissolved solids from 650 to 1615 mg/L (Figure 1). However, during the remaining months, both chloride and total dissolved solids concentrations increased substantially, reaching peak chloride levels of 2520 mg/L on 12 March 2023 and peak total dissolved solids levels of 3240 mg/L on 13 December 2022. While minimum values of 18 mg/L for chloride (5 May 2023) and 321 mg/L for total dissolved solids (9 May 2023) were also recorded, the overall trend indicates periods of elevated salinity in the RW source.

3.2. Effect of RW on Soil Physical and Chemical Properties

Table S2 presents a summary of soil properties, including the pH, electricity conductivity, bulk density, porosity, concentrations of water-soluble salt, available phosphorus, total phosphorus, total nitrogen, total potassium, cation exchange capacity, and organic matter in the soil samples collected from the plots irrigated with either RW or TW.

3.2.1. Soil Salinity

The salinity of RW significantly influences soil salinity. The pH (Figure 2a) levels of soil in the RW plot varied from 6.50 to 8.72, with a median of 7.88, while pH in the TW plot ranged from 6.45 to 8.22, with a median of 7.35. A significant difference in pH between the RW and TW plots was observed throughout the study (significant difference, paired t-test, p < 0.01), and was particularly notable during January, February, and March of 2023 and in January 2024 (t-test, p < 0.05).

Water-soluble salt content (Figure 2b) also demonstrated a significant disparity between RW and TW plots (paired t-test, p < 0.01), ranging from 0.3 to 2.9 g/kg (median = 0.8 g/kg) in the RW plot, compared to 0.1 to 1.7 g/kg (median = 0.4 g/kg) in the TW plot. The TW plot maintained relatively stable salt levels until October 2023, when salts increased to 0.73 g/kg in November and 1.20 g/kg in December 2023, before decreasing to 0.83 g/kg in January 2024. In contrast, the RW plot showed fluctuating concentrations, reaching 1.37 g/kg in March 2023, then reduced and maintained a low-level (0.3–0.9 g/kg) from April to August 2023, and increased further to 0.90 g/kg in September and 2.43 g/kg in December, which were 2.1 and 5.8 times higher than the initial values of 0.42 g/kg (Table S1).

Similarly, electricity conductivity (Figure 2c) in soil irrigated by RW (99.2–989.0 μs/cm, median = 317.0 μs/cm) was significantly higher (paired t-test, p < 0.01) than that by TW (82.3–542.0 μs/cm, median = 172.4 μs/cm). The trend in electricity conductivity of the RW plot increased from January to March 2023, fluctuating between 213.3 μs/cm and 365.0 μs/cm from March to June. And then, electricity conductivity in the RW plot undergoes significant variations, decreasing from 359.0 μs/cm to 185.0 μs/cm until September, then increasing to a peak of 762.0 μs/cm by December, and finally decreasing to 535.3 μs/cm in January 2024.

Figure 3 illustrates the effect of irrigation water (RW vs. TW) and soil depth (topsoil 0–20 cm vs. deep soil 20–40 cm) on soil pH, water-soluble salt, and electricity conductivity following 13 months of irrigation. Significant differences were observed in the topsoil between RW and TW plots in terms of water-soluble salts, electricity conductivity, and pH (t-test, p < 0.05). However, no significant differences were observed at deep soil (t-test, p > 0.05). In the RW plot, notable differences were measured in water-soluble salt (average concentrations of 1.67 g/kg in topsoil compared to 0.47 g/kg in deeper soil) and electricity conductivity (average value at 535.3 μs/cm in the topsoil versus 297.0 μs/cm in deeper soil), while differences were not apparent in the TW plot. Overall, both topsoil and deeper soils exhibited increased salinity compared to initial conditions documented in Table S1 (pH = 7.2; electricity conductivity = 167.74 μs/cm; soluble salts = 1.42 g/kg).

3.2.2. Soil Nutrients

RW significantly affected the nitrogen, phosphorus, and organic matter in the soil. Soil organic matter levels gradually increased until June 2023 in both plots, reaching values of 9.43 g/kg in RW and 6.45 g/kg in TW by May (Figure 2d). From June 2023 to January 2024, organic matter in RW consistently remained higher than in TW, and that in RW fluctuated between 1.85 and 17.61 g/kg (median = 9.17 g/kg), while in TW, it varied from 0.98 to 16.81 g/kg (median = 4.67 g/kg). Significant differences in soil organic matter were observed throughout the experimental period (paired t-test, p < 0.05). Similarly, the cation exchange capacity was significantly higher in RW plots than that in the TW plot (paired t-test, p < 0.05), with the most significant differences recorded in October 2023 (t-test, p < 0.05) (Figure 2e). Total phosphorus levels were significantly lower in RW plots compared to TW plots (paired t-test, p < 0.001) (Figure 2i), with total phosphorus concentrations ranging from 0.09 to 0.40 g/kg (median = 0.23 g/kg) in RW and 0.13 to 0.64 g/kg (median = 0.31 g/kg) in TW. From January to June 2023, total phosphorus concentrations at TW were about 1.7 times higher than those at RW. From July to November, total phosphorus concentrations at both RW and TW decreased, narrowing the gap, which may be related to rainfall. Additionally, available phosphorus levels were marginally higher in RW plots compared to TW plots, although this difference did not reach significance (paired t-test, p = 0.066) (Figure 2g). Irrigation with RW also significantly increased soil total nitrogen levels (paired t-test, p < 0.05). Soil total nitrogen levels at the RW site were higher from February to October 2023 and lower in January and November, with average total nitrogen concentrations being consistently higher in RW plots than TW plot after March 2023, particularly in July 2023. However, there was no significant difference in total potassium levels between the two irrigation water sources.

3.2.3. Soil Enzyme Activities

Enzyme activities, specifically soil saccharase, soil acid phosphatase, and soil urease, were assessed as typical representative enzymes involved in the C, P, and N cycles. Soil Urease, which hydrolyzes urea to produce ammonium and carbonic acid, reflects soil nitrogen availability. Soil sucrose activity, indicating the breakdown of sucrose into monosaccharides for organismal uptake, is a key indicator of soil fertility. Soil acid phosphatases catalyze the mineralization of organic phosphorus, providing insights into phosphorus bioavailability. As illustrated in Figure 4, RW irrigation enhanced the activities of these soil enzymes, which is consistent with the expected results. By January 2024, significant increases in enzyme activities between RW and TW were observed, as for soil saccharase (RW: 16.0 U/g soil; TW: 10.8 U/g soil; p = 0.09), soil acid phosphatase (RW: 9923.9 U/g soil; TW: 6868.2 U/g soil; p < 0.001), and soil urease (RW: 861.9 U/g soil; TW: 196.6 U/g soil; p < 0.001). Notably, soil enzyme activities in deeper soils differed significantly from those in topsoil, especially within RW plot.

3.2.4. Soil Structure

Figure S4a illustrates that soil porosity and bulk density showed no significant monthly (t-test, p > 0.05) or overall differences (paired t-test, p > 0.05). The bulk density ranged from 1.24 to 1.83 g/cm³ in RW and from 1.22 to 1.86 g/cm³ in the TW plot, both with a median of 1.55 g/cm³. In September 2023, the average bulk density peaked at 1.81 g/cm³ for RW and 1.75 g/cm³ for TW, followed by a gradual decline over the subsequent four months, reaching minimum levels of 1.32 g/cm³ and 1.25 g/cm³ in January 2024, respectively. Soil porosity, however, showed a decreasing-then-increasing trend, with RW plot ranging from 18% to 54% (median = 39%) and TW from 23% to 54% (median = 38%). The lowest porosity values for RW and TW were recorded in July and May 2023, respectively, with average values of 28% and 31%. A negative correlation between soil bulk density and porosity was observed, with a Spearman’s correlation coefficient of −0.692 (p < 0.01).

3.2.5. Soil Heavy Metals

Figure S5 illustrates the heavy metal concentrations of the soil samples from both RW and TW plots. Generally, reclaimed watering did not lead a significant (t-test, p > 0.05) accumulation of heavy metals, such as Pb, Cu, As, Zn, and Ni, compared to TW irrigation during the current experimental cycle. Notably, the contents of Pb and Ni were elevated in both RW and TW. Pb concentrations increased from 35.4 to 57.8 mg/kg after irrigation by RW and from 33.2 to 81.4 mg/L by TW, while Ni concentrations rose from 6.9 to 10.2 mg/kg for RW and 4.7 to 5.9 mg/kg during the same period. No significant differences were observed in heavy metal concentrations between topsoil and deep soil after one year of irrigation. However, Ni concentrations in deeper soils were 14.0 mg/kg and 10.7 mg/kg, representing increases of 3.8 mg/kg and 4.8 mg/kg compared to the topsoil.

3.3. Plant Growth Response

Plant growth responses to RW and TW irrigation revealed nuanced interspecific variations and temporal dynamics. While overall growth showed no statistically significant differences between RW and TW treatments across eight species in January 2024, certain shrub species in RW plots displayed visible stress symptoms during the initial transplant (March–April 2023) (Table S3, Figure S7). N. cordifolia and C. fruticose exhibited leaf necrosis, marginal scorching, premature defoliation, wilting, and occasional mortality mainly under RW irrigation (Figure S7). This stress response was more evident in C. fruticose, with a lower percentage of thriving individuals (54.3% in RW vs. 73.1% in TW) and reduced density (16 plants/m² in RW vs. 27 plants/m² in TW) observed on 21 April 2023 (Table S4). However, C. fruticose in RW plots showed marked recovery by June and August 2023.

Despite initial stress, subsequent growth data demonstrated resilience in these shrub species (Figure S7). By June 2023, N. cordifolia and C. fruticose in RW plots reached heights of 39.0 cm and 48.8 cm, respectively, representing smaller increased compared to their TW counterparts (41.7 cm and 67.7 cm). This disparity was statistically significant for C. fruticose (t-test, p < 0.001). Continued growth was observed by August 2023, with N. cordifolia and C. fruticose in RW plots reaching 46.7 cm and 76.9 cm, respectively. Although these height increases remained lower than those observed in TW plots (56.6 cm and 90.5 cm), the difference was less pronounced. Similar trends were observed in other shrub species. In contrast, arbor species, including C. officinarum, B. javanica, and P. rubra, showed no significant differences in growth rate, defoliation, or leaf necrosis between RW and TW treatments. These results suggest a differential response to water source, with the tested shrub species exhibiting greater initial sensitivity to RW irrigation than the arbor species.

4. Discussion

This study investigated the impact of RW irrigation on soil properties and the growth of eight landscape plant species over 13 months, comparing it to TW irrigation. Our findings highlight the complex interplay between RW characteristics, soil properties, and plant health. While no detrimental effects on plant growth were observed after 13 months, the study reveals potential long-term risks associated with elevated salinity in RW.

4.1. RW Quality and Its Implication for Irrigation

RW used in this study originates primarily from domestic wastewater. The observed fluctuations in chloride and total dissolved solids of RW suggest a strong influence of tidal cycles and rainfall (Figure 2). The apparent increases in chloride concentrations around the 2nd/3rd and 17th/18th days of the lunar calendar (Figure 1a) align with periods of higher astronomical tides, which likely exacerbate seawater intrusion into the sewage network, thereby increasing salinity in the influent of the wastewater [38], which is the RW source. This cyclical pattern highlights the vulnerability of coastal RW sources to saline intrusion and underscores the importance of continuous monitoring and appropriate management strategies.

The average chloride (909.3 mg/L) and total dissolved solids (1788.0 mg/L) concentrations in our RW, surpass recommended irrigation guidelines established by the Chinese Standard GB/T 24599-2010 (chloride < 250 mg/L, total dissolved solids < 1000 mg/L) [31] and USEPA (chloride < 355 mg/L, total dissolved solids < 2000 mg/L) [39]. This elevated salinity poses potential risks to plant health and soil quality, especially over the long-term irrigation [40].

4.2. Soil Salinity Variations and Influencing Factors

The salinity of RW significantly influences soil properties. As anticipated, RW irrigation led to significant increases in soil electricity conductivity (Figure 2c), soluble salt content (Figure 2b), pH (Figure 2a), and Na concentrations (Figure S8) compared to TW irrigation [41]. This increase is consistent with the accumulation of Na⁺, Cl⁻ and other ions in the soil, as well as their subsequent transformations [42]. It might induce micronutrient deficiencies (e.g., Fe and Mn), potentially leading to chlorosis in susceptible plants [43].

RDA revealed that RW characteristics explained 30.42% and 9.03% of the total variation in soil indicators along the first and the second canonical axes, respectively. The soil characteristics of RW were strongly affected by pH (permutation test, p = 0.006) and total dissolved solids (p = 0.076) (Figure 5a). This confirms the dominant influence of salinity on soil response to RW irrigation. The long-term implications of increased salinity include reduced soil permeability altered structure, and decreased hydraulic conductivity, ultimately impacting soil quality and productivity [41,44].

Furthermore, RDA demonstrated that rainfall and temperature (Figure 5b) accounted for 16.60% of the variability in soil indicators along the first and the second canonical axes. The temporal dynamics of soil electrical conductivity and rainfall highlight the significant influence of weather on soil chemical properties by RW irrigation. As expected, salt accumulation is exacerbated in arid and semi-arid environments with high evaporative demand and low natural precipitation [45]. Our monthly analysis of soil electrical conductivity and rainfall provides insights into the short-term salinity dynamics (Figure 6). During periods of low rainfall (e.g., November 2023–January 2024, average monthly rainfall = 4.3 mm) (Figure S2), soil electrical conductivity in RW plot reached peak values (762 μs/cm in December 2023), representing a 4.5-fold increase from the initial value and 1.9 times the corresponding TW value. Conversely, during the typhoon season (July–September 2023), characterized by high rainfall (average monthly rainfall = 228.25 mm), soil electrical conductivity in both RW and TW plots decreased. Specifically, the lowest electrical conductivity values were observed at 184.97 μs/cm in the RW plot and 125.3 μs/cm in the TW plot in September 2023, coinciding with the highest cumulative rainfall of 452.5 mm between 16 August and 17 September 2023. Substantial rainfall events three days prior to sampling in April and June also corresponded to reductions in soil electrical conductivity (Figure 6). This negative correlation between rainfall and soil electrical conductivity (r = −0.589, p < 0.05, Spearman’s two-tailed test) suggests that rainfall can efficiently mitigate the salinity increase associated with RW irrigation by leach soluble salts [14,46]. However, this leaching process potentially affects the deeper soil layers [47].

Salt accumulation patterns within the soil profile are complex, influenced by the interplay of irrigation, evaporation, leaching, and drainage [48,49]. While some studies reported greater salt accumulation at depth [46], our findings, consistent with Phogat [43,50], revealed higher salt concentrations in the topsoil of both RW and TW plots, with a more pronounced difference in RW plot (Figure 2). This suggests that RW irrigation, which potentially reduced infiltration rates [7], affect the salt accumulations in the present work.

Although previous research has demonstrated that potential of RW to modify the soil pore structure and alter bulk density [5,47], our findings showed no statistically significant differences between RW and TW treatments, likely due to the limited experimental period (Figure S4). Compared to water quality, rainfall often affects soil properties by increasing surface runoff [51], which leads to fluctuations in soil bulk density and porosity throughout different months.

The use of RW in coastal cities poses unique challenge due to the potential for sweater intrusion impacting RW quality. In this study, although plants successfully acclimatized to RW and exhibited healthy growth, the observed slight salt stress underscores the importance of careful monitoring and management. Previous research has demonstrated the role of rainfall in mitigating soil salinity [14,46] and the potential for enhanced salt tolerance in coastal plants due to exposure to seawater aerosols [52,53]. However, relying solely on these natural mitigating factors may be insufficient to prevent salinity stress, especially during dry periods.

4.3. Influence of RW Irrigation on Soil Nutrient Dynamics

RW irrigation influences soil nutrient dynamics through both direct nutrient input and indirect effects on soil properties and biological processes. While RW elevate soil nutrient levels [54], its high salinity can also indirectly affect nutrient availability by altering microbial communities and enzyme activity [55], modifying nutrient cycling pathway [56], and potentially reducing the efficiency of plant nutrient uptake [54]. Our findings reveal that RW irrigation resulted in significantly higher soil organic matter and total nitrogen, but lower total phosphorus content compared to TW plot soil (Table S2).

This observed pattern is further corroborated by the soil enzyme activity. Specifically, soil saccharase, soil urease, and soil acid phosphatase activities were significantly higher in the topsoil of RW plot compared to TW plot in January 2024 (Figure 3). Soil enzymes play an important role in regulating C, N, and P cycling and nutrient transformation [57]. Their activity is influenced by various factors, including plant root interaction [58], litter decomposition [59], soil properties (electrical conductivity and pH), and nutrient content (soil organic matter, total nitrogen, and total phosphorus), with soil organic matter exhibiting the strongest influence followed by total nitrogen, total phosphorus, pH, and electrical conductivity [58]. The higher enzyme activity in RW topsoil likely reflects the combined effects of increased plant litter inputs and direct nutrient contribution from RW itself, consistent with observations of increased soil organic matter accumulation in RW-irrigated soils [60]. For example, RW plot exhibited lower total phosphorus concentrations and concurrently higher available phosphorus content, suggesting enhanced soil acid phosphatase activity mediating phosphorus mineralization [18]. The vertical distribution of enzyme activity also differed significantly between RW and TW plots. In January 2024, enzyme activities were markedly lower in deeper soil layers compared to the topsoil in both treatments (Figure 3). This is likely due to reduced root and litter influence. This effect was more pronounced in RW, potentially attributed to the higher salinity inhibiting enzyme activity. The interaction between RW irrigation, soil enzyme activity, soil nutrients, and plant growth is complex. While some studies have shown increased nutrient availability [14] and microbial activity [61] with RW irrigation, the specific effects depend on numerous factors, including soil type, RW quality, and plant species. Planting landscape plants may contribute to enhanced soil enzyme activity, improved soil quality, and mitigation of salt accumulation [62]. However, it is crucial to acknowledge the potential negative consequences of RW irrigation, particularly concerning salt accumulation and subsequent leaching via rainfall, which can lead to significant losses of nutrient including nitrogen [18] and monovalent cation including K⁺ [47]. Overall, this experiment demonstrated that RW irrigation promotes nutrient cycling, enhancing the concentration of total nitrogen, total phosphate, and organic matter, as well as related enzyme activities, thereby reducing the need for additional fertilizer application. Simultaneously, it minimizes the risk of nutrient loss. Future research should focus on optimizing RW management strategies to maximize water resources and nutrient benefits.

4.4. Environmental Significance of RW Irrigation

The long-term use of RW for irrigation presents both opportunities and challenges from an environmental perspective. While RW offers valuable alternative water source, particularly in water-stressed regions, its elevated salinity raises concerns regarding soil health, plant growth, and potential pollutant accumulation.

While RW can provide valuable nutrients to plants, the interplay between nutrient input and salinity is complex. For the two applications of RW irrigation per week during the dry season in the present study, each delivering 2 tons of RW, and considering the measured nutrient concentrations, our estimated weekly input of total nitrogen and total phosphorus was 35.24 and 0.64 g, respectively. This nutrient input could potentially stimulate plant growth. However, the elevated salinity in RW can inhibit plant uptake nutrient, potentially reducing the benefits of nutrient addition [7,40]. Our findings indicate that RW plot exhibited lower soil total phosphorus concentrations and higher available phosphorus under the influence of soil acid phosphatase compared to TW plots. This suggests a potential shift in phosphorus cycling dynamics under RW irrigation [18,19].

Prolonged irrigation with RW can negatively impact soil properties. Increased salinity reduces soil permeability and alters soil structure, ultimately impacting soil quality and productivity [7]. These changes can hinder root development, restrict water and nutrient uptake by plants, and potentially lead to soil degradation in the long term [8]. In addition, the potential for pollutant accumulation in soil and plants through RW irrigation is another environmental concern. While our study did not detect significant heavy metal accumulation in the soil, long-term monitoring is essential. Heavy metal inputs from atmospheric deposition can often exceed those from RW [16], but the cumulative effect of multiple sources, including RW, should be considered. Furthermore, emerging contaminants such as pharmaceuticals and personal care products, which may not be effectively removed by conventional wastewater treatment processes, require further investigation regarding their potential impacts on ecosystems [63].

To mitigate the potential negative impacts of RW irrigation and maximize its environmental benefits, a comprehensive management approach is necessary, including, but not limited to, first, blending RW with lower-salinity water to reduce salinity levels and minimize the risk of salt accumulation in the soil; second, the careful management of irrigation frequency and volume, incorporating leaching fractions to remove excess salts from the root zone and prevent salinity buildup; third, the continuous monitoring of soil salinity and other parameters to enable early detection of potential problems and facilitate timely adjustments to irrigation practices; fourth, choosing plant species with higher salt tolerance; and last but not the least, exploring cost-efficient desalination treatments, which may be necessary to remove specific contaminants of concern.

5. Conclusions

This study demonstrates the significant effects of RW irrigation on soil salinity (pH, electrical conductivity, and water-soluble salt), nutrient contents (soil organic matter, total nitrogen, and total phosphorus), and enzyme activity (soil saccharase, soil urease, and soil acid phosphatase) in a coastal environment. The 13-month experiment with monthly high-frequency sampling revealed the complexities and dynamic nature associated with RW irrigation, particularly in a monsoon climate. Rainfall serves as a crucial factor in mitigating soil salinity accumulation, while also being a major contributor to greater variability of soil salinity in RW plot. The landscape plants studied (Z. japonica, D. erecta, C. officinarum, B. javanica, P. rubra, B. spectabilis) exhibited adaptability to RW irrigation and the associated soil changes. Certain species, such as N. cordifolia and C. fruticose, displayed signs of salt stress within the first six months. This study verifies the feasibility of using saline RW for irrigating landscape plants under natural outdoor conditions. This approach offers a viable alternative water source, contributing to sustainable water management practices. However, it emphasized the need for careful management of RW irrigation, regular monitoring of soil properties (especially during dry periods), appropriate plant selection, and tailored irrigation management strategies. Future research should delve deeper into the effects of RW on microbial communities and other key indicators of soil health, validating the conclusions of this experiment over the long term and across a wider range of plant species.