Biochar-based slow-release fertilizer for improving soil fertility of citrus orchard and layered application method of biochar-based slow-release fertilizer

Abstract

The invention relates to the technical field of citrus planting, and discloses a biochar-based slow-release fertilizer for improving soil fertility of a citrus orchard and a layered application method thereof.The biochar-based slow-release fertilizer sequentially comprises a core layer, a coating layer and an outer layer from inside to outside; the core layer comprises biochar, an organic fertilizer, a compound fertilizer and an additive; the coating layer comprises polylactic acid and biochar; the outer layer comprises a sodium alginate microcapsule fungicide. The charcoal-based slow-release fertilizer with the core-shell structure forms a dual slow-release mechanism of physical adsorption and chemical slow release, reduces nutrient loss, and is applied to shallow, middle and deep three-layer citrus soil in combination with a layered fertilization technology, so that nutrients directly act on a root system dense area, and the yield and quality of citrus are improved.

Description

Technical Field

[0001] This invention relates to the field of citrus cultivation technology, specifically to a biochar-based slow-release fertilizer for improving soil fertility in citrus orchards and its method for layered application. Background Technology

[0002] Citrus (Citrus reticulata Blanco), a specialty economic crop in southern my country, has a cultivation history dating back to the Xia and Shang dynasties. Belonging to the Rutaceae family, this crop is rich in vitamin C and flavonoids, making it edible and medicinal. The pomace and peel can be used to extract pectin, citric acid, and other industrial raw materials, offering benefits such as promoting digestion, moisturizing the lungs and relieving coughs, protecting the cardiovascular system, and beautifying the skin and preventing disease. Citrus thrives in warm, humid, and sunny environments. It has strict soil requirements, needing slightly acidic (pH 5.5-6.5), loose, well-drained, and fertile soil, but dislikes waterlogging and soil compaction.

[0003] Currently, the core production areas of citrus are mainly distributed in the hilly and mountainous areas of the south. The soil in these areas generally suffers from physical defects such as a soil layer thickness of less than 40cm, weak water retention and drought resistance (water content <12% in the dry season), organic matter content of less than 1.5%, chemical imbalance such as easy loss of nitrogen, phosphorus and potassium nutrients, and management difficulties such as fertilizer utilization rate of less than 35% due to traditional broadcasting and ineffective utilization of deep nutrients. These factors seriously restrict the high-quality, high-yield and sustainable development of citrus.

[0004] Therefore, developing fertilizers and matching application techniques that combine soil improvement and precise nutrient supply to meet the synergistic absorption needs of citrus shallow and deep root systems, thereby improving the quality and yield of citrus, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] This invention provides a biochar-based slow-release fertilizer for improving soil fertility in citrus orchards and its stratified application method, in order to solve the problem that existing conventional fertilizers are difficult to meet crop needs and have limited effects on soil improvement.

[0006] In a first aspect, the present invention provides a biochar-based slow-release fertilizer for improving soil fertility in citrus orchards, comprising, from the inside out: a core layer, a coating layer, and an outer layer; In one optional embodiment, the core layer includes biochar, organic fertilizer, compound fertilizer, and additives; In one alternative embodiment, the coating layer comprises polylactic acid and biochar; In one alternative embodiment, the outer layer comprises sodium alginate microcapsule bacterial agent.

[0007] In one optional embodiment, the mass ratio of biochar, organic fertilizer, compound fertilizer and additives in the core layer is 20~40:20~40:10~30:1~5; In one optional embodiment, the additive comprises Bacillus subtilis, humic acid, and bentonite in a mass ratio of 1~2:5~6:1~2; In one optional embodiment, the organic fertilizer comprises well-rotted sheep manure and bagasse in a mass ratio of 50-95:5-50. In one optional embodiment, the compound fertilizer comprises nitrogen-phosphorus-potassium compound fertilizer and EDTA, with a mass ratio of 98~99:1~2; In one optional embodiment, the mass ratio of N, P2O5 and K2O in the nitrogen-phosphorus-potassium compound fertilizer is 16~18:8~10:12~18; In one optional embodiment, the raw materials for preparing the biochar include pruned branches from citrus trees and pomace from citrus juice extraction, in a mass ratio of 50-80:20-50.

[0008] In one optional embodiment, the mass ratio of the core layer, the outer membrane layer, and the outer layer is 80~95:5~15:1~5; In one optional embodiment, the mass ratio of polylactic acid to biochar in the coating layer is 50-80:20-50; In one optional embodiment, the sodium alginate microcapsule bacterial agent in the outer layer comprises Bifidobacterium adolescentis and Bacillus subtilis, with a live bacteria ratio of 1 to 3:1 and a total live bacteria concentration of 2 × 10⁻⁶. 8 ~2×10 9 CFU / mL.

[0009] Secondly, the present invention provides a method for preparing the above-mentioned biochar-based slow-release fertilizer for improving soil fertility in citrus orchards, comprising the following steps: S1. Preparation of biochar; S2. Preparation of the core layer: Biochar, organic fertilizer, compound fertilizer and additives are mixed according to the mass ratio and granulated; S3. Preparation of coating layer: Take polylactic acid and biochar according to the mass ratio, mix them, and coat them onto the surface of the core layer; S4. Spraying the outer layer: Prepare sodium alginate microcapsule bacterial agent and coat it onto the surface of the coating layer.

[0010] In one optional embodiment, in step S1, the citrus branches pruned from the citrus tree and the pomace after juicing the citrus fruit are taken in the specified mass ratio, mixed, crushed, dried, carbonized at a constant temperature, pyrolyzed, and finally activated by steam, modified, cooled, and sieved.

[0011] In an optional embodiment, in step S1, the branches pruned from citrus trees and the pomace after citrus juice extraction are mixed in the specified mass ratio, crushed to a particle size ≤3cm, and dried at 100~105℃ to a moisture content <10%; under nitrogen protection, the mixture is heated to 700~800℃ at a constant temperature of 1~10℃ / min, with a pyrolysis pressure of 0.05~0.11MPa and a pyrolysis time of 30~90min; then activated with steam at 800~850℃ for 30~45min, modified by impregnation with 1wt%~2wt% phosphoric acid solution, cooled, and then crushed through an 80-mesh sieve. In one optional embodiment, the biochar has a particle size of 0.5~1 mm.

[0012] In one optional embodiment, in step S2, the mixing speed is 20-30 rpm and the time is 10-15 min; In an optional embodiment, step S2 further includes spraying water after mixing to adjust the moisture content of the core layer to 15wt%~20wt%. In one optional embodiment, in step S2, the granulation conditions are a temperature of 50~70℃, a pressure of 15~30MPa, and a particle size of 1.5~2.5mm.

[0013] In an optional implementation, step S3, before mixing, further includes the following step: S31. Dissolve polylactic acid in an 8wt%~10wt% aqueous solution of ethyl acetate and stir until completely dissolved; S32. The biochar is activated by impregnation with a phosphoric acid solution of concentration of 1wt%~2wt%; S33. Add the impregnated and activated biochar to a polylactic acid solution and ultrasonically disperse for 20-40 minutes to obtain a mixture of polylactic acid and biochar.

[0014] In one optional embodiment, step S3, before spraying, further includes preheating the particles of the core layer to 50~70°C; In one optional embodiment, in step S3, the spraying conditions are: rotation speed 10~20 rpm, pressure 0.1~0.2 MPa, and atomization temperature 50~70℃. In an optional embodiment, in step S3, during the spraying process, the mixture of polylactic acid and biochar prepared in step S33, polyethylene glycol, and nano silica are added, with a mass ratio of 70~80:10~20:5~10. In one optional embodiment, during step S3, the spraying process involves drying the product to a moisture content of 2wt% to 5wt% under hot air circulation at 40-60°C.

[0015] In an optional embodiment, step S4 further includes a step of vacuum drying the core layer and the coating layer at 30~40°C for 10~15 hours before spraying. In an optional embodiment, the method for preparing sodium alginate microcapsule bacterial agent includes: preparing a mixed bacterial suspension of *Bifidobacterium adolescentis* and *Bacillus subtilis* according to the stated viable count ratio, and adding it to a 0.5wt%~1.5wt% sodium alginate solution to achieve a total viable count of 2×10⁻⁶ for *Bifidobacterium adolescentis* and *Bacillus subtilis*. 8 ~2×10 9 CFU / mL; In one optional embodiment, in step S4, the spraying process is electrostatic spraying technology with a voltage of 5~20kV.

[0016] Thirdly, the present invention provides an application of the biochar-based slow-release fertilizer for improving soil fertility in citrus orchards, or the biochar-based slow-release fertilizer for improving soil fertility in citrus orchards prepared by the above preparation method, in the planting of citrus fruit trees.

[0017] Fourthly, the present invention provides a method for applying the above-mentioned biochar-based slow-release fertilizer for improving soil fertility in citrus orchards, or the biochar-based slow-release fertilizer for improving soil fertility in citrus orchards prepared by the above-mentioned preparation method, characterized by comprising the following steps: (1) Excavate fertilizer trenches: On the outside of the drip line of the citrus canopy, dig strip-shaped fertilizer trenches along the rows or between the trees of citrus fruit trees. The trenches are 20-40cm wide and 40-60cm deep. The trench length for a single citrus tree is 1-2 meters. (2) Layered fertilization: The fertilization trench is divided into a bottom layer, a middle layer and an upper layer; first, 50wt% to 70wt% of the total amount of biochar-based slow-release fertilizer is applied to the bottom of the trench, and then 1 to 5cm of soil is backfilled to cover it as the bottom layer; then 20wt% to 30wt% of the total amount of biochar-based slow-release fertilizer is applied above the bottom layer, and then 1 to 5cm of soil is backfilled to cover it as the middle layer; finally, 10wt% to 20wt% of the total amount of biochar-based slow-release fertilizer is applied above the middle layer, and then soil is backfilled until the trench is full and slightly lower than or level with the original ground level as the upper layer. (3) Field management: including phased topdressing, irrigation and drainage, pruning, weeding, biological control of pests and diseases, fruit thinning and winter frost protection measures.

[0018] In one optional embodiment, the total amount of the biochar-based slow-release fertilizer is 3-6 kg / plant / year; In one optional embodiment, the biochar-based slow-release fertilizer is applied from the time of autumn harvest to the time of spring budding. In one optional embodiment, the staged topdressing includes applying 8-12 kg / mu of potassium fertilizer, 5-7 kg / mu of phosphorus fertilizer, and 10-15 kg / mu of nitrogen fertilizer during the autumn shoot period of citrus trees, and foliar spraying with 0.1 wt% magnesium sulfate and 0.2 wt% chelated calcium. And / or, during the flower bud differentiation period of citrus trees, stop applying nitrogen fertilizer, increase the application of phosphorus fertilizer by 4-6 kg / mu and potassium fertilizer by 8-12 kg / mu, and spray the leaves with 0.1wt% borax and 0.3wt% potassium dihydrogen phosphate. In one optional implementation, the irrigation and drainage are guided by data from smart agriculture sensors to maintain soil moisture content at 55wt%~65wt%, and 30cm deep ditches are dug for drainage during the rainy season. In one optional implementation, the biological control of pests and diseases includes setting up insect-attracting boards and releasing predatory mites, Trichogramma wasps, and lacewings. In one alternative implementation, the pruning includes pruning diseased and insect-infested branches, crossing branches, and drooping branches; In one optional embodiment, the fruit thinning includes retaining fruit at a leaf-to-fruit ratio of 30-50:1, thinning out small, deformed, and terminal fruits, and controlling the yield per acre to ≤3000 kg; In one optional embodiment, the winter antifreeze measures include applying a whitewash to the trunk and first main branch of the citrus tree using a mixture of quicklime, lime-sulfur, salt, animal oil, and water in a mass ratio of 5-10:0.5-1:0.3-0.5:0.05-0.5:30-40; and / or covering the soil around the tree base with straw.

[0019] The technical solution of this invention has the following advantages: 1. This invention provides a biochar-based slow-release fertilizer for improving soil fertility in citrus orchards, comprising: a core layer, a coating layer, and an outer layer; the core layer includes biochar, organic fertilizer, compound fertilizer, and additives; the coating layer includes polylactic acid and biochar; and the outer layer includes sodium alginate microcapsule bacterial agent. The biochar in the core layer is prepared from pruned citrus branches and juice after juicing, achieving resource recycling, providing a porous structure, and loading Bacillus subtilis, organic fertilizer, and inorganic fertilizer, offering advantages such as slow-release fertilizer effect, soil improvement, and reduced nutrient loss; the polylactic acid in the coating layer provides a biodegradable polymer coating, while the biochar provides porous structural support, a slow-release nutrient carrier, and a microbial habitat, enhancing slow-release fertilizer effect, activating microorganisms, and environmental friendliness; the outer layer is sodium alginate microcapsule bacterial agent, containing Bifidobacterium adolescentis, which promotes root development, enhances plant resistance, and improves fruit quality; together with Bacillus subtilis, it provides a dual "root-soil" protection system for plant growth. The core-shell structured biochar-based slow-release fertilizer of this invention has the effect of slow-release of nutrients, which together constitute a dual slow-release mechanism of "physical adsorption + chemical slow release" to reduce nutrient loss.

[0020] 2. This invention provides a stratified fertilization technology for biochar-based slow-release fertilizer to improve soil fertility in citrus orchards, aiming to solve key problems in current citrus fertilization. Traditional fertilization methods—whether single deep application (depth > 30cm) or shallow broadcasting (depth < 15cm)—fail to effectively match the spatial distribution and nutrient absorption patterns of citrus vertical roots (mainly concentrated in the 15-45cm soil layer), resulting in problems such as high fertilizer application rates, low fertilizer utilization, and severe nutrient loss. Through stratified fertilization technology, biochar-based slow-release fertilizer is applied proportionally to three layers of citrus soil, such as shallow, medium, and deep layers, allowing nutrients to directly act on the densely rooted areas, reducing fertilizer application and lowering production costs. Attached Figure Description

[0021] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the overall process of the present invention; Figure 2 This is a core-shell structure diagram of the biochar-based slow-release fertilizer for improving soil fertility in citrus orchards according to the present invention; Figure 3 This is a distribution diagram of tiered fertilization for citrus trees in Embodiment 4 of the present invention; Figure 4 This is a planting layout diagram of a citrus orchard according to Embodiment 4 of the present invention. Detailed Implementation

[0023] The following embodiments are provided to better understand the present invention, but the following embodiments do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the scope of protection of the present invention.

[0024] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0025] Bacillus subtilis powder was purchased from the China General Microbiological Culture Collection Center, with an effective viable count of 2 × 10⁻⁶. 10CFU / g; Bifidobacterium adolescentis powder was purchased from the China General Microbiological Culture Collection Center, with an effective viable count of 1×10⁻⁶. 9 CFU / g; Polylactic acid was purchased from China National Pharmaceutical Group Co., Ltd., CAS No. 26100-51-6, molecular weight 5000kDa; Polyethylene glycol was purchased from China National Pharmaceutical Group Co., Ltd., CAS No. 25322-68-3, molecular weight 1000kDa.

[0026] Example 1 This embodiment provides a method for preparing a biochar-based slow-release fertilizer, comprising the following steps: (1) Preparation of biochar Collect pomace from citrus tree pruning branches and juice extraction (mixed at a mass ratio of 70:30), crush to less than 3 cm, dry at 105℃ until moisture content <10%, carbonize at a constant temperature of 750℃ under nitrogen protection with a heating rate of 10℃ / min, a pyrolysis pressure of 0.11 MPa, and a pyrolysis time of 60 min, then activate with steam at 850℃ for 30 min, add 1 wt% phosphoric acid solution for impregnation and modification to increase the specific surface area and pore volume of biochar, cool and crush through an 80-mesh sieve to obtain a particle size of 0.5~1 mm; (2) Preparation of the core layer The organic fertilizer is made from fully decomposed sheep manure (C / N=25:1), with a moisture content of ≤15% and no pathogen residues. Sugarcane bagasse is added to adjust the carbon-nitrogen ratio. The mass ratio of decomposed sheep manure to sugarcane bagasse is 95:5. In compound fertilizer, the mass ratio of nitrogen, phosphorus and potassium compound fertilizer (N:P2O5:K2O=16:8:12, mass ratio) to EDTA is 98:2. EDTA is used to chelate micronutrients. The additives are Bacillus subtilis, humic acid, and bentonite, with a mass ratio of 1.5:5:1.5, which are used to promote plant absorption, alleviate soil compaction, slow release of nutrients, and improve fertilizer utilization efficiency. Biochar, organic fertilizer, compound fertilizer, and additives were added to a twin-shaft paddle mixer at a mass ratio of 35:35:25:5 and mixed at 30 r / min for 15 minutes to ensure uniform mixing. Water was sprayed to adjust the moisture content of the core layer to 20 wt% to ensure cohesion of the components. Granulation was performed using a disc granulator at a temperature of 60℃ and a pressure of 30 MPa to achieve a particle size of 2 mm. (3) Preparation of the coating layer Polylactic acid (PLA) and biochar were prepared in a mass ratio of 70:30. PLA was dissolved in a 10 wt% ethyl acetate aqueous solution and stirred until completely dissolved. Biochar was impregnated and activated with a 2 wt% phosphoric acid solution to improve the interfacial bonding between PLA and biochar and reduce the risk of coating layer cracking. The impregnated and activated biochar was added to the PLA solution and ultrasonically dispersed for 20-40 min to obtain a mixture of PLA and biochar. Before spraying, the prepared core layer particles were preheated to 70°C. Polyethylene glycol and nano-silica were added to a mixture of polylactic acid and biochar (the mass ratio of the three was 15:10:70). The fluidized bed speed was set to 15 rpm, the spraying pressure to 0.15 MPa, the atomization temperature to 60°C, and the mixture was dried to a moisture content of 5% under hot air circulation at 40°C. (4) Preparation of outer layer Bacillus subtilis and Bifidobacterium adolescentis were activated and cultured to prepare a mixed bacterial suspension (the ratio of viable bacteria to Bacillus subtilis was 3:1). This suspension was then added to a 1 wt% sodium alginate solution to achieve a concentration of 2 × 10⁻⁶ for both Bacillus adolescentis and Bacillus subtilis. 9 CFU / mL, prepared into sodium alginate-based microcapsule bacterial agent; Before spraying, the core layer and the coating layer are vacuum dried at 30°C for 12 hours to increase the crystallinity of polylactic acid to 35%. Electrostatic spraying technology (voltage 5kV) is used to apply sodium alginate-based microcapsule bacterial agent to the coating layer to prolong the fertilizer effect and reduce nutrient loss. The biochar-based slow-release fertilizer prepared according to the above preparation method has a core layer, coating layer, and outer layer with a mass ratio of 90:7:3.

[0027] Example 2 This embodiment provides a method for preparing a biochar-based slow-release fertilizer, comprising the following steps: (1) Preparation of biochar Collect pomace from citrus tree pruning branches and juice extraction (mixed in a 50:50 mass ratio), crush to less than 3 cm, dry at 103℃ until moisture content <10%, carbonize at a constant temperature of 700℃ with a heating rate of 1℃ / min under nitrogen protection, pyrolysis pressure of 0.08 MPa, pyrolysis time of 30 min, then activate with steam at 800℃ for 40 min, add 2wt% phosphoric acid solution for impregnation and modification to increase the specific surface area and pore volume of biochar, cool and crush through an 80 mesh sieve with a particle size of 0.5~1 mm; (2) Preparation of the core layer The organic fertilizer should be fully decomposed sheep manure (C / N=25:1), with a moisture content of ≤15% and no pathogen residues. Sugarcane bagasse should be added to adjust the carbon-nitrogen ratio. The mass ratio of decomposed sheep manure to sugarcane bagasse should be 50:50. In compound fertilizer, the mass ratio of nitrogen, phosphorus and potassium compound fertilizer (N:P2O5:K2O=17:10:16, mass ratio) to EDTA is 99:1. EDTA is used to chelate trace elements. The additives are Bacillus subtilis, humic acid and bentonite, with a mass ratio of 1:5:2, which are used to promote plant absorption, alleviate soil compaction, slow release of nutrients and improve fertilizer utilization efficiency. Biochar, organic fertilizer, compound fertilizer, and additives were added to a twin-shaft paddle mixer at a mass ratio of 20:40:10:1 and mixed at 20 r / min for 10 minutes to ensure uniform mixing. Water was sprayed to adjust the moisture content of the core layer to 15 wt% to ensure cohesion of the components. Granulation was performed using a disc granulator at a temperature of 50℃ and a pressure of 20 MPa to achieve a particle size of 1.5 mm. (3) Preparation of the coating layer Polylactic acid (PLA) and biochar were prepared in a mass ratio of 50:50. PLA was dissolved in an 8 wt% ethyl acetate aqueous solution and stirred until completely dissolved. Biochar was impregnated and activated with a 1.5 wt% phosphoric acid solution to improve the interfacial bonding between PLA and biochar and reduce the risk of coating layer cracking. The impregnated and activated biochar was added to the PLA solution and ultrasonically dispersed for 40 min to obtain a mixture of PLA and biochar. Before spraying, the prepared core layer particles were preheated to 55°C. Polyethylene glycol and nano-silica were added to a mixture of polylactic acid and biochar (the mass ratio of the three was 20:5:80). The fluidized bed speed was set to 10 rpm, the spraying pressure to 0.2 MPa, and the atomization temperature to 70°C. The mixture was then dried to a moisture content of 2% under hot air circulation at 70°C. (4) Preparation of outer layer Bacillus subtilis and Bifidobacterium adolescentis were activated and cultured to prepare a mixed bacterial suspension (the ratio of viable bacteria to Bacillus subtilis was 2:1). This suspension was then added to a 0.5 wt% sodium alginate solution to ensure that the concentrations of both Bacillus adolescentis and Bacillus subtilis were 2 × 10⁻⁶. 8 CFU / mL, prepared into sodium alginate-based microcapsule bacterial agent; Before spraying, the core layer and the coating layer are vacuum dried at 35°C for 15 hours to increase the crystallinity of polylactic acid to 35%. Electrostatic spraying technology (voltage 15kV) is used to apply sodium alginate-based microcapsule bacterial agent to the coating layer to prolong the fertilizer effect and reduce nutrient loss. The biochar-based slow-release fertilizer prepared according to the above preparation method has a core layer, coating layer, and outer layer with a mass ratio of 95:4:1.

[0028] Example 3 This embodiment provides a method for preparing a biochar-based slow-release fertilizer, comprising the following steps: (1) Preparation of biochar Collect pomace from citrus tree pruning branches and juice extraction (mixed at a mass ratio of 80:20), crush to less than 3 cm, dry at 100℃ until moisture content <10%, carbonize at a constant temperature of 800℃ under nitrogen protection with a heating rate of 5℃ / min, a pyrolysis pressure of 0.05 MPa, and a pyrolysis time of 90 min, then activate with steam at 825℃ for 45 min, and impregnate with 1.5 wt% phosphoric acid solution to increase the specific surface area and pore volume of biochar. After cooling, crush and pass through an 80-mesh sieve to obtain a particle size of 0.5~1 mm. (2) Preparation of the core layer The organic fertilizer should be fully decomposed sheep manure (C / N=25:1), with a moisture content of ≤15% and no pathogen residues. Sugarcane bagasse should be added to adjust the carbon-nitrogen ratio. The mass ratio of decomposed sheep manure to sugarcane bagasse is 95:50. In compound fertilizer, the mass ratio of nitrogen, phosphorus and potassium compound fertilizer (N:P2O5:K2O=18:9:16, mass ratio) to EDTA is 98:2. EDTA is used to chelate micronutrients. The additives are Bacillus subtilis, humic acid and bentonite, with a mass ratio of 2:6:1.5, which are used to promote plant absorption, alleviate soil compaction, slow release of nutrients and improve fertilizer utilization efficiency. Biochar, organic fertilizer, compound fertilizer, and additives were added to a twin-shaft paddle mixer at a mass ratio of 40:40:30:5 and mixed at 15 r / min for 15 minutes to ensure uniform mixing. Water was sprayed to adjust the moisture content of the core layer to 20 wt% to ensure the cohesion of the components. Granulation was carried out using a disc granulator at a temperature of 70℃ and a pressure of 15 MPa to achieve a particle size of 2.5 mm. (3) Preparation of the coating layer Polylactic acid (PLA) and biochar were prepared in a mass ratio of 80:20. PLA was dissolved in a 9 wt% ethyl acetate aqueous solution and stirred until completely dissolved. Biochar was impregnated and activated with a 1 wt% phosphoric acid solution to improve the interfacial bonding between PLA and biochar and reduce the risk of coating layer cracking. The impregnated and activated biochar was added to the PLA solution and ultrasonically dispersed for 20 min to obtain a mixture of PLA and biochar. Before spraying, the prepared core layer particles were preheated to 50°C. Polyethylene glycol and nano-silica were added to a mixture of polylactic acid and biochar (the mass ratio of the three was 10:10:75). The fluidized bed speed was set to 20 rpm, the spraying pressure to 0.1 MPa, and the atomization temperature to 50°C. The mixture was then dried to a moisture content of 3% under hot air circulation at 50°C. (4) Preparation of outer layer Bacillus subtilis and Bifidobacterium adolescentis were activated and cultured to prepare a mixed bacterial suspension (the live bacteria ratio of both was 1:1). This suspension was then added to a 1.5 wt% sodium alginate solution to ensure that the concentrations of both Bacillus adolescentis and Bacillus subtilis were 1 × 10⁻⁶. 9 CFU / mL, prepared into sodium alginate-based microcapsule bacterial agent; During spraying, the core layer and the coating layer are vacuum dried at 40℃ for 10 hours to increase the crystallinity of polylactic acid to 35%. Electrostatic spraying technology (voltage 20kV) is used to apply sodium alginate-based microcapsule bacterial agent to the coating layer to prolong the fertilizer effect and reduce nutrient loss. The biochar-based slow-release fertilizer prepared according to the above preparation method has a core layer, coating layer, and outer layer with a mass ratio of 80:15:5.

[0029] Example 4 This embodiment provides a method for applying a biochar-based slow-release fertilizer, specifically as follows: Figure 3 and Figure 4 As shown, it includes the following steps: (1) Excavate fertilization trenches On the outer side of the drip line of the citrus tree canopy, dig strip-shaped fertilization trenches along the citrus rows. The trenches are 30cm wide and 50cm deep, and the trench length for a single citrus tree is 2 meters. (2) Layered fertilization The application of biochar-based slow-release fertilizer should be done from autumn harvest to spring bud break each year, with a total application rate of 5 kg per citrus tree. Specifically: For the lower layer application: apply 60% of the total biochar-based slow-release fertilizer to the bottom of the trench (30-40 cm deep), then backfill with 1-5 cm of soil and lightly compact. For the middle layer application: when the trench has been backfilled to a depth of approximately 15-25 cm, apply 30% of the total biochar-based slow-release fertilizer to the middle layer, then backfill with 5 cm of soil and lightly compact. For the upper layer application: apply the remaining 10% of the total biochar-based slow-release fertilizer to the upper layer (5-10 cm deep), backfill with soil until the trench is full, and lightly compact, ensuring the trench surface is slightly lower than or level with the original ground level. Cover the trench surface with straw to reduce soil erosion. (3) Field management After applying fertilizer in layers, additional nitrogen, phosphorus, and potassium fertilizers should be applied in stages, including: During the autumn shoot growth period of citrus trees (September-October), apply 8 kg / mu of potassium fertilizer, 7 kg / mu of phosphorus fertilizer, and 15 kg / mu of nitrogen fertilizer. Also, spray the leaves with 0.1 wt% magnesium sulfate and 0.2 wt% chelated calcium. During the flower bud differentiation period of citrus trees (November-December), stop applying nitrogen fertilizer, increase the application of phosphorus fertilizer by 5 kg / mu and potassium fertilizer by 12 kg / mu, and spray the leaves with 0.1wt% borax and 0.3wt% potassium dihydrogen phosphate. Irrigation is guided by intelligent agricultural sensors (buried at a depth of 30cm) to maintain soil moisture content of 55wt%-65wt%. Deep ditches (30cm) are dug for drainage during the rainy season. Insect traps are set up, and predatory mites, Trichogramma wasps, and lacewings are released for biological control of pests and diseases. Manual weeding, pruning of diseased, crossed, and drooping branches are carried out. Fruits are retained according to a leaf-to-fruit ratio of 30:1, and small, deformed, and top fruits are thinned out to control the yield to ≤3000kg per mu.

[0030] In winter, take good care of overwintering management, whitewash the tree trunk (10kg quicklime + 1kg lime-sulfur mixture + 0.3kg salt + 0.05kg animal oil + 30kg water), apply the whitewash to the main trunk and the first main branch, and cover the tree basin with soil and straw.

[0031] Example 5 This embodiment provides a method for applying a biochar-based slow-release fertilizer. The method includes the following steps: (1) Excavate fertilization trenches On the outer side of the drip line of the citrus tree canopy, dig strip-shaped fertilization trenches along the citrus rows. The trenches are 20cm wide and 60cm deep, and the trench length for a single citrus tree is 1 meter. (2) Layered fertilization The application of biochar-based slow-release fertilizer should be done from autumn harvest to spring bud break each year, with a total application rate of 3 kg per citrus tree. Specifically: For the lower layer application: apply 70% of the total biochar-based slow-release fertilizer to the bottom of the trench (30-40 cm deep), then backfill with 1-5 cm of soil and lightly compact. For the middle layer application: when the trench has been backfilled to a depth of approximately 15-25 cm, apply 20% of the total biochar-based slow-release fertilizer to the middle layer, then backfill with 5 cm of soil and lightly compact. For the upper layer application: apply the remaining 10% of the total biochar-based slow-release fertilizer to the upper layer (5-10 cm deep), backfill with soil until the trench is full, and lightly compact, ensuring the trench surface is slightly lower than or level with the original ground level. Cover the trench surface with straw to reduce soil erosion. (3) Field management After applying fertilizer in layers, it is necessary to add nitrogen, phosphorus and potassium fertilizers in different proportions in stages. During the autumn shoot growth period of citrus trees (September-October), apply 10 kg / mu of potassium fertilizer, 5 kg / mu of phosphorus fertilizer, and 10 kg / mu of nitrogen fertilizer. Also, spray the leaves with 0.1 wt% magnesium sulfate and 0.2 wt% chelated calcium. During the flower bud differentiation period of citrus trees (November-December), stop applying nitrogen fertilizer, increase the application of phosphorus fertilizer by 4 kg / mu and potassium fertilizer by 8 kg / mu, and spray the leaves with 0.1wt% borax and 0.3wt% potassium dihydrogen phosphate. Irrigation is guided by intelligent agricultural sensors (buried 30cm deep) to maintain soil moisture content at 55%-65%, and deep ditches (30cm) are dug for drainage during the rainy season. Insect-attracting boards are installed, and predatory mites, Trichogramma wasps, and lacewings are released for biological control of pests and diseases. Manual weeding, pruning of diseased, crossing, and drooping branches are employed. Fruits are retained at a leaf-to-fruit ratio of 50:1, and small, deformed, and terminal fruits are thinned to control yield to ≤3000kg per mu. In winter, take good care of overwintering management, whitewash the tree trunk (5kg quicklime + 0.5kg lime-sulfur mixture + 0.5kg salt + 0.05kg animal oil + 30kg water), apply the whitewash to the main trunk and the first main branch, and cover the tree basin with soil and straw.

[0032] Example 6 This embodiment provides a method for applying a biochar-based slow-release fertilizer. The method includes the following steps: (1) Excavate fertilization trenches On the outer side of the drip line of the citrus canopy, dig strip-shaped fertilization trenches along the citrus rows. The trenches are 40cm wide and 40cm deep, and the trench length for a single citrus tree is 2 meters. (2) Layered fertilization The application of biochar-based slow-release fertilizer should be done from autumn harvest to spring bud break each year, with a total application rate of 6 kg per citrus tree. Specifically: For the lower layer application: apply 50% of the total biochar-based slow-release fertilizer to the bottom of the trench (30-40 cm deep), then backfill with 1-5 cm of soil and lightly compact. For the middle layer application: when the trench has been backfilled to a depth of approximately 15-25 cm, apply 30% of the total biochar-based slow-release fertilizer to the middle layer, then backfill with 5 cm of soil and lightly compact. For the upper layer application: apply the remaining 20% ​​of the total biochar-based slow-release fertilizer to the upper layer (5-10 cm deep), backfill with soil until the trench is full, and lightly compact, ensuring the trench surface is slightly lower than or level with the original ground level. Cover the trench surface with straw to reduce soil erosion. (3) Field management After applying fertilizer in layers, it is necessary to add nitrogen, phosphorus and potassium fertilizers in different proportions in stages. During the autumn shoot growth period of citrus trees (September-October), apply 12 kg / mu of potassium fertilizer, 6 kg / mu of phosphorus fertilizer, and 12.5 kg / mu of nitrogen fertilizer. Also, spray the leaves with 0.1 wt% magnesium sulfate and 0.2 wt% chelated calcium. During the flower bud differentiation period of citrus trees (November-December), stop applying nitrogen fertilizer, increase the application of phosphorus fertilizer by 6 kg / mu and potassium fertilizer by 10 kg / mu, and spray the leaves with 0.1wt% borax and 0.3wt% potassium dihydrogen phosphate. Irrigation is guided by intelligent agricultural sensors (buried 30cm deep) to maintain soil moisture content at 55%-65%, and deep ditches (30cm) are dug for drainage during the rainy season. Insect-attracting boards are installed, and predatory mites, Trichogramma wasps, and lacewings are released for biological control of pests and diseases. Manual weeding, pruning of diseased, crossing, and drooping branches are employed. Fruits are retained at a leaf-to-fruit ratio of 40:1, and small, deformed, and terminal fruits are thinned to control yield to ≤3000kg per mu. In winter, take good care of overwintering management, whitewash the tree trunk (7.5kg quicklime + 0.75kg lime-sulfur mixture + 0.4kg salt + 0.5kg animal oil + 40kg water), apply the whitewash to the main trunk and the first main branch, and cover the tree basin with soil and straw.

[0033] Example 7 This embodiment provides a method for applying a biochar-based slow-release fertilizer. The application of the biochar-based slow-release fertilizer prepared in Example 1 includes the following steps: (1) The total amount of biochar-based slow-release fertilizer applied to a single citrus tree is 5 kg. It is spread on the upper layer (5-10 cm deep), backfilled with soil until the trench is full, and lightly compacted so that the trench surface is slightly lower than or level with the original ground. The trench surface is covered with straw to reduce soil erosion. (2) Field management After applying fertilizer in layers, additional nitrogen, phosphorus, and potassium fertilizers should be applied in stages, including: During the autumn shoot growth period of citrus trees (September-October), apply 8 kg / mu of potassium fertilizer, 7 kg / mu of phosphorus fertilizer, and 15 kg / mu of nitrogen fertilizer. Also, spray the leaves with 0.1 wt% magnesium sulfate and 0.2 wt% chelated calcium. During the flower bud differentiation period of citrus trees (November-December), stop applying nitrogen fertilizer, increase the application of phosphorus fertilizer by 5 kg / mu and potassium fertilizer by 12 kg / mu, and spray the leaves with 0.1wt% borax and 0.3wt% potassium dihydrogen phosphate. Irrigation is guided by intelligent agricultural sensors (buried 30cm deep) to maintain soil moisture content at 55wt%-65wt%, and deep ditches (30cm) are dug for drainage during the rainy season; insect traps are hung, and predatory mites, Trichogramma wasps, and lacewings are released for biological control of pests and diseases; manual weeding, pruning of diseased, crossed, and drooping branches are carried out; fruits are retained according to a leaf-to-fruit ratio of 30:1, and small, deformed, and top fruits are thinned out to control the yield to ≤3000kg per mu; In winter, take good care of overwintering management, whitewash the tree trunk (10kg quicklime + 1kg lime-sulfur mixture + 0.3kg salt + 0.05kg animal oil + 30kg water), apply the whitewash to the main trunk and the first main branch, and cover the tree basin with soil and straw.

[0034] Comparative Example 1 This comparative example provides a method for applying a commonly available compound fertilizer purchased from Hubei Yishizhuang Agricultural Technology Co., Ltd., registration number HBFHFL 2021-00847. The mass ratio of nitrogen, phosphorus, and potassium in this compound fertilizer is 15:7:13. The fertilization scheme is the same as in Example 4, specifically including: (1) Excavate fertilization trenches On the outer side of the drip line of the citrus tree canopy, dig strip-shaped fertilization trenches along the citrus rows. The trenches are 30cm wide and 50cm deep, and the trench length for a single citrus tree is 2 meters. (2) Layered fertilization The application of compound fertilizer should be done between autumn harvest and spring bud break, with a total fertilizer application of 5 kg per citrus tree. Specifically: For the lower layer fertilization: apply 60% of the total compound fertilizer to the bottom of the trench (30-40 cm deep), then backfill with 1-5 cm of soil and lightly compact. For the middle layer fertilization: when the trench has been backfilled to a depth of approximately 15-25 cm, apply 30% of the total compound fertilizer to the middle layer, then backfill with 5 cm of soil and lightly compact. For the upper layer fertilization: apply the remaining 10% of the total compound fertilizer to the upper layer (5-10 cm deep), backfill with soil until the trench is full, and lightly compact, ensuring the trench surface is slightly lower than or level with the original ground level. Cover the trench surface with straw to reduce soil erosion. (3) Field management After applying fertilizer in layers, additional nitrogen, phosphorus, and potassium fertilizers should be applied in stages, including: During the autumn shoot growth period of citrus trees (September-October), apply 8 kg / mu of potassium fertilizer, 7 kg / mu of phosphorus fertilizer, and 15 kg / mu of nitrogen fertilizer. Also, spray the leaves with 0.1 wt% magnesium sulfate and 0.2 wt% chelated calcium. During the flower bud differentiation period of citrus trees (November-December), stop applying nitrogen fertilizer, increase the application of phosphorus fertilizer by 5 kg / mu and potassium fertilizer by 12 kg / mu, and spray the leaves with 0.1wt% borax and 0.3wt% potassium dihydrogen phosphate. Irrigation is guided by intelligent agricultural sensors (buried 30cm deep) to maintain soil moisture content at 55wt%-65wt%, and deep ditches (30cm) are dug for drainage during the rainy season; insect traps are hung, and predatory mites, Trichogramma wasps, and lacewings are released for biological control of pests and diseases; manual weeding, pruning of diseased, crossed, and drooping branches are carried out; fruits are retained according to a leaf-to-fruit ratio of 30:1, and small, deformed, and top fruits are thinned out to control the yield to ≤3000kg per mu; In winter, take good care of overwintering management, whitewash the tree trunk (10kg quicklime + 1kg lime-sulfur mixture + 0.3kg salt + 0.05kg animal oil + 30kg water), apply the whitewash to the main trunk and the first main branch, and cover the tree basin with soil and straw.

[0035] Comparative Example 2 This comparative example provides a method for applying a commonly available compound fertilizer purchased from Hubei Yishizhuang Agricultural Technology Co., Ltd., registration number HBFHFL 2021-00847. The mass ratio of nitrogen, phosphorus, and potassium in this compound fertilizer is 15:7:13. The fertilization scheme is the same as in Example 7, specifically including: (1) The total amount of compound fertilizer applied to a single citrus tree is 5 kg. Spread it all on the upper layer (5-10 cm deep), backfill the soil until the trench is full, and compact it slightly so that the trench surface is slightly lower than or level with the original ground. Cover the trench surface with straw to reduce soil erosion. (2) Field management It is also necessary to apply nitrogen, phosphorus, and potassium fertilizers in different proportions in stages; During the autumn shoot growth period of citrus trees (September-October), apply 12 kg / mu of potassium fertilizer, 6 kg / mu of phosphorus fertilizer, and 12.5 kg / mu of nitrogen fertilizer. Also, spray the leaves with 0.1 wt% magnesium sulfate and 0.2 wt% chelated calcium. During the flower bud differentiation period of citrus trees (November-December), stop applying nitrogen fertilizer, increase the application of phosphorus fertilizer by 6 kg / mu and potassium fertilizer by 10 kg / mu, and spray the leaves with 0.1wt% borax and 0.3wt% potassium dihydrogen phosphate. Irrigation is guided by intelligent agricultural sensors (buried 30cm deep) to maintain soil moisture content at 55%-65%, and deep ditches (30cm) are dug for drainage during the rainy season. Insect-attracting boards are hung, and predatory mites, Trichogramma wasps, and lacewings are released for biological control of pests and diseases. Weeding, pruning of diseased, crossed, and drooping branches are carried out manually. Fruits are retained according to a leaf-to-fruit ratio of 40:1, and small, deformed, and terminal fruits are thinned out to control the yield to ≤3000kg per mu.

[0036] In winter, take good care of overwintering management, whitewash the tree trunk (7.5kg quicklime + 0.75kg lime-sulfur mixture + 0.4kg salt + 0.5kg animal oil + 40kg water), apply the whitewash to the main trunk and the first main branch, and cover the tree basin with soil and straw.

[0037] Experimental Example 1 1. Selection of test site In Yiling District, Yichang, Hubei Province (soil type: purple soil, initial organic matter 0.8%, bulk density 1.45 g / cm³) 3 A 5-mu (approximately 0.33 hectares) 10-year-old navel orange orchard was selected for the experiment. The biochar-based slow-release fertilizer was applied from the time of autumn harvest to the time of spring bud break (January 2025 - March 2025).

[0038] 2. Grouping and Processing The aforementioned 5-mu navel orange orchard was randomly divided into 6 zones, which were then divided into 6 groups.

[0039] The specific fertilization plan is shown in Table 1.

[0040] Table 1 Fertilization plans for each group

[0041] 3. Indicator Testing Soil samples were collected using the S-shaped sampling method in November, with three soil samples collected from each group. Soil physical, chemical, and biological indicators were measured, and the average value was taken. Five days before the citrus harvest, citrus fruits were collected using the sampling method, with five citrus fruits collected from each group. The quality indicators of the citrus fruits were measured, and the average value was taken. The yield of citrus fruits in each group was calculated.

[0042] The analytical methods are shown in Table 2.

[0043] Table 2 Indicator Detection Methods

[0044] 4. Results Analysis In both the examples and comparative examples, the same soil property monitoring and fertilizer testing were carried out simultaneously.

[0045] Table 3 Soil properties, citrus yield, and fertilizer status in citrus orchards

[0046] The specific results are shown in Table 3. This invention, through the slow-release and stratified application technology of biochar-based slow-release fertilizer, reduced soil bulk density and increased soil moisture content, pH, organic matter, total nitrogen, total phosphorus, available phosphorus, available potassium, urease, sucrase, alkaline phosphatase, and acid phosphatase. The single fruit weight, edible rate, juice rate, soluble solids content, and acid-solid ratio of citrus were also improved, thus achieving improved soil fertility and high-quality and high-yield citrus.

[0047] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A biochar-based slow release fertilizer for enhancing soil productivity in citrus orchards, characterized in that, From inside to outside, the core layer, the coating layer and the outer layer are sequentially included; The core layer includes biochar, organic fertilizer, compound fertilizer and additive; The coating layer includes polylactic acid and biochar; The outer layer includes sodium alginate microcapsule fungicide.

2. The biochar-based slow release fertilizer for enhancing soil productivity in citrus orchard according to claim 1, characterized in that, In the core layer, the mass ratio of biochar, organic fertilizer, compound fertilizer and additive is 20-40:20-40:10-30:1-5; Optionally, the additive includes bacillus subtilis, fulvic acid and bentonite, and the mass ratio is 1-2:5-6:1-2; Optionally, the organic fertilizer includes matured sheep manure and bagasse, and the mass ratio is 50-95:5-50; Optionally, the compound fertilizer includes nitrogen, phosphorus and potassium compound fertilizer and EDTA, and the mass ratio is 98-99:1-2; Optionally, in the nitrogen, phosphorus and potassium compound fertilizer, the mass ratio of N, P2O5 and K2O is 16-18:8-10:12-18; Optionally, the raw materials for preparing the biochar include pruned branches of citrus trees and citrus pomace after juice extraction, and the mass ratio is 50-80:20-50.

3. The biochar-based slow release fertilizer for enhancing soil productivity in citrus orchard according to claim 1 or 2, characterized in that, The mass ratio of the core layer, the coating layer and the outer layer is 80-95:4-15:1-5; Optionally, in the coating layer, the mass ratio of polylactic acid and biochar is 50-80:20-50; Optionally, in the outer layer, the sodium alginate microcapsule fungicide comprises Bifidobacterium adolescentis and Bacillus subtilis, the ratio of viable bacterial count is 1-3:1, and the total viable bacterial count concentration is 2x10 8 ~2x10 9 CFU / mL.

4. A process for the preparation of the biochar-based slow release fertilizer for enhancing soil health in citrus orchard as claimed in any one of claims 1 to 3, characterized in that, The method comprises the following steps: S1. Preparing biochar; S2. Preparing the core layer: taking biochar, organic fertilizer, compound fertilizer and additive according to the mass ratio, mixing and granulating; S3. Preparing the coating layer: taking polylactic acid and biochar according to the mass ratio, mixing and coating on the surface of the core layer; S4. Spraying the outer layer: preparing sodium alginate microcapsule fungicide and coating on the surface of the coating layer.

5. The method of claim 4, wherein the biochar-based slow release fertilizer is prepared by, In the step S1, the pruned branches of citrus trees and the citrus pomace after juice extraction are mixed according to the mass ratio, crushed, dried, carbonized at constant temperature, pyrolyzed, finally steam activated, modified, cooled and sieved.

6. The method of claim 4 or 5, wherein the biochar-based slow release fertilizer for improving soil productivity in citrus orchard is prepared by the steps of: In the step S1, the pruned branches of citrus trees and the citrus pomace after juice extraction are mixed according to the mass ratio, crushed to a particle size of ≤3 cm, dried at 100-105℃ to a water content of <10%, carbonized at constant temperature under nitrogen protection at a temperature rising rate of 1-10℃ / min to 700-800℃, pyrolysis pressure is 0.05-0.11MPa, pyrolysis time is 30-90min, then steam activated at 800-850℃ for 30-45min, modified by immersing in 1wt%-2wt% phosphoric acid solution, cooled and crushed through an 80 mesh sieve; Optionally, the particle size of the biochar is 0.5-1mm.

7. The process for preparing biochar-based slow release fertilizer for enhancing soil productivity in citrus orchard as claimed in claim 5 wherein, In the step S2, the mixing speed is 20-30rpm and the mixing time is 10-15min; Optionally, the step S2 further comprises a step of spraying water after mixing to adjust the water content of the core layer to 15wt%-20wt%; Optionally, in the step S2, the granulation conditions are temperature 50-70℃, pressure 15-30MPa, and the granulation particle size is 1.5-2.5mm.

8. The process for preparing biochar-based slow release fertilizer for enhancing soil productivity in citrus orchard as claimed in claim 5 wherein, In the step S3, before mixing, the following step is further included: S31. Dissolve polylactic acid in an ethyl acetate aqueous solution with a concentration of 8wt%-10wt%, and stir until completely dissolved; S32. The biochar is activated by impregnating with a phosphoric acid solution having a concentration of 1wt%-2wt%; S33. The activated biochar is added to a polylactic acid solution and ultrasonically dispersed for 20-40min to obtain a mixture of polylactic acid and biochar.

9. The process for preparing biochar-based slow release fertilizer for enhancing soil productivity in citrus orchard as claimed in claim 5 wherein, In the step S3, before the spraying, the particles of the core layer are preheated to 50-70℃; Optionally, in the step S3, the spraying is performed at a rotation speed of 10-20rpm, a pressure of 0.1-0.2MPa, and an atomization temperature of 50-70℃; Optionally, in the step S3, during the spraying, the mixture of polylactic acid and biochar prepared in the step S33, polyethylene glycol, and nano-silicon dioxide are added, and the mass ratio of the three is 70-80:10-20:5-10; Optionally, in the step S3, during the spraying, drying is performed at 40-60℃ under hot air circulation until the water content is 2wt%-5wt%.

10. The process for preparing biochar-based slow release fertilizer for enhancing soil productivity in citrus orchard as claimed in claim 5 wherein, In the step S4, before the spraying, the core layer and the coating layer are vacuum dried at 30-40℃ for 10-15h; Optionally, the method for preparing the sodium alginate microcapsule microbial agent comprises: preparing a mixed bacteria suspension according to the ratio of viable bacteria of Bifidobacterium adolescentis and Bacillus subtilis, adding 0.5wt%-1.5wt% of a sodium alginate solution, so that the total viable bacteria of Bifidobacterium adolescentis and Bacillus subtilis is 2×10 8 ~2×10 9 CFU / mL. Optionally, in the step S4, the spraying process is electrostatic spraying technology, and the voltage is 5-20kV.

11. Use of the biochar-based slow-release fertilizer for improving soil fertility in citrus orchard according to any one of claims 1-3 or prepared by the preparation method of any one of claims 4-10 in the cultivation of citrus trees.

12. A method of applying the biochar-based slow release fertilizer for improving soil productivity in citrus orchard according to any one of claims 1 to 3 or prepared by the method according to any one of claims 4 to 10, characterized in that, The method comprises the following steps: (1) Excavating a fertilization trench: a strip-shaped fertilization trench is excavated outside the drip line of the citrus tree crown along the inter-row or inter-plant space of the citrus trees, the trench has a width of 20-40cm, a depth of 40-60cm, and a length of 1-2m corresponding to a single citrus plant; (2) Layered fertilization: the fertilization trench is divided into a bottom layer, an intermediate layer, and an upper layer; 50wt%-70wt% of the total amount of the biochar-based slow-release fertilizer is applied to the bottom of the trench, and then 1-5cm of soil is backfilled to cover the bottom layer; 20wt%-30wt% of the total amount of the biochar-based slow-release fertilizer is applied above the bottom layer, and then 1-5cm of soil is backfilled to cover the intermediate layer; and 10wt%-20wt% of the total amount of the biochar-based slow-release fertilizer is applied above the intermediate layer, and then the soil is backfilled until the trench is full and slightly lower than or level with the original ground surface; (3) Field management: including measures such as stage-wise topdressing, irrigation and drainage, pruning, weeding, biological pest control, fruit thinning, and winter frost prevention.

13. The method of applying a biochar-based slow release fertilizer according to claim 12, wherein, The total amount of the biochar-based slow-release fertilizer is 3-6kg / plant / year; Optionally, the biochar-based slow-release fertilizer is applied from after fruit harvesting in the autumn to before sprouting in the spring each year; Optionally, the stage-wise topdressing includes, during the autumn shoot period of the citrus trees, applying 8-12kg / acre of potassium fertilizer, 5-7kg / acre of phosphorus fertilizer, and 10-15kg / acre of nitrogen fertilizer, and spraying 0.1wt% magnesium sulfate and 0.2wt% chelated calcium on the leaves; And / or, during the flower bud differentiation period of the citrus trees, stopping the application of nitrogen fertilizer, increasing the application of phosphorus fertilizer by 4-6kg / acre and potassium fertilizer by 8-12kg / acre, and spraying 0.1wt% borax and 0.3wt% dihydrogen potassium phosphate on the leaves; Optionally, the irrigation and drainage is guided by data of intelligent agricultural sensors to maintain soil water content of 55wt%-65wt%, and 30cm deep ditch drainage is performed in rainy season; Optionally, the biological pest control includes hanging insect luring plates, releasing predatory mites, trichogramma and grass locusts; Optionally, the pruning includes pruning diseased branches, crossing branches and drooping branches; Optionally, the fruit thinning includes leaving fruits according to a leaf to fruit ratio of 30-50:1, thinning small fruits, deformed fruits and top fruits, and controlling yield per mu to be less than or equal to 3000kg; Optionally, the winter anti-freezing measures include brushing the main stem to the first main branch of the citrus tree with a white coating agent having a mass ratio of quicklime, stone sulfur mixture, salt, animal oil and water of 5-10:0.5-1:0.3-0.5:0.05-0.5:30-40; and / or covering the tree pit with straw.

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