Nano-zinc oxide coupled charcoal-based slow-release fertilizer and preparation method thereof

Abstract

The invention relates to a nano-zinc oxide coupled charcoal-based slow-release fertilizer and a preparation method thereof, and belongs to the technical field of agricultural fertilizers. Nano-zinc oxide is loaded on the surface of biochar, the biochar and a compound fertilizer are mixed and granulated, a biochar-based compound fertilizer core is obtained, then the surface of the biochar-based compound fertilizer core is coated with a polyvinyl alcohol / starch coating layer through a polyvinyl alcohol / starch coating technology, and the nano-zinc oxide coupled biochar-based slow-release fertilizer is obtained. According to the slow-release fertilizer, a polyvinyl alcohol / starch coating layer serves as a first barrier for fertilizer release, biochar with the surface loaded with nano-zinc oxide serves as a second barrier for fertilizer release, and effective control over the nutrient release rate and improvement of the nutrient absorption and utilization rate of crops are achieved at the same time through the synergistic effect of the two barriers; the slow-release fertilizer can improve the physicochemical properties of soil and increase the crop yield, conforms to the concepts of green agriculture and sustainable development, and has wide application prospects.

Description

Technical Field

[0001] The present invention relates to a nano-zinc oxide coupled biochar-based slow-release fertilizer and a preparation method thereof, belonging to the technical field of agricultural fertilizers. Background Art

[0002] Chemical fertilizers play a crucial role in modern agriculture. However, the overuse of chemical fertilizers has caused serious environmental problems, such as soil degradation, water pollution, and greenhouse gas emissions. Currently, biochar-based slow-release fertilizers have gradually attracted attention as a promising solution. Biochar is a carbon-rich material produced by pyrolyzing biomass under limited oxygen conditions. It can improve soil properties, including water retention, porosity, and microbial activity, and can act as a potential nutrient adsorbent to reduce nutrient leaching and promote the gradual release of nutrients to plants. However, the nutrient adsorption capacity of biochar is still weak, and the existing biochar-based slow-release fertilizers are limited by the physical and chemical properties of biochar, and their nutrient release patterns are not ideal.

[0003] At present, introducing zinc additives into the preparation of slow-release fertilizers has become a promising strategy to enhance the performance of slow-release fertilizers and optimize nutrient release. Zinc also plays a crucial role in plant nutrition, participating in enzyme activation, protein synthesis, and various metabolic processes. Moreover, zinc ions in zinc additives can have a strong interaction with phosphate anions, playing a certain role in slowing down the release of fertilizer efficiency. Chinese Patent CN119350078A discloses a zinc-containing diammonium phosphate, its preparation method, and a fertilizer including the zinc-containing diammonium phosphate. This slow-release fertilizer adds chelated zinc to the diammonium phosphate product, solving problems such as low water-soluble zinc content ratio, low agronomic effect, and high cost of adding integrated zinc fertilizer in diammonium phosphate products. However, the preparation technology of this compound fertilizer is relatively complex, and the price of chelated zinc fertilizer is much higher than that of ordinary zinc fertilizer, increasing the input costs of fertilizer production enterprises and consumers and restricting the application of this technical solution. Chinese Patent CN113735088A discloses a zinc-aluminum oxide modified biochar supported phosphate composite material, including zinc-aluminum oxide modified biochar and phosphate nanominerals supported on the zinc-aluminum oxide. Among them, the zinc-aluminum oxide modified biochar includes biochar and zinc-aluminum oxide deposited on the surface of the biochar. The zinc-aluminum oxide includes two phases: zinc-aluminum layered double hydroxide and zinc oxide. The phosphate nanominerals include zinc phosphate tetrahydrate composite nanosheets. Although this patent shows that the biochar-based composite material containing zinc-aluminum oxide can be used for slow-release applications of phosphate fertilizers, this patent has not actually implemented it in field experiments, and the barrier from laboratory to practical application has not been broken. This is because a single zinc-aluminum oxide modified biochar supported phosphate material may still have problems such as poor slow-release effect of fertilizers or too strong slow-release effect resulting in low nutrient utilization efficiency. Its practical application still requires design and breakthroughs in many aspects such as fertilizer degradability, slow-release effect, and nutrient utilization efficiency. Therefore, there is still a lack of a biochar-based slow-release fertilizer modified with zinc additives and its preparation method that have both good fertilizer slow-release performance and nutrient utilization efficiency in the prior art. Summary of the Invention

[0004] Aiming at the existing problems, the present invention provides a nano-zinc oxide coupled biochar-based slow-release fertilizer and its preparation method. By loading nano-zinc oxide on biochar and adopting a polyvinyl alcohol / starch coating technology, a nano-zinc oxide coupled biochar-based slow-release fertilizer is prepared. The prepared slow-release fertilizer has good slow-release effect, good water retention, and can add trace elements required by plants, effectively promoting the yield increase of wheat and improving the physical and chemical properties of the soil.

[0005] The technical solution of the present invention is as follows:

[0006] The present invention provides a nano-zinc oxide coupled biochar-based slow-release fertilizer, which is composed of a biochar-based compound fertilizer core and a polyvinyl alcohol / starch coating layer coated on the surface of the biochar-based compound fertilizer core;

[0007] Among them, the biochar-based compound fertilizer core is spherical particles formed by mixing and granulating biochar with a surface load of nano-zinc oxide and compound fertilizer.

[0008] Among them, the polyvinyl alcohol / starch coating layer is composed of polyvinyl alcohol, starch, glycerol, potassium persulfate and water. By mass percentage, polyvinyl alcohol is 2% - 6%, corn starch is 3% - 7%, glycerol is 1%, potassium persulfate is 0.04% - 0.08%, and water is 89% - 95%.

[0009] Furthermore, the mass percentage of zinc element in the biochar with a surface load of nano-zinc oxide is between 2 and 3 wt%.

[0010] Furthermore, the compound fertilizer is a nitrogen, phosphorus and potassium compound fertilizer with a total nutrient content of ≥45%.

[0011] The present invention also provides a preparation method of a nano-zinc oxide-coupled biochar-based slow-release fertilizer, which includes the following steps:

[0012] S1. Immerse the biochar in a zinc chloride solution and continuously stir to fully impregnate it. Then, adjust the pH of the obtained solution to 10 with a NaOH solution, and continue to stir for at least 24 hours. After the product is washed and dried, biochar with a surface load of nano-zinc oxide is obtained.

[0013] S2. Mix the biochar with a surface load of nano-zinc oxide obtained in step S1 with compound fertilizer, and granulate it with a granulator to obtain a biochar-based compound fertilizer core.

[0014] S3. Stir and dissolve starch in water at 80 - 90°C, and stir and dissolve polyvinyl alcohol in water at 90 - 100°C. Then, mix the two solutions, keep the temperature of the mixed solution at 60 - 85°C, add glycerol and a potassium persulfate solution thereto, and continuously stir and react sufficiently at 60 - 85°C to obtain a polyvinyl alcohol / starch coating solution.

[0015] S4. Put the biochar-based compound fertilizer core obtained in step S2 into a coating machine, and evenly spray and coat the polyvinyl alcohol-starch coating solution obtained in step S3 on the surface of the biochar-based compound fertilizer core to form a polyvinyl alcohol-starch coating solution layer on the surface of the biochar-based compound fertilizer core, thereby obtaining the nano-zinc oxide-coupled biochar-based slow-release fertilizer.

[0016] Furthermore, the biochar in step S1 is prepared by pyrolyzing crop straw; the crop straw includes one or more of wheat straw, rice straw, and corn straw.

[0017] Furthermore, in step S1, the concentration of the zinc chloride solution is 0.01 mol / L, and the mass ratio of the added biochar to the volume of the zinc chloride solution is 1 g:16 mL.

[0018] Further, in step S2, the mass ratio of biochar to compound fertilizer is 1:3.

[0019] Further, in step S3, the concentration of potassium persulfate solution is 0.14 mol / L.

[0020] Different from the prior art, the present invention has the following beneficial effects:

[0021] 1. In the present invention, nano-zinc oxide is loaded on the surface of biochar and mixed with compound fertilizer for granulation to obtain a biochar-based compound fertilizer core, and then a polyvinyl alcohol / starch coating layer is coated on the surface of the biochar-based compound fertilizer core by using a polyvinyl alcohol / starch coating technology, thereby obtaining a nano-zinc oxide-coupled biochar-based slow-release fertilizer; zinc oxide has a strong interaction with phosphate anions in the soil and can form an inner complex, thereby enhancing the adsorption and retention ability of biochar for phosphates and ammonium ions. The present invention significantly improves the adsorption ability of biochar for nutrients such as nitrogen and phosphorus through the combination of zinc oxide and biochar, effectively controls the nutrient release rate, extends the fertilizer release period, and at the same time, the addition of zinc oxide also enhances the nutrient absorption of the soil and increases a stable zinc supply to the soil, effectively improving the physical and chemical properties of the soil, improving the fertilizer utilization rate and reducing the application cost.

[0022] 2. The slow-release fertilizer provided by the present invention uses a polyvinyl alcohol / starch coating layer as the first barrier for fertilizer release, and biochar with nano-zinc oxide loaded on the surface as the second barrier for fertilizer release. Compared with the existing biochar slow-release fertilizers, the present invention effectively controls the fertilizer nutrient release rate and improves the absorption and utilization rate of crop fertilizer nutrients through the synergistic effect of the double barriers, realizes the balance between the two properties of fertilizer slow-release and fertilizer effect release, and finally improves the field yield. Both the static water release and soil column leaching experiments show that the total nitrogen and phosphorus release amounts of the slow-release fertilizer prepared by the present invention are significantly lower than those of traditional compound fertilizers and coated fertilizers without adding nano-zinc oxide in the same time. The nano-zinc oxide-coupled biochar-based slow-release fertilizer provided by the present invention has a better fertilizer slow-release effect. Description of the Drawings

[0023] Figure 1 It is the nutrient release curves of nitrogen and phosphorus of the nano-zinc oxide-coupled biochar-based slow-release fertilizer in Example 1 and the fertilizers described in Comparative Examples 1-2 in the soil column leaching test.

[0024] Figure 2 It is the degradability and water retention of the nano-zinc oxide-coupled biochar-based slow-release fertilizer in Example 1 and the fertilizers described in Comparative Examples 1-2.

[0025] Figure 3 It is the adsorption amounts of nitrogen and phosphorus in the compound fertilizer adsorbed by nano-zinc oxide-coupled biochar.

[0026] Figure 4 XPS spectra of the O 1s and P 2p orbitals before and after the adsorption of nitrogen and phosphorus in the nano-zinc oxide-coupled biochar composite fertilizer.

[0027] Figure 5 Key agronomic parameters of wheat under different fertilization treatments.

[0028] Figure 6 Soil nitrogen, phosphorus and potassium contents of different fertilization treatments in the filling stage and maturity stage.

[0029] Figure 7 Soil fertility indexes of different fertilization treatments. Detailed implementation mode

[0030] The present invention will be further described below in conjunction with the accompanying drawings and preferred embodiments. The provided embodiments are only for clarifying the present invention and not for limiting the scope of the present invention.

[0031] The materials, reagents, etc. used in the following embodiments can be obtained from commercial channels without special instructions; the methods in the following embodiments are conventional methods without special instructions.

[0032] The crop straws involved in the embodiments of the present invention can be prepared by the following method: First, clean the crop straws, then perform blanching and the first homogenization treatment in sequence, and finally obtain crop straw powders. Among them, the crop straws include but are not limited to one or more of rice straws, wheat straws and nut shells; the cleaning process can adopt conventional techniques in the art without special limitations. The blanching temperature is preferably 105 °C, and the blanching time is 20 - 40 minutes, preferably 30 minutes in the embodiments. The first homogenization treatment includes drying and cutting steps. The drying temperature is 60 - 90 °C, preferably 80 °C in the embodiments; the drying time is 24 - 48 hours, preferably 24 hours in the embodiments; the cutting length can be 5 - 10 cm, preferably 5 cm in the embodiments to ensure the uniformity of the pyrolysis reaction. The above process is beneficial to obtaining crop straw raw materials suitable for subsequent operations.

[0033] In the embodiments of the present invention, the biochar involved is prepared by pyrolyzing the above crop straws, and the specific steps are as follows: The crop straws are pyrolyzed under oxygen-limited conditions, followed by rinsing and a second homogenization treatment, and finally the required biochar is obtained. Among them, the pyrolysis temperature is 500-600 °C, preferably 550 °C; the pyrolysis time is 0.5-2 hours, and preferably 2 hours in the embodiments; the pyrolysis equipment can adopt conventional devices in the art. After pyrolysis, the obtained biochar is rinsed, preferably with tap water, and the number of rinsing times and the method are based on the pH value of the rinsing solution reaching 6.0-8.0, more preferably 7.0. The second homogenization treatment includes drying the rinsed biochar to obtain a uniform biochar product.

[0034] Example 1

[0035] This example provides a nano-zinc oxide-coupled biochar-based slow-release fertilizer, which is composed of a biochar-based compound fertilizer core and a polyvinyl alcohol / starch coating layer coated on the surface of the biochar-based compound fertilizer core;

[0036] Among them, the biochar-based compound fertilizer core is a spherical particle with a diameter of 2-4 mm formed by mixing biochar with nano-zinc oxide loaded on the surface and compound fertilizer;

[0037] Among them, the polyvinyl alcohol / starch coating layer is composed of polyvinyl alcohol, starch, glycerol, potassium persulfate and water. By mass percentage, polyvinyl alcohol is 2%-2.5%, corn starch is 3%-4%, glycerol is 1%, potassium persulfate is 0.07%-0.08%, and water is 93%-95%.

[0038] The preparation method of the nano-zinc oxide-coupled biochar-based slow-release fertilizer provided in this example includes the following steps:

[0039] S1. Immerse 100 g of biochar in 1600 mL of 0.01 mol / L zinc chloride solution (stirring speed is 700 rpm), stir for 24 hours, adjust the pH to 10 with 1 mol / L NaOH, continue to stir for 24 hours, wash and dry to obtain biochar with nano-zinc oxide loaded on the surface; the mass percentage of zinc element in the biochar with nano-zinc oxide loaded on the surface prepared in this example is between 2-3 wt%.

[0040] S2. Mix the biochar with nano-zinc oxide loaded on the surface and compound fertilizer in a mass ratio of 1:3, and granulate with a granulator to obtain a biochar-based compound fertilizer core; the compound fertilizer used in this example is a nitrogen-phosphorus-potassium compound fertilizer (N-P2O5-K2O 15-15-15) with a total nutrient content of ≥45%.

[0041] S3. Stir and dissolve 12 g of starch in water at 85°C, and stir and dissolve 8 g of polyvinyl alcohol in water at 90°C. The stirring speed is 500 rpm, and the stirring time is 4 h. Then mix the two solutions, keep the temperature of the mixed solution at 85°C, add 1 mL of glycerol and 2.5 mL of 0.14 mol / L potassium persulfate solution thereto, and stir at 300 rpm at 85°C for 2 h to obtain the polyvinyl alcohol / starch coating solution;

[0042] S4. Preheat the coating machine to 50°C, put the biochar-based compound fertilizer core into the coating machine, rotate at a speed of 30 rpm, evenly spray and coat the polyvinyl alcohol-starch coating solution on the surface of the urea fertilizer core particles to form a polyvinyl alcohol-starch coating solution layer on the surface of the urea fertilizer core particles. After drying, the nano-zinc oxide-coupled biochar-based slow-release fertilizer is obtained.

[0043] Example 2

[0044] This example provides a nano-zinc oxide-coupled biochar-based slow-release fertilizer. The nano-zinc oxide-coupled biochar-based slow-release fertilizer has a structure similar to that of Example 1, which will not be elaborated here. The difference from Example 1 is that:

[0045] The polyvinyl alcohol / starch coating layer of the slow-release fertilizer is composed of polyvinyl alcohol, starch, glycerol, potassium persulfate and water. By mass percentage, polyvinyl alcohol is 2.5% - 3%, corn starch is 4% - 5%, glycerol is 1%, potassium persulfate is 0.06% - 0.07%, and water is 91% - 93%.

[0046] The preparation method of the nano-zinc oxide-coupled biochar-based slow-release fertilizer provided in this example includes the following steps:

[0047] S1. Immerse 100 g of biochar in 1600 mL of 0.01 mol / L zinc chloride solution (stirring speed is 700 rpm). After stirring for 48 hours, adjust the pH to 10 with 1 mol / L NaOH, continue stirring for 48 hours, wash and dry to obtain biochar with nano-zinc oxide loaded on the surface; the mass percentage of zinc element in the biochar with nano-zinc oxide loaded on the surface prepared in this example is between 2 - 3 wt%;

[0048] S2. Mix the biochar with nano-zinc oxide loaded on the surface and the compound fertilizer at a mass ratio of 1:3, and granulate with a granulator to obtain a biochar-based compound fertilizer core; the compound fertilizer used in this example is a nitrogen, phosphorus and potassium compound fertilizer with a total nutrient content ≥ 45% (N-P2O5-K2O 15-15-15);

[0049] S3. Stir and dissolve 12 g of starch in water at 80°C, and stir and dissolve 8 g of polyvinyl alcohol in water at 95°C. The stirring speed is 300 rpm and the stirring time is 6 h. Then mix the two solutions, keep the temperature of the mixed solution at 80°C, add 1 mL of glycerol and 2.5 mL of 0.14 mol / L potassium persulfate solution thereto, and stir at 300 rpm at 85°C for 2 h to obtain the polyvinyl alcohol / starch coating solution;

[0050] S4. Preheat the coating machine to 40°C, put the biochar-based compound fertilizer core into the coating machine, rotate at a speed of 20 rpm, evenly spray and coat the polyvinyl alcohol-starch coating solution on the surface of the urea fertilizer core particles to form a polyvinyl alcohol-starch coating solution layer on the surface of the urea fertilizer core particles. After drying, the nano-zinc oxide-coupled biochar-based slow-release fertilizer is obtained.

[0051] Example 3

[0052] This example provides a nano-zinc oxide-coupled biochar-based slow-release fertilizer. This nano-zinc oxide-coupled biochar-based slow-release fertilizer has a similar structure to that of Example 1, which will not be elaborated here. The difference from Example 1 is as follows:

[0053] The polyvinyl alcohol / starch coating layer of this slow-release fertilizer is composed of polyvinyl alcohol, starch, glycerol, potassium persulfate and water. By mass percentage, polyvinyl alcohol is 3% - 6%, corn starch is 5% - 7%, glycerol is 1%, potassium persulfate is 0.04% - 0.06%, and water is 89% - 91%.

[0054] The preparation method of the nano-zinc oxide-coupled biochar-based slow-release fertilizer includes the following steps:

[0055] S1. Immerse 100 g of biochar in 1600 mL of 0.01 mol / L zinc chloride solution (stirring speed is 700 rpm). After stirring for 72 hours, adjust the pH to 10 with 1 mol / L NaOH, continue stirring for 72 hours, wash and dry to obtain biochar with nano-zinc oxide loaded on the surface; the mass percentage of zinc element in the biochar with nano-zinc oxide loaded on the surface prepared in this example is between 2 - 3 wt%;

[0056] S2. Mix the biochar with nano-zinc oxide loaded on the surface and the compound fertilizer at a mass ratio of 1:3, and granulate with a granulator to obtain a biochar-based compound fertilizer core; the compound fertilizer used in this example is a nitrogen, phosphorus and potassium compound fertilizer (N-P2O5-K2O 15-15-15) with a total nutrient content of ≥45%;

[0057] S3. Stir and dissolve 12 g of starch in water at 90 °C, and stir and dissolve 8 g of polyvinyl alcohol in water at 100 °C. The stirring speed is 400 rpm and the stirring time is 5 h. Then mix the two solutions, keep the temperature of the mixed solution at 60 °C, add 1 mL of glycerol and 2.5 mL of 0.14 mol / L potassium persulfate solution thereto, and stir at 300 rpm at 85 °C for 2 h to obtain the polyvinyl alcohol / starch coating solution;

[0058] S4. Preheat the coating machine to 60 °C, put the biochar-based compound fertilizer core into the coating machine, rotate at a speed of 80 rpm, evenly spray and coat the polyvinyl alcohol-starch coating solution on the surface of the urea fertilizer core particles to form a polyvinyl alcohol-starch coating solution layer on the surface of the urea fertilizer core particles. After drying, the nano-zinc oxide-coupled biochar-based slow-release fertilizer is obtained.

[0059] Comparative Example 1

[0060] This comparative example provides a slow-release compound fertilizer, which is different from Example 1 in that:

[0061] This slow-release fertilizer is composed of a compound fertilizer core and a polyvinyl alcohol / starch coating layer coated on the surface of the biochar-based compound fertilizer core. The compound fertilizer core is spherical fertilizer particles (N-P2O5-K2O 15-15-15) with a diameter of 2-4 mm, and the total nutrient content is ≥45%;

[0062] The composition of the polyvinyl alcohol / starch coating layer of this slow-release fertilizer is the same as that in Example 1, and will not be elaborated here.

[0063] The preparation method of the slow-release compound fertilizer includes the following steps:

[0064] S1. Stir and mix 12 g of starch with 150 mL of water at 85 °C (stirring speed is 500 rpm), stir and mix 8 g of polyvinyl alcohol with 150 mL of water at 90 °C (stirring speed is 500 rpm), then mix the two solutions, add 1 mL of glycerol and 2.5 mL of 0.14 mol / L potassium persulfate solution to the mixed solution, and heat and stir at 300 rpm at 85 °C for 2 h to obtain the fertilizer coating solution;

[0065] S2. Preheat the coating machine to 40 °C, put in spherical compound fertilizer core particles with a diameter of 2-4 mm, and rotate at a speed of 20 rpm; evenly spray and coat the polyvinyl alcohol-starch coating solution on the surface of the core particles to form a polyvinyl alcohol-starch coating solution layer on the surface of the core particles, thereby obtaining the slow-release compound fertilizer.

[0066] The slow-release compound fertilizer prepared by the preparation method provided in this example has a composition and structure similar to those in Example 1.

[0067] Comparative Example 2

[0068] This comparative example provides a compound fertilizer, which is spherical fertilizer granules (N-P2O5-K2O 15-15-15) with a diameter of 2-4 mm, and the total nutrient content is ≥ 45%.

[0069] Performance test

[0070] In order to illustrate the performance of the nano-zinc oxide-coupled biochar-based slow-release fertilizer of the present invention, taking the nano-zinc oxide-coupled biochar-based slow-release fertilizer sample provided in Example 1 as an example, the slow-release performance of the fertilizer, the nutrient adsorption performance of the fertilizer and the field test were carried out, and the results are as follows:

[0071] (1) Soil column leaching test

[0072] The fertilizer samples prepared in Example 1 and Comparative Examples 1-2 were respectively loaded into the soil column device for leaching test, and 3 replicate samples were set for each group of tests. Starting from the time when the fertilizer was applied to the soil column and the first watering was carried out, the nutrient release dynamics of each treatment group were continuously monitored for 28 days. The contents of nitrogen and phosphorus elements in the leachate were determined by ion chromatography, and the nutrient release kinetic curve was drawn (as Figure 1 shown). The test results showed that within the 30-day test cycle of the coated fertilizer prepared by the present invention, the cumulative nitrogen release amount was 5.82 ± 0.01 mg, and the cumulative phosphorus release amount was 1.22 ± 0.03 mg. Compared with the uncoated control group, the nitrogen and phosphorus release amounts were reduced by 62.7% and 70.0% respectively, indicating that the coated fertilizer has significant slow-release characteristics.

[0073] (2) Water retention and biodegradability tests of different fertilizers

[0074] 1) Water retention

[0075] The fertilizer samples prepared in Example 1 and Comparative Examples 1-2 were respectively subjected to soil water retention tests, and 3 replicate samples were set for each test group. The change in soil water content was measured by the weighing method, and the test cycle was 26 days. The test results are as Figure 2 shown. The nano-zinc oxide-coupled biochar-based slow-release fertilizer prepared by the present invention showed the best water retention performance, and its soil water content was increased by up to 5.01% compared with the traditional compound fertilizer control group. Analysis showed that the synergistic effect of the PVA / ST composite film layer and the nano-zinc oxide-coupled biochar matrix in the fertilizer significantly enhanced the soil water holding capacity, confirming that the slow-release fertilizer prepared by the present invention has good soil water retention performance.

[0076] 2) Degradability

[0077] First, determine the initial mass of the coated fertilizer before it is applied to the soil, and calculate the mass of the film material by the difference method. After burying the samples in the soil, samples are taken on the 9th, 18th, 27th, 36th, and 45th days respectively, and independent experimental groups are set at each time point. After sampling, the film material is obtained by mechanical separation method, and its mass change is measured after drying treatment.

[0078] As Figure 2 shown, the test results indicate that the mass loss rate of the film material of the conventional slow-release compound fertilizer reaches 61.4 ± 0.3% within 0 - 18 days, while the mass loss rate of the film material of the nano-zinc oxide coupled biochar-based slow-release fertilizer prepared in the present invention is only 46.9 ± 1.4%. It is analyzed that the hydrophobic property of the nano-zinc oxide coupled biochar matrix delays the initial degradation, but its activation effect on the soil microbial community promotes the later degradation. The degradation rate at 45 days is 10.5% higher than that of the conventional slow-release compound fertilizer, which confirms that the slow-release fertilizer prepared in the present invention has controllable biodegradable properties.

[0079] As Figure 2 shown, the test results indicate that the mass loss rate of the film material of the conventional slow-release compound fertilizer reaches 61.4 ± 0.3% within 0 - 18 days, while the mass loss rate of the film material of the nano-zinc oxide coupled biochar-based slow-release fertilizer prepared in the present invention is only 46.9 ± 1.4%. It is analyzed that the hydrophobic property of the nano-zinc oxide coupled biochar matrix delays the initial degradation, but its activation effect on the soil microbial community promotes the later degradation. The degradation rate at 45 days is 10.5% higher than that of the conventional slow-release compound fertilizer, which confirms that the slow-release fertilizer prepared in the present invention has controllable biodegradable properties.

[0080] (3) Adsorption performance test of nano-zinc oxide coupled biochar and ordinary biochar on nitrogen and phosphorus

[0081] The present invention determines the fertilizer nutrient adsorption performance of the nano-zinc oxide coupled biochar of the sample in Example 1. The method for determining the fertilizer nutrient adsorption performance includes the following steps: Take 40.0 ± 0.05 mg of the original biochar and the nano-zinc oxide modified biochar samples respectively, add them to 400 mL of a compound fertilizer solution with a concentration of 0.3 g / L, and place them in a 500 mL conical flask. Place the mixed system in a constant temperature oscillator and oscillate and adsorb at a speed of 180 r / min under the condition of 25 ± 0.5 °C. Take 3 mL of samples at the time points of 10, 20, 30, 60, 180, 420, 1000, and 1440 minutes respectively. After filtering through a 0.22 μm microporous membrane, use ultraviolet-visible spectrophotometry to measure the nitrogen and phosphorus element contents in the filtrate. The adsorption kinetic data is fitted and analyzed using the pseudo-second-order kinetic model to evaluate its role in delaying the release of nutrients from the nano-zinc oxide coupled biochar slow-release fertilizer. Its mathematical expression is:

[0082]

[0083] where t is the adsorption time (min), q t is the adsorption capacity at time t (mg / g), q e is the equilibrium adsorption capacity (mg / g), and k2 is the pseudo-second-order adsorption rate constant (g / (mg·min)). Through this measurement method, the adsorption characteristics of different biochar materials for fertilizer nutrients can be accurately evaluated.

[0084] As Figure 3 shown, the results of the adsorption kinetics study indicate that the adsorption process of the raw biochar for nitrogen and phosphorus elements conforms to the physical adsorption mechanism, and the kinetic fitting correlation coefficient R 2 = 0.97. The main adsorption mechanisms include pore filling effect and electrostatic interaction. In contrast, the adsorption behavior of the nano-ZnO modified biochar conforms to the pseudo-second-order kinetic model, indicating that its adsorption process is dominated by chemical adsorption, especially the formation of stable chemical bonds with phosphate ions. Quantitative analysis shows that the theoretical saturated adsorption capacity of the nano-ZnO modified biochar for nitrogen element reaches 19.93 ± 0.05 mg / g, and the adsorption capacity for phosphorus element is 29.4 ± 0.3 mg / g, which are increased by 25.0% and 100.0% respectively compared with the raw biochar. The experimental data confirm that the nutrient adsorption performance of the biochar material can be significantly improved by nano-ZnO modification, making it have significant advantages in the application of slow-release fertilizer carriers.

[0085] (4) XPS spectra of nano-ZnO coupled biochar before and after adsorption

[0086] The surface chemical composition of the prepared nano-ZnO coupled biochar samples before and after adsorbing compound fertilizer was analyzed by X-ray photoelectron spectroscopy. The spectral results of O 1s and P 2p XPS are as Figure 4 shown. The peak at 529.06 eV in O 1s corresponds to the lattice oxygen in metal oxides, pointing to zinc oxide in the nano-ZnO coupled biochar. Therefore, the existing form of zinc loaded on the biochar is zinc oxide. The energy spectrum of the adsorbed material shows that the peak position moves from 529.06 eV to 531.97 eV. The literature indicates that this is due to the formation of zinc phosphate. This change indicates the existence of chemical interaction between the nano-ZnO coupled biochar and phosphate ions. In addition, the new peak at 140.5 eV in the P 2p spectrum confirms that phosphorus exists in the nano-ZnO coupled biochar after adsorbing compound fertilizer, which verifies that the nano-ZnO coupled biochar has successfully adsorbed the phosphate in the fertilizer. These XPS results provide direct evidence for the chemical process of the nano-ZnO coupled biochar adsorbing nitrogen and phosphorus, and this characteristic enables the nano-ZnO coupled biochar-based slow-release fertilizer to effectively delay the release rate of compound fertilizer during actual application.

[0087] (5) Field experiment

[0088] Select the non-grain cultivated land in Lin'an District, Hangzhou City, Zhejiang Province as the test area, delimit a rectangular test area of 5m×4m, separated by grooves 30cm wide and 40cm deep, for observing the effect of the slow-release fertilizer on the growth and development of wheat. The non-grain cultivated land has typical soil fertility characteristics, with the soil organic matter content lower than the conventional tillage standard value and insufficient supply of available nutrients. This characteristic makes the response of the test area to different fertilization treatments significantly different, which is conducive to accurately evaluating the fertilizer efficiency. The slow-release fertilizer prepared by the preparation method of the nano-zinc oxide coupled biochar-based slow-release fertilizer described in Example 1 and the fertilizers described in Comparative Examples 1-2 are respectively applied to the sample plots, and a blank control group (without fertilizer) is set as a control treatment.

[0089] 1) Performance evaluation of different fertilizers on wheat growth

[0090] The nano-zinc oxide coupled biochar-based slow-release fertilizer prepared in Example 1, the slow-release compound fertilizer in Comparative Example 1, and the compound fertilizer in Comparative Example 2 are respectively applied to the test plots, and 3 repeated test units are set for each treatment group. The fertilization amount is determined according to the "Scientific Fertilization Guidance for Winter Wheat and Rapeseed in Zhejiang in the Autumn and Winter of 2022" and standardized by equal nitrogen amount conversion; the fertilization time is selected during the greening stage of winter wheat. At two key growth stages, the filling stage and the maturity stage of wheat, wheat plants and soil samples are collected from each treatment plot respectively. The wheat samples are naturally air-dried and then threshed to measure agronomic trait indexes such as the number of grains per spike, the weight of per spike, and the 1000-grain weight, and the theoretical yield of wheat under different fertilization treatments is calculated based on the above measured data. The soil samples are dried and passed through a 100-mesh sieve for standby, and are used to measure the physical and chemical properties of the soil under different fertilization treatments.

[0091] Comparison of key agronomic parameters of the nano-zinc oxide coupled biochar-based slow-release fertilizer in Example 1 and the fertilizers described in Comparative Examples 1-2 is as Figure 4 shown. The wheat plants in the sample plots treated with the nano-zinc oxide coupled biochar-based slow-release fertilizer have a higher number of grains per spike at the filling stage and the maturity stage, which are 44.8 and 45.4 grains respectively. In contrast, the wheat plants in the sample plots treated with ordinary compound fertilizer and slow-release compound fertilizer have fewer grains per spike. At the filling stage, they are 31.8 and 41.3 grains respectively, and at the maturity stage, they are 20.8 and 31.4 grains respectively. This difference is mainly attributed to the rapid dissolution and nutrient release of commercial compound fertilizers, resulting in low nutrient use efficiency. On the other hand, the nano-zinc oxide coupled biochar in the nano-zinc oxide coupled biochar-based slow-release fertilizer has a strong nutrient adsorption capacity, and the outer PVA / ST film can effectively delay nutrient release. This is particularly important during key growth periods such as flowering and filling, ensuring that wheat obtains sufficient nutrients at each growth stage. This stable nutrient supply promotes the growth of wheat, enhances the development of spikes, and ultimately increases the number of grains per spike.

[0092] Figure 5 The results showed that the grain weight per spike of the control group 1 and the example 1 group were significantly higher than those of the control group 2 during the filling stage and the maturity stage, with an increase of 70.6% - 87.1% and 20.8% - 30.3% respectively. This improvement was mainly attributed to the sustained and stable nutrient supply provided by the slow-release fertilizer, which minimized the fluctuations in nutrient release and ensured that wheat received sufficient nutrients during the critical growth stages, ultimately improving the photosynthesis efficiency and nutrient accumulation. After applying the nano-zinc oxide coupled biochar-based slow-release fertilizer, the 1000-grain weight of wheat was 31.1 g and 41.3 g during the filling stage and the maturity stage respectively. However, due to the insufficient nutrients in the original soil, the increase in 1000-grain weight brought about by the nano-zinc oxide coupled biochar-based slow-release fertilizer was relatively small.

[0093] The results showed that the wheat yields in the plots treated with ordinary compound fertilizer and slow-release compound fertilizer were 2095.7 kg / ha and 3691.9 kg / ha respectively, while the yield in the plot treated with the nano-zinc oxide coupled biochar-based slow-release fertilizer reached 3928.9 kg / ha, an increase of 87.5% and 6.4% compared with the control groups. This increase in yield was mainly attributed to the excellent nutrient slow-release performance of the nano-zinc oxide coupled biochar-based slow-release fertilizer, which ensured a stable nutrient supply and avoided the nutrient waste and unbalanced absorption caused by the rapid release of traditional fertilizers. This improvement in fertilizer efficiency contributed to the growth and yield of wheat, indicating that the addition of nano-zinc oxide coupled biochar could effectively improve the nutrient slow-release effect of slow-release fertilizers and increase the crop utilization rate.

[0094] 2) Effects of different fertilizers on improving soil properties

[0095] Figure 6 The contents of soil nitrogen, phosphorus, and potassium during the filling stage and the maturity stage of wheat were shown. The results indicated that the total nitrogen content in the soil treated with the nano-zinc oxide coupled biochar-based slow-release fertilizer was 5.6 mg / g during the filling stage, which was approximately 3.1% - 69.8% higher than that of other groups (blank group, ordinary compound fertilizer group, and slow-release compound fertilizer group). During the maturity stage, the total nitrogen content was 4.9 mg / g, still 7.7% - 19.5% higher than that of other groups. This was because the nano-zinc oxide coupled biochar-based slow-release fertilizer had excellent nitrogen slow-release performance, which enabled it to slowly release nitrogen during the early stage and filling stage of wheat growth, thus meeting the peak nitrogen demand of the crop. As wheat approached maturity, the nutrient absorption of the plant slowed down, and part of the nitrogen was transferred to the grains, resulting in a decrease in soil nitrogen content.

[0096] Regarding phosphorus, after applying slow-release compound fertilizer and nano-zinc oxide coupled biochar-based slow-release fertilizer, the total phosphorus content in the soil at the maturity stage decreased by 21.1% and 31.9% respectively compared to the filling stage, while the treatment with ordinary compound fertilizer increased by 10.6%. During the filling stage, plants actively absorb phosphorus, and most of the phosphorus in the soil exists in a soluble form, which is easily absorbed by plants. However, at the maturity stage, phosphorus is converted into a form that is difficult for plants to absorb, resulting in a decrease in the total phosphorus content in the soil. During the filling stage, the total potassium contents in the soil of the slow-release compound fertilizer and nano-zinc oxide coupled biochar-based slow-release fertilizer groups were 8.6 and 9.9 mg / g respectively, and 8.5 and 8.7 mg / g respectively at the maturity stage, which were significantly higher than those of ordinary commercial compound fertilizers. The nano-zinc oxide coupled biochar in the nano-zinc oxide coupled biochar-based slow-release fertilizer enhanced the ion exchange capacity of the soil, reduced the leaching loss of potassium, and thus improved the effective utilization of potassium in the soil. As wheat matured, most of the potassium was absorbed by plants and transferred to the grains, resulting in a decrease in the available potassium content in the soil.

[0097] As Figure 7 shown, after applying slow-release compound fertilizer and nano-zinc oxide coupled biochar-based slow-release fertilizer, the organic matter contents in the soil at the filling stage were 22.9 and 22.6 g / kg respectively, which were about 20.4%-25.4% higher than those of other control groups. At the maturity stage, the organic matter contents were 22.4 and 21.4 g / kg respectively, still 25% higher than those of other control groups. In addition, after applying these two slow-release fertilizers, the organic matter content in the soil at the maturity stage decreased. Biochar, as an organic carbon source, increased the soil organic matter content, while nano-zinc oxide coupled biochar improved nutrient fixation. During the filling stage, the root activity was strong, resulting in a relatively high organic matter mineralization rate, thus increasing the organic matter content. However, as wheat matured, plant growth slowed down, root activity weakened, and the organic matter mineralization rate slowed down, resulting in the stabilization of the organic matter content in the soil.

[0098] After applying nano-zinc oxide coupled biochar-based slow-release fertilizer, the cation exchange capacities (CEC values) of the soil at the filling stage and the maturity stage were 5.4 and 6.6 cmol + / kg respectively, while the CECs of the slow-release compound fertilizer treatment at the filling stage and the maturity stage were 6.5 and 6.0 cmol + / kg, significantly higher than ordinary commercial compound fertilizers. In addition, after applying the nano-zinc oxide coupled biochar-based slow-release fertilizer, the CEC at the maturity stage increased by about 19.7% compared to the filling stage. Biochar enhanced the cation retention capacity of the soil through its surface functional groups and pore structure, thus increasing the CEC. The persistent nutrient supply mechanism of the nano-zinc oxide coupled biochar-based slow-release fertilizer also contributed to regulating soil ion concentration and indirectly affecting the CEC. During the filling stage, due to the high crop growth demand, ion exchange activities were more intense, resulting in a lower CEC value. At the maturity stage, as root activities weakened, the CEC value gradually increased. Finally, regarding the change in soil pH, the pH values of all groups were similar at the filling stage, but compared with ordinary commercial compound fertilizers, the pH values of the slow-release compound fertilizer and nano-zinc oxide coupled biochar-based slow-release fertilizer groups increased at the maturity stage. This was due to the adsorption of acidic substances by zinc-modified biochar, the slow release of alkaline substances, and the long-term nutrient supply mechanism of the coated fertilizer, which could effectively regulate soil pH and slow down soil acidification.

[0099] In summary, the nano-zinc oxide coupled biochar-based slow-release fertilizer significantly improved the slow-release performance and nutrient utilization efficiency of the fertilizer by combining nano-zinc oxide modified biochar with compound fertilizer and adopting the polyvinyl alcohol / starch coating technology. This fertilizer can not only effectively control the release rate of nutrients such as nitrogen and phosphorus, extend the fertilizer efficiency period, but also has good water retention and biodegradability, can improve soil physical and chemical properties, and promote crop growth. The experimental results showed that this fertilizer significantly increased the number of grains per spike and the yield in wheat cultivation, and had a yield increase effect compared with traditional compound fertilizers and slow-release compound fertilizers. In addition, the preparation process of this fertilizer is simple, the raw materials are widely available, and the cost is low, meeting the concepts of green agriculture and sustainable development, and having broad market application prospects.

Claims

1. A nano zinc oxide coupled biochar-based slow-release fertilizer, characterized in that: The slow-release fertilizer is composed of a biochar-based compound fertilizer core and a polyvinyl alcohol / starch coating layer coated on the surface of the biochar-based compound fertilizer core; The biochar-based compound fertilizer core is a spherical particle formed by mixing and granulating biochar with nano zinc oxide loaded on the surface and compound fertilizer; The polyvinyl alcohol / starch coating layer is composed of polyvinyl alcohol, starch, glycerol, potassium persulfate and water. By mass percentage, the polyvinyl alcohol is 2% to 6%, corn starch is 3% to 7%, glycerol is 1%, potassium persulfate is 0.04% to 0.08%, and water is 89% to 95%.

2. The nano zinc oxide coupled biochar-based slow-release fertilizer according to claim 1, characterized in that: The mass percentage of zinc element in the biochar with nano zinc oxide loaded on the surface is between 2 and 3 wt%.

3. The nano zinc oxide coupled biochar-based slow-release fertilizer according to claim 1, characterized in that: The compound fertilizer is a nitrogen, phosphorus and potassium compound fertilizer with a total nutrient content of ≥45%.

4. A method for preparing the nano zinc oxide coupled biochar-based slow-release fertilizer according to any one of claims 1 to 3, characterized in that: The steps include: S1, immersing the biochar in a zinc chloride solution and continuously stirring to fully immerse it, and then adjusting the pH of the resulting solution to 10 with a NaOH solution, and continuously stirring for at least 24 hours, and after washing and drying the product, obtaining biochar with nano zinc oxide loaded on the surface; S2, mixing the biochar with nano zinc oxide loaded on the surface obtained in step S1 with compound fertilizer, and granulating the mixture with a granulator to obtain a biochar-based compound fertilizer core; S3, stirring and dissolving starch in water at 80-90°C, stirring and dissolving polyvinyl alcohol in water at 90-100°C, then mixing the two solutions, maintaining the temperature of the mixed solution at 60-85°C, adding glycerol and potassium persulfate solution thereto, and continuously stirring at 60-85°C for sufficient reaction, to obtain a polyvinyl alcohol / starch coating solution; S4. Put the biochar-based compound fertilizer core obtained in step S2 into a coating machine, and evenly spray the polyvinyl alcohol-starch coating liquid obtained in step S3 on the surface of the biochar-based compound fertilizer core to form a polyvinyl alcohol-starch coating liquid layer on the surface of the biochar-based compound fertilizer core, thereby obtaining the nano zinc oxide coupled biochar-based slow-release fertilizer.

5. The method for preparing nano zinc oxide coupled biochar-based slow-release fertilizer according to claim 4, characterized in that: In step S1, the biochar is prepared by pyrolysis of crop straw; the crop straw includes one or more of wheat straw, rice straw, and corn straw.

6. The method for preparing nano zinc oxide coupled biochar-based slow-release fertilizer according to claim 4, characterized in that: The concentration of the zinc chloride solution in step S1 is 0.01 mol / L, and the ratio of the mass of the added biochar to the volume of the zinc chloride solution is 1 g:16 mL.

7. The method for preparing nano zinc oxide coupled biochar-based slow-release fertilizer according to claim 4, characterized in that: The mass ratio of biochar to compound fertilizer in step S2 is 1:

3.

8. The method for preparing nano zinc oxide coupled biochar-based slow-release fertilizer according to claim 4, characterized in that: The concentration of the potassium persulfate solution in step S3 is 0.14 mol / L.

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