Slow-release compound microbial fertilizer and preparation method thereof

Abstract

This invention discloses a slow-release compound microbial fertilizer and its preparation method, relating to the field of agricultural bio-fertilizers. It comprises the following components: an acrylic acid-potassium humate-acrylamide terpolymer, a composite carrier of bentonite and biochar, polylactic acid, compound microbial powder, sodium alginate, urea, diammonium phosphate, potassium sulfate, and trace element components; the compound microbial powder includes *Azotobacter chrysophagus* powder, *Bacillus megaterium* powder, and *Bacillus mucilaginosus* powder. The preparation method includes: preparation of the ternary copolymer slow-release material; preparation of fertilizer core layer particles; preparation of the microbial-film-carrier co-coating solution; and surface coating of the fertilizer core layer particles. This invention, through the synergistic effect of the ternary copolymer slow-release material, the composite protective carrier, and the polylactic acid / sodium alginate microbial film system, achieves high microbial survival rate, stable slow release of nutrients, and a significant improvement in fertilizer utilization rate without the need for a complex multilayer structure.

Description

Technical Field

[0001] This invention relates to the field of agricultural bio-fertilizer technology, specifically to a slow-release compound microbial fertilizer and its preparation method. Background Technology

[0002] Compound microbial fertilizer is a new type of fertilizer that organically combines chemical fertilizers, organic carriers and beneficial microorganisms. It combines the rapid effect of chemical fertilizers, the improvement effect of organic matter and the functional activity of microorganisms. It has significant advantages in reducing the application of chemical fertilizers, improving soil fertility and controlling soil-borne diseases, and has become an important development direction for green agricultural inputs.

[0003] In the existing technology, there are several main methods for preparing compound microbial fertilizers.

[0004] Post-spraying method: After granulating chemical fertilizer granules, microbial liquid or powder is sprayed onto the surface of the granules. This method is simple, but the microorganisms are directly exposed to the granule surface, making them susceptible to inactivation by moisture, high temperature and ultraviolet radiation during storage and transportation; at the same time, after being applied to the soil, the microorganisms lack a protective substrate and are difficult to successfully colonize in the rhizosphere.

[0005] Blending method: Microbial agents are physically mixed with chemical fertilizers and organic materials and then directly granulated. In this method, the high osmotic pressure of the chemical fertilizers and the high temperature (usually >60℃) during the granulation process severely kill the microorganisms. The number of effective live bacteria in the product is often far lower than the labeled value, and the shelf life is short.

[0006] Coating protection method: Some existing technologies use biodegradable polymer materials to individually encapsulate microorganisms or set up multi-layer coating structures to isolate chemical fertilizers from microorganisms. However, these solutions are usually complex in structure and involve many preparation processes, requiring multiple coatings, zone inoculation, low-temperature drying, etc., resulting in large equipment investments and high production costs, which is not conducive to large-scale promotion. At the same time, the design of multi-layer structures often requires significant modifications to existing fertilizer production lines, which has low acceptance among enterprises.

[0007] Therefore, developing a slow-release compound microbial fertilizer with a simple structure, easy process, high microbial survival rate, good slow-release effect, and easy industrial production remains a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] The purpose of this invention is to provide a slow-release compound microbial fertilizer and its preparation method. Through the synergistic effect of components such as ternary copolymer slow-release material, composite protective carrier, and polylactic acid / sodium alginate biofilm system, a high survival rate of microorganisms, stable slow release of nutrients, and a significant improvement in fertilizer utilization are achieved simultaneously without the need for a complex multilayer structure. This solves the problems of low microbial survival rate and poor slow-release effect in existing compound microbial fertilizers.

[0009] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0010] A slow-release compound microbial fertilizer, by weight, comprises the following components: 50-82 parts of chemical fertilizer, 5-10 parts of ternary copolymer slow-release material, 5-15 parts of protective carrier, 2-5 parts of polylactic acid, 1-3 parts of compound microbial powder, and 0.5-1 parts of sodium alginate.

[0011] The chemical fertilizer comprises the following components: 20-30 parts urea, 15-25 parts diammonium phosphate, 15-25 parts potassium sulfate, and 0.1-2 parts trace elements.

[0012] The ternary copolymer slow-release material is an acrylic acid-potassium humate-acrylamide ternary copolymer;

[0013] The protective carrier is a composite carrier of bentonite and biochar, with a mass ratio of bentonite to biochar of 1:0.5~3.

[0014] The composite microbial powder includes Azotobacter chrysophyte powder, Bacillus megaterium powder, and Bacillus mucilaginosus powder, with a mass ratio of 1:0.5~2:0.5~2.

[0015] To address the problems of low microbial survival rate and asynchronous nutrient release and microbial metabolism in existing compound microbial fertilizers, this invention provides a slow-release compound microbial fertilizer with simplified components and synergistic functions.

[0016] Existing technologies often employ post-coating or simple mixing, leading to the deactivation of microbial powders due to high osmotic pressure and the presence of water and oxygen. This invention incorporates polylactic acid, sodium alginate, composite microbial powder, and a protective carrier into a single formulation: polylactic acid forms a hydrophobic barrier to isolate water and oxygen; sodium alginate absorbs water to form a hydration layer to buffer osmotic pressure; and the bentonite / biochar composite carrier provides attachment sites and adsorbs ammonium and potassium ions to reduce local salinity. The synergistic effect of these three components ensures that the effective viable bacteria count is ≥1×10⁻⁶ after 6 months of storage at room temperature. 8 CFU / g, significantly better than existing products.

[0017] To address the misalignment between nutrient release and the bacterial powder's metabolic cycle, this invention employs a ternary copolymer slow-release material (acrylic acid-potassium humate-acrylamide) in synergy with polylactic acid. This ternary copolymer possesses a highly absorbent three-dimensional network, swelling upon application to the soil and controlling the release of nitrogen, phosphorus, and potassium through physical retention and ion exchange, with a release cycle of up to 90 days. The polylactic acid coating layer undergoes slow hydrolysis, its degradation rate synergistic with the swelling-release curve of the ternary copolymer. Early on, the intact membrane prevents sudden nutrient release, while in the mid-to-late stages, micropores appear in the membrane, and the ternary copolymer regulates the release. This dual slow-release mechanism of "outer barrier - inner matrix" ensures stable nutrient supply, matching the bacterial powder's colonization and metabolic cycle, resulting in a fertilizer utilization rate more than 12.6% higher than conventional slow-release compound fertilizers.

[0018] In existing technologies, polyacrylic acid-humic acid copolymers are mostly used as water-retaining agents or coating materials. The former lacks nutrient regulation, while the latter becomes ineffective once the coating is damaged. This invention breaks through this conventional thinking by directly and uniformly mixing the terpolymer into chemical fertilizers for co-granulation. Even if the granules break, the slow-release function is maintained, and the slow-release period is well matched with the metabolic cycle of the bacterial powder.

[0019] Bentonite and biochar were mixed in a ratio of 1:0.5-3. Biochar's pores and oxygen-containing functional groups adsorbed ammonia nitrogen and organic carbon, providing nutrients. Bentonite's high expansibility and negatively charged interlayer structure adsorbed potassium and ammonium ions, slowing their release and buffering pH. The two complemented each other: biochar improved the permeability of bentonite, while bentonite compensated for the insufficient fertilizer retention of biochar, together constructing a micro-ecological environment with stable humidity and buffered salinity. Experiments showed that the composite carrier increased the colonization rate of functional bacteria by more than 13.2% compared to the single carrier, and significantly enhanced the slow-release effect of phosphorus and potassium.

[0020] This invention achieves a unified approach to high microbial survival rate, stable nutrient release, and improved fertilizer utilization with a simplified composition through the application of microbial film protection, dual slow release, composite carrier synergy, and terpolymer.

[0021] The polylactic acid used in this invention is industrial-grade polylactic acid of grade 2003D produced by NatureWorks, Inc. in the United States, with a weight-average molecular weight (Mw) of 87~97kDa.

[0022] Furthermore, the trace element components include zinc sulfate, boric acid, ferrous sulfate, and manganese sulfate, in a mass ratio of 1:0.4~1:0.2~0.8:0.2~0.5.

[0023] The micronutrient ratio in this invention is based on the physiological synergy and anti-antagonistic balance of each element. Zinc promotes photosynthesis and auxin synthesis, boron ensures flowering and fruiting, iron participates in chlorophyll formation, and manganese activates the photosystem. The appropriate ratio can avoid ion antagonism, achieve increased yield, and significantly improve indicators such as plant height and ear length.

[0024] Simultaneously, this formulation produces multiple synergistic effects with other components in the fertilizer. The ternary copolymer slow-release material is rich in carboxyl and amide groups, and adsorbs Zn through complexation. 2+ Fe 2+ Mn 2+ After being applied to the soil, it is slowly released through ion exchange, with a release period of 30-90 days, avoiding the low utilization rate caused by precipitation or fixation of conventional micronutrient fertilizers. Simultaneously, the potassium humate further chelates trace elements, enhancing their anti-fixation ability. In the protective carrier, the high cation exchange capacity of bentonite actively adsorbs Zn. 2+ Mn 2+ Its layered structure protects Fe 2+It is protected from oxidation; the surface functional groups of biochar complex with trace elements and provide attachment sites for microorganisms. The controlled release of iron and manganese on the carrier can activate phosphate-solubilizing and potassium-solubilizing bacteria to secrete organic acids, thereby improving the leaching efficiency of phosphorus and potassium. The polylactic acid coating layer slowly hydrolyzes in the soil, and its degradation rate is synergistic with the swelling-release curve of the terpolymer. It avoids the sudden release of trace elements in the early stage and forms micropores in the middle and late stages, so that the release curve is stable and lasts for 90 days, preventing early toxicity and late deficiency.

[0025] Furthermore, the compound microbial powder and trace elements can achieve bidirectional activation. Zinc and iron are key cofactors for nitrogenase and organic acid metabolism. Appropriate amounts of trace elements can enhance the nitrogen-fixing, phosphorus-solubilizing, and potassium-solubilizing activities of the powder. Simultaneously, the organic acids produced by bacterial metabolism can chelate and dissolve iron, manganese, zinc, etc., fixed in the soil, increasing the utilization rate of trace elements by more than 15%. Sodium alginate stably disperses trace element salts in the coated emulsion, preventing precipitation, and its hygroscopic layer protects Fe. 2+ It is free from oxidation and, after being applied to the soil, interpenetrates with the terpolymer hydrogel network to form hydrophilic channels, which can promote the directional diffusion of trace elements.

[0026] A method for preparing a slow-release compound microbial fertilizer includes the following steps:

[0027] S100. An elastic gel polymer is prepared by a one-step aqueous solution polymerization method. The elastic gel polymer is dried and pulverized to obtain a ternary copolymer sustained-release material.

[0028] S200: Mix urea, diammonium phosphate, potassium sulfate, trace elements, some ternary copolymer slow-release materials and binder evenly, granulate, dry, and obtain chemical fertilizer core layer granules.

[0029] S300. Dissolve polylactic acid in an organic solvent, add the remaining ternary copolymer slow-release material, protective carrier, and sodium alginate aqueous solution, stir evenly to obtain a coating emulsion; add the composite microbial powder to the above coating emulsion, stir and disperse evenly to obtain a microbial-membrane-carrier co-coating solution;

[0030] S400. The bacterial-film-carrier blend coating solution is sprayed onto the surface of the core layer particles of the above-mentioned chemical fertilizer for coating. After coating, it is dried at low temperature to obtain granular slow-release compound microbial fertilizer.

[0031] In this invention, the terpolymer sustained-release material obtained by one-step aqueous solution polymerization in step S100 has its undried elastic gel portion directly used as the granulation binder in step S200. This avoids the introduction of exogenous binders and utilizes the polymer's own viscosity to enhance particle strength. The dried and pulverized powder is then split into two parts, used respectively for the core matrix sustained release in step S200 and the coating layer active carrier in step S300. This material design greatly simplifies the material system.

[0032] The chemical fertilizer core layer particles formed in step S200 have a surface affinity that matches the bacteria-film-carrier blend coating solution prepared in step S300. The polylactic acid organic solvent phase in the coating solution can moderately wet the core layer surface, while the sodium alginate aqueous phase provides a polar interface to ensure uniform adhesion of the coating layer.

[0033] Step S300 involves preparing the coating emulsion at low temperature, which is seamlessly integrated with the low-temperature spraying and drying (18~28℃) of step S400. All processes involving contact with the bacterial powder maintain mild conditions, preventing high-temperature inactivation and ensuring the bacterial powder is uniformly distributed in a dormant state within the coating layer. Existing technologies typically involve spraying the bacterial powder onto the surface of the finished particles afterward, or preparing the coating layer first and then inoculating the bacterial powder. This invention directly blends polylactic acid, the remaining terpolymer, a protective carrier, sodium alginate aqueous solution, and the composite microbial powder to prepare the coating emulsion, simultaneously completing film formation, inoculation, and bacterial cell protection in a single spraying process. The aqueous sodium alginate phase encapsulates the bacterial powder into micro-regions, preventing damage to the bacterial cells from organic solvents.

[0034] The coating process in step S400 complements the slow-release function of the core layer in a sequential manner. The coating layer first releases the bacterial powder in a controlled manner, and the core layer then slowly releases the nutrients. The release curve is pre-adjusted through the material distribution in the process.

[0035] The various steps of this invention are highly coordinated in terms of material reuse, process conditions, and functional division of labor, achieving a unified approach to microbial protection, nutrient slow release, and large-scale production with a simplified process.

[0036] Further, in step S100, the one-step aqueous solution polymerization method for preparing the elastic gel polymer includes the following steps:

[0037] Step 1: Dissolve acrylic acid (AA) in deionized water, add KOH solution while stirring, adjust the neutralization degree to 60%~75%, and let it cool naturally to room temperature;

[0038] Step 2: Add potassium humate (KHA) and acrylamide (AM) in sequence, stir to dissolve, then add N,N'-methylenebisacrylamide (MBA), mix thoroughly to obtain monomer mixture;

[0039] Step 3: Heat the above monomer mixture to 70~75℃, add potassium persulfate (KPS), and continue the reaction until the reactants are in an elastic gel state to obtain an elastic gel polymer.

[0040] Further, the mass ratio of acrylic acid, potassium humate, and acrylamide is 3.5~7:0.5~1.5:1; the mass ratio of acrylic acid to deionized water is 1:1.5~2.5; the amount of crosslinking agent N,N'-methylenebisacrylamide is 0.1%~0.5% of the total mass of monomers; and the amount of initiator potassium persulfate is 0.08~0.15% of the total mass of monomers.

[0041] In existing technologies, polyacrylic acid-humic acid binary copolymers are mostly used as water-retaining agents, while polyacrylic acid-acrylamide binary copolymers focus on salt tolerance. Neither technology has used potassium humate, acrylic acid, and acrylamide in a three-component copolymer for slow-release fertilizers. Furthermore, existing copolymers often use sodium humate, which easily precipitates humic acid after acid neutralization, leading to uneven reactions and low grafting rates. This invention is the first to systematically employ a ternary copolymerization of potassium humate, acrylic acid, and acrylamide at a mass ratio of 3.5~7:0.5~1.5:1. Potassium humate not only provides potassium ions for direct crop absorption, but its aromatic rings can also form a more stable network structure through π-π stacking and hydrogen bonding with the amide groups, while simultaneously avoiding the problems of uneven reactions and low grafting rates caused by humic acid precipitation.

[0042] In terms of process, existing technologies typically optimize the water absorption ratio individually, but a high water absorption rate does not necessarily equate to excellent sustained release. Insufficient crosslinking leads to a loose network and poor nutrient retention, while excessive neutralization damages the hydrogen bond network and reduces the duration of controlled release. This invention, through the synergistic limitation of neutralization degree, crosslinking agent dosage, initiator dosage, and reaction temperature, abandons the traditional approach of solely pursuing a high water absorption ratio. It achieves a balance between water absorption and swelling, nutrient retention, ion exchange sustained release performance, and preparation reproducibility, stabilizing the product's water absorption ratio at 150-400 times while allowing the nutrient release cycle to be controlled to 30-90 days.

[0043] Further, in step S100, the elastic gel polymer is dried in a drying oven at 80~100℃ until the moisture content is less than 5%, and then pulverized to a particle size of 80~120 mesh to obtain a ternary copolymer sustained-release material.

[0044] Further, the obtained elastic gel polymer is taken and prepared into a gel solution as a binder by stirring at a mass ratio of elastic gel polymer to deionized water of 1:0.5~2.

[0045] The dry weight of the elastic gel polymer used to prepare the binder accounts for 5% to 15% of the total mass of the ternary copolymer slow-release material; the remaining ternary copolymer slow-release material is divided into a first part and a second part. The first part is used to mix and granulate with chemical fertilizer in step S200, and the second part is used to dissolve with polylactic acid in step S300. The mass ratio of the first part to the second part is 4 to 7:1.

[0046] Further, in step S200, the granulation temperature is ≤55℃, and granulation is carried out in a cycle until the particle size is 2~4mm; the particles are dried at 45~52℃ until the moisture content is less than 5% to obtain the core layer particles of chemical fertilizer.

[0047] Further, in step S300, the organic solvent is selected from chloroform or dichloromethane, and the mass ratio of polylactic acid to organic solvent is 1:5~15; the temperature of the coating emulsion preparation process is controlled at 20~30℃.

[0048] Sodium alginate is dissolved in deionized water to prepare a sodium alginate solution; the mass ratio of sodium alginate to deionized water is 1:10~20.

[0049] Further, in step S400, the above-mentioned bacteria-film-carrier blend coating liquid is uniformly sprayed onto the surface of the core layer particles at 20~30℃, and the coating layer thickness is 15~60μm; it is then ventilated and dried at 18~28℃ until the coating layer surface is dry and solidified, thus obtaining the slow-release compound microbial fertilizer.

[0050] Compared with the prior art, the beneficial effects of the present invention are:

[0051] This invention effectively solves the core problems of low microbial survival rate and asynchronous nutrient release and microbial powder metabolism. Through the synergistic protection of polylactic acid, sodium alginate, and bentonite / biochar composite carrier, the effective viable bacteria count of the fertilizer still reaches 1.6 × 10⁻⁶ after 6 months of storage at room temperature. 8 The dual slow-release system, constructed from a terpolymer and polylactic acid, maintains a 30-day nitrogen release rate of 51.2%, achieving a corn nitrogen utilization rate of 43.6%, which is over 12.6% higher than conventional slow-release compound fertilizers. The composite carrier enables a functional bacteria colonization rate of 41.2%, over 13.2% higher than single bentonite or biochar carriers. The utilization rate of the trace element zinc reaches 26.3%, increasing field corn yield to 685 kg / mu. The raw materials of this invention are biodegradable, requiring no special low-temperature equipment throughout the process, making it easy for industrial-scale promotion. This slow-release compound microbial fertilizer is suitable for grain crops, cash crops, fruits and vegetables, and oil crops. Detailed Implementation

[0052] Example 1

[0053] The preparation method of ternary copolymer sustained-release material includes the following steps:

[0054] Step 1: Dissolve acrylic acid in deionized water, add 40% KOH solution while stirring, adjust the neutralization to 70%, and let it cool naturally to room temperature;

[0055] Step 2: Add potassium humate and acrylamide in sequence, stir to dissolve, then add N,N'-methylenebisacrylamide, mix thoroughly to obtain monomer mixture;

[0056] Step 3: Heat the above monomer mixture to 72°C, add potassium persulfate, and continue the reaction for 30 minutes until the reactant becomes an elastic gel. Stop heating to obtain an elastic gel polymer.

[0057] Step 4: Cut the elastic gel polymer into small pieces, dry at 90°C until the moisture content is less than 5%, crush and pass through a 100-mesh sieve to obtain the ternary copolymer sustained-release material.

[0058] The mass ratio of acrylic acid, potassium humate, and acrylamide is 5.2:1:1; the mass ratio of acrylic acid to deionized water is 1:2; the amount of crosslinking agent N,N'-methylenebisacrylamide is 0.3% of the total mass of monomers; and the amount of initiator potassium persulfate is 0.12% of the total mass of monomers.

[0059] Example 2

[0060] The preparation method of ternary copolymer sustained-release material includes the following steps:

[0061] Step 1: Dissolve acrylic acid in deionized water, add 40% KOH solution while stirring, adjust the neutralization to 60%, and let it cool naturally to room temperature;

[0062] Step 2: Add potassium humate and acrylamide in sequence, stir to dissolve, then add N,N'-methylenebisacrylamide, mix thoroughly to obtain monomer mixture;

[0063] Step 3: Heat the above monomer mixture to 70°C, add potassium persulfate, and continue the reaction for 40 minutes until the reactant becomes an elastic gel. Stop heating to obtain an elastic gel polymer.

[0064] Step 4: Cut the elastic gel polymer into small pieces, dry them at 80°C until the moisture content is less than 5%, crush them, and pass them through an 80-mesh sieve to obtain the ternary copolymer sustained-release material.

[0065] The mass ratio of acrylic acid, potassium humate, and acrylamide is 3.5:0.5:1; the mass ratio of acrylic acid to deionized water is 1:1.5; the amount of crosslinking agent N,N'-methylenebisacrylamide is 0.1% of the total mass of monomers; and the amount of initiator potassium persulfate is 0.08% of the total mass of monomers.

[0066] Example 3

[0067] The preparation method of ternary copolymer sustained-release material includes the following steps:

[0068] Step 1: Dissolve acrylic acid in deionized water, add 40% KOH solution while stirring, adjust the neutralization to 75%, and let it cool naturally to room temperature;

[0069] Step 2: Add potassium humate and acrylamide in sequence, stir to dissolve, then add N,N'-methylenebisacrylamide, mix thoroughly to obtain monomer mixture;

[0070] Step 3: Heat the above monomer mixture to 75°C, add potassium persulfate, and continue the reaction for 20 minutes until the reactant becomes an elastic gel. Stop heating to obtain an elastic gel polymer.

[0071] Step 4: Cut the elastic gel polymer into small pieces, dry them at 100°C until the moisture content is less than 5%, crush them, and pass them through a 120-mesh sieve to obtain the ternary copolymer sustained-release material.

[0072] The mass ratio of acrylic acid, potassium humate, and acrylamide is 7:1.5:1; the mass ratio of acrylic acid to deionized water is 1:2.5; the amount of crosslinking agent N,N'-methylenebisacrylamide is 0.5% of the total mass of monomers; and the amount of initiator potassium persulfate is 0.15% of the total mass of monomers.

[0073] Comparative Example 1

[0074] In step 2, potassium humate is not added; instead, acrylic acid and acrylamide are used as monomers for copolymerization. The amounts of other raw materials and process conditions are exactly the same as in Example 1.

[0075] Comparative Example 2

[0076] In Example 1, potassium humate was replaced with sodium humate, while the amounts of other raw materials and process conditions were exactly the same as in Example 1.

[0077] Comparative Example 3

[0078] The mass ratio of acrylic acid, potassium humate, and acrylamide was adjusted to 2:1:1. The amounts of other raw materials and process conditions were exactly the same as in Example 1.

[0079] The performance of the ternary copolymer sustained-release materials prepared by the methods of Examples 1-3 and Comparative Examples 1-3 is shown in Table 1.

[0080] Grafting rate: The crude product was extracted with methanol using a Soxhlet extractor for 48 hours to remove ungrafted potassium humate, unreacted monomers, and homopolymers. After drying to constant weight, the product was weighed. Grafting rate = (dry weight after purification - dry weight before purification) / dry weight after purification × 100%.

[0081] Carboxyl content: determined by direct titration with NaOH standard solution.

[0082] Amide group content: The total nitrogen content is calculated by subtracting the nitrogen content in potassium humate from the total nitrogen content using the Kjeldahl method.

[0083] Network crosslink density: The effective crosslink density is calculated using the Flory-Rehner equation and the volume expansion rate at swelling equilibrium, which directly reflects the spatial density of crosslink points inside the three-dimensional network.

[0084] Swelling ratio (combined density and water absorption capacity): The maximum volume of water that the three-dimensional network can hold is estimated by estimating the equilibrium water absorption ratio in deionized water using the tea bag method as specified in JIS K7223.

[0085] Water retention rate determination: After the gel has fully swollen, it is left to stand naturally at room temperature, and its mass change is measured periodically. Water retention rate = mass of gel after standing / mass of initial gel × 100%.

[0086] Table 1. Performance of the ternary copolymer sustained-release materials prepared by the methods of Examples 1-3 and Comparative Examples 1-3

[0087]

[0088] Note: "—" indicates that it does not exist or is not applicable.

[0089] As shown in Table 1, in Example 1, potassium humate, acrylic acid, and acrylamide were copolymerized in a ratio of 5.2:1:1. The aromatic rings of potassium humate formed π-π stacking and hydrogen bonding with the amide groups, promoting the grafting reaction and achieving a grafting rate of 86.3%. Furthermore, physical crosslinking points were appropriately introduced, controlling the network crosslinking density at 2.85 × 10⁻⁶. -4 mol / cm 3 The swelling ratio was 283 g / g, the water retention rate was 72.6%, and the network morphology was uniform and dense. Example 2, due to its lower degree of neutralization (60%) and smaller amount of crosslinking agent (0.1%), showed a grafting rate (78.5%) and crosslinking density (2.31 × 10⁻⁶). -4 mol / cm 3 The swelling ratio was slightly lower, at 251 g / g, but the network remained relatively uniform, exhibiting good sustained-release performance. Example 3, by increasing the proportion of acrylic acid, the amount of crosslinking agent, and the reaction temperature, achieved the highest grafting rate (88.2%) and crosslinking density (3.12 × 10⁻⁶). -4 mol / cm 3 It has the highest content of carboxyl and amide groups, a swelling rate of 306 g / g, a water retention rate of 75.1%, a dense and highly cross-linked network, and the best overall performance.

[0090] Comparative Example 1, due to the absence of aromatic rings in the physical crosslinking, has a network crosslinking density of only 1.42 × 10⁻⁶. -4 mol / cm 3The structure is loose and porous, with a swelling rate as high as 355 g / g, but a water retention rate of only 54.2% and poor nutrient retention capacity. Comparative Example 2 (sodium humate substitution) suffers from a decrease in grafting rate (69.4%) and carboxyl content (6.84 mmol / g) due to the easy precipitation of sodium humate in acidic systems, resulting in localized network aggregation, a swelling rate of 238 g / g, and a water retention rate of 61.5%. Comparative Example 3 suffers from excessive interference with copolymerization due to humic acid, resulting in a grafting rate of only 61.8%, a carboxyl content reduced to 5.67 mmol / g, uneven network collapse, a swelling rate of only 162 g / g, and a water retention rate of 58.9%.

[0091] Example 4

[0092] A slow-release compound microbial fertilizer comprises the following components: 65g of chemical fertilizer, 8g of ternary copolymer slow-release material (dry weight of the elastic gel polymer used as a binder), 10g of protective carrier, 4g of polylactic acid, 2g of compound microbial powder, and 0.8g of sodium alginate.

[0093] The chemical fertilizer comprises the following components: 25g urea, 19g diammonium phosphate, 20g potassium sulfate, and 1g trace element components; the trace element components include zinc sulfate, boric acid, ferrous sulfate, and manganese sulfate in a mass ratio of 1:0.7:0.5:0.4.

[0094] The protective carrier is a composite carrier of bentonite and biochar, with a mass ratio of bentonite to biochar of 1:1.8.

[0095] The composite microbial powder includes Azotobacter chrysophyte powder, Bacillus megaterium powder, and Bacillus mucilaginosus powder, in a mass ratio of 1:1:1.

[0096] The preparation method of the slow-release compound microbial fertilizer includes the following steps:

[0097] S100. An elastic gel polymer and a ternary copolymer sustained-release material were prepared using the method of Example 1.

[0098] S200: Mix urea, diammonium phosphate, potassium sulfate, trace elements, some ternary copolymer slow-release materials and binder evenly, granulate at ≤55℃, and granulate in a cyclic manner until the particle size is 2~4mm; dry at 50℃ until the moisture content is less than 5% to obtain the core layer particles of chemical fertilizer.

[0099] The obtained elastic gel polymer was mixed with deionized water at a mass ratio of 1:1.2 to prepare a gel solution as a binder. The dry weight of the elastic gel polymer used to prepare the binder accounted for 10% of the total mass of the ternary copolymer slow-release material.

[0100] S300. Dissolve polylactic acid in chloroform, add the remaining ternary copolymer slow-release material, protective carrier, and sodium alginate aqueous solution, and stir until homogeneous to obtain a coating emulsion; add the composite microbial powder to the above coating emulsion, stir and disperse until homogeneous to obtain a microbial-membrane-carrier blend coating solution; control the system temperature at 20~30℃ (fluctuating within this range) during the coating emulsion preparation process.

[0101] The remaining ternary copolymer slow-release material is divided into a first part and a second part. The first part is used to mix and granulate with chemical fertilizer in step S200, and the second part is used to dissolve with polylactic acid in step S300. The mass ratio of the first part to the second part is 5:1.

[0102] The mass ratio of polylactic acid to chloroform is 1:10; sodium alginate is pre-dissolved in deionized water to prepare a sodium alginate solution; the mass ratio of sodium alginate to deionized water is 1:15.

[0103] S400. At 28°C, the above-mentioned bacteria-film-carrier blended coating solution is uniformly sprayed onto the surface of the core layer particles for coating, with a coating layer thickness of 15~60μm. After coating, the coating layer is dried in a ventilated environment at 23°C until the surface of the coating layer is dry and solidified, thus obtaining the slow-release compound microbial fertilizer.

[0104] Example 5

[0105] A slow-release compound microbial fertilizer comprises the following components: 50g of chemical fertilizer, 5g of ternary copolymer slow-release material (dry weight of the elastic gel polymer used as a binder), 5g of protective carrier, 2g of polylactic acid, 1g of compound microbial powder, and 0.5g of sodium alginate.

[0106] The chemical fertilizer comprises the following components: 19.9g urea, 15g diammonium phosphate, 15g potassium sulfate, and 0.1g trace element components; the trace element components include zinc sulfate, boric acid, ferrous sulfate, and manganese sulfate, in a mass ratio of 1:0.4:0.2:0.2.

[0107] The protective carrier is a composite carrier of bentonite and biochar, with a mass ratio of bentonite to biochar of 1:0.5.

[0108] The composite microbial powder includes Azotobacter chrysophyte powder, Bacillus megaterium powder, and Bacillus mucilaginosus powder, in a mass ratio of 1:0.5:0.5.

[0109] The preparation method of the slow-release compound microbial fertilizer includes the following steps:

[0110] S100. An elastic gel polymer and a ternary copolymer sustained-release material were prepared using the method of Example 1.

[0111] S200: Mix urea, diammonium phosphate, potassium sulfate, trace elements, some ternary copolymer slow-release materials and binder evenly, granulate at ≤55℃, and granulate in a cyclic manner until the particle size is 2~4mm; dry at 45℃ until the moisture content is less than 5% to obtain the core layer particles of chemical fertilizer.

[0112] The obtained elastic gel polymer was mixed with deionized water at a mass ratio of 1:0.5 to prepare a gel solution as a binder; the dry weight of the elastic gel polymer used to prepare the binder accounted for 5% of the total mass of the ternary copolymer slow-release material.

[0113] S300. Dissolve polylactic acid in dichloromethane, add the remaining ternary copolymer slow-release material, protective carrier, and sodium alginate aqueous solution, and stir until homogeneous to obtain a coating emulsion; add the composite microbial powder to the above coating emulsion, stir and disperse until homogeneous to obtain a microbial-membrane-carrier blend coating solution; control the system temperature at 20~30℃ (fluctuating within this range) during the coating emulsion preparation process.

[0114] The remaining ternary copolymer slow-release material is divided into a first part and a second part. The first part is used to mix and granulate with chemical fertilizer in step S200, and the second part is used to dissolve with polylactic acid in step S300. The mass ratio of the first part to the second part is 4:1.

[0115] The mass ratio of polylactic acid to dichloromethane is 1:5; sodium alginate is pre-dissolved in deionized water to prepare a sodium alginate solution; the mass ratio of sodium alginate to deionized water is 1:10.

[0116] S400. At 20°C, the above-mentioned bacteria-film-carrier blended coating solution is uniformly sprayed onto the surface of the core layer particles for coating, with a coating layer thickness of 15~60μm. After coating, the coating layer is dried in a ventilated environment at 18°C ​​until the surface of the coating layer is dry and solidified, thus obtaining the slow-release compound microbial fertilizer.

[0117] Example 6

[0118] A slow-release compound microbial fertilizer comprises the following components: 82g of chemical fertilizer, 10g of ternary copolymer slow-release material (including the dry weight of the elastic gel polymer used as a binder), 15g of protective carrier, 5g of polylactic acid, 3g of compound microbial powder, and 1g of sodium alginate.

[0119] The chemical fertilizer comprises the following components: 30g urea, 25g diammonium phosphate, 25g potassium sulfate, and 2g trace element components; the trace element components include zinc sulfate, boric acid, ferrous sulfate, and manganese sulfate in a mass ratio of 1:1:0.8:0.5.

[0120] The protective carrier is a composite carrier of bentonite and biochar, with a mass ratio of bentonite to biochar of 1:3.

[0121] The composite microbial powder includes Azotobacter chrysophyte powder, Bacillus megaterium powder, and Bacillus mucilaginosus powder, in a mass ratio of 1:2:2.

[0122] The preparation method of the slow-release compound microbial fertilizer includes the following steps:

[0123] S100. An elastic gel polymer and a ternary copolymer sustained-release material were prepared using the method of Example 1.

[0124] S200: Mix urea, diammonium phosphate, potassium sulfate, trace elements, some ternary copolymer slow-release materials and binder evenly, granulate at ≤55℃, and granulate in a cyclic manner until the particle size is 2~4mm; dry at 52℃ until the moisture content is less than 5% to obtain the core layer particles of chemical fertilizer.

[0125] The obtained elastic gel polymer was mixed with deionized water at a mass ratio of 1:2 to prepare a gel solution as a binder; the dry weight of the elastic gel polymer used to prepare the binder accounted for 15% of the total mass of the ternary copolymer slow-release material.

[0126] S300. Dissolve polylactic acid in chloroform, add the remaining ternary copolymer slow-release material, protective carrier, and sodium alginate aqueous solution, and stir until homogeneous to obtain a coating emulsion; add the composite microbial powder to the above coating emulsion, stir and disperse until homogeneous to obtain a microbial-membrane-carrier blend coating solution; control the system temperature at 20~30℃ (fluctuating within this range) during the coating emulsion preparation process.

[0127] The remaining ternary copolymer slow-release material is divided into a first part and a second part. The first part is used to mix and granulate with chemical fertilizer in step S200, and the second part is used to dissolve with polylactic acid in step S300. The mass ratio of the first part to the second part is 7:1.

[0128] The mass ratio of polylactic acid to chloroform is 1:15; sodium alginate is pre-dissolved in deionized water to prepare a sodium alginate solution; the mass ratio of sodium alginate to deionized water is 1:20.

[0129] S400. At 30°C, the above-mentioned bacteria-film-carrier blend coating solution is uniformly sprayed onto the surface of the core layer particles for coating, with a coating layer thickness of 15~60μm. After coating, the coating layer is dried in a ventilated environment at 28°C until the surface of the coating layer is dry and solidified, thus obtaining the slow-release compound microbial fertilizer.

[0130] In Examples 4-6, the *Azotobacter chrysophagus* powder had a viable count ≥3 × 10⁻⁶. 9CFU / g, spore count ≥95%; Bacillus megater powder: viable count ≥1×10⁻⁶ 10 CFU / g, spore count ≥95%; Bacillus mucilaginosus powder: viable count ≥1×10⁻⁶ 10 CFU / g, spore rate ≥95%. The contamination rate of the three types of bacterial powder is ≤2%, and the moisture content is ≤8%.

[0131] Comparative Example 4

[0132] No ternary copolymer slow-release material is used: No ternary copolymer slow-release material is used in either the core layer or the coating layer. The binder is replaced with an equal amount of bentonite slurry, with a bentonite:water ratio of 1:5. The rest is the same as in Example 4.

[0133] Comparative Example 5

[0134] The ternary copolymer sustained-release material was replaced with the sustained-release material prepared by the method in Comparative Example 1. The rest was the same as in Example 4.

[0135] Comparative Example 6

[0136] The ternary copolymer sustained-release material was replaced with the sustained-release material prepared by the method in Comparative Example 2. The rest were the same as in Example 4.

[0137] Comparative Example 7

[0138] All of the ternary copolymer slow-release material is added to step S300. The binder for the core layer is replaced with an equal amount of bentonite slurry, with a bentonite:water ratio of 1:5. The rest is the same as in Example 4.

[0139] Comparative Example 8

[0140] All the ternary copolymer sustained-release material from step S300 is added to step S200. The rest is the same as in Example 4.

[0141] Comparative Example 9

[0142] Polylactic acid is not added, and no coating layer is applied. That is, the core layer is granulated directly and used as the finished product, without going through the S300 and S400 steps. The composite microbial powder is sprayed onto the particle surface in the same proportion after the core layer is granulated, and then dried at low temperature. The rest is the same as in Example 4.

[0143] Comparative Example 10

[0144] The protective carrier uses only bentonite, without biochar; that is, the biochar is replaced with an equal mass of bentonite. The rest is the same as in Example 4.

[0145] Comparative Example 11

[0146] The protective carrier uses only biochar, without adding bentonite; that is, the bentonite is replaced with an equal mass of biochar. The rest is the same as in Example 4.

[0147] Comparative Example 12

[0148] The mass ratio of the compound microbial powder, including *Azotobacter chrysophagus* powder, *Bacillus megaterium* powder, and *Bacillus mucilaginosus* powder, is...

[0149] Comparative Example 13

[0150] The bacterial powder formulation does not contain Bacillus mucilaginosus powder; it consists only of Azotobacter chrysophyte powder and Bacillus megaterium powder in a 1:1 mass ratio, and the total amount of bacterial powder added remains unchanged. The remaining components and preparation process are exactly the same as in Example 4.

[0151] Comparative Example 14

[0152] The compound microbial powder was replaced with equal mass ratios of *Azotobacter chrysophagus* powder, *Pseudomonas fluorescens* powder, and *Bacillus subtilis* powder, with a mass ratio of 1:1:1. The viable cell count specifications of the powder remained the same as in Example 4. Everything else was the same as in Example 4.

[0153] Comparative Example 15

[0154] Ordinary commercially available slow-release compound microbial fertilizer.

[0155] The performance of the slow-release compound microbial fertilizers prepared by the methods in Examples 4-6 and Comparative Examples 4-15 is shown in Table 2.

[0156] The performance testing method is as follows:

[0157] Determination of viable count: According to the method in Appendix A of NY / T798-2015, the sample was sealed in an aluminum foil bag and stored at room temperature (25±2℃) and relative humidity of 60%. Samples were taken at 0, 3 and 6 months, and the total number of viable bacteria (CFU / g) was determined by plate counting method. The average value of three replicates for each sample was taken.

[0158] Nitrogen cumulative release rate: Accurately weigh 10.00g of fertilizer sample, place it in a nylon mesh bag, put it in a 250mL wide-mouth bottle, add 200mL of deionized water, seal, and let it stand in a 25℃ constant temperature incubator. Take water samples periodically (5, 15, 30, 60, and 90 days), determine the nitrogen content in the water samples using the Kjeldahl method, and calculate the nitrogen cumulative release rate (nitrogen released / total nitrogen in the sample × 100%). Each sample is tested in triplicate.

[0159] Nitrogen use efficiency in potted maize: A pot experiment was conducted using maize (Zhengdan 958). Each pot contained 5 kg of air-dried soil, and the nitrogen application rate was calculated at 0.2 g N / kg soil (all treatments were nitrogen-equal). A control group without nitrogen application was included. Maize was harvested after 60 days of growth, and the total nitrogen content of the plants was measured. The nitrogen fertilizer utilization rate was calculated as (nitrogen uptake by the plant - nitrogen uptake by the control) / nitrogen application rate × 100%. Five pots were used per treatment.

[0160] Rhizosphere bacterial colonization rate: After the maize pot experiment, rhizosphere soil samples were taken, and the 16S rDNA copy number of Azotobacter chrysophagus, Bacillus megaterium, and Bacillus mucilaginosus were detected by qPCR. The colonization rate (rhizosphere copy number / applied copy number × 100%) was calculated based on the initial copy number of the inoculum powder.

[0161] Micronutrient (zinc) utilization rate: Corn pot plants were used, and equal amounts of zinc were applied (the amount of zinc sulfate added to each treatment was the same). After harvest, the zinc content of the plants was measured, and the zinc utilization rate was calculated (the amount of zinc absorbed by the plant / the amount of zinc applied × 100%).

[0162] Maize kernel yield: Field trial, plot area 20m² 2 Repeat 3 times, with routine field management, and measure yield after harvest.

[0163] Table 2. Performance of the slow-release compound microbial fertilizers prepared by the methods in Examples 4-6 and Comparative Examples 4-15

[0164]

[0165] Note: The unit for the number of live bacteria after 6 months of storage is (×10). 8 CFU / g); Initial viable count in Example 4 ≥ 2.4 × 10⁻⁶ 8 CFU / g.

[0166] As shown in Table 2, the data from Examples 4 to 6 indicate that Example 4 exhibits the best overall performance, with a viable bacterial count of 1.6 × 10⁶ after 6 months of storage. 8 CFU / g, nitrogen utilization rate 43.6%, yield 685 kg / mu; all indicators in Example 5 were the lowest, indicating that the lower amount of slow-release material and protective carrier weakened the ability of the bacterial powder to protect and control the release of nutrients; the performance of Example 6 was close to that of Example 4, but the number of viable bacteria and the yield were slightly lower, indicating that excessively high nutrient concentration may cause slight stress to the microenvironment of the bacterial powder.

[0167] In Comparative Example 4, the nitrogen release rate after 30 days reached as high as 78%, while the nitrogen utilization rate of corn decreased to 28%, a reduction of 15.6 percentage points compared to Example 4; the viable bacterial count decreased to 1.2 × 10⁻⁶. 8 The levels were still higher than the standard because polylactic acid still provided some protection, but the colonization rate of the strain was only 20%, and the zinc utilization rate was only 12%. This proves that the dual slow-release and trace element complexation effects of the terpolymer in the core layer and the coating layer are irreplaceable.

[0168] In Comparative Example 5, the grafting rate was low, the network was loose, the 30-day release rate was 75%, the nitrogen utilization rate was 30%, and the zinc utilization rate was 15%, all significantly worse than in Example 4. This indicates that potassium humate is crucial for constructing a stable three-dimensional network, providing potassium nutrition, and chelating trace elements.

[0169] Comparative Example 6 exhibits performance between Comparative Example 5 and Example 4, with a 30-day release rate of 65%, nitrogen utilization rate of 35%, and zinc utilization rate of 20%, still lower than Example 4. This indicates that sodium humate suffers from uneven network structure due to precipitation issues, and has an extremely low potassium content (only 0.2%), resulting in inferior sustained-release and complexation effects compared to potassium humate.

[0170] Comparative Example 7, lacking a slow-release material in its core layer, resulted in a 70% release rate after 30 days, significant early nutrient burst release, a nitrogen utilization rate of only 32%, and a yield of 560 kg / mu. This demonstrates that slow-release of the core layer matrix is ​​crucial for "inner layer slow-release."

[0171] In Comparative Example 8, the coating layer lacked terpolymers, resulting in accelerated mid-term release of polylactic acid after degradation (92% release rate after 90 days), decreased viable bacterial count and colonization rate, and zinc utilization rate of only 16%. This indicates that the terpolymers in the coating layer play an important role in bacterial community protection and late-stage sustained release. Example 4 achieved synergistic function of the inner and outer layers through a 4-7:1 ratio.

[0172] Comparative Example 9 showed a sharp drop in viable bacterial count to 0.1 × 10⁻⁶ after 6 months of storage. 8 The yield was far below the standard; the 30-day release rate was as high as 85%, the nitrogen utilization rate was only 25%, and the yield was 500 kg / mu. This directly proves that the polylactic acid hydrophobic barrier is the core of the long-term survival of the bacterial powder and the realization of the dual slow-release structure of the outer barrier and the inner matrix.

[0173] In Comparative Example 10, the number of viable bacteria was 1.4 × 10⁻⁶. 8 The colonization rate was 28%, lower than that of Example 4; the release rate after 30 days was 55% (slightly higher), and the yield was 620 kg. Although bentonite can adsorb ions, it lacks the porous structure and carbon source of biochar, resulting in insufficient strain attachment and nutrient supply.

[0174] In Comparative Example 11, the number of viable bacteria was 1.5 × 10⁻⁶. 8 The colonization rate was 25%, but the fertilizer retention capacity was weak, with a 30-day release rate of 58% and a yield of 600 kg. Biochar improved aeration but had poor fertilizer retention. When the two were combined (Example 4), the number of viable bacteria, colonization rate, slow release, and yield were all significantly improved, demonstrating synergistic effects.

[0175] Comparative Example 12, although the viable bacterial count was acceptable (1.5 × 10⁻⁶), 8 However, the high proportion of Bacillus megaterium and Bacillus mucilaginosus led to metabolic competition and nutrient consumption, reducing the soil's available potassium enhancement rate to 25% (lower than 32.6% in Example 4), nitrogen utilization rate to only 36%, and yield to 610 kg. This indicates that the bacterial strain ratio must be within the limits defined in this invention to achieve a balance and synergy in nitrogen fixation, phosphorus solubilization, and potassium release.

[0176] In Comparative Example 13, the soil available potassium enhancement rate was only 8.5%, a decrease of 74% compared to Example 4; nitrogen use efficiency was 34%, and yield was 570 kg. This demonstrates that the potassium-solubilizing function of Bacillus mucilaginosus is irreplaceable, and all three functional bacteria are indispensable.

[0177] In Comparative Example 14, Bacillus subtilis and Pseudomonas fluorescens showed antagonism, with a colonization rate of only 20% and a viable count of 1.3 × 10⁻⁶. 8 The nitrogen utilization rate was 32%, the zinc utilization rate was 18%, and the yield was 550 kg. This indicates that the Bacillus megaterium and Bacillus mucilaginosus screened in this invention not only have complementary functions but also no antagonism, and exhibit better rhizosphere adaptability.

[0178] Comparative Example 15 showed a decrease in viable bacteria count to 0.5 × 10⁻⁶ after 6 months of storage at room temperature. 8 CFU / g is lower than the ≥0.2×10⁻⁶ requirement of the NY / T798-2015 standard. 8 CFU / g, compared to Example 4 (1.6 × 10⁻⁶). 8 The CFU / g ratio decreased by 68.8%, indicating that the conventional coating / carrier system of commercially available products has limited long-term protection capabilities for the microbial powder. This invention, through a multi-physical-chemical protection mechanism synergistically constructed using polylactic acid hydrophobic barrier, sodium alginate hydration layer, and bentonite / biochar composite carrier, provides significantly better protection for the microbial powder. The nitrogen release rate reached 72% at 30 days and 94% at 90 days, with a generally steep release curve and insufficient stability of the slow-release effect. The nitrogen utilization rate of corn was 31%, and the zinc utilization rate was only 14%, significantly lower than in Example 4. This indicates that although commercially available products have a certain slow-release effect and contain microbial powder, the overall temporal coordination between microorganisms and nutrients is not ideal, and the lack of ternary copolymers greatly limits the complexation-slow-release micronutrient function. The rhizosphere colonization rate of functional bacteria in Comparative Example 14 was only 18%, and the soil available potassium enhancement rate was only 20%, a decrease of 56.3% and 38.7% respectively compared to Example 4. This indicates that commercially available products lack efficient colonization carriers and microecological sites to support microorganisms in the rhizosphere environment. This invention utilizes a bentonite / biochar composite protective carrier to provide attachment sites and carbon sources for the bacterial strains, significantly improving the rhizosphere colonization environment. The corn yield was only 550 kg / mu, a 19.7% decrease compared to Example 4 (685 kg / mu), indicating that the yield-increasing effect of commercially available slow-release compound microbial fertilizers in field applications is significantly limited by the survival period and functional synergy of the bacterial community.

Claims

1. A slow-release complex microbial fertilizer, characterized by, By weight, it includes the following components: 50-82 parts chemical fertilizer, 5-10 parts ternary copolymer slow-release material, 5-15 parts protective carrier, 2-5 parts polylactic acid, 1-3 parts compound microbial powder, and 0.5-1 parts sodium alginate. The chemical fertilizer comprises the following components: 20-30 parts urea, 15-25 parts diammonium phosphate, 15-25 parts potassium sulfate, and 0.1-2 parts trace elements. The ternary copolymer slow-release material is an acrylic acid-potassium humate-acrylamide ternary copolymer; The protective carrier is a composite carrier of bentonite and biochar, with a mass ratio of bentonite to biochar of 1:0.5~3. The composite microbial powder includes Azotobacter chrysophyte powder, Bacillus megaterium powder, and Bacillus mucilaginosus powder, with a mass ratio of 1:0.5~2:0.5~2.

2. The slow release complex microbial fertilizer according to claim 1, characterized in that, The trace element components include zinc sulfate, boric acid, ferrous sulfate, and manganese sulfate, in a mass ratio of 1:0.4~1:0.2~0.8:0.2~0.

5.

3. The method of claim 1 or 2, wherein the slow-release complex microbial fertilizer is prepared by the steps of: Includes the following steps: ​ S100. Prepare an elastic gel polymer. After drying and pulverizing the elastic gel polymer, obtain a ternary copolymer sustained-release material. S200: Mix urea, diammonium phosphate, potassium sulfate, trace elements, some ternary copolymer slow-release materials and binder evenly, granulate, dry, and obtain chemical fertilizer core layer granules. S300. Dissolve polylactic acid in an organic solvent, add the remaining ternary copolymer slow-release material, protective carrier, and sodium alginate aqueous solution, stir evenly to obtain a coating emulsion; add the composite microbial powder to the above coating emulsion, stir and disperse evenly to obtain a microbial-membrane-carrier co-coating solution; S400. After spraying the bacterial-film-carrier blended coating liquid onto the surface of the core layer particles of the above-mentioned chemical fertilizer for coating, it is dried at low temperature to obtain granular slow-release compound microbial fertilizer.

4. The preparation method according to claim 3, characterized in that, In step S100, the one-step aqueous solution polymerization method for preparing the elastic gel polymer includes the following steps: Step 1: Dissolve acrylic acid (AA) in deionized water, add KOH solution while stirring, adjust the neutralization degree to 60%-75%, and let it cool naturally to room temperature; Step 2: Add potassium humate and acrylamide in sequence, stir to dissolve, then add N,N'-methylenebisacrylamide, mix thoroughly to obtain monomer mixture; Step 3: Heat the above monomer mixture to 70~75℃, add potassium persulfate, and continue the reaction until the reactants are in an elastic gel state to obtain an elastic gel polymer.

5. The preparation method according to claim 4, characterized in that, The mass ratio of acrylic acid, potassium humate, and acrylamide is 3.5~7:0.5~1.5:1; the mass ratio of acrylic acid to deionized water is 1:1.5~2.5; the amount of crosslinking agent N,N'-methylenebisacrylamide is 0.1%~0.5% of the total mass of monomers; and the amount of initiator potassium persulfate is 0.08~0.15% of the total mass of monomers.

6. The preparation method according to claim 3, characterized in that, In step S100, the elastic gel polymer is dried in a drying oven at 80~100℃ until the moisture content is less than 5%, and then pulverized to a particle size of 80~120 mesh to obtain a ternary copolymer sustained-release material.

7. The preparation method according to claim 3, characterized in that, Take the obtained elastic gel polymer and prepare a gel solution as a binder by stirring at a mass ratio of elastic gel polymer to deionized water of 1:0.5~2. The dry weight of the elastic gel polymer used to prepare the binder accounts for 5% to 15% of the total mass of the ternary copolymer slow-release material; the remaining ternary copolymer slow-release material is divided into a first part and a second part. The first part is used to mix and granulate with chemical fertilizer in step S200, and the second part is used to dissolve with polylactic acid in step S300. The mass ratio of the first part to the second part is 4 to 7:

1.

8. The preparation method according to claim 3, characterized in that, In step S200, the granulation temperature is ≤55℃, and granulation is carried out in a cycle until the particle size is 2~4mm; the particles are dried at 45~52℃ until the moisture content is less than 5% to obtain the core layer particles of chemical fertilizer.

9. The preparation method according to claim 3, characterized in that, In step S300, the organic solvent is selected from chloroform or dichloromethane, and the mass ratio of polylactic acid to organic solvent is 1:5~15; the temperature of the coating emulsion preparation process is controlled at 20~30℃. Sodium alginate is dissolved in deionized water to prepare a sodium alginate solution; the mass ratio of sodium alginate to deionized water is 1:10~20.

10. The preparation method according to claim 3, characterized in that, In step S400, the above-mentioned bacteria-film-carrier blended coating liquid is uniformly sprayed onto the surface of the core layer particles at 20~30℃, with a coating layer thickness of 15~60μm; and then ventilated and dried at 18~28℃ until the coating layer surface is dry and solidified to obtain the slow-release compound microbial fertilizer.

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