Slow-release controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture and preparation method thereof

Abstract

The invention relates to the technical field of fertilizer preparation, and discloses a slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture and a preparation method thereof.The slow-release and controlled-release multifunctional yield-increasing fertilizer comprises a core layer, a middle layer, a shell layer and an outer attachment layer, and the preparation method comprises the steps that a multi-layer coating design is adopted, and a nutrient core layer is formed through melt blending granulation in sequence; a slow-release and carbon-sequestration synergistic intermediate layer is constructed by biochar-humic acid dynamic cross-linked gel, a photoresponse shell layer is formed by electrostatic spraying of nitrogen-doped nano-zinc oxide composite membrane liquid, and finally, the microbial capsule is accurately anchored on the surface in a micro-aerobic environment through an enzymatic grafting process. Through nano adsorption of a core layer, dynamic crosslinking of a middle layer and multi-level collaborative design of a shell light response film, nutrient controlled release and soil carbon sequestration functions are integrated; a multi-response mechanism is constructed based on environmental signal perception, and dynamic matching of the release rate and crop requirements is realized; an enzymatic grafting technology is used for anchoring a microbial inoculum to break through the bottleneck of activity protection, and an ecological closed loop is formed by combining with a full-degradable material.

Description

Technical Field

[0001] The present invention relates to the technical field of fertilizer preparation, in particular to a slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture and a preparation method thereof. Background Art

[0002] With the deepening of the green and low-carbon transformation of agriculture, the integration of slow-release fertilizers and carbon sequestration efficiency enhancement technologies has become a key direction for improving the quality of cultivated land and reducing emissions and carbon fixation. Traditional slow-release fertilizers mostly rely on a single mechanism of coating physical barrier or chemical adsorption to achieve nutrient controlled release, while carbon sequestration efficiency enhancement is mainly achieved through the separate application of materials such as biochar and humic acid. However, existing technologies often regard "nutrient controlled release" and "carbon fixation" as independent functional modules, lacking collaborative design between material systems, resulting in problems such as misalignment of slow-release cycle and carbon sequestration cycle, and delayed environmental response in actual applications. It is difficult to meet the integrated needs of "fertilizer preservation, carbon sequestration, and efficiency improvement" under complex field conditions.

[0003] In the existing technology, the controlled-release performance of slow-release fertilizers is easily affected by fluctuations in environmental temperature and humidity. Under high temperature and high humidity, the coating layer is prone to irreversible degradation, causing an explosive release of nutrients; and under low-temperature dry conditions, the release rate is too low to match the needs of crops. At the same time, the pore structure of carbon sink materials such as biochar is easily destroyed by high temperatures during fertilizer processing, resulting in a decrease in CO2 adsorption capacity, and its simple compounding with slow-release components makes it difficult to achieve a temporal match between nutrient release and humic acid activity enhancement. In addition, the actual application effect of multifunctional fertilizers is further restricted by problems such as the attenuation of the activity of microbial agents due to insufficient carrier protection during fertilizer storage, and the poor controlled-release stability caused by the low efficiency of light-responsive materials in utilizing visible light.

[0004] The root cause of these problems lies in the failure of existing technologies to build a multi-layered, coordinated material system, and to achieve environmental adaptive response through structural design and functional coupling. To address this issue, the present invention proposes a slow-release, controlled-release, multifunctional yield-increasing fertilizer based on carbon sequestration agriculture and its preparation method. Summary of the Invention

[0005] In response to the deficiencies of the existing technology, the present invention provides a slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture and a preparation method thereof, which solves the problems of traditional fertilizers having a single function, delayed environmental response and insufficient material synergy.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solutions:

[0007] The first aspect of the present invention provides a slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture, comprising the following layer structures in percentage by weight:

[0008] The nuclear layer is 50-70%, composed of urea, potassium dihydrogen phosphate, potassium chloride and nano-hydroxyapatite;

[0009] The middle layer, 8-20%, consists of biochar, humic acid, sodium alginate, borax, and citric acid;

[0010] The outer shell layer is 5-15%, composed of hydroxypropyl starch, chitosan and nitrogen-doped nano-zinc oxide;

[0011] The outer layer is 0.5-6%, and is composed of nitrogen-fixing bacteria and phosphate-dissolving bacteria wrapped in poly-γ-glutamic acid, and carboxymethyl cellulose nanofibers.

[0012] Preferably, the urea accounts for 50-65% of the core layer, the potassium dihydrogen phosphate accounts for 15-25% of the core layer, the potassium chloride accounts for 8-12% of the core layer, and the nano-hydroxyapatite accounts for 1-3% of the core layer.

[0013] The core layer is composed of urea, potassium dihydrogen phosphate and potassium chloride as the main nutrients. A homogeneous melt is formed by melt blending and spray granulation to form microspheres with a particle size of 1-3 mm. Nanohydroxyapatite (nHA) is embedded in the core layer as a sustained-release regulator. Its nano-scale porous structure can absorb some nutrient ions (such as NH4 + , K + ) and forms hydrogen bonds with nutrient ions via surface hydroxyl groups, slowing nutrient dissolution. The introduction of nHA changes the traditional sustained-release mechanism of the core layer, which relies solely on physical coating, to achieve a dual sustained-release effect of chemical adsorption and physical barrier.

[0014] Preferably, the biochar occupies 50-70% of the middle layer, the humic acid occupies 20-30% of the middle layer, the sodium alginate occupies 5-10% of the middle layer, the borax occupies 0.2-0.5% of the middle layer, and the citric acid occupies 0.5-1.5% of the middle layer. The biochar in the middle layer is prepared by oxygen-limited pyrolysis of agricultural straw, the pyrolysis temperature is 250-350°C, and the specific surface area is ≥300m 2 / g, the mass ratio of borax to citric acid in the middle layer is 1:2-1:4, forming a dynamic cross-linking system.

[0015] The middle layer is composed of agricultural straw oxygen-limited pyrolysis biochar as the core, which has a high specific surface area (≥300m 2 / g) and a rich pore structure can absorb CO2 and free nutrients, improving carbon sequestration efficiency. Humic acid and sodium alginate form a dynamic gel network through a borax-citric acid double cross-linking system:

[0016] Borax (Na2B4O7) provides borate ions, which form coordination bonds with the carboxyl groups of sodium alginate;

[0017] Citric acid regulates the pH of the gel through protonation and partially dissociates when the humidity is >30%, releasing humic acid.

[0018] This dynamic cross-linking design enables the middle layer to maintain structural stability under dry conditions, while releasing humic acid in a wet environment, promoting the formation of soil aggregates, thereby simultaneously achieving carbon fixation and nutrient controlled release.

[0019] Preferably, the hydroxypropyl starch accounts for 50-70% of the outer shell layer, the chitosan accounts for 20-30% of the outer shell layer, the nitrogen-doped nano-zinc oxide accounts for 0.5-2% of the outer shell layer, the nitrogen doping concentration of the nitrogen-doped nano-zinc oxide in the outer shell layer is 1.5-2.2at%, the particle size is 20-50nm, and the degree of substitution of the hydroxypropyl starch in the outer shell layer is ≥80%.

[0020] The outer shell uses a hydroxypropyl starch and chitosan composite membrane as a degradable matrix. The hydrogen bonds and hydrophobic interactions between its molecular chains form a dense network, which slows the penetration of water into the outer layer. Nitrogen-doped nanozinc oxide (N-ZnO) is used as a light-responsive functional material. By introducing lattice defects through doping, its light absorption range is extended to the visible light region (400-600nm). Under light conditions, N-ZnO produces reactive oxygen species (ROS), which attack the glycosidic bonds of the hydroxypropyl starch molecular chains, triggering the controlled degradation of the membrane layer. This design dynamically matches nutrient release with light intensity, avoiding the limitations of traditional light-responsive materials that rely solely on ultraviolet light.

[0021] Preferably, the poly-gamma-glutamic acid accounts for 40-60% of the outer layer, the carboxymethyl cellulose nanofibers account for 20-40% of the outer layer, the total amount of the nitrogen-fixing bacteria and the phosphate-solubilizing bacteria accounts for 0.5-2% of the outer layer, the carboxymethyl cellulose nanofibers are grafted onto the surface of the poly-gamma-glutamic acid capsule through laccase catalysis, and the grafting rate is ≥85%, the nitrogen-fixing bacteria is Azotobacter chroococcum, and the phosphate-solubilizing bacteria is Pseudomonas fluorescens.

[0022] The outer layer is encapsulated with nitrogen-fixing bacteria and phosphate-dissolving bacteria by poly-γ-glutamic acid, forming a microbial capsule. Laccase-catalyzed oxidation of carboxymethyl cellulose nanofibers forms a covalent bond between the phenolic hydroxyl groups on its surface and the amino groups of the poly-γ-glutamic acid capsule, achieving directional grafting. The pH-responsive layer is formed after grafting of carboxymethyl cellulose nanofibers:

[0023] In alkaline soil (pH>7), the carboxyl groups of CMC nanofibers ionize to generate electrostatic repulsion, maintaining the integrity of the capsule;

[0024] In the slightly acidic environment of the root system (pH = 5-6.5), the carboxyl groups are protonated, and the carboxymethyl cellulose nanofibers dissolve and release the bacterial agent.

[0025] Laccase-catalyzed grafting technology avoids the toxicity of chemical cross-linkers to bacteria, while ensuring a grafting rate of ≥85%, significantly improving the survival rate of bacterial agents in the field.

[0026] Preferably, the core layer is a melt-blended microsphere with a particle size of 1-3 mm, the middle layer has a thickness of 200-400 μm, and the outer shell layer has a thickness of 50-100 μm.

[0027] The second aspect of the present invention provides a method for preparing a slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture, which is prepared using the slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture, comprising the following steps:

[0028] S1. Preparation of core layer: melt-blending urea, potassium dihydrogen phosphate, and potassium chloride, adding nano-hydroxyapatite, and spray granulating to form microspheres;

[0029] S2, middle layer construction: biochar is mixed with humic acid-sodium alginate dynamic cross-linked gel and coated on the surface of the core layer;

[0030] S3, outer shell construction: hydroxypropyl starch, chitosan and nitrogen-doped nano zinc oxide are prepared into a membrane solution, which is then electrostatically sprayed on the surface of the middle layer;

[0031] S4. Microbial capsule attachment: nitrogen-fixing bacteria and phosphate-solubilizing bacteria are embedded in poly-gamma-glutamic acid capsules. In a microaerobic environment, the dissolved oxygen concentration is ≤2mg / L, the reaction temperature is 35-45°C, and carboxymethyl cellulose nanofibers are enzymatically grafted and fixed to the outer shell layer;

[0032] S5. Post-processing: low-temperature drying and screening.

[0033] The core layer melting temperature is ≤65℃ to avoid high temperature decomposition of urea;

[0034] The cross-linking temperature of the middle layer gel is ≤45°C, preserving the pore structure of the biochar;

[0035] The temperature during the entire preparation process of microbial capsules is ≤45°C, and a microaerobic environment (dissolved oxygen ≤2 mg / L) is used to maintain the metabolic activity of the bacteria.

[0036] The preparation sequence and temperature control of each layer ensure the structural stability and activity retention of the functional materials, and ultimately form a "core-shell-bacteria" synergistic enhancement system.

[0037] Preferably, in step S1:

[0038] The melting temperature is 55-65°C, the inlet temperature of spray granulation is 70-85°C, and the outlet temperature is 35-45°C;

[0039] The added amount of nano-hydroxyapatite is 1-3% of the total weight of the core layer.

[0040] Preferably, in step S2:

[0041] The preparation conditions of the dynamic cross-linked gel are: pH = 6.0-7.0, temperature 35-45°C, stirring time ≥ 30 minutes;

[0042] The inlet air temperature of the fluidized bed coating is 35-45°C, and the coating thickness is 200-400μm.

[0043] Preferably, in step S3:

[0044] The voltage of electrostatic spraying is 25-35kV, and the spraying distance is 10-20cm;

[0045] The concentration of nitrogen-doped nano zinc oxide in the film-forming solution is 0.5-2%.

[0046] The present invention provides a slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture and a preparation method thereof.

[0047] It has the following beneficial effects:

[0048] 1. The present invention achieves multi-dimensional adaptation of nutrient release rate to soil environment through the hierarchical design of core layer nanomaterial adsorption, middle layer dynamic cross-linking and outer shell light-responsive membrane, breaking through the single controlled-release mode of traditional fertilizers. The synergistic effect of humic acid and biochar delays nutrient loss while enhancing soil carbon sequestration capacity, forming an agricultural technology path with the dual effects of "fertilizer preservation and carbon sequestration".

[0049] 2. Based on the dynamic perception of environmental signals such as humidity, pH and light, the present invention constructs a multi-response system of borax-citric acid reversible cross-linking network, enzymatic grafted capsules and nitrogen-doped photosensitive film. This mechanism enables nutrient release to accurately match crop needs and environmental changes, overcoming the problem of uncontrolled release of conventional slow-release fertilizers due to environmental fluctuations, and significantly improving fertilizer utilization.

[0050] 3. The present invention uses enzymatic grafting technology to anchor the microbial agent in a carboxymethyl cellulose-alginate composite matrix, isolating it from external environmental stress through a double barrier of chemical bonding and physical encapsulation. This strategy ensures a high survival rate of the microbial agent while achieving targeted release in acidic soil environments, solving the technical bottleneck of the asynchronous attenuation and release of microbial fertilizer activity.

[0051] 4. From the pore protection of low-temperature pyrolysis biochar to the degradable film formation of hydroxypropyl starch, the present invention uses a green modification process for each component to avoid energy-consuming processes such as high temperature and high pressure. After completing the controlled-release function, the material system can be naturally degraded in the soil, which not only reduces the risk of microplastic pollution, but also improves the soil structure through the continuous action of humic acid and biochar, forming an ecological benign cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 The figure is a flow chart of the preparation method of the present invention. DETAILED DESCRIPTION

[0053] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the present specification. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0054] Please see the attached Figure 1 The present invention provides a slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture and a preparation method thereof through three embodiments. The embodiments are as follows:

[0055] Example 1:

[0056] Raw material ratio:

[0057] Nuclear layer (60%): urea 55%, potassium dihydrogen phosphate 20%, potassium chloride 10%, nanohydroxyapatite 2%;

[0058] Middle layer (12%): biochar 60% (pyrolysis temperature 300°C), humic acid 25%, sodium alginate 8%, borax 0.3%, citric acid 1.0%;

[0059] Shell layer (7.8%): 60% hydroxypropyl starch, 25% chitosan, 1.5% nitrogen-doped nano-zinc oxide (nitrogen doping concentration 1.8 at%);

[0060] Outer layer (2%): PGA 50%, carboxymethyl cellulose nanofiber 30%, nitrogen-fixing bacteria and phosphate-dissolving bacteria 1.5%.

[0061] Preparation steps:

[0062] Nuclear layer preparation:

[0063] Urea, potassium dihydrogen phosphate, and potassium chloride were melted at 60°C, and 2% nHA was added and stirred for 30 minutes;

[0064] Spray granulation (inlet temperature 80°C, outlet temperature 40°C) was performed to obtain microspheres with a particle size of 1.5 mm.

[0065] Middle layer construction:

[0066] Biochar (pyrolyzed at 300 °C) was mixed with humic acid-sodium alginate gel (pH = 6.5, stirred at 40 °C for 40 min);

[0067] Fluidized bed coating (inlet air temperature 40°C), middle layer thickness 250μm.

[0068] Shell construction:

[0069] Hydroxypropyl starch, chitosan and 1.5% N-ZnO were mixed into a membrane solution, which was electrostatically sprayed (voltage 30 kV, spraying distance 15 cm) to a membrane thickness of 70 μm.

[0070] Microbial capsule attached:

[0071] Nitrogen-fixing bacteria and phosphate-solubilizing bacteria were encapsulated in poly-γ-glutamic acid capsules and grafted onto carboxymethyl cellulose nanofibers catalyzed by laccase (dissolved oxygen 1.5 mg / L, reaction at 40°C for 4 h);

[0072] The capsules are adsorbed on the surface of the outer shell.

[0073] Post-treatment: Dry at 40℃ to a moisture content of <5%, and sieve to obtain 2-4 mm particles.

[0074] Example 2:

[0075] Raw material ratio:

[0076] Nuclear layer (50%): urea 50%, potassium dihydrogen phosphate 15%, potassium chloride 8%, nanohydroxyapatite 1%;

[0077] Middle layer (8%): biochar 50% (pyrolysis temperature 250°C), humic acid 20%, sodium alginate 5%, borax 0.2%, citric acid 0.5%;

[0078] Shell layer (5%): hydroxypropyl starch 50%, chitosan 20%, nitrogen-doped nano-zinc oxide 0.5% (nitrogen doping concentration 1.5at%);

[0079] Outer layer (0.5%): PGA 40%, carboxymethyl cellulose nanofiber 20%, nitrogen-fixing bacteria and phosphate-dissolving bacteria 0.5%.

[0080] Preparation steps:

[0081] Nuclear layer preparation:

[0082] The raw materials were melted at 55°C and spray granulated (inlet temperature 70°C, outlet temperature 35°C) to obtain microspheres with a particle size of 1 mm.

[0083] Middle layer construction:

[0084] Biochar (pyrolyzed at 250 °C) was mixed with gel (pH = 6.0, stirred at 35 °C for 30 min);

[0085] Fluidized bed coating (inlet air temperature 35°C), middle layer thickness 200 μm.

[0086] Shell construction:

[0087] Electrostatic spraying (voltage 25kV, spraying distance 10cm), film thickness 50μm.

[0088] Microbial capsule attached:

[0089] Laccase-catalyzed grafting (dissolved oxygen ≤ 2 mg / L, reaction at 35°C for 5 hours).

[0090] Post-treatment: drying at 35°C and sieving into 2-4 mm particles.

[0091] Example 3:

[0092] Raw material ratio:

[0093] Nuclear layer (70%): urea 65%, potassium dihydrogen phosphate 25%, potassium chloride 12%, nanohydroxyapatite 3%;

[0094] Middle layer (15%): biochar 70% (pyrolysis temperature 350°C), humic acid 30%, sodium alginate 10%, borax 0.5%, citric acid 1.5%;

[0095] Shell layer (10%): 70% hydroxypropyl starch, 30% chitosan, 2% nitrogen-doped nano-zinc oxide (nitrogen doping concentration 2.2 at%);

[0096] Outer layer (3%): PGA 60%, carboxymethyl cellulose nanofiber 40%, nitrogen-fixing bacteria and phosphate-dissolving bacteria 2%.

[0097] Preparation steps:

[0098] Nuclear layer preparation:

[0099] The raw materials were melted at 65°C and spray granulated (inlet temperature 85°C, outlet temperature 45°C) to obtain microspheres with a particle size of 3 mm.

[0100] Middle layer construction:

[0101] Biochar (pyrolyzed at 350 °C) was mixed with gel (pH = 7.0, stirred at 45 °C for 50 min);

[0102] Fluidized bed coating (inlet air temperature 45°C), middle layer thickness 400 μm.

[0103] Shell construction:

[0104] Electrostatic spraying (voltage 35kV, spraying distance 20cm), film thickness 100μm.

[0105] Microbial capsule attached:

[0106] Laccase-catalyzed grafting (dissolved oxygen ≤ 2 mg / L, reaction at 45°C for 3 hours).

[0107] Post-treatment: drying at 45°C and sieving into 2-4 mm particles.

[0108] Comparative Examples 1-7:

[0109] Comparative Example 1:

[0110] Compared with Example 1, the difference is that: no nanohydroxyapatite (nHA) is added to the core layer, and it is composed only of urea, potassium dihydrogen phosphate, and potassium chloride in the original proportion. The other steps and parameters are the same.

[0111] Comparative Example 2:

[0112] Compared with Example 1, the difference is that the nitrogen-doped nano-zinc oxide (N-ZnO) in the outer shell layer is replaced by ordinary nano-zinc oxide (not doped with nitrogen), and the other steps and parameters are the same.

[0113] Comparative Example 3:

[0114] Compared with Example 1, the difference is that no borax and citric acid crosslinking agents are added to the middle layer, only sodium alginate and humic acid are physically mixed, and the other steps and parameters are the same.

[0115] Comparative Example 4:

[0116] Compared with Example 1, the difference is that in the microbial capsule attachment step, laccase is not used to catalyze the grafting of carboxymethyl cellulose nanofibers, and the carboxymethyl cellulose nanofibers are directly physically mixed with the poly-γ-glutamic acid capsules before adsorption. The other steps and parameters are the same.

[0117] Comparative Example 5:

[0118] Compared with Example 1, the difference is that the pyrolysis temperature of the middle layer biochar is 400°C (beyond the range of 250-350°C in the claims), and the other steps and parameters are the same.

[0119] Comparative Example 6:

[0120] Compared with Example 1, the difference is that the degree of substitution of hydroxypropyl starch in the outer shell layer is 70% (lower than the requirement of ≥80% in the claims), and the other steps and parameters are the same.

[0121] Comparative Example 7:

[0122] Compared with Example 1, the difference is that the total amount of nitrogen-fixing bacteria and phosphate-dissolving bacteria in the outer layer is 3% (exceeding the range of 0.5-2% in the claims), and the other steps and parameters are the same.

[0123] Test Example 1-4:

[0124] Test Example 1: Comparative Experimental Description of Nutrient Slow Release Performance and Humic Acid Release

[0125] Purpose of the experiment:

[0126] Verify the synergistic effect of core layer nanohydroxyapatite (nHA) and middle layer dynamic cross-linking system on nutrient sustained release and humic acid humidity response release.

[0127] Experimental steps:

[0128] Sample grouping:

[0129] Example 1: Fertilizer granules prepared according to claim 1 and Example 1;

[0130] Comparative Example 1: Fertilizer granules without nHA added to the core layer;

[0131] Comparative Example 3: Fertilizer granules without the addition of borax-citric acid cross-linking agent in the middle layer.

[0132] Nutrient release test in water:

[0133] Soaking conditions: Place 10g of fertilizer granules in 500mL of deionized water at a constant temperature of 25°C in the dark.

[0134] Sampling time: 7th day, 30th day, 60th day;

[0135] Detection method:

[0136] Take 5mL of solution and centrifuge and filter, and determine NH4 by spectrophotometry. + -N、PO4 3- -P, K + concentration;

[0137] Calculate the cumulative release rate (%) = (dissolution amount / total content) × 100.

[0138] Humic acid humidity response release test:

[0139] Simulate soil environment: Mix fertilizer granules with quartz sand (simulated soil) at a ratio of 1:10 and place in a sealed container;

[0140] Humidity Control:

[0141] Low humidity group: 30% humidity (adjusted with saturated NaCl solution);

[0142] High humidity group: 60% humidity (adjusted by saturated KNO3 solution);

[0143] Sampling and testing: 2 g of the mixture was centrifuged every day, and the humic acid release was determined by ultraviolet spectrophotometry (280 nm).

[0144] Experimental data:

[0145] Table 1 Comparison of nutrient slow-release performance and humic acid release

[0146]

[0147] Data Description:

[0148] Sustained release performance:

[0149] Due to the sustained release effect of nHA, the nitrogen release rate of Example 1 over 60 days was 13.2% lower than that of Comparative Example 1;

[0150] Due to the lack of dynamic cross-linking, the nutrient release rate of Comparative Example 3 is between Example 1 and Comparative Example 1.

[0151] Humic acid humidity response:

[0152] The humic acid release of Example 1 under high humidity (60%) is 283% higher than that of Comparative Example 3;

[0153] The humic acid release of Comparative Example 1 (only the core layer difference) is close to that of Example 1, indicating that the dynamic cross-linking system dominates the humidity response.

[0154] Experimental Summary: This experiment demonstrates that the introduction of nanohydroxyapatite (nHA) into the core layer significantly slows the rapid dissolution of nutrients. nHA adsorbs nitrogen, potassium, and other ions through its nanopores and forms hydrogen-bonding networks with molecules like urea. Through a dual mechanism of physical barrier and chemical adsorption, the 60-day nitrogen release rate is controlled at 78.9%, a 13.2% decrease compared to the control group without nHA. This result demonstrates the synergistic enhancement of the sustained-release properties of traditional melt granulation by nanomaterials, overcoming the technical limitations of relying solely on a coating layer for controlled release.

[0155] The humidity-responsive nature of the dynamic cross-linking system in the intermediate layer was clearly demonstrated in experiments. When the ambient humidity rose to 60%, the borax-citric acid cross-linking network partially dissociated due to the protonation of citric acid, releasing humic acid at a rate 283% higher than that of the control group without a cross-linker. The slow release of humic acid dynamically matched soil moisture, preventing ineffective release in dry conditions while promoting soil aggregate formation in moist environments, achieving simultaneous optimization of carbon fixation and controlled nutrient release.

[0156] In addition, the coordinated design of the core layer and the middle layer further enhances the overall slow-release effect. The adsorption of nutrients in the core layer by nHA slows the initial dissolution rate, while the dynamic cross-linking system in the middle layer inhibits nutrient loss through the release of humic acid in the middle and late stages. The two complement each other in time and space, enabling the fertilizer to maintain a stable nutrient supply for 60 days while reducing the risk of uncontrolled release due to environmental fluctuations. This multi-level synergistic mechanism provides the core support for the invention to achieve long-term slow release and carbon sequestration efficiency.

[0157] Test Example 2: Comparative Experimental Description of Carbon Sequestration Efficiency and Controlled Release Synergy

[0158] Purpose of the experiment:

[0159] Verify the effects of the dynamic cross-linking system of the intermediate layer and the pyrolysis temperature of biochar on the synergy of carbon sequestration efficiency and nutrient controlled release.

[0160] Experimental steps:

[0161] Sample grouping:

[0162] Example 1: Fertilizer granules prepared according to Example 1 (biochar pyrolysis temperature 300°C);

[0163] Comparative Example 3: Fertilizer granules without a borax-citric acid crosslinking agent in the middle layer;

[0164] Comparative Example 5: Fertilizer granules with a biochar pyrolysis temperature of 400°C (other conditions are the same as those in Example 1).

[0165] Soil organic carbon content test:

[0166] Soil culture: Mix fertilizer granules with sandy loam at a ratio of 1:20 and place in an incubator (25°C constant temperature, 60% humidity);

[0167] Sampling time: 30th day, 60th day;

[0168] Detection method:

[0169] 5 g of soil sample was taken and the organic carbon content (g / kg) was determined by potassium dichromate oxidation method;

[0170] Calculate the organic carbon increment (%) = (experimental group-blank group) / blank group×100.

[0171] CO2 static adsorption test

[0172] Sample processing: Take the middle layer of biochar and grind it to 200 mesh separately;

[0173] Adsorption conditions: 0.5 g of biochar was placed in a closed adsorption apparatus and pure CO2 gas (pressure 1 atm, 25 °C) was introduced;

[0174] Detection method: Record the CO2 adsorption amount (mg / g) within 30 minutes.

[0175] Experimental data:

[0176] Table 2 Comparison of carbon sink efficiency and CO2 adsorption performance

[0177]

[0178] Data Description:

[0179] Carbon sequestration efficiency:

[0180] In Example 1, due to the release of humic acid by the dynamic cross-linking system, the organic carbon increment over 60 days was 116% higher than that of Comparative Example 3, indicating the promoting effect of humic acid on soil carbon fixation;

[0181] In Comparative Example 5, the pores of the biochar collapsed due to high-temperature pyrolysis (400° C.), and the carbon sequestration efficiency decreased by 32% compared with Example 1.

[0182] CO2 adsorption capacity:

[0183] The CO2 adsorption capacity of the biochar of Example 1 (pyrolysis at 300°C) is 93% higher than that of the biochar of Comparative Example 5 (pyrolysis at 400°C), which verifies the protective effect of low-temperature pyrolysis on the pore structure;

[0184] In Comparative Example 3, due to the lack of dynamic cross-linking, the humic acid release is low, and the CO2 adsorption capacity depends solely on biochar, but is still lower than that of Example 1.

[0185] Experimental Summary: The mechanism by which the dynamic cross-linking system improves carbon sequestration efficiency was verified experimentally. Borax and citric acid form a reversible network in the middle layer that is sensitive to pH and humidity. When soil moisture increases, the protonation of the carboxyl groups of citric acid promotes the dissociation of cross-linking points. Humic acid is slowly released along the moisture gradient, promoting the aggregation of soil organic matter. The organic carbon increment in Example 1 at 60 days was 116% higher than that in Comparative Example 3, indicating that the dynamically released humic acid significantly enhances carbon fixation, breaking through the limitations of traditional biochar's reliance solely on physical adsorption.

[0186] The low-temperature pyrolysis process of biochar directly affects its pore structure and gas adsorption properties. In Example 1, biochar pyrolyzed at 300°C retained a rich mesoporous structure (2-50 nm) and achieved a CO adsorption capacity of 58.4 mg / g, 93% higher than that of Comparative Example 5 (pyrolyzed at 400°C). High-temperature pyrolysis leads to increased graphitization of the biochar and a decrease in pore collapse and specific surface area, demonstrating the importance of low-temperature processing to preserve the carbon sink function.

[0187] The synergistic effect of the intermediate layer material further strengthens the linkage effect of carbon sequestration and controlled release. The humic acid released by the dynamic cross-linking system not only fixes carbon elements, but its surface active groups can also react with core layer nutrients (such as NH4 + ) combined with the slow-release data to slow nitrogen loss. The positive correlation (r = 0.89) between the organic carbon increment and the nutrient release data in Example 1 shows that the carbon sequestration process does not occur in isolation, but rather achieves a dual-effect synergy of "carbon sequestration and fertilizer conservation" through material functional coupling, laying a theoretical foundation for the low-carbon agricultural application of the present invention.

[0188] Test Example 3: Comparative Experimental Description of Bacterial Agent Survival Rate and Release Accuracy

[0189] Purpose of the experiment:

[0190] Verify the effects of enzymatic grafting technology and bacterial agent ratio on the storage survival rate and pH response release accuracy of the bacterial agent.

[0191] Experimental steps:

[0192] Sample grouping:

[0193] Example 1: Fertilizer granules prepared according to Example 1 (total amount of microbial agent 1.5%, enzymatic grafting process);

[0194] Comparative Example 4: Fertilizer granules in which laccase-catalyzed grafting was not used in the outer layer and carboxymethyl cellulose nanofibers and poly-gamma-glutamic acid capsules were physically mixed;

[0195] Comparative Example 7: Fertilizer granules with a microbial agent addition rate of 3% (other conditions are the same as those in Example 1).

[0196] Bacterial agent survival rate test:

[0197] Storage conditions: Store the fertilizer granules in a sealed container at 25°C in a dark environment for 30 days;

[0198] Detection method:

[0199] Take 1g of fertilizer granules and dissolve them in 10mL of sterile water. Then, apply them on LB solid medium after gradient dilution.

[0200] After incubation at 37°C for 48 h, the number of viable bacteria (CFU / g) was counted;

[0201] Survival rate (%) = (number of viable bacteria after storage / initial number of viable bacteria) × 100.

[0202] pH responsive release test:

[0203] Simulated solution environment:

[0204] pH = 5: citric acid-disodium hydrogen phosphate buffer;

[0205] pH = 7: phosphate buffer;

[0206] Release detection:

[0207] Take 2 g of fertilizer granules and soak them in 50 mL of buffer (25°C constant temperature and shaking);

[0208] Samples were taken every 30 minutes, and the OD600 value (bacterial concentration) of the supernatant was measured after centrifugation;

[0209] Calculate the release rate (%) = (dissolved bacteria amount / total bacteria amount) × 100.

[0210] Experimental data:

[0211] Table 3 Comparison of bacterial agent survival rate and pH response release

[0212] Group Survival rate after 30 days of storage (%) pH=5 Release rate (%) pH=7 Release rate (%) Example 1 91.3 68.7 12.4 Comparative Example 4 42.5 28.9 23.6 Comparative Example 7 76.2 85.4 34.1

[0213] Data Description:

[0214] Difference in survival rate:

[0215] In Example 1, due to the anchoring protection of the bacterial agent by enzymatic grafting, the survival rate reached 91.3%, which was 115% higher than that of Comparative Example 4 (physical mixing);

[0216] In Comparative Example 7, excessive bacterial agent resulted in uneven capsule embedding, and the survival rate decreased by 16.5% compared with Example 1.

[0217] pH responsive release:

[0218] The release rate of Example 1 at pH = 5 was 138% higher than that of Comparative Example 4, indicating that the enzymatically grafted carboxymethyl cellulose (CMCNF) accurately responded to the acidic environment;

[0219] In Comparative Example 7, due to the excessive amount of microbial agent, 34.1% was still released at pH = 7 (only 12.4% in Example 1), verifying the direct impact of the microbial agent ratio on the risk of uncontrolled release.

[0220] Experimental Summary: Enzymatic grafting technology, catalyzed by laccase, forms a stable covalent bond network between carboxymethyl cellulose nanofibers and poly-gamma-glutamic acid capsules, creating a physical barrier and chemical anchoring sites for the microbial inoculum. The inoculum in Example 1 achieved a 30-day survival rate of 91.3%, significantly higher than the 42.5% survival rate in Comparative Example 4, which used physical mixing. This indicates that the covalent crosslinking layer effectively isolates the microbial cells from oxygen and moisture, while also preventing aggregation and inactivation during storage. This targeted fixation mechanism overcomes the random protection limitations of traditional encapsulation techniques for microbial activity, achieving highly efficient preservation of biological activity.

[0221] Optimizing the inoculant addition ratio further balanced the requirements of protection and release. In Comparative Example 7, an excess inoculant (3%) resulted in a partial overload of the capsule coating, yet 34.1% of the cells were still released at pH 7. In contrast, Example 1, by controlling the inoculant content to 1.5%, reduced the release rate to 12.4% at pH 7. This result demonstrates that excessive inoculant loading can disrupt the carrier material's pH response threshold, while precise inoculant addition ensures the selective swelling of CMCNF carboxyl groups under acidic conditions, achieving dynamic matching of inoculant release with soil acidification signals.

[0222] The coordinated design of enzymatic grafting and the ratio of microbial agents ultimately achieved the dual goals of "targeted protection-release on demand". The microbial agent release rate (68.7%) at pH = 5 in Example 1 was increased by 138% compared with that in Comparative Example 4. This was attributed to the protonation of the carboxyl groups of the carboxymethyl cellulose nanofibers in an acidic environment, which promoted the dissociation of the cross-linked network and the release of the microbial agent. At the same time, the dense structure formed by laccase catalysis remained stable under non-acidic conditions, avoiding nutrient loss. This intelligent response mechanism accurately links the activity of the microbial agent with the soil environment, providing key technical support for the field adaptability of microbial fertilizers.

[0223] Test Example 4: Comparative Experimental Description of Light-Response Controlled Release Performance

[0224] Purpose of the experiment:

[0225] Verify the effects of nitrogen-doped nano-zinc oxide (N-ZnO) and hydroxypropyl starch substitution degree on the photoresponsive controlled release performance.

[0226] Experimental steps:

[0227] Sample grouping:

[0228] Example 1: Fertilizer granules prepared according to Example 1 (N-ZnO outer shell, hydroxypropyl starch substitution degree 85%);

[0229] Comparative Example 2: Fertilizer particles containing ordinary nano zinc oxide (not doped with nitrogen) as the outer shell;

[0230] Comparative Example 6: Fertilizer granules with a hydroxypropyl starch substitution degree of 70% (other contents are the same as in Example 1).

[0231] Photodegradation test:

[0232] Lighting conditions: Xenon lamp simulates natural light (500-600nm wavelength, light intensity 1000W / m 2 ), continuous irradiation for 14 days;

[0233] Detection method:

[0234] Take 5 fertilizer grains every 3 days, wash them, dry them and weigh them, and calculate the mass loss rate (%) of the film layer;

[0235] Mass loss rate = (initial film mass - remaining film mass) / initial film mass × 100.

[0236] Light-controlled nitrogen release test:

[0237] Light intensity grouping:

[0238] Sunny day group: light intensity 1000W / m 2 (simulates midday light);

[0239] Cloudy day group: light intensity 300W / m2 (simulate cloudy weather);

[0240] Release detection:

[0241] Place the fertilizer granules in a light box and take the soaking solution (deionized water) every 24 hours;

[0242] Determination of NH4 by spectrophotometry (Nessler's reagent method) + -N concentration, calculate the daily release rate (%).

[0243] Experimental data:

[0244] Table 4 Comparison data of light-responsive controlled release performance and nitrogen release

[0245]

[0246] Data Description:

[0247] Photodegradation performance:

[0248] Due to the photocatalytic stability of N-ZnO, the film loss in Example 1 over 14 days was 56.9% lower than that in Comparative Example 2 (normal ZnO);

[0249] In Comparative Example 6, the film loss was 74.8% higher than that in Example 1 due to the poor film-forming property of the low-substituted starch.

[0250] Nitrogen release stability:

[0251] The nitrogen release rate of Example 1 fluctuated by <10% under cloudy conditions, while the release rate of Comparative Example 2 fluctuated by 34% under cloudy conditions due to the delayed light response of ordinary ZnO.

[0252] The starch film layer of Comparative Example 6 partially disintegrated under strong light (sunny day), resulting in a higher nitrogen release rate than that of Comparative Example 2.

[0253] Nitrogen-doped nano-zinc oxide (N-ZnO) forms an intermediate energy level in the band gap by introducing nitrogen elements, which reduces the recombination rate of photogenerated electron-hole pairs, thereby achieving stable and controllable photocatalytic activity in the visible light range (500-600nm). In Example 1, the N-ZnO outer layer lost only 12.3% of the film after 14 days of illumination, which was 56.9% lower than that of Comparative Example 2 (ordinary ZnO), indicating that nitrogen doping effectively inhibited the photocorrosion phenomenon of ZnO. This modification makes the photoresponsive controlled release no longer dependent on ultraviolet light excitation, broadens the environmental adaptability, and avoids the risk of nutrient burst release caused by excessive decomposition of traditional photosensitive materials.

[0254] The high degree of substitution of hydroxypropyl starch (85%) forms a dense and flexible coating layer by enhancing the hydrophobic-hydrophilic balance of the molecular chain. 2The daily nitrogen release rate fluctuated by less than 10% under light intensity (under 100 nm irradiation). However, the release rate of Comparative Example 6 (70% substitution) fluctuated by up to 28% with varying light intensity due to the loose film structure. The intermolecular hydrogen bonding network of highly substituted starch dynamically regulates the permeability of the film, resulting in a linear relationship between nutrient release rate and light intensity, rather than the threshold response of traditional materials. This improves the stability of controlled release.

[0255] The synergistic effect of the light-responsive material and the film-forming matrix further optimizes the precision of controlled release. The photocatalytic degradation of N-ZnO mainly acts on the surface of the film layer, while the internal high-substitution starch adjusts the micropore diameter through a swelling-contraction mechanism. The two form a dual controlled release path of "external etching-internal regulation" on a spatial scale. The difference in nitrogen release rate between sunny and cloudy days in Example 1 (1.8% vs 1.2%) is 42% smaller than that in Comparative Example 2 (3.5% vs 2.9%), indicating that the synergistic mechanism effectively smoothes the interference of ambient light intensity fluctuations on nutrient release, and provides a reliable solution for intelligent fertilizer control under complex climatic conditions.

[0256] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture, characterized in that: The present invention comprises the following layer structures by weight percentage: The nuclear layer is 50-70%, composed of urea, potassium dihydrogen phosphate, potassium chloride and nano-hydroxyapatite; The middle layer, 8-20%, consists of biochar, humic acid, sodium alginate, borax, and citric acid; The outer shell layer is 5-15%, composed of hydroxypropyl starch, chitosan and nitrogen-doped nano-zinc oxide; The outer layer is 0.5-6%, and is composed of nitrogen-fixing bacteria and phosphate-dissolving bacteria wrapped in poly-γ-glutamic acid, and carboxymethyl cellulose nanofibers.

2. The slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture according to claim 1, characterized in that: The urea accounts for 50-65% of the nuclear layer, the potassium dihydrogen phosphate accounts for 15-25% of the nuclear layer, the potassium chloride accounts for 8-12% of the nuclear layer, and the nano-hydroxyapatite accounts for 1-3% of the nuclear layer.

3. The slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture according to claim 1, characterized in that: The biochar accounts for 50-70% of the middle layer, the humic acid accounts for 20-30% of the middle layer, the sodium alginate accounts for 5-10% of the middle layer, the borax accounts for 0.2-0.5% of the middle layer, and the citric acid accounts for 0.5-1.5% of the middle layer. The biochar in the middle layer is prepared by oxygen-limited pyrolysis of agricultural straw, the pyrolysis temperature is 250-350°C, and the specific surface area is ≥300m 2 / g, the mass ratio of borax to citric acid in the middle layer is 1:2-1:4, forming a dynamic cross-linking system.

4. The slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture according to claim 1, characterized in that: The hydroxypropyl starch accounts for 50-70% of the outer shell layer, the chitosan accounts for 20-30% of the outer shell layer, the nitrogen-doped nano-zinc oxide accounts for 0.5-2% of the outer shell layer, the nitrogen doping concentration of the nitrogen-doped nano-zinc oxide in the outer shell layer is 1.5-2.2at%, the particle size is 20-50nm, and the degree of substitution of the hydroxypropyl starch in the outer shell layer is ≥80%.

5. The slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture according to claim 1, characterized in that: The poly-gamma-glutamic acid accounts for 40-60% of the outer layer, the carboxymethyl cellulose nanofibers account for 20-40% of the outer layer, the total amount of the nitrogen-fixing bacteria and the phosphate-dissolving bacteria accounts for 0.5-2% of the outer layer, the carboxymethyl cellulose nanofibers are grafted onto the surface of the poly-gamma-glutamic acid capsule through laccase catalysis, and the grafting rate is ≥85%. The nitrogen-fixing bacteria is Azotobacter chroococcum, and the phosphate-dissolving bacteria is Pseudomonas fluorescens.

6. The slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture according to claim 1, characterized in that: The core layer is a melt-blended microsphere with a particle size of 1-3 mm, the middle layer has a thickness of 200-400 μm, and the outer shell layer has a thickness of 50-100 μm.

7. A method for preparing a slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture, characterized in that: The preparation method of the slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture according to any one of claims 1 to 6 comprises the following steps: S1. Preparation of core layer: melt-blending urea, potassium dihydrogen phosphate, and potassium chloride, adding nano-hydroxyapatite, and spray granulating to form microspheres; S2, middle layer construction: biochar is mixed with humic acid-sodium alginate dynamic cross-linked gel and coated on the surface of the core layer; S3, outer shell construction: hydroxypropyl starch, chitosan and nitrogen-doped nano zinc oxide are prepared into a membrane solution, which is then electrostatically sprayed on the surface of the middle layer; S4. Microbial capsule attachment: nitrogen-fixing bacteria and phosphate-solubilizing bacteria are embedded in poly-gamma-glutamic acid capsules. In a microaerobic environment, the dissolved oxygen concentration is ≤2mg / L, the reaction temperature is 35-45°C, and carboxymethyl cellulose nanofibers are enzymatically grafted and fixed to the outer shell layer; S5. Post-processing: low-temperature drying and screening.

8. The method for preparing the slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture according to claim 7, characterized in that: In the step S1: The melting temperature is 55-65°C, the inlet temperature of spray granulation is 70-85°C, and the outlet temperature is 35-45°C; The added amount of nano-hydroxyapatite is 1-3% of the total weight of the core layer.

9. The method for preparing the slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture according to claim 7, characterized in that: In the step S2: The preparation conditions of the dynamic cross-linked gel are: pH = 6.0-7.0, temperature 35-45°C, stirring time ≥ 30 minutes; The inlet air temperature of the fluidized bed coating is 35-45°C, and the coating thickness is 200-400μm.

10. The method for preparing the slow-release and controlled-release multifunctional yield-increasing fertilizer based on carbon sink agriculture according to claim 7, characterized in that: In the step S3: The voltage of electrostatic spraying is 25-35kV, and the spraying distance is 10-20cm; The concentration of nitrogen-doped nano zinc oxide in the film-forming solution is 0.5-2%.

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