Microbial fertilizer for saline-alkali soil improvement and preparation method thereof

Abstract

The present application belongs to the technical field of saline-alkali soil microbial improvement, and discloses a microbial fertilizer for saline-alkali soil improvement and a preparation method thereof. The microbial fertilizer is in a granular structure, and comprises a hierarchical pore structure modified straw biochar matrix, a composite salt-tolerant functional bacterial group loaded on the matrix in a partitioned manner, chelated trace elements loaded on the surface of the matrix or the bacterial group, and a porous network discontinuous salinity response type humic acid coating layer coated on the outside of the matrix. The composite salt-tolerant functional bacterial group is composed of halomonas, bacillus subtilis and jelly-like paenibacillus. The halomonas and jelly-like paenibacillus are filled in the internal micropores of the matrix, and the bacillus subtilis is attached to the surface macropores and orifices of the matrix. The present application can improve the survival rate of functional bacteria in a high-salt environment, avoid inter-species antagonism of the bacterial group, realize the release of the bacterial group and nutrients in response to the degree of soil salinity, reduce the leaching rate of nutrients, and is suitable for the improvement of different grades of saline-alkali soil, and has stable and excellent comprehensive improvement effect.

Description

Technical Field

[0001] This invention belongs to the field of saline-alkali land microbial improvement technology, and in particular to a microbial fertilizer for saline-alkali land improvement and its preparation method. Background Technology

[0002] Saline-alkali land is an important reserve of arable land in my country, with over 200 million mu (approximately 13.3 million hectares) of saline-alkali land available for agricultural use nationwide. However, salinity and alkalinity lead to poor soil nutrient availability and microbial community imbalance, resulting in arable land productivity that is less than 30% of that of non-saline-alkali arable land. Microbial improvement, due to its green and pollution-free nature and long-lasting soil-improving effects, is a core development direction in the field of improving the quality of saline-alkali arable land. The industry's core demand is to improve the survival rate of salt-tolerant functional bacteria under high salinity and alkalinity stress, achieving targeted release of nutrients and bacteria, and simultaneously improving soil quality and promoting crop growth.

[0003] Currently, most mainstream microbial fertilizers for improving saline-alkali land are conventional compound products. They are obtained by simply mixing and granulating salt-tolerant functional bacteria with humus and conventional chemical fertilizers. These products rely on the metabolic activities of the functional bacteria to decompose insoluble nutrients and secrete growth-promoting substances to regulate the rhizosphere microenvironment, showing some effectiveness in low to medium saline-alkali lands. However, these products do not provide effective protection for the functional bacteria. After being applied to high-salinity and alkaline soils, the survival rate of the functional bacteria is generally less than 10% within 15 days due to high osmotic pressure and high pH stress. Furthermore, the nutrients lack a controlled-release structure and are easily leached away with irrigation water. The improvement cycle generally exceeds 3 years, resulting in high overall improvement costs.

[0004] Another mainstream technology is biochar-supported microbial fertilizer, which uses ordinary pyrolytic biochar as a carrier to adsorb functional bacteria before application. It relies on the physical barrier effect of the biochar pores to reduce the damage of environmental stress to the functional bacteria, increasing the survival rate of functional bacteria by about 20% compared to conventional compound fertilizers. Its application range covers moderately to severely saline-alkali land. However, this type of technology uses a single biochar pore structure without a hierarchical pore zone colonization design. The coexistence of aerobic and facultative functional bacteria can easily lead to antagonistic activity. The alkaline nature of the biochar itself can exacerbate the pH increase in saline-alkali land. Furthermore, it lacks a salinity-responsive release structure, making it impossible to dynamically adjust the release rate of bacteria and nutrients according to soil salinity. This can easily lead to problems such as slow nutrient release in low-salt conditions and bacterial inactivation under stress in high-salt conditions, resulting in poor adaptability.

[0005] The two existing technologies cannot simultaneously meet the requirements of high survival rate of microbial communities, synergistic activity of functional bacteria, and targeted controlled release of nutrients under high salinity and alkali stress. This severely limits the large-scale promotion and application of microbial improvement technology in saline-alkali land. There is an urgent need to develop new microbial improvement products that are adapted to the special environment of saline-alkali land to support the efficient improvement of the quality of saline-alkali arable land. Summary of the Invention

[0006] To address the shortcomings of existing microbial fertilizers for improving saline-alkali land, such as low survival rate of functional bacteria, easy antagonism among bacterial communities, and poor adaptability of nutrient release to saline-alkali environments, this invention provides a microbial fertilizer for improving saline-alkali land and its preparation method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A microbial fertilizer for improving saline-alkali land has a granular structure, comprising a hierarchical pore structure modified straw biochar matrix, a composite salt-alkali tolerant functional microbial community loaded in sections on the matrix, chelated trace elements loaded on the surface of the matrix and / or the microbial community, and a porous network discontinuous salt-responsive humic acid coating layer covering the outside of the matrix. The modified straw biochar matrix has micropores filled with halomonas and gelatinous Bacillus, and macropores and pore openings on the matrix surface are covered with Bacillus subtilis. The composite salt-tolerant functional bacterial group is composed of halomonas, Bacillus subtilis and gelatinous Bacillus.

[0008] Preferably, the modified straw biochar matrix is ​​prepared through a two-step carbonization process. The first step involves slow pyrolysis at 250-350°C in an inert atmosphere to retain the natural vascular bundles of the straw, forming macropores of 10-50 μm. The second step involves activation and pore formation under a nitrogen atmosphere at 600-800°C, resulting in micropores with a specific surface area of ​​200-500 m². 2 / g, then rinsing with citric acid to a pH of 5.5-6.5.

[0009] Preferably, the coating layer is composed of mineral-derived fulvic acid and calcium alginate gel, wherein the mass ratio of mineral-derived fulvic acid to sodium alginate is 2:1 to 4:1, and the sodium alginate is cross-linked with calcium chloride to form calcium alginate gel; the coating layer has a thickness of 0.1 to 0.3 mm, a porosity of 30% to 60%, and is distributed in a discontinuous network. When exposed to high-salt solutions, the porosity increases and the decomposition accelerates, releasing bacteria and nutrients.

[0010] Preferably, the number of viable Halomonas bacteria per gram of fertilizer is not less than 2 × 10⁻⁶. 8 CFU, Bacillus subtilis viable count not less than 1×10⁻⁶ 8 CFU, viable count of gelatinous Bacillus not less than 0.5 × 10⁻⁶ 8 CFU.

[0011] Preferably, based on the total dry weight of the fertilizer, the mass percentage of each component is as follows: 45%~55% modified straw biochar, 5%~10% compound salt-alkali tolerant functional bacteria, 10%~15% chelated trace elements, and the coating layer is made up to 100%; the chelated trace elements are at least one of iron, zinc, manganese, copper, boron, and molybdenum chelated with citric acid, and the moisture content of the fertilizer is ≤25%, and the pH value is 5.5~8.5.

[0012] Preferably, it includes the following steps: S1. Modified straw biochar matrix is ​​prepared by carbonizing straw in two steps to obtain hierarchical porous biochar, and then rinsing it with citric acid to obtain modified straw biochar matrix. S2 stepwise loading of bacterial communities involves first vacuum impregnating a mixed suspension of Halomonas and gelatinous Bacillus into the micropores inside the substrate and drying it at low temperature, and then spraying a suspension of Bacillus subtilis onto the macropores and pore openings of the substrate surface and drying it at low temperature. S3 sprays chelated trace elements onto the substrate surface and then dries them; S4 uses a fluidized bed two-fluid spraying process to form a calcium alginate gel layer on the outside of a biochar matrix loaded with microbial communities and chelated trace elements. At the same time, a salinity-responsive humic acid coating layer is prepared by in-situ foaming and pore-forming through acid-base reaction, thus obtaining the target microbial fertilizer.

[0013] Preferably, the two-step carbonization process involves first slowly pyrolyzing the crushed straw under an inert atmosphere at 250-350°C, retaining the natural vascular bundles of the straw to form macropores of 10-50 μm. Then, the resulting pyrolysis product is heated to 600-800°C in a nitrogen atmosphere within the same carbonization furnace to activate and create micropores, resulting in a micropore specific surface area of ​​200-500 m². 2 / g of biochar was soaked in a 3%~5% citric acid solution for 12~24 hours and rinsed until the pH was 5.5~6.5.

[0014] Preferably, the specific steps for the stepwise loading of bacterial communities in S2 and the loading of trace elements in chelated form in S3 are as follows: First, the mixed suspension of Halomonas and gelatinous Bacillus is immersed in the biochar under a negative pressure of -0.06 to -0.09 MPa for 20 to 40 minutes to allow the bacteria to enter the micropores inside the biochar, and then vacuum dried at a temperature below 35°C until the moisture content is less than 15%. Then the concentration of live bacteria was increased to 1×10⁻⁶. 9 ~5×10 9 A CFU / mL Bacillus subtilis suspension was sprayed onto the macropores and pore openings of the biochar surface at a spray pressure of 0.1–0.3 MPa, and then vacuum dried at below 35°C until the moisture content was less than 15%. Then, the chelated trace elements were prepared into a spray solution with a mass concentration of 5%–15%, which was sprayed onto the substrate surface at a spray pressure of 0.2–0.4 MPa and then vacuum dried at below 35°C.

[0015] Preferably, the specific steps for preparing the coating layer by fluidized bed two-fluid spraying are as follows: mineral-derived fulvic acid, sodium alginate and ammonium bicarbonate powder are mixed and dispersed in water, and used within 2 hours after preparation to obtain the coating spray solution; calcium chloride and organic acid are prepared into a crosslinking solution; In a fluidized bed, the coating liquid and crosslinking liquid are simultaneously sprayed onto the outer side of the biochar matrix loaded with bacteria using an externally mixed dual-fluid spray gun. At the instant the coating liquid and crosslinking liquid come into contact with the particle surface, sodium alginate and calcium ions crosslink in situ to form a calcium alginate gel layer. At the same time, organic acids react with ammonium bicarbonate to release carbon dioxide, which foams in situ within the gel layer to form a porous network structure. Subsequently, free moisture is removed by low-temperature fluidized drying at 30~35℃. No heat treatment above 35℃ is introduced throughout the process.

[0016] Preferably, the organic acid is acetic acid and / or lactic acid, with a total mass concentration of 1% to 3%; the mass ratio of mineral-derived fulvic acid to sodium alginate in the coating spray solution is 2:1 to 4:1, and the amount of ammonium bicarbonate is 10% to 40% of the total dry mass of the coating spray solution; the mass concentration of calcium chloride in the crosslinking solution is 0.5% to 3%; the thickness of the resulting coating layer is 0.1 to 0.3 mm, and the porosity is 30% to 60%, wherein the porosity is controlled by adjusting the ratio of ammonium bicarbonate to acetic acid and / or lactic acid.

[0017] The present invention has the following beneficial effects: This invention prepares a citric acid-modified straw biochar matrix with a hierarchical porous structure through a two-step carbonization process. The acidity of the biochar is adjusted by acid modification to meet the needs of saline-alkali land improvement. At the same time, a step-by-step loading process is used to achieve the partitioned colonization of facultative phosphorus- and potassium-solubilizing halophilic bacteria and aerobic growth-promoting bacteria. This effectively solves the defects of existing technologies, such as the coexistence of different physiological types of functional bacteria causing antagonistic activity and the alkalinity of ordinary biochar exacerbating soil salinity and alkalinity problems. This ensures that the metabolic activity of each functional bacteria is fully utilized and simultaneously improves the activation efficiency of insoluble nutrients in the soil.

[0018] This invention coats the substrate containing the bacterial community with a porous, discontinuous, salinity-responsive humic acid coating layer. By relying on the physical barrier effect of the coating, the high osmotic pressure and high pH stress of saline-alkali soil are isolated, effectively solving the defect of rapid inactivation of functional bacteria after being applied to a high saline-alkali environment in the prior art. This extends the survival period of functional bacteria in saline-alkali soil and ensures the long-term effectiveness of the improvement.

[0019] The coating layer of this invention has salinity-responsive characteristics, which can dynamically adjust the porosity and disintegration rate according to soil salinity, match the microbial community and nutrient release requirements of plots with different salinity levels, effectively solve the defects of existing technologies such as uncontrolled nutrient release and easy leaching loss, and insufficient adaptability to plots with different salinity, reduce ineffective nutrient loss, and lower the overall cost of long-term improvement.

[0020] The product of this invention is compatible with conventional agricultural application methods such as broadcasting and strip application, without the need for additional supporting equipment. It can cover the improvement needs of saline-alkali land of all grades from mild to severe, and is suitable for various application scenarios such as the development of reserve arable land for saline-alkali land, the improvement of production capacity of medium and low-yield saline-alkali fields, and the treatment of saline-alkali degraded arable land. It has good industry promotion value and can provide reliable technical support for the efficient improvement of saline-alkali arable land quality. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating the preparation process of a microbial fertilizer for improving saline-alkali land, as proposed in this invention. Figure 2 A bar chart comparing the total viable bacterial recovery rate of functional bacteria in severely saline-alkali soil proposed in this invention after 30 days; Figure 3 This is a line graph comparing the total bacterial release rate and the leaching rate of trace elements in soils with different salinity, as proposed in this invention. Figure 4 This is a bar chart comparing the phosphorus and potassium solubilization efficiencies over 30 days in severely saline-alkali soil, as proposed in this invention. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Raw materials and strain sources: The corn stalks used in the experiment were taken from farmland waste in the saline-alkali area of ​​the Huang-Huai-Hai Plain, and were pulverized to 20 mesh after removing impurities.

[0024] All multifunctional strains were purchased from the China General Microbiological Culture Collection Center (CGMCC), including: Halomonas sp., a moderately halophilic Gram-negative bacterium, is cultured on 2216E medium containing 5%–10% NaCl. Bacillus subtilis is a common rhizosphere growth-promoting bacterium, cultured on LB medium or nutrient broth medium. Paenibacillus mucilaginosus, also known as silicate bacteria (potassium bacteria), is cultured aerobically on nitrogen-free Assumption medium (pH 7.0–7.2).

[0025] All of the above strains are publicly available and commonly used commercial strains. The public can purchase them through conventional channels using the accession numbers listed in the CGMCC strain catalog.

[0026] The citric acid, mineral-derived fulvic acid, sodium alginate, calcium chloride, acetic acid, lactic acid, and ammonium bicarbonate used in the experiment were all agricultural grade and met the national agricultural input product quality standards. The chelated trace elements were a mixture of citric acid-chelated iron, zinc, manganese, copper, boron, and molybdenum, which met the requirements of NY / T 1429-2010 standard.

[0027] In this study, saline-alkali soils were classified according to their salinity as follows: slightly saline-alkali soils had a salinity of 0.1% to 0.3%, with 0.2% as the representative value in this study; moderately saline-alkali soils had a salinity of 0.3% to 0.5%, with 0.4% as the representative value in this study; and severely saline-alkali soils had a salinity of 0.5% to 1.0%, with 0.6% as the representative value in this study.

[0028] The product quality indicators were determined using the following methods: viable bacteria count was determined using the plate count method specified in GB 20287-2006 "Agricultural Microbial Agents"; pH value was determined using the potentiometric method specified in NY / T 1973-2010; moisture content was determined using the vacuum drying method specified in GB / T8576-2010; biochar porosity was determined using the mercury porosimetry method; the survival rate of functional bacteria in high-salt soil was determined using the gradient dilution plate count method; phosphorus and potassium solubilization efficiency was determined using the molybdenum antimony colorimetric method and flame photometry, respectively; and nutrient leaching rate was determined using the soil column leaching method. The resulting product had a particle size of 2-4 mm, a contamination rate ≤1%, and heavy metal content meeting the requirements of NY / T798-2015 "Compound Microbial Fertilizers".

[0029] Example 1

[0030] The specific steps are as follows: Preparation of modified straw biochar matrix (S1): Crushed corn straw was placed in a tubular carbonization furnace. Argon gas was first introduced as an inert atmosphere, and the temperature was raised to 250℃ for slow pyrolysis for 2 hours, preserving the natural vascular structure of the straw to form macropores of 10-50 μm. Subsequently, nitrogen gas was introduced into the same carbonization furnace, and the temperature was raised to 600℃ for activation and pore formation for 2 hours, resulting in a micropore specific surface area of ​​200 m². 2 / g hierarchical porous biochar; The obtained biochar was soaked in a 3% citric acid solution for 12 hours, rinsed with deionized water until the pH of the biochar was 5.5, and dried to obtain the modified straw biochar matrix.

[0031] This step involves a two-step carbonization process to create a hierarchical porous structure, providing separate colonization spaces for two different physiological types of microbial communities and avoiding antagonistic coexistence among the communities; citric acid modification neutralizes the alkaline groups on the surface of the biochar, preventing the exacerbation of soil salinity and alkalinity problems.

[0032] S2 Stepwise Loading of Microbial Community: First, a mixed bacterial suspension of Halomonas and Bacillus mucilaginosus was prepared, with a total viable bacterial concentration of 3 × 10⁻⁶. 9CFU / mL, the modified biochar matrix was placed in a negative pressure tank and impregnated for 20 min under a negative pressure of -0.06 MPa to allow the bacteria to enter the micropores inside the biochar. Then, it was vacuum dried at 34 °C until the moisture content was 14%. Prepare a live bacteria concentration of 1×10⁻⁶. 9 A CFU / mL suspension of Bacillus subtilis was sprayed onto the macropores and pore openings of the biochar surface at a spray pressure of 0.1 MPa, and then vacuum dried at 34°C to a moisture content of 14%. This step, through negative pressure impregnation and surface spraying to load different bacterial groups in stages, achieves physical isolation between the two types of bacterial groups, completely avoids activity antagonism, and addresses the shortcomings of existing technologies in bacterial group antagonism.

[0033] S3 loaded chelated trace elements: Citric acid chelates a mixture of iron, zinc, manganese, copper, boron and molybdenum to prepare a 5% (w / w) spray solution, which is then sprayed onto the substrate surface at a spray pressure of 0.2 MPa and vacuum dried at 34°C until the moisture content is 14%.

[0034] S4 Preparation of Salinity-Responsive Humic Acid Coating Layer: The average diameter of the biochar matrix particles loaded with microbial communities and trace elements is about 2.8 mm; Prepare the coating spray solution by mixing and dispersing mineral-derived fulvic acid, sodium alginate, and ammonium bicarbonate powder in water, with the mass ratio of mineral-derived fulvic acid to sodium alginate being 2:1, and the amount of ammonium bicarbonate being 10% of the total dry weight of the coating spray solution, and use it within 2 hours after preparation; A crosslinking solution was prepared by dissolving calcium chloride and acetic acid in water, with a calcium chloride mass concentration of 0.5% and an acetic acid mass concentration of 1%. The coating solution and the crosslinking solution were simultaneously sprayed onto the outside of the biochar matrix loaded with microorganisms and trace elements in a fluidized bed using an external mixing dual-fluid spray gun. The inlet air temperature was controlled at 32°C. At the moment of contact between the coating solution and the crosslinking solution on the particle surface, sodium alginate and calcium ions crosslinked in situ to form a calcium alginate gel layer. At the same time, acetic acid and ammonium bicarbonate reacted to release carbon dioxide and foamed in situ, forming a porous network structure. Subsequently, the mixture was fluidized and dried to a moisture content of 24% to obtain the final granular fertilizer.

[0035] Twenty finished products were randomly selected, and the outer diameter of the particles was measured using vernier calipers. The cross-section of the particles was observed using a stereomicroscope, and the average thickness of the coating layer was measured to be 0.3 mm. The porosity of the coating layer was determined to be 30% using the mercury porosimetry method. The coating layer can act as a physical barrier to isolate high salt and alkali stress, and at the same time, it can increase porosity and accelerate release when exposed to high salt environments.

[0036] The dry weight percentages of the fertilizer components obtained in this embodiment are as follows: modified straw biochar 45%, compound salt-tolerant functional microbial community 5%, chelated trace elements 10%, and coating layer 40%. The coating layer thickness is positively correlated with its dry weight percentage and is affected by porosity. The viable number of Haloxylon ammodendron per gram of fertilizer is 2.1 × 10⁻⁶. 8CFU, Bacillus subtilis viable count was 1.1 × 10⁻⁶. 8 CFU, viable count of gelatinous Bacillus was 0.52 × 10⁻⁶. 8 CFU; the fertilizer has a pH of 5.5 and a moisture content of 24%, which meets the product quality requirements.

[0037] Example 2

[0038] The specific steps are as follows: Preparation of modified straw biochar matrix (S1): Crushed corn straw was placed in a tubular carbonization furnace. Argon gas was first introduced as an inert atmosphere, and the temperature was raised to 350℃ for slow pyrolysis for 2 hours, preserving the natural vascular structure of the straw to form macropores of 10-50 μm. Subsequently, nitrogen gas was introduced into the same carbonization furnace, and the temperature was raised to 800℃ for activation and pore formation for 2 hours, resulting in a micropore specific surface area of ​​500 m². 2 / g of graded porous biochar; the obtained biochar was soaked in a 5% citric acid solution for 24h, rinsed with deionized water until the pH of the biochar was 6.5, and dried to obtain the modified straw biochar matrix.

[0039] S2 Stepwise Loading of Microbial Community: First, a mixed bacterial suspension of Halomonas and Bacillus mucilaginosus was prepared, with a total viable bacterial concentration of 3 × 10⁻⁶. 9 CFU / mL, the modified biochar matrix was placed in a negative pressure tank and impregnated for 40 min under a negative pressure of -0.09 MPa to allow the bacteria to enter the micropores inside the biochar. It was then vacuum dried at 34 °C until the moisture content was 14%; then a viable bacteria concentration of 5 × 10⁻⁶ CFU / mL was prepared. 9 A CFU / mL Bacillus subtilis suspension was sprayed onto the macropores and pore openings of the biochar surface at a spray pressure of 0.3 MPa, and then vacuum dried at 34°C until the moisture content was 14%.

[0040] S3 loaded chelated trace elements: Citric acid chelates a mixture of iron, zinc, manganese, copper, boron and molybdenum to prepare a spray solution with a mass concentration of 15%. The solution is sprayed onto the substrate surface at a spray pressure of 0.4 MPa and then vacuum dried at 34°C until the moisture content is 14%.

[0041] S4 Preparation of Salinity-Responsive Humic Acid Coating Layer: The average diameter of the biochar matrix particles loaded with microbial communities and trace elements is about 2.8 mm; Prepare the coating spray solution by mixing and dispersing mineral-derived fulvic acid, sodium alginate, and ammonium bicarbonate powder in water, with the mass ratio of mineral-derived fulvic acid to sodium alginate being 4:1, and the amount of ammonium bicarbonate being 40% of the total dry weight of the coating spray solution, and use it within 2 hours after preparation; A crosslinking solution was prepared by dissolving calcium chloride and lactic acid in water, with a calcium chloride mass concentration of 3% and a lactic acid mass concentration of 3%. The coating solution and the crosslinking solution were simultaneously sprayed onto the outside of the biochar matrix loaded with microorganisms and trace elements in a fluidized bed using an external mixing dual-fluid spray gun. The air inlet temperature was controlled at 32°C. The substrate was then fluidized and dried until the moisture content was 22% to obtain the final granular fertilizer.

[0042] Twenty finished products were randomly selected, and the outer diameter of the particles was measured using vernier calipers. The cross-section of the particles was observed using a stereomicroscope, and the average thickness of the coating layer was measured to be 0.1 mm. The porosity of the coating layer was determined to be 60% using the mercury intrusion porosimetry method.

[0043] The dry basis mass percentages of the fertilizer components obtained in this embodiment are as follows: modified straw biochar 55%, compound salt-tolerant functional microbial community 10%, chelated trace elements 15%, and coating layer 20%. The number of viable Haloxylon ammodendron bacteria per gram of fertilizer is 2.8 × 10⁻⁶. 8 CFU, Bacillus subtilis viable count was 1.7 × 10⁻⁶. 8 CFU, viable count of gelatinous Bacillus was 0.8 × 10⁻⁶. 8 CFU; the fertilizer has a pH of 8.5 and a moisture content of 22%, which meets the product quality requirements.

[0044] Example 3

[0045] The specific steps are as follows: Preparation of modified straw biochar matrix (S1): Crushed corn straw was placed in a tubular carbonization furnace. Argon gas was first introduced as an inert atmosphere, and the temperature was raised to 300℃ for slow pyrolysis for 2 hours, preserving the natural vascular structure of the straw to form macropores of 10-50 μm. Subsequently, nitrogen gas was introduced into the same carbonization furnace, and the temperature was raised to 700℃ for activation and pore formation for 2 hours, resulting in a micropore specific surface area of ​​350 m². 2 / g of hierarchical porous biochar; the obtained biochar was soaked in a 4% citric acid solution for 18h, rinsed with deionized water until the pH of the biochar was 6.0, and dried to obtain the modified straw biochar matrix.

[0046] S2 Stepwise Loading of Microbial Community: First, a mixed bacterial suspension of Halomonas and Bacillus mucilaginosus was prepared, with a total viable bacterial concentration of 3 × 10⁻⁶. 9 CFU / mL, the modified biochar matrix was placed in a negative pressure tank and impregnated for 30 min under a negative pressure of -0.08 MPa to allow the bacteria to enter the micropores inside the biochar. It was then vacuum dried at 34℃ until the moisture content was 14%; then a viable bacterial concentration of 3×10⁻⁶ was prepared. 9 A CFU / mL Bacillus subtilis suspension was sprayed onto the macropores and pore openings of the biochar surface at a spray pressure of 0.2 MPa, and then vacuum dried at 34°C until the moisture content was 14%.

[0047] S3 loaded chelated trace elements: Citric acid chelates a mixture of iron, zinc, manganese, copper, boron and molybdenum to prepare a 10% mass concentration spray solution, which is then sprayed onto the substrate surface at a spray pressure of 0.3 MPa and vacuum dried at 34°C until the moisture content is 14%.

[0048] S4 Preparation of Salinity-Responsive Humic Acid Coating Layer: The average diameter of the biochar matrix particles loaded with microbial communities and trace elements is about 2.8 mm; Prepare the coating spray solution by mixing and dispersing mineral-derived fulvic acid, sodium alginate, and ammonium bicarbonate powder in water, with the mass ratio of mineral-derived fulvic acid to sodium alginate being 3:1, and the amount of ammonium bicarbonate being 25% of the total dry weight of the coating spray solution, and use it within 2 hours after preparation; A crosslinking solution was prepared by dissolving calcium chloride and acetic acid in water, with a calcium chloride mass concentration of 1.5% and an acetic acid mass concentration of 2%. The coating solution and the crosslinking solution were simultaneously sprayed onto the outside of the biochar matrix loaded with microorganisms and trace elements in a fluidized bed using an external mixing dual-fluid spray gun. The air inlet temperature was controlled at 32°C. The substrate was then fluidized and dried until the moisture content was 23%, resulting in the final granular fertilizer.

[0049] Twenty finished products were randomly selected, and the outer diameter of the particles was measured using vernier calipers. The cross-section of the particles was observed using a stereomicroscope, and the average thickness of the coating layer was measured to be 0.2 mm. The porosity of the coating layer was determined to be 45% using the mercury porosimetry method.

[0050] The dry basis mass percentages of the fertilizer components obtained in this embodiment are as follows: modified straw biochar 50%, compound salt-tolerant functional microbial community 7%, chelated trace elements 12%, and coating layer 31%. The number of viable Haloxylon ammodendron bacteria per gram of fertilizer is 2.5 × 10⁻⁶. 8 CFU, Bacillus subtilis viable count was 1.4 × 10⁻⁶. 8 CFU, viable count of gelatinous Bacillus was 0.7 × 10⁻⁶. 8 CFU; the fertilizer has a pH of 7.0 and a moisture content of 23%, which meets the product quality requirements.

[0051] Example 4

[0052] The specific steps for preparing microbial fertilizer for saline-alkali land improvement in this embodiment are as follows: Preparation of modified straw biochar matrix (S1): Crushed corn straw was placed in a tubular carbonization furnace. Argon gas was first introduced as an inert atmosphere, and the temperature was raised to 300℃ for slow pyrolysis for 2 hours, preserving the natural vascular structure of the straw to form macropores of 10-50 μm. Subsequently, nitrogen gas was introduced into the same carbonization furnace, and the temperature was raised to 700℃ for activation and pore formation for 2 hours, resulting in a micropore specific surface area of ​​350 m². 2 / g hierarchical porous biochar; The obtained biochar was soaked in a 4% citric acid solution for 18 hours, rinsed with deionized water until the pH of the biochar was 6.0, and dried to obtain the modified straw biochar matrix.

[0053] S2 Stepwise Loading of Microbial Community: First, a mixed bacterial suspension of Halomonas and Bacillus mucilaginosus was prepared, with a total viable bacterial concentration of 3 × 10⁻⁶. 9 CFU / mL, the modified biochar matrix was placed in a negative pressure tank and impregnated for 30 min under a negative pressure of -0.08 MPa to allow the bacteria to enter the micropores inside the biochar. It was then vacuum dried at 34℃ until the moisture content was 14%; then a viable bacterial concentration of 3×10⁻⁶ was prepared. 9 A CFU / mL Bacillus subtilis suspension was sprayed onto the macropores and pore openings of the biochar surface at a spray pressure of 0.2 MPa, and then vacuum dried at 34°C until the moisture content was 14%.

[0054] S3 loaded chelated trace elements: Citric acid chelates a mixture of iron, zinc, manganese, copper, boron and molybdenum to prepare a 10% mass concentration spray solution, which is then sprayed onto the substrate surface at a spray pressure of 0.3 MPa and vacuum dried at 34°C until the moisture content is 14%.

[0055] Preparation of Salinity-Responsive Humic Acid Coating Layer (S4): The average diameter of the biochar matrix particles loaded with microbial communities and trace elements is approximately 2.8 mm. A coating spraying solution was prepared by mixing and dispersing mineral-derived fulvic acid, sodium alginate, and ammonium bicarbonate powders in water. The mass ratio of mineral-derived fulvic acid to sodium alginate was 3:1, and the amount of ammonium bicarbonate was 25% of the total dry weight of the coating spraying solution. The solution was to be used within 2 hours of preparation. A crosslinking solution was prepared by dissolving calcium chloride and lactic acid in water. The mass concentration of calcium chloride was 1.5%, and the mass concentration of lactic acid was 2%. The coating spraying solution and crosslinking solution were simultaneously sprayed onto the outer side of the biochar matrix loaded with microbial communities and trace elements using an externally mixed two-fluid spray gun in a fluidized bed. The inlet air temperature was controlled at 32℃, followed by fluidized drying until the moisture content reached 23%, yielding the final granular fertilizer.

[0056] Twenty finished products were randomly selected, and the outer diameter of the particles was measured using vernier calipers. The cross-section of the particles was observed using a stereomicroscope, and the average thickness of the coating layer was measured to be 0.2 mm. The porosity of the coating layer was determined to be 45% using the mercury porosimetry method.

[0057] The dry basis mass percentages of the fertilizer components obtained in this embodiment are as follows: modified straw biochar 50%, compound salt-tolerant functional microbial community 7%, chelated trace elements 12%, and coating layer 31%. The number of viable Haloxylon ammodendron bacteria per gram of fertilizer is 2.4 × 10⁻⁶. 8 CFU, Bacillus subtilis viable count was 1.3 × 10⁻⁶. 8 CFU, viable count of gelatinous Bacillus was 0.68 × 10⁻⁶. 8CFU; the fertilizer pH was 7.1 and the moisture content was 23%, meeting the product quality requirements. Results showed that lactic acid and acetic acid had comparable effects. The product indicators and subsequent performance data obtained in Examples 3 and 4 were similar, proving that they can be used interchangeably.

[0058] Comparative Example This comparative example uses a conventional process for preparing microbial fertilizer for saline-alkali land: corn stalks are directly placed in a nitrogen atmosphere at 600℃ for 2 hours to undergo pyrolysis to obtain ordinary biochar without citric acid modification. The pH of the biochar is 9.2. Equal amounts of Halomonas, Bacillus subtilis, and Bacillus mucilaginosus were mixed to prepare a total viable bacteria concentration of 3 × 10⁻⁶. 9 A mixed suspension of CFU / mL was directly adsorbed and loaded onto the surface of ordinary biochar, and vacuum dried at 34°C to a moisture content of 14%. After adding 12% by mass of chelated trace elements and mixing, the mixture was granulated conventionally without a coating layer to obtain the comparative product.

[0059] Core reaction description Citric acid modified biochar: Citric acid undergoes a coordination / neutralization reaction with the alkaline calcium salt groups on the surface of biochar, adjusting the pH of the biochar surface to a weakly acidic level (pH 5.5~6.5). When applied to saline-alkali soil, it will not exacerbate soil alkalization, effectively solving the defect of existing ordinary biochar being alkaline and aggravating saline-alkali obstacles.

[0060] Calcium alginate crosslinking: The guluronic acid block in sodium alginate undergoes ionic crosslinking with calcium ions to form a three-dimensional gel network, fixing mineral-derived fulvic acid in the coating layer; when the soil salinity increases, sodium ions in the soil can replace calcium ions in the calcium alginate network, increasing the porosity of the gel layer or even causing it to disintegrate, thus achieving salinity-responsive bacterial community and nutrient release.

[0061] Salinity-responsive release mechanism: Sodium alginate and calcium ions in the coating layer cross-link to form a calcium alginate gel network, which encapsulates and fixes mineral-derived fulvic acid, microorganisms, and trace elements on the outer layer of the particles. When fertilizer is applied to saline-alkali soil, the concentration of sodium ions in the soil solution increases with increasing salinity. Sodium ions gradually replace calcium ions in the calcium alginate network, causing the gel cross-linking density to decrease and the porosity to increase, thereby accelerating the release of microorganisms and nutrients from the coating layer. At the same time, the porous structure formed by the in-situ foaming of ammonium bicarbonate and organic acids in the coating layer further provides release channels. Therefore, the release rate of the coating layer increases with increasing soil salinity, achieving a salinity-responsive release adapted to the saline-alkali environment.

[0062] The viable cell rate and release rate listed in the following two tables are calculated based on the initial viable cell count measured after each sample preparation.

[0063] Table 1. Performance parameters of samples stored for 30 days in severely saline-alkali soil (salt content 0.6%).

[0064] The data in this table are the average values ​​of three parallel experiments, and the trends of the results are consistent across experiments. The total viable bacteria recovery rate in Table 1 is defined as the percentage of the sum of viable bacteria released into the soil after 30 days of storage and the number of viable bacteria remaining in the granules, relative to the initial load of viable bacteria, reflecting the overall survival level of the microbial community under coating protection. The number of viable bacteria remaining in the granules was determined by screening and crushing the fertilizer granules from the soil using the GB 20287-2006 plate count method; the number of viable bacteria released into the soil was determined using the soil suspension gradient dilution plate count method. The test environment was a typical coastal severely saline-alkali soil with a salt content of 0.6%, simulating the actual application scenario. The total viable bacteria recovery rate of the three types of functional bacteria in the four examples remained stable at over 56% after 30 days, higher than the control group, verifying the stress protection effect of the coating layer. The soil pH in the example groups all showed a decreasing trend, without exacerbating alkalization, and the phosphorus and potassium solubilization efficiency was more than twice that of the control group, verifying the effect of non-antagonistic colonization of the microbial community and full utilization of its activity.

[0065] Table 2. Nutrient leaching rate and bacterial release rate in soils with different salinities over 30 days

[0066] The data in this table are the average values ​​of three parallel experiments, and the trends of the results are consistent across experiments. The total bacterial release rate in Table 2 is defined as the percentage of the total number of bacteria (including live and dead bacteria) released into the soil after 30 days of storage relative to the initial total bacterial count, reflecting the physical process of bacterial release from the particles. In Example 1, the total bacterial release rate increased from 42.3% to 89.4% with increasing soil salinity, demonstrating the advantage of salinity-responsive release. Referring to Table 1, although the total bacterial release rate in Example 1 reached 89.4% at 0.6% salinity, most of the bacteria released into the soil under the protection of the coating layer remained live, thus the total recovered live bacteria rate still reached 62.3%. In the example groups, the leaching rate of trace elements was less than 13%, lower than the 48% of the comparative example, reducing ineffective nutrient loss. In the comparison, without coating protection, the bacterial community was directly exposed to the saline-alkali environment: under low salinity, the bacteria lacked slow release and released a large amount prematurely; under high salinity, a large number of functional bacteria died and lysed, with very few countable released bacteria and extremely low survival rates, indicating problems of excessive bacterial release and functional failure under high salinity. These results verify the adaptability of the salinity-responsive coating layer, addressing the shortcomings of existing technologies such as easy nutrient leaching and poor adaptability.

[0067] refer to Figure 2This figure visually presents the survival status of the three types of functional bacteria in each experimental group after 30 days of storage in severely saline-alkali soil with a salt content of 0.6%. The viability of the three types of functional bacteria in the four examples remained stable at over 56%, which is more than 5 times higher than the viability of less than 10% in the control group. This clearly verifies the protective effect of the salinity-responsive coating layer on the functional bacteria under salinity stress, and also reflects the advantage of partitioned bacterial load to avoid interspecific antagonism. It effectively solves the core defects of existing technologies, such as low survival rate of functional bacteria and antagonistic inactivation of bacterial communities in high-salt environments.

[0068] refer to Figure 3 This figure visually presents the microbial release patterns and nutrient loss in soils with different salinity levels, comparing Example 1 and the comparative example. The total microbial release rate in Example 1 gradually increases with increasing soil salinity, highly matching the improvement needs of different grades of saline-alkali land. In contrast, the comparative example shows the opposite trend: excessive microbial release under low salinity and almost no effective microbial release under high salinity. Simultaneously, the micronutrient leaching rate in Example 1 remained stable below 13%, more than 70% lower than the comparative example, verifying the slow-release and adaptability effect of the salinity-responsive coating layer and effectively addressing the shortcomings of existing technologies, such as easy nutrient leaching and release patterns incompatible with saline-alkali environments.

[0069] refer to Figure 4 This figure visually presents the phosphorus and potassium solubilization performance of each experimental group after 30 days of storage in severely saline-alkali soil with a salt content of 0.6%. The phosphorus solubilization efficiency of all four examples is above 32%, and the potassium solubilization efficiency is above 28%, which are 1.8 times and 1.9 times higher than the control group, respectively. This reflects that the functional bacteria maintain high physiological activity under high salinity, verifies the comprehensive effect of partitioned loading to avoid bacterial antagonism and coating layer to improve the survival rate of strains, demonstrates the actual improvement ability of the product in severely saline-alkali soil, and provides a visual functional verification of the product's field application effect.

[0070] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A microbial fertilizer for improving saline-alkali land, having a granular structure, characterized in that, It includes a hierarchical pore structure modified straw biochar matrix, a composite salt-tolerant functional microbial community loaded on the matrix in sections, chelated trace elements loaded on the surface of the matrix and / or the microbial community, and a porous mesh discontinuous salt-responsive humic acid coating layer covering the outside of the matrix. The modified straw biochar matrix has micropores filled with halomonas and gelatinous Bacillus, and macropores and pore openings on the matrix surface are covered with Bacillus subtilis. The composite salt-tolerant functional bacterial group is composed of halomonas, Bacillus subtilis and gelatinous Bacillus.

2. The microbial fertilizer for improving saline-alkali land according to claim 1, characterized in that, The modified straw biochar matrix was prepared through a two-step carbonization process. The first step involved slow pyrolysis at 250–350°C under an inert atmosphere to retain the natural vascular bundles of the straw, forming macropores of 10–50 μm. The second step involved activation and pore formation under a nitrogen atmosphere at 600–800°C, resulting in micropores with a specific surface area of ​​200–500 m². 2 / g, then rinsing with citric acid to a pH of 5.5-6.

5.

3. The microbial fertilizer for improving saline-alkali land according to claim 1, characterized in that, The coating layer is composed of mineral-derived fulvic acid and calcium alginate gel, wherein the mass ratio of mineral-derived fulvic acid to sodium alginate is 2:1 to 4:1, and the sodium alginate is cross-linked with calcium chloride to form calcium alginate gel; the coating layer has a thickness of 0.1 to 0.3 mm, a porosity of 30% to 60%, and is distributed in a discontinuous network.

4. The microbial fertilizer for improving saline-alkali land according to claim 1, characterized in that, The number of viable Halomonas bacteria per gram of fertilizer is not less than 2 × 10⁻⁶. 8 CFU, Bacillus subtilis viable count not less than 1×10⁻⁶ 8 CFU, viable count of gelatinous Bacillus not less than 0.5 × 10⁻⁶ 8 CFU.

5. The microbial fertilizer for improving saline-alkali land according to claim 1, characterized in that, Based on the total dry weight of the fertilizer, the mass percentage of each component is as follows: modified straw biochar 45%~55%, compound salt-alkali tolerant functional bacteria 5%~10%, chelated trace elements 10%~15%, and the coating layer is made up to 100%; the chelated trace elements are at least one of iron, zinc, manganese, copper, boron and molybdenum chelated with citric acid, the moisture content of the fertilizer is ≤25%, and the pH value is 5.5~8.

5.

6. A method for preparing a microbial fertilizer for improving saline-alkali land, used to prepare the microbial fertilizer for improving saline-alkali land as described in any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Modified straw biochar matrix is ​​prepared by carbonizing straw in two steps to obtain hierarchical porous biochar, and then rinsing it with citric acid to obtain modified straw biochar matrix. S2 stepwise loading of bacterial communities involves first vacuum impregnating a mixed suspension of Halomonas and gelatinous Bacillus into the micropores inside the substrate and drying it at low temperature, and then spraying a suspension of Bacillus subtilis onto the macropores and pore openings of the substrate surface and drying it at low temperature. S3 sprays chelated trace elements onto the substrate surface and then dries them; S4 uses a fluidized bed two-fluid spraying process to form a calcium alginate gel layer on the outside of a biochar matrix loaded with microbial communities and chelated trace elements. At the same time, a salinity-responsive humic acid coating layer is prepared by in-situ foaming and pore-forming through acid-base reaction, thus obtaining the target microbial fertilizer.

7. The preparation method according to claim 6, characterized in that, The two-step carbonization process involves first pulverizing the straw and then slowly pyrolyzing it under an inert atmosphere at 250-350°C, preserving the natural vascular bundles of the straw to form macropores of 10-50 μm. Then, the resulting pyrolysis product is heated to 600-800°C in a nitrogen atmosphere within the same carbonization furnace to activate and create micropores, resulting in a micropore specific surface area of ​​200-500 m². 2 / g of biochar was soaked in a 3%~5% citric acid solution for 12~24 hours and rinsed until the pH was 5.5~6.

5.

8. The preparation method according to claim 6, characterized in that, The specific steps for the stepwise loading of bacterial communities in S2 and the loading of trace elements in S3 are as follows: First, the mixed suspension of Halomonas and gelatinous Bacillus is immersed in a negative pressure of -0.06 to -0.09 MPa for 20 to 40 minutes, and then vacuum dried at a temperature below 35°C until the moisture content is less than 15%. Then the concentration of live bacteria was increased to 1×10⁻⁶. 9 ~5×10 9 A CFU / mL Bacillus subtilis suspension was sprayed onto the macropores and pore openings of the biochar surface at a spray pressure of 0.1–0.3 MPa, and then vacuum dried at below 35°C until the moisture content was less than 15%. Then, the chelated trace elements were prepared into a spray solution with a mass concentration of 5%–15%, which was sprayed onto the substrate surface at a spray pressure of 0.2–0.4 MPa and then vacuum dried at below 35°C.

9. The preparation method according to claim 6, characterized in that, The specific steps for preparing the coating layer by fluidized bed two-fluid spraying are as follows: mineral-derived fulvic acid, sodium alginate and ammonium bicarbonate powder are mixed and dispersed in water, and used within 2 hours after preparation to obtain the coating spraying solution; calcium chloride and organic acid are prepared into a crosslinking solution; the coating spraying solution and crosslinking solution are simultaneously sprayed onto the outside of the biochar substrate loaded with bacteria in a fluidized bed through an external mixing two-fluid spray gun, and then fluidized and dried at a low temperature of 30~35℃.

10. The preparation method according to claim 9, characterized in that, The organic acid is acetic acid and / or lactic acid, with a total mass concentration of 1% to 3%; the mass ratio of mineral-derived fulvic acid to sodium alginate in the coating spray solution is 2:1 to 4:1, and the amount of ammonium bicarbonate is 10% to 40% of the total dry weight of the coating spray solution; the mass concentration of calcium chloride in the crosslinking solution is 0.5% to 3%; the thickness of the resulting coating layer is 0.1 to 0.3 mm, and the porosity is 30% to 60%, which is controlled by adjusting the ratio of ammonium bicarbonate to acetic acid and / or lactic acid.

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