Alkaline charcoal coated slow-release fertilizer as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of fertilizers, and particularly relates to an alkaline biochar coated slow release fertilizer and a preparation method and application thereof, the preparation method comprises the following steps: S1, preparing modified biochar: modifying traditional Chinese medicine residues with calcium chloride, drying, and putting into a vacuum tube furnace to prepare the alkaline biochar; s2, preparation of fertilizer glue beads: dissolving a fertilizer in a sodium alginate solution, standing and defoaming, dropwise adding the mixed solution into a calcium chloride solution, and cross-linking to obtain the fertilizer glue beads; s3, coating and granulating: uniformly coating the modified biochar and fertilizer mixture on fertilizer glue beads by using a binder to obtain the fertilizer. The alkaline biochar coated slow-release fertilizer disclosed by the invention is high in utilization rate and lasting in fertilizer efficiency, and can relieve soil acidification, reduce cadmium pollution, reduce fertilization cost and environmental pollution caused by the fertilizer, effectively promote crop growth and improve crop yield and quality.

Description

Technical Field

[0001] The present invention belongs to the technical field of fertilizers, and particularly relates to an alkaline biochar-coated slow-release fertilizer, a preparation method thereof, and an application thereof. Background Art

[0002] Biochar contains a large amount of carbon and plant nutrients, and has characteristics such as a stable carbon framework structure, a developed pore structure, and a large specific surface area. In recent years, biochar has received increasing attention due to its multifunctionality, including aspects such as carbon sequestration, improving soil fertility, environmental remediation, and removing heavy metal ions in the environment. Because of its low cost and good effect, it is considered by many to be a substitute for activated carbon in the future. Existing studies have found that biochar can improve the acidic red soil environment and promote crop growth. Returning biochar to the field can also increase the content of soil organic carbon and improve the soil compaction condition. The coated slow-release fertilizer prepared with biochar as the coating material can improve the soil, such as reducing the soil bulk density and density, increasing the water holding capacity, improving the soil acidity and alkalinity, solidifying heavy metals in the soil, and can also retain nutrients through electrostatic attraction, adsorption, and forming chemical bonds.

[0003] However, the physical and chemical properties of traditional biochar have many defects and lack of pertinence. In order to improve the safety, efficiency, and repeatability of biochar, it is selected to be modified. Compared with the original biochar, the pore volume, specific surface area, types and amounts of surface functional groups, and physical and chemical properties of the modified biochar will all change. Numerous studies have shown that the modified biochar has a higher adsorption capacity and stronger ability to improve the physical and chemical properties of the soil. At present, the modification of biochar is still in the laboratory research stage, and the stability and environmental risks of the modified biochar are unknown. Therefore, developing a modified biochar with stable properties and environmental friendliness is a research hotspot in the future.

[0004] The information disclosed in this background art section is only intended to enhance the overall understanding of the present invention and should not be regarded as an admission or any form of implication that this information constitutes prior art already known to those of ordinary skill in the art. Summary of the Invention

[0005] The present invention aims to provide an alkali-modified biochar-coated slow-release fertilizer and a preparation method thereof. This fertilizer not only has a high fertilizer utilization rate and a long slow-release period, but also can effectively alleviate soil acidification, reduce cadmium pollution, and reduce the damage to the soil environment.

[0006] To achieve the above object, the technical solution of the present invention is as follows:

[0007] The first object of the present invention is to provide a preparation method of an alkaline biochar-coated slow-release fertilizer, including the following steps:

[0008] S1. Preparation of modified biochar: Prepare modified biochar from modified Chinese medicine residues.

[0009] S2. Preparation of fertilizer gel beads: Prepare fertilizer gel beads from sodium alginate and urea.

[0010] S3. Coating granulation: Mix the modified biochar, calcium chloride and fertilizer gel beads evenly, place them in a granulator, then add a starch binder, and pass through a 10-mesh sieve after uniform coating to obtain an alkaline biochar-coated slow-release fertilizer.

[0011] Preferably, in step S1, the preparation method of the modified Chinese medicine residues is as follows:

[0012] Dissolve 1 part by weight of calcium chloride in 40 parts by weight of deionized water and stir evenly, then add 10 parts by weight of Chinese medicine residues and continue to stir for 2 h until fully integrated. After that, place it in an electrothermal constant temperature forced-air drying oven at a temperature of 80 °C and dry for 72 h until completely dry to obtain the modified Chinese medicine residues.

[0013] The use of waste Chinese medicine residues to prepare biochar realizes the resource utilization of Chinese medicine residues, reduces the solid waste generated in the Chinese medicine processing process, and reduces environmental pollution. The rich organic matter and various bioactive components contained in Chinese medicine residues, such as flavonoids and phenols, are retained in the biochar, which may enhance the soil improvement and plant growth promotion effects of the biochar. At the same time, the unique pore structure and surface chemical properties are formed during the preparation of Chinese medicine residue biochar, improving its adsorption performance, which helps to remove pollutants and slow-release nutrients in the soil.

[0014] In addition, the biomass organic carbon and silicon components contained in the alkali-modified biochar itself can also play a role in loosening the soil and improving the soil structure, providing a suitable environment for the growth and reproduction of beneficial microorganisms. Especially during the preparation of the alkali-modified biochar, after a series of treatments, the flavonoids, phenols and other substances contained in the Chinese medicine residues have been converted into small molecules that are more easily absorbed by crops. After being absorbed by crops, they can improve the antioxidant effect of crops, protect crop cells from free radical damage, and also promote the growth of crop roots and leaves, and improve the nutrient absorption efficiency of crops.

[0015] Preferably, the Chinese medicine residue is one of licorice, poria or coptis.

[0016] Preferably, in step S1, after passing through a 2-mm sieve, the modified Chinese medicine residues are placed in a corundum ark, and then the corundum ark is placed in the middle of the quartz tube of a vacuum tube furnace. Under a nitrogen atmosphere, the temperature is raised to 400 °C at a heating rate of 5 °C / min and maintained at 400 °C for 3 h. After the heat preservation is completed, it is naturally cooled to room temperature under nitrogen protection to obtain the modified biochar.

[0017] Biochar modification enhances its adsorption capacity, increases specific surface area and pore volume, thereby improving the adsorption efficiency of pollutants, especially in the removal of heavy metals and organic pollutants. Alkali-modified biochar can improve soil pH, neutralize acidic soil, enhance soil fertility and water retention, and promote plant growth and nutrient uptake. It also helps to improve the availability of nutrients in the soil, enhance carbon sequestration capacity, combat global climate change, and improve soil biological activity and ecosystem health by promoting soil microbial activity.

[0018] Preferably, in step S2, 1 part by weight of sodium alginate and 10 parts by weight of urea are dissolved in 50 parts by weight of deionized water, stirred evenly, and left standing for 30 min to remove air bubbles. Then, a syringe with a pore size of 2 mm is used to suck the mixed solution and evenly drop it into 500 mL of a 2% calcium chloride solution by mass for crosslinking for 30 min to obtain transparent gel beads. After rinsing 2 - 3 times with deionized water and air-drying, the fertilizer gel beads are obtained.

[0019] Sodium alginate is a polysaccharide compound of marine organisms and has the ability to form a gel-like coating, which is beneficial to the slow penetration and diffusion of water and nutrients, achieving a slow-release effect, reducing nutrient loss, and improving fertilizer utilization efficiency. As a biodegradable natural polymer, its degradation products are harmless to the environment, do not cause pollution to the environment, can provide nutrients after degradation, improve the soil environment, stimulate plant growth, and enhance the stress resistance of crops.

[0020] Preferably, in step S3, the preparation method of the starch binder is as follows:

[0021] 60 g of cassava starch is dissolved in 480 mL of deionized water and stirred evenly. Then, 3 mL of H 2 O 2 and 6 mL of a 6% FeSO 4 solution are added in sequence, stirred for 45 min, then 3 mL of a 10% NaOH solution by mass is added, stirred in a water bath for 10 min, left standing for 5 min, and then 12 mL of a 1 mol / L Na 2 SO 3 solution is added, and stirred in a water bath for 10 min to obtain the starch binder.

[0022] The main raw material of the starch binder is starch. Starch is a natural renewable resource, with low cost and harmless to the environment during the degradation process.

[0023] Preferably, in step S3, the modified biochar, calcium chloride, and fertilizer gel beads are mixed evenly according to a weight ratio of 4:2:3.

[0024] The second object of the present invention is to provide an alkaline biochar-coated slow-release fertilizer prepared by using the preparation method described in any one of the above.

[0025] The third object of the present invention is to provide an application of an alkaline biochar-coated slow-release fertilizer in slow-release fertilizers and / or improving fertilizer utilization rate and / or alleviating soil acidification and / or reducing fertilizer environmental pollution.

[0026] Due to the adoption of the above technical solution, the beneficial effects of the present invention are as follows:

[0027] The present invention provides an alkaline biochar-coated slow-release fertilizer and its preparation method and application. The main components of the alkaline biochar slow-release fertilizer of the present invention are alkali-modified biochar and nitrogen fertilizers such as urea. Compared with the simple application of nitrogen fertilizers (chemical fertilizers), the practice of biochar slow-release fertilizers can achieve the gradual and slow release of nitrogen fertilizers. Crops can absorb and balance nutrients, improving the utilization rate of fertilizers. It can not only increase the yield and quality of crops, but also alleviate soil acidification, reduce the application of fertilizers, and avoid the impact of excessive use of chemical fertilizers on the soil environment. Description of the Drawings

[0028] Figure 1 It is a scanning electron microscope image of biochar;

[0029] Figure 2 It is a pH histogram of the hydrolysis desorption test;

[0030] Figure 3 It is a histogram of calcium ion concentration in the hydrolysis desorption test;

[0031] Figure 4 It is a histogram of the total nitrogen content in the hydrolysis desorption test;

[0032] Figure 5 It is a pH histogram of the soil culture test;

[0033] Figure 6 It is a histogram of the mass fraction of ammonium nitrogen in the soil culture test Figure 1 ;

[0034] Figure 7 It is a histogram of the mass fraction of ammonium nitrogen in the soil culture test Figure 2 ;

[0035] Figure 8 It is a histogram of the mass fraction of nitrate nitrogen in the soil culture test. Detailed Embodiments

[0036] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0037] Some raw materials in this application are introduced as follows:

[0038] Chinese medicine residue, from the Chinese medicine production base in Lingshan County, Guangxi Zhuang Autonomous Region;

[0039] Calcium chloride, analytical pure, purchased from Sinopharm Chemical Reagent Co., Ltd., product number: 10005861, molecular weight: 110.98;

[0040] Urea, analytical pure, purchased from Sinopharm Chemical Reagent Co., Ltd., product number: 10023218, molecular weight: 60.06;

[0041] Sodium alginate, chemically pure, purchased from Sinopharm Chemical Reagent Co., Ltd., product number: 30164428, molecular weight: 198.11;

[0042] Cassava starch, purchased from Nanjing Ganzhiyuan Sugar Industry Co., Ltd.

[0043] The present application is further illustrated by the following examples, but is not limited by these examples

[0044] Example 1

[0045] A preparation method of an alkaline biochar-coated slow-release fertilizer, comprising the following steps:

[0046] S1. Prepare modified biochar: After passing the modified Chinese medicine residue through a 2 mm sieve, place it in a corundum ark, and then place the corundum ark in the middle position of the quartz tube of a vacuum tube furnace. Under a nitrogen atmosphere, heat it to 400 °C at a heating rate of 5 °C / min, and keep it at 400 °C for 3 h. After the heat preservation is completed, naturally cool it to room temperature under nitrogen protection to obtain the modified biochar, and the product yield is 44.9%;

[0047] S2. Prepare fertilizer beads: Dissolve 1 part by weight of sodium alginate and 10 parts by weight of urea in 50 parts by weight of deionized water, stir evenly, and let it stand for 30 min to eliminate bubbles. Then use a syringe with a pore size of 2 mm to suck the mixed solution and evenly drop it into 500 mL of a 2% calcium chloride solution by mass for crosslinking for 30 min to obtain transparent beads. After rinsing 2-3 times with deionized water and air-drying, obtain the fertilizer beads;

[0048] S3. Coating granulation: Mix the modified biochar, calcium chloride and fertilizer beads evenly according to the weight ratio of 4:2:3, place them in a granulator, then add a starch binder, and after uniform coating, pass through a 10-mesh sieve to obtain the alkaline biochar-coated slow-release fertilizer.

[0049] The preparation method of the modified Chinese medicine residue is:

[0050] Dissolve 1 part by weight of calcium chloride in 40 parts by weight of deionized water and stir evenly. Then add 10 parts by weight of Chinese medicine residues and continue to stir for 2 hours to fully blend. After that, place it in an electrothermal constant temperature forced air drying oven at a temperature of 80 °C and dry for 72 hours until completely dry to obtain modified Chinese medicine residues; the Chinese medicine residues are one of licorice, poria cocos, or coptis chinensis.

[0051] The preparation method of the starch binder is as follows:

[0052] Dissolve 60 g of tapioca starch in 480 mL of deionized water and stir evenly. Then add 3 mL of H 2 O 2 and 6 mL of a 6% FeSO 4 solution, stir for 45 minutes, then add 3 mL of a 10% NaOH solution by mass percentage, stir in a water bath for 10 minutes and then let it stand for 5 minutes. Then add 12 mL of 1 mol / L Na 2 SO 3 solution, stir in a water bath for 10 minutes to obtain the starch binder.

[0053] Example 2

[0054] Characterization analysis of calcium-modified biochar: Send the sample to the Science Compass Guangzhou Laboratory. The structure and surface morphology of the soil conditioner will affect the function of the conditioner.

[0055] The scanning electron microscope image of the biochar is shown in Figure 1 .

[0056] It can be seen from Figure 1 that by analyzing the surface image of the biochar, it is found that the biochar has a small particle size, many and obvious pores, and a hollow shape inside. This indicates that the biochar has many adsorption sites and can adsorb more functional groups, thereby increasing the cation exchange capacity of the biochar.

[0057] The specific surface area and pore volume of the material play a very important role in the adsorption performance of the material. The pore structure parameters of the biochar are shown in Table 1.

[0058] Table 1 Pore structure parameters of biochar

[0059]

[0060] It can be seen from Table 1 that the specific surface area of the biochar is 3.722 cm 2 / g. The increase in the specific surface area represents that the biochar surface contains more adsorption sites, which is beneficial to the adsorption and solidification of heavy metals. The micropore volume in the biochar is 0.001 cm 2 / g, and the mesopore volume is 0.01 cm 2 / g. This biochar was prepared in an environment of high temperature and nitrogen gas passing through. The high temperature enables the organic matter in the biochar to be fully cracked, and the nitrogen gas brings out volatile organic compounds and ash, resulting in a significant increase in the micropore volume of the biochar. This is consistent with Figure 1 the results observed in the scanning electron microscope.

[0061] Example 3

[0062] Hydrolysis and desorption test of alkaline biochar-coated slow-release fertilizer

[0063] Specific test method:

[0064] Mix 1 g of alkaline biochar-coated slow-release fertilizer with 400 mL of deionized water and shake at room temperature (25 ± 0.5 °C). The experimental time is 48 h. Extracts are collected every 1, 2, 3, 4, 5, 12, 24, and 48 h, filtered, and 400 mL of deionized water is added. The concentrations of NH4 * and PO 4 3- are measured by an ultraviolet-visible spectrophotometer after centrifuging at 4000 rpm for 5 minutes in a centrifuge. Finally, samples of nitrogen and phosphorus removal from the alkaline biochar-coated slow-release fertilizer are obtained by filtration.

[0065] Water sample measurement indicators:

[0066] pH value: The filtered water sample is directly measured with a pH meter.

[0067] The measurement results are shown in Figure 2 .

[0068] As Figure 2 can be seen, the alkaline biochar-coated slow-release fertilizer exhibits obvious slow-release characteristics in the hydrolysis and desorption test. In the initial stage, the pH value rapidly rises to 11.19, then rapidly drops to 8.57 within the first 5 hours, and then the release rate slows down. By 48 hours, the pH value stabilizes at 7.38, indicating that the fertilizer can quickly neutralize acidic soil and maintain a stable release for a long time. During the whole process, the change range of the pH value gradually decreases, and the values of the standard deviation and standard error indicate good consistency of the experimental data, further confirming the stability and slow-release effect of the slow-release fertilizer.

[0069] Calcium ion concentration: Pipette 0 mL, 0.50 mL, 1.00 mL, 2.50 mL, 4.00 mL, 5.00 mL, and 7.50 mL of calcium standard solution into 50-mL volumetric flasks respectively. Add 5 mL of lanthanum solution and 2.5 mL of sodium solution to each flask, and make up the volume to the mark with water to obtain the calcium standard series solutions. Measure the absorption values of the calcium standard series solutions on an atomic absorption spectrophotometer (Model 6810), and list the regression equation or plot the calibration curve. Pipette 20 mL of the test solution accurately into a 50-mL volumetric flask, add 5.0 mL of lanthanum solution and 2.5 mL of sodium solution, make up the volume to the mark with water, and measure the absorption values of calcium ions in the test solution and the sample blank at the wavelength of 422.7 nm (calcium) on the atomic absorption spectrophotometer. Determine the calcium ion concentration in the test solution from the calibration curve.

[0070] The determination results are shown in Figure 3 .

[0071] It can be seen from Figure 3 that the alkaline biochar-coated slow-release fertilizer exhibits significant slow-release characteristics in the hydrolysis desorption test. The calcium ion concentration rises rapidly at 1 hour and 2 hours, then decreases significantly within 5 hours, and then remains relatively stable during the period from 12 hours to 48 hours, indicating that the fertilizer has the ability to control nutrient release, which helps to continuously provide calcium ions to alleviate soil acid stress and promote plant growth. At the same time, the consistency of the experimental data shows that the release behavior of the slow-release fertilizer is stable and reliable, which has a positive impact on soil health and plant nutrient supply.

[0072] Total water-soluble nitrogen content: Take 10 mL of urea stock solution and make up the volume to 1000 mL with deionized water to obtain the urea sample; Determination method: Pipette 0.00, 1.00, 2.00, 3.00, 4.00, 5.00, 6.00, 7.00, 8.00, 9.00, and 10.00 mL of urea sample solution into 25-mL colorimetric tubes respectively, dilute to 10.00 mL with deionized water, stopper the ground-glass part of the colorimetric tube, and place it in an autoclave (Model HVA-100). Turn on the autoclave, set the temperature to 124 °C, and the time to 30 min. Take out the colorimetric tube, cool it, add 1 mL of hydrochloric acid solution, and make up the volume to 25 mL with deionized water. Measure the absorbance at wavelengths of 220 nm and 275 nm, calculate the corrected absorbance and the corresponding urea content, and plot the standard curve. Pipette 10 mL of the test solution into a 25-mL colorimetric tube, add 5 mL of alkaline potassium persulfate solution. Stopper the ground-glass part of the colorimetric tube and place it in the autoclave. Turn on the autoclave, set the temperature to 124 °C, and the time to 30 min. Take out the colorimetric tube, cool it, add 1 mL of hydrochloric acid solution, and make up the volume to 25 mL with deionized water. Measure the absorbance at wavelengths of 220 nm and 275 nm respectively, and calculate the obtained corrected absorbance A, A = A 220 - 2A 275Substitute A into the standard curve to obtain the total nitrogen content.

[0073] The measurement results are shown in Figure 4 .

[0074] From Figure 4 it can be seen that the nitrogen release of the alkaline biochar-coated slow-release fertilizer shows a trend of first fast and then slow in the hydrolysis desorption test. At 1 hour and 2 hours, the total nitrogen concentration reaches the highest, indicating that the fertilizer has a relatively fast nitrogen release rate in the initial stage, which may be due to the rapid dissolution or rupture of the coating. After 3 hours, the total nitrogen concentration decreases significantly, which may be due to the protective effect of the coating beginning to appear, slowing down the nitrogen release rate. Then it rises again at 4 hours, but the overall trend is a gradual slowdown in release, showing a good slow-release effect. From 12 hours to 48 hours, the nitrogen concentration changes little, indicating that the fertilizer can maintain the relative stability of the nitrogen concentration during hydrolysis, which is beneficial to the stable growth of plants. Since the nitrogen concentration remains at a certain level for a long time, it shows that the fertilizer has good effectiveness and can continuously provide nitrogen for plants.

[0075] Example 4

[0076] Soil culture experiment of alkaline biochar-coated slow-release fertilizer

[0077] Specific test method:

[0078] The experiment is divided into nine treatment groups: ① applying 1% slow-release fertilizer; ② applying 3% slow-release fertilizer; ③ applying 1% calcium-modified biochar; ④ applying 3% calcium-modified biochar; ⑤ applying 1% urea; ⑥ applying 3% urea; ⑦ applying 1% calcium chloride; ⑧ applying 3% calcium chloride; ⑨ blank group. Each treatment has 3 replicates, for a total of 27 pots. 800 g of soil is added to each pot, and the addition amount of the modifier is 8 g / pot for 1% and 24 g / pot for 3%. Mix the soil in each pot with the additive and add water to 60% saturated water content. The experiment lasts for 40 days. Keep the soil water content at 60% saturated water content. Samples are taken on the 0th, 5th, 10th, 15th, 20th, 25th, 30th, 35th, and 40th days to measure the soil pH, nitrate nitrogen, and ammonium nitrogen.

[0079] Soil sample measurement indicators:

[0080] pH value: Pass the soil sample through a test sieve with a 2 mm aperture, weigh 10.0 g ± 0.1 g of the sample, place it in a 100 mL plastic bottle, and add 25 mL of water. After sealing the container, shake it with a shaker (ZWYR-D2403 type) for 5 minutes, then let it stand for 1 h - 3 h, and then measure it with a pH meter (testo 205 type).

[0081] The measurement results are shown in Figure 5 .

[0082] From Figure 5It can be seen that, compared with the blank group, the addition of the modifier can significantly increase the pH value of the soil. The pH change in the 3% treatment group is smaller than that in the 1% treatment group, and the overall value is more stable. Among them, the overall pH of the calcium chloride treatment group (1% and 3%) is the highest; the pH of the calcium-modified biochar treatment group shows an upward trend, and the effect on improving acidic soil is the most significant; the change trend of the alkaline biochar-coated slow-release fertilizer treatment group is similar to that of the urea treatment group, but the overall pH of the alkaline biochar-coated slow-release fertilizer treatment group is higher, and the effect on alleviating soil acid stress is better.

[0083] Ammonium nitrogen content: ① Phenol solution: 10 g of phenol + 0.1 g of nitroprusside, made up to 1 L in a volumetric flask and stored at 4 °C in a brown bottle; ② Sodium hypochlorite solution: 25 mL of sodium hypochlorite (10%) + 10 g of sodium hydroxide + 7.06 g of disodium hydrogen phosphate heptahydrate + 31.8 g of sodium phosphate dodecahydrate made up to 1 L in a volumetric flask and stored at 4 °C in a brown bottle (optional: 5 mL of sodium hypochlorite (10%) + 10 g of trisodium citrate + 4.4 g of sodium hydroxide, made up to 200 mL in a volumetric flask); ③ Masking agent: Mix equal volumes of a 400 g / L solution of potassium sodium tartrate and a 100 g / L solution of disodium EDTA. Add 0.5 mL to 1 mL of a 10 mol / L sodium hydroxide solution to every 100 mL of the mixed solution until the mixed solution is clear; ④ Extract with a 2 mol / L solution, 2 g of soil + 40 mL of potassium chloride, extract on a shaker (ZWYR-D2403 type) at 200 r / min, then centrifuge with a centrifuge (TD5 type) at 4000 rpm for 3 min, and take the supernatant; ⑤ Add 100 μL of the sample solution + 50 μL of the phenol solution + 50 μL of the sodium hypochlorite solution to a microplate, cover and react for 1 hour, add 50 μL of the masking agent, react for 1 hour, and measure at 630 nm on an enzyme-linked immunosorbent assay reader (A51119600C type). Substitute the absorbance into the standard curve to obtain the ammonium nitrogen concentration. Substitute the value of the absorbance into the standard curve to obtain the ammonium nitrogen concentration.

[0084] Drawing of the standard curve: Pipette 0.00, 2.00, 4.00, 6.00, 8.00, 10.00 mL of NH4 + -N standard solution into 50 mL volumetric flasks, add 10 mL of potassium chloride solution to each, and perform colorimetric determination according to the above steps. Take the corrected absorbance as the ordinate and the corresponding nitrate nitrogen concentration as the abscissa to draw the standard curve.

[0085] The calculation formula is as follows

[0086]

[0087] In the formula:

[0088] ω(N) — mass fraction of nitrate nitrogen (NO 3 -1 -N) in the soil, mg·kg -1 ;

[0089] ρ—the concentration of nitrogen in the color-developing solution obtained from the working curve, mg·kg -1 ;

[0090] V—the volume of the color-developing solution, mL;

[0091] ts—the partitioning multiple;

[0092] 10 -3 —the coefficient for converting mL to L;

[0093] m—the mass of the soil sample, g;

[0094] 1000—to convert to the content per kg of soil.

[0095] The measurement results are shown in Figures 6 - 7 .

[0096] From Figures 6 - 7 it can be seen that the mass fraction of nitrate nitrogen in the 3% treatment group is higher than that in the 1% treatment group. The changes in ammonium nitrogen in all treatment groups show a sharp downward trend. Relatively speaking, the alkaline biochar-coated slow-release fertilizer treatment group shows a relatively stable ammonium nitrogen release curve, indicating that the slow-release fertilizer can provide a stable nitrogen supply and help plants absorb nutrients stably in the long term. The calcium-modified biochar treatment group shows a relatively high ammonium nitrogen mass fraction at the initial stage of the experiment, which may be due to the alkaline properties of the modified biochar contributing to the release of nitrogen. The urea treatment group shows a relatively high ammonium nitrogen mass fraction at the initial stage of the experiment and then drops rapidly, which may be due to the rapid decomposition of urea in the soil and the release of ammonium nitrogen, but this rapid release may lead to the volatilization of nitrogen and reduce the effectiveness of nitrogen. The calcium chloride treatment group may have a relatively small impact on the ammonium nitrogen mass fraction because calcium chloride mainly affects the soil environment by increasing the soil pH value rather than directly providing nitrogen.

[0097] Mass fraction of nitrate nitrogen: Extracted with 2 mol / L solution, 2 g of soil + 40 mL of potassium chloride, shaken on a shaker (ZWYR-D2403 type) at 200 r / min, then centrifuged for 3 min with a centrifuge (TD5 type), and the supernatant was taken; 200 μL of the sample solution was taken into a microplate, and measured on an enzyme-linked immunosorbent assay instrument (A51119600C type) at 230 nm and 275 nm respectively. The difference in absorbance was brought into the standard curve to obtain the nitrate nitrogen concentration.

[0098] Drawing of the standard curve: Respectively take 10 μg / mL NO 3——For the N standard solutions of 0, 0.5, 1, 2, 5, 10, and 15 mL, place them in a 50 mL volumetric flask, dilute to the mark with distilled water, and use a 1 cm quartz cuvette to measure the absorbance at 220 nm and 275 nm respectively. Take the corrected absorbance as the ordinate and the corresponding nitrate nitrogen concentration as the abscissa to plot the standard curve. The calculation formula is as follows:

[0099] A = A 220 - 2.23×A 275

[0100]

[0101] In the formula:

[0102] A—Corrected absorbance;

[0103] A 220 —Absorbance at 220 nm wavelength;

[0104] A 275 —Absorbance at 275 nm wavelength;

[0105] 2.23—f value, an empirical correction factor obtained from laboratory measurements of various types of soil;

[0106] ω(N)—Mass fraction of ammonium nitrogen (NH 4 + -N) in the soil, mg·kg -1 ;

[0107] ρ—Concentration of nitrogen in the color-developed solution obtained from the working curve, mg·kg -1 ;

[0108] V—Volume of the color-developed solution, mL;

[0109] ts—Partition multiple;

[0110] 10 -3 —Coefficient for converting mL to L;

[0111] m—Mass of the soil sample, g;

[0112] 1000—Conversion to the content per kg of soil.

[0113] The measurement results are shown in Figure 8 .

[0114] From Figure 8It can be seen that the mass fraction of nitrate nitrogen in each treatment group shows a fluctuating upward trend, which may be related to the conversion of ammonium nitrogen to nitrate nitrogen and the leaching loss of nitrate nitrogen. The mass fraction of nitrate nitrogen in the alkaline biochar-coated slow-release fertilizer treatment group is significantly higher than that in other treatment groups, but the pH value is not lower than that in other treatment groups. This may be because the nitrate nitrogen in other treatment groups is easily leached, while the nitrate nitrogen in the alkaline biochar slow-release fertilizer treatment group is not easily leached due to the characteristics of the slow-release fertilizer. At the same time, the alkaline substances in the alkali-modified biochar maintain the pH at a relatively high level through cationic interactions, effectively alleviating soil acidification and nitrogen loss.

[0115] Overall, the alkaline biochar-coated slow-release fertilizer combines the comprehensive advantages of calcium chloride, urea, and alkaline biochar. While alleviating soil acidification, the alkaline biochar-coated slow-release fertilizer can more effectively reduce nitrogen volatilization and leaching, thereby improving nitrogen use efficiency and reducing potential environmental impacts. This helps protect groundwater and prevent water eutrophication, showing its environmentally friendly characteristics.

[0116] The above description is a detailed description of the preferred feasible embodiments of the present invention, but the embodiments are not intended to limit the scope of the patent application of the present invention. Any equivalent changes or modifications made under the technical spirit prompted by the present invention shall fall within the scope of the patent covered by the present invention.

Claims

1. A method for preparing an alkaline biochar coated slow-release fertilizer, characterized in that: The steps include: S1, preparing modified biochar; preparing modified Chinese medicine residue into modified biochar; S2, preparing fertilizer colloid beads; preparing fertilizer colloid beads from sodium alginate and urea; S3, coating and granulation: the modified biochar, calcium chloride and fertilizer beads are mixed evenly and placed in a granulator, and then starch binder is added, and after uniform coating, the mixture is passed through a 10-mesh sieve to obtain an alkaline biochar coated slow-release fertilizer.

2. The method for preparing an alkaline biochar coated slow-release fertilizer according to claim 1, characterized in that: In step S1, the preparation method of the modified Chinese medicinal residue is: 1 part by weight of calcium chloride is dissolved in 40 parts by weight of deionized water and stirred evenly, and then 10 parts by weight of Chinese medicinal residues are added and stirred for 2 hours to fully blend. After the mixture is dried in an electric constant temperature blast drying oven at 80° C. for 72 hours until completely dried, thereby obtaining the modified Chinese medicinal residues.

3. The method for preparing an alkaline biochar coated slow-release fertilizer according to claim 2, characterized in that: The Chinese medicinal residue is one of liquorice, tuckahoe or coptis root.

4. The method for preparing an alkaline biochar coated slow-release fertilizer according to claim 1, characterized in that: In step S1, the preparation method of the modified biochar is: The modified Chinese medicine residue is passed through a 2 mm sieve and placed in a corundum ark, which is then placed in the middle of a quartz tube of a vacuum tube furnace. In a nitrogen environment, the temperature is raised to 400°C at a heating rate of 5°C / min and maintained at 400°C for 3 hours. After the insulation is completed, the residue is naturally cooled to room temperature under nitrogen protection to obtain the modified biochar.

5. The method for preparing an alkaline biochar coated slow-release fertilizer according to claim 1, characterized in that: In step S2, the method for preparing the fertilizer rubber beads is as follows: Dissolve 1 part by weight of sodium alginate and 10 parts by weight of urea in 50 parts by weight of deionized water, stir evenly and let stand for 30 minutes to eliminate bubbles, then use a syringe with a pore size of 2 mm to absorb the mixture, evenly drop it into 500 mL of 2% by weight calcium chloride solution and cross-link for 30 minutes to obtain transparent rubber beads, rinse with deionized water 2-3 times and dry in the shade to obtain the fertilizer rubber beads.

6. The method for preparing an alkaline biochar coated slow-release fertilizer according to claim 1, characterized in that: In step S3, the starch binder preparation method is: 60g of cassava starch was dissolved in 480mL of deionized water and stirred evenly, 3mL of H2O2 and 6mL of 6% by mass FeSO4 solution were added in sequence, stirred for 45min, 3mL of 10% by mass NaOH solution was added, stirred in a water bath for 10min and then allowed to stand for 5min, then 12mL of 1mol / L Na2SO3 solution was added, stirred in a water bath for 10min, and the starch binder was obtained.

7. The method for preparing an alkaline biochar coated slow-release fertilizer according to claim 1, characterized in that: In step S3, the modified biochar, calcium chloride and fertilizer beads are evenly mixed in a weight ratio of 4:2:

3.

8. An alkaline biochar coated slow-release fertilizer, characterized in that: The preparation method is described in any one of claims 1 to 7.

9. Use of an alkaline biochar coated slow-release fertilizer according to claim 8 for slow-release fertilizer and / or improving fertilizer utilization and / or alleviating soil acidification and / or reducing fertilizer environmental pollution.

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