A slow-release plant bio-stimulant hydrogel and a preparation method and application thereof

Abstract

The application belongs to the technical field of agricultural hydrogel development, and particularly relates to a slow-release plant biological stimulant hydrogel and a preparation method and application thereof. The hydrogel is obtained by cross-linking polyglutamic acid and 2-keto-L-gulonic acid fermentation liquor. The hydrogel has high mechanical strength and good soil moisture adsorption performance, and shows good water storage and retention capacity, thereby effectively reducing the irrigation water consumption. In addition, the hydrogel can continuously release small-molecule organic matters with plant biological stimulant function such as glutamic acid, 2-keto-L-gulonic acid, D-gulonic acid-gamma-lactone and citric acid in the soil microenvironment, effectively improves the content of the soil available organic nutrients, enhances the microbial activity, improves the crop rhizosphere soil, strengthens the water and fertilizer synergistic effect, thereby effectively promotes the growth of crops, improves the yield and accumulates vitamin C, and has good application value in the fields of agriculture and environment.

Description

Technical Field

[0001] This invention belongs to the field of agricultural hydrogel development technology, specifically relating to a hydrogel that releases plant biostimulants, its preparation method, and its application. Background Technology

[0002] Severe soil conditions significantly reduce crop quality, particularly vitamin C content, and lead to decreased land productivity, reduced crop yields, and increased production costs. Meanwhile, against the backdrop of global warming, my country is experiencing a significant increase in the frequency of extreme weather events such as droughts and high temperatures, posing a serious challenge to agricultural efficiency. Therefore, developing novel plant biostimulant products that combine soil improvement, water retention, drought resistance, and enhanced crop resistance to abiotic stresses has become an urgent need in the agricultural sector.

[0003] 2-Keto-L-gulonic acid is a key precursor in the industrial production of vitamin C. Currently, industrial production of vitamin C utilizes a two-step microbial mixed-culture fermentation method developed by Chinese engineers to prepare 2-keto-L-gulonic acid. During fermentation, a fermentation broth containing a high concentration of 2-keto-L-gulonic acid (10%–15%) is obtained, along with a concentrated fermentation broth (containing 15%–20% 2-keto-L-gulonic acid) remaining after 2-keto-L-gulonic acid extraction. Our team's research found that 2-keto-L-gulonic acid and its fermentation broth significantly improve crop resistance to abiotic stresses, yield, and the content of vitamins, amino acids, and active secondary metabolites. However, the water-soluble organic carbon in 2-keto-L-gulonic acid and its fermentation broth is low molecular weight (100-300 Daltons (Da)). Once applied to the soil, it is readily and rapidly degraded by microorganisms, severely impacting its growth-promoting and quality-improving effects. If it could be slowly dissolved and released into the soil, its effects on soil, plants, and soil microorganisms could be enhanced. Summary of the Invention

[0004] The purpose of this invention is to provide a hydrogel containing a sustained-release plant biostimulant, its preparation method, and its application.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A hydrogel containing a slow-release plant biostimulant, wherein the hydrogel is obtained by crosslinking a polyglutamic acid aqueous solution and a 2-keto-L-gulonic acid fermentation broth; wherein the ratio of the 2-keto-L-gulonic acid fermentation broth to the polyglutamic acid aqueous solution is in the range of 1:3 to 1:10 (mass ratio); and the polyglutamic acid has a molecular weight range of 50 kDa to 200 kDa (preferably 70 kDa).

[0007] A method for preparing a hydrogel containing the sustained-release plant biostimulant involves mixing an aqueous solution of polyglutamic acid with a fermentation broth of 2-keto-L-gulonic acid, adjusting the pH of the mixture to 4.0-6.0 with a 0.1 mol / L hydrochloric acid solution, adding a crosslinking agent to the system, mixing thoroughly, and then carrying out a crosslinking reaction at 50-70 °C to obtain the hydrogel; wherein the fermentation broth of 2-keto-L-gulonic acid and the aqueous solution of polyglutamic acid are mixed at a mass ratio of 1:3 to 1:10.

[0008] The 2-keto-L-gulonic acid fermentation broth is an industrially produced fermentation broth containing 10%–15% (mass-volume ratio) of 2-keto-L-gulonic acid, obtained by fermentation using sorbitol as a substrate; or, the uncrystallized residual concentrate containing 10%–25% (mass-volume ratio) of 2-keto-L-gulonic acid obtained by further ultrafiltration, concentration, and crystallization of the above-mentioned 2-keto-L-gulonic acid fermentation broth.

[0009] The 2-keto-L-gulonic acid fermentation broth is an industrially produced fermentation broth that uses sorbitol as a substrate and is converted into 2-keto-L-gulonic acid in a dual-strain fermentation system of common keto-gulonic acid bacillus and Bacillus megaterium; or the uncrystallized concentrate remaining after ultrafiltration, concentration, and crystallization of the 2-keto-L-gulonic acid fermentation broth.

[0010] The final concentration of polyglutamic acid (γ-PGA) in the polyglutamic acid aqueous solution is 10-20% (mass-volume ratio).

[0011] The crosslinking agent is ethylene glycol diglycidyl ether with a purity ≥ 99%; its dosage is 1~6% of the total reaction volume (mass-volume ratio).

[0012] The drying process following the crosslinking reaction is carried out by one of the following methods: vacuum drying, freeze drying, supercritical drying, and atmospheric pressure drying, with a drying temperature not exceeding 50°C.

[0013] The dried hydrogel is pulverized and then passed through a 30-80 mesh sieve. The sieved hydrogel product is obtained.

[0014] Application of a hydrogel containing a slow-release plant biostimulant, wherein the hydrogel is used in slow-release plant biostimulants in poor farmland soil.

[0015] The hydrogel can be mixed with fertilizer or seeds and applied to farmland soil. The application rate of the hydrogel is 0.5 to 5.0 kg per acre.

[0016] The farmland soil mainly refers to sandy loam and slightly saline soil in arid or semi-arid regions.

[0017] Advantages of this invention:

[0018] This invention involves adding polyglutamic acid to the fermentation broth of 2-keto-L-gulonic acid to obtain a hydrogel through a chemical reaction. This effectively improves the slow release and efficient utilization of low molecular weight organic acids, lactones, and other organic substances in the 2-keto-L-gulonic acid fermentation broth, while also retaining water and improving the rhizosphere soil environment. The prepared hydrogel not only allows the effective components in the 2-keto-L-gulonic acid fermentation broth to be released slowly, but also better exerts the effects of its plant biostimulants and the biological effects of water-soluble organic carbon, ultimately achieving the dual functions of water retention and slow release of low molecular weight organic carbon. Detailed Implementation

[0019] To better understand the preparation method and application of the hydrogel containing a sustained-release plant biostimulant according to the present invention, specific embodiments of the present invention are listed below, providing a clear and complete description of the technology and application solutions of the present invention. Obviously, the described embodiments are only some examples of the implementation of the present invention. All other embodiments obtained by those skilled in the art without inventive effort should fall within the scope of protection of the present invention.

[0020] This invention involves adding polyglutamic acid to the fermentation broth of 2-keto-L-gulonic acid and conducting a chemical settling reaction under acidic conditions, with the acidic environment being H2O. + As a catalyst, it can promote the protonation of the carboxyl group (-COOH) on the γ-PGA molecule, forming a carboxylic acid hydrogen ion (-COOH). + This process increases the ionization of the carboxyl group, enhancing reactivity. The protonated carboxylic acid acts as a Lewis acid, attacking the epoxy group of ethylene glycol glycidyl ether, leading to ring-opening of the epoxy ring. After ring-opening, the oxygen atom on the epoxy group becomes negatively charged, forming an alkoxy anion. This alkoxy anion, acting as a nucleophile, attacks the carbonyl carbon atom (C=O) on the γ-PGA molecule, forming a new ester bond (-COO-). Through the ring-opening and ester bond formation of multiple ethylene glycol glycidyl ether molecules, chemical bonds are formed between the γ-PGA molecular chains, constructing a three-dimensional cross-linked network structure. With the formation of this cross-linked network, water molecules and acid radicals from the 2-keto-L-gulonic acid fermentation broth are immobilized within the network, thus forming a stable hydrogel structure.

[0021] In the following examples, the 2-keto-L-gulonic acid fermentation broth was prepared using a mixed-culture two-step fermentation method reported in the literature (Li Qi, Li Bingchao, Lü Shuxia, et al., Improving 2-keto-L-gulonic acid production by controlling ventilation in stages, China Brewing [J], 2012, 31(4): 144-147). The production strain was a mixed strain of common keto-gulonic acid-producing Bacillus and Bacillus megaterium (strain number 2980). Fermentation used two media: a seed culture medium and a fermentation medium. The compositions were as follows: Seed culture medium (g / L): L-sorbose 20 g, urea 1 g, corn steep liquor 5 g, glucose 2 g, calcium carbonate 2 g, brought to a final volume of 1 L; pH adjusted to 6.8–7.0; sterilized at 121°C for 30 min. Fermentation medium (g / L): L-sorbose 80 g, urea 12 g, calcium carbonate 5 g, magnesium sulfate 0.1 g, potassium dihydrogen phosphate 1 g, corn steep liquor 15 g, brought to a final volume of 1 L; pH adjusted to 6.8–7.0; sterilized at 121°C for 30 min. The seed culture conditions were: shaking culture at 29°C and 220 r / min for 18 h to obtain the seed culture. Fermentation process: The seed culture was added to the fermentation medium at an inoculum rate of 13% (v / v). Each shake flask (250ml) contained 20ml of fermentation broth. Fermentation was carried out at 29 ± 1℃ and 220r / min for 40-48 hours to obtain 2-keto-L-gulonic acid fermentation broths of different times and batches. The fermentation broth was then concentrated under reduced pressure to approximately 1 / 2 to 1 / 3 of its original volume using a rotary evaporator at 55–60℃ to obtain the 2-keto-L-gulonic acid fermentation broth (gulonic acid content 15%–25% (m / v)). The gulonic acid content in the 2-keto-L-gulonic acid fermentation broth was determined using the iodometric method.

[0022] Example 1: Experiment A: Preparation of Hydrogels for Sustained-Release Plant Biostimulants

[0023] Weigh 2.0 g of 70 kDa γ-PGA, add 18.0 g of water, stir thoroughly, add 2.14 g of 2-keto-L-gulonic acid fermentation broth (concentration 15%), stir thoroughly to dissolve, adjust the pH to 4.5 with 0.1 mol / L hydrochloric acid, add 0.5 g of ethylene glycol diglycidyl ether crosslinking agent, and let it react at 60℃ for 24 h. After the reaction, a hydrogel is obtained, which is dried in a vacuum freeze dryer for 12 h, and after returning to room temperature, it is pulverized and passed through a 30-mesh sieve to obtain 2.62 g of the finished hydrogel.

[0024] A 0.2 g sample of the hydrogel product obtained above was subjected to a swelling equilibrium test in 1 L of pure water with a pH of 7.00. The water absorption ratio was measured to be 432.6 times, and the pH of the solution decreased to 6.12 after swelling equilibrium. A 0.2 g sample of the hydrogel product obtained above was also subjected to a swelling equilibrium test in 1 L of physiological saline with a pH of 7.00. The saline absorption ratio was measured to be 102.6 times, and the pH of the solution decreased to 6.36 after swelling equilibrium.

[0025] Example 2: Experiment B on the preparation of hydrogels containing sustained-release plant biostimulants

[0026] Weigh 2.5g of 70kDa γ-PGA, add 17.5g of water, stir thoroughly, add 6.56g of 2-keto-L-gulonic acid fermentation broth (10% concentration), stir evenly, then add 0.25g of ethylene glycol diglycidyl ether crosslinking agent, stir thoroughly to dissolve, adjust the pH to 4.3 with 0.1mol / L hydrochloric acid, and let it stand at 65℃ for 24h. The resulting hydrogel was dried in a vacuum freeze dryer for 12h, then pulverized after returning to room temperature, and passed through a 30-mesh sieve to obtain 3.06g of the final hydrogel product.

[0027] After 0.2 g of the hydrogel product obtained above swelled and equilibrated in pure water at pH 7.00, the saline absorption ratio was measured to be 446.2 times, and the pH of the solution decreased to 5.72 after swelling equilibrium. After 0.2 g of the hydrogel product obtained above swelled and equilibrated in 1 L of physiological saline at pH 7.00, the saline absorption ratio was measured to be 131.4 times, and the pH of the solution decreased to 6.27 after swelling equilibrium.

[0028] Example 3: Preparation Experiment of Hydrogel for Sustained-Release Plant Biostimulants C

[0029] Weigh 3.0 g of 70 kDa γ-PGA, add 17.0 g of water, stir thoroughly, add 2.82 g of 2-keto-L-gulonic acid concentrate (20% concentration), stir evenly, then add 1.5 g of ethylene glycol diglycidyl ether crosslinking agent, stir thoroughly to dissolve, adjust the pH to 4.8 with 0.1 mol / L hydrochloric acid, and let it stand at 70℃ for 24 h. After the reaction, a hydrogel is obtained, which is dried in a vacuum freeze dryer for 12 h, then pulverized with a pulverizer after returning to room temperature, and passed through a 30-mesh sieve to obtain 4.12 g of the finished hydrogel.

[0030] Taking 0.2 g of the hydrogel product obtained above, after swelling and equilibration in 1 L of pH 7.00 pure water, the water absorption ratio was measured to be 395.3 times, and the pH of the solution decreased to 6.14 after swelling and equilibration. Taking 0.2 g of the hydrogel product obtained above, after swelling and equilibration in 1 L of pH 7.00 physiological saline, the saline absorption ratio was measured to be 112.5 times, and the pH of the solution decreased to 6.29 after swelling and equilibration.

[0031] Example 4: Preparation Experiment of Hydrogel for Sustained-Release Plant Biostimulants D

[0032] Weigh 4.0 g of 70 kDa γ-PGA, add 16.0 g of water, stir thoroughly, add 3.26 g of the remaining concentrated solution of 2-keto-L-gulonic acid (25% concentration) that has not crystallized, stir evenly, then add 0.75 g of ethylene glycol diglycidyl ether crosslinking agent, stir thoroughly to dissolve, adjust the pH to 5.0 with 0.1 mol / L hydrochloric acid, and react at 65℃ for 24 h. After the reaction, a hydrogel is obtained, which is dried in a vacuum freeze dryer for 12 h, and after returning to room temperature, it is pulverized and passed through a 30-mesh sieve to obtain 5.30 g of the finished hydrogel.

[0033] 0.2 g of the hydrogel product obtained above was swollen and equilibrated in 1 L of pH 7.00 pure water. The saline absorption ratio was measured to be 481.1 times, and the pH of the solution decreased to 5.64 after swelling and equilibration. After swelling and equilibration in 1 L of pH 7.00 physiological saline, the saline absorption ratio was measured to be 142.6 times, and the pH of the solution decreased to 6.03 after swelling and equilibration.

[0034] Application Example 1: Comparative Test of Water Retention Capacity of Hydrogels

[0035] Soil water retention tests were conducted to compare the water retention capacity of different water-retaining agents. The soil used in the tests was brown soil, with the following main physicochemical properties: pH 7.8, electrical conductivity EC 2.6 mS·cm. -1 The cation exchange capacity (CEC) is 17.8 cmol·kg⁻¹. -1 The total carbon content is 12.6 g·kg⁻¹. -1 Total nitrogen is 1.1 g·kg⁻¹ -1 Total phosphorus content is 0.27 g·kg⁻¹. -1 The soil water holding capacity was 27.4%. Five treatments with different water-retaining agents were set up: ultrapure water treatment (CK), 2-keto-L-gulonic acid-γ-PGA hydrogel treatment (RP), 2-keto-L-gulonic acid treatment (K, pure 2-keto-L-gulonic acid with a purity greater than 99%), γ-PGA hydrogel treatment (P, γ-PGA hydrogel purchased from the agricultural input market), and polyacrylic acid water-retaining agent treatment (PAA, polyacrylic acid water-retaining agent purchased from the agricultural input market). The hydrogel used in the RP treatment was the finished hydrogel prepared according to the method described in Example 2. Except for the control group (CK), the K treatment added pure 2-keto-L-gulonic acid (0.1g of pure 2-keto-L-gulonic acid per 100g of air-dried soil), and the other three treatments added the corresponding water-retaining agent (0.1g of water-retaining agent per 100g of air-dried soil). After addition, the soil in each treatment was thoroughly mixed with the additive.

[0036] The thoroughly mixed soil samples from each treatment were placed in square plastic containers (8 cm high × 7.5 cm long × 7.0 cm wide). Three 0.1 kg portions of the mixed soil were weighed from each treatment and placed into three separate containers, resulting in three replicates per treatment. Five treatments were performed with three replicates, for a total of 15 containers. 25 ml of deionized water was added to each container, and all containers were incubated at 25°C for 7 days. The soil water retention capacity under different treatments was monitored and measured over these 7 days.

[0037] Table 1. Effects of various treatments on soil water retention capacity (unit: g)

[0038]

[0039] Note: Values ​​represent the total weight of soil and soil moisture after deducting the container weight, expressed as mean ± standard deviation. Different lowercase letters indicate statistically significant differences between treatments (one-way ANOVA). p < 0.05)

[0040] As shown in Table 1, compared with the control (CK) and conventional γ-PGA hydrogel treatment (P), 2-keto-L-gulonic acid-γ-PGA hydrogel treatment (RP) significantly improved soil water retention capacity in the range of 1–7 days. p < 0.05), and remained significantly different throughout the experiment. Notably, the soil moisture content of the treatment with 2-keto-L-gulonic acid alone (K) was not significantly different from that of the control (CK) at any time point, indicating that it did not have a significant water-retention effect. On day 7, the differences among the treatments widened further: the soil moisture content of the RP treatment still exceeded 10%, significantly higher than all other treatments. Compared with CK, K, P, and PAA, the soil moisture content of the RP treatment increased by 10.4%, 10.6%, 8.7%, and 3.5%, respectively, and all differences were statistically significant. p < 0.05). The above results collectively indicate that the water retention capacity of the hydrogel (RP) prepared in this invention is significantly higher than that of commercially available polyglutamic acid hydrogels and PAA hydrogels, and it has a superior water retention capacity.

[0041] Application Example 2: Experiment on the effects of hydrogels on the growth, biomass, and quality of Chinese cabbage.

[0042] Using the soil from Application Example 1, and following the same mixing ratio of hydrogel (or water-retaining agent) to soil as in the experimental protocol of Application Example 1, the settings for each treatment were also the same as in Application Example 1. The effects of different hydrogels (or water-retaining agents) on the growth, biomass, and quality of pakchoi were investigated through pot experiments.

[0043] Specific experimental procedure: 0.6 kg of mixed soil was placed in cylindrical plastic pots (15 cm high × 20 cm top diameter × 15 cm bottom diameter), and Chinese cabbage was planted. Five treatments were set up: ultrapure water treatment (CK), 2-keto-L-gulonic acid-γ-PGA hydrogel treatment (RP), 2-keto-L-gulonic acid treatment (K), γ-PGA hydrogel treatment (P), and polyacrylic acid water-retaining agent treatment (PAA). Each pot contained 40 seeds, and each treatment was repeated 4 times. Germination was conducted in a culture room at 25℃ with 8 h of light and 16 h of darkness, and a light intensity of 750 μmol / m³. -2 On the 7th day after sowing, the number of germinations and the germination rate were counted. The experiment was set up with 4 replicates. The standard for germination of pak choi seeds was that they grew into normal seedlings after 7 days of germination. On the 28th day after sowing, the fresh weight, dry weight, leaf area, and quality indicators (crude protein, soluble sugar, vitamin C) and stress resistance indicators (proline) of pak choi under different treatments were measured (see Table 2-4).

[0044] Table 2 Effects of various treatments on the germination of Chinese cabbage seeds

[0045]

[0046] Note: Different lowercase letters indicate statistically significant differences between treatments (one-way ANOVA). p <0.05)

[0047] Table 3. Effects of each treatment on fresh weight, dry weight, and leaf area of ​​Chinese cabbage.

[0048]

[0049] Note: Results were obtained from samples taken on day 28 after the bok choy sprouted. Different lowercase letters indicate statistically significant differences between treatments (one-way ANOVA). p < 0.05)

[0050] Table 4 Effects of different treatments on stress resistance and quality indicators of Chinese cabbage

[0051]

[0052] Note: Results were obtained from samples taken on day 28 after the bok choy sprouted. Different lowercase letters indicate statistically significant differences between treatments (one-way ANOVA). p < 0.05)

[0053] According to the results in Table 2, compared with CK, P, K, and PAA treatments, the 2-keto-L-gulonic acid-γ-PGA hydrogel (RP) prepared in this embodiment of the invention can significantly improve the germination rate of pak choi. p < 0.05). The germination rate of RP was 62% higher than that of CK, 53% higher than that of P, and approximately 10% higher than that of PAA (81.88%), making it the highest among all treatments. In contrast, the germination rate of treatment with 2-keto-L-gulonic acid (K) alone was not significantly different from that of CK and P. p The value > 0.05 indicates that the promoting effect of 2-keto-L-gulonic acid on seed germination can only be fully realized when it forms a hydrogel with polyglutamic acid.

[0054] The data in Table 3 further show that the RP treatment was the most significant in promoting the early biomass of pak choi. p <0.05). The dry weight of RP increased by 191% and 52% compared to P and PAA, respectively, and the leaf area increased by 102%, 16%, and 74% compared to CK, P, and PAA, respectively. K treatment showed a moderate promoting effect in the early stages, with a slightly higher dry weight (0.28 g) than CK and a significantly higher leaf area (7.02 cm²) than CK and PAA, but significantly weaker than RP. This indicates that although 2-keto-L-gulonic acid can promote leaf expansion and some dry matter accumulation, the lack of a stable moisture environment provided by hydrogels limits its effectiveness.

[0055] According to Table 4, RP treatment significantly increased the content of soluble sugars, proteins, and vitamin C, with vitamin C increasing by 173%, 47%, and 67% compared to CK, P, and PAA, respectively. K treatment also improved quality indicators, particularly vitamin C (447.15 μg / g), which was 106% higher than CK, but still significantly lower than RP. p < 0.05). In terms of stress resistance, the proline content of group K was significantly higher than that of CK and PAA, indicating that it has a certain osmotic regulation promoting effect, but its effect is much weaker than that of RP (16.70 μg / g).

[0056] In summary, the hydrogel with slow-release plant biostimulant function described in this invention exhibits significant adsorption performance for soil moisture, demonstrating excellent water storage and retention capacity. Furthermore, by releasing small-molecule organic compounds with plant biostimulant function, such as 2-keto-L-gulonic acid, into the soil microenvironment, this hydrogel not only performs its water-retention function but also effectively increases the content of readily available small-molecule carbon in the soil, optimizes the supply of organic carbon nutrients in the rhizosphere soil, and promotes the synergistic effect of carbon, nitrogen, water, and fertilizer, thereby effectively promoting crop growth, yield increase, and vitamin C accumulation. This novel hydrogel technology and product with slow-release plant biostimulant function described in this invention has promising application prospects and value in the agricultural and environmental fields, contributing to the promotion of green and sustainable agricultural development.

Claims

1. A hydrogel containing a sustained-release plant biostimulant, characterized in that: The hydrogel is obtained by crosslinking polyglutamic acid aqueous solution and 2-keto-L-gulonic acid fermentation broth; wherein the ratio of 2-keto-L-gulonic acid fermentation broth to polyglutamic acid aqueous solution is in the range of 1:3 to 1:10 by mass; and the molecular weight of polyglutamic acid is in the range of 50 kDa to 200 kDa.

2. A method for preparing a hydrogel containing a sustained-release plant biostimulant as described in claim 1, characterized in that: A polyglutamic acid aqueous solution was mixed with 2-keto-L-gulonic acid fermentation broth, and the pH of the mixture was adjusted to 4.0-5.0 with 0.1 mol / L hydrochloric acid solution. Then, a cross-linking agent was added to the system, and after mixing, a cross-linking reaction was carried out at 50℃-70℃ to obtain a hydrogel. The 2-keto-L-gulonic acid fermentation broth and polyglutamic acid aqueous solution were mixed at a mass ratio of 1:3 to 1:

10.

3. The method for preparing the hydrogel of sustained-release plant biostimulants according to claim 2, characterized in that: The 2-keto-L-gulonic acid fermentation broth is an industrially produced fermentation broth containing 10%–15% 2-keto-L-gulonic acid by fermentation using sorbitol as a substrate; or, the remaining uncrystallized concentrate containing 10%–25% 2-keto-L-gulonic acid after the above 2-keto-L-gulonic acid fermentation broth has undergone ultrafiltration, concentration, and crystallization.

4. The method for preparing the hydrogel of sustained-release plant biostimulants according to claim 3, characterized in that: The 2-keto-L-gulonic acid fermentation broth is the industrially produced fermentation broth that uses sorbitol as a substrate and is converted into 2-keto-L-gulonic acid in a dual-strain fermentation system of common keto-gulonic acid bacillus and Bacillus megaterium; or the uncrystallized concentrate remaining after the above-mentioned 2-keto-L-gulonic acid fermentation broth has undergone ultrafiltration, concentration, and crystallization.

5. The method for preparing the hydrogel of sustained-release plant biostimulants according to claim 2, characterized in that: The final concentration of polyglutamic acid in the polyglutamic acid aqueous solution is 10%~20% by mass-volume ratio.

6. The method for preparing the hydrogel of sustained-release plant biostimulants according to claim 2, characterized in that: The crosslinking agent is ethylene glycol diglycidyl ether, and its dosage is 1% to 6% of the total reaction volume by mass.

7. The application of a hydrogel containing a sustained-release plant biostimulant as described in claim 1, characterized in that: The application of the hydrogel in slow-release plant biostimulants for poor farmland soil.

8. The application of the hydrogel containing the sustained-release plant biostimulant according to claim 7, characterized in that: The hydrogel can be mixed with fertilizer or seeds and applied to barren farmland soil. The application rate of the hydrogel is 0.5 to 5.0 kg per acre.

9. The application of the hydrogel containing the sustained-release plant biostimulant according to claim 7 or 8, characterized in that: The farmland soil mainly refers to sandy loam soil in arid or semi-arid regions.

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