Nitrogen-fixing-phosphorus-dissolving-potassium-promoting multifunctional microbial fertilizer and preparation method thereof

Abstract

The application discloses a kind of nitrogen fixation-phosphorus solubilization-potassium promotion multifunctional microbial fertilizer and preparation method thereof, it is related to microbial fertilizer technical field, including multifunctional microbial inoculant core and PVA composite hydrogel coating layer;The preparation method includes: 1) the preparation of multifunctional microbial inoculant core;2) the preparation of PVA composite hydrogel;3) coating process;The application is designed by molecule, so that coating layer can accurately identify the salicylic acid signal secreted by root system, and specific degradation occurs in root-fertilizer interface microdomain, realizes the on-demand supply and spatial targeting of nutrient;This mechanism greatly avoids soil fixation, leaching loss and microbial ineffective competition;At the same time, the system integrates nitrogen fixation, phosphorus solubilization, potassium promotion multifunctional microbial inoculant, combined with the immune excitation function of salicylic acid itself, constructs the rhizosphere microecology of plant-microorganism synergistic effect, realizes the unity of reducing fertilizer and green sustainable development.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of microbial fertilizer, in particular to a nitrogen-fixing-phosphorus-dissolving-potassium-promoting multifunctional microbial fertilizer and a preparation method thereof. BACKGROUND

[0002] The slow-release fertilizer has become an important direction of agricultural fertilizer saving and efficiency increasing by delaying nutrient release through the coating technology.

[0003] The existing technology mainly relies on physical coating, such as polyurethane, sulfur, mineral materials, or non-specific response coating, and the core limitation is that the nutrient release and plant demand are difficult to accurately match: the physical coating type fertilizer realizes release through water permeation, membrane rupture or pore diffusion, and the release rate is fixed by the material properties, which cannot respond to the dynamic demand of the plant growth cycle; even for the products aimed at the rhizosphere, the nutrients are still diffused in the root zone soil, which is easy to be fixed by microorganisms or leached with water, resulting in a large amount of invalid loss and competition, and the fertilizer utilization rate is limitedly improved. In addition, the trigger signal of the existing pH / enzyme sensitive type fertilizer widely exists in the soil, and the specificity is poor, which is easy to be interfered by humic acid, polysaccharide and other hydroxyl-containing substances to trigger, resulting in non-targeted release.

[0004] The above-mentioned technology lacks accurate recognition and response mechanism to plant demand, and is difficult to stably play a role in complex soil environment, which restricts the application effect and popularization value of the slow-release fertilizer. SUMMARY

[0005] The present application relates to the technical field of microbial fertilizer, in particular to a nitrogen-fixing-phosphorus-dissolving-potassium-promoting multifunctional microbial fertilizer and a preparation method thereof.

[0006] In order to achieve the above-mentioned purpose, the technical scheme is adopted as follows:

[0007] The present application firstly proposes a nitrogen-fixing-phosphorus-dissolving-potassium-promoting multifunctional microbial fertilizer, which comprises a multifunctional microbial agent core and a PVA composite hydrogel coating layer.

[0008] The multifunctional microbial agent is prepared by compounding microbial liquid, organic carrier and nutrient medium in a mass ratio of 10-15:80-85:5-8, wherein the microbial liquid is mixed by nitrogen-fixing bacteria, phosphorus-dissolving bacteria and potassium-promoting bacteria in a volume ratio of 4:4:3.

[0009] The PVA composite hydrogel is prepared by compounding polyvinyl alcohol (PVA), carboxymethyl chitosan (CMCS), glycerol (GL) and 2,6-dimethylphenylboronic acid (DMPBA) in a mass ratio of 70-75:10-15:8-10:4-5.

[0010] Preferably, the organic carrier is a mixture of rotten straw powder and soybean meal powder; the nutrient substrate is glucose, dipotassium hydrogen phosphate and magnesium sulfate; the effective viable count of the nitrogen-fixing bacteria needs to be ≥2.0×10 9 CFU / g; the effective viable count of the phosphorus-dissolving bacteria needs to be ≥2.0×10 9 CFU / g; and the effective viable count of the potassium-promoting bacteria needs to be ≥1.5×10 9 CFU / g.

[0011] Preferably, the preparation process of the PVA composite hydrogel comprises the following steps:

[0012] Polyvinyl alcohol powder is added to deionized water at 85°C and stirred until completely dissolved and transparent to obtain a PVA solution, which is cooled to room temperature; carboxymethyl chitosan is dissolved in deionized water at room temperature and stirred until completely dissolved to obtain a carboxymethyl chitosan solution; the PVA solution, the carboxymethyl chitosan solution and glycerol are mixed in proportion and stirred uniformly, and 2,6-dimethylphenylboronic acid is added under continuous stirring, and the pH is adjusted to 6.5-7.0 with dilute NaOH to obtain a coating solution; the PVA composite hydrogel is obtained by deaeration at 4°C.

[0013] The boronic acid group (-B(OH)2) of 2,6-dimethylphenylboronic acid belongs to Lewis acid (the central B atom has an empty orbital), which undergoes two-step reactions at neutral pH (6.5-7.0):

[0014] The first step (coordination binding): the lone pair of electrons of the hydroxyl group (-OH) attacks the empty orbital of the B atom to form a coordination bond (-O→B-), at which time the B atom changes from sp 2 hybridization to sp 3 hybridization, forming a four-coordinated intermediate;

[0015] The second step (dehydration condensation): the -OH connected to the B atom in the intermediate and the -H of the hydroxyl donor undergo dehydration to form a stable boron ester bond (-O-B-O-).

[0016] Further, the -B(OH)2 of 2,6-dimethylphenylboronic acid can simultaneously bind to two hydroxyl donors, such as the hydroxyl group of one polyvinyl alcohol molecule and the hydroxyl group of one carboxymethyl chitosan molecule, to form a bidentate boron ester bond, thereby "cross-linking" different polymer molecular chains and constructing a three-dimensional network structure.

[0017]

[0018] The steric hindrance effect of 2,6-dimethyl can reduce the self-polymerization of 2,6-dimethylphenylboronic acid molecules, while enhancing the selectivity of the boron ester bond, combining with polyhydroxyl compounds and avoiding non-specific reactions with single hydroxyl substances in the soil, such as humic acid monohydroxyl, thereby laying a foundation for subsequent salicylic acid targeted response.

[0019] After cross-linking reaction, multiple non-covalent interactions still exist in the system, which cooperates with borate ester bond to form a dense network, such as the hydrogen bond between the -OH of PVA and the -COOH of CMCS; the hydrogen bond between the -OH of PVA and the -NH2 of CMCS; the hydrogen bond between the -OH of PVA and the -OH of glycerol; the hydrogen bond between the -COOH of CMCS and the -NH2 of CMCS; these hydrogen bonds fill the gaps of borate ester bond cross-linking, improve the network density and mechanical strength, and at the same time do not affect the reversibility of borate ester bond.

[0020] The three primary hydroxyl groups of glycerol form hydrogen bonds with the hydroxyl groups of PVA and CMCS, which are inserted between the polymer molecular chains, weaken the intermolecular forces (such as hydrogen bonds between PVA molecular chains), increase the flexibility of the chains, avoid the subsequent coating layer from being too hard and brittle, and at the same time improve the swelling performance of the hydrogel (facilitating the subsequent permeation of salicylic acid to trigger degradation).

[0021] The multiple hydroxyl groups (3 -OH) of glycerol can act as a "cross-linking bridge" - the multiple hydroxyl groups of one glycerol molecule can form borate ester bonds with the boronic acid groups of different DMPBA molecules, connecting two independent PVA / CMCS molecular chains, making up for the problem of insufficient cross-linking density of PVA hydroxyl groups (isolated secondary hydroxyl groups), and making the hydrogel network more uniform and dense.

[0022] Preferably, the solid content of the coating liquid is 10-12%.

[0023] The application also provides a preparation method of the aforementioned nitrogen-fixing, phosphorus-releasing and potassium-promoting multifunctional microbial fertilizer, which comprises the following steps:

[0024] S1, preparation of the multifunctional microbial agent core: inoculate the three strains into corresponding liquid culture media respectively, and cultivate at 30℃ and 150rpm for 24-48h on a shaking table; mix the three bacterial liquids, inoculate into the sterilized solid-state fermentation medium, and the inoculation amount is 10%; after fermentation at 28℃ for 3-5 days, dry the material at a low temperature below 35℃ under sterile conditions, control the moisture content below 15%; use a granulator to make the dried microbial agent into uniform particles with a diameter of 2-3mm, and obtain the multifunctional microbial agent core;

[0025] S2, coating process: add the multifunctional microbial agent core into the fluidized bed, set the air inlet temperature to 40℃, preheat the multifunctional microbial agent core and achieve stable fluidization state, spray the coating liquid into the fluidized bed at a rate of 8mL / min, uniformly cover the surface of the fluidized fertilizer particles, and evaporate the water to form a PVA composite hydrogel coating film; after the coating is completed, continue to run for 10-15min at a lower fluidization air volume, so as to further solidify and shape the coating layer, take out the finished product, seal and package, and store in a cool and dry place.

[0026] The coating layer is mainly composed of polyvinyl alcohol and 2,6-dimethylphenylboronic acid, the boronic acid group forms a borate ester bond with the hydroxyl group of PVA to form a three-dimensional network structure, which gives the coating layer initial mechanical strength to coat the fertilizer;

[0027] The plant root system is the main part of salicylic acid secretion, and especially when subjected to biological or non-biological stress, the salicylic acid concentration around the rhizosphere is significantly higher than that in the non-rhizosphere soil;

[0028] When the coated fertilizer particles are located in the rhizosphere of plants, the salicylic acid molecules secreted by the root system will diffuse into the coating layer, and the salicylic acid molecules have a carboxyl group and a phenolic hydroxyl group adjacent thereto, forming an ortho-dihydroxy structure;

[0029] The ortho-dihydroxy structure of salicylic acid competes with the ortho-dihydroxy structure of PVA to bind to the boronic acid center, and since the complex formed by salicylic acid and DMPBA is a stable six-membered ring, its thermodynamic stability is higher than that of the borate ester bond formed by DMPBA and PVA, so the reaction will proceed in the direction of generating a salicylic acid-boric acid complex:

[0030]

[0031] The competitive binding reaction causes the borate ester bond originally used for cross-linking PVA to be destroyed, and the three-dimensional polymer network is decomposed into shorter, soluble polymer chain fragments, the coating layer changes from a cross-linked state to a swollen state, and finally completely disintegrates under the mechanical friction and water erosion of the root, after the disintegration of the coating layer, the multifunctional microbial inoculant wrapped inside is released and directly acts on the rhizosphere microenvironment, and is immediately utilized by plants.

[0032] Ordinary phenylboronic acid (PBA) will react with any hydroxyl substance, and the methylation modification (introducing a methyl group at the ortho position of the boronic acid group, i.e., DMPBA) allows the small and matched salicylic acid to enter and bind through the steric hindrance effect, and blocks the large or unmatched soil monohydroxy substances such as humic acid fragments from approaching the boronic acid center, which greatly improves the specificity of the starting mechanism and ensures the specificity of the salicylic acid triggered release.

[0033] Preferably, in S1, the concentration of the bacterial solution in the liquid medium needs to reach 10 8 CFU / mL or more.

[0034] Preferably, in S2, the coating layer accounts for 10-15% of the total weight of the particles, and the thickness is 50-100 μm.

[0035] Compared with the prior art, the beneficial effects of the present application are:

[0036] 1. The prior art relies on water penetration, membrane pressure increase leading to membrane rupture or slow physical diffusion through membrane pores for nutrient release; the release rate is determined by the material itself and is difficult to match the plant demand cycle; the present application relies on the specific chemical reaction of salicylic acid with crosslinking agent to destroy the crosslinking network; when the plant needs (when salicylic acid is secreted), it releases quickly, and when it does not need, it stops, and the release curve is synchronized with the plant growth demand.

[0037] 2. The controlled release fertilizer of the prior art is released by coating to delay release, hoping that the fertilizer will dissolve around the root system, but this cannot prevent nutrients from being fixed by microorganisms in the rhizosphere area or being lost with water, and there is a large amount of invalid space and microbial competition in the whole root zone soil. The release behavior of the present application is directly controlled by the salicylic acid secreted by the root system, and degradation only occurs in the micron level range where the fertilizer particles are in close contact with the roots and the salicylic acid concentration is high enough. Nutrients are only released at the root surface and directly absorbed by plants, greatly reducing soil fixation, leaching and gas volatilization, and greatly improving fertilizer utilization rate.

[0038] 3. The pH / enzyme sensitive fertilizer in the prior art has a trigger signal widely present in the soil, low specificity, is easily affected by environmental fluctuations, has low selectivity, and is easily mis-triggered, resulting in a large amount of non-targeted release. The present application uses steric hindrance effect through molecular structure design, so that it can only combine with specific molecules (salicylic acid) with ortho-dihydroxy group with high efficiency, has high selectivity, and can effectively exclude the interference of most hydroxyl-containing substances in soil such as humic acid and ordinary polysaccharide. The present application gives the coating material a very high signal recognition accuracy, ensuring the stability and reliability of the system in complex soil environment, and only starts when the correct biological command is received, avoiding the invalid loss of nutrients.

[0039] The present application can accurately recognize the salicylic acid signal secreted by the root system through molecular design, and specifically degrades at the root-fertilizer interface microdomain, realizing the "on-demand supply" and "spatial targeting" of nutrients. This mechanism greatly avoids soil fixation, leaching loss and microbial invalid competition, and improves the fertilizer utilization rate to a new height. At the same time, the system integrates nitrogen-fixing, phosphorus-dissolving and potassium-promoting multifunctional microbial agents, combined with the immune stimulating function of salicylic acid itself, to construct a plant-microorganism synergistic rhizosphere microecology, realizing the unity of reducing fertilizer and increasing efficiency and green and sustainable development. BRIEF DESCRIPTION OF DRAWINGS

[0040] Figure 1 Possible mechanism diagram of the nitrogen-fixing, phosphorus-dissolving and potassium-promoting multifunctional microbial fertilizer produced by the present application. DETAILED DESCRIPTION

[0041] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the embodiments of the present application and related drawings. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments of the present application.

[0042] Preparation Example 1: Preparation process of PVA composite hydrogel, including the following steps:

[0043] Polyvinyl alcohol powder is added to deionized water at 85℃ and stirred until completely dissolved and transparent to obtain a PVA solution, which is cooled to room temperature; carboxymethyl chitosan is dissolved in deionized water at room temperature and stirred until completely dissolved to obtain a carboxymethyl chitosan solution; the PVA solution, the carboxymethyl chitosan solution, and glycerol are mixed in proportion and stirred uniformly, and 2,6-dimethylphenylboronic acid is added under continuous stirring to obtain a coating solution, i.e., a PVA composite hydrogel; the pH is adjusted to 6.5-7.0 with dilute NaOH, and the material is deaerated at 4℃.

[0044] The solid content of the coating solution is 10-12% in the PVA composite hydrogel. The mass ratio of polyvinyl alcohol, carboxymethyl chitosan, glycerol, and 2,6-dimethylphenylboronic acid is 75:10:10:4.

[0045] Preparation Example 2: The preparation method is the same as that in Preparation Example 1, but the solid content of the coating solution is 10-12% in the PVA composite hydrogel. The mass ratio of polyvinyl alcohol, carboxymethyl chitosan, glycerol, and 2,6-dimethylphenylboronic acid is 72.5:12.5:9:4.5.

[0046] Preparation Example 3: The preparation method is the same as that in Preparation Example 1, but the solid content of the coating solution is 10-12% in the PVA composite hydrogel. The mass ratio of polyvinyl alcohol, carboxymethyl chitosan, glycerol, and 2,6-dimethylphenylboronic acid is 70:15:8:5.

[0047] Example 1: A preparation method of a nitrogen-fixing-phosphorus-dissolving-potassium-promoting multifunctional microbial fertilizer, including the following steps:

[0048] S1, preparation of a multifunctional microbial agent core: inoculate the bacterial liquid of nitrogen-fixing bacteria, phosphorus-dissolving bacteria, and potassium-promoting bacteria mixed in a volume ratio of 4:4:3 into the corresponding liquid culture medium, and cultivate at 30℃ and 150rpm on a shaking table for 24-48h; inoculate the mixed bacterial liquid into a solid-state fermentation medium made of organic carriers and nutrient substrates in a mass ratio of 10:85:5, and the inoculation amount is 10%; after fermentation at 28℃ for 4 days, dry the material at a low temperature below 35℃ under sterile conditions, and control the moisture content below 15%; use a granulator to make the dried bacterial agent into uniform particles with a diameter of 2-3mm to obtain a multifunctional microbial agent core.

[0049] S2, coating process: using the coating solution obtained in preparation example 3, the multifunctional microbial agent core is added to the fluidized bed, the inlet temperature is set to 40 DEG C, the multifunctional microbial agent core is preheated and reaches a stable fluidized state, the coating solution is sprayed into the fluidized bed at a rate of 8 mL / min, uniformly covers the surface of the fluidized fertilizer particles, the water is rapidly evaporated, and a PVA composite hydrogel coating film is formed; after the coating is completed, continue to run for 10-15 min under a lower fluidization air volume, so that the coating layer is further solidified and shaped, take out the finished product, seal and package, and store in a cool and dry place.

[0050] In the S1, the concentration of the bacterial solution in the liquid culture medium needs to reach 10 8 CFU / mL or more.

[0051] In the S2, the coating layer accounts for 10-15% of the total weight of the particles, and the thickness is 50-100 mu m.

[0052] Example 2: the preparation method is the same as example 1, but the coating solution obtained in preparation example 2 is used, but the mass ratio of composite microbial solution, organic carrier and nutrient medium is 12.5:82.5:6.5.

[0053] Example 3: the preparation method is the same as example 1, but the coating solution obtained in preparation example 1 is used, but the mass ratio of composite microbial solution, organic carrier and nutrient medium is 15:80:8.

[0054] Also designed:

[0055] Comparative example 1: the same as preparation example 2 formula and experimental method, but no carboxymethyl chitosan is added, and polyvinyl alcohol with the same molar mass is used instead;

[0056] Comparative example 2: the same as preparation example 2 formula and experimental method, but no 2,6-dimethylphenylboronic acid is added, and benzene boronic acid with the same molar mass is used instead;

[0057] Comparative example 3: the same as example 2 formula and experimental method, but using sulfur as the coating material;

[0058] Comparative example 4: the same as example 2 formula and experimental method, but the coating layer accounts for 25% of the total weight of the particles;

[0059] Comparative example 5: the same as example 2 formula and experimental method, but the coating layer accounts for 5% of the total weight of the particles.

[0060] The possible action mechanism of the nitrogen-fixing-phosphorus-dissolving-potassium-promoting multifunctional microbial fertilizer produced by the application is as shown in Figure 1 The root system releases salicylic acid, and the salicylic acid combines with the boron acid group in the coating of the nearby microbial fertilizer particles, disassembles the coating layer, and releases the fertilizer.

[0061] The release performance, environmental safety indicators and other related performances of the application were tested according to GB / T-23348-2020 and GB-23400-2009 for each embodiment and comparative example. At the same time, the response efficiency was detected through setting the salicylic acid concentration related comparative test, and the response specificity was verified by adding common hydroxyl substances in the soil. The film adhesion was detected through the oscillation test, and the corresponding results are shown in Table 1.

[0062] Table 1. Performance detection data of microbial fertilizer

[0063]

[0064] Data analysis:

[0065] The extremely low initial release rate of the embodiment proves that the PVA-borate-carboxymethyl chitosan triple network forms a dense coating layer, effectively blocking the rapid penetration of water and the free diffusion of nutrients. In contrast, Comparative Example 1 (without carboxymethyl chitosan) lacks hydrogen bond enhancement, resulting in a more loose network and more severe initial leakage. Comparative Example 5 (coating layer 5%) loses the slow-release function due to the over-thin and incomplete coating layer. The embodiment shows a high cumulative release rate at 28 days and 60 days, indicating that its release is continuous and controllable. Comparative Example 4 (coating layer 25%) has a too thick coating layer, which severely hinders the diffusion of water and signal molecules, resulting in delayed release and may not meet the nutrient needs of crops during the critical growth period. The release mechanism of Comparative Example 3 (sulfur coating) is physical barrier and microbial degradation, and its release curve has low correlation with plant demand and low efficiency.

[0066] In the presence of salicylic acid, the coating layer of the embodiment degrades rapidly within 72h (>70%), which is direct evidence that salicylic acid competes with PVA chains and destroys borate crosslinking. The release of nutrients is no longer dependent on slow physical diffusion, but on rapid chemical triggering. Comparative Example 2 (using ordinary phenylboronic acid) has low response efficiency in the presence of salicylic acid, but high degradation rate in the absence of salicylic acid (or in humic acid), which proves that ordinary PBA lacks steric hindrance and is easily combined with various hydroxyl-containing substances in the soil, resulting in "mis-triggered degradation" of the coating layer, and when it needs to respond, the crosslinking points have been occupied and the reaction is slow. On the contrary, the steric hindrance of the methyl group of 2,6-dimethylphenylboronic acid in the embodiment effectively screens out interfering molecules, ensuring its high specificity and high responsiveness to salicylic acid. The degradation rate of Comparative Example 1 (without carboxymethyl chitosan) in humic acid (16.5%) is higher than that of the embodiment (≈10%), which shows that carboxymethyl chitosan further densifies the network through hydrogen bonds, improving the "anti-interference" ability of the coating layer.

[0067] The excellent compressive strength and low shedding rate of Examples 1-3 are attributed to the interpenetrating / synergic network formed by covalently cross-linked borate ester bonds and physically cross-linked hydrogen bonds, which has both the strength of covalent bonds and the toughness of hydrogen bonds. The network mechanical properties of Comparative Example 1 are significantly reduced due to the lack of hydrogen bond enhancement of carboxymethyl chitosan, and it is more prone to break under mechanical external force. Comparative Example 5 has the lowest natural strength and is prone to shedding due to the too thin coating layer and incomplete structure.

[0068] Examples 1-3 use low-temperature fluidized bed coating, and the hydrogel material is hydrophilic and non-toxic, providing a good microenvironment for microorganisms, so the survival rate is extremely high. However, in the high-temperature melting coating process of Comparative Example 3 (sulfur coating), most of the microorganisms are killed, losing the function of microbial fertilizer. PVA and carboxymethyl chitosan are biodegradable polymers that can eventually decompose into CO2 and H2O under the action of soil microorganisms, avoiding white pollution. Sulfur coating degrades slowly and may cause soil acidification.

[0069] Examples 1-3 release nutrients through salicylic acid signal triggering, releasing nutrients at the right time (when crops need them), in the right place (around the roots), and in the right form (directly available for absorption), greatly reducing soil fixation, leaching, and volatile loss. Comparative Example 1 has reduced utilization due to premature leakage and false triggering. Comparative Example 3, as a traditional slow-release fertilizer, does not synchronize with crop demand, and cannot protect microorganisms, so it has the worst effect.

[0070] The above description is only a preferred specific embodiment of the present application, but the protection scope of the present application is not limited thereto. Any skilled person in the art can make equivalent replacements or changes within the technical scope disclosed by the present application and according to the technical solutions and inventive concepts of the present application, which should be covered within the protection scope of the present application.

Claims

1. A nitrogen-fixing-phosphorus-releasing-potassium-promoting multifunctional microbial fertilizer, characterized in that, The multifunctional microbial agent core and a PVA composite hydrogel coating layer; the multifunctional microbial agent is prepared from a composite microbial liquid, an organic carrier and a nutrient medium in a mass ratio of 10-15:80-85:5-8, wherein the composite microbial liquid is a microbial liquid mixed by nitrogen-fixing bacteria, phosphorus-dissolving bacteria and potassium-promoting bacteria in a volume ratio of 4:4:3; and the PVA composite hydrogel is prepared from polyvinyl alcohol, carboxymethyl chitosan, glycerol and 2,6-dimethylbenzene boronic acid in a mass ratio of 70-75:10-15:8-10:4-5.

2. The nitrogen-fixing, phosphorus-dissolving, and potassium-promoting multifunctional microbial fertilizer according to claim 1, characterized in that, The organic carrier is a mixture of rotten straw powder and soybean meal powder; the nutrient substrate is glucose, dipotassium hydrogen phosphate and magnesium sulfate; the effective viable count of the nitrogen-fixing bacteria needs to be ≥2.0×10 9 CFU / g; the effective viable count of the phosphorus-dissolving bacteria needs to be ≥2.0×10 9 CFU / g; Potassium-promoting bacteria effective viable cell number needs to be ≥ 1.5 x 10 9 CFU / g.

3. The nitrogen-fixing, phosphorus-dissolving, and potassium-promoting multifunctional microbial fertilizer according to claim 1, characterized in that, The preparation process of the PVA composite hydrogel comprises the following steps: adding polyvinyl alcohol powder into deionized water at 85 DEG C, stirring until completely dissolved and transparent to obtain a PVA solution, and cooling to room temperature; dissolving carboxymethyl chitosan in deionized water at room temperature, stirring until completely dissolved to obtain a carboxymethyl chitosan solution; mixing the PVA solution, the carboxymethyl chitosan solution and glycerol in proportion, stirring uniformly, adding 2,6-dimethylbenzene boronic acid under continuous stirring, adjusting the pH to 6.5-7.0 with dilute NaOH to obtain a coating liquid; and standing at 4 DEG C to remove bubbles to obtain the PVA composite hydrogel.

4. The nitrogen-fixing, phosphorus-dissolving, and potassium-promoting multifunctional microbial fertilizer according to claim 3, characterized in that, The solid content of the coating liquid is 10-12%.

5. The method for preparing the multifunctional microbial fertilizer for nitrogen fixation-phosphorus solubilization-potassium promotion according to any one of claims 1-4, characterized in that, The method comprises the following steps: S1, preparation of the multifunctional microbial agent core: inoculating nitrogen-fixing bacteria, phosphorus-dissolving bacteria and potassium-promoting bacteria into corresponding liquid culture media respectively, and culturing at 30 DEG C and 150 rpm for 24-48 h; inoculating the mixed bacterial liquid into sterilized solid-state fermentation medium at an inoculation amount of 10%; fermenting at 28 DEG C for 3-5 days, and then drying the material at a low temperature below 35 DEG C under sterile conditions to control the moisture content below 15%; and using a granulator to prepare the dried microbial agent into uniform particles with a diameter of 2-3 mm to obtain the multifunctional microbial agent core; S2, coating process: adding the multifunctional microbial agent core into a fluidized bed, setting the air inlet temperature to 40 DEG C to preheat and achieve stable fluidization of the multifunctional microbial agent core, spraying the PVA composite hydrogel into the fluidized bed at a rate of 8 mL / min to uniformly cover the surface of the fluidized fertilizer particles, and rapidly evaporating the water to form a PVA composite hydrogel coating layer; continuing to run at a lower fluidization air volume for 10-15 min after the coating is completed to further solidify and shape the coating layer, taking out the finished product, sealing and packaging, and storing in a cool and dry place.

6. The method of claim 5, wherein the method comprises the steps of: a) inoculating the nitrogen-fixing, phosphorus-dissolving, and potassium- promoting microorganism into the culture medium; b) culturing the microorganism in the culture medium; c) collecting the microorganism; d) mixing the microorganism with the carrier; and e) drying the mixture. The concentration of the bacterial liquid in the liquid culture medium needs to reach 10 8 CFU / mL above; the solid-state fermentation medium is made of organic carriers and nutrient substrates.

7. The method of claim 5, wherein the method comprises the steps of: a) inoculating the nitrogen-fixing, phosphorus-dissolving, and potassium- promoting microorganism into the culture medium; b) culturing the microorganism in the culture medium; c) collecting the microorganism; d) mixing the microorganism with the carrier; and e) drying the mixture. In the step S2, the coating layer accounts for 10-15% of the total weight of the particles, and the thickness is 50-100 mu m.

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