Controlled-release nitrogen fertilizer containing organic acid salt signal substance

Abstract

The invention discloses a controlled-release nitrogen fertilizer containing an organic acid salt signal substance. The bio-based urea-trace element composite fertilizer is prepared from urea particles, a cross-linked substance-trace element mixture, molten urea and a bio-based polyurethane coating material in sequence from inside to outside, the cross-linked substance-trace element mixture is a porous cross-linked material loaded with trace elements; the porous cross-linked material is obtained by cross-linking alginic acid and organic acid salt through a cross-linking agent; the organic acid salt is selected from at least one of sodium citrate, sodium fumarate, sodium maleate, sodium malonate and sodium succinate; the trace elements are selected from at least one of zinc, cobalt, molybdenum and iron. The novel controlled-release fertilizer obtained through alginic acid-organic acid salt crosslinking, trace element precise loading and a multilayer coating technology has the characteristics of pH responsiveness, environmental resistance and efficient nutrient utilization; nutrient release can be dynamically regulated and controlled according to soil pH and crop requirements, nutrient loss and environmental pollution are reduced, the requirements of green agriculture development are met, and an innovative solution is provided for improving the fertilizer utilization rate and the crop yield.

Description

Technical Field

[0001] The present invention relates to the technical field of novel fertilizers, and in particular to a controlled-release nitrogen fertilizer containing an organic acid salt signal substance. Background Art

[0002] Corn, wheat, and rice are important global food crops, and their high and stable yields are crucial to food security. Nitrogen fertilizer, as the primary means of nitrogen supplementation, is widely used in agricultural production, significantly improving crop yield and quality. However, the global average nitrogen fertilizer utilization rate is low. This is primarily due to a variety of factors. First, nitrogen forms are easily lost. Ammonium and nitrate ions are easily transformed, leached, or volatilized in the soil, resulting in nitrogen losses. Second, the complex soil environment. Factors such as clay minerals, organic matter, and microbial activity in the soil all affect nitrogen availability, making it difficult for crops to fully absorb and utilize it. Low nitrogen fertilizer utilization leads to excess nitrogen in the soil, thinning crop cell walls, weakening tissues, reducing stress resistance, and increasing the risk of pests and diseases. It also affects the absorption of trace elements such as zinc and iron, resulting in reduced yields and fruit quality. Excessive nitrogen can also easily cause soil acidification and salinization, releasing ammonia and nitric acid gases, which are toxic to the growing environment of crops. Therefore, rational nitrogen fertilizer application, improving utilization efficiency, and reducing overapplication are key to ensuring sustainable agricultural development.

[0003] Nutrient signaling substances can affect the absorption and assimilation of nitrogen by plants by regulating the expression of genes related to nitrogen utilization in plants. These signaling substances can act as sensors of nitrogen status in plants, triggering a series of changes in gene expression, thereby regulating the preferential absorption of nitrogen forms by plant roots and the transport and distribution of nitrogen in plants. At the same time, plants can recruit specific microbial communities that can participate in the transformation of nitrogen in the soil. Substances secreted by plants through their roots, such as organic acids, can attract microorganisms with functions such as nitrogen fixation, nitrification, and denitrification. These microorganisms can transform nitrogen forms in the soil, such as converting nitrogen gas (N2) in the atmosphere into ammonia nitrogen (NH4 + ) or nitrate nitrogen (NO3 - ). Under the action of microorganisms, nitrogen in the soil is absorbed and assimilated by plants and stored in the plant body as organic nitrogen. During plant growth, some organic nitrogen is returned to the soil through root secretion, decomposition of plant residues, and metabolic excretion, becoming part of soil organic matter, thereby affecting soil fertility and microbial activity. This process is a crucial link in the nitrogen cycle, ensuring a sustainable supply of soil nitrogen and the health of the ecosystem. Therefore, nutrient signaling substances regulate plant gene expression, promote the conversion and uptake of nitrogen by microorganisms, and return organic nitrogen to the soil through interactions between plants and microorganisms, thereby maintaining the nitrogen cycle and soil fertility.

[0004] Patent document CN118271130A discloses a fertilizer composition with reduced fertilizer use and enhanced efficiency, and its application. This invention utilizes acetate, oxalate, and phosphate salts to prepare a fertilizer specifically for Chuanxiong cultivation. This fertilizer can reduce fertilizer usage and improve fertilizer utilization. However, it does not utilize these organic acid salts as nitrogen signaling substances, and the raw fertilizer composition is unstable in form (e.g., solid or liquid), which can affect the fertilizer's storage stability and application effectiveness. Organic acid salts have poor thermal stability and are prone to decomposition or hydrolysis at high temperatures. Furthermore, oxalates such as calcium oxalate are prone to crystal transformation at high temperatures, which can compromise their structural stability. Sodium citrate, on the other hand, has a high critical relative humidity and is prone to adhesion after moisture absorption. Some organic acid salts also exhibit reduced sustained-release properties after prolonged contact with carriers (e.g., bentonite). Therefore, the application of organic acid salts as a single component or in combination lacks a suitable matrix for the environment, making it impossible to dynamically regulate nutrient release rates based on soil pH. This makes it difficult to meet the needs of crops at different growth stages, resulting in low fertilizer utilization and potentially causing soil nutrient imbalances. Therefore, there is a need for a controlled-release nitrogen fertilizer using organic acid salts as nitrogen signal substances, so that the fertilizer has good thermal stability and avoids the decomposition or deterioration of the signal substances during the high-temperature heating process of fertilizer preparation, thereby maintaining the quality and effect of the fertilizer. Summary of the Invention

[0005] In response to the above-mentioned prior art, the present invention aims to provide a controlled-release nitrogen fertilizer containing an organic acid salt signaling substance. This novel controlled-release fertilizer, prepared through alginic acid-organic acid salt crosslinking, precise trace element loading, and multi-layer coating technology, combines pH responsiveness, environmental resistance, and efficient nutrient utilization. In agricultural production, this fertilizer can dynamically regulate nutrient release based on soil pH and crop demand, reducing nutrient loss and environmental pollution. This approach meets the needs of green agricultural development and provides an innovative solution for improving fertilizer utilization and crop yields.

[0006] To achieve the above object, the present invention adopts the following technical solutions: The first aspect of the present invention provides a controlled-release nitrogen fertilizer containing an organic acid salt signal substance, which comprises, from the inside to the outside, urea granules, a cross-linked material-trace element mixture, molten urea, and a bio-based polyurethane coating material; The cross-linked material-trace element mixture is a porous cross-linked material loaded with trace elements; The porous cross-linked material is obtained by cross-linking alginic acid and an organic acid salt through a cross-linking agent; The organic acid salt is selected from at least one of sodium citrate, sodium fumarate, sodium maleate, sodium malonate, and sodium succinate; The trace element is selected from at least one of zinc, cobalt, molybdenum and iron.

[0007] Preferably, the porous cross-linked material is prepared by the following method: Alginic acid is dissolved in deionized water and the pH is adjusted to 6.5 to obtain an alginate solution; an organic acid salt is dissolved in deionized water to obtain an organic acid salt solution; the organic acid salt solution is slowly added to the alginate solution and stirred, and after stirring evenly, a crosslinker solution is added dropwise, stirred to react at room temperature, allowed to stand and solidify to form a uniform gel, which is filtered, washed, and then freeze-dried to obtain a porous cross-linked material.

[0008] Preferably, the concentration of the alginate solution is 3 wt %; the concentration of the organic acid salt solution is 2-10 wt %; and the molar ratio of the organic acid salt to alginate is 1:0.5-2.

[0009] Preferably, the cross-linking agent solution comprises CaCl2 solution, EDC and NHS; the concentration of the CaCl2 solution is 2wt%; 2+ The molar ratio of EDC and NHS is 1:5:2; the CaCl2 contains Ca 2+ The molar ratio of COO⁻ to alginic acid is 3:10.

[0010] Preferably, the stirring reaction time is 30 minutes; the static solidification time is 12 hours; the freeze-drying temperature is -50°C and the time is 24 hours; and the pore size of the porous cross-linked material is 50-200 nm.

[0011] Preferably, the cross-linked material-trace element mixture is obtained by immersing the porous cross-linked material in a solution containing trace elements and then drying it.

[0012] Preferably, the solution containing trace elements is selected from at least one of zinc sulfate solution, ferric chloride solution, sodium molybdate solution, and cobalt nitrate solution; the mass ratio of the porous cross-linked material and the trace elements is 10:1; the immersion pressure is -0.1 MPa and the time is 30 minutes; and the drying is performed at 60°C until the moisture content is 5%.

[0013] Preferably, the controlled-release nitrogen fertilizer containing the organic acid salt signal substance is prepared by the following method: (1) mixing the cross-linked material-trace element mixture with urea particles and spraying PVA solution as an adhesive so that the cross-linked material-trace element mixture is evenly coated on the surface of the urea particles to obtain cross-linked material-trace element mixture coated urea particles; (2) spraying molten urea onto the surface of the cross-linked material-trace element mixture to form a dense layer to obtain coated urea particles; (3) Spraying the bio-polyurethane prepolymer onto the surface of the coated urea particles and curing to obtain a controlled-release nitrogen fertilizer containing organic acid salt signal substances.

[0014] Preferably, the mass ratio of the cross-linked material-trace element mixture to the urea granules is 1:5; the concentration of the PVA solution is 5wt%; the mass ratio of the molten urea and the cross-linked material-trace element mixture coating the urea granules is 1:10; the bio-polyurethane prepolymer is synthesized from castor oil-based polyol and hexamethylene diisocyanate and prepared with acetone as solvent; the mass ratio of the bio-polyurethane prepolymer to the coated urea granules is 1:15.

[0015] The bio-polyurethane prepolymer is prepared into a solution with a solid content of 30 wt% using acetone as a solvent and then sprayed. A second aspect of the present invention provides the use of a signal substance containing an organic acid salt in at least one of the following 1) to 5): 1) Improve nitrogen fertilizer utilization efficiency; 2) Improve Rhizobiales relative abundance; 3) Inhibit denitrifying bacteria Burkholderiales; 4) Enrichment of growth-promoting bacteria Sphingomonas and Pseudomonas relative abundance; 5) Improve the responsiveness to pH.

[0016] Beneficial effects of the present invention: (1) This invention utilizes alginate-organic acid salt cross-linking, precise trace element loading, and multi-layer coating technology to prepare a novel controlled-release fertilizer that combines pH responsiveness, environmental resistance, and efficient nutrient utilization. In agricultural production, this fertilizer can dynamically regulate nutrient release based on soil pH and crop demand, reducing nutrient loss and environmental pollution. This approach meets the needs of green agricultural development and provides an innovative solution for improving fertilizer utilization and crop yields.

[0017] (2) The alginic acid used in the present invention is derived from marine brown algae resources. Its natural colloidal properties not only enhance the slow-release performance of the fertilizer, but also strengthen nitrogen signal transmission by stimulating the activity of root secretions. In addition, by integrating marine biomass resources, this technology provides a green solution to reduce the problem of offshore eutrophication caused by the loss of land-based nitrogen fertilizers, reflecting the sustainable development concept of coordinated utilization of land and sea resources.

[0018] (3) The present invention combines the synergistic characteristics of controlled-release nitrogen fertilizers and nutrient signal substances. Compared with traditional controlled-release nitrogen fertilizers, controlled-release nitrogen fertilizers containing nitrogen signal substances for crops can continuously release nutrient signal substances into the soil while releasing nitrogen. On the one hand, it can meet the crop's demand for nitrogen nutrients, and the released nutrient signal substances can regulate the metabolic activities of root cells, enhance the root system's ability to absorb soil nutrients, and thus improve the crop's nitrogen utilization efficiency. On the other hand, marine extract alginate can stimulate plant roots to secrete organic acids, and synergize with the organic acid salts in the present invention to regulate the rhizosphere pH value, enhance the solubility and availability of trace elements in the soil, and alginate recruits specific rhizosphere microorganisms, and synergizes with organic acid salts to promote nitrogen activation, reduce nitrogen loss, and improve nitrogen utilization efficiency. In addition, nutrient signal substances and alginate have a synergistic effect, by activating plant endogenous hormones, promoting the transport of photosynthetic products to grains, improving grain filling efficiency, thereby increasing crop yields, and having a high-yield and high-quality effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 : Comparison of nitrogen release curves of each fertilizer group with the nitrogen requirement of corn; Figure 2 : Nitrogen release dynamics of Example 1 in soils with different pH values; Figure 3 : Corn yield under different fertilizer treatments; Figure 4 : Analysis of rhizosphere microbial communities in field experiments (a) order level; (b) genus level. DETAILED DESCRIPTION

[0020] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present application belongs.

[0021] As discussed in the background technology section, organic acid salts have poor thermal stability and are prone to decomposition or hydrolysis at high temperatures. Furthermore, oxalates, such as calcium oxalate, are prone to crystal transformation at high temperatures, destabilizing their structure. Sodium citrate, on the other hand, has a high critical relative humidity and tends to stick after absorbing moisture. Furthermore, some organic acid salts suffer from a decrease in sustained-release properties due to long-term contact with carriers (such as bentonite). Consequently, the application of organic acid salts as a single component or in combination lacks a suitable matrix for the environment, making it impossible to dynamically regulate nutrient release rates based on soil pH. This makes it difficult to meet the needs of crops at different growth stages, resulting in low fertilizer utilization efficiency and potentially causing soil nutrient imbalances.

[0022] Based on this, the present invention aims to provide a controlled-release nitrogen fertilizer containing an organic acid salt signaling substance. The nutrient signaling substance in this invention is an organic acid salt, whose functional groups primarily exist in the form of sodium carboxylates (-COONa), with two sodium carboxylates attached to either side of a carbon bond. The nutrient signaling substance is mixed with trace elements such as zinc, cobalt, molybdenum, and iron, which improve nitrogen utilization efficiency, and a marine extract, alginic acid, to form a composite cross-linked material. A dense layer of molten urea is formed outside the cross-linked material coating, resulting in rounded and smooth fertilizer particles. A bio-based polyurethane film is then sprayed onto the coating surface to produce a controlled-release nitrogen fertilizer containing a nitrogen signaling substance. Cross-linking the organic acid salt with alginic acid enhances controlled release and improves environmental responsiveness. The cross-linked network can adjust porosity and control the release rate of nutrients (such as nitrogen, phosphorus, and potassium) through diffusion resistance. For example, the "egg-box" structure formed by cross-linking alginic acid with calcium ions can extend the urea release period to 60–90 days. The cross-linking material (such as a pH-sensitive polymer) can dynamically adjust the network structure based on soil conditions (such as pH and moisture), achieving intelligent controlled release. Alginic acid contains β-D-mannuronic acid (M) and α-L-guluronic acid (G) linked by 1→4 glycosidic bonds to form a linear polysaccharide. The molecular chain is rich in carboxylic acid groups (-COOH, pKa≈3.5). The carboxyl groups of the G units easily form an "egg-box" structure between adjacent chain segments, which is the core site of ionic crosslinking. Organic acid salts such as sodium citrate (C6H5O7 3 ⁻)、Sodium fumarate(C4H2O4 2 ⁻), sodium succinate (C4H4O4 2 ⁻), whose anionic structure contains carboxyl groups (-COO⁻), and some also contain hydroxyl groups (-OH), which form a synergistic coordination effect with the carboxyl groups of alginic acid. At pH 6.0-6.5, the cross-linked network forms a dense structure, significantly improving mechanical strength (compressive strength >5 MPa). This effectively inhibits the penetration of small molecules (such as water molecules), slows nutrient diffusion, and achieves a nitrogen release rate of <0.5 mg / cm²・h. It also significantly improves thermal stability (decomposition temperature increases from 180°C to 240°C) and hygroscopic resistance (moisture absorption rate <3% after 30 days of storage at 40°C / 75% RH).

[0023] When the soil pH is less than 5 (acidic soil), the carboxyl groups of alginate are partially protonated, causing the cross-linked network to swell (expansion rate >150%), accelerating nutrient release at a rate of >2mg / cm²・h. Under alkaline conditions of pH 7.0-8.5, the carboxyl groups fully dissociate, forming a stable, high-strength controlled-release membrane that allows for slow nutrient release.

[0024] The present invention combines the synergistic characteristics of controlled-release nitrogen fertilizers and nutrient signal substances. Compared with traditional controlled-release nitrogen fertilizers, controlled-release nitrogen fertilizers containing nitrogen signal substances specifically for crops can continuously release nutrient signal substances into the soil while releasing nitrogen. On the one hand, it can meet the crop's demand for nitrogen nutrients, and the released nutrient signal substances can regulate the metabolic activities of root cells, enhance the root system's ability to absorb soil nutrients, and thus improve the crop's nitrogen utilization efficiency. On the other hand, marine extract alginate can stimulate plant roots to secrete organic acids, and synergize with the organic acid salts in the present invention to regulate the rhizosphere pH value, enhance the solubility and availability of trace elements in the soil, and alginate recruits specific rhizosphere microorganisms, and synergizes with organic acid salts to promote nitrogen activation, reduce nitrogen loss, and improve nitrogen utilization efficiency. In addition, nutrient signal substances and alginate have a synergistic effect, which activates plant endogenous hormones, promotes the transport of photosynthetic products to grains, improves grain filling efficiency, and thus increases crop yield, with high-yield and high-quality effects.

[0025] The present invention constructs a controlled-release nitrogen fertilizer system through the synergistic effect of multiple components. The coating technology can realize the simultaneous sowing of crop seeds and fertilizers. With nutrient signal substances as the core, the rhizosphere acidification effect mediated by carboxylate ions activates insoluble phosphorus, iron and other elements in the soil, while upregulating the expression of nitrogen transport protein genes to enhance the nitrogen absorption capacity of the root system, stimulating the crop root system and the efficient absorption and utilization of nitrogen; alginic acid, as a marine extract, activates the MAPK signaling pathway through oligosaccharide fragments to promote lateral root development, and its three-dimensional network structure forms a high-strength controlled-release membrane through hydrogen bonds and ionic cross-linking. Alginic acid, as a marine extract, activates the MAPK signaling pathway through oligosaccharide fragments to promote lateral root development; the rich carboxyl groups in its molecular chain activate the MAPK signaling pathway through Ca 2+The cross-linking mechanism forms an 'egg-box' structure, which, combined with the synergistic coordination of organic acid salts, self-assembles into a three-dimensional network framework, forming a pH-responsive, high-strength, controlled-release membrane. This delays nutrient release to 60-70 days and simultaneously drives the transport of photosynthate products into the grain by upregulating sucrose transport genes. Trace elements, acting as cofactors for nitrogen metabolic enzymes, chelate with organic acid salts to form stable complexes, preventing soil fixation and enhancing bioavailability. The active ingredients are protected from thermal decomposition during the high-temperature granulation process, and their cross-linking with alginic acid further optimizes the slow-release kinetics. The outer bio-based polyurethane membrane precisely regulates nitrogen release through a hydrophobic-hydrophilic microphase separation structure, matching the nutrient requirements of corn from the jointing and tasseling stages to the grain filling stage. Its biodegradability, combined with the adsorption and interception capacity of alginic acid, synergistically reduces the total nitrogen concentration in runoff from the Yellow River Basin, significantly alleviating the risk of offshore eutrophication and ultimately achieving the trinity of agricultural green transformation: increased production, increased efficiency, and reduced emissions. In addition, compared with the shortcomings of ordinary coated nitrogen fertilizer particles, such as uneven surface and large amount of film material used, the present invention mixes nutrient signal substances with trace elements such as zinc, cobalt, molybdenum, iron, etc. that improve nitrogen utilization efficiency, as well as marine extract alginate to form a composite cross-linked substance, and forms a dense layer of molten urea outside the cross-linked coating layer, making the fertilizer particles round and smooth, improving the smoothness of the fertilizer surface, reducing the amount of film material used, and reducing production costs.

[0026] A combined treatment of alginic acid and sodium organic acid can increase both starch accumulation and 100-kernel weight in corn kernels, with a synergistic effect significantly superior to that of a single component. Furthermore, the three-dimensional network structure of alginic acid forms a high-strength controlled-release membrane. Combined with the slow-release properties of bio-based polyurethane, this precisely aligns the nitrogen release cycle with the fertilizer requirements of corn from jointing to grain filling, improving fertilizer utilization.

[0027] In order to enable those skilled in the art to more clearly understand the technical solution of the present application, the technical solution of the present application will be described in detail below with reference to specific embodiments.

[0028] The test materials used in the examples of the present invention are all conventional test materials in the art and can be purchased through commercial channels.

[0029] Example 1: Preparation of controlled-release nitrogen fertilizer containing organic acid salt signal substance (1) Cross-linked material-trace element mixture 3g of alginate powder with a degree of polymerization of 50-2000 was added to 97g of deionized water and slowly dissolved at 40°C and 300rpm under stirring conditions. The initial pH was adjusted to 6.5 with 0.1M NaOH to prepare a 3 wt% alginate solution. 2.5g of sodium citrate and 1.0g of sodium fumarate were dissolved in 96.5g of deionized water to prepare 100g of organic acid salt solution (sodium citrate concentration 2.5wt%, sodium fumarate concentration 1.0wt%). Under stirring conditions, 100g of organic acid salt solution was slowly added to 100g of alginate solution, stirred at 300rpm for 10 minutes, and 28.3g of 2 wt% CaCl2 solution (containing 0.566g of CaCl2, nCaCl2) was gradually added. 2+ =0.0051 mol), and a composite system of EDC 4888 mg (0.0255 mol) and NHS 1174 mg (0.0102 mol). Stir at room temperature (25°C) for 30 minutes and let it stand for 12 hours to solidify to form a uniform gel. After the reaction is completed, the cross-linked product is separated by filtration or centrifugation, washed with deionized water to remove unreacted impurities, and then freeze-dried (-50°C, 24 h) to obtain porous cross-linked particles with a pore size of 50-200 nm and a specific surface area of ​​>25 m 2 / g.

[0030] The porous cross-linked particles were mixed with a trace element solution (ZnSO₄ solution, FeCl₃ solution, sodium molybdate solution, and cobalt nitrate solution, with a molar ratio of 1:0.2:0.1:0.5 for zinc, cobalt, molybdenum, and iron) at a mass ratio of 10:1. The particles were immersed in a vacuum of -0.1 MPa for 30 minutes to allow the trace elements to fully adsorb into the pores of the cross-linked particles. The mixture was then dried at 60°C to a moisture content of <5%, yielding a uniformly dispersed cross-linked material-trace element complex.

[0031] (2) Urea granule coating and molten urea spraying A cross-linked material-trace element mixture with a particle size of 0.5-1 mm and urea particles with a particle size of 2-3 mm were added to a fluidized bed coating machine in a mass ratio of 1:5, and a 5wt% PVA solution (polyvinyl alcohol PVA-1788 powder was added to 80°C deionized water, soaked and swelled at room temperature for 30 minutes, and then magnetically stirred at 500 rpm in an 80°C water bath for 60 minutes until completely dissolved, and cooled to 25°C to form a transparent viscous solution) was sprayed as a binder. The spraying rate was controlled to 5 mL / min and the atomization pressure to 0.3 MPa so that the mixture was evenly coated on the surface of the urea particles to obtain urea particles coated with the cross-linked material-trace element mixture.

[0032] Molten urea (140°C) was atomized and sprayed onto the surface of the cross-linked material-trace element mixture-coated urea particles. The mass ratio of molten urea to the cross-linked material-trace element mixture-coated urea particles was 1:10, forming a 50-100 μm dense layer to obtain coated urea particles.

[0033] (3) Bio-based polyurethane film wrapping A bio-based polyurethane prepolymer was synthesized by mixing castor oil-based polyol (hydroxyl value 160 mgKOH / g) with hexamethylene diisocyanate (HDI) in a molar ratio of NCO:OH = 2:1 and reacting at 70°C for 3 hours. The bio-based polyurethane prepolymer was diluted with acetone to a solid content of 30 wt% to prepare a bio-based polyurethane prepolymer solution.

[0034] Take the bio-based polyurethane prepolymer solution and the coated urea particles in a mass ratio of 1:15 and add them into the fluidized bed coating machine. Control the preheating temperature at 50 °C, the atomization pressure at 0.3 MPa, and the hot air flow rate at 40 m 3 / h, and spraying with an inlet air temperature of 60°C, and curing at 60°C for 2 hours to form a uniform outer wrapping film with a thickness of 20-30 μm, and finally obtain a controlled-release nitrogen fertilizer containing organic acid salt signal substances.

[0035] Comparative Example 1 Urea particles (1000 g) with a particle size of 2-3 mm were placed in a fluidized bed and sprayed with a bio-based polyurethane prepolymer solution with a solid content of 30% (prepared by the same method as step (3) of Example 1). The mass ratio of bio-based polyurethane prepolymer solution to urea particles was 1:15. The solution was cured at 60°C for 2 hours to form a uniform outer coating with a thickness of 20-30 μm, thereby obtaining coated urea.

[0036] Comparative Example 2 Urea particles (1000 g) with a particle size of 2-3 mm were placed in a fluidized bed and first sprayed with 100 g of an organic acid salt solution (2.5 wt% sodium citrate + 1 wt% sodium fumarate, 3.5 g total organic acid salts). Then, a bio-based polyurethane prepolymer solution with a solid content of 30% (prepared by the same method as step (3) of Example 1) was sprayed. The mass ratio of the bio-based polyurethane prepolymer solution to the urea particles was 1:15. The mixture was cured at 60 °C for 2 h to form a uniform outer coating with a thickness of 20-30 μm, thereby obtaining coated urea.

[0037] Comparative Example 3 Urea particles (1000g) with a particle size of 2-3mm were placed in a fluidized bed, first sprayed with 100g of 3wt% alginate solution, and then sprayed with a bio-based polyurethane prepolymer solution with a solid content of 30%; the mass ratio of bio-based polyurethane prepolymer solution: urea particles was 1:15. The particles were cured at 60°C for 2 hours to form a uniform outer coating with a film thickness of 20-30 μm, thereby obtaining coated urea.

[0038] Comparative Example 4 The difference from Example 1 is that no trace elements are added. Finally, a controlled-release nitrogen fertilizer containing organic acid salt signal substances is prepared.

[0039] Test Example 1: Still Water Test Group comparison Example 1 and Comparative Examples 1-4.

[0040] The temperature was set at 25°C, the solution was deionized water (pH 6.5), the fertilizer dosage was 5 g / group (repeated 3 times), the sampling intervals were 1, 3, 7, 14, 21, 28, 35, 42, 49, 56, 63, and 70 days, and the detection method was the Kjeldahl method.

[0041] The results of Test Example 1 can be expressed as Figure 1 , the nitrogen release curves of each group of fertilizers were compared with the nitrogen requirement of corn.

[0042] During the seedling stage (0-20 days), Example 1's 15.0% release rate precisely matched the 13% nitrogen requirement, while the comparative treatments exhibited a significant imbalance—Comparative Example 1 released 40.3% and Comparative Example 2 released 64.7%, leading to the risk of excessive growth. Comparative Example 3 released only 9.5%, restricting seedling development. This indicates that the molten urea layer in Example 1 provides a steady release of 0.29% / day during this stage, perfectly matching the corn's initial nitrogen requirement of 0.25% / day.

[0043] By the jointing stage (days 21-35), Example 1's release rate of 57.1% moderately exceeded the nitrogen requirement of 55%, meeting peak stem and leaf growth. However, Comparative Example 1 released 88.3%, while Comparative Example 2 released 95.4%, resulting in waste. Comparative Example 3 released 50.7%, limiting biomass accumulation. This indicates that at this stage, the cross-linked layer in Example 1, through pH response, increased the release slope to 1.60% / day, precisely matching the nitrogen requirement of 1.43% / day.

[0044] During the tasseling period (days 36-49), Example 1 released 89.7% of nitrogen, which closely matched the nitrogen requirement of 87%, ensuring ear differentiation. However, Comparative Example 1 released 96.1% and Comparative Example 2 released 97.1%, indicating premature depletion. Comparative Example 3 released 81.7%, inhibiting ear development. This indicates that the cross-linked network in Example 1 dynamically adjusts porosity during this period, maintaining a stable error of <3.7%.

[0045] By the grain filling stage (50-70 days), Example 1's release rate of 98.1% was nearly consistent with the nitrogen requirement of 98%, while the control treatments deviated significantly—Comparative Example 1 released 99.6%, Comparative Example 2 released 99.8%, and Comparative Example 4 released 99.8%, leading to premature ripening. Comparative Example 3 released 94.8%, limiting grain filling. This indicates that the polyurethane film in Example 1 controlled the release slope of 0.24% / day during this stage, which was very close to the nitrogen requirement slope of 0.28% / day.

[0046] This demonstrates that the multilayer structure prepared by the present invention achieves precise controlled release through triple synergy: ① The pH responsiveness of the alginate-organic acid salt cross-linked network regulates the basal release rate; ② The molten urea layer quickly responds to the peak of nitrogen demand during the seedling stage; ③ The bio-based polyurethane membrane achieves mid- and late-stage linear release through hydrophobic-hydrophilic microphase separation.

[0047] This spatiotemporal coupled release characteristic makes the nitrogen supply highly consistent with the "less-more-less" nitrogen requirement pattern of corn, fundamentally solving the technical bottleneck of traditional controlled-release fertilizers "insufficient in the early stage, surplus in the middle stage, and ineffectiveness in the late stage", and providing experimental support for improving the utilization rate of nitrogen fertilizers.

[0048] Test Example 2 1. Field Experiment Design In order to investigate the effect of the controlled-release nitrogen fertilizer containing nitrogen signal substances prepared by the present invention on crop yield, the present invention selected corn as the test crop and carried out field experiments at Shandong Agricultural University. The corn variety used was "Zhengdan 958".

[0049] Each plot in the experimental site is 20m 2 , six treatments, each replicated four times; nitrogen fertilizer was applied to the soil at the same time as corn seeds; soil composition remained consistent across treatments, and sampling occurred at the jointing stage (35 days), tasseling stage (60 days), and grain filling stage (90 days); corn was harvested after maturity, and grain yield was calculated as corn yield based on a moisture content of 14%. The results of the field experiment can be expressed as Figure 3 , Example 1, Comparative Example 1 to 4 groups, plus blank control, a total of 6 groups of corn yield.

[0050] 2. Microbial Community Analysis Rhizosphere sampling: Take the rhizosphere soil (0-5 mm from the root surface).

[0051] Detection methods: 16S rRNA sequencing (bacterial community); ITS sequencing (fungal community), etc.

[0052] According to the obtained data, the results can be expressed as Figure 4 , Analysis of rhizosphere microbial communities in field experiments.

[0053] 3. pH responsiveness verification In situ soil monitoring: Three plots with different pH values ​​were selected for bag burying treatment, with five ion exchange resin bags (depth 20 cm) buried in each plot.

[0054] Regular sampling (7, 14, 21, 28, 35, 42, 49, 56, 63, 70 days) to determine: NH4 adsorbed by resin + and NO3 - ; Dynamic changes in rhizosphere soil pH.

[0055] According to the results of the field test, it can be expressed as Figure 2 , nitrogen release dynamics of Example 1 in soils with different pH values.

[0056] Set up processing: Treatment NPK: Conventional nitrogen, phosphorus and potassium fertilizers, including urea, superphosphate and potassium sulfate (equivalent fertilizer amount per hectare is N-P2O5-K2O=192-60-75kg).

[0057] Treatment of NF (Example 1): Apply the controlled-release fertilizer obtained in Example 1, superphosphate, and potassium sulfate (equivalent to a fertilizer rate of N-P2O5-K2O = 192-60-75 kg per hectare).

[0058] Treatment of CRU (Comparative Example 1): Apply the controlled-release fertilizer obtained in Comparative Example 1, superphosphate, and potassium sulfate (equivalent to a fertilizer rate of N-P2O5-K2O = 192-60-75 kg per hectare).

[0059] Treatment of OA (Comparative Example 2): Apply the controlled-release fertilizer obtained in Comparative Example 2, superphosphate, and potassium sulfate (equivalent to a fertilizer application rate of N-P2O5-K2O = 192-60-75 kg per hectare).

[0060] Treatment AA (Comparative Example 3): Apply the controlled-release fertilizer obtained in Comparative Example 3, superphosphate, and potassium sulfate (equivalent to a fertilizer application rate of N-P2O5-K2O = 192-60-75 kg per hectare).

[0061] Treatment NW (Comparative Example 4): Apply the controlled-release fertilizer obtained in Comparative Example 4, superphosphate, and potassium sulfate (equivalent to a fertilizer rate of N-P2O5-K2O = 192-60-75 kg per hectare).

[0062] Figure 2 It is shown that the fertilizer of Example 1 exhibits significant differences in nutrient release in soils with different pH values: Acidic soil (pH 5.2): The cumulative release rate reaches 99.5% in 70 days (alginic acid protonation → network swelling → accelerated release); Neutral soil (pH 6.8): 70-day cumulative release rate is 98.2% (between acidic and alkaline); Alkaline soil (pH 8.1): The cumulative release rate over 70 days is only 62.6% (carboxyl group dissociation → dense structure → delayed release).

[0063] The above results show that the release rate of the controlled-release nitrogen fertilizer is regulated by soil pH: acidic environments accelerate release, alkaline environments significantly delay release, and neutral environments are somewhere in between. This shows that the fertilizer of the present invention has a clear pH responsiveness.

[0064] The corn yield results of each treatment are as follows Figure 3 As shown in the figure, the application of the controlled-release fertilizer obtained in Example 1 promoted the growth of corn. Compared with the NPK treatment, the yield of the NF treatment increased significantly by 17.76%; the yield of the CRU treatment increased by 8.47%; the yield of the OA treatment increased by 9.48%; the yield of the AA treatment increased by 12.75%; and the yield of the NW treatment increased by 13.76%. Overall, the application of controlled-release urea containing alginate-organic acid salt can significantly increase the biomass of corn. The controlled-release nitrogen fertilizer containing organic acid salt signal substances developed by the present invention shows important application potential for significantly improving corn yield during corn cultivation.

[0065] Analysis of rhizosphere microbial communities in field experiments Figure 4 As shown in Figure 2, the fertilizer of the present invention significantly changed the structure of the corn rhizosphere microbial community. Figure 4 a), the nitrogen-fixing cells in the treatment group of Example 1 Rhizobiales The relative abundance of Rhizobiales reached 28.5%, showing a significant enrichment trend in each treatment group; at the same time, it inhibited the denitrifying bacteria group. Burkholderiales (Burkholderiales) to 9.3%, which is the lowest level among all treatment groups. Figure 4 b) Example 1: Specific enrichment of growth-promoting bacteria Sphingomonas (Sphingomonas spp., 12.1%) and Pseudomonas (Pseudomonas, 15.3%), the relative abundance of the two at the genus level was significantly higher than that of the NPK group and other treatment groups, and they can synergistically promote root development by secreting indoleacetic acid. The above changes in microbial communities verified the mechanism described in the above instructions: Sphingomonas ( Sphingomonas ) synergistically activates nifH nitrogen fixation gene expression with organic acid salts, while Pseudomonas ( Pseudomonas ) activated the insoluble phosphorus in the soil through rhizosphere acidification (pH decreased by 0.8 units), and finally achieved a nitrogen fertilizer utilization rate of 74.7 kg / kg in the treatment group of Example 1, which was significantly higher than the NPK of 42.1 kg / kg in the control group (p<0.01).

[0066] The foregoing description is merely a preferred embodiment of the present application and is not intended to limit the present application. Persons skilled in the art will readily appreciate that various modifications and variations are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application.

Claims

1. A controlled-release nitrogen fertilizer containing an organic acid salt signal substance, characterized in that: From the inside to the outside, they are urea granules, cross-linked material-trace element mixture, molten urea, and bio-based polyurethane coating material; The cross-linked material-trace element mixture is a porous cross-linked material loaded with trace elements; The porous cross-linked material is obtained by cross-linking alginic acid and an organic acid salt through a cross-linking agent; The organic acid salt is selected from at least one of sodium citrate, sodium fumarate, sodium maleate, sodium malonate and sodium succinate; The trace element is selected from at least one of zinc, cobalt, molybdenum and iron.

2. The controlled-release nitrogen fertilizer containing an organic acid salt signal substance according to claim 1, characterized in that: The porous cross-linked material is prepared by the following method: Alginic acid is dissolved in deionized water and the pH is adjusted to 6.5 to obtain an alginate solution; an organic acid salt is dissolved in deionized water to obtain an organic acid salt solution; the organic acid salt solution is slowly added to the alginate solution and stirred, and after stirring evenly, a crosslinker solution is added dropwise, stirred to react at room temperature, allowed to stand and solidify to form a uniform gel, which is filtered, washed, and then freeze-dried to obtain a porous cross-linked material.

3. The controlled-release nitrogen fertilizer containing an organic acid salt signal substance according to claim 2, characterized in that: The concentration of the alginic acid solution is 3 wt %; the concentration of the organic acid salt solution is 2-10 wt %; and the molar ratio of the organic acid salt to the alginic acid is 1:0.5-2.

4. The controlled-release nitrogen fertilizer containing an organic acid salt signal substance according to claim 2, characterized in that: The cross-linking agent solution comprises CaCl2 solution, EDC and NHS; the concentration of the CaCl2 solution is 2 wt%; the Ca 2+ The molar ratio of EDC and NHS is 1:5:2; the CaCl2 contains Ca 2+ The molar ratio of COO⁻ to alginic acid is 3:

10.

5. The controlled-release nitrogen fertilizer containing an organic acid salt signal substance according to claim 2, characterized in that: The stirring reaction time is 30 minutes; the static solidification time is 12 hours; the freeze-drying temperature is -50°C and the time is 24 hours; the pore size of the porous cross-linked material is 50-200 nm.

6. The controlled-release nitrogen fertilizer containing an organic acid salt signal substance according to claim 1, characterized in that: The cross-linked material-trace element mixture is obtained by immersing a porous cross-linked material in a solution containing trace elements and then drying the solution.

7. The controlled-release nitrogen fertilizer containing an organic acid salt signal substance according to claim 6, characterized in that: The solution containing trace elements is selected from at least one of zinc sulfate solution, ferric chloride solution, sodium molybdate solution, and cobalt nitrate solution; the mass ratio of the porous cross-linked material to the trace elements is 10:1; the immersion pressure is -0.1 MPa and the time is 30 minutes; and the drying is performed at 60°C until the moisture content is 5%.

8. The controlled-release nitrogen fertilizer containing an organic acid salt signal substance according to claim 1, characterized in that: The controlled-release nitrogen fertilizer containing the organic acid salt signal substance is prepared by the following method: (1) mixing the cross-linked material-trace element mixture with urea particles and spraying PVA solution as an adhesive so that the cross-linked material-trace element mixture is evenly coated on the surface of the urea particles to obtain cross-linked material-trace element mixture coated urea particles; (2) spraying molten urea onto the surface of the cross-linked material-trace element mixture to form a dense layer to obtain coated urea particles; (3) Spraying the bio-polyurethane prepolymer onto the surface of the coated urea particles and curing to obtain a controlled-release nitrogen fertilizer containing organic acid salt signal substances.

9. The controlled-release nitrogen fertilizer containing an organic acid salt signal substance according to claim 8, characterized in that: The mass ratio of the cross-linked material-trace element mixture to the urea granules is 1:5; the concentration of the PVA solution is 5wt%; the mass ratio of the molten urea and the cross-linked material-trace element mixture to the urea granules is 1:10; the bio-polyurethane prepolymer is synthesized from castor oil-based polyol and hexamethylene diisocyanate and prepared using acetone as a solvent; the mass ratio of the bio-polyurethane prepolymer to the coated urea granules is 1:

15.

10. Use of the organic acid salt-containing signal substance according to any one of claims 1 to 9 in at least one of the following 1) to 5): 1) Improve nitrogen fertilizer utilization efficiency; 2) Improve Rhizobiales relative abundance; 3) Inhibit denitrifying bacteria Burkholderiales; 4) Enrichment of growth-promoting bacteria Sphingomonas and Pseudomonas relative abundance; 5) Improve the responsiveness to pH.

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