A multi-nuclear nested controlled-release micro-fertilizer for efficiently reducing cadmium absorption by rice and a preparation method thereof

Abstract

The application discloses a kind of multi-nuclear nested controlled-release micro-fertilizer for efficiently reducing cadmium absorption of rice and a preparation method, comprising inner layer fertilizer core, inner layer coating material, middle layer fertilizer core, middle layer coating material, outer layer fertilizer core and outer layer coating material.The inner layer fertilizer core is zinc sulfate heptahydrate round particle, and the inner layer coating material is thick water-based polyacrylate.The middle layer fertilizer core is ferrous sulfate heptahydrate, and the middle layer coating material is thin water-based polyacrylate.The outer layer fertilizer core is manganese sulfate monohydrate, and the outer layer coating material is chitosan glutaraldehyde crosslinking compound.The prepared multi-nuclear nested controlled-release micro-fertilizer has a cadmium reduction rate of rice brown rice as high as 54.1%~61.8%, which is 35 percentage points higher than conventional iron, manganese and zinc fertilizers, and achieves the technical effect of slow release of manganese fertilizer at seedling stage, slow release of iron fertilizer at tillering stage and slow release of zinc fertilizer at heading and filling stage.

Description

Technical Field

[0001] This invention relates to the field of heavy metal contaminated soil remediation technology, and in particular to safe production technology for paddy fields with mild to moderate cadmium contamination. Background Technology

[0002] Cadmium pollution in farmland directly threatens the quality and safety of agricultural products and the sustainable development of agriculture in my country, as exemplified by the cadmium-contaminated rice incidents in Guangdong and Hunan provinces. Given the characteristics of low cadmium pollution levels, large areas of farmland, and agricultural use in my country, there is an urgent need to develop efficient, economical, rapid, simple, replicable, and easily scalable technologies for the safe utilization of farmland. Currently, there is considerable research on the remediation and safe utilization of cadmium-contaminated soil, including the breeding of low-cadmium-accumulating varieties, the development and application of passivating agents, and adjustments to planting systems and agronomic techniques. However, low-cadmium-accumulating varieties often struggle to simultaneously achieve both yield and quality, and their large-scale application is not common. Many passivating agents have been developed, including lime, organic fertilizer, biochar, sepiolite, and hydroxyapatite. These agents primarily work by increasing soil pH, altering the form of cadmium, and reducing the content of available cadmium in the soil, which can reduce cadmium absorption by rice to some extent. However, most of these are costly, and long-term application can adversely affect the physical and chemical properties of the soil. For example, long-term application of lime can cause soil compaction, leading to reduced fertility and impacting crop yield.

[0003] Agronomic techniques are relatively simple and easy to implement; good cadmium reduction effects can be achieved through field water regulation and the application of micronutrient fertilizers. During the absorption and translocation of cadmium in rice, the manganese transporter OsNramp5 is the main factor controlling cadmium absorption by roots throughout the entire growth period, while the zinc transporters OsHMA2 / 3 and OsZIP1 / 7 during the heading and grain-filling stages are key factors controlling the translocation of cadmium from roots to grains. Additionally, the iron oxide film (iron film) on the rice root surface acts as a natural barrier against cadmium absorption. Therefore, the levels of soil micronutrients such as manganese, zinc, and iron have a significant impact on cadmium absorption in rice. Current research confirms that applying manganese, zinc, and iron fertilizers alone not only results in low fertilizer utilization but also has limited cadmium reduction efficiency in rice. Increasing the application of manganese, zinc, and iron fertilizers can improve cadmium reduction efficiency, but this increases costs and may lead to environmental risks.

[0004] To improve fertilizer utilization, traditional agriculture typically employs fertilizers with slow dissolution rates or utilizes organic polymers and inorganic mineral coatings to enhance fertilizer slow-release performance and efficiency. However, current slow-release fertilizers only achieve the slow release of a single fertilizer, often concentrated within a specific timeframe, resulting in limited improvement in micronutrient utilization and cadmium reduction efficiency. Previous studies have shown that manganese regulates cadmium translocation expression in the early stages of rice growth, iron enhances the cadmium-blocking effect of the root surface iron film during the mid-tillering stage, and zinc inhibits cadmium translocation in the later stages of rice growth, achieving highly efficient cadmium reduction. Therefore, there is an urgent need for a method that overcomes the drawbacks of rapid, concentrated release of micronutrients, releasing appropriate amounts of manganese in the early stages, iron in the mid-stages, and zinc in the later stages to improve rice's utilization of micronutrients, maximize cadmium reduction efficiency in brown rice, and simultaneously reduce the ecological and environmental risks caused by excessive micronutrient application. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a highly efficient multi-core nested controlled-release micronutrient fertilizer for reducing cadmium absorption in rice and its preparation method. This invention utilizes the cadmium-reducing patterns of zinc, iron, and manganese micronutrients at different stages of rice growth. It leverages the strong water absorption and permeability of the chitosan-glutaraldehyde cross-linked composite coating material, the weak water absorption and permeability of the water-based polyacrylate coating material, and the dispersing and adhesive properties of modified cassava starch. By designing parameters such as nesting sequence, coating thickness, and fertilizer core size, the multi-core nested material described in this invention is obtained. This achieves the technical effect of slow release of manganese fertilizer during the seedling stage, slow release of iron fertilizer during the mid-tillering stage, and slow release of zinc fertilizer during the heading and grain-filling stage, thereby improving the bioavailability and cadmium-reducing efficiency of zinc, iron, and manganese fertilizers and solving the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: On one hand, this invention provides a multi-core nested controlled-release micro-fertilizer that efficiently reduces cadmium absorption in rice, comprising an inner fertilizer core, an inner coating material, a middle fertilizer core, a middle coating material, an outer fertilizer core, and an outer coating material; wherein, the inner fertilizer core is zinc sulfate heptahydrate spherical particles, and the inner coating material is a thick water-based polyacrylate; the middle fertilizer core is ferrous sulfate heptahydrate, and the middle coating material is a thin water-based polyacrylate; the outer fertilizer core is manganese sulfate monohydrate, and the outer coating material is a chitosan-glutaraldehyde crosslinked composite.

[0007] Optionally, the particle size of the inner fertilizer core is 3~5mm, and the particle size of the middle fertilizer core and the outer fertilizer core is 0.1~0.2mm.

[0008] Optionally, the middle and outer fertilizer cores also include modified starch as a dispersant and adhesive.

[0009] On the other hand, the present invention provides a method for preparing a multi-core nested controlled-release micro-fertilizer, comprising the following steps: S1. Preparation of water-based polyacrylate coating: using zinc sulfate heptahydrate granules as fertilizer cores, coating is carried out at a fertilizer core-coating mass ratio of 1:0.9~1.2 to obtain thick water-based polyacrylate coated zinc fertilizer. S2. Cassava starch powder and pure water are mixed in a mass ratio of 1:0.5-3. Then, sodium hypochlorite with a mass fraction of 5%~8% and nickel sulfate oxidation catalyst with a mass fraction of 0.05~0.05% are added. After stirring at room temperature, the mixture is allowed to stand for oxidation to obtain oxidized starch. Then, sodium hydroxide with a mass fraction of 3%~6% is added and stirred for 20-50 minutes to obtain gelatinized starch. Ferrous sulfate heptahydrate and gelatinized starch are mixed and stirred evenly in a mass ratio of 2~3:1. The mixture is then adhered to the surface of a thick water-based polyacrylate-coated zinc fertilizer. After drying, a zinc and iron double-layer nested micro-fertilizer is obtained. S3. Prepare water-based polyacrylate coating. Using the zinc and iron nested micro-fertilizer obtained in S2 as the fertilizer core, coat it with a fertilizer core coating mass ratio of 1:0.3~0.5 to obtain zinc and iron double-core nested coated micro-fertilizer. Add talc powder and dry for later use. S4. Using the gelatinized starch obtained in S2 as a dispersant and adhesive, the mixture of manganese sulfate monohydrate and gelatinized starch in a mass ratio of 2 to 3:1 is stirred evenly and then adhered to the surface of zinc and iron double-core nested micro-fertilizer. After drying, zinc, iron and manganese multi-layer nested micro-fertilizer is obtained. S5. Glutaraldehyde and chitosan are dissolved in acetic acid solution to prepare glutaraldehyde and chitosan solutions respectively. Glutaraldehyde is added dropwise to the chitosan solution under constant stirring to obtain a chitosan-glutaraldehyde cross-linked composite coating material. The zinc, iron, and manganese nested micro-fertilizer prepared in S4 is used as the fertilizer core. The zinc, iron, and manganese multi-core nested controlled-release micro-fertilizer is prepared by coating at a fertilizer core-coating mass ratio of 1:0.3~0.5. After drying, a multi-core nested controlled-release micro-fertilizer that effectively reduces cadmium absorption by rice is obtained.

[0010] Optionally, in steps S1 and S3, an in-situ melting method is used to prepare a water-based polyacrylate coating.

[0011] Optionally, in step S2, after adding the oxidation catalyst, stir at room temperature for 5-30 minutes and let it stand for oxidation for 8-12 hours.

[0012] Optionally, in step S2, the thickness of the modified starch-embedded iron fertilizer layer after drying is 1-2 mm; the particle size of the zinc and iron double-layer nested micro-fertilizer after drying is 4-6 mm; in step S4, the thickness of the modified starch-embedded manganese fertilizer after drying is 0.8-1 mm; the particle size of the zinc, iron, and manganese multi-layer nested micro-fertilizer after drying is 5-7 mm.

[0013] Optionally, in step S5, the concentration of the acetic acid solution is 1%, the concentration of the glutaraldehyde solution is 0.2%, and the concentration of the chitosan solution is 2.5%. The glutaraldehyde is added dropwise to the continuously stirred chitosan solution at a volume ratio of 1:2~5.

[0014] Optionally, talc powder is added before drying in steps S1, S3, and S5.

[0015] On the other hand, the present invention provides the application of the above-mentioned multi-core nested controlled-release micro-fertilizer in paddy fields with mild to moderate cadmium contamination.

[0016] Beneficial effects This invention utilizes the strong water absorption and permeability of chitosan-glutaraldehyde cross-linked composite coating material, and leverages the dispersing and adhesive properties of modified cassava to embed manganese fertilizer, which regulates cadmium transport proteins in rice seedlings, in the outer layer, achieving slow release of manganese fertilizer in the early stages of rice growth. It also utilizes the moderate water absorption and permeability of a thin water-based polyacrylate coating material, embedding iron fertilizer, which regulates the formation of an iron film on the rice root surface, in the middle layer, achieving slow release of iron fertilizer in the middle growth stage. Finally, it utilizes the weak water absorption and permeability of a thick water-based polyacrylate coating material, embedding zinc fertilizer, which regulates cadmium transport from the rice cell to the grain, in the inner layer, achieving slow release of zinc fertilizer in the later stages of growth. Based on the release patterns and cadmium-reducing properties of the three micronutrients, the embedding thickness of different core micronutrients is optimized to obtain the multi-core nested controlled-release micronutrients described in this invention, which can achieve both slow release of each fertilizer and precise release at different growth stages of rice. The multi-core nested controlled-release micro-fertilizer described in this invention has a cadmium reduction rate of up to 54.1%~61.8% in brown rice, which is more than 35 percentage points higher than that of conventional iron, manganese and zinc fertilizers. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the preparation method steps of the present invention; Figure 2 This is a schematic diagram of the structure of the multi-core nested controlled-release microfertilizer prepared according to the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying tables. 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.

[0019] Example 1: like Figure 1 As shown, according to Figure 1 The following is a flowchart illustrating fertilizer preparation: S1. Preparation of water-based polyacrylate inner coating material using in-situ melting method: 8.24 g of nonionic emulsifier polyoxyethylene alkylphenyl ether (OP-10), 4.12 g of anionic emulsifier sodium dodecylbenzenesulfonate (SDBS), and 31 g of hydrolysis inhibitor ethylene glycol (EG) were dissolved in 248 g of deionized water to obtain an aqueous phase; 110 g of butyl acrylate (BA), 90 g of methyl methacrylate (MMA), and 3.5 g of functional monomer methacrylic acid (MAA) were mixed to obtain an organic phase. The two phases were stirred at 1000 rpm for 30 min at room temperature using a magnetic stirrer to obtain an oil-water mixture; the polymerization reaction was carried out at 80℃ and 400 rpm. Initially, 25% of the oil-water mixture was added to a three-necked flask, and the remaining oil-water mixture and initiator solution (52 mL, 0.013 g mL) were added. -1 K2S2O8 (polymerization accelerator) was added to the reaction system in four alternating batches over 3 hours. After the addition was complete, 5g of methyl silicone oil was added to a round-bottom flask and the reaction continued for another 3 hours. After the reaction was complete, the heater was turned off, and the emulsion was filtered through a 100-mesh nylon screen and collected when the temperature dropped to 40℃. The emulsion was then processed three times by a homogenizer at 900 bar pressure to obtain a water-based polyacrylate emulsion coating material. Using 3mm zinc sulfate heptahydrate round particles as the fertilizer core, 1kg of zinc sulfate heptahydrate round particles and 0.9kg of water-based polyacrylate emulsion coating material were used. A bottom-spray fluidized bed coating device was used, with the peristaltic pump speed set at 0.6 rpm, the material chamber temperature at 25℃, and the nozzle pressure at 0.4MPa to prepare a thick water-based polyacrylate coated zinc fertilizer. Talc powder was added and the fertilizer was dried for later use. S2. Cassava starch powder and pure water are mixed in a mass ratio of 1:1. Then, 5% sodium hypochlorite and 0.01% nickel sulfate oxidation catalyst are added. The mixture is stirred at room temperature for 10 minutes and allowed to stand for oxidization for 10 hours to obtain oxidized starch. Then, 3% sodium hydroxide is added and stirred for 30 minutes to obtain gelatinized starch. Ferrous sulfate heptahydrate (particle size 0.1 mm) and gelatinized starch are mixed in a mass ratio of 2:1 and stirred evenly. The mixture is then adhered to the surface of a thick water-based polyacrylate-coated zinc fertilizer. After drying, a modified starch-nested iron fertilizer layer with a thickness of 2 mm is obtained, which is a zinc and iron double-layer nested micro-fertilizer.

[0020] S3. Water-based polyacrylate middle layer coating material is prepared by in-situ melting method. The zinc and iron nested micro-fertilizer obtained in S2 is used as fertilizer core. The fertilizer core and coating are coated with thin water-based polyacrylate coated zinc and iron double core nested micro-fertilizer at a fertilizer core to coating mass ratio of 2:1. After adding talc powder, it is dried for later use. S4. Using the gelatinized starch obtained in S2 as a dispersant and binder, manganese sulfate monohydrate (particle size of 0.1 mm) and gelatinized starch are mixed and stirred evenly in a mass ratio of 2 to 3. The mixture is then adhered to the surface of the zinc and iron double-core nested coating micro-fertilizer. After drying, a modified starch-nested manganese fertilizer with a thickness of 0.8 mm is obtained, which is a zinc, iron and manganese multi-layer nested micro-fertilizer.

[0021] S5. Glutaraldehyde and chitosan were dissolved in a 1% acetic acid solution to prepare 0.2% glutaraldehyde and 2.5% chitosan. Glutaraldehyde was added dropwise to the continuously stirred chitosan solution at a volume ratio of 1:4 to obtain the outer coating material of the chitosan-glutaraldehyde cross-linked composite. Using the zinc, iron, and manganese nested micro-fertilizer prepared in S4 as the fertilizer core, a multi-core, dual-core nested controlled-release micro-fertilizer of zinc, iron, and manganese was prepared by coating at a fertilizer core-coating mass ratio of 1:0.5. After adding talc powder and drying, a highly efficient multi-core nested controlled-release micro-fertilizer for reducing cadmium absorption by rice was obtained. Figure 2 As shown.

[0022] In a rice paddy in Guixi, Jiangxi Province, with a pH of 4.73, total cadmium of 0.75 mg / kg, total zinc of 55.8 mg / kg, total iron of 14.1 g / kg, and total manganese of 0.43 g / kg, the rice variety was *Yexiang Youlis*. The multi-core nested controlled-release micronutrient fertilizer (Example 1) was applied as a basal fertilizer at a rate of 3 kg / mu. The application method was as follows: two days before rice transplanting, the multi-core nested controlled-release micronutrient fertilizer was applied to the soil as a basal fertilizer under flooded conditions. Water and fertilizer management in the field remained consistent with traditional methods. The fertilizer was applied after the rice matured and was harvested.

[0023] Rice yield measurement: The yield is measured at the rice harvest stage.

[0024] Method for determining cadmium content in brown rice: Take 0.1g of brown rice powder sample, add 5mL of nitric acid and 3.0mL of hydrogen peroxide solution, cover and let it digest overnight. Digest using a graphite digester for 2h. The cadmium concentration in the digestion solution is analyzed and determined by inductively coupled plasma mass spectrometry.

[0025] Eleven comparative cases were set up based on Example 1, as follows: Comparative Example 1: Soil without added micronutrients served as a blank control, and the water and fertilizer management method in the paddy field was the same as in Example 1; Comparative Example 2: The same amount (effective amount) of conventional zinc fertilizer (zinc sulfate heptahydrate), iron fertilizer (ferrous sulfate heptahydrate), and manganese fertilizer (manganese sulfate monohydrate) were applied to the soil as in Example 1, and the application method and paddy field water and fertilizer management method were the same as in Example 1. Comparative Example 3: The middle coating material in Example 1 was replaced with gelatin combined with polyvinyl alcohol (iron). The preparation process was as follows: 10% gelatin solution and 10% polyvinyl alcohol solution were mixed in a ratio of 1:4 to prepare gelatin combined with polyvinyl alcohol coating material, and 5% glycerol was added. The mixing conditions were: stirring at 40°C water bath temperature for 30 minutes and standing for 6 hours to remove bubbles. Other process parameters and the coating of each fertilizer remained unchanged. A zinc, iron and manganese multi-core nested controlled-release micro-fertilizer was prepared. The fertilizer application rate (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 1. Comparative Example 4: The inner coating material in Example 1 was replaced with gelatin combined with polyvinyl alcohol (zinc) (the preparation process was the same as in Comparative Example 3), while other process parameters and the coating of each fertilizer remained unchanged. A multi-core nested controlled-release micro-fertilizer of zinc, iron and manganese was prepared. The amount of fertilizer (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 1. Comparative Example 5: The coating materials in Example 1 were changed in the following order: outer layer thick water-based polyacrylate (manganese), middle layer chitosan glutaraldehyde crosslinking complex (iron), and inner layer thin water-based polyacrylate (zinc). Other process parameters remained unchanged. A zinc, iron, and manganese multi-core nested controlled-release micro-fertilizer was prepared. The fertilizer application rate (effective amount), application method, and paddy field water and fertilizer management method were the same as in Example 1. Comparative Example 6: The coating materials in Example 1 were changed in the following order: outer thin water-based polyacrylate (manganese), middle thick water-based polyacrylate (iron), and inner chitosan glutaraldehyde crosslinking complex (zinc). Other process parameters remained unchanged. A zinc, iron, and manganese multi-core nested controlled-release micro-fertilizer was prepared. The fertilizer application rate (effective amount), application method, and paddy field water and fertilizer management method were the same as in Example 1. Comparative Example 7: The modified cassava starch in Example 1 was replaced with polyacrylic acid, while other process parameters and the coating of each fertilizer remained unchanged. A multi-core nested controlled-release micro-fertilizer containing zinc, iron, and manganese was prepared. The amount of fertilizer (effective amount), application method, and paddy field water and fertilizer management method were the same as in Example 1. Comparative Example 8: The zinc, iron, and manganese three-layer nested controlled-release micro-fertilizer in Example 1 was replaced with a double-layer nested controlled-release micro-fertilizer material of iron and manganese. Ferric sulfate heptahydrate was used as the inner iron fertilizer, and the inner coating material was thin water-based polyacrylate. The fertilizer core to coating mass ratio was 2:1. Other process parameters and the coating of each fertilizer remained unchanged. A multi-core nested controlled-release micro-fertilizer of iron and manganese was prepared. The fertilizer application rate (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 1. Comparative Example 9: The zinc, iron, and manganese three-layer nested controlled-release micro-fertilizer in Example 1 was replaced with a zinc and iron double-layer nested controlled-release micro-fertilizer material. Other process parameters and the coating of each fertilizer remained unchanged. Iron and manganese multi-core nested controlled-release micro-fertilizer was prepared. The fertilizer application rate (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 1. Comparative Example 10: The nesting order of the multi-core nested controlled-release micro-fertilizer in Example 1 from the inner layer to the outer layer was changed to "iron-manganese-zinc". Other process parameters and the coating of each fertilizer remained unchanged. A multi-core nested controlled-release micro-fertilizer of iron, manganese and zinc was prepared. The fertilizer application rate (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 1. Comparative Example 11: The nesting order of the multi-core nested controlled-release micro-fertilizer in Example 1 from the inner layer to the outer layer was changed to "iron-zinc-manganese". Other process parameters and the coating of each fertilizer remained unchanged. A multi-core nested controlled-release micro-fertilizer of iron, zinc and manganese was prepared. The amount of fertilizer (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 1.

[0026] The experimental results are shown in Table 1: Table 1. Differences in rice yield and cadmium content in brown rice among treatments in Example 1.

[0027] Note: Except for the treatment method, all other conventional cultivation methods are the same for the groups in the table.

[0028] In this application case, there was no significant difference in rice yield between Example 1, Comparative Examples 2-11, and the blank control (Comparative Example 1). Compared to the blank control (Comparative Example 1), the reduction in Cd content of brown rice in Example 1 was the largest, reaching 54.1%, which was 36.1% higher than the cadmium reduction rate of basal application of ordinary zinc, iron, and manganese fertilizer (Comparative Example 2). Changing the membrane component in the multi-core nested material (Comparative Examples 3-4) resulted in a cadmium reduction rate of only 32.8%~34.4%; changing the membrane sequence in the multi-core nested material (Comparative Examples 5-6) resulted in a cadmium reduction rate of only 27.9%~39.3%; changing the adhesive material in the multi-core nested material (Comparative Example 7) resulted in a cadmium reduction rate of only 37.7%; changing the tri-core nested material to a bi-core nested material (Comparative Examples 8-9) resulted in a cadmium reduction rate of only 21.3%~22.9%; and changing the nesting sequence of micronutrients in the multi-core nested material (Comparative Examples 10-11) resulted in a cadmium reduction rate of only 29.5%~32.78%. The results showed that, compared with the comparative example, the multi-core nested micro-fertilizer in Example 1 had the best cadmium reduction efficiency for rice and the highest rice yield.

[0029] Example 2: The inner core consists of 4 mm zinc sulfate heptahydrate spherical particles coated with a fertilizer core coating at a mass ratio of 1:1.2 to obtain a thick water-based polyacrylate coated zinc fertilizer. The middle layer consists of modified cassava starch nested with 0.2 mm ferrous sulfate heptahydrate particles, and the modified starch nested with iron fertilizer layer is 2 mm thick. The fertilizer core coating (thin polyacrylate) is applied at a mass ratio of 1:0.3 to obtain a zinc and iron dual-core nested coated fertilizer. The outer layer consists of modified cassava starch nested with 0.2 mm manganese sulfate monohydrate particles, with a thickness of 0.9 mm. The fertilizer core coating (polysaccharide glutaraldehyde crosslinking complex) is applied at a mass ratio of 1:0.3 to obtain a zinc, iron, and manganese multi-core nested controlled-release micro-fertilizer. Other preparation processes are the same as in Example 1.

[0030] In a paddy field in Tongling, Anhui Province, with a pH of 5.32, total cadmium of 0.66 mg / kg, total zinc of 91.3 mg / kg, total iron of 8.3 g / kg, and total manganese of 0.34 g / kg, the rice variety Jingliangyou Huazhan was planted. The multi-core nested controlled-release micronutrient fertilizer (Example 2) was applied as a basal fertilizer at a rate of 3 kg / mu. The application method was as follows: two days before rice transplanting, under flooded conditions, the multi-core nested controlled-release micronutrient fertilizer was applied as a basal fertilizer to the soil. Water and fertilizer management in the field remained consistent with traditional methods. The rice was harvested after maturity.

[0031] Rice yield measurement: The yield is measured at the rice harvest stage.

[0032] Method for determining cadmium content in brown rice: Take 0.1g of brown rice powder sample, add 5mL of nitric acid and 3.0mL of hydrogen peroxide solution, cover and let it digest overnight. Digest using a graphite digester for 2h. The cadmium concentration in the digestion solution is analyzed and determined by inductively coupled plasma mass spectrometry.

[0033] Eleven comparative examples were set up based on Example 2, as follows: Comparative Example 1: Soil without added micronutrients served as a blank control, and the water and fertilizer management method in the paddy field was the same as in Example 2; Comparative Example 2: The same amount (effective amount) of conventional zinc fertilizer (zinc sulfate heptahydrate), iron fertilizer (ferrous sulfate heptahydrate), and manganese fertilizer (manganese sulfate monohydrate) were applied to the soil as in Example 1, and the application method and paddy field water and fertilizer management method were the same as in Example 2. Comparative Example 3: The middle coating material in Example 2 was replaced with gelatin combined with polyvinyl alcohol (iron), while other process parameters and fertilizer coating remained unchanged. A zinc, iron and manganese multi-core nested controlled-release micro fertilizer was prepared. The fertilizer application rate (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 2. Comparative Example 4: The inner coating material in Example 2 was replaced with gelatin combined with polyvinyl alcohol (zinc), while other process parameters and fertilizer coating remained unchanged. A multi-core nested controlled-release micro-fertilizer of zinc, iron and manganese was prepared. The fertilizer application rate (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 2. Comparative Example 5: The coating materials in Example 2 were changed in the following order: outer layer thick water-based polyacrylate (manganese), middle layer chitosan glutaraldehyde crosslinking complex (iron), and inner layer thin water-based polyacrylate (zinc). Other process parameters remained unchanged. A zinc, iron, and manganese multi-core nested controlled-release micro-fertilizer was prepared. The fertilizer application rate (effective amount), application method, and paddy field water and fertilizer management method were the same as in Example 2. Comparative Example 6: The coating materials in Example 2 were changed in the following order: outer thin water-based polyacrylate (manganese), middle thick water-based polyacrylate (iron), and inner chitosan glutaraldehyde crosslinking complex (zinc). Other process parameters remained unchanged. A zinc, iron, and manganese multi-core nested controlled-release micro-fertilizer was prepared. The fertilizer application rate (effective amount), application method, and paddy field water and fertilizer management method were the same as in Example 2. Comparative Example 7: The adhesive-modified cassava starch in Example 2 was replaced with polyacrylic acid (0.2%), while other process parameters and the coating of each fertilizer remained unchanged. A zinc, iron, and manganese multi-core nested controlled-release micro-fertilizer was prepared. The fertilizer application rate (effective amount), application method, and paddy field water and fertilizer management method were the same as in Example 2. Comparative Example 8: The zinc, iron, and manganese three-layer nested controlled-release micro-fertilizer in Example 2 was replaced with iron and manganese two-layer nested controlled-release micro-fertilizer material. The inner iron fertilizer was ferrous sulfate heptahydrate with a particle size of 0.2 mm. Other process parameters and the coating of each fertilizer remained unchanged. Iron and manganese two-layer nested controlled-release micro-fertilizer was prepared. The fertilizer application rate (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 2. Comparative Example 9: The zinc, iron, and manganese three-layer nested controlled-release micro-fertilizer in Example 2 was replaced with a zinc and iron double-layer nested controlled-release micro-fertilizer material. Other process parameters and the coating of each fertilizer remained unchanged. A zinc and iron double-layer nested controlled-release micro-fertilizer was prepared. The fertilizer application rate (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 2. Comparative Example 10: The nesting order of the multi-core nested controlled-release micro-fertilizer in Example 2 from the inner layer to the outer layer was changed to "iron-manganese-zinc". Other process parameters and the coating of each fertilizer remained unchanged. A multi-core nested controlled-release micro-fertilizer of iron, manganese and zinc was prepared. The fertilizer application rate (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 2. Comparative Example 11: The nesting order of the multi-core nested controlled-release micro-fertilizer in Example 2 from the inner layer to the outer layer was changed to "iron-zinc-manganese". Other process parameters and the coating of each fertilizer remained unchanged. A multi-core nested controlled-release micro-fertilizer of iron, zinc and manganese was prepared. The amount of fertilizer (effective amount), application method and paddy field water and fertilizer management method were the same as in Example 2.

[0034] The total effective content of micronutrients applied to the soil in the above comparative examples and embodiments is consistent. For example, in Examples 1 and 2, the fertilizer application rate is 3 kg / mu of multi-core nested controlled-release micronutrients. In some comparative examples, the fertilizer application rate per mu is calculated based on the micronutrient content to ensure that the total effective content of micronutrients applied to the soil is consistent.

[0035] The experimental results are shown in Table 1: Table 2. Differences in rice yield and cadmium content in brown rice among treatments in Example 2.

[0036] Note: Except for the treatment method, all other conventional cultivation methods are the same for the groups in the table.

[0037] In this application case, there was no significant difference in rice yield between Example 2, Comparative Examples 2-11, and the blank control (Comparative Example 1). Compared to the blank control (Comparative Example 1), the reduction in Cd content of brown rice in Example 2 was the largest, reaching 61.8%, which was 38.2% higher than the cadmium reduction rate of basal application of ordinary zinc, iron, and manganese fertilizer (Comparative Example 2). Changing the membrane component in the multi-core nested material (Comparative Examples 3-4) resulted in a cadmium reduction rate of only 30.9%~32.7%; changing the membrane sequence in the multi-core nested material (Comparative Examples 5-6) resulted in a cadmium reduction rate of only 34.5%~38.2%; changing the adhesive material in the multi-core nested material (Comparative Example 7) resulted in a cadmium reduction rate of only 36.4%; changing the tri-core nested material to a bi-core nested material (Comparative Examples 8-9) resulted in a cadmium reduction rate of only 25.5%~29.1%; and changing the nesting sequence of micronutrients in the multi-core nested material (Comparative Examples 10-11) resulted in a cadmium reduction rate of only 21.8%~29.1%. The results showed that, compared with the comparative example, the multi-core nested micro-fertilizer in Example 2 had the best cadmium reduction efficiency for rice and increased rice yield.

[0038] Therefore, those skilled in the art will recognize that although embodiments of the present invention have been shown and described in detail herein, many other variations or modifications conforming to the principles of the present invention can be directly determined or derived from the disclosure of the present invention without departing from the spirit and scope of the invention. Thus, the scope of the present invention should be understood and recognized as covering all such other variations or modifications.

Claims

1. A multi-nuclear nested controlled-release micro-fertilizer for reducing cadmium absorption in rice, characterized in that, The fertilizer comprises an inner fertilizer core, an inner coating material, a middle fertilizer core, a middle coating material, an outer fertilizer core, and an outer coating material; wherein, the inner fertilizer core is zinc sulfate heptahydrate spherical particles, and the inner coating material is water-based polyacrylate; the middle fertilizer core is ferrous sulfate heptahydrate, and the middle coating material is water-based polyacrylate; the outer fertilizer core is manganese sulfate monohydrate, and the outer coating material is chitosan glutaraldehyde crosslinked composite. The preparation method of the multi-core nested controlled-release micro-fertilizer includes the following steps: S1. Preparation of water-based polyacrylate coating: using zinc sulfate heptahydrate granules as fertilizer cores, coating is carried out at a fertilizer core-coating mass ratio of 1:0.9~1.2 to obtain water-based polyacrylate coated zinc fertilizer. S2. Cassava starch powder and pure water are mixed in a mass ratio of 1:0.5-3. Then, sodium hypochlorite with a mass fraction of 5%~8% and nickel sulfate oxidation catalyst with a mass fraction of 0.05~0.05% are added. After stirring at room temperature, the mixture is allowed to stand for oxidation to obtain oxidized starch. Then, sodium hydroxide with a mass fraction of 3%~6% is added and stirred for 20~50 minutes to obtain gelatinized starch. Ferrous sulfate heptahydrate and gelatinized starch are mixed and stirred evenly in a mass ratio of 2~3:

1. The mixture is then adhered to the surface of water-based polyacrylate-coated zinc fertilizer and dried to obtain zinc and iron double-layer nested micro-fertilizer. S3. Prepare water-based polyacrylate coating. Using the zinc and iron nested micro-fertilizer obtained in S2 as the fertilizer core, coat it with a fertilizer core coating mass ratio of 1:0.3~0.5 to obtain zinc and iron double-core nested coated micro-fertilizer. Add talc powder and dry for later use. S4. Using the gelatinized starch obtained in S2 as a dispersant and adhesive, the mixture of manganese sulfate monohydrate and gelatinized starch in a mass ratio of 2~3:1 is stirred evenly and then adhered to the surface of zinc and iron double-core nested micro-fertilizer. After drying, zinc, iron and manganese multi-layer nested micro-fertilizer is obtained. S5. Glutaraldehyde and chitosan are dissolved in acetic acid solution to prepare glutaraldehyde and chitosan solutions respectively. The glutaraldehyde solution is added dropwise to the chitosan solution under constant stirring to obtain a chitosan-glutaraldehyde cross-linked composite coating material. The zinc, iron, and manganese nested micro-fertilizer prepared in S4 is used as the fertilizer core. The zinc, iron, and manganese multi-core and dual-core nested controlled-release micro-fertilizer is prepared by coating at a fertilizer core coating mass ratio of 1:0.3~0.

5. After drying, the multi-core nested controlled-release micro-fertilizer that reduces cadmium absorption by rice is obtained.

2. The multi-nuclear nested controlled-release micro-fertilizer for reducing cadmium absorption in rice according to claim 1, characterized in that, The inner fertilizer core has a particle size of 3-4 mm, and the middle and outer fertilizer cores have a particle size of 0.1-0.2 mm.

3. The multi-nuclear nested controlled-release micro-fertilizer for reducing cadmium absorption in rice according to claim 1, characterized in that, The middle and outer fertilizer cores also include gelatinized starch as a dispersant and adhesive.

4. The multi-nuclear nested controlled-release micro-fertilizer for reducing cadmium absorption in rice according to claim 1, characterized in that, In steps S1 and S3, an in-situ melting method is used to prepare a water-based polyacrylate coating.

5. The multi-nuclear nested controlled-release micro-fertilizer for reducing cadmium absorption in rice according to claim 1, characterized in that, In step S2, after adding the oxidation catalyst, stir at room temperature for 5-30 minutes and let it stand for oxidation for 8-12 hours.

6. The multi-nuclear nested controlled-release micro-fertilizer for reducing cadmium absorption in rice according to claim 1, characterized in that: In step S5, the acetic acid solution has a concentration of 1%, the glutaraldehyde solution has a concentration of 0.2%, and the chitosan solution has a concentration of 2.5%. The glutaraldehyde solution is added dropwise to the continuously stirred chitosan solution at a volume ratio of 1:2~5.

7. The application of a multi-core nested controlled-release micro-fertilizer according to any one of claims 1 to 6 in paddy fields with mild to moderate cadmium contamination.

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