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Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)12793-4
10.1016/j.heliyon.2024.e36762
e36762
Research Article
Ameliorative effect of poly-γ-glutamic acid biopreparation on coastal saline soil
Liu Pei a
Chen Lihua chenlihua@hhu.edu.cn
a⁎
Hamoud Yousef Alhaj b
Zheng Jinhai c
Chang Tingting a
Ali Jawad a
Huang He ad
Shaghaleh Hiba e
a College of Agricultural Science and Engineering, Hohai University, Nanjing, 210098, China
b College of Hydrology and Water Resources, Hohai University, Nanjing, 210098, China
c College of Harbour, Coastal and Offshore Engineering, Hohai University, Nanjing, 210098, China
d College of Forestry, Nanjing Forestry University, Nanjing, 210037, China
e College of Environment, Hohai University, Nanjing, 210098, China
⁎ Corresponding author. College of Agricultural Science and Engineering, Hohai University, Nanjing, 210098, China. chenlihua@hhu.edu.cn
23 8 2024
15 9 2024
23 8 2024
10 17 e3676212 3 2024
21 8 2024
21 8 2024
© 2024 The Authors. Published by Elsevier Ltd.
2024

https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
To investigate the effect of poly-γ-glutamic acid (γ-PGA) biopreparation on ameliorating coastal saline soil, three treatments were established: soil salt washed treatment (CK), soil salt washed with added γ-PGA (PGA), soil salt washed with added γ-PGA biopreparation (PGAB). This study determined the effects of γ-PGA on coastal saline soil by analyzing soil aggregate, soil evaporation, soil vertical water and salt distribution, and soil cation content, soil pH, soil nutrient content and soil microorganism quantity. Results showed that γ-PGA had an ameliorative effect on saline soil, with the PGAB treatment exhibiting the most pronounced ameliorative effect compared to CK. Adding PGAB reduced soil evaporation by 30.45 %, soil salt content by 27.91 %, meanwhile increasing plant height by 33.86 %, plant fresh weight by 98.54 %, soil aggregate diameter by 6.68 times, soil water content by 26.47 % (P < 0.05). Additionally, soil total nitrogen was increased by 50.0 % in PGAB treatment, and available nitrogen and phosphorus contents were increased by 1.68 times and 85.83 % (P < 0.05), respectively. Populations of soil-culturable bacteria and fungi of PGAB treatment increased by 65.96 % and 1.23 times, respectively (P < 0.05). After salt-washing process, adding PGAB improved soil physicochemical properties, which altered the ecological environment of rhizosphere soil and promoted plant growth. The results can provide a practical approach for ameliorating coastal saline soils.

Keywords

Coastal silty saline soil
Poly-γ-glutamic acid
Salt distribution
Bacillus amyloliquefaciens
Soil amelioration
==== Body
pmc1 Introduction

The coastal saline soil exhibits highly mineralized groundwater and significant seasonal cyclic evaporation. Although washing with fresh water can reduce the soil salinity below the tolerated threshold for crops in a short period, the dry periods during autumn, winter, and summer lead to greater soil water evaporation than irrigation, resulting in re-salination by carrying salt back from the lower layer to the upper surface by evaporation [1]. The cyclical processes of freshwater leaching desalination and evaporative re-salination of coastal beach saline soil areas seriously restrict the yield increase with high salinity content. To facilitate a continuous soil desalination process in the coastal silty saline soil area, it's necessary to enhance the soil water conductivity. Therefore, freshwater can infiltrate downward due to gravity and reduce the upward movement of salt and water from the bottom of the soil to the surface. When the salt leaching from the topsoil decreases, the soil water retention capacity needs to be enhanced. This, in turn, reduces soil evaporation, effectively decreasing the movement of saline water from underground to the surface soil [2,3].

Poly-γ-glutamic acid (γ-PGA) is a natural polymer with versatile applications, as it is biocompatible, moisturizing, and biodegradable [4]. Several strains of Bacillus, including Bacillus subtilis and Bacillus amyloliquefaciens, can synthesize γ-PGA [5,6]. γ-PGA has been found to reduce soil-saturated hydraulic conductivity, soil evaporation, and infiltration capacity, in which improving soil water retention is achievable through increased saturated water content and field water holding capacity [[7], [8], [9]], applying γ-PGA yields significant increases in soil water storage capacity in the root zone, leading to a higher proportion of soil water stability aggregates. The efficacy of these functions has been demonstrated, but the interaction between of γ-PGA and synthesize strains remains to be unraveled. γ-PGA accelerates biomass and production, increasing plants' nitrogen and phosphorus absorption rate and increasing crop water productivity [[10], [11], [12]]. Moreover, γ-PGA can potentially improve the physical properties of saline soil, such as increased soil structure stability, water capacity, and retention of water and soil aggregates in the field [13].

Under drought stress caused by water shortage, exogenous γ-PGA can increase levels of plant abscisic acid, soluble sugar, proline, and chlorophyll. These increases promote root system development, improve plant tolerance to drought stress, and augment microorganism abundance that stimulates growth. This enhances the plant absorption rate of ions against the external hypertonic environment, thereby improving the growth of the plants under drought conditions [14,15]. γ-PGA can enhance the ability to scavenge active oxygen by increasing the activity of antioxidant enzymes like peroxidase, ascorbate peroxidase, superoxide dismutase, and catalase in plants. This finally relieves the malondialdehyde-induced damage and improves plant tolerance to drought and salt stress [16]. γ-PGA positively affects soil water retention and growth promotion in various soil types. However, indigenous microorganisms can easily degrade it, which weakens its water retention. According to the report based on γ-PGA and its fermented compounds, the continuous production of γ-PGA in a specific environment by γ-PGA-producing bacteria enhances its role in promoting water retention and plant growth. During the γ-PGA fermentation production process, the organism metabolizes numerous related substances due to ample substrate. In an oligotrophic environment, γ-PGA serves as a crucial energy source, and its synthetic strains produce degrading enzymes, facilitating the utilization of metabolized substances, including itself [17].

In various environments, strains display variable behavior. Coastal silty saline soil hosts fewer microorganisms, exhibiting a weak bioenergy exchange process between plants and microorganisms. Additionally, the soil has a low mineral nutrient content, necessitating the application of γ-PGA to improve water retention and inhibit salination in coastal silty saline soil. In this process, it remains unreported whether the microbial strains that metabolize γ-PGA strengthen or diminish its function by producing or degrading it.

2 Materials and methods

2.1 Experimental materials

The soil used for the experiment was collected from the Tiaozini reclamation area in Dongtai City, Jiangsu Province, China. The soil category is siltic sodic solonchaks [18]. Its chemical properties were determined after the soil was naturally air-dried and sifted through a 100-mesh sieve, as shown in Table 1.Table 1 Chemical properties comparation of the original saline soil and the washed saline soil.

Table 1Index	Soil-soluble salt (g kg−1)	pH value	Soil-soluble sodium (g kg −1)	Soil-soluble potassium (mg kg −1)	Soil-available nitrogen (mg kg −1)	Soil-available phosphorus (mg kg −1)	Soil-available potassium (mg kg −1)	Soil total nitrogen (g kg −1)	Soil total phosphorus (g kg −1)	Soil total potassium (g kg −1)	Soil organic matter (g kg −1)	
Original saline soil	3.40 ± 0.25	8.14 ± 0.03	0.71 ± 0.15	10.60 ± 0.31	19.35 ± 0.58	7.94 ± 0.25	133.68 ± 0.51	0.29 ± 0.02	0.75 ± 0.02	12.18 ± 0.03	4.29 ± 0.03	
Washed saline soil	1.94 ± 0.05	8.04 ± 0.04	0.51 ± 0.02	7.98 ± 0.29	14.25 ± 0.18	5.63 ± 0.13	98.88 ± 0.41	0.20 ± 0.01	0.57 ± 0.01	8.27 ± 0.04	4.21 ± 0.02	

Salt-tolerant plant Pennisetum purpureum was planted in the experiment. The γ-PGA sample used in the experiment was from the lab of Hohai University with a purity of 95 %, it was extracted [19] in functional microbe co-fermented substrate that consisted of corn flour, cane molasses, urea, K2HPO4, MgSO4, MnSO4, and pig dung compost. The γ-PGA biopreparation (PGAB) was created by the lab of Hohai University, in which the functional strain was Bacillus amyloliquefaciens IAE635 (kept in China Center for Type Culture Collection with accession No. of CCTCC 2013086), a high-yield strain of γ-PGA could produce the liquid fermentation output of up to 15 g L−1 and was present at a concentration of 1.2 × 1011 CFU g−1. The weight of γ-PGA content in PGAB was 50 %, and the remaining 50 % comprised CaCO3 powder.

2.2 Experimental design

The experiment was carried out in the Jiangning Campus of Hohai University. A 200 mm diameter soil column was utilized, with a PVC pipe bottom sealed to prevent leakage. The column was loaded with 100 cm of soil, and the soil bulk density was determined at 1.45 g cm−3. The experiment was conducted using a completely randomized design. Three treatments were designed in the experiment.

Control (CK): The 0–20 cm soil layer of the soil column was filled with saline soil that has been washed, while the lower layer remains unwashed.

PGA Treatment: 1 % of γ-PGA was added to the soil after salt-washing in the 0–20 cm column and mixed thoroughly. Maintaining the rest procedures same as CK treatment.

PGAB Treatment: 2 % of PGAB was added to the soil after salt washing in the 0–20 cm column and mixed thoroughly, ensuring that the γ-PGA amount in the soil remains 1 %. Maintaining the rest procedures same as CK treatment. Three replicates were set up for each treatment. A total of 12 P. purpureum seeds were sown, and there was no replanting step after germination. For the 1st to 30th days of the experiment, 150 mL of freshwater was irrigated every third day to facilitate plant growth, and the irrigation was stopped from the 30th to 60th day. No fertilizers were applied during the experiment. Soil evaporation between plants was measured every day. After the experiment, the biomass of P. purpureum was measured, and the soil's physical, chemical, and microbial properties were sampled. 12 samples were uniformly taken from each treatment for statistical analysis.

2.3 Measurement indicators and methods

Soil inter-plant evaporation was measured using the weighing method. Soil soluble salt content was determined by the residue drying-weighing method. Soil pH was measured using the pH meter method. Soil-soluble sodium and potassium were determined by flame photometry. Soil available nitrogen was determined by the alkaline solution diffusion method. Soil available phosphorus was determined by 0.5 mol L−1 NaHCO3 lixiviate and molybdenum antimony colorimetric method. Soil available potassium was determined by 1 mol L−1 NH4OAc lixiviate and flame photometry. The determination of soil total nitrogen adopts the semi-micro Kjeldahl method. Soil total phosphorus was determined by NaOH fusion and molybdenum antimony colorimetric method. Soil total potassium was determined by NaOH fusion and flame photometry. Soil organic matter was measured by potassium dichromate volumetric method with external heating. Soil bulk density was determined by the cutting ring weighing method. Soil moisture was determined by the drying and weighing method. Cultivable soil microorganisms were enumerated by the dilution-plate method.

2.4 Data analysis

Data analysis was performed using SPSS Statistics 26 (SPSS Institute, Inc., Cary, NC, USA). The collected data were subjected to analysis of variance (ANOVA), with a significance level of 0.05. Prior to ANOVA, the data were exposed to a Levene's test to assess homogeneity of variance, which indicated variance homogenized. A Duncan's test at 95 % probability was used for multiple comparisons. Graphs were created using Origin 2022.

3 Results

3.1 Development of soil aggregates in different treatments

The electron microscope scans of the soil section are shown in Fig. 1. It was observed that the volume of soil aggregates increased under the adding of PGAB. Incorporating γ-PGA and PGAB into the soil significantly improved the soil structure, promoting soil aggregate formation. Between 0 and 10 cm soil layer, the CK comprises silt particles (Fig. 1a), and both the PGA and PGAB treatment profiles stimulated the formation of numerous aggregates and pores within the soil (Fig. 1b and c). Compared with the CK treatment, the aggregate diameter increased by 3.71 times and 6.68 times in the PGA and PGAB treatment, respectively (P < 0.05).Fig. 1 Soil electron microscope scanning shows the soil aggregate in CK (a), PGA (b), and PGAB (c) treatments after 60 days experiment. The magnification factor is 1000.

Fig. 1

3.2 Effects of different treatments on soil evaporation

γ-PGA and PGAB significantly affected soil inter-plant evaporation (Fig. 2). During the initial thirty days of the experiment (Fig. 2a), the soil evaporation of each treatment was higher due to sufficient irrigation. However, after the 30th day, there was no irrigation, and the evaporation decreased in each treatment. The average daily inter-plant evaporation for each treatment during 0–30 days was CK 1.36 mm d−1, PGA 0.96 mm d−1, and PGAB 0.93 mm d−1. Compared with the CK treatment, the PGA and PGAB treatments showed a decrease of 29.44 % and 31.29 % respectively (P < 0.05). Each treatment's average daily inter-plant evaporation (Fig. 2b) at 30th to 60th days was: CK 0.41 mm d−1, PGA 0.27 mm d−1, PGAB 0.42 mm d−1. Comparing these values with CK, the PGA treatment reduced evaporation by 33.18 %, while the PGAB treatment increased it by 2.44 % (P < 0.05). Compared to the CK, the PGA and PGAB treatments demonstrated cumulative evaporation reductions of 30.45 % and 22.16 %, respectively (P < 0.05). Additionally, the PGAB treatment showed lower cumulative evaporation than the PGA treatment. During the 60-day experimental period, with the addition of γ-PGA, the soil inter-plant evaporation was reduced significantly, and soil water loss was curbed.Fig. 2 Daily evaporation of the soil with CK, PGA, and PGAB treatments in the first 30 days (a) with irrigation and second 30 days (b) without irrigation.

Fig. 2

3.3 Different treatments impact the distribution of soil water and salt

After 60 days of experimentation, Fig. 3a shows alterations in soil water content. CK soil water content shows an upward trend within the soil layer depth. The CK and PGA treatments feature a decline followed by a rise, while the PGAB treatment features an initial rise followed by a decline in moisture content.Fig. 3 Soil vertical water (a) and salinity content (b) distribution with CK, PGA, and PGAB treatments after 60 days experiment.

Fig. 3

From 0 to 20 cm soil layer, the soil water content increased by 18.75 % within PGA treatment compared to CK treatment (P < 0.05), while it decreased by 24.70 % within PGAB treatment (P < 0.05). This suggests that γ-PGA addition enhances the water retention capacity of the topsoil. However, applying γ-PGA in the soil added with B. amyloliquefaciens IAE635 strain was found to reduce the soil water retention capacity. In the 20–40 cm soil layer, compared with the CK treatment, the soil water content decreased by 6.89 % in the PGA treatment, and in the PGAB treatment, it increased by 27.60 % (P < 0.05). In the 40–60 cm soil layer, the soil water contents of PGAB and PGA treatments were 7.74 % and 13.61 % less than that of CK treatment, respectively (P < 0.05). In the 60–80 cm soil layer, the soil water content in each treatment was significantly different; compared with the CK treatment, the PGA and the PGAB treatments decreased by 12.63 % and 22.11 %, respectively (P < 0.05). The 80–100 cm soil layer showed similar results as the 60–80 cm soil layer.

The vertical distribution of soil-soluble salt content recorded after 60 days of the experiment is shown in Fig. 3b. CK, PGA, and PGAB exhibited parallel variation patterns, but CK displayed more extensive variation, initially, it decreased and then increased within the 0–60 cm soil layer. In the 0–20 cm soil layer, the lowest soil soluble salt content was observed in the PGA treatment. The PGA and PGAB treatments decreased 14.72 % and 8.76 %, respectively, compared with the CK treatment (P < 0.05). The soluble salt content of different treatments shows varying degrees of topsoil accumulation. When compared with the soil collected at the start of the experiment, the soluble salt content of the topsoil in CK, PGA, and PGAB treatments exhibited an increase of 35.63 %, 15.66 %, and 23.75 %, respectively (P < 0.05).

In the 20–40 cm soil layer, the PGA and PGAB treatments increased soluble salt content by 12.37 % and 19.40 %, respectively, compared with CK treatment (P < 0.05). Furthermore, in the 40–60 cm soil layer, the PGA treatment exhibited a 6.48 % reduction in soluble salt content compared to the CK treatment. In contrast, the PGAB treatment demonstrated an increase of 2.93 % over the CK treatment (P < 0.05). In the 60–80 cm soil layer, the soil soluble salt content decreased by 10.21 % and 9.16 % in PGA and PGAB treatments, compared with CK, respectively (P < 0.05), and there was no significant difference in soil soluble salt content between PGA and PGAB treatments (P > 0.05). In the 80–100 cm soil layer, the CK treatment had the highest soluble salt content, while there was no significant difference between PGA and PGAB treatments, and the PGAB treatment resulted in an 8.20 % decrease compared to CK (P < 0.05).

3.4 Effects of different treatments on the physical and chemical properties of surface soil

Table 2 shows the pH value and base cation content of 0–20 cm surface soil. The soil pH values of soil in the PGA and PGAB treatments were 0.63 % and 2.39 % lower compared to that of the CK treatment, respectively (P < 0.05). Similarly, the soluble sodium level in PGA and PGAB treatment decreased by 26.82 % and 13.41 %, respectively, compared with the CK treatment (P < 0.05). The variations in soil soluble potassium and soluble sodium contents among the treatments were similar, which was higher in CK than in PGA and PGAB. In detail, the PGA and PGAB treatments showed a decrease of 32.06 % and 22.73 %, respectively (P < 0.05).Table 2 Soil cation content and pH in 0–20 cm layer after 60 days experiment.

Table 2Treatment	pH value	Soluble sodium (mg kg −1)	Soluble potassium (mg kg −1)	
CK	7.95 ± 0.02 a	0.82 ± 0.03 a	10.73 ± 0.21 a	
PGA	7.90 ± 0.04 b	0.60 ± 0.04 c	7.29 ± 0.21 c	
PGAB	7.76 ± 0.02 c	0.71 ± 0.03 b	8.29 ± 0.10 b	
Note: Different low case letters indicate statistical differences at P < 0.05.

The soil available nutrient and organic matter contents in the 0–20 cm soil layer experienced significant changes in the PGA and PGAB treatments compared with the CK treatment (P < 0.05). Specifically, the soil alkaline nitrogen increased by 1.69 times and 1.3 times, and available phosphorus increased by 1.1 times and 85.83 %, respectively. However, the available potassium decreased by 22.6 % and 9.2 %, respectively. Conversely, the soil organic matter content increased by 12.41 % and 21.43 %, respectively (Table 3).Table 3 Soil nutrient content in 0–20 cm layer after 60 days experiment.

Table 3Treatment	Available nitrogen (mg kg −1)	Available phosphorus (mg kg −1)	Available potassium (mg kg −1)	Organic matter (g kg −1)	
CK	10.84 ± 0.35 c	4.87 ± 0.21 c	132.1 ± 2.95 a	5.32 ± 0.32b	
PGA	29.16 ± 2.04 a	9.05 ± 0.21 b	102.19 ± 5.1 c	5.98 ± 0.25a	
PGAB	24.95 ± 1.57 b	10.35 ± 0.22 a	119.9 ± 4.47 b	6.46 ± 0.23a	
Note: Different low case letters indicate statistical differences at P < 0.05.

3.5 Effects of different treatments on the number of soil microorganisms

The results of determining the cultivable microorganisms in the soil are shown in Table 4. The population of soil bacteria in PGA and PGAB treatments increased by 65.96 % and 4.01 times compared with CK treatment, respectively (P < 0.05). The population of soil actinomycetes in PGA treatment increased by 54.88 % compared with CK (P < 0.05), while no significant difference was counted between CK and PGAB treatment. The fungal population increased by 1.24 times and 3.37 times in PGA and PGAB treatments, compared with CK treatment (P < 0.05).Table 4 Number of culturable microorganisms in the 0–20 cm soil layer after 60 days experiment.

Table 4Treatment	Bacteria (lg CFU g−1)	Actinomyces (lg CFU g−1)	Fungi (lg CFU g−1)	
CK	6.83 ± 0.02c	4.06 ± 0.05b	4.71 ± 0.04c	
PGA	7.05 ± 0.03b	4.25 ± 0.01a	5.06 ± 0.04b	
PGAB	7.53 ± 0.02a	4.10 ± 0.04b	5.35 ± 0.02a	
Note: Different low case letters indicate statistical differences at P < 0.05.

3.6 Effects of different treatments on the biomass of P. purpureum

The fresh weight and plant height of P. purpureum are displayed in Fig. 4. The PGAB treatment resulted in a 19.17 % increase in plant height compared with the PGA treatment and a 33.86 % increase compared with the CK treatment (P < 0.05). Meanwhile, plant height in the PGA treatment increased by 12.33 % compared with the CK treatment (P < 0.05). The PGAB treatment had 40.21 % and 98.54 % increases in fresh weight compared with PGA and CK treatments, respectively (P < 0.05). In the PGA treatment, fresh weight increased by 41.60 % compared with CK (P < 0.05). Adding γ-PGA significantly increased the biomass of P. purpureum, and adding B. amyloliquefaciens further stimulated its growth.Fig. 4 Growth conditions of P. purpureum with CK, PGA, and PGAB treatments after 60 days experiment. Different low case letters indicate statistical differences at P < 0.05.

Fig. 4

4 Discussion

γ-PGA is a biopolymer that forms through the amide bond between the α-amino group and the γ-carboxyl group in the glutamic acid monomer. γ-PGA can be synthesized by fermentation of several strains of Bacillus in the soil [20,21]. The synthesis of γ-PGA involves dehydration and condensation of L-glutamic acid and D-glutamic acid [9]. The addition of B. amyloliquefaciens to the soil may enhance the continuous synthesis and improvement of γ-PGA. It is soluble in water and can increase water viscosity [22,23].

The hydrophilic properties of γ-PGA, stemming from its single-chain molecular structure and a hydrophilic group, make it a beneficial soil supplement. Adding γ-PGA to soil can alter its water-holding characteristics, subsequently improving its water retention capacity. In addition, applying γ-PGA increases soil water suction, ultimately reducing the evaporation rate [21]. γ-PGA exhibits excellent biodegradability. Bacteria that produce γ-PGA and certain other strains are capable of secreting specific depolymerase, utilizing γ-PGA as a nutrient source, and facilitating its rapid degradation [17]. Therefore, the proliferation of soil microorganisms or the enrichment of the population may result in an unsustainable improvement.

Plant root exudates, microbial metabolites, and other substances can enhance soil aggregate composition [24]. The cohesive function of γ-PGA resulted in a significant increase in soil aggregate diameter. This study demonstrated that microorganisms could accelerate the formation of aggregates in silty soil, evidenced by the significant increase in soil aggregate diameter in the presence of γ-PGA and its metabolizing strains. The increase in soil macroaggregates resulted in greater soil porosity and was somewhat correlated with increased evaporation from treatment with microorganisms. The hydrophilic structure of γ-PGA allows it to hold a substantial volume of water in its three-dimensional construction. The high molecular weight, negative charge, three-dimensional cross-linked construction, and hydrophilic functional groups of γ-PGA retain the water in its structure [8]. Adding γ-PGA significantly increased soil-saturated and soil water content [25] while reducing soil water infiltration. This leads to higher water retention in the γ-PGA added layer, strongly correlated with reduced soil evaporation observed in this study. Improving the water retention capacity of the topsoil can reduce salt accumulation on the surface [26,27]. The high water content and low evaporation of the topsoil create a barrier to the upward movement of water, which hinders base ions transport and thus inhibits salt migration [28]. This study found a strong correlation between the reduction of surface salt with water retention in the PGA and PGAB treatments. To sustain the salt-washing effect of coastal saline soil, it is necessary to consume large quantities of freshwater on a regular basis [1]. The amendment capacity of PGAB may potentially reduce the demand for freshwater resources.

Salt migration due to evaporation typically occurs in the topsoil [29]. The process of water transport by evaporation and infiltration results in the vertical transport of water-soluble salts. In the irrigation stage of this study, the surface soil water retention capacity increased, which led to a decrease in soil salt migration toward the bottom of the soil column and resulted in salt accumulation in shallower soil. During the evaporation phase later in the experiment, these salts migrated toward the surface, leading to less change in the moisture and salinity at the bottom of the soil column [30]. In the later stage of the experiment, the decrease in soil water content caused a reduction in soil matric potential, resulting in decreased soil evaporation. Consequently, transpiration became the primary method of water consumption in plants, which explains the higher soluble salt content and lower topsoil water content in the treatments with better plant growth. The soil water and salt vertical distribution indicated that all treatments experienced infiltration to the bottom of the soil column during the experiment's irrigation stage, leading to the accumulation of soluble salt and water. Infiltration volume varies across treatments, with the PGA treatment resulting in the smallest infiltration volume and the most uniform distribution of soluble salt content. The addition of Ca2+ can facilitate the exchange of Na+ from cation exchange sites in the soil, thereby facilitating the leaching of the exchanged sodium from the soil [31,32]. This process results in more sodium leaching into the underlying soil layer. The soil salinity in the three treatments mostly accumulated in the 40–60 cm soil layer during the irrigation stage. After ceasing irrigation, different degrees of accumulation emerged in the surface soil. Movement of water and salt in the middle and lower layers of the soil column toward the surface was relatively limited. After the irrigation stops, soil moisture moves upward due to evaporation and plant transpiration. The upward migration process in the PGA treatment was weaker than the other two treatments, consequent to the soluble salt content of the surface soil was the lowest. The soil water content on the surface of the PGAB treatment was lower than that of the CK treatment. Conversely, the surface soil water content on the PGA treatment was higher than that of the CK treatment. Additionally, the upper layer of the PGAB treatment had a higher soluble salt content than the PGA treatment. This may be related to increased soil porosity and evaporation in the PGAB treatment, the upward migration of soluble salt at the bottom of soil columns and the exchangeability of calcium contained in PGAB resulted in the soluble salt content of this treatment exhibiting the highest concentration at a depth of 40–60 cm. Further research is needed to ascertain the specific underlying reasons.

Soil nitrogen can be categorized as organic nitrogen or inorganic nitrogen. Plant absorption and utilization require the conversion of organic nitrogen into inorganic nitrogen via mineralization [33]. The significant increase of soil alkaline nitrogen in γ-PGA treatment can be attributed to γ-PGA decomposition. The alkaline nitrogen content in the PGA treatment was higher than that in the PGAB treatment, potentially because of plant absorption and utilization, which is aligned with the highest plant biomass observed in the PGAB treatment. Microorganisms influence the hydrolysis of soil phosphorus into available phosphorus. Soil with sufficient water and a rich microorganism population tends to have higher phosphorus availability [20]. Increased microbial activity can accelerate the release of other nutrients into the soil, thus increasing the soil's available nutrient content [34]. Therefore, the highest available phosphorus content in PGAB treatment matched the highest microbial activity in PGAB treatment. The background available potassium content in the soil was high, strengthening crops to resist the stress of sodium ions. Selective absorption of potassium ions led to larger plant biomass and greater absorption capability. The determination of the available potassium content corresponded with changes in plant biomass. The reduction in soil bulk density enhanced the soil permeability, water content, porosity, and other conditions, thereby facilitating soil biochemical activities. The Ca2+ remaining in the surface layer due to cation exchange and CaCO3 release serves as an essential nutrient for plant growth and promotes plant growth [35]. Increasing soil water content enhanced the enzymatic activity of urease and phosphatase, promoted the decomposition of organic matter into compounds such as humic acid, and increased soil organic matter content. Additionally, applying γ-PGA heightened the soil organic matter content, while PGAB, rich in B. amyloliquefaciens, further increased the soil organic matter content. γ-PGA can absorb saturated water and form a hydrogel, which provides a continuous water supply and enhances plant root activity and nutrient absorption [10]. Overall, γ-PGA promotes plant growth under salt and drought stresses.

Microorganisms are essential for maintaining ecosystem stability, productivity, and environmental resiliency [36]. Adding γ-PGA to the soil has led to a significant increase in bacterial diversity and biomass. Furthermore, a negative correlation was found between soil microbial biomass and soil soluble salt content [37], with a decrease in the surface soil soluble salt content increasing the number of soil microorganisms. Adding PGAB increases the number of soil microorganisms, including the improvement of various kinds of microorganisms, which leads to increased soil plant nutrients and organic matter [[38], [39], [40]]. The ratio between soil bacteria and fungi indicates whether the cultivable microorganisms are balanced. The probability of soil-borne diseases increases with a lower ratio [41]. The application of B. amyloliquefaciens bacteria enhanced the ecological conditions of the rhizosphere soil and contributed to the higher biomass of PGAB plants.

5 Conclusion

Using γ-PGA has demonstrated various benefits for coastal saline soil amelioration, as shown by the following results. When γ-PGA metabolizing microorganism B. amyloliquefaciens is present, it promotes the formation of large aggregates. The evaporation of the later stage in the experiment increases, leading to enhanced upward movement of soil salinity, while the PGAB cumulative evaporation decreases. Additionally, PGAB increases the organic matter in the topsoil, available phosphorus content, soil bacteria, and fungi, leading to a significant increase in plant biomass. In conclusion, the γ-PGA metabolizing microorganism B. amyloliquefaciens strengthened the soil improvement effect of γ-PGA by water retaining and salt suppressing in coastal silty saline soil.

Data availability statement

The data associated with this study has not been deposited into a publicly available repository.

The data supporting the findings of this study are available by contacting the corresponding author upon a reasonable request.

CRediT authorship contribution statement

Pei Liu: Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation. Lihua Chen: Writing – review & editing, Resources, Project administration, Methodology, Funding acquisition, Conceptualization. Yousef Alhaj Hamoud: Writing – review & editing, Resources, Project administration, Methodology, Funding acquisition, Conceptualization. Jinhai Zheng: Supervision. Tingting Chang: Resources. Jawad Ali: Writing – review & editing, Formal analysis. He Huang: Validation. Hiba Shaghaleh: Writing – review & editing, Formal analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This research work was financially supported by the 10.13039/501100012166 National Key Research and Development Program of China (2020YFD09007001 ), the Jiangsu Provincial 10.13039/501100018581 Water Conservancy Science and Technology Project (2021079 ), the 10.13039/501100004610 Jiangsu Provincial Ecological Environment Research Project (2022010 ).
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