Coastal saline-alkali soil crop growth-promoting modifier with pH response release characteristic as well as preparation method and application of coastal saline-alkali soil crop growth-promoting modifier

Abstract

The invention belongs to the technical field of saline-alkali soil improvement, and particularly relates to a coastal saline-alkali soil-oriented intelligent pH-responsive controlled-release crop growth-promoting soil conditioner as well as a preparation method and application thereof. The modifier is biochar-based composite microspheres, an inner core is formed by compounding biochar, silicon quantum dots and a nitrogen-phosphorus-potassium compound fertilizer, an outer layer is coated with a pH response coating formed by chitosan, and an integrated system of environmental perception, release regulation and accurate fertilizer supply is constructed. After the fertilizer is applied to soil, a coating layer swells under the rhizosphere micro-domain acidic condition, nutrient diffusion is intelligently regulated and controlled, and leaching loss and volatilization are reduced. The biochar porous structure adsorbs and fixes salt ions, improves aggregate and ventilation and water permeability, and enhances water retention and fertilizer conservation; the silicon quantum dots improve the photosynthetic efficiency and enhance the oxidation resistance; the compound fertilizer is continuously supplied to nitrogen, phosphorus and potassium. The components synergistically improve soil microstructure and water and salt migration, relieve salt ion poison, improve nutrient utilization and crop salt tolerance, promote yield increase and stability and are suitable for saline-alkali soil remediation.

Description

Technical Field

[0001] This invention belongs to the field of saline-alkali soil improvement technology, specifically relating to a pH-responsive crop growth promoter for coastal saline-alkali soil, its preparation method, and its application. Background Technology

[0002] Soil salinization is a major obstacle to sustainable agricultural development globally, especially in coastal areas. High salinity leads to fragile soil ecosystems, seriously threatening crop growth and food security. Coastal saline-alkali soils generally suffer from poor water retention, lack of organic matter, and high salt content. High salinity significantly inhibits plant growth and development through mechanisms such as osmotic stress, ion toxicity, nutrient imbalance, oxidative damage, and inhibition of cell division and metabolic activity, resulting in reduced crop yields and quality, and consequently impacting regional ecological security and agricultural development. Therefore, developing efficient and environmentally friendly technologies for improving coastal saline-alkali soils is of great significance.

[0003] Currently, saline-alkali soil improvement mainly relies on physical, chemical, and biological methods, such as soil replacement, leaching for desalination, application of soil conditioners, or planting salt-tolerant crops. While these traditional methods can partially alleviate soil salinity, they generally suffer from long improvement cycles, high costs, complex operations, and the potential for secondary pollution. In recent years, the development of functional soil conditioners has become a new direction for saline-alkali soil remediation, with the core being the synergistic effect of soil structure optimization, salinity regulation, and plant growth promotion through material innovation. However, existing conditioners still have the following significant limitations: 1. Limited Functionality and Insufficient Synergy: Common materials are often designed to address specific problems in saline-alkali soils, lacking systematic integration. For example, biochar can improve soil structure and water retention capacity, but its selective adsorption capacity for salt ions is limited, and its direct effect on enhancing plant physiological resistance is weak; silicon quantum dots (Si-QDs) can enhance plant antioxidant capacity and photosynthetic efficiency, but their effect on improving soil physicochemical properties is not significant; conventional compound fertilizers can supplement nutrients, but their utilization rate is low in saline-alkali soils due to leaching or fixation. These materials, used alone or in simple combinations, cannot simultaneously achieve the multi-objective synergistic effect of "improving soil—alleviating stress—promoting growth."

[0004] 2. Poor environmental adaptability and uncontrollable release: Coastal saline-alkali soils have a unique environment of high salinity and high pH. Under these conditions, most soil amendments have poor stability, their functions are easily degraded, or they cannot respond intelligently to changes in the soil environment. For example, ordinary fertilizers or active ingredients are released too quickly in alkaline soils, making them prone to loss or inactivation; the adsorption function of biochar may be weakened under alkaline conditions; traditional encapsulation materials lack pH responsiveness, making it difficult to achieve on-demand nutrient release, resulting in short-lasting and untargeted improvement effects.

[0005] Therefore, developing a new type of composite material that can adapt to coastal saline-alkali soil environments and integrate multiple functions such as structural improvement, salinity regulation, intelligent nutrient release, and plant resistance enhancement has become a key technical problem that urgently needs to be solved in this field.

[0006] Biochar, with its porous structure, high specific surface area, and abundant surface functional groups, is widely used for soil improvement, effectively enhancing the water-holding capacity of saline-alkali soils, adsorbing salt ions, and promoting microbial activity. Silicon quantum dots (Si-QDs), as novel nanomaterials, possess excellent biocompatibility and photoregulatory properties, and can alleviate salt stress by enhancing plant antioxidant capacity and photosynthetic efficiency. Compared to other nanoparticles, they have smaller volume, better water solubility and stability, and good biocompatibility, allowing them to better penetrate plant tissues and influence plant physiological activities, demonstrating promising application prospects in agriculture. Furthermore, nitrogen, phosphorus, and potassium compound fertilizers are essential nutrient sources for crop growth, but their utilization rate is low in saline-alkali soils due to leaching or fixation. Efficiently integrating these functional materials and achieving controlled nutrient release is key to improving the effectiveness of saline-alkali soil improvement. Against this backdrop, the natural polymer chitosan exhibits unique value. Chitosan possesses good biodegradability, film-forming properties, and abundant active groups, making it an excellent microsphere encapsulation matrix and binder. More importantly, the amino groups on its molecular chain endow it with pH-responsive properties, readily undergoing protonation and swelling under acidic conditions, while tending to shrink under alkaline conditions, thus providing the possibility for "adaptive release" of fertilizer and active ingredients. Combining chitosan with biochar, silicon quantum dots, and compound fertilizers to construct an environmentally responsive composite microsphere system holds promise for synergistically addressing multiple problems in saline-alkali soils, such as poor structure, salt stress, and inefficient nutrient utilization. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention proposes a pH-responsive crop growth promoter for coastal saline-alkali soils, along with its preparation method and application. Based on a biochar-silicon quantum dot-compound fertilizer microsphere system (BCA) and employing chitosan pH-responsive coating, the core-shell microsphere structure design achieves a synergistic effect of water retention, slow nutrient release, and plant salt stress resistance. It is particularly suitable for the ecological restoration and crop yield increase of high-salt, high-pH coastal saline soils.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: a soil conditioner for promoting crop growth in coastal saline-alkali soil with pH-responsive release characteristics, wherein the soil conditioner is composed of biochar-based microspheres composed of biochar, silicon quantum dots and nitrogen-phosphorus-potassium compound fertilizer, and its surface is covered with a coating formed by chitosan with pH-responsive characteristics.

[0009] Furthermore, The weight parts of each component are: 100-300 parts of biochar; 1-10 parts of silicon quantum dots; and 1-10 parts of nitrogen, phosphorus, and potassium compound fertilizer.

[0010] Furthermore, The silicon quantum dots have a particle size of 2.58 ± 0.65 nm, and the biochar-based microspheres have a particle size of 3~5 mm.

[0011] Furthermore, The molar ratio of nitrogen, phosphorus, and potassium compound fertilizer is N:P:K = 17:16:19.

[0012] A method for preparing a pH-responsive crop growth promoter for coastal saline-alkali soils, comprising the following steps: (1) Preparation of biochar It is prepared using corn stalks as raw material and an anaerobic slow pyrolysis method; (2) Preparation of silicon quantum dots Silicon quantum dots were prepared using ascorbic acid and N-[3-(trimethoxysilyl)propyl]ethylenediamine; (3) Preparation of biochar-based microspheres Biochar, silicon quantum dots, and NPK compound fertilizer were thoroughly mixed in a specific ratio; this mixture was then added to a sodium alginate solution and ultrasonically vibrated to form a combined solution; the combined solution was then added dropwise to a CaCl2 solution, where the alginate reacted with the CaCl2. 2+ Cross-linking forms biochar-based microspheres, which are then naturally air-dried to constant weight. (4) Preparation of chitosan coating Chitosan solution was dissolved in acetic acid solution, and air-dried biochar-based microspheres were added to the chitosan solution to form a coating layer on the surface of the microspheres, thus obtaining soil conditioner microspheres with pH-responsive release characteristics.

[0013] Furthermore, The specific steps of (1) include: The corn stalks were dried in an oven to constant weight, then pulverized into powder using a pulverizer and passed through a 2 mm standard sieve. The pretreated waste biomass raw materials were weighed and placed in a tube furnace. Nitrogen gas was introduced, and the air in the system was purged. The temperature was raised from room temperature to 500 ℃ and maintained for 3 h. After pyrolysis, the temperature was naturally cooled to room temperature under nitrogen protection, air-dried, pulverized, and passed through a 1 mm sieve.

[0014] Furthermore, The specific steps of (2) include: Weigh 2.3 g of ascorbic acid and dissolve it in 8 mL of ultrapure water. Then add 2 mL of N-[3-(trimethoxysilyl)propyl]ethylenediamine. Heat and stir at 80 °C for 8 h. Transfer the solution to a 1000 Da dialysis bag and dialyze it in ultrapure water for 48 h. The collected aqueous solution is silicon quantum dots.

[0015] Furthermore, In (3), the mass fraction of sodium alginate solution is 2-5%, the mass fraction of CaCl2 solution is 2%, and 1-3 mL of sodium alginate solution is added to each gram of the mixed solid mixture of biochar, silicon quantum dots and nitrogen, phosphorus and potassium compound fertilizer. In step (4), the mass fraction of the chitosan solution is 1-5%, and the concentration of the acetic acid solution is 20%.

[0016] Furthermore, In step (1), the drying temperature is 95°C and the nitrogen gas introduction rate is 500 mL / min. -1 Time: 20 min, Heating rate: 5℃ / min -1 .

[0017] Application of a pH-responsive soil conditioner for promoting crop growth in coastal saline-alkali soils, wherein the soil conditioner is used to improve saline-alkali soils and promote plant growth.

[0018] The beneficial technical effects of this invention are as follows: When the pH-responsive soil conditioner microspheres of this invention are applied to saline-alkali soil, their chitosan coating intelligently regulates the nutrient release rate according to the soil pH environment, achieving precise fertilization. The components of the conditioner work synergistically to improve soil water retention, enhance soil microstructure, mitigate salt ion toxicity, promote efficient nutrient utilization, and strengthen crop resistance. The porous structure of biochar effectively adsorbs salt and improves soil permeability, silicon quantum dots enhance plant photosynthetic efficiency and antioxidant capacity, and the NPK compound fertilizer continuously provides essential nutrients. The application of this conditioner can reduce fertilizer application by more than 30%, lower the risk of agricultural non-point source pollution, and the preparation process is simple, low-cost, and environmentally friendly. Attached Figure Description

[0019] Figure 1 This is a diagram illustrating the structure and usage principle of the modifier of the present invention; Figure 2 The nitrogen release rate curves of the modifier of the present invention under different pH conditions are shown. Figure 3 The phosphorus release rate curves of the modifier of the present invention under different pH conditions are shown. Figure 4 The potassium release rate curves of the modifier of the present invention under different pH conditions are shown. Figure 5 This shows the growth of maize plant height after soil treatment with the soil conditioner of this invention; Figure 6 The aboveground fresh weight of corn after soil treatment with the soil conditioner of this invention; Figure 7 The chlorophyll content of corn leaves after soil treatment with the soil conditioner of this invention; Figure 8 The content of proline in maize leaves after soil treatment with the soil conditioner of this invention; Figure 9 The length of corn roots after soil treatment with the soil conditioner of this invention; Figure 10 The soil electrical conductivity (EC) after soil treatment with the soil conditioner of this invention; Figure 11 The soil alkalinity (ESP) after soil treatment with the soil conditioner of this invention; Figure 12 The available nitrogen content in the soil after treatment with the soil conditioner of this invention; Figure 13 The available phosphorus content in the soil after treatment with the soil conditioner of this invention; Figure 14 The available potassium content in the soil after treatment with the soil conditioner of this invention; Figure 15 The soil cation exchange capacity (CEC) after soil treatment with the soil amendment of this invention. Figure 16 The soil exchangeable sodium content (EX-Na) after soil treatment with the soil conditioner of this invention. Detailed Implementation

[0020] The specific implementation method will be further described below with reference to the accompanying drawings. Example 1:

[0021] A soil conditioner for promoting crop growth in coastal saline-alkali soil with pH-responsive release properties, wherein the soil conditioner is composed of biochar-based microspheres made of biochar, silicon quantum dots and nitrogen-phosphorus-potassium compound fertilizer, and its surface is covered with a coating of chitosan with pH-responsive characteristics.

[0022] The components are present in the following weight proportions: 300 parts biochar; 2 parts silicon quantum dots; and 5 parts nitrogen-phosphorus-potassium compound fertilizer. The silicon quantum dots have a particle size of 2.58 nm, and the biochar-based microspheres have a particle size of 4 mm. The molar ratio of the nitrogen-phosphorus-potassium compound fertilizer is N:P:K = 17:16:19.

[0023] A method for preparing a pH-responsive crop growth promoter for coastal saline-alkali soils, comprising the following steps: (1) Preparation of biochar

[0024] Biochar was prepared from corn stalks using an anaerobic slow pyrolysis method to obtain corn stalk biochar (BC). The corn stalks were dried in a 95℃ oven for 72 h to constant weight, then pulverized into powder using a grinder and passed through a 2 mm standard sieve. 125 g of the pretreated corn stalk powder was weighed and placed in a tube furnace, and then heated at 500 mL / min. -1 Nitrogen gas was continuously introduced at a rate of 20 min for 20 min, and after the air in the system was purged, it was introduced at 5℃ for 1 min. -1 The temperature was increased from room temperature to 500°C and held for 3 hours. After pyrolysis, the mixture was naturally cooled to room temperature under nitrogen protection.

[0025] (2) Preparation of silicon quantum dots Weigh 2.3 g of ascorbic acid and dissolve it in 8 mL of ultrapure water. Then add 2 mL of N-[3-(trimethoxysilyl)propyl]ethylenediamine and heat and stir at 80 °C for 8 h. After that, transfer the solution to a 1000 Da dialysis bag and dialyze it in ultrapure water for 48 h. The aqueous solution collected after this process is silicon quantum dots.

[0026] (3) Preparation of biochar-based microspheres The biochar, silicon quantum dots, and NPK compound fertilizer were thoroughly mixed. This mixture was then added to a 5% sodium alginate solution, with the volume (mL) of the sodium alginate solution being 1:1 in mass (g) of the solid mixture of biochar, silicon quantum dots, and NPK compound fertilizer. The mixture was thoroughly stirred to form a homogeneous slurry, which was then ultrasonically vibrated for 120 min (100 W, 40 kHz) in an ultrasonic cleaner. Finally, the mixture was dripped into a 2% CaCl2 solution using a syringe. The alginate and CaCl2... 2+ Cross-linking forms biochar-based microspheres; the microspheres are air-dried naturally to constant weight, and the particle size of the microspheres is between 4 mm.

[0027] (4) Preparation of chitosan coating 5 g of chitosan was dissolved in a 20% acetic acid solution to prepare a 5% chitosan solution. The air-dried microspheres were then added to the chitosan solution, allowing the chitosan to form a coating layer on the surface of the microspheres, thus obtaining soil conditioner microspheres with pH-responsive release characteristics.

[0028] Figure 1 This is a diagram illustrating the structure and usage principle of the modifier of the present invention.

[0029] Experimental verification: The soil conditioner prepared in the examples was named BCA-CS. Meanwhile, the uncoated conditioner BCA was used as a control. Nutrient release experiments and saline-alkali soil improvement experiments were conducted under different pH conditions. 1. Test soil Soil samples were collected from the coastal saline-alkali soil area of ​​Dongying City, Shandong Province (118°39'E, 37°24'N). The topsoil (0-20 cm) was obtained using the five-point sampling method, and after natural air drying, it was sieved through a 2 mm sieve to remove stones and plant debris.

[0030]

[0031] 2. Test plants Maize was selected as the test plant.

[0032] 3. Nutrient release experiment Accurately weigh 50 mg of BCA-CS and BCA into separate 40 mL headspace vials, then add buffer solutions with pH values ​​of 5, 7, and 9, respectively. Perform triplicate for each group, and incubate in the dark with shaking at 25°C. Take 1 mL samples at 1 h, 6 h, 24 h, 72 h, and 7 d (simultaneously replenishing with an equal volume of fresh buffer solution), and determine the concentration using the indophenol blue method. Concentration was determined by ultraviolet spectrophotometry. Concentration was determined using the molybdenum-antimony spectrophotometric method. Concentration was determined directly using ICP-OES for K. + concentration.

[0033] 4. Potted plant experiment Pot experiments were conducted using the improved method described in the examples, where BCA-CS and BCA each accounted for 1.5% (w / w) of the air-dried soil. A blank treatment without added amendments served as a control, denoted as CK. Each pot contained 500 g of original soil or mixed soil, with four replicates per treatment group. The plants were pre-cultured in the dark for 3 days at 60% of their maximum water holding capacity.

[0034] Corn plants were sown using the hole-sowing method. During the growing season, watering was done by weighing to maintain soil moisture at 60% of maximum water holding capacity. The potted plants were randomly repositioned weekly. The plant growth cycle was 40 days.

[0035] 5. Collection and analysis of plant samples Plant height: measured from stem base to tip with a ruler; Biomass: blanched at 105℃ for 2 h, dried at 70℃ to constant weight, and weighed using an electronic balance; Chlorophyll: measured at the middle of the leaf using a SPAD-502 chlorophyll meter; Proline: sulfosalicylic acid method (UV spectrophotometer, 520nm). 6. Soil sample collection and analysis EC and pH: determined by the extract (water-soil ratio 5:1); Nitrogen: diffusion method; Phosphorus: molybdenum-antimony colorimetric method (UV-Vis, 700nm); Potassium: ammonium acetate extraction-flame photometry; CEC and EX-Na: ammonium acetate exchange-flame photometry; ESP: calculated by CEC and EX-Na.

[0036] 7. Test Results (1) Figures 2-4 Nutrient release curves for BCA and BCA-CS.

[0037] Based on experimental data, BCA-CS microspheres exhibited significant nutrient controlled release behavior under different pH conditions: Under acidic conditions (pH 5.0): N, P, and K were rapidly released within 24 hours (N: 35.2±3.0%, P: 28.1±2.5%, K: 40.5±3.8%), and nearly completely released after 7 days (N: 89.7±7.1%, P: 80.2±6.5%, K: 94.6±8.0%). The main release mechanism of BCA-CS at this time was the swelling of the chitosan (CS) membrane under acidic conditions. (Formation), pores increase, leading to a sustained rapid release after the initial release phase (0-24h).

[0038] Under neutral conditions (pH 7.0): the release rate is slow, with a 24-hour release amount (N: 15.4±1.8%, P: 12.2±1.2%, K: 20.1±2.0%), reaching a plateau after 7 days (N: 70.3±6.0%, P: 60.5±5.5%, K: 84.7±7.3%). The release mechanism is primarily diffusion-controlled, conforming to a linear release pattern.

[0039] Under alkaline conditions (pH 9.0): the release is slowest, with extremely low release levels after 24 hours (N: 5.1±0.8%, P: 4.0±0.6%, K: 8.3±1.0%), and some nutrients are still retained after 7 days (N: 49.5±4.8%, P: 40.3±4.0%, K: 59.7±5.8%). The release mechanism is CS membrane contraction (-NH2 deprotonation), which closes the pores and inhibits nutrient diffusion.

[0040] Release behavior of the uncoated BCA control group: No pH responsiveness: Release rates were similar across the pH range of 5.0–9.0 (7-day N release: pH 5.0 95.2 ± 1.8% vs pH 9.0 93.8 ± 1.5%).

[0041] Significant short-term release effect: N release reached 41.5%~44.8% within 24 hours, far higher than BCA-CS (pH 7.0: 15.4±1.8%), proving that chitosan coating effectively delays nutrient release.

[0042]

[0043] At pH 5.0, the release index n = 0.59, indicating that non-Fick diffusion dominates the dissolution of the chitosan film, thereby promoting nutrient release. For pH 7.0–9.0: it conforms to the Higuchi diffusion model (R0). 2 > 0.95), but the rate constant (k) decreased by 56% under alkaline conditions, confirming that membrane contraction inhibited release.

[0044] The pH response factor (PRF) of BCA-CS was calculated. PRF = cumulative release under acidic conditions / cumulative release under alkaline conditions. A PRF > 1.5 indicates that the material has strong pH responsiveness. The PRF values ​​for nitrogen, phosphorus, and potassium in BCA-CS were 1.81, 1.99, and 1.58, respectively, indicating that BCA-CS has strong pH responsiveness. It is particularly effective in controlling the release of N and P.

[0045]

[0046] (2) The effect of BCA-CS on maize growth in saline-alkali soil.

[0047] Figure 5 and Figure 6 The growth of maize under different treatments is shown.

[0048] Compared with the control group (CK), BCA-CS significantly promoted the increase of maize plant height and aboveground fresh weight. Specifically: Plant height: The maize plant height in the BCA-CS treatment group was 67.5% higher than that in the CK group and 16.0% higher than that in the BCA group. Aboveground fresh weight: The BCA-CS group was 132.7% higher than that in the CK group and 32.6% higher than that in the BCA group. Possible reasons include: the pH-responsive release characteristics of BCA-CS in saline-alkali soil (pH 8-9) can maintain a stable supply of nitrogen, phosphorus, and potassium, alleviating the inhibitory effect of salt stress on maize growth; the light conversion effect of silicon quantum dots: converting ultraviolet light into visible light (especially 450 nm blue light and 660 nm red light), improving photosynthetic efficiency; and the cell wall strengthening effect of nano-silicon, promoting silicification deposition and enhancing the mechanical strength of the stem.

[0049] Figure 7 and Figure 8 The changes in chlorophyll content and proline content in maize leaves were shown respectively.

[0050] Chlorophyll SPAD value: The BCA-CS group showed a 49.3% increase compared to the CK group and a 16.8% increase compared to the BCA group. Proline content: The BCA-CS group showed a 43.4% decrease compared to the CK group, indicating a significant reduction in salt stress. Possible reasons include: improved photosynthetic efficiency: chitosan coating reduces nutrient loss, promotes nitrogen assimilation, and enhances chlorophyll synthesis; antioxidant response: the porous structure of biochar adsorbs Na+. +It reduces ion toxicity and oxidative damage; the internalization of silicon quantum dots enhances the antioxidant system of corn.

[0051] Figure 9 The growth parameters of the maize root system are displayed.

[0052] The root length of the BCA-CS treatment group increased by 74.8% compared to the control (CK) and by 23.4% compared to BCA. Possible reasons include: improved soil structure (the microporous-mesoporous structure of BCA-CS increases soil permeability and promotes root extension); and water regulation (chitosan's water-retention properties reduce salt migration to the root surface, thus lowering osmotic stress).

[0053] (3) The effect of BCA-CS on the improvement of saline-alkali soil.

[0054] Figure 10 and Figure 11 The changes in soil EC and ESP are shown.

[0055] After 40 days of BCA-CS treatment, soil EC decreased by 31.7% compared to CK, indicating that soluble salts were adsorbed or leached. Soil ESP decreased from 18.4% of CK to 7.6% (a reduction of 58.7%), meeting the standard for improvement of slightly saline-alkali soil (ESP < 10%). Reasoning: Na + Immobilization, the high CEC of biochar adsorbs Na through ion exchange. + Aggregate formation; chitosan binds to soil particles, reducing Na+. + The mobility of nanocrystalline silicon; the ion sieving effect of nanocrystalline silicon: selective adsorption of Na through size exclusion (quantum dot pore size 0.5-0.8 nm). + .

[0056] Figure 12-14 The changes in available nitrogen, phosphorus, and potassium were shown: Available nitrogen: 76.7% higher in the BCA-CS group compared to the control group. Available phosphorus: 96.8% higher, due to reduced phosphorus fixation caused by pH-responsive release. Available potassium: 88.7% higher, with chitosan delaying potassium release. + Leaching. Possible causes include: the controlled-release effect of BCA-CS, which accelerates nutrient release in the weakly acidic microenvironment of the root system; and the biochar carrier, whose porous structure protects nutrients from rapid fixation by the soil.

[0057] Figure 15 and 16 The changes in cation exchange capacity (CEC) and exchangeable sodium are shown: For CEC: the BCA-CS group showed an 82.4% increase compared to the CK group, enhancing soil fertility retention. For EX-Na: a 57.9% decrease, confirming effective passivation of Na⁺. Reasoning: The oxygen-containing functional groups (-COOH, -OH) of biochar and the amino groups of chitosan (… Together they provide adsorption sites.

[0058] The above comparative experiments show that: (1) pH-responsive controlled release characteristics: BCA-CS microspheres exhibited significant nutrient controlled release behavior under different pH conditions. Under acidic conditions (pH 5.0), the release rates of N, P, and K reached 89.7%, 80.2%, and 94.6% within 7 days, respectively, while the release rates were significantly reduced (49.5%, 40.3%, and 59.7%) under alkaline conditions (pH 9.0). The pH response factors (PRF) were 1.81 (N), 1.99 (P), and 1.58 (K), respectively, confirming the intelligent responsiveness of chitosan coating.

[0059] (2) Significantly promoted maize growth: In saline-alkali soil (pH 8-9), the maize plant height in the BCA-CS treatment group increased by 67.5% compared with the CK, the aboveground fresh weight increased by 132.7%, the chlorophyll content increased by 49.3%, the proline content decreased by 43.4%, and the root length increased by 74.8%, which was attributed to the light conversion effect of silicon quantum dots, continuous nutrient supply and salt stress relief effect.

[0060] (3) High-efficiency improvement of saline-alkali soil: BCA-CS reduced soil EC by 31.7%, ESP from 18.4% to 7.6% (a decrease of 58.7%), and increased available nitrogen, phosphorus and potassium contents by 76.7%, 96.8% and 88.7% respectively. CEC increased by 82.4%, and EX-Na decreased by 57.9%. The mechanism includes the synergistic adsorption of Na by biochar-chitosan. + Nano-silicon ion sieve effect and pH-responsive nutrient release.

[0061] This application combines biochar, Si-QDs, and compound fertilizer into microspheres and uses chitosan to form a pH-responsive coating. It synergistically improves saline-alkali soil through the following mechanisms: (1) biochar optimizes soil structure and water retention; (2) Si-QDs enhance plant salt tolerance; (3) compound fertilizer continuously supplies nutrients; and (4) chitosan coating enables pH-dependent intelligent release, avoiding nutrient waste. This addresses the obstacles to crop growth in coastal saline-alkali soils, such as poor water retention, high salt content, and low nutrient content.

[0062] This study indicates that the pH-dependent controlled-release behavior of chitosan coating primarily stems from its protonation / deprotonation equilibrium as a weakly basic polyelectrolyte. The primary amino groups on the chitosan molecular chain (… Reversible transformation can occur under different pH conditions. This mechanism alters the charge state and network structure of the coating layer: Under slightly alkaline conditions (e.g., pH 8–9), primary amines are predominantly deprotonated, enhancing intersegmental interactions in the polymer chain. This results in a more compact coating structure and increased diffusion resistance, thus slowing the outward diffusion of nutrient ions from the microspheres, leading to sustained release. When the ambient pH decreases to weakly acidic conditions (e.g., pH approximately 5.0), the protonation degree of primary amines increases, enhancing electrostatic repulsion between chain segments. The coating layer swells and forms more open pores / diffusion channels, accelerating the nutrient release rate and achieving pH-responsive regulation of nutrient release kinetics. Based on this mechanism, this study constructed a coating system using alginate... Cross-linked spherical microspheres serve as a carrier, composite-loaded with biochar, silicon quantum dots, and NPK compound fertilizer, and coated with chitosan to form a biochar-based composite microsphere soil conditioner. This system integrates functions such as salt regulation, soil structure improvement, slow-release nutrient supply, and crop growth promotion. It synergistically addresses the problems of high salinity, high alkalinity, poor structure, and unstable nutrient supply in coastal saline soils: the porous structure of biochar enhances soil permeability and water retention, and alleviates salt ion stress; the microsphere loading method reduces direct nutrient loss and improves its effective retention in the soil; the chitosan coating regulates nutrient release according to soil / rhizosphere pH changes, making the fertilization process more aligned with crop needs; and silicon quantum dots, as a functional component, further enhance crop resistance and promote growth. Overall, this conditioner demonstrates excellent comprehensive effects in improving the physicochemical properties of saline soils, reducing the impact of salt damage, improving nutrient use efficiency, and promoting crop growth, providing a feasible material and technical pathway for the improvement and agricultural application of coastal saline and alkaline soils.

[0063] The above embodiments are merely illustrative of the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made based on the essence of the content of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A pH-responsive crop growth promoter for coastal saline-alkali soils, characterized by: The soil conditioner is a biochar-based microsphere composed of biochar, silicon quantum dots, and nitrogen, phosphorus, and potassium compound fertilizer. The biochar-based composite microspheres are made from alginate and... The microspheres are formed by cross-linking and curing, and a pH-responsive coating layer of chitosan is formed on the outer surface of the microspheres. The pH-responsive coating layer swells or shrinks under different pH conditions, thereby regulating the release rate of nutrients in the microspheres.

2. The pH-responsive soil growth promoter for coastal saline-alkali soil crops according to claim 1, characterized in that: The weight parts of each component are: 100-300 parts of biochar; 1-10 parts of silicon quantum dots; and 1-10 parts of nitrogen, phosphorus, and potassium compound fertilizer.

3. The pH-responsive soil growth promoter for coastal saline-alkali soil crops according to claim 1, characterized in that: The silicon quantum dots have a particle size of 2-4 nm, and the biochar-based microspheres have a particle size of 3-5 mm.

4. The pH-responsive soil growth promoter for coastal saline-alkali soil crops according to claim 1, characterized in that: The molar ratio of nitrogen, phosphorus, and potassium compound fertilizer is N:P:K = 17:16:

19.

5. The method for preparing a pH-responsive crop growth promoter for coastal saline-alkali soils according to any one of claims 1-4, characterized in that: The preparation steps include, (1) Preparation of biochar It is prepared using corn stalks as raw material and an anaerobic slow pyrolysis method; (2) Preparation of silicon quantum dots Silicon quantum dots were prepared using ascorbic acid and N-[3-(trimethoxysilyl)propyl]ethylenediamine; (3) Preparation of biochar-based microspheres Biochar, silicon quantum dots, and nitrogen-phosphorus-potassium compound fertilizer are thoroughly mixed in proportion; then added to sodium alginate solution and ultrasonically dispersed to form a mixed solution; The mixed solution was added dropwise to the CaCl2 solution, where alginate reacted with Ca... 2+ Cross-linking forms biochar-based microspheres, which are then naturally air-dried to constant weight. (4) Preparation of chitosan coating Chitosan solution was dissolved in acetic acid solution, and air-dried biochar-based microspheres were added to the chitosan solution to form a coating layer on the surface of the microspheres, thus obtaining soil conditioner microspheres with pH-responsive release characteristics.

6. The preparation method of the pH-responsive crop growth promoter for coastal saline-alkali soils according to claim 5, characterized in that: The specific steps of (1) include: The corn stalks were dried in an oven to constant weight, then pulverized into powder using a pulverizer and passed through a 2 mm standard sieve. The pretreated waste biomass raw material was weighed and placed in a tube furnace. Nitrogen gas was introduced, and the air in the system was purged. The temperature was raised from room temperature to 500°C and maintained for 3 hours. After pyrolysis, the material was naturally cooled to room temperature under nitrogen protection, air-dried, pulverized, and passed through a 1 mm sieve.

7. The preparation method of the pH-responsive crop growth promoter for coastal saline-alkali soils according to claim 5, characterized in that: The specific steps of (2) include: Weigh 2.3 g of ascorbic acid and dissolve it in 8 mL of ultrapure water. Then add 2 mL of N-[3-(trimethoxysilyl)propyl]ethylenediamine and heat and stir at 80 °C for 8 h. After that, transfer the solution to a 1000 Da dialysis bag and dialyze it in ultrapure water for 48 h. The aqueous solution collected is silicon quantum dots.

8. The preparation method of the pH-responsive crop growth promoter for coastal saline-alkali soils according to claim 5, characterized in that: In (3), the mass fraction of sodium alginate solution is 2%~5%, the mass fraction of CaCl2 solution is 2%, and 1~3 mL of sodium alginate solution is added to each gram of the mixed solid mixture of biochar, silicon quantum dots and nitrogen, phosphorus and potassium compound fertilizer. In step (4), the mass fraction of the chitosan solution is 1% to 5%, and the concentration of the acetic acid solution is 20%.

9. The preparation method of the pH-responsive crop growth promoter for coastal saline-alkali soils according to claim 6, characterized in that: In step (1), the drying temperature is 95°C and the nitrogen gas introduction rate is 500 mL / min. -1 Time: 20 min, Heating rate: 5℃ / min -1 .

10. The application of the pH-responsive release agent for coastal saline-alkali soil crops according to any one of claims 1-4, characterized in that: The soil conditioner is used to improve saline-alkali soil and promote plant growth.

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