Long-acting slow-release compound fertilizer containing polyglutamic acid and preparation method thereof

Abstract

This invention relates to the technical field of compound fertilizer production and preparation, and particularly to a long-acting slow-release compound fertilizer containing polyglutamic acid and its preparation method. By weight, it comprises the following components: 30-45 parts urea, 20-35 parts monoammonium phosphate, 15-30 parts potassium sulfate, 5-15 parts ammonium chloride, 0.5-2 parts γ-polyglutamic acid, 3-8 parts composite modified biochar, 1-3 parts humic acid powder, 2-5 parts magnesium sulfate, and 1.1-4.3 parts a mixture of trace elements. The composite modified biochar uses porous biochar as a carrier, with polyaspartic acid covalently grafted onto its surface and adsorbing fermentation broth containing γ-polyglutamic acid. This invention achieves multiple effects, including controlled nutrient release, effective retention of bioactive components, and continuous soil improvement, effectively overcoming the core defects of traditional compound fertilizers such as short fertilizer effect, high nitrogen loss rate, and inactivation of bioactive components during granulation.

Description

Technical Field

[0001] This invention relates to the technical field of compound fertilizer production and preparation, and in particular to a long-acting slow-release compound fertilizer containing polyglutamic acid and its preparation method. Background Technology

[0002] Compound fertilizers are an indispensable nutrient input in modern agriculture, providing crops with nitrogen, phosphorus, potassium, and trace elements simultaneously. They offer significant advantages such as comprehensive nutrient coverage and ease of use. High-tower granulation technology, with its advantages of high granulation rate, good particle strength, and adaptability to complex formulations, has become one of the mainstream compound fertilizer production methods in China. Its core process involves melting solid raw materials into a uniform slurry in a melting tank at 130–160°C, then spraying the slurry through granulation nozzles at the top of a high tower. The slurry droplets solidify into spherical particles during free fall, followed by cooling, sieving, and anti-caking coating treatment to finally obtain the finished compound fertilizer granules.

[0003] However, existing high-tower granulation compound fertilizers face several prominent problems in practical applications. Firstly, the nutrient release rate is too rapid. Shortly after application, a large amount of nitrogen is leached away in the form of nitrate or ammonium nitrogen with irrigation water or rainfall. In rainy southern regions, the nitrogen loss rate can exceed 40%, and nitrogen fertilizer utilization is generally below 35%, resulting in resource waste and ecological risks such as eutrophication. Secondly, the activity of bioactive substances such as γ-polyglutamic acid (γ-PGA) is significantly damaged during high-temperature granulation (above 130℃). Current practices of directly mixing γ-PGA with molten slurry lack effective activity protection mechanisms, preventing the full realization of its root-promoting, water-retaining, and nutrient-chelating effects. Thirdly, existing slow-release compound fertilizers typically use resin or sulfur coatings for slow release. These coating materials are expensive, and the coating is difficult to degrade after breakage, posing a significant risk of soil residue. Furthermore, long-term continuous application may exacerbate soil compaction, adversely affecting soil physical properties and the health of the microbial community.

[0004] Chinese patent CN114276181A discloses a multifunctional fertilizer synergist composed of poly-γ-glutamic acid of different molecular weights. This synergist mixes poly-γ-glutamic acid of four molecular weight specifications—low molecular weight (1–10 kDa), medium molecular weight (10–300 kDa), high molecular weight (300–1500 kDa), and ultra-high molecular weight (1500–8000 kDa)—in a specific ratio, and combines it with γ-aminobutyric acid, urease inhibitor NBPT, and nitrification inhibitor DCD. By utilizing the difference in degradation rate of poly-γ-glutamic acid of different molecular weights in the soil, it achieves both short-term and long-term slow release of nutrients. At the same time, it utilizes the strong adsorption and chelation ability of poly-γ-glutamic acid to inhibit the chemical precipitation reaction between anions and cations in the soil, thereby improving nutrient utilization. Field verification results show that this synergist can increase corn yield by 12.7% when used in compound fertilizer. However, the γ-PGA involved in this technical solution is only combined with fertilizer through physical mixing or surface spraying, lacking a stable functional slow-release carrier structure. Under soil irrigation or rainfall leaching conditions, γ-PGA is rapidly released and lost along with soluble nutrients, resulting in a short fertilizer effect duration and low nitrogen fertilizer utilization rate. At the same time, this synergist does not solve the problem of protecting the activity of γ-PGA during the high-temperature process of high-tower granulation in the integrated preparation of polyglutamic acid functional components and compound fertilizer basic formula. Summary of the Invention

[0005] In view of this, the present invention proposes a long-acting slow-release compound fertilizer containing polyglutamic acid and its preparation method, in order to solve the technical problems of insufficient fertilizer efficiency caused by the easy inactivation of bioactive components due to high temperature and insufficient nutrient release regulation capacity in the existing compound fertilizer production process.

[0006] The technical solution of this invention is implemented as follows: This invention provides a long-acting slow-release compound fertilizer containing polyglutamic acid, comprising the following components by weight: 30-45 parts urea, 20-35 parts monoammonium phosphate, 15-30 parts potassium sulfate, 5-15 parts ammonium chloride, 0.5-2 parts γ-polyglutamic acid, 3-8 parts composite modified biochar, 1-3 parts humic acid powder, 2-5 parts magnesium sulfate, and 1.1-4.3 parts trace element mixture; wherein the composite modified biochar uses porous biochar as a carrier, with polyaspartic acid covalently grafted onto its surface and adsorbing fermentation broth containing γ-polyglutamic acid.

[0007] Specifically, the long-acting slow-release compound fertilizer of this invention uses urea, monoammonium phosphate, potassium sulfate, and ammonium chloride as the basic sources of nitrogen, phosphorus, and potassium macronutrients, supplemented with magnesium sulfate to supplement a mixture of medium and micronutrients (ferrous sulfate, zinc sulfate, manganese sulfate, and borax) to meet the nutritional needs of crops for iron, zinc, manganese, and boron. Humic acid powder, with its rich active functional groups, chelates and retains soil nutrients and improves soil aggregate structure. Exogenous γ-polyglutamic acid plays a role in promoting root growth, water retention, and nutrient chelation and efficiency enhancement. Composite modified biochar serves as the core functional carrier, constructing a dual nutrient slow-release mechanism of "pore retention-chemical adsorption" through the physical interception of its porous structure and the chemical adsorption of its surface carboxyl groups. Combined with the physical barrier of the low-melting-point paraffin outer membrane, it synergistically achieves long-term stable release of nitrogen in the soil, promotes nitrogen fertilizer utilization, and simultaneously takes into account the health of the soil microbial community and the long-term improvement of soil physical properties, thus promoting crop growth.

[0008] Based on the above technical solutions, preferably, the preparation method of the composite modified biochar includes the following steps: S1. Pre-treat wheat gluten powder, then disperse the pre-treated wheat gluten powder in distilled water, add citric acid and glycerol, stir evenly, inoculate with microbial bacteria, and anaerobic ferment at 35-37℃ for 48-72 hours. After the fermentation culture is completed, sterilize, filter, and obtain fermentation broth. S2. The biochar is calcined at high temperature to obtain porous biochar; the porous biochar is dispersed in an ethanol aqueous solution, 3-aminopropyltriethoxysilane is added, the pH of the system is adjusted to 6.5-8.0, and the reaction is stirred at 78-85℃ for 4-6 hours to obtain aminated biochar. S3. Dissolve polyaspartic acid in MES buffer solution with pH 5.0-6.0, add EDC and NHS, stir and activate at 20-30℃ for 30-60 min, then add aminated biochar, stir and react at 20-30℃ for 2-4 h to obtain modified biochar. S4. Disperse the modified biochar in the fermentation broth, adjust the pH of the system to 6.0-7.0, and shake and adsorb at 25-35℃ for 4-8 hours. After adsorption is completed, filter, wash and dry to obtain composite modified biochar.

[0009] Based on the above technical solutions, preferably, in step S1, the pretreatment of wheat gluten powder includes: dispersing wheat gluten powder in water to prepare a dispersion with a mass fraction of 6% to 8%, adding protease, and pre-hydrolyzing at 50 to 55°C for 2 to 4 hours.

[0010] Specifically, in step S1, wheat gluten powder is used as a substrate and is first pre-hydrolyzed with protease to break down the protein chains into free glutamic acid and small functional peptides, providing sufficient precursor substances for subsequent bacterial strains. Citric acid and glycerol are used as auxiliary carbon sources to activate the γ-PGA synthase activity in Bacillus licheniformis, promoting its enzymatic polymerization to generate γ-PGA using free glutamic acid as a substrate. Simultaneously, microbial bacteria are inoculated for fermentation, and the final fermentation broth is rich in endogenous γ-PGA, functional peptides, extracellular polysaccharides, and free amino acids, among other bioactive metabolites. In step S2, porous biochar undergoes a silylation reaction with 3-aminopropyltriethoxysilane, anchoring the amino-containing tricarbon aminopropyl segments onto the surface of the biochar, giving it uniformly distributed active amino sites. In step S3, the activated carboxyl groups of the polyaspartic acid (PASP) side chain react with the amino groups on the surface of aminated biochar via an amidation reaction to form stable covalent amide bonds, thus grafting PASP segments onto the biochar surface. After grafting, the remaining carboxyl groups on the PASP segments provide hydrogen bonding and electrostatic interaction sites for the adsorption loading of active substances in the fermentation broth, and can also interact with Ca in the soil. 2+ Mg 2+ High-valent cation chelation inhibits clay particle bridging and compaction, while also endowing the carrier with certain hydrophilic water-retention functions. In step S4, under suitable temperature and shaking conditions, functional peptides, extracellular polysaccharides, free amino acids, and endogenous γ-PGA in the fermentation broth are stably loaded onto the biochar pores and surface through the physical retention of the porous biochar channels, as well as multiple non-covalent interactions such as hydrogen bonds formed between PASP free carboxyl groups and –NH and –OH groups on the active metabolite peptide chains, and electrostatic attraction between positively charged functional peptides and negatively charged PASP carboxyl groups. The resulting composite modified biochar combines the physical slow-release function of the porous carrier, the chemical adsorption slow-release function of the PASP covalent graft layer, and the bio-enhancing function of the loaded active metabolites. After being applied to the soil, it is slowly released along with nutrients, achieving a long-term and continuous supply of active substances such as γ-PGA and functional peptides. It works synergistically with exogenously supplemented γ-PGA to exert multiple effects such as root promotion and water retention, nutrient chelation, and improvement of soil microecology.

[0011] Based on the above technical solutions, preferably, in step S1, the amount of protease added is 0.5% to 1.0% of the mass of wheat gluten, the amount of citric acid added is 5 to 10 g / L, the amount of glycerol added is 20 to 30 g / L, and the amount of microbial inoculation is 2% to 5% (by volume). The microbial bacteria include Bacillus amyloliquefaciens and Bacillus licheniformis, and the inoculation volume ratio of Bacillus amyloliquefaciens to Bacillus licheniformis is 1:1 to 1:2.

[0012] Bacillus licheniformis is a high-yield strain of γ-PGA, capable of synthesizing and secreting γ-PGA via enzymatic polymerization using free glutamate in the culture medium as a direct precursor substrate. Bacillus amyloliquefaciens, on the other hand, leverages its strong protease secretion capacity to continuously hydrolyze residual macromolecules of glutamate in the culture medium during fermentation, dynamically replenishing free glutamate and functional peptides, thus maintaining a continuous and sufficient supply of glutamate precursors in the fermentation system. Simultaneously, both strains secrete extracellular polysaccharides during fermentation, which, together with functional peptides, free amino acids, and endogenous γ-PGA, constitute a multi-functional metabolite complex in the fermentation broth.

[0013] Based on the above technical solutions, the preferred method for preparing porous biochar in step S2 includes: activating the biochar raw material by impregnation in potassium hydroxide solution, and then calcining it at 700-900℃ for 2-4 hours under a nitrogen protective atmosphere.

[0014] By impregnating and activating biochar with KOH and then calcining it at high temperature in an inert atmosphere, KOH undergoes an etching reaction with the carbon skeleton at high temperature, creating a large number of pores and forming a highly developed microporous / mesoporous structure, which significantly increases the specific surface area and pore volume, and is beneficial for the subsequent physical retention and slow release of active substances.

[0015] Based on the above technical solutions, preferably, in step S2, the amount of 3-aminopropyltriethoxysilane added is 5% to 10% of the mass of porous biochar.

[0016] Based on the above technical solutions, preferably, in step S3, the mass ratio of aminated biochar, polyaspartic acid, EDC and NHS is 1:(0.5~1.5):(2~6):(1~4).

[0017] Based on the above technical solutions, preferably, in step S4, the solid-liquid ratio of biochar to fermentation broth is 1:10 to 1:20 (g / mL).

[0018] Based on the above technical solutions, the preferred trace element mixture includes 0.5 to 2 parts ferrous sulfate, 0.3 to 1 part zinc sulfate, 0.2 to 0.8 parts manganese sulfate, and 0.1 to 0.5 parts borax.

[0019] This invention also provides a method for preparing a long-acting slow-release compound fertilizer containing polyglutamic acid, comprising the following steps: Urea, monoammonium phosphate, and ammonium chloride are added to a melting tank according to the formula ratio and heated at 130-145℃ to fully melt them into a uniform slurry. Potassium sulfate and magnesium sulfate are added and stirred evenly. Humic acid powder is added and stirred to mix. The moisture content of the slurry is controlled at 1%-3%. After spray granulation, basic compound fertilizer granules are obtained. The basic compound fertilizer granules, ammoniation pretreated trace element mixture, γ-polyglutamic acid and composite modified biochar are thoroughly mixed at a low temperature of 50-70℃ for 15-20 minutes, cooled to 45-55℃, sprayed with molten low-melting-point paraffin, rolled and coated, cooled and shaped, and then screened by a vibrating screen and treated to prevent agglomeration to obtain the finished product of long-acting slow-release compound fertilizer granules containing polyglutamic acid.

[0020] The long-acting slow-release compound fertilizer containing polyglutamic acid and its preparation method of the present invention have the following advantages over the prior art: (1) This invention provides a long-acting slow-release compound fertilizer. By using composite modified biochar as a functional structural carrier, it works synergistically with exogenous γ-polyglutamic acid, humic acid and a variety of trace elements to achieve multiple effects such as slow-release of nutrients, effective retention of bioactive components and continuous improvement of soil. It effectively overcomes the core defects of traditional compound fertilizers such as short fertilizer effect, high nitrogen loss rate and inactivation of bioactive components during granulation.

[0021] (2) This invention constructs a triple synergistic slow-release structure of "physical retention of pores - chemical adsorption of PASP carboxyl groups - physical barrier of paraffin film" by grafting polyaspartic acid onto the surface of porous biochar with covalent amide bonds. Compared with the slow-release method of relying solely on physical coating or surface chelation in the prior art, the covalently grafted PASP segments are not easily detached in the soil environment. Its dense free carboxyl groups can form a continuous chemical adsorption to retain water-soluble nitrogen. Combined with the physical retention of porous biochar pores and the hydrophobic barrier of the low-melting-point paraffin film, the release rate of nutrients into the soil solution is significantly slowed down, effectively reducing nitrogen leaching loss under rainy conditions and improving nitrogen fertilizer utilization.

[0022] (3) The present invention adopts a staged temperature control process, which postpones the addition of γ-polyglutamic acid and composite modified biochar to the low temperature stage, thus fundamentally avoiding the damage to bioactive components caused by the high temperature of high tower granulation. At the same time, the microbial endogenous γ-PGA, functional peptides, extracellular polysaccharides and other active metabolites in the fermentation broth are stably loaded into the PASP grafted biochar carrier through hydrogen bonding and electrostatic interaction. They are slowly released into the soil along with the carrier and work synergistically with the exogenously supplemented γ-PGA to promote root growth, retain water and chelate nutrients, thus achieving a long-term and continuous supply of bioactive substances. Detailed Implementation

[0023] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] It should be noted that, in this invention, the wheat gluten powder was purchased from Henan Zaohua Flour Industry Co., Ltd., and is food-grade wheat gluten powder with a protein content ≥80%. The Bacillus amyloliquefaciens was purchased from Chaoyang Huaxing Bioengineering Co., Ltd., with a bacterial count ≥10. 9 CFU / g (lyophilized powder), Bacillus licheniformis purchased from Weifang Yihao Biotechnology Co., Ltd., bacterial count ≥10. 9 CFU / g (lyophilized powder), biochar is prepared from agricultural waste (corn stalks, rice husks, sawdust, etc.), polyaspartic acid is purchased from Shandong Taihe Technology Co., Ltd., high-purity water-soluble powder, content ≥98%, relative molecular mass 1000~5000Da.

[0025] Example 1 This embodiment provides a long-acting slow-release compound fertilizer containing polyglutamic acid and its preparation method. The compound fertilizer includes the following components (each part is 1 kg in this embodiment): 38 parts urea, 28 parts monoammonium phosphate, 22 parts potassium sulfate, 8 parts ammonium chloride, 1.2 parts γ-polyglutamic acid, 6 parts composite modified biochar, 2 parts humic acid powder, 3.5 parts magnesium sulfate, 1.2 parts ferrous sulfate, 0.6 parts zinc sulfate, 0.5 parts manganese sulfate, and 0.3 parts borax. The preparation methods of composite modified biochar include: S1. Disperse wheat gluten in water to prepare a dispersion with a mass fraction of 7%. Add 0.8% of the mass of wheat gluten protease and pre-hydrolyze at 53℃ for 3 hours. After pre-hydrolysis, sterilize at 120℃ for 30 minutes and cool to obtain pretreated wheat gluten. Then, the pretreated wheat gluten powder was dispersed in distilled water, and 8 g / L citric acid and 25 g / L glycerol were added and stirred evenly. Bacillus amyloliquefaciens and Bacillus licheniformis were inoculated at an inoculation rate of 3.5% (volume ratio), with the inoculation volume ratio adjusted to 1:1.5. Anaerobic fermentation was carried out at 36℃ and 200 rpm for 60 h. After fermentation, the mixture was sterilized at 120℃ for 30 min, cooled to room temperature, and filtered through a 0.22 μm microfiltration membrane to remove the bacterial cells, thus obtaining the fermentation broth. S2. Disperse the biochar raw material in KOH solution (prepared as a 25% aqueous solution by mass), with the amount of KOH solution being 9 times the mass of the biochar. After thorough impregnation and stirring, let it stand for 21 hours, dry it at 105℃, and then calcine and activate it at 900℃ for 3 hours under a nitrogen protective atmosphere. After activation, wash it thoroughly with deionized water until the pH of the washing solution is neutral, and dry it to obtain porous biochar. 100g of porous biochar was dispersed in 500ml of ethanol aqueous solution (volume ratio 4:1), 8g of 3-aminopropyltriethoxysilane was added, the mixture was stirred for 30min, the pH of the system was adjusted to 7.0, and the reaction was stirred at 80℃ for 5h. After the reaction was completed, the mixture was filtered, washed 3 times with ethanol, and dried under vacuum at 60℃ to obtain aminated biochar. S3. Dissolve 100g of polyaspartic acid in MES buffer (0.1 mol / L, 2-morpholinoethanesulfonic acid) at pH 5.5 to prepare a 3.5% polyaspartic acid solution. Add 400g of EDC and 250g of NHS, and stir at 25℃ for 45min to activate. Then add 100g of aminated biochar and stir at 25℃ for 3h. After the reaction is complete, wash thoroughly with deionized water 4 times and dry under vacuum at 50℃ to obtain modified biochar. S4. Disperse 100g of modified biochar in 1500ml of fermentation broth, adjust the pH of the system to 6.5, and shake and adsorb at 30℃ for 6h. After adsorption is complete, filter, wash once with a small amount of deionized water, and dry under vacuum at 50℃ to obtain composite modified biochar.

[0026] Ferrous sulfate, zinc sulfate, manganese sulfate, and borax are mixed in the above formula ratio. Dilute ammonia water with a mass fraction of 10% is sprayed evenly into the mixture for ammoniation treatment. The ammoniation temperature is controlled at 50℃ and the ammoniation treatment time is 45min. After the ammoniation pretreatment is completed, the water content of the mixture is controlled to not exceed 1% to obtain an ammoniation pretreated trace element mixture. Urea, monoammonium phosphate, and ammonium chloride are added to a melting tank according to the formula ratio and heated at 140°C to fully melt them into a uniform slurry. Potassium sulfate and magnesium sulfate are added and stirred evenly. Humic acid powder is added and stirred for 10 minutes. The slurry temperature is controlled at 140°C and the slurry moisture content is controlled at 2%. After spray granulation, the particle size is controlled at 2-4 mm to obtain basic compound fertilizer granules. The basic compound fertilizer granules were cooled to 60°C, and then ammoniation pretreated trace element mixture, γ-polyglutamic acid and composite modified biochar were added in sequence. The mixture was thoroughly mixed at 60°C for 20 minutes, cooled to 50°C, and molten low-melting-point paraffin wax was sprayed in at a rate of 0.8% of the total weight of the granules. The mixture was then rolled and coated, cooled and shaped, and then sieved by a vibrating screen and treated to prevent caking, thus obtaining long-acting slow-release compound fertilizer granules.

[0027] Example 2 This embodiment provides a long-acting slow-release compound fertilizer containing polyglutamic acid and its preparation method. The compound fertilizer includes the following components (each part is 1 kg in this embodiment): 30 parts urea, 20 parts monoammonium phosphate, 15 parts potassium sulfate, 5 parts ammonium chloride, 0.5 parts γ-polyglutamic acid, 3 parts composite modified biochar, 1 part humic acid powder, 2 parts magnesium sulfate, 0.5 parts ferrous sulfate, 0.3 parts zinc sulfate, 0.2 parts manganese sulfate, and 0.1 parts borax. The preparation methods of composite modified biochar include: S1. Disperse wheat gluten in water to prepare a dispersion with a mass fraction of 6%. Add 0.5% of the mass of wheat gluten protease and pre-hydrolyze at 50℃ for 4 hours. After pre-hydrolysis, sterilize at 120℃ for 30 minutes and cool to obtain pretreated wheat gluten. Then, the pretreated wheat gluten powder was dispersed in distilled water, and 5 g / L citric acid and 20 g / L glycerol were added and stirred evenly. Bacillus amyloliquefaciens and Bacillus licheniformis were inoculated at an inoculation rate of 2% (volume ratio) respectively, and the inoculation volume ratio of the two was adjusted to 1:1. Anaerobic fermentation was carried out at 35℃ and 200 rpm for 72 h. After fermentation, the mixture was sterilized at 120℃ for 30 min, cooled to room temperature, and filtered through a 0.22 μm microfiltration membrane to remove the bacterial cells, thus obtaining the fermentation broth. S2. Disperse the biochar raw material in KOH solution (prepared as a 25% aqueous solution by mass), with the amount of KOH solution being 8 times the mass of the biochar. After thorough impregnation and stirring, let it stand for 18 hours, dry it at 105℃, and then calcine and activate it at 700℃ for 4 hours under a nitrogen protective atmosphere. After activation, wash it thoroughly with deionized water until the pH of the washing solution is neutral, and dry it to obtain porous biochar. 100g of porous biochar was dispersed in 500ml of ethanol aqueous solution (volume ratio 4:1), 5g of 3-aminopropyltriethoxysilane was added, the mixture was stirred for 30min, the pH of the system was adjusted to 6.5, and the reaction was stirred at 78℃ for 6h. After the reaction was completed, the mixture was filtered, washed 3 times with ethanol, and dried under vacuum at 60℃ to obtain aminated biochar. S3. Dissolve 50g of polyaspartic acid in MES buffer (0.1 mol / L, 2-morpholinoethanesulfonic acid) at pH 5.0 to prepare a 2% polyaspartic acid solution. Add 200g of EDC and 100g of NHS, and stir to activate at 20℃ for 60min. Then add 100g of aminated biochar and stir to react at 20℃ for 4h. After the reaction is complete, wash thoroughly with deionized water 4 times and dry under vacuum at 50℃ to obtain modified biochar. S4. Disperse 100g of modified biochar in 1000ml of fermentation broth, adjust the pH of the system to 6.0, and shake and adsorb at 25℃ for 8h. After adsorption is complete, filter, wash once with a small amount of deionized water, and dry under vacuum at 45℃ to obtain composite modified biochar.

[0028] Ferrous sulfate, zinc sulfate, manganese sulfate, and borax are mixed in the above formula ratio. Dilute ammonia water with a mass fraction of 10% is sprayed evenly into the mixture for ammoniation treatment. The ammoniation temperature is controlled at 40℃ and the ammoniation treatment time is 60min. After the ammoniation pretreatment is completed, the water content of the mixture is controlled to not exceed 1% to obtain an ammoniation pretreated trace element mixture. Urea, monoammonium phosphate, and ammonium chloride are added to a melting tank according to the formula ratio and heated at 130°C to fully melt them into a uniform slurry. Potassium sulfate and magnesium sulfate are added and stirred evenly. Humic acid powder is added and stirred for 10 minutes. The slurry temperature is controlled at 130°C and the slurry moisture content is controlled at 1%. After spray granulation, the particle size is controlled at 2-4 mm to obtain basic compound fertilizer granules. The basic compound fertilizer granules were cooled to 50°C, and then ammoniation pretreated trace element mixture, γ-polyglutamic acid and composite modified biochar were added in sequence. The mixture was thoroughly mixed at 50°C for 20 minutes, cooled to 45°C, and molten low-melting-point paraffin was sprayed in at a rate of 0.5% of the total weight of the granules. The mixture was then rolled and coated, cooled and shaped, and then sieved by a vibrating screen and treated to prevent caking, to obtain long-acting slow-release compound fertilizer granules.

[0029] Example 3 This embodiment provides a long-acting slow-release compound fertilizer containing polyglutamic acid and its preparation method. The compound fertilizer includes the following components (each part is 1 kg in this embodiment): 45 parts urea, 35 parts monoammonium phosphate, 30 parts potassium sulfate, 15 parts ammonium chloride, 2 parts γ-polyglutamic acid, 8 parts composite modified biochar, 3 parts humic acid powder, 5 parts magnesium sulfate, 2 parts ferrous sulfate, 1 part zinc sulfate, 0.8 parts manganese sulfate, and 0.5 parts borax. The preparation methods of composite modified biochar include: S1. Disperse wheat gluten in water to prepare a dispersion with a mass fraction of 8%. Add 1.0% of the mass of wheat gluten protease and pre-hydrolyze at 55℃ for 2 hours. After pre-hydrolysis, sterilize at 120℃ for 30 minutes and cool to obtain pretreated wheat gluten. Then, the pretreated wheat gluten powder was dispersed in distilled water, and 10 g / L citric acid and 30 g / L glycerol were added and stirred evenly. Bacillus amyloliquefaciens and Bacillus licheniformis were inoculated at an inoculation rate of 5% (volume ratio) respectively, with the inoculation volume ratio adjusted to 1:2. Anaerobic fermentation was carried out at 37℃ and 200 rpm for 48 h. After fermentation, the mixture was sterilized at 120℃ for 30 min, cooled to room temperature, and filtered through a 0.22 μm microfiltration membrane to remove the bacterial cells, thus obtaining the fermentation broth. S2. Disperse the biochar raw material in KOH solution (prepared as a 25% aqueous solution by mass), with the amount of KOH solution being 10 times the mass of the biochar. After thorough impregnation and stirring, let it stand for 24 hours. Dry it at 105℃, and then calcine and activate it at 900℃ for 2 hours under a nitrogen protective atmosphere. After activation, wash it thoroughly with deionized water until the pH of the washing solution is neutral, and then dry it to obtain porous biochar. 100g of porous biochar was dispersed in 500ml of ethanol aqueous solution (volume ratio 4:1), 10g of 3-aminopropyltriethoxysilane was added, the mixture was stirred for 30min, the pH of the system was adjusted to 8.0, and the reaction was stirred at 85℃ for 4h. After the reaction was completed, the mixture was filtered, washed 3 times with ethanol, and dried under vacuum at 60℃ to obtain aminated biochar. S3. Dissolve 150g of polyaspartic acid in MES buffer (0.1 mol / L, 2-morpholinoethanesulfonic acid) at pH 5.0-6.0 to prepare a 5% polyaspartic acid solution. Add 600g of EDC and 400g of NHS, and stir at 30℃ for 30min to activate. Then add 100g of aminated biochar and stir at 30℃ for 2h. After the reaction is complete, wash thoroughly with deionized water 4 times and vacuum dry at 50℃ to obtain modified biochar. S4. Disperse 100g of modified biochar in 2000ml of fermentation broth, adjust the pH of the system to 7.0, and shake and adsorb at 35℃ for 4h. After adsorption is complete, filter, wash once with a small amount of deionized water, and dry under vacuum at 60℃ to obtain composite modified biochar.

[0030] Ferrous sulfate, zinc sulfate, manganese sulfate, and borax are mixed according to the above formula ratio. Dilute ammonia water with a mass fraction of 10% is sprayed evenly into the mixture for ammoniation treatment. The ammoniation temperature is controlled at 60℃ and the ammoniation treatment time is 30min. After the ammoniation pretreatment is completed, the water content of the mixture is controlled to not exceed 1%, and an ammoniation pretreated trace element mixture is obtained. Urea, monoammonium phosphate, and ammonium chloride are added to a melting tank according to the formula ratio and heated at 145°C to fully melt them into a uniform slurry. Potassium sulfate and magnesium sulfate are added and stirred evenly. Humic acid powder is added and stirred for 10 minutes. The slurry temperature is controlled at 145°C and the slurry moisture content is controlled at 3%. After spray granulation, the particle size is controlled at 2-4 mm to obtain basic compound fertilizer granules. The basic compound fertilizer granules were cooled to 70°C, and then ammoniation pretreated trace element mixture, γ-polyglutamic acid and composite modified biochar were added in sequence. The mixture was thoroughly mixed at 70°C for 15 minutes, cooled to 55°C, and molten low-melting-point paraffin wax was sprayed in at a rate of 1.0% of the total weight of the granules. The mixture was then rolled and coated, cooled and shaped, and then sieved by a vibrating screen and treated to prevent caking, to obtain long-acting slow-release compound fertilizer granules.

[0031] Comparative Example 1 This comparative example provides a long-acting slow-release compound fertilizer containing polyglutamic acid and its preparation method. The components and preparation method of the compound fertilizer are the same as in Example 1, except that the modified biochar is not loaded with fermentation broth. The preparation method includes: S1. Disperse the biochar raw material in KOH solution (prepared as a 25% aqueous solution by mass), with the amount of KOH solution being 9 times the mass of the biochar. After thorough impregnation and stirring, let it stand for 21 hours. Dry it at 105℃, and then calcine and activate it at 900℃ for 3 hours under a nitrogen protective atmosphere. After activation, wash it thoroughly with deionized water until the pH of the washing solution is neutral, and then dry it to obtain porous biochar. 100g of porous biochar was dispersed in 500ml of ethanol aqueous solution (volume ratio 4:1), 8g of 3-aminopropyltriethoxysilane was added, the mixture was stirred for 30min, the pH of the system was adjusted to 7.0, and the reaction was stirred at 80℃ for 5h. After the reaction was completed, the mixture was filtered, washed 3 times with ethanol, and dried under vacuum at 60℃ to obtain aminated biochar. S2. Dissolve 100g of polyaspartic acid in MES buffer (0.1 mol / L, 2-morpholinoethanesulfonic acid) at pH 5.0-6.0 to prepare a 3.5% polyaspartic acid solution. Add 400g of EDC and 250g of NHS, and stir to activate at 25℃ for 45min. Then add 100g of aminated biochar and stir to react at 25℃ for 3h. After the reaction is complete, wash thoroughly with deionized water 4 times and dry under vacuum at 50℃ to obtain modified biochar, which is the composite modified biochar.

[0032] Comparative Example 2 This comparative example provides a long-acting, slow-release compound fertilizer containing polyglutamic acid and its preparation method. The components and preparation method of the compound fertilizer are the same as in Example 1, except that only Bacillus amyloliquefaciens is inoculated into the fermentation broth of the modified biochar. The preparation method includes: S1. Disperse wheat gluten in water to prepare a dispersion with a mass fraction of 7%. Add 0.8% of the mass of wheat gluten protease and pre-hydrolyze at 53℃ for 3 hours. After pre-hydrolysis, sterilize at 120℃ for 30 minutes and cool to obtain pretreated wheat gluten. The pretreated wheat gluten was then dispersed in distilled water, and 8 g / L citric acid and 25 g / L glycerol were added and stirred evenly. Bacillus amyloliquefaciens was inoculated at an inoculation rate of 3.5% (volume ratio). The mixture was then anaerobic fermented at 36℃ and 200 rpm for 60 h. After fermentation, the mixture was sterilized at 120℃ for 30 min, cooled to room temperature, and filtered through a 0.22 μm microfiltration membrane to remove the bacterial cells, thus obtaining the fermentation broth.

[0033] The remaining steps are the same as in Example 1.

[0034] Comparative Example 3 This comparative example provides a long-acting slow-release compound fertilizer containing polyglutamic acid and its preparation method. The components and preparation method of the compound fertilizer are the same as in Example 1, except that the modified biochar is inoculated only with Bacillus licheniformis, and its preparation method includes: S1. Disperse wheat gluten in water to prepare a dispersion with a mass fraction of 7%. Add 0.8% of the mass of wheat gluten protease and pre-hydrolyze at 53℃ for 3 hours. After pre-hydrolysis, sterilize at 120℃ for 30 minutes and cool to obtain pretreated wheat gluten. The pretreated wheat gluten was then dispersed in distilled water, and 8 g / L citric acid and 25 g / L glycerol were added and stirred evenly. Bacillus licheniformis was inoculated at an inoculation rate of 3.5% (volume ratio). The mixture was then anaerobic fermented at 36℃ and 200 rpm for 60 h. After fermentation, the mixture was sterilized at 120℃ for 30 min, cooled to room temperature, and filtered through a 0.22 μm microfiltration membrane to remove the bacterial cells, thus obtaining the fermentation broth.

[0035] The remaining steps are the same as in Example 1.

[0036] Comparative Example 4 This comparative example provides a long-acting slow-release compound fertilizer containing polyglutamic acid and its preparation method. The components and preparation method of the compound fertilizer are the same as in Example 1, except that the modified biochar is not grafted with polyaspartic acid. The preparation method includes: S1~S2 are the same as in Example 1; S3. Disperse 100g of aminated biochar in 1500ml of fermentation broth, adjust the pH of the system to 6.5, and shake and adsorb at 30℃ for 6h. After adsorption is completed, filter, wash once with a small amount of deionized water, and dry under vacuum at 50℃ to obtain composite modified biochar.

[0037] Comparative Example 5 This comparative example provides a long-acting slow-release compound fertilizer containing polyglutamic acid and its preparation method. The components and preparation method of the compound fertilizer are the same as in Example 1, except that PASP in the composite modified biochar is added in the form of physical blending. The preparation method includes: S1~S2 are the same as in Example 1; S3. Disperse 100g of aminated biochar in 1500ml of fermentation broth, add 100g of polyaspartic acid, adjust the pH of the system to 6.5, and shake and adsorb at 30℃ for 6h. After adsorption is completed, filter, wash once with a small amount of deionized water, and dry under vacuum at 50℃ to obtain composite modified biochar.

[0038] Performance testing The performance of the polyglutamic acid-containing long-acting slow-release compound fertilizers prepared in the examples and comparative examples was tested. The test indicators included slow-release performance, nitrogen leaching loss rate, apparent nitrogen utilization rate, hygroscopicity, and anti-caking properties. The slow-release performance included two indicators: initial nutrient release rate and 28-day cumulative nutrient release rate, both performed according to GB / T 23348-2009 "Slow-Release Fertilizers". The initial nutrient release rate was determined by soaking the sample in still water at 25℃ for 24 hours, filtering, and determining the total nitrogen content in the filtrate using the Kjeldahl method. The nutrient release rate within 24 hours was calculated and was required to be no more than 15%. The 28-day cumulative nutrient release rate was determined by continuously extracting the sample in still water at 25℃, changing the water every 7 days, and measuring the cumulative total nitrogen release for a total of 28 days. The 28-day cumulative release rate was required to be no more than 75%.

[0039] Nitrogen leaching loss rate was determined using a soil column leaching simulation test: the test soil was filled into a leaching column, and each treatment sample was leached with an equal amount of nitrogen. The sample was leached once every 7 days with 1000 mL of simulated rainfall each time, for a total of 4 leachings. After collecting all the leachate, the total nitrogen content was determined by the Kjeldahl method, and the nitrogen leaching loss rate was calculated. The standard method of soil column leaching was followed by the Institute of Soil and Fertilizer Research, Chinese Academy of Agricultural Sciences.

[0040] Nitrogen fertilizer apparent utilization rate was assessed using a pot experiment combined with... 15 The nitrogen isotope tracer method was used for determination: wheat was used as the test crop, and fertilizer was applied at the same amount of nitrogen. After 60 days of growth, the nitrogen content of the aboveground parts and roots of the plant was measured, and the percentage of nitrogen absorbed by the plant to the total amount of nitrogen applied was calculated. The method was carried out in accordance with NY / T 3038-2016 "Determination of nitrogen, phosphorus and potassium utilization rate of fertilizer".

[0041] For hygroscopicity and anti-caking properties, the samples were placed in a constant temperature and humidity chamber (30℃, 80% relative humidity) for 72 hours, and the mass difference before and after placement was measured to calculate the hygroscopic rate. The particle agglomeration force was measured using a pressure sensor. The acceptable criteria were a hygroscopic rate not exceeding 1.5% and an agglomeration force not exceeding 50N.

[0042] The test results are shown in Table 1.

[0043] Table 1 Performance Test Data

[0044] As shown in Table 1, the compound fertilizer prepared in the embodiments of the present invention is significantly superior to the comparative examples in terms of slow-release performance, nutrient utilization rate, and soil improvement effect. Comparative Example 1, with its modified biochar, was not loaded with fermentation broth, resulting in a lack of bio-enhancing effects from the active metabolites in the fermentation broth. This led to a decrease in the actual nitrogen uptake by the plants, a relatively high concentration of residual inorganic nitrogen in the soil, and increased losses under leaching conditions. Comparative Example 2 was inoculated only with Bacillus amyloliquefaciens, and the fermentation broth lacked γ-PGA synthesized by Bacillus licheniformis, resulting in an incomplete combination of active metabolites. However, the functional peptides and extracellular polysaccharides produced by the continuous hydrolysis of gluten by proteases still partially exerted a enhancing effect. Therefore, all indicators were between those of Example 1 and Comparative Example 1. Comparative Example 3 was inoculated only with Bacillus licheniformis, which theoretically possesses the ability to synthesize γ-PGA. However, due to the lack of continuous replenishment of glutamate precursors by Bacillus amyloliquefaciens, the γ-PGA production was limited, and the overall concentration and diversity of active metabolites in the fermentation broth were lower than those of the dual-culture system. Comparative Examples 2 and 3 demonstrate that the combination of diverse active metabolites produced by the synergistic fermentation of two strains can produce better results, and no single strain can completely replace the effect of the two-strain system.

[0045] Comparative Example 4, without PASP covalent grafting, retained only native amino groups and a small amount of oxygen-containing functional groups on the biochar surface, leading to increased initial nutrient release rate and 28-day cumulative release rate. Simultaneously, nitrogen leaching loss rate increased, and apparent nitrogen fertilizer utilization decreased, showing a significant difference from Example 1. This indicates that the PASP covalently grafted layer plays an irreplaceable core role in constructing a stable chemically slow-release structure and reducing nutrient loss. Furthermore, due to the loss of the electrostatic adsorption function of PASP segments, the hygroscopic rate of Comparative Example 4 also increased significantly. Comparative Example 5 introduced PASP through physical blending, but it was not covalently anchored to the biochar surface. Under the continuous action of soil moisture and leaching water, PASP segments gradually dissolved and leached from the particle surface, and the chemical adsorption and slow-release capacity continued to decline over time. This mechanism was clearly reflected in the 28-day cumulative nutrient release rate and nitrogen leaching loss rate, indicating that while physically mixed PASP has some initial effect, its long-term stability is significantly insufficient. Meanwhile, the apparent utilization rate of nitrogen fertilizer was lower than that in Example 1, further indicating that the PASP physical mixing method can still partially exert its effectiveness in the early stage, but cannot achieve the long-term sustained release performance conferred by covalent grafting.

[0046] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A long-acting, slow-release compound fertilizer containing polyglutamic acid, characterized in that, The product comprises the following components by weight: 30-45 parts urea, 20-35 parts monoammonium phosphate, 15-30 parts potassium sulfate, 5-15 parts ammonium chloride, 0.5-2 parts γ-polyglutamic acid, 3-8 parts composite modified biochar, 1-3 parts humic acid powder, 2-5 parts magnesium sulfate, and 1.1-4.3 parts trace element mixture; wherein the composite modified biochar uses porous biochar as a carrier, with polyaspartic acid covalently grafted onto its surface and adsorbing fermentation broth containing γ-polyglutamic acid.

2. The long-acting slow-release compound fertilizer containing polyglutamic acid as described in claim 1, characterized in that: The preparation method of the composite modified biochar includes the following steps: S1. Pre-treat wheat gluten powder, then disperse the pre-treated wheat gluten powder in distilled water, add citric acid and glycerol, stir evenly, inoculate with microbial bacteria, and anaerobic ferment at 35-37℃ for 48-72 hours. After the fermentation culture is completed, sterilize, filter, and obtain fermentation broth. S2. The biochar is calcined at high temperature to obtain porous biochar; the porous biochar is dispersed in an ethanol aqueous solution, 3-aminopropyltriethoxysilane is added, the pH of the system is adjusted to 6.5-8.0, and the reaction is stirred at 78-85℃ for 4-6 hours to obtain aminated biochar. S3. Dissolve polyaspartic acid in MES buffer solution with pH 5.0-6.0, add EDC and NHS, stir and activate at 20-30℃ for 30-60 min, then add aminated biochar, stir and react at 20-30℃ for 2-4 h to obtain modified biochar. S4. Disperse the modified biochar in the fermentation broth, adjust the pH of the system to 6.0-7.0, and shake and adsorb at 25-35℃ for 4-8 hours. After adsorption is completed, filter, wash and dry to obtain composite modified biochar.

3. The long-acting slow-release compound fertilizer containing polyglutamic acid as described in claim 2, characterized in that: In step S1, the pretreatment of wheat gluten includes: dispersing wheat gluten in water to prepare a dispersion with a mass fraction of 6% to 8%, adding protease, and pre-hydrolyzing at 50 to 55°C for 2 to 4 hours.

4. The long-acting slow-release compound fertilizer containing polyglutamic acid as described in claim 3, characterized in that: In step S1, the amount of protease added is 0.5% to 1.0% of the mass of wheat gluten, the amount of citric acid added is 5 to 10 g / L, the amount of glycerol added is 20 to 30 g / L, and the amount of microbial inoculation is 2% to 5% by volume. The microbial bacteria include Bacillus amyloliquefaciens and Bacillus licheniformis, and the inoculation volume ratio of Bacillus amyloliquefaciens to Bacillus licheniformis is 1:1 to 1:

2.

5. The long-acting slow-release compound fertilizer containing polyglutamic acid as described in claim 2, characterized in that: The preparation method of porous biochar in step S2 includes: impregnating and activating the biochar raw material in potassium hydroxide solution, and then calcining and activating it at 700-900℃ for 2-4 hours under a nitrogen protective atmosphere.

6. The long-acting slow-release compound fertilizer containing polyglutamic acid as described in claim 2, characterized in that: In step S2, the amount of 3-aminopropyltriethoxysilane added is 5% to 10% of the mass of the porous biochar.

7. The long-acting slow-release compound fertilizer containing polyglutamic acid as described in claim 2, characterized in that: In step S3, the mass ratio of aminated biochar, polyaspartic acid, EDC and NHS is 1:0.5-1.5:2-6:1-4.

8. The long-acting slow-release compound fertilizer containing polyglutamic acid as described in claim 2, characterized in that: In step S4, the solid-liquid ratio of biochar to fermentation broth is 1:10 to 1:20 g / mL.

9. A long-acting slow-release compound fertilizer containing polyglutamic acid as described in claim 1, characterized in that: The trace element mixture includes 0.5 to 2 parts ferrous sulfate, 0.3 to 1 part zinc sulfate, 0.2 to 0.8 parts manganese sulfate, and 0.1 to 0.5 parts borax.

10. A method for preparing a long-acting, slow-release compound fertilizer containing polyglutamic acid as described in any one of claims 1 to 9, characterized in that: Includes the following steps: Urea, monoammonium phosphate, and ammonium chloride are added to a melting tank according to the formula ratio and heated at 130-145℃ to fully melt them into a uniform slurry. Potassium sulfate and magnesium sulfate are added and stirred evenly. Humic acid powder is added and stirred to mix. The moisture content of the slurry is controlled at 1%-3%. After spray granulation, basic compound fertilizer granules are obtained. The basic compound fertilizer granules, ammoniation pretreated trace element mixture, γ-polyglutamic acid and composite modified biochar are thoroughly mixed at a low temperature of 50-70℃ for 15-20 minutes, cooled to 45-55℃, sprayed with molten low-melting-point paraffin, rolled and coated, cooled and shaped, and then screened by a vibrating screen and treated to prevent agglomeration to obtain the finished product of long-acting slow-release compound fertilizer granules containing polyglutamic acid.

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