Effect of Planting Portulaca oleracea L. on Improvement of Salt-Affected Soils

Abstract

Saline–alkali land is a critical factor limiting agricultural production and ecological restoration. Utilizing salt-tolerant plants for bioremediation represents an environmentally friendly and sustainable approach to soil management. This study employed the highly salt-tolerant crop Portulaca oleracea L. cv. “Su Ma Chi Xian 3” as the test material. A plot experiment was established in coastal saline soils with planting P. a- oleracea (P) and no planting (CK) under three blocks with the different salt levels (S1: 2.16 g/kg; S2: 4.08 g/kg; S3: 5.43 g/kg) to systematically evaluate its salt accumulation capacity and effects on soil physicochemical properties. The results demonstrated that P. oleracea exhibited adaptability across all three salinity levels, with aboveground biomass following the trend PS2 > PS3 > PS1. The ash salt contents removed through harvesting were 1.29, 2.03, and 1.74 t/ha, respectively, in PS1, PS2, and PS3. Compared to no planting, a significant reduction in bulk density was observed in the 0–10 and 10–20 cm soil layers (p < 0.05). A significant increase in porosity by 9.72%, 16.29%, and 12.61% was found under PS1, PS2, and PS3, respectively, in the 0–10 cm soil layer. Soil salinity decreased by 34.20%, 50.23%, and 48.26%, in the 0–10 cm soil layer and by 14.43%, 32.30%, and 26.42% in the 10–20 cm soil layer under PS1, PS2, and PS3, respectively. The pH exhibited a significant reduction under the planting treatment in the 0–10 cm layer. A significant increase in organic matter content by 13.70%, 12.44%, and 13.55%, under PS1, PS2, and PS3, respectively, was observed in the 0–10 cm soil layer. The activities of invertase and urease were significantly enhanced in the 0–10 and 10–20 cm soil layers, and the activity of alkaline phosphatase also exhibited a significant increase in the 0–10 cm layer under the planting treatment. This study indicated that cultivating P. oleracea could effectively facilitate the improvement of coastal saline soils by optimizing soil structure, reducing salinity, increasing organic matter, and activating the soil enzyme system, thereby providing theoretical and technical foundations for ecological restoration and sustainable agricultural utilization of saline–alkali lands.

1. Introduction

Soil salinization has emerged as a globally escalating environmental crisis that threatens agricultural sustainability and food security [1]. Current estimates indicate that approximately 1 billion hectares of land worldwide are affected by salinization, with this figure continuing to rise at an alarming rate [2]. In recent decades, the rapid advancement of urbanization has led to a large amount of cultivated land becoming occupied, resulting in continuous reduction in cultivated land area, which undoubtedly brings huge pressure to food production [3]. For China, a country with scarce land resources, a small cultivated land area, and a wide distribution of salt-affected soil, saline–alkali land management is an important measure to ensure national food security from a strategic height and an important link to alleviate the contradiction between people and land and promote the sound and rapid development of the national economy [4,5]. Conventional amelioration approaches, including hydraulic engineering measures and chemical amendments, have demonstrated limited effectiveness in addressing salinization challenges [6,7,8]. These traditional methods are frequently constrained by high implementation costs, operational complexity, and the potential for secondary environmental contamination. In contrast, the utilization of salt-tolerant plants for phytoremediation represents a promising alternative strategy. This phytoremediation approach offers distinct advantages through its cost-effectiveness, environmental compatibility, and potential economic returns from cultivated species. As such, it embodies a sustainable, eco-friendly, and economically viable solution for saline–alkali soil reclamation and can be widely applied worldwide for the restoration of such degraded lands [9].

Studies have demonstrated that certain halophytic species can absorb saline ions from salt-affected soils and enhance both the physicochemical properties and structural characteristics of saline soils [10,11]. Purslane, Portulaca oleracea L, is an herbaceous plant belonging to the Portulacaceae family that exhibits strong stress resistance (including tolerance to salinity, drought, and poor soil conditions), high photosynthetic efficiency, minimal susceptibility to pests and diseases, and low requirements for pesticides and fertilizers [12,13]. It is widely distributed across the globe. Under suitable temperature and light conditions, P. oleracea develops a unique high-yield growth pattern due to its C4 photosynthetic advantage, strong regenerative capacity, and efficient resource utilization [14]. With a short growth cycle, P. oleracea can trigger an axillary bud regeneration mechanism by retaining 2–3 basal nodes during harvesting, resulting in multiple regenerative branches and allowing for repeated harvesting.

In recent years, studies have revealed that P. oleracea possesses significant potential for the remediation of saline–alkali soils. P. oleracea demonstrates the ability to grow normally under certain levels of salt stress, exhibiting strong salt tolerance [15,16]. Remarkably, it can absorb salts from the soil and thrive and complete its life cycle even in high salt-affected conditions [17]. Nevertheless, the studies on P. oleracea have primarily focused on its physiological traits, with limited exploration of its role as a dominant species for ecological restoration in salt–alkali land. The effects of planting P oleracea in the ecological remediation of coastal salt-affected soils remain poorly understood. Therefore, the objectives of this study were to (1) determine P. oleracea capacity of salt removal in coastal saline soils with different salinity concentrations, (2) compare its effects on the physicochemical properties of salt-affected soils, and (3) further clarify its phytoremediation potential in saline–alkali lands. The findings are expected to provide a scientific basis for the application of purslane in saline–alkali soil remediation and establish a theoretical foundation for the biological improvement of saline–alkali soils.

2. Materials and Methods

2.1. Characteristics of the Experimental Site

The experimental site is located at the tidal flat experimental base of Jinhai Farm in Dafeng District, Yancheng City, Jiangsu Province, China (32°59′42″ N, 120°49′32″ E), approximately 6 km east of the Yellow Sea. The site lies within the northern subtropical monsoon climate zone, with an annual precipitation of 900–1300 mm, predominantly concentrated between June and August. Three field blocks with different salinity levels (S1: 2.16‰; S2: 4.08‰; S3: 5.43‰) were selected as experimental sites. The soil used in the experiment is classified as alluvial saline soil. The basic properties of three experimental blocks in 0–20 cm soil are presented in

Table 1. Basic properties of three blocks with different salt levels in 0–20 cm soil.

Blocks	Soil Bulk Density	Soil Salinity	pH	Soil Organic Matter	Available Nitrogen	Available Phosphorus	Available Potassium
	(g/cm3)	(g/kg)		(g/kg)	(mg/kg)	(mg/kg)	(mg/kg)
S1	1.47 ± 0.03	2.16 ± 0.08	8.01 ± 0.01	8.34 ± 0.03	28.59 ± 0.80	10.42 ± 0.27	195.53 ± 4.92
S2	1.51 ± 0.02	4.08 ± 0.07	7.95 ± 0.02	6.93 ± 0.04	26.45 ± 0.81	9.23 ± 0.29	187.67 ± 9.51
S3	1.48 ± 0.03	5.43 ± 0.09	8.05 ± 0.02	6.64 ± 0.06	24.72 ± 0.75	8.92 ± 0.26	176.6 ± 7.41

Note: The data is presented as mean ± SD (n = 3).

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2.2. Experimental Design and Setup

The experiment was conducted from April to August 2024. The six treatments were two planting treatments (planting P. oleracea [(P) and non-planting (CK)] × three salinity levels (S1, S2, and S3). Each treatment was replicated three times. A total of 18 experimental plots (4 m × 10 m) were established. The P. oleracea variety “Su Ma Chi Xian 3” was chosen as the test material, which was bred by Jiangsu Coastal Area Institute of Agricultural Sciences, Yancheng, China. The seeding rate is 15 kg/ha. No fertilization was applied during the experiment. The experiment commenced on 15 April 2024, with two cuttings of P. oleracea taken on 13 June and 13 July, retaining approximately 8 cm of the rootstock each time. The final harvest was conducted on 16 August 2024. Soil samples were collected after the harvest of P. oleracea.

2.3. Measurement Methods

2.3.1. Plant Biomass

The fresh and dry biomass of P. oleracea aboveground parts was measured after each cutting event and at the final harvest to estimate the total aboveground biomass. Fresh plant materials were weighed, then plant samples of about 2 kg were taken in triplicate to measure the moisture content of the plants. The plant samples were deactivated at 105 °C for 0.5 h and dried at 80 °C for more than 48 h until a constant weight was achieved to determine the dry weight. The dry weight was calculated according to fresh weight time moisture content.

2.3.2. Ash Content in Plant and Salt Accumulation by Plants

The plant materials were ashed in a muffle furnace at 550 °C for 8 h. After cooling, the weight of the ash was measured to determine the ash content. We used biomass of aboveground plant time ash content in aboveground plants to calculate the salt accumulation capacity of plants based on Wang et al. [18]. The salt accumulation capacity of P. oleracea was calculated using the following formula:

Total amount of salt accumulation (kg/ha) = Total aboveground biomass (kg/ ha) × Ash content (%)

(1)

2.3.3. Soil Physicochemical and Biological Analysis

The salt content of the soil profile from 0 to 60 cm was determined, with sampling depths at 0–10, 10–20, 20–40, and 40–60 cm. The total soil salt content was determined by the weight loss of a 5:1 (water–soil mass ratio) extract after being oven-dried at 105 °C to a constant weight [19].

The measurements of soil bulk density, porosity, pH, organic matter content (SOM), and enzyme activity were conducted across three soil layers (0–10, 10–20, and 20–40 cm). The soil bulk density and porosity were measured by inserting steel cylinders. Soil pH was measured by drying and grinding the soil samples, followed by extraction with a 5:1 (water–soil mass ratio) solution and then determining the pH of the extract using a pH meter (Sartorius, PB-10, Gottingen, Germany). The soil organic matter content was determined using a potassium dichromate oxidation technique combined with external heating [20].

Soil invertase activity was determined using 3, 5-dinitrosalicylic acid colorimetry, soil urease activity was measured by the phenol–sodium colorimetric method, and soil alkaline phosphatase activity was calculated by colorimetry using p-nitrophenyl phosphate disodium as the substrate [21,22].

We measured soil physicochemical and biologic properties across three soil layers (0–10, 10–20, and 20–40 cm), except for salt content, which was measured across four layers (0–10, 10–20, 20–40, and 40–60 cm). The objective of this study was mainly focused on phytoremediation of salt from soils. The salt in the soils is very easy to move up and down with water, but the other parameters are relatively stable and less changed at 40–60 cm soil depth, since P. oleracea is a shallow-rooted plant.

2.4. Statistical Analysis

A one-way analysis of variance (ANOVA) was performed to analyze the significant difference of biomass, ash content in plants, and salt accumulation by plants among treatments, and a two-way ANOVA was conducted to analyze significant changes of soil physiochemical and biological properties among treatments through SPSS Statistics 20.0 software. Duncan’s test was used to test significant differences between treatments when ANOVA showed significant effects (p < 0.05). Graphs were created using Excel 2016.

3. Results

3.1. Aboveground Biomass of P. oleracea

The highest fresh weight of aboveground part among treatments was observed in PS2 treatment, which significantly increased by 56.75% and 15.15%, respectively, compared to PS1 and PS3. The aboveground dry weight of P. oleracea under the three salt levels (PS1, PS2, and PS3) reached 5.00 t/ha, 7.61 t/ha, and 6.54 t/ha, respectively. Notably, the aboveground biomass of P. oleracea was the highest in PS2, exceeding that of PS1 and PS3 by 52.20% and 16.36%, respectively (

Table 2. Aboveground biomass of P. oleracea in the salt-affected soils.

Treatment	Fresh Weight (t/ha)				Dry Weight (t/ha)
	First Harvest	Second Harvest	Third Harvest	Total	First Harvest	Second Harvest	Third Harvest	Total
PS1	11.54 ± 0.20c	14.78 ± 0.21c	30.42 ± 0.41c	56.74 ± 0.43c	1.02 ± 0.02c	1.30 ± 0.02c	2.68 ± 0.04c	5.00 ± 0.04c
PS2	17.44 ± 0.13a	23.71 ± 0.36a	47.79 ± 0.38a	88.94 ± 0.73a	1.49 ± 0.01a	2.03 ± 0.03a	4.09 ± 0.03a	7.61 ± 0.06a
PS3	14.48 ± 0.28b	20.07 ± 0.47b	42.69 ± 0.31b	77.24 ± 0.47b	1.23 ± 0.02b	1.70 ± 0.04b	3.62 ± 0.03b	6.54 ± 0.04b

Note: Means ± SD (n = 3) with different letters within columns are significantly different (p < 0.05).

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3.2. Accumulation of Salt by P. oleracea

The effect of planting P. oleracea on salt accumulation is shown in

Table 3. Ash content in plants and salt accumulation by P. oleracea in the salt-affected soils.

Treatment	Ash Content (%)			Salt Accumulation (t/ha)
	First Harvest	Second Harvest	Third Harvest	First Harvest	Second Harvest	Third Harvest	Total
PS1	26.68 ± 0.53a	25.81 ± 0.48a	25.55 ± 0.56a	0.27 ± 0.01c	0.34 ± 0.01c	0.69 ± 0.02c	1.29 ± 0.02c
PS2	27.03 ± 0.31a	26.35 ± 0.34a	26.57 ± 0.77a	0.40 ± 0.01a	0.53 ± 0.01a	1.09 ± 0.04a	2.03 ± 0.05a
PS3	26.90 ± 0.39a	26.25 ± 0.46a	26.51 ± 0.50a	0.33 ± 0.01b	0.45 ± 0.02b	0.96 ± 0.02b	1.74 ± 0.01b

Note: Means ± SD (n = 3) with different letters within columns are significantly different (p < 0.05).

. There was no significant difference in ash content in plants among the treatments. The salt accumulation in aboveground P. oleracea was significantly influenced by soil salinity levels. The salt accumulation reached 1.29, 2.03, and 1.74 t/ha, respectively, in PS1, PS2, and PS3 treatments.

3.3. Effect of Planting P. oleracea on Soil Physiochemical Properties in Salt-Affected Soils

3.3.1. Salinity Distribution in Soils

The salt content was significantly lower in planting soils than non-planting soils at 0–10 and 10–20 cm soil layers but not significantly different at 20–40 and 40–60 cm (Figure 1). Compared to non-planting, the salt contents significantly decreased by 34.20%, 50.23%, and 48.26% in the 0–10 cm soil layer and by 14.43%, 32.30%, and 26.42% in the 10–20 cm layer under PS1, PS2, and PS3 treatments, respectively. The differences in salt content gradually decreased with the increase in soil depth between the planting and non-planting soils.

3.3.2. Soil Bulk Density

The soil bulk density in the 0–10 and 10–20 cm layers was significantly higher in planting soils than non-planting soils (p < 0.05) (Figure 2). Compared to CK, the bulk density in the 0–10 cm and 10–20 cm soil layers significantly decreased by 8.05% and 7.05% in PS1, by 12.81% and 7.97% in PS2, and by 10.27% and 7.16% in PS3, respectively. The soil bulk density in the 20–40 cm layer decreased to different degrees in the planting treatments, but the difference was not significant.

3.3.3. Soil Porosity

Compared to non-planting soil, the soil porosity of all layers increased to varying degrees after planting P. oleracea (Figure 3). The soil porosity was significant higher in planting soils than non-planting soils in the 0–10 and 10–20 cm soil layers (p < 0.05) but not in the 20–40 cm soil layer. The soil porosity at 0–10 cm increased by 9.72%, 16.29%, and 12.61%, respectively, in PS1, PS2, and PS3.

3.3.4. Soil pH

The soil pH at 0–10 cm was significantly lower in planting soil than non-planting soil, but there was no significant difference in pH value in the 10–20 and 20–40 cm soil layers (p > 0.05) (Figure 4). Compared to non-planting, the pH value at 0–10 cm decreased by 1.96%, 1.81%, and 1.22%, respectively, in the planting soils with three salt levels.

3.3.5. Soil Organic Matter

The organic matter contents were significantly higher in planting than non-planting soils at 0–10 and 10–20 cm but not at 20–40 cm (Figure 5). Compared to non-planting, the organic matter contents in the 0–10 cm soil layer significantly increased by 13.75%, 12.34%, and 13.56%, respectively, in PS1, PS2, and PS3, while they increased by 4.59%, 6.90%, and 7.92% in the 10–20 cm soil layer.

3.4. Effect of Planting P. oleracea on Soil Enzyme Activity

3.4.1. Soil Invertase Activity

The soil invertase activity showed a significant increase in planting treatments in the 0–10 and 10-20 cm soil layers, but no significant difference was observed in the 20–40 cm layer (Figure 6). Compared to non-planting, the invertase activity in the 0–10 cm soil layer exhibited 2.22-, 2.11-, and 1.92-fold increases in three salt-level planting treatments, while demonstrating comparatively smaller increases at 10–20 cm.

3.4.2. Soil Urease Activity

The soil urease activity significantly increased in planting treatments in the 0–10 and 10–20 cm soil layers, while there was no significant difference in the 20–40 cm soil layer (Figure 7). Compared to non-planting, the activity of urease in the 0–10 cm soil layer increased by 56.25%, 61.36%, and 47.62%, respectively, in PS1, PS2, and PS3, and by 55.56%, 50.00%, and 41.38%, respectively, in the 10–20 cm soil layer.

3.4.3. Soil Alkaline Phosphatase Activity

The soil alkaline phosphatase activity significantly increased in planting treatments in the 0–10 cm soil layer, while there was no significant difference in the 10–20 and 20–40 cm soil layers (Figure 8). Compared to non-planting, the activity of alkaline phosphatase in the 0–10 cm soil layer increased by 13.88%, 10.00%, and 17.03%, respectively, in PS1, PS2, and PS3.

4. Discussion

4.1. Phytoremediation by P. oleracea in Salt-Affected Soils

In the phytoremediation of salt-affected soil, the efficiency of salt extraction highly depends on the characteristics of the selected halophyte species [23]. Ideal remediation plants should possess high biomass yield and strong salt absorption capacity. Therefore, when selecting remediation species, priority should be given to halophytes of the salt-accumulating type to fully utilize their efficient salt absorption and accumulation capabilities [24]. This study further revealed the unique ecological functions of P. oleracea in saline environments, particularly its ability to accumulate salt in its aboveground parts. It was found that the trend of ash salt content accumulated by P. oleracea in the different treatments was PS2 > PS3 > PS1. When harvested, the salt removal reached 1.29, 2.03, and 1.74 t/ha, respectively, in planting treatments under three salinity levels (Table 3). Notably, the ash salt accumulation (2.03 t/ha) in the PS2 treatment was particularly significant, equivalent to half of the ash salt accumulation of Suaeda salsa. Meanwhile, the ash salt accumulation in the PS3 treatment (1.74 t/ha) was comparable to that of S. salsa at the seedling stage [18]. A similar study reported by Kilic et al. indicated that P. oleracea can effectively absorb salt in the soil, which can effectively remove 65 kg/ha of Na⁺ and 210 kg/ha of Cl⁻ in a growing season and accumulate these salts in aboveground parts, thereby reducing soil salt content [17]. These results indicate that P. oleracea exhibits similar salt-tolerance mechanisms to typical halophytes such as Halogeton glomeratus and Suaeda glauca [25]. Notably, P. oleracea not only adapts well to high-salinity environments but also effectively reduces soil salt content by accumulating salt in its aboveground biomass. This characteristic gives it significant application value in salt-affected soil remediation. Through regular cutting, P. oleracea can serve as an efficient phytoremediation tool, helping to reduce soil salinity levels while providing a sustainable solution for the ecological restoration of salt-affected soil. Additionally, the salt accumulation capacity of P. oleracea complements its salt tolerance mechanisms, further highlighting its adaptability and ecological value in adverse environments. Meanwhile, the salinity in the 0–10 and 10–20 cm soil layers decreased after harvest (Figure 1), demonstrating the strong desalination capacity of P. oleracea. The possible mechanisms of desalination included the following: (1) the absorption and translocation of salt from the soil to the aboveground parts through transpiration, ultimately removing salt through harvesting; (2) the secretion of organic acids and other substances by P. oleracea roots, which promote the dissolution and leaching of salt ions in the soil; (3) the promotion of soil aggregate formation by root exudates, reducing surface salt accumulation.

4.2. Planting P. oleracea Improves Soil Physiochemical and Biological Properties in Salt-Affected Soils

Planting P. oleracea could improve soil structure in salt-affected soil. Soil bulk density is an important physical indicator reflecting soil compactness and structural condition, often referred to as the physical structural fertility of soil [26]. Excessive bulk density indicates reduced soil porosity, which may lead to excessive water retention, thereby affecting the growth and development of plant roots. In this study, after planting P. oleracea, the bulk density significantly decreased (Figure 2), and porosity increased in the 0–10 and 10–20 cm soil layers (Figure 3), indicating that the root growth of P. oleracea effectively improved soil structure and enhanced soil aeration and water permeability. Studies have demonstrated that cultivating salt-tolerant plants can mitigate salt buildup in soils by enhancing their structural properties. For instance, Araya et al. found that halophyte cultivation reduced soil bulk density while enhancing porosity and hydraulic conductivity, facilitating the leaching of salts from saline–alkali soils [27]. Similarly, Qadir et al. observed that the plants improved soil physicochemical properties, promoting the removal of Na⁺ and soluble salts through enhanced leaching [28].

Planting P. oleracea could increase soil fertility in salt-affected soil. In the present study, the significant decrease in pH in the 0–10 cm soil layer (Figure 4) could be related to the secretion of organic acids by roots and the acidic substances produced by the decomposition of soil organic matter [29,30]. The reduction in pH helps improve the availability of nutrients such as phosphorus, iron, and zinc, promoting plant growth [31]. Additionally, the organic matter content in the 0–10 cm soil layer increased (Figure 5), indicating that P. oleracea primarily increases surface soil organic matter content through aboveground litter and root exudates. The increase in organic matter content helps improve soil structure, enhance water and nutrient retention, and promote soil microbial activity, thereby improving soil fertility and promoting the health of the soil ecosystem [31]. Soil enzyme activity is an important indicator reflecting soil fertility and ecological functions [32]. In this study, the activities of invertase and urease in the 0–20 cm soil layer significantly increased (Figure 6 and Figure 7), indicating that the cultivation of P. oleracea promoted soil carbon and nitrogen cycling. The enhancement of invertase activity facilitated the decomposition of carbohydrates in the soil, providing an energy source for soil microorganisms. The increase in urease activity, on the other hand, promotes the decomposition of nitrogen-containing organic compounds such as urea, releasing ammonium nitrogen that is readily absorbable by plants [33]. Additionally, the activity of alkaline phosphatase in the 0–10 cm soil layer showed a significant increase (Figure 8), suggesting that the cultivation of P. oleracea enhanced the mineralization of organic phosphorus in the soil, thereby improving the availability of soil phosphorus [34]. The soil microbial community composition possibly changed under the ameliorative effect of various halophytes or salt-tolerant plants on saline soils [25]. Hence, it is necessary to conduct metagenomic analysis to reveal the phytoremediation effects of halophytes or salt-tolerant plants in saline soils in future studies.

5. Conclusions

This study demonstrated that planting P. oleracea significantly reduced the pH values and soluble salt contents in salt-affected soil while improving soil structure and increasing soil organic matter contents and enzyme activities. These multifunctional characteristics highlighted the significant application value of P. oleracea for salt-affected soil improvement. Future studies should further elucidate the molecular mechanisms underlying the salt tolerance and remediation functions of P. oleracea. Additionally, optimizing agronomic management practices (such as cutting frequency and fertilizer management) could enhance its ecological remediation efficiency, thereby promoting the large-scale application of this technology in the management of coastal saline–alkali lands.