A bifunctional biochar for acidic soils, its preparation method and application

Abstract

This invention provides a bifunctional biochar for acidic soils, its preparation method, and its application. The preparation method includes: placing biochar raw material and a boron-containing compound in an acidic solution with a pH of 3.5-4.5, and heating at 120-180°C for 4-12 hours to obtain a solid product. The biochar raw material is a precursor containing tannin-based polyphenols. The solid product is washed, dried, and then placed in an inert gas atmosphere and pyrolyzed at 480-520°C for 1-2 hours to obtain the bifunctional biochar for acidic soils. The biochar in this application can simultaneously address the problems of active aluminum toxicity and available boron deficiency in acidic soils.

Description

Technical Field

[0001] This application relates to the field of biochar technology, and in particular to a bifunctional biochar for acidic soils, its preparation method, and its application. Background Technology

[0002] Acidic soils are widely distributed in southern my country. These soils commonly exhibit two key limiting factors: reactive aluminum toxicity and available boron deficiency. Reactive aluminum inhibits root cell division and elongation, severely damaging the root system and affecting the absorption of water and all nutrients; while boron is easily leached under acidic conditions, and its deficiency directly impacts crop reproductive growth and yield formation. Currently, there is still a lack of solutions that can simultaneously address both reactive aluminum toxicity and available boron deficiency in acidic soils. Summary of the Invention

[0003] In view of this, the purpose of this application is to propose a bifunctional biochar for acidic soils, its preparation method and application, in order to solve the current problems of active aluminum toxicity and available boron deficiency in acidic soils.

[0004] To achieve the above objectives, this application provides a method for preparing bifunctional biochar for acidic soils, comprising: Biochar raw material and boron-containing compound are placed in an acidic solution with pH 3.5-4.5 and heated at 120-180℃ for 4-12 hours to obtain a solid product, wherein the biochar raw material is a precursor raw material containing tannin-type polyphenols. The solid product is washed, dried, and then placed in an inert gas atmosphere and pyrolyzed at 480-520°C for 1-2 hours to obtain bifunctional biochar for acidic soils.

[0005] Optionally, the mass ratio of the biochar raw material to the boron-containing compound is 1:(10-30).

[0006] Optionally, the solid-liquid ratio of the biochar raw material to the acidic solution is 1:(3-8)g / mL.

[0007] Optionally, the acidic solution includes one or more of the following: citric acid-sodium citrate buffer solution, tartaric acid solution, and potassium dihydrogen phosphate solution.

[0008] Optionally, the boron-containing compound is boric acid or sodium tetraborate.

[0009] Optionally, the precursor raw material containing tannin-type polyphenols includes one or more of tea residue, coffee grounds, and pomegranate peel.

[0010] Optionally, the heating at 120-180°C for 4-12 hours to obtain a solid product includes: Heating at 140-160℃ for 6-10 hours yields a solid product.

[0011] Optionally, the solid product is washed, dried, placed in an inert gas atmosphere, and pyrolyzed at 480-520°C for 1-2 hours to obtain bifunctional biochar for acidic soils, comprising: The solid product is washed, dried, and then placed in an inert gas atmosphere and pyrolyzed at 490-510°C for 1.2-1.8 hours to obtain bifunctional biochar for acidic soils.

[0012] Based on the same inventive concept, this disclosure also provides a bifunctional biochar for acidic soils, prepared by any of the above preparation methods.

[0013] Optionally, a bifunctional biochar for acidic soils is applied to acidic soils where aluminum toxicity exists and available boron is deficient.

[0014] As can be seen from the above, the present application provides a method for preparing bifunctional biochar for acidic soils, comprising: placing biochar raw material and a boron-containing compound in an acidic solution with a pH of 3.5-4.5, and heating at a temperature of 120-180℃ for 4-12 hours to obtain a solid product, wherein the biochar raw material is a precursor raw material containing tannin-type polyphenols; washing and drying the solid product, placing it in an inert gas, and pyrolyzing it at a temperature of 480-520℃ for 1-2 hours to obtain bifunctional biochar for acidic soils. The biochar raw material in this application is rich in tannin polyphenols. The acidic environment of pH 3.5-4.5 can gently activate the tannin polyphenols in the biochar raw material, hydrolyze and expose a large number of phenolic hydroxyl groups, which serve as active sites for subsequent specific complexation of aluminum ions to alleviate aluminum toxicity in the soil. At the same time, the acidic environment can catalyze the hydrolysis of boron-containing compounds to generate boric acid, and promote the esterification reaction between boric acid and the polysaccharides, cellulose and the above-mentioned phenolic hydroxyl groups dissolved in the raw material to form a boric acid ester bond with high thermal stability, thereby achieving chemical pre-fixation of boron and solving the problem of easy leaching of boron in acidic soil. After hydrothermal treatment with the above-mentioned acidic solution, a solid product is obtained from the biochar raw material. The surface of the solid product is loaded with phenolic hydroxyl groups and borate ester bonds. Pyrolysis of the solid product at a temperature of 480-520℃ can balance the development of the carbon skeleton and the retention of functional groups. This ensures that the biochar forms a porous structure with a high specific surface area, while maximizing the retention of phenolic hydroxyl groups and borate ester bonds. The resulting biochar has the dual functions of aluminum passivation and boron slow release, avoiding the limitations of single-function materials. When applied to acidic soils, it can simultaneously solve the problems of aluminum toxicity and boron deficiency. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in this application or related technologies, the drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, the drawings described below are only embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 The flowchart illustrates a method for preparing bifunctional biochar according to an embodiment of this application. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with specific embodiments and the accompanying drawings.

[0018] It should be noted that, unless otherwise defined, the technical or scientific terms used in the embodiments of this application should have the ordinary meaning understood by one of ordinary skill in the art to which this application pertains. The terms "first," "second," and similar terms used in the embodiments of this application do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed after the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are only used to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0019] As mentioned in the background, acidic soils are widely distributed in southern my country, covering more than 40% of the country's arable land, and are the main growing medium for food and cash crops in this region. However, due to long-term leaching from hot and rainy climates and improper fertilization, these soils generally suffer from two key limiting factors that restrict agricultural production: active aluminum (Al₂O₃). 3+The synergistic effects of aluminum toxicity and available boron deficiency severely restrict crop yield and quality improvement. On the one hand, under acidic conditions (especially when pH < 5.5), a large amount of aluminum adsorbed in soil colloids will desorb and transform into free active aluminum, which has extremely strong biotoxicity. It can penetrate the cell walls of crop roots, directly inhibiting root cell division and elongation, leading to root tip necrosis, sparse root hairs, and damage to the integrity of the root system. Taking rapeseed as an example, high concentrations of active aluminum in acidic red soil can shorten root length by 30%-50% and significantly reduce root surface area, thereby severely hindering the crop's absorption of water and various nutrients such as nitrogen, phosphorus, and potassium from the soil, ultimately resulting in weak crop growth and reduced stress resistance. On the other hand, boron, an essential micronutrient for crop growth and development, mainly exists in the form of boric acid in acidic soils. However, the soil's ability to adsorb boric acid is extremely weak in acidic environments. In addition, with abundant rainfall in southern regions, boric acid is easily leached away by rainwater or irrigation water, resulting in the soil's available boron content generally being lower than the critical value required by crops. Boron deficiency directly affects crop pollen germination, pollen tube elongation, and cell wall pectin synthesis. For boron-sensitive crops such as rapeseed and citrus, boron deficiency can easily lead to problems such as flowering without fruiting and deformed fruit, resulting in yield loss.

[0020] To address the aforementioned problems, existing technologies have consistently failed to achieve a synergistic solution: Traditional methods, while applying lime can raise soil pH to passivate active aluminum, exacerbate the binding and fixation of boron with soil minerals, further worsening boron deficiency and creating a contradiction of aluminum suppression and boron depletion; applying soluble boron fertilizer alone results in rapid leaching and low utilization (boron leaching rates can reach over 50% in acidic soils), failing to provide long-term boron supply and offering no mitigation of aluminum toxicity; recent advancements in biochar remediation technologies focus on single functions, such as enhancing aluminum adsorption through strong alkali modification or simply mixing boron fertilizers to achieve boron loading, often falling into the dilemma of solving boron deficiency exacerbating aluminum toxicity, or mitigating aluminum toxicity exacerbating boron deficiency. Therefore, existing technologies still lack an integrated solution that can simultaneously address the root causes of active aluminum toxicity and available boron deficiency in acidic soils. To solve these problems, this application provides a bifunctional biochar for acidic soils, its preparation method, and its application.

[0021] The following is combined with the appendix Figure 1 The embodiments of this application are described in detail.

[0022] like Figure 1 As shown, a method for preparing bifunctional biochar for acidic soils includes: S100: Place biochar raw material and boron-containing compound in an acidic solution with pH 3.5-4.5 and heat at 120-180℃ for 4-12 hours to obtain a solid product, wherein the biochar raw material is a precursor raw material containing tannin-type polyphenols; S200: The solid product is washed, dried, and then placed in an inert gas and pyrolyzed at 480-520℃ for 1-2 hours to obtain bifunctional biochar for acidic soils.

[0023] In this embodiment, the biochar raw material is rich in tannin polyphenols. The acidic environment of pH 3.5-4.5 can gently activate the tannin polyphenols in the biochar raw material, hydrolyze and expose a large number of phenolic hydroxyl groups, which serve as active sites for subsequent specific complexation of aluminum ions to alleviate aluminum toxicity in the soil. At the same time, the acidic environment can also catalyze the hydrolysis of boron-containing compounds to generate boric acid, and promote the esterification reaction between boric acid and the polysaccharides, cellulose and the above-mentioned phenolic hydroxyl groups dissolved in the raw material to form a boric acid ester bond with high thermal stability, thereby achieving chemical pre-fixation of boron and solving the problem of easy leaching of boron in acidic soil. After hydrothermal treatment with the above-mentioned acidic solution, a solid product is obtained from the biochar raw material. The surface of the solid product is loaded with phenolic hydroxyl groups and borate ester bonds. Pyrolysis of the solid product at a temperature of 480-520℃ can balance the development of the carbon skeleton and the retention of functional groups. This ensures that the biochar forms a porous structure with a high specific surface area, while maximizing the retention of phenolic hydroxyl groups and borate ester bonds. The resulting biochar has the dual functions of aluminum passivation and boron slow release, avoiding the limitations of single-function materials. When applied to acidic soils, it can simultaneously solve the problems of aluminum toxicity and boron deficiency.

[0024] In some embodiments, the mass ratio of the biochar feedstock to the boron-containing compound is 1:(10-30).

[0025] In this embodiment, the mass ratio of the biochar raw material to the boron-containing compound is 1:(10-30). This mass ratio ensures a sufficient supply of boron-containing compound, guaranteeing that it can fully react with the active groups in the biochar raw material during the hydrothermal pretreatment stage to form a sufficient amount of boron ester structure to meet the subsequent boron slow-release requirements; it also avoids waste and agglomeration problems caused by excessive boron-containing compound. Simultaneously, the reasonable ratio allows boron to form uniformly distributed functional sites with the tannins and polyphenols in the raw material, ensuring that aluminum passivation and boron slow-release functions work synergistically on the biochar framework, improving the improvement effect on acidic soils.

[0026] In some embodiments, the solid-liquid ratio of the biochar raw material to the acidic solution is 1:(3-8)g / mL.

[0027] In this embodiment, the solid-liquid ratio of the biochar raw material to the acidic solution is 1:(3-8) g / mL. This solid-liquid ratio provides a suitable reaction environment for the hydrothermal pretreatment reaction. If the solid-liquid ratio is too low, the biochar raw material and boron-containing compounds will not be able to fully contact, and the activation of phenolic hydroxyl groups and the formation of boron esters will be incomplete. If the solid-liquid ratio is too high, the reaction efficiency will be reduced, and energy consumption and subsequent processing costs will be increased. At this solid-liquid ratio, the biochar raw material is fully dispersed in the acidic solution, so that the protonation and catalytic effects of the acidic medium act uniformly on the surface of the raw material, ensuring that the biochar raw material can be fully activated and loaded with boron.

[0028] In some embodiments, the acidic solution includes one or more of the following: citric acid-sodium citrate buffer solution, tartaric acid solution, and potassium dihydrogen phosphate solution.

[0029] In this embodiment, the citric acid-sodium citrate buffer solution, tartaric acid solution, and potassium dihydrogen phosphate solution provide a mild and stable acidic environment. This environment satisfies the reaction requirements for activating tannin polyphenols and catalyzing the formation of boron esters during the hydrothermal pretreatment stage, without causing excessive damage to the structure of the biochar raw materials due to excessive acidity. Specifically, the citric acid-sodium citrate buffer solution effectively maintains pH stability during the reaction, preventing pH fluctuations from affecting the formation efficiency of functional groups. The tartaric acid solution and potassium dihydrogen phosphate solution help enhance the binding stability of boron with the raw materials, further improving the slow-release performance of boron and its aluminum complexing ability in the biochar.

[0030] In some embodiments, the boron-containing compound is boric acid or sodium tetraborate.

[0031] In this embodiment, both boric acid and sodium tetraborate exhibit good reactivity, efficiently hydrolyzing in acidic solutions to generate boric acid molecules. These molecules then undergo esterification with phenolic hydroxyl groups and polysaccharide groups in the biochar raw material, forming stable borate ester bonds and achieving chemical fixation of boron. Compared to other boron-containing compounds, these two have advantages such as wide availability, low cost, and mild reaction. Furthermore, the resulting borate ester structure exhibits strong stability during pyrolysis, is not easily decomposed or lost, and can slowly release available boron through hydrolysis in acidic soil environments. Simultaneously, these boron-containing compounds leave no toxic residues and will not adversely affect soil ecology or crop growth, meeting the safety requirements for agricultural environmental materials.

[0032] In some embodiments, the precursor raw material containing tannin-type polyphenols includes one or more of tea residue, coffee grounds, and pomegranate peel.

[0033] In this embodiment, the precursor raw materials containing tannin-type polyphenols include one or more of tea residue, coffee grounds, and pomegranate peel. These raw materials are naturally rich in tannin-type polyphenols, providing abundant phenolic hydroxyl precursors for biochar without the need for additional chemical modifiers. This reduces preparation costs and achieves resource utilization of agricultural waste, aligning with green and environmentally friendly principles. The tannin-type polyphenols in tea residue, coffee grounds, and pomegranate peel have stable structures and are easily activated and hydrolyzed during acidic hydrothermal pretreatment, exposing a large number of phenolic hydroxyl groups. Furthermore, the raw materials themselves possess a certain degree of porous structure, which, after pyrolysis, forms a biochar framework with a larger specific surface area and stronger adsorption capacity. This provides ample spatial sites for aluminum ion complexation and boron slow release, significantly enhancing the dual-functionality of the biochar.

[0034] In some embodiments, the heating at 120-180°C for 4-12 hours to obtain a solid product includes: Heating at 140-160℃ for 6-10 hours yields a solid product.

[0035] In this embodiment, the optimal combination of temperature (140-160℃) and reaction time (6-10h) for the hydrothermal pretreatment step accelerates the activation of tannin-based polyphenols and promotes rapid exposure of phenolic hydroxyl groups. Compared to the upper limit, this lower temperature avoids the decomposition and loss of effective components in the raw materials due to high temperatures. Simultaneously, the 6-10h reaction time ensures that the boron-containing compounds fully undergo esterification with the activated raw materials, forming structurally stable boron esters. This reduces the residue of unreacted functional groups, resulting in a more uniform distribution of functional sites and more stable performance of the biochar obtained from subsequent pyrolysis, further enhancing the synergistic effect of aluminum passivation and boron slow release.

[0036] In some embodiments, the solid product is washed, dried, placed in an inert gas atmosphere, and pyrolyzed at 480-520°C for 1-2 hours to obtain bifunctional biochar for acidic soils, comprising: The solid product is washed, dried, and then placed in an inert gas atmosphere and pyrolyzed at 490-510°C for 1.2-1.8 hours to obtain bifunctional biochar for acidic soils.

[0037] In this embodiment, the pyrolysis temperature of 490-510℃ can more fully promote the carbonization of the precursor, forming a carbon skeleton with a well-developed porous structure and a larger specific surface area, providing a stable support for functional sites; and it can effectively prevent the loss of phenolic hydroxyl groups and borate ester bonds due to high-temperature decomposition, maximizing the preservation of bifunctional active sites. The holding time of 1.2-1.8h ensures that the carbonization process is thorough and gentle, reducing the collapse of the biochar structure and the destruction of functional groups. The inert gas environment can prevent the biochar from being oxidized during pyrolysis, ensuring the structural integrity and functional stability of the biochar. The final biochar can more efficiently exert the synergistic effect of aluminum passivation and boron slow release in acidic soils.

[0038] Based on the same inventive concept, this disclosure also provides a bifunctional biochar for acidic soils, which is prepared by the preparation method described in any of the above embodiments and has corresponding beneficial effects, which will not be repeated here.

[0039] A bifunctional biochar for acidic soils has been developed for application in soils with both aluminum toxicity and available boron deficiency. After application, the phenolic hydroxyl groups on the biochar surface rapidly complex free active aluminum, reducing aluminum damage to crop roots, protecting root structure integrity, and promoting root absorption of water and nutrients. Simultaneously, the borate ester structure slowly hydrolyzes in the acidic environment, releasing available boron and continuously supplementing the crop with boron nutrition, improving reproductive growth, and increasing yield and quality. Furthermore, this biochar application does not require additional amendments, avoiding the contradiction of aluminum suppression and boron consumption in traditional methods, and solving the problem of limited effectiveness of single-function materials. It addresses the dual obstacles of acidic soils at their root, providing an efficient and convenient improvement solution for agricultural production in acidic soil regions, and contributing to sustainable agricultural development.

[0040] The above embodiments are described below with reference to specific examples and comparative examples.

[0041] The raw materials and pharmaceuticals used in this embodiment and comparative example are all commercially available products.

[0042] Example 1 Methods for preparing biochar include: (1) Take 100g of dried and pulverized tea residue (passed through a 40-mesh sieve) and 5g of boric acid, add them together to 500mL of citric acid-sodium citrate buffer solution with pH=4.0, stir evenly, and then transfer to a 1L hydrothermal reactor lined with polytetrafluoroethylene. Place the reactor in an oven and react at 150℃ for 8 hours; (2) After the reaction is completed, the mixture in the reactor is cooled naturally and filtered to obtain a solid product. The solid product is washed with deionized water until the filtrate is close to neutral. Then it is placed in an oven at 80°C and dried to constant weight to obtain a dark brown block precursor. (3) The dried precursor was ground and placed in a quartz boat in a tubular furnace, and high-purity nitrogen (flow rate 200 mL / min) was introduced as a protective gas. The temperature was programmed to rise to 500 °C at a rate of 8 °C / min and held at that temperature for 1.5 hours. After pyrolysis, the product was cooled to room temperature under continuous nitrogen purging, and then ground through a 100-mesh sieve to obtain biochar, denoted as BC-Syn.

[0043] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that boric acid is not added in step (1), while the remaining steps are the same as in Example 1, and the resulting biochar is denoted as BC-Al.

[0044] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the citric acid-sodium citrate buffer solution with pH=4.0 was replaced with deionized water with pH=7.0. All other steps were the same as in Example 1, and the resulting biochar was designated as BC-B.

[0045] Comparative Example 3 Biochar 1 was prepared according to the preparation method of Comparative Example 1; Ordinary boron-carbon with only boron loading function was prepared by commercial impregnation method and is referred to as biochar 2. Biochar 1 and biochar 2 are physically mixed uniformly at a mass ratio of 1:1 to obtain biochar, which is denoted as BC-Al+B-Mix.

[0046] The biochar prepared in Example 1 and Comparative Examples 1-2 were characterized, and the results are as follows: In Fourier transform infrared spectroscopy (FT-IR) analysis, the infrared spectrum of BC-Syn exhibits clear characteristic functional group signals: at approximately 3400 cm⁻¹ -1 A broad and strong OH stretching vibration peak appears at approximately 1720 cm⁻¹, corresponding to the phenolic hydroxyl structure; -1 A weak absorption peak was observed at the carboxyl group C=O; more importantly, a weak absorption peak was observed at approximately 1320 cm⁻¹. -1 The presence of a distinct BOC stretching vibration characteristic peak at 1320 cm⁻¹ clearly indicates the existence of a boronic ester structure, directly proving that both phenolic hydroxyl groups and boronic esters, two core functional groups, coexist on the carbon skeleton of BC-Syn. In contrast, BC-Al, which only possesses aluminum passivation functionality, shows a peak at 1320 cm⁻¹. -1 No characteristic peak of BOC was observed at this wavenumber, while the characteristic peak intensity of BC-B loaded with boron alone at this wavenumber was much lower than that of BC-Syn, and its 3400 cm⁻¹ peak intensity was also significantly lower. -1 The absorption peak of the phenolic hydroxyl group at that location was also significantly weaker.

[0047] X-ray photoelectron spectroscopy (XPS) analysis further verified the existence of boron. The B 1s spectrum of BC-Syn showed a characteristic binding energy peak at about 191.8 eV, which was attributed to the BOC chemical bond. This confirmed that boron does not exist in a physically adsorbed form, but rather forms a stable chemical bond with the carbon skeleton of biochar through boron ester bonds.

[0048] Boehm titration results showed that the phenolic hydroxyl content on the surface of BC-Syn was as high as 1.92 mmol / g, which was significantly higher than that of BC-B (0.71 mmol / g). This indicates that the preparation method in Example 1 can more efficiently activate tannin polyphenols in biochar raw materials and retain more active sites for complexing aluminum ions.

[0049] In summary, the series of material characterization results fully confirm that, through the synergistic preparation process of acidic hydrothermal pretreatment and precise temperature-controlled pyrolysis, Example 1 successfully constructed a coexisting and stable bifunctional structure of phenolic hydroxyl groups and borate esters on a single carbon skeleton of biochar, and the preparation of the target product BC-Syn achieved the expected design effect.

[0050] To systematically verify the practical application effect of the biochar prepared in Example 1 and Comparative Examples 1-3, a pot experiment was conducted in acidic soil. The specific experimental characterization process is as follows: 1. Test soil Typical acidic red soil samples were collected from southern my country, air-dried, and then sieved through a 2mm sieve to remove impurities such as stones and plant debris. The basic properties were determined using conventional soil physicochemical analysis methods: pH value 4.7, exchangeable aluminum content 1.85 cmol / kg, and available boron content 0.22 mg / kg.

[0051] 2. Experimental Design Rapeseed, which is sensitive to aluminum toxicity, was selected as the indicator crop. A total of six treatments were set up: ① Blank control (CK): no biochar was added; ② BC-Al treatment: biochar with only aluminum passivation function prepared in Comparative Example 1 was added; ③ BC-B treatment: biochar with only boron loading function prepared in Comparative Example 2 was added; ④ BC-Al+B-Mix treatment: a physical mixture of aluminum passivated biochar and ordinary boron char prepared in Comparative Example 3 was added; ⑤ BC-Syn treatment: bifunctional synergistic biochar prepared in Example 1 was added. The application rate of all biochar materials was uniformly set at 1.5% (w / w) of the soil dry weight to ensure consistent material addition standards across treatment groups and to eliminate the interference of application rate differences on the experimental results.

[0052] 3. Cultivation Management Each pot was filled with 2.0 kg of pretreated test soil, and the corresponding biochar material was added according to the design of each treatment group. After thorough mixing, rapeseed was sown. During the experiment, a uniform conventional water and fertilizer management mode was adopted, and environmental conditions such as light, temperature, and irrigation were kept consistent to avoid the influence of non-treatment factors on crop growth. After 45 days of growth, the rapeseed was harvested uniformly, and relevant indicators were measured.

[0053] 4. Measurement Indicators and Methods (1) Determination of soil physicochemical properties After harvest, rhizosphere soil samples were collected from each treatment group, and the following key indicators were measured: Soil pH value: determined by potentiometry; Exchangeable aluminum content: determined by 1M KCl extraction-inductively coupled plasma atomic emission spectrometry; Available boron content: determined by hot water extraction-methyleneimine colorimetric method.

[0054] (2) Measurement of plant growth and nutrient absorption indicators The harvested rapeseed plants were subjected to the following index measurements: Root-related indicators: Root length and root surface area were measured using a root scanner; Biomass index: Determination of aboveground dry weight; Element content in the plant: Boron content in leaves was determined by the dry ash-curcumin colorimetric method.

[0055] The experimental results are shown in Table 1: Table 1. Effects of different biochar treatments on the physicochemical properties of acidic soil and rapeseed growth indicators.

[0056] Based on Table 1, in terms of aluminum toxicity mitigation, the exchangeable aluminum content in the soil decreased to 0.89 cmol / kg after treatment with the BC-Syn group. This was not only significantly lower than the blank control CK group (1.82 cmol / kg), but also slightly better than the BC-Al group (0.98 cmol / kg) which only has aluminum passivation function and the physically mixed BC-Al+B-Mix group (1.02 cmol / kg). This indicates that the phenolic hydroxyl groups enriched on its surface have a stronger complexing ability for active aluminum. On the other hand, the BC-B group treatment with only boron loading did not significantly improve the exchangeable aluminum content (1.65 cmol / kg), further confirming that boron alone cannot alleviate aluminum toxicity.

[0057] In terms of available boron supply, the available boron content in the soil increased to 0.48 mg / kg after treatment in the BC-Syn group, which was significantly higher than 0.41 mg / kg in the BC-B group and 0.39 mg / kg in the BC-Al+B-Mix group, and much higher than 0.23 mg / kg in the CK group, truly resolving the contradiction of boron consumption due to aluminum pressure in traditional amelioration methods.

[0058] In terms of crop growth promotion, the synergistic effect of BC-Syn biochar led to a comprehensive lead in rapeseed growth indicators: the root surface area of ​​rapeseed treated with BC-Syn reached 182 cm², which is 2.14 times that of the CK group, and significantly higher than the BC-Al, BC-Al+B-Mix, and BC-B groups. This is because BC-Syn biochar not only protects the root structure by passivating aluminum toxicity, but also provides necessary nutrients for root development through slow release of boron. The corresponding aboveground dry weight of rapeseed also reached 6.2 g / pot, which is 2.21 times that of the CK group, and significantly higher than the BC-Al, BC-Al+B-Mix, and BC-B groups, fully demonstrating the synergistic promoting effect of BC-Syn biochar on crop growth.

[0059] In terms of crop nutrient absorption, the BC-Syn group also exhibited unique advantages: on the one hand, the significant reduction in soil exchangeable aluminum, combined with the inhibition of aluminum absorption by robust roots, effectively reduced aluminum accumulation in rapeseed leaves; on the other hand, the boron content in rapeseed leaves treated with the BC-Syn group reached as high as 75%. The concentration of boron ester in BC-Syn biochar was 3.13 times that of the control group (CK), significantly higher than that of the BC-B and BC-Al+B-Mix groups. This is attributed to the slow-release properties of the boron ester structure in BC-Syn biochar and the enhanced boron absorption capacity of the roots due to the relief of aluminum toxicity. BC-B biochar achieves physical binding of boron only through simple loading. This type of boron lacks stable chemical bond constraints in acidic soils and is easily leached away quickly with irrigation or rainfall, failing to provide boron continuously to crops. In contrast, BC-Al+B-Mix biochar is a physical mixture of BC-Al biochar with only aluminum passivation function and ordinary boron char with only boron loading function. The two types of materials are independently dispersed in the soil and do not form an integrated functional structure. When BC-Al biochar plays an aluminum passivation role around the roots, ordinary boron char may be distributed in areas far from the roots, failing to provide boron to the roots that have recovered from aluminum toxicity relief simultaneously. This spatial misalignment and functional asynchrony directly leads to the inefficient acquisition of boron by crops, ultimately resulting in a significant decrease in boron absorption.

[0060] In summary, the BC-Syn biochar prepared by this invention achieves the dual functions of aluminum passivation and boron slow release. In acidic soils, it can effectively alleviate aluminum toxicity and provide long-term boron supply, while promoting crop root development and nutrient absorption. Its comprehensive effect is significantly better than that of single-function biochar and physical mixtures. It has a significant improvement effect on acidic soils with aluminum toxicity and available boron deficiency.

[0061] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of this application is limited to these examples; under the concept of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of the embodiments of this application as described above, which are not provided in detail for the sake of brevity.

[0062] Although this application has been described in conjunction with specific embodiments thereof, many substitutions, modifications and variations of these embodiments will be apparent to those skilled in the art from the foregoing description.

[0063] The embodiments of this application are intended to cover all such substitutions, modifications, and variations that fall within the broad scope of the claims of this application. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the embodiments of this application should be included within the protection scope of this application.

Claims

1. A method for preparing bifunctional biochar for acidic soils, characterized in that, include: Biochar raw material and boron-containing compound are placed in an acidic solution with pH 3.5-4.5 and heated at 120-180℃ for 4-12 hours to obtain a solid product, wherein the biochar raw material is a precursor raw material containing tannin-type polyphenols. The solid product is washed, dried, and then placed in an inert gas atmosphere and pyrolyzed at 480-520°C for 1-2 hours to obtain bifunctional biochar for acidic soils.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the biochar raw material to the boron-containing compound is 1:(10-30).

3. The preparation method according to claim 1, characterized in that, The solid-liquid ratio of the biochar raw material to the acidic solution is 1:(3-8)g / mL.

4. The preparation method according to claim 1, characterized in that, The acidic solution includes one or more of the following: citric acid-sodium citrate buffer solution, tartaric acid solution, and potassium dihydrogen phosphate solution.

5. The preparation method according to claim 1, characterized in that, The boron-containing compound is boric acid or sodium tetraborate.

6. The preparation method according to claim 1, characterized in that, The precursor raw materials containing tannin-type polyphenols include one or more of tea residue, coffee grounds, and pomegranate peel.

7. The preparation method according to claim 1, characterized in that, The solid product obtained by heating at 120-180℃ for 4-12 hours includes: Heating at 140-160℃ for 6-10 hours yields a solid product.

8. The preparation method according to claim 1, characterized in that, The solid product is washed, dried, and then placed in an inert gas atmosphere and pyrolyzed at 480-520°C for 1-2 hours to obtain bifunctional biochar for acidic soils, comprising: The solid product is washed, dried, and then placed in an inert gas atmosphere and pyrolyzed at 490-510°C for 1.2-1.8 hours to obtain bifunctional biochar for acidic soils.

9. A bifunctional biochar for acidic soils, characterized in that, It is prepared by the preparation method according to any one of claims 1-8.

10. The application of biochar according to claim 9, characterized in that, It can be applied to acidic soils where aluminum is toxic and available boron is lacking.

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