Magnesium-iron biochar-based soil conditioner for salt fixation and alkali reduction of saline alkali soil as well as preparation method and application of magnesium-iron biochar-based soil conditioner

Abstract

The invention relates to a preparation method and application of a magnesium-iron biochar-based soil conditioner for salt fixation and alkali reduction of saline alkali soil. The preparation method comprises the following steps: crushing waste biomass; adding a magnesium mineralizer into the biomass, carrying out wet ball milling, and drying; performing oxygen-limited pyrolysis on the dried product, and extracting and separating a heat source organic matter solution with pure water to obtain a charcoal precursor; adding an iron-based mineralizing agent, an organic dispersing agent and mixed organic acid into the biochar precursor, blending, carrying out three-stage gradient hydrothermal treatment, and filtering to obtain nano magnesium-iron biochar; adding the nano magnesium-iron biochar into a heat source organic matter solution, directly mixing, compounding and granulating, and adjusting the acid-base of a hydrothermal solution to prepare a conditioner; the conditioner is mixed with saline-alkali soil, and the'salt fixation and alkali reduction 'remediation of the soil is realized through a physical remediation means. Compared with the prior art, the method has the advantages that the production cost is low, the commercial value is high, industrial application is easy, and sustainable restoration and treatment of saline-alkali soil are greatly promoted.

Description

Technical Field

[0001] This invention relates to the field of saline-alkali soil remediation, and in particular to a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soil, its preparation method, and its application. Background Technology

[0002] Soil salinization in farmland is a key obstacle to agricultural production. Its main cause lies in the excessive accumulation of soluble salt ions such as sodium, calcium, and magnesium in the soil, leading to soil structure degradation and decreased fertility. This hinders plant growth, manifesting as difficulty in water absorption, ion toxicity, and osmotic stress, ultimately resulting in reduced crop yields and posing a serious threat to sustainable agricultural development. Therefore, there is an urgent need to develop economical and efficient saline-alkali soil improvement technologies.

[0003] Among existing soil amendment methods, biochar is a commonly used amendment due to its low cost, availability, well-developed pores, large specific surface area, and negative charge. It can improve soil microclimate, hydrological conditions, microbial communities, and nutrient cycling, and its research and applications are widespread, as exemplified by the technical solutions disclosed in CN119462285 A and CN119463888 A. However, using biochar alone or in combination with organic matter generally suffers from low salt-reducing efficiency, slow onset of action, and complex sources of organic additives. In contrast, the combined application of biochar and mineralizers shows potential for rapid, simple, and efficient results. The introduction of specific mineralizers can achieve "salt fixation and alkali reduction" through a synergistic mechanism of cationic leaching and anionic fixation, while simultaneously supplementing the mineral nutrients required by crops. However, traditional direct mixing or co-calcination methods easily lead to large product particle size, difficulty in releasing mineral elements, and lack of organic nutrients, resulting in problems such as poor soil salt drainage (anions), poor alkali reduction (sodium ions), and low crop nutrient absorption efficiency, as mentioned in the CN patents.

[0004] To address the aforementioned issues, existing research has attempted to optimize performance through process improvements. For example, CN117965177A discloses a method for preparing humic acid amendments by catalytically pyrolyzing biomass from iron-containing solid waste using saline-alkali soil to provide an alkaline environment. While this method can lower soil pH and inhibit alkali metal ion migration through humic acid, its preparation process involves only simple mixing and calcination. The resulting product lacks targeted modification, and the improvement effect relies more on the neutralization and dilution effects of humic acid, failing to establish a clear mineral fixation mechanism. Therefore, it is insufficient in terms of the strength and persistence of "salt fixation and sodium removal." Another example is CN118725871A, which uses a compound of hydrothermal carbon, biochar, and activated lignite to improve the stability of soil organic matter and aggregates. However, this technology lacks key activation steps such as oxygen-limited catalytic pyrolysis, resulting in limited activity of the prepared material. Its core improvement still focuses on carbon input and aggregate formation, similarly failing to introduce efficient mineral fixation components, making it difficult to achieve strong and persistent salt fixation and sodium ion removal.

[0005] In summary, while the use of biochar and mineralizing agents for saline-alkali soil remediation shows great promise, existing technologies still have significant limitations in terms of material composite methods, mineral fixation mechanisms, and long-term effectiveness. Therefore, there is an urgent need in this field to develop a novel biochar-based soil conditioner preparation method that is simple, efficient, and easily industrialized. This method should be able to achieve rapid salt fixation and alkali reduction in saline-alkali soils, simultaneous replenishment of organic matter, slow release of mineral elements, and ultimately, effective improvement in crop yield. Summary of the Invention

[0006] The purpose of this invention is to solve at least one of the above-mentioned problems by providing a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils, its preparation method, and its application.

[0007] The objective of this invention can be achieved through the following technical solution: a method for preparing a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils, comprising the following steps: (1) The waste biomass is crushed to obtain biomass powder; (2) Add magnesium mineralizer to the biomass powder obtained in step (1) and perform wet ball milling, adjust the pH with hydrothermal liquid, and evaporate and dry; (3) The product obtained in step (2) is subjected to oxygen-limited pyrolysis. The pyrolysis product is separated from the heat source organic matter solution by water extraction to obtain biochar precursor. Low-temperature catalytic pyrolysis with limited oxygen is adopted, and the product is separated into two phases, "heat source organic matter solution" and "biochar precursor", by water extraction, forming a two-component technical route of "liquid phase rapid fertilization + solid phase long-term carrier". (4) Add iron-based mineralizers and organic dispersants to the biochar precursor obtained in step (3), mix with organic acids, and carry out a three-stage gradient hydrothermal reaction to obtain hydrothermal liquid and nano-magnesium-iron biochar. The hydrothermal liquid is reused as an acid-base regulator in step (2). Add iron-based mineralizers, organic dispersants, and mixed organic acids to the biochar precursor, and use a low-medium-high temperature three-stage hydrothermal reaction to make the carbon body nano-sized and enrich Mg-Fe active sites to obtain nano-scale magnesium-iron biochar. Construct a "magnesium-based mineralizer + iron-based mineralizer" system to directionally form magnesium-iron layered double hydroxide (Mg-FeLDH) in the soil, which serves "salt fixation + Na replacement". + Mineralization remediation by "alkalinity reduction"; hydrothermal fluid is used as an acid-base regulator in each step of the process, achieving virtually no waste liquid discharge and precisely controlling the pH of the final product to the acidic range, thus obtaining an acidic magnesium-iron biochar-based soil conditioner.

[0008] (5) The nano-magnesium-iron biochar obtained in step (4) is directly mixed with the heat source organic matter solution, and then a compounding agent is added. The pH is adjusted by hydrothermal fluid to prepare an acidic magnesium-iron biochar-based soil conditioner. The resulting product is an acidic magnesium-iron biochar-based soil conditioner that is compounded and granulated with "nano-magnesium-iron biochar + heat source organic matter solution + various mineral additives". Structurally, it has nano-carbon carrier, mineralization active sites and readily available organic matter.

[0009] Furthermore, the waste biomass in step (1) includes one or more of corn, cotton, rice husks, biogas residue, and fruit wood, preferably corn; Biomass crushing needs to reach a mesh size of 50 to 300, preferably including one or more of 50 mesh, 100 mesh, 200 mesh, and 300 mesh, and more preferably 200 mesh.

[0010] Furthermore, in step (2), the magnesium mineralizer is composed of one or more of magnesium oxide, magnesium sulfate, magnesium chloride, magnesium carbonate, and magnesium ammonium phosphate, preferably magnesium chloride and magnesium sulfate are mixed in equal mass; The mass ratio of biomass powder to magnesium-based mineralizer is 1:(0.05~0.20), with a preferred mass ratio of 1:0.15; The wet ball milling method involves adding water to the ball, with a ball-to-material mass ratio of (1~10):1, a milling time of 2~4 hours, and water addition controlled at a mass-to-volume ratio of 1:(0.01~0.05) kg / m³. 3 The preferred ball-to-material mass ratio is 5:1, the ball milling time is 2.5 h, and the mass-to-volume ratio of the mixture to water is 1:0.03 Kg / m³. 3 ; The pH of the ball-milled product is adjusted to 3.0 ~ 6.0 using hydrothermal fluid; the evaporation and drying temperature is 65 ~ 100 ℃, and the drying time is 0.5 ~ 1.5 h. Preferably, the pH of the ball-milled product is 5.50, the drying temperature is 75 ℃, and the drying time is 1.0 h.

[0011] Furthermore, in step (3), the pyrolysis temperature of the oxygen-limited pyrolysis is 150~300 ℃; the pyrolysis time is 3~6 h; the oxygen-limited atmosphere is a mixture of oxygen and nitrogen with a volume ratio of 1:(5~10); and the gas flow rate is controlled at 50~150 m³ / h. 3 ·h -1 The preferred pyrolysis temperature is 240 °C, the pyrolysis time is 3 h, the volume ratio of oxygen to nitrogen in the pyrolysis atmosphere is 1:9, and the gas flow rate is 100 m³ / h. 3 ·h -1 ; The specific steps for extracting the pyrogen organic matter solution from the pyrolysis products with water are as follows: pure water is mixed with the pyrolysis products, stirred, and then filtered. The volume mass ratio of liquid to solid is (2 ~ 5): 1 L / Kg. The stirring speed is controlled at 200 ~ 500 rpm, and the stirring time is controlled at 0.5 ~ 2.0 h. Preferably, the volume mass ratio of water to pyrolysis products is 3:1 L / Kg, the stirring speed is 350 rpm, and the stirring time is 1.5 h.

[0012] Further, in step (4), the iron-based mineralizing agent is composed of one or more of ferric chloride, ferric acetate, ferric sulfate, ferric nitrate, polyferric sulfate, and ferric oleate, preferably ferric chloride and polyferric sulfate with equal molar amounts of iron. The organic dispersant is composed of one or more of cationic surfactants, anionic surfactants, or nonionic surfactants; wherein the cationic surfactants include hexadecyltrimethylammonium bromide (CTAB), the anionic surfactants include ammonium dodecylbenzenesulfonate (SDBS) and sodium dodecyl sulfate (SDS), and the nonionic surfactants include one or more of polyvinylpyrrolidone (PVP-k30) and Triton (X-100), preferably a mixed solution of equal masses of hexadecyltrimethylammonium bromide, sodium dodecyl sulfate, and Triton; The mixed organic acid is composed of one or more of acetic acid, citric acid, oxalic acid, and lactic acid, preferably a mixed solution of acetic acid and citric acid of equal volume; The mass ratio of biochar precursor to iron-based mineralizer ranges from 1:(0.01~0.05), preferably 1:0.02; the solid-to-liquid mass-volume ratio of biochar precursor, organic dispersant, and mixed organic acid is 1:(0.001~0.005):(0.01~0.05) Kg / L / L, preferably 1:0.002:0.3 Kg / L / L. The three-stage gradient hydrothermal reaction is as follows: the material is placed in a high-temperature and high-pressure hydrothermal reactor with a material filling degree of 30% to 50%, preferably 40%; the hydrothermal reactor is heated to 80 to 100 ℃ and reacted for 1 to 3 hours, then heated to 150 to 180 ℃ and reacted for 1 to 3 hours, and finally heated to 210 to 230 ℃ and reacted for 1 to 3 hours. The hydrothermal temperature is set in three gradient ranges, preferably 90 ℃, 160 ℃, and 220 ℃, and the same hydrothermal time range is uniformly set to 1 to 3 hours, preferably 1.5 hours.

[0013] Further, in step (5), the mass-to-volume ratio of the mixed solid and liquid of the nano-magnesium iron biochar and the heat source organic matter solution is 1:(1~5) Kg / L, preferably 1:2 Kg / L; The compounding agent is composed of one or more of the following: wood ash, attapulgite clay, eggshells, furfural residue, zeolite, and biogas residue, preferably an equal mass mixture of attapulgite clay and furfural residue. The mass ratio of nano-magnesium iron biochar to the compounding agent is controlled at 1:(0.2~0.4), with a preferred mass ratio of 1:0.3; The pH range of the hydrothermal liquid-adjusted compound granulation product is 3 to 6, preferably 4.5.

[0014] The present invention also provides a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils, prepared by the above method.

[0015] The present invention also provides an application of a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soil, wherein the magnesium-iron biochar-based soil conditioner is applied to saline-alkali soil.

[0016] Further, proceed as follows: mix magnesium-iron biochar-based soil conditioner with saline-alkali soil, and achieve "salt fixation and alkali reduction" remediation of saline-alkali soil through physical remediation methods.

[0017] Furthermore, the magnesium-iron biochar-based soil conditioner is mixed with saline-alkali soil at a certain mass ratio of (1~5):100, preferably 3:100. Saline-alkali soil types include one of carbonate alkaline soil, sulfate alkaline soil, and chloride alkaline soil, with carbonate alkaline soil being the preferred choice.

[0018] Furthermore, the physical repair methods include one or more of leaching and concealed pipe treatment, with leaching being preferred.

[0019] Compared with the prior art, the present invention has the following beneficial effects: 1. Stronger and more lasting "salt fixation and alkali reduction" effect: Relying on the Mg-FeLDH formed by nano-magnesium iron biochar in the soil, this invention can simultaneously fix CO3. 2-- SO4 2- Cl - Anionic salts, and through Mg 2+ / Fe 3+ Replacement of Na + It significantly reduces soil electrical conductivity and water-soluble sodium salts, lowering soil pH from strongly alkaline to near neutral and maintaining relative stability; the resulting conditioner rapidly and effectively achieves salt anion fixation and Na+ reduction under harsh soda carbonate alkaline conditions. + The replacement of alkalinity and the simultaneous restoration of organic carbon / minerals enable crops to achieve high survival rates under conditions where they would otherwise have zero survival.

[0020] 2. "Rapid-acting fertilization + long-term mineralization remediation" dual-timescale improvement: The heat source organic matter solution provides soluble humic acid / fulvic acid and other quick carbon sources and nutrients, rapidly improving microbial activity and rhizosphere environment in the short term; Nano-magnesium iron biochar maintains its remediation effect in the medium and long term through mineral slow release, continuous LDH generation and structural improvement, achieving "fast-acting + long-lasting" time-gradient conditioning; 3. Higher interfacial activity and nutrient availability: Due to the nano-sized particles and the enrichment of Mg-Fe active sites on the surface, the magnesium-iron biochar of this invention has a larger specific surface area and stronger interfacial reactivity, and is more resistant to salt and Na+. + It has a more complete adsorption, exchange and complexation effect with soil organic / inorganic components, and can significantly increase the content of available Mg, Fe and other trace elements in the soil; 4. Enhanced comprehensive improvement of soil structure and aggregates: Nano-carbon, in synergy with heat source organic matter and various mineral additives, helps to form stable aggregates, improve pore structure, enhance water and fertilizer retention and aeration, and can still provide a suitable physical environment for roots in a strongly saline-alkali environment. 5. This invention achieves high-yield "heat source organic matter" through oxygen-limited low-temperature pyrolysis + Mg coating, while simultaneously obtaining a biochar precursor with high soluble humic acid / fulvic acid content and retained active functional groups, realizing "dual-channel carbon fixation and improvement through soluble organic matter + solid carbon". Nano-Mg-Fe carbon with abundant organic functional groups is obtained through a three-stage hydrothermal process + Fe-based mineralizer + organic acid / surfactant. This triple synergistic design significantly enhances the ability of subsequent products to form LDH and aggregates in the soil. Finally, the conditioner obtained by granulation of nano-Mg-Fe carbon + heat source organic matter solution + natural minerals is a composite soil conditioner integrating "salt fixation, alkalinity reduction, fertilization, and agglomeration". In field trials, pH, EC, and water-soluble sodium salts decreased significantly, porosity and available Fe / Mg significantly increased, and corn survival rate reached 93-98%. This comprehensive improvement effect is significantly stronger than the gradual improvement of conventional organic / mineral conditioners.

[0021] 6. This invention proposes a magnesium-iron biochar-based soil conditioner for saline-alkali soil remediation, prepared by mixing nano-sized magnesium-iron modified biochar, a heat-source organic matter solution, a hydrothermal fluid, and a compounding agent. During soil improvement, the magnesium-iron biochar-based soil conditioner releases magnesium and iron ions. First, the magnesium and iron ions react with anionic salts in the soil to form magnesium-iron double hydroxides, fixing the anions and achieving soil salt fixation, such as carbonates, sulfates, and chlorides. Second, the magnesium and iron ions are also displaced by sodium salts through ion exchange and cation bridging. Finally, the magnesium and iron ions, exogenous organic matter, soil organic matter, soil mineral particles, and soil microorganisms aggregate, improving soil physical structure, aeration, and drainage, such as porosity. Because the magnesium-iron biochar has a nano-sized particle size, the content of available magnesium and available iron in the soil increases, further stimulating plant growth and supplementing the plant's mineral needs.

[0022] 7. This invention significantly reduces the amount of added acid / alkali and fresh water used through closed-loop hydrothermal circulation and oxygen-limited low-temperature pyrolysis, resulting in virtually no wastewater discharge. It balances efficient resource utilization with environmental friendliness, making it suitable for industrial scale-up. A magnesium-iron biochar-based soil conditioner for saline-alkali soil remediation is prepared using a simple process. This conditioner possesses advantages such as abundant soluble organic matter, sufficient mineral nutrients, and small particle size. Through repeated experiments and research, a low-cost, low-energy-consumption, economically efficient, and environmentally friendly saline-alkali soil mineralization remediation solution has been developed. This solution achieves resource utilization of waste materials, improves the application effect of biochar in saline-alkali soils, and promotes the synergistic effect of biochar and magnesium-iron mineralizers.

[0023] 8. This invention has multiple functions, including improving the physical and chemical properties of saline-alkali soil, increasing soil organic matter, and providing mineral nutrients for crop growth, effectively improving crop resistance and yield. Attached Figure Description

[0024] Figure 1 A flowchart illustrating the steps of a method for preparing a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils, provided by this invention. Figure 2 This is a three-dimensional fluorescence spectrum of the heat source organic matter in Example 1 of the present invention; Figure 3 This is a scanning electron microscope image of the biochar precursor in Example 1 of the present invention; Figure 4 The infrared spectrum of the biochar precursor in Example 1 of this invention shows the presence of magnesium oxygen functional groups. Figure 5 This is a scanning electron microscope image of the nano-magnesium iron biochar in Example 1 of the present invention; Figure 6 The infrared spectrum of the magnesium-oxygen and iron-oxygen functional groups present on the nano-magnesium-iron biochar in Example 1 of the present invention. Figure 7 The X-ray diffraction peaks of saline-alkali soil repaired by magnesium-iron biochar-based soil conditioner in Examples 1, 2, and 3 of this invention are shown. Figure 8 This is a diagram showing the planting effect of the soil used in the field experiment of this invention. Detailed Implementation

[0025] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0026] For simplicity, this application only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly stated; and any lower limit can be combined with other lower limits to form a range not explicitly stated, just as any upper limit can be combined with any other upper limit to form a range not explicitly stated. Furthermore, although not explicitly stated, each point or individual value between the endpoints of a range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit and combined with any other point or individual value, or combined with other lower or upper limits, to form a range not explicitly stated. In the description of this application, it should be noted that, unless otherwise stated, "above" includes the stated number, and "multiple" in "one or more" means two or more.

[0027] The foregoing description of this application is not intended to describe every disclosed implementation or method. Instead, the following description provides more specific examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments that can be used in various combinations. The examples listed are representative only and should not be construed as exhaustive.

[0028] Numerous details are explored in the following description to provide a more thorough explanation of embodiments of this application; however, it will be apparent to those skilled in the art that embodiments of this application may be practiced without these specific details.

[0029] The purpose of this invention is to address the shortcomings of existing technologies by providing a simple, efficient, waste-free, and easily industrialized method for a magnesium-iron biochar-based saline-alkali soil conditioner. Another purpose is to provide a good soil conditioner that can simultaneously achieve soil desalination and alkali reduction, soil carbon sequestration, mineral replenishment, and promotion of crop growth.

[0030] The mechanism of this invention is as follows: The acidity of the hydrothermal fluid promotes the release of magnesium ions from the magnesium-based mineralizer, which then fully adhere to the surface of the biomass particles to form a coating. During oxygen-limited pyrolysis, cellulose, hemicellulose, lignin, and protein macromolecules in the waste biomass undergo a series of reactions such as deoxygenation, decarboxylation, and dehydroxylation under thermochemical conditions. Among these, the Maillard reaction and glycoamine condensation reaction induced by thermochemical processes are crucial for the formation of pyrogen-generating organic matter from biomass decomposition. Due to the magnesium-containing mineral coating on the biomass particles, under high temperature and low oxygen conditions, the magnesium metal minerals in the magnesium-based mineralizer can promote electron transfer between biomass particles, promote the full decomposition of biomass, and accelerate the Maillard and glycoamine condensation reactions to form more pyrogen-generating dissolved organic matter, which can supplement the soil organic carbon lacking in saline-alkali soils. Because the initial roasting temperature does not exceed 300℃, the porosity characteristics of the biochar precursor are not obvious, and the biomass particle size can only reach the micron level. The three-stage gradient hydrothermal reaction involves three stages: the first stage of heating promotes the full encapsulation of iron ions into the biochar particles; the second stage of heating further decomposes recalcitrant lignin and cellulose to form heat-generating organic matter; and the third stage of heating intensifies the physical bombardment of steam and the chemical digestion of organic acids to transform micron-sized biochar particles into nano-magnesium-iron biochar. Finally, the nano-magnesium-iron biochar, the heat-generating organic matter solution, and the compounding agent are mixed and granulated, and the pH is adjusted using a hydrothermal solution to form a magnesium-iron biochar-based soil conditioner.

[0031] Magnesium-iron biochar-based soil conditioners release magnesium and iron ions during soil improvement. First, these ions react with anionic salts in the soil to form magnesium-iron double hydroxides, fixing the anions and thus consolidating salts such as carbonates, sulfates, and chlorides. Second, these ions are also released through ion exchange and cation bridging. Finally, the aggregation of magnesium-iron ions, exogenous organic matter, soil organic matter, soil mineral particles, and soil microorganisms improves soil physical structure, aeration, and drainage, such as porosity. Because the magnesium-iron biochar has a nano-sized particle size, the content of available magnesium and iron in the soil increases, further stimulating plant growth and supplementing the plant's mineral needs.

[0032] To further understand the present invention, the following embodiments are provided. It is worth noting that, unless otherwise specified, all raw materials used in the present invention are commercially available; and all methods and equipment used are common in the art. Specifically, the polyferric sulfate is selected from commercially available products and conforms to GB14591-2016; the saline-alkali soils are classified as chloride-type, carbonate-type, and sulfate-type saline-alkali soils, respectively.

[0033] Example 1: A method for preparing a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils, such as... Figure 1 As shown, it includes the following steps: (1) Crush the waste corn stalks to a mesh size of 200 to obtain waste corn biomass powder; (2) Magnesium chloride and magnesium sulfate were mixed at a mass ratio of 1 kg:1 kg to obtain a magnesium-based mineralizer. The biomass powder obtained in step (1) and the magnesium-based mineralizer were mixed at a mass ratio of 1 kg:0.15 kg and then wet-milled with water. The mass ratio of the mixture to the material was 5 kg:1 kg, the milling time was 2.5 h, and the mass-to-volume ratio of the mixture to water was 1 kg:0.03 m³. 3 After ball milling, the pH of the sample was adjusted to 5.50 using an acidic hydrothermal solution (1 mol·L⁻¹ for the first time). -1 Adjust with hydrochloric acid, and then use the hydrothermal liquid obtained in step (4) and dry it by evaporation at 75 °C.

[0034] (3) The dried sample obtained in step (2) is dried in a mixed atmosphere (oxygen and nitrogen volume ratio 1:9, gas flow rate 100m³ / h). 3 ·h -1The pyrolysis was performed in a tubular furnace under limited oxygen conditions at 240 °C for 3 h to obtain the pyrolysis products. The pyrolysis products were then mixed with pure water, stirred, and filtered to obtain a pyrogen organic matter solution and a biochar precursor. The liquid-to-solid volume-to-mass ratio was controlled at 3 L:1 Kg, the stirring speed was controlled at 350 rpm, and the stirring time was controlled at 1.5 h. Three-dimensional fluorescence detection was performed on the obtained pyrogen organic matter solution. Figure 2 As shown in the figure, the organic matter solution from the pyrogen source was qualitatively identified using three-dimensional fluorescence spectroscopy. Under room temperature conditions, the colorimetric region of the pyrogen source organic matter between excitation wavelengths of 200–350 nm and emission wavelengths of 400–600 nm overlaps with the fluorescence emission region of humic acid-based organic compounds. This result indicates that the organic matter in the pyrogen source organic matter solution belongs to the humic acid class. Infrared spectroscopy and scanning electron microscopy were performed on the obtained biochar precursor, as shown... Figure 3 and Figure 4 As shown, the functional group composition and particle size distribution of the biochar precursor were identified by infrared spectroscopy and scanning electron microscopy. It was found that the average particle size of the biochar precursor was in the micrometer range and contained magnesium oxygen functional groups.

[0035] (4) Iron-based mineralizers were prepared by mixing ferric chloride and polyferric sulfate in equal molar amounts; organic dispersants were prepared by mixing hexadecyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Triton X-100 solution in equal masses; mixed organic acids were prepared by mixing acetic acid and citric acid in equal volumes; 1 kg of biochar precursor, 0.02 kg of iron-based mineralizer, 0.002 L of organic dispersant, and 0.03 L of mixed organic acid were mixed together; the resulting mixed reactants were subjected to a three-stage gradient hydrothermal reaction: the material filling degree of the high-temperature and high-pressure hydrothermal reactor was set to 40%, the hydrothermal reactor was heated to 90 °C and the reaction was maintained for 1.5 h, then the temperature was raised to 160 °C and the reaction was maintained for 1.5 h, and finally the temperature was raised to 220 °C and the reaction was maintained for 1.5 h. Nano-magnesium iron biochar was obtained through hydrothermal reaction, and the filtered liquid was the hydrothermal liquid. The obtained nano-magnesium iron biochar was detected by infrared spectroscopy and scanning electron microscopy, such as Figure 5 and Figure 6 As shown, infrared spectroscopy and scanning electron microscopy images show that the particle size of the nano-magnesium iron biochar reaches the nanoscale and contains magnesium oxygen and iron oxygen functional groups. (5) Mix attapulgite and furfural residue in a mass ratio of 1:1 to prepare a compound agent; mix the nano-magnesium iron biochar obtained in step (4), the heat source organic matter solution obtained in (5), and the solid compound agent obtained in (3) in a mass-volume ratio of 1 Kg: 2L: 0.3 Kg for compounding and granulation; finally, use hydrothermal liquid to adjust the pH range of the compounded granulated product to 4.5 to prepare the product magnesium iron biochar-based soil conditioner; as shown in Table 1, the magnesium iron biochar-based soil conditioner products after compounding and granulation all meet the standard of acidic humic acid soil conditioner in HG / T 5782-2020 "Humic Acid Soil Conditioner".

[0036] Table 1. Standards for acidic humic acid soil conditioners in "Humic Acid Soil Conditioners" (6) In this case study, carbonate-alkali soil was collected from an uncultivated plot of land in the western part of the Songnen Plain in Jilin Province, China (43°53′3.5″N, 125°19′5.1″E). This area is rich in sodium carbonate and sodium bicarbonate. The soil was collected from a plot of wasteland that had not been used for maize production for at least 5 years. The basic properties of the soil are shown in Table 2. The soil pH is extremely alkaline at 10.35, and the salinity is 9.58 ds·m -1 It belongs to severely saline-alkali soil.

[0037] The mass ratio of the magnesium-iron biochar-based soil conditioner obtained in step (5) to the 10 cm carbonate-alkali soil layer was controlled at 3:100. A zoned irrigation method was adopted. After applying the soil conditioner and preparing the land, raised beds (2 m × 5 m) were constructed according to the plot, with the bed surface kept basically level. Clean water was slowly introduced from a high position to form a 5-10 cm still water layer on the surface. The flow rate was controlled to avoid erosion. Each leaching session lasted 8-12 hours, with intermittent water replenishment depending on the infiltration situation. Water was stopped for 24 hours after each leaching cycle, and infiltration samples were collected after the water had infiltrated and the underground pipe drainage had basically stabilized. The underground pipes were perforated PVC pipes, buried at a depth of approximately 0.9 m, spaced 4-5 m apart, with a slope of approximately 0.3%. They were wrapped with non-woven fabric and covered with 10 cm of medium-coarse sand, then backfilled with the original soil to prevent blockage. The outlet was connected to a collection well, and the drainage volume and water quality were recorded regularly.

[0038] Table 2. Basic soil physicochemical properties before and after leaching and underground pipe remediation of carbonate-alkali soils. Soil pH was determined using the potentiometric method as specified in NY / T 1121.2-2006 "Soil Testing Part 2: Determination of Soil pH"; soil conductivity was determined using the electrode method as specified in HJ 802-2016 "Determination of Soil Conductivity", with a soil-to-water ratio of 1:5, and the conductivity of the extract was measured using a conductivity electrode; soil water-soluble sodium salt content was determined by preparing a soil water extract according to NY / T 1121.16-2006 "Soil Testing Part 16: Determination of Total Soil Water-Soluble Salts", and the water-soluble sodium content was determined using flame photometry or inductively coupled plasma atomic emission spectrometry. + Content; soil bulk density was determined by the ring cutter method in NY / T 1121.4-2006 "Soil Testing Part 4: Determination of Soil Bulk Density"; soil exchangeable cations were determined by HJ 889-2017 "Determination of Soil Cation Exchange Capacity - Hexaamminecobalt Trichloride Extraction-Spectrophotometric Method"; water-soluble carbonate (CO3) 2- HCO3 can be obtained simultaneously - Soil water-soluble carbonate and bicarbonate were determined using a dual-indicator neutralization titration method, referring to LY / T 1251-1999 "Analysis of Water-Soluble Salts in Forest Soils"; and the exchangeable magnesium content was determined using ammonium acetate extraction-EDTA titration or spectrometry, referring to NY / T 1121.13-2006 "Soil Testing Part 13: Determination of Exchangeable Calcium and Magnesium in Soils". The content of exchangeable magnesium was determined according to NY / T 890-2004 "Determination of Available Zinc, Manganese, Iron, and Copper Content in Soils - Diethylenetriaminepentaacetic Acid (DTPA) Extraction Method", using DTPA-CaCl2-triethanolamine buffer solution to extract soil samples. The Fe content was quantified by atomic absorption spectrometry or ICP-OES and expressed as available iron content. These soil indicators were measured before remediation (before application of conditioners and leaching treatment) and after remediation (at the end of leaching remediation) to evaluate the "salt-fixing and alkali-reducing" effect on saline-alkali soils. X-ray testing was performed on the carbonate-alkali soil remediated by the magnesium-iron biochar-based soil conditioner. Figure 7 As shown, soil minerals exhibit distinct diffraction peaks at 10.5°, 21.6°, 26.5°, and 59.5°, with the triplet diffraction peaks corresponding to the diffraction peaks (003, 006, 0012, 110) of magnesium-iron layered double hydroxide (Mg-Fe LDH) crystals. This indicates the presence of newly formed Mg-Fe LDH in the soil minerals. Combined with Table 2, it can be seen that the carbonates (CO3-) in the soil... 2- The free carbonate anions in the soil are mineralized to form Mg-Fe LDH, thus becoming part of the stable minerals and achieving "salt fixation" in carbonate-alkali soils. Table 2 shows the pH and water-soluble Na+ in the soil cations. +The significant reduction demonstrates that the magnesium-iron biochar-based soil conditioner prepared in this embodiment can effectively reduce the alkalinity of carbonate-alkali soils.

[0039] Depend on Figures 2-7 As can be seen from Tables 1 and 2, the magnesium-iron biochar-based soil conditioner prepared by the preparation method provided by this invention meets the standards for acidic humic acid soil conditioners in HG / T5782-2020 "Humic Acid Soil Conditioners" in terms of the mass fraction of total humic acid and the mass fraction of activated humic acid. It effectively improves the physical structure of the soil, improves aeration and drainage, and supplements the mineral needs of plants, thus effectively improving the quality of carbonate and saline-alkali soils.

[0040] Example 2: (1) Waste cotton stalks are crushed to a mesh size of 200 to obtain waste cotton biomass powder; (2) The magnesium-based mineralizer was prepared by mixing magnesium sulfate and magnesium phosphate at a mass ratio of 1 kg: 1 kg. The product obtained in (1) and the magnesium-based mineralizer were mixed at a mass ratio of 1 kg: 0.10 kg and then subjected to wet ball milling with water. The mass ratio of ball to material was 6:1, the milling time was 2 h, and the mass-volume ratio of the mixture to water was 1 kg: 0.03 m³. 3 After ball milling, the pH of the sample was adjusted to 5.0 using an acidic hydrothermal solution (1 mol·L⁻¹ for the first time). -1 Adjust with hydrochloric acid, and then use the hydrothermal liquid obtained in step (4) and dry it by evaporation at 85°C.

[0041] (3) The reaction products obtained in (2) are subjected to a mixed atmosphere (oxygen and nitrogen volume ratio 1:9, gas flow rate 120 m³ / h). 3 ·h -1 The pyrolysis was carried out in a tubular furnace with limited oxygen at a temperature of 220 °C for 4 h to obtain the pyrolysis products. The pyrolysis products were mixed with pure water, stirred and filtered to obtain a heat source organic matter solution and a biochar precursor. The volume-to-mass ratio of liquid to solid was controlled at 4 L: 1 Kg, the stirring speed was controlled at 350 rpm and the stirring time was controlled at 1 h.

[0042] (4) The iron-based mineralizer is prepared by mixing ferric acetate and ferric sulfate in equal molar amounts of iron; the organic dispersant is prepared by mixing a solution of a preferred cationic surfactant, cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and a nonionic surfactant, polyvinylpyrrolidone (PVP-k30); the mixed organic acid is prepared by mixing equal volumes of lactic acid and citric acid; 1 kg of the biochar precursor obtained in step (3), 0.01 kg of the iron-based mineralizer, 0.001 kg of the organic dispersant, and 0.02 kg of the mixed organic acid are mixed together; the resulting mixed reactants are subjected to a three-stage gradient hydrothermal reaction: the material filling degree of the high-temperature and high-pressure reactor is set to 50%, the hydrothermal reactor is heated to 100 °C and the reaction is maintained for 1.0 h, then the temperature is raised to 180 °C and the reaction is maintained for 1.0 h, and finally the temperature is raised to 230 °C and the reaction is maintained for 1.0 h. Nano-magnesium iron biochar and hydrothermal liquid are obtained through hydrothermal reaction.

[0043] (5) The compound agent is prepared by mixing eggshells and biogas residue in a mass ratio of 1:1; the nano-magnesium iron biochar obtained in (4), the heat source organic matter solution obtained in (3), and the compound agent obtained in (5) are mixed and granulated in a mass and volume ratio of 1 Kg: 1 Kg: 0.2 Kg; finally, the pH range of the compound granulated product is adjusted to 5.0 using hydrothermal liquid to prepare the product magnesium iron biochar-based soil conditioner. The granulated acidic magnesium iron biochar-based soil conditioner products all meet the standards of acidic humic acid soil conditioners in HG / T 5782-2020 "Humic Acid Soil Conditioner".

[0044] (6) In this implementation case, chloride-salt-alkali soil was collected from a reclaimed agricultural plot in the Dongtan Wetland coastal saline-alkali soil area of ​​Chongming District, Shanghai, China (31°52′586″N, 121°92′272″E). This area is rich in sodium chloride. The collected soil came from a wasteland used for corn production. The basic properties of the soil are shown in Table 3. The soil pH is extremely alkaline at 8.35, and the salinity is 1.85 ds·m -1 It belongs to slightly saline-alkali soil.

[0045] The mass ratio of the magnesium-iron biochar-based soil conditioner obtained from (5) to the 10 cm chloride-salt-alkali soil layer was controlled at 2:100. A leaching-based zoned irrigation method was adopted. After applying the soil conditioner and preparing the land, ridges (2 m × 5 m) were built according to the plot, and the ridge surface was kept basically level. Clean water was slowly introduced from a high place to form a 5-10 cm static water layer on the surface. The flow rate was controlled to avoid scouring. Each leaching was carried out for 8-12 hours, and water was replenished intermittently according to the infiltration situation. After each round of leaching, the water was stopped for 24 hours. After the water infiltrated and the drainage of the underground pipe was basically stable, the infiltration water samples were collected. The underground pipe was a perforated PVC pipe, buried at a depth of about 0.9 m, with a spacing of 4-5 m and a slope of about 0.3%. It was wrapped with non-woven fabric and covered with 10 cm of medium-coarse sand, and then backfilled with the original soil to prevent blockage. The outlet was connected to a collection well, and the drainage volume and water quality were recorded regularly.

[0046] Table 3. Basic soil physicochemical properties before and after leaching remediation of chloride-type saline-alkali soils using underground pipes. X-ray testing was performed on the chloride-salt-alkali soil remediated by the magnesium-iron biochar-based soil conditioner. (The results are as follows...) Figure 7 As shown, soil minerals exhibit distinct diffraction peaks at 10.5°, 21.6°, 26.5°, and 59.5°, with the triplet diffraction peaks corresponding to the diffraction peaks (003, 006, 0012, 110) of magnesium-iron layered double hydroxide (Mg-Fe LDH) crystals. This indicates the presence of newly formed Mg-Fe LDH in the soil minerals. Chloride (Cl) in the soil... - The chloride anions in the soil are mineralized to form Mg-Fe LDH, and the free chloride anions become part of the stable minerals, thus achieving "salt fixation" in chloride-alkali soils. Table 3 shows the acidity / alkalinity and water-soluble Na+ in the soil cations. + Significantly reduced, achieving "alkalinity reduction" of chloride-salt-alkali soil. The detection method is the same as in Example 1. As can be seen from Table 3, the magnesium-iron biochar-based soil conditioner prepared by the preparation method provided by this invention can effectively improve the quality of chloride-type saline-alkali soil.

[0047] Example 3: (1) Waste fruit wood straw is crushed to a mesh size of 200 to obtain waste fruit wood biomass powder; (2) The magnesium-based mineralizer was prepared by mixing magnesium oxide and magnesium phosphate in a 1:1 ratio. The product obtained in (1) and the magnesium-based mineralizer were mixed at a mass ratio of 1 kg : 0.20 kg and then subjected to wet ball milling with water. The mass ratio of ball to material was 4:1, the milling time was 4 h, and the mass-volume ratio of the mixture to water was 1 kg : 0.01 m³. 3 After ball milling, the pH of the sample was adjusted to 4.0 using an acidic hydrothermal solution (1 mol·L⁻¹ for the first time). -1Adjust with hydrochloric acid, and then use the hydrothermal liquid obtained in step (4) and dry it by evaporation at 90 °C.

[0048] (3) The reaction products obtained in (2) are subjected to a mixed atmosphere (oxygen and nitrogen volume ratio 1:9, gas flow rate 120 m³ / h). 3 ·h -1 The pyrolysis was carried out in a tubular furnace with limited oxygen at a temperature of 210 °C for 5 h to obtain the pyrolysis product. The pyrolysis product was mixed with pure water, stirred and filtered to obtain a heat source organic matter solution and a biochar precursor. The volume-to-mass ratio of liquid to solid was controlled at 3 kg: 1 kg, the stirring speed was controlled at 350 rpm and the stirring time was controlled at 1 h.

[0049] (4) The iron-based mineralizer is prepared by mixing equimolar amounts of ferric acetate and ferric sulfate; the organic dispersant is prepared by mixing an equimolar amount of cationic surfactant cetyltrimethylammonium bromide (CTAB) and nonionic surfactant polyvinylpyrrolidone (PVP-k30) solution; the mixed organic acid is prepared by mixing an equivolute amount of lactic acid and citric acid; 1 kg of the biochar precursor prepared in step (3), 0.02 kg of the iron-based mineralizer, 0.001 kg of the organic dispersant, and 0.02 kg of the mixed organic acid are mixed together; the resulting mixed reactants are subjected to a three-stage gradient hydrothermal reaction: the material filling degree of the high-temperature and high-pressure reactor is set to 30%, the hydrothermal reactor is heated to 100 ℃ and the reaction is maintained for 3.0 h, then the temperature is raised to 160 ℃ and the reaction is maintained for 3.0 h, and finally the temperature is raised to 210 ℃ and the reaction is maintained for 3.0 h. Nano-magnesium iron biochar and hydrothermal liquid are obtained through hydrothermal reaction.

[0050] (5) The compound agent is prepared by mixing eggshells and zeolite in a mass ratio of 1:1; the nano-magnesium-iron biochar obtained in (4), the heat source organic matter solution obtained in (3), and the compound agent obtained in (5) are mixed and granulated in a mass-volume ratio of 1 Kg: 0.5 Kg: 0.2 Kg; finally, the pH range of the compound granulated product is adjusted to 6.0 using hydrothermal liquid to prepare the product magnesium-iron biochar-based soil conditioner. The granulated acidic magnesium-iron biochar-based soil conditioner products all meet the standards for acidic humic acid soil conditioners in HG / T 5782-2020 "Humic Acid Soil Conditioner".

[0051] (6) In this implementation case, sulfate-saline soil was collected from a reclaimed agricultural plot in Inner Mongolia Autonomous Region, China (108°66′684″N, 40°69′415″E). This area is rich in sodium sulfate. The soil was collected from a wasteland used for corn production. The basic properties of the soil are shown in Table 4. The soil pH is extremely alkaline at 8.85, and the salinity is 8.45 ds·m. -1 It belongs to severely saline-alkali soil.

[0052] The mass ratio of the magnesium-iron biochar-based soil conditioner obtained from (5) to the 10 cm sulfate-alkali soil layer was controlled at 2:100. A leaching-based zoned irrigation method was adopted. After applying the soil conditioner and preparing the land, ridges (2 m × 5 m) were built according to the plot, and the ridge surface was kept basically level. Clean water was slowly introduced from a high place to form a 5-10 cm static water layer on the surface. The flow rate was controlled to avoid scouring. Each leaching was carried out for 8-12 hours, and water was replenished intermittently according to the infiltration situation. After each round of leaching, the water was stopped for 24 hours. After the water infiltrated and the drainage of the underground pipe was basically stable, the infiltration water sample was collected. The underground pipe was a perforated PVC pipe, buried at a depth of about 0.9 m, with a spacing of 4-5 m and a slope of about 0.3%. It was wrapped with non-woven fabric and covered with 10 cm of medium-coarse sand, and then backfilled with the original soil to prevent blockage. The outlet was connected to a collection well, and the drainage volume and water quality were recorded regularly.

[0053] Table 4. Basic soil physicochemical properties before and after leaching remediation of sulfate-type saline-alkali soils using underground pipes. X-ray testing was performed on the sulfate-saline soil remediated by the magnesium-iron biochar-based soil conditioner. Figure 7 As shown, soil minerals exhibit distinct diffraction peaks at 10.5°, 21.6°, 26.5°, and 59.5°, with the triplet diffraction peaks corresponding to the diffraction peaks (003, 006, 0012, 110) of magnesium-iron layered double hydroxide (Mg-Fe LDH) crystals. This indicates the presence of newly formed Mg-Fe LDH in the soil minerals. Sulfates (SO4) in the soil... 2- The free sulfate anions in the soil are mineralized to form Mg-Fe LDH, thus becoming part of the stable minerals and achieving "salt fixation" in sulfate-saline-alkali soils. Table 3 shows the pH and water-soluble Na+ in the soil cations. + Significantly reduced, achieving "alkalinity reduction" of sulfate-alkali soils. The detection method is the same as in Example 1. As can be seen from Table 4, the magnesium-iron biochar-based soil conditioner prepared by the preparation method provided by this invention can effectively improve the quality of sulfate-type saline-alkali soils.

[0054] Example 4: Field planting trials were conducted as follows: (1) Experimental soil and treatment setup Field trials were conducted on the three types of saline-alkali soil corresponding to Examples 1-3: Example 1 used carbonate-type saline-alkali soil, Example 2 used chloride-type saline-alkali soil, and Example 3 used sulfate-type saline-alkali soil. Three treatments were set up for each soil type. ① Blank control (Comparative Examples 1, 2, 3): Original saline-alkali soils collected in the corresponding examples were selected respectively. No soil conditioners were applied, and the land was cultivated and prepared according to local conventional methods to reflect the saline-alkali hazards of the original soil. ② Treatments of the present invention (Examples 1, 2, 3): Magnesium-iron biochar-based soil conditioner was applied to the corresponding saline-alkali soils according to the mass ratios described in Examples 1, 2, and 3 (carbonate type 3:100, chloride type 2:100, sulfate type 2:100), and thoroughly mixed with the 0–10 cm topsoil before land preparation. Except for the differences mentioned above, other fertilization, irrigation, and field management conditions remained consistent for each treatment.

[0055] (2) Field layout and leaching underground pipe conditions Each treatment employed a randomized block design, with plots measuring 2 m × 5 m. Ridges were constructed along the raised beds, maintaining a generally level surface. After applying a conditioner or ordinary biochar and mixing it with the soil, a subsurface drainage system was installed as described in Examples 1-3: perforated PVC subsurface pipes were buried at a depth of 0.9 m, spaced 4-5 m apart, with a slope of approximately 0.3%. The pipes were covered with non-woven fabric and then covered with 10 cm of medium-coarse sand before backfilling with the original soil. Pre-leaching was then carried out using a "zoned irrigation and leaching" method: water was slowly introduced from a higher elevation to create a 5-10 cm still water layer on the surface. Each leaching cycle lasted 8-12 hours, with water replenished as needed based on infiltration levels. After each leaching cycle, water was stopped for 24 hours until the subsurface drainage stabilized, at which point the drainage system was discontinued. Pre-leaching was performed at least 2-3 times to remove free soluble salts and ensure that the initial salinity of each treatment remained stable.

[0056] (3) Corn sowing and growth period management Sowing should be carried out after leaching has ended and the soil surface has been naturally air-dried to a suitable moisture content. Locally grown medium-maturing maize varieties should be selected. Hill sowing should be used, with a row spacing of 40 cm and a plant spacing of 20 cm. Two seeds should be sown per hill, resulting in approximately 50-60 seeds per plot. Cover with 2-3 cm of soil after sowing. No additional soil conditioners should be applied throughout the entire growth period. All treatments should receive the same amount of basal fertilizer (e.g., 150 kg / hm² of pure nitrogen). -2 P2O5 75 kg·hm -2 K2O 75 kg·hm -2 Field management (weeding, pest control, and supplemental irrigation) was carried out according to local conventional agronomic practices. The number of seedlings emerging was recorded during the seedling stage (7–14 days after sowing), and the number and height of surviving corn seedlings were recorded at 30 days to calculate the "corn seedling survival rate." The experimental growth period lasted 30 days, and the average plant height of each plot was also recorded in this example. By comparing the differences between the blank control and the treatment of this invention in the above indicators, the comprehensive remediation effect of the magnesium-iron biochar-based soil conditioner on promoting corn emergence in different types of saline-alkali soils was systematically evaluated, as shown in Table 5.

[0057] Table 5 Growth index of maize seedlings at 30 days In the field trials, the only difference between the treatments was in the soil improvement measures: Comparative Examples 1-3 were blank controls for the corresponding saline-alkali soils without any improvement; Examples 1-3 involved applying the magnesium-iron biochar-based soil conditioner prepared in this invention to different types of saline-alkali soils, while other crop varieties, sowing densities, fertilization, and irrigation management conditions remained consistent. Therefore, the differences in maize seedling growth indicators in the table can be directly attributed to the different improvement measures.

[0058] The data in the table show that, in the three types of saline-alkali soils (carbonate, chloride, and sulfate), the survival rate of maize seedlings in the blank control and comparative examples was only 0-5%, the average plant height was 8-10 cm, the average root length was only 8-10 cm, and the average fresh weight and dry weight were extremely low. This indicates that under conditions of strong salinity and ion toxicity, it is difficult to restore soil productivity by relying solely on natural leaching or ordinary biochar.

[0059] In contrast, in Examples 1-3, where the magnesium-iron biochar-based soil conditioner of this invention was applied, the survival rate of maize seedlings significantly increased to 93-98%, the average plant height stabilized at 35-38 cm, the average root length reached 26-28 cm, the average fresh weight increased to 4.05-5.41 g, and the average dry weight increased to 2.04-3.45 g. Compared with their respective corresponding proportions, the fresh and dry weight of seedlings increased by more than an order of magnitude, the plant height and root length were about 2.5-3 times that of the control, and the survival rate improved from almost zero to nearly full emergence.

[0060] Combined with the aforementioned changes in soil physicochemical properties (significant decrease in pH and electrical conductivity, and decrease in water-soluble Na+), + and CO3 2- / Cl - SO4 2- (significantly reduced and significantly increased water holding capacity) The above results indicate that, under three different salinity conditions, the magnesium-iron biochar-based soil conditioner of this invention can effectively alleviate salt stress, significantly improve the rhizosphere environment, and restore or even rebuild the crop productivity of saline-alkali soils, while the unmodified soil in the comparative example could not achieve this remediation effect.

[0061] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils, comprising the following steps: (1) The waste biomass is crushed to obtain biomass powder; (2) Add magnesium mineralizer to the biomass powder obtained in step (1) and perform wet ball milling, adjust the pH with hydrothermal liquid, and evaporate and dry; (3) The product obtained in step (2) is subjected to oxygen-limited pyrolysis, and the pyrolysis product is extracted with water to separate the heat source organic matter solution to obtain biochar precursor; (4) Add iron-based mineralizer, organic dispersant and mixed organic acid to the biochar precursor obtained in step (3) and mix them together, and carry out a three-stage gradient hydrothermal reaction to obtain hydrothermal liquid and nano-magnesium iron biochar. The hydrothermal liquid is reused as an acid-base regulator in step (2). (5) The nano-magnesium iron biochar obtained in step (4) is directly mixed with the heat source organic matter solution, and then a compounding agent is added. The acidity and alkalinity are adjusted by hydrothermal liquid to prepare an acidic magnesium iron biochar-based soil conditioner.

2. The method for preparing a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils according to claim 1, characterized in that, The waste biomass in step (1) includes one or more of the following: corn, cotton, rice husks, biogas residue, and fruit wood; Biomass is crushed to 50-300 mesh.

3. The method for preparing a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soil according to claim 1, characterized in that, In step (2), the magnesium mineralizer is composed of one or more of magnesium oxide, magnesium sulfate, magnesium chloride, magnesium carbonate, and magnesium ammonium phosphate. The mass ratio of biomass powder to magnesium-based mineralizer is 1:(0.05~0.20); Wet ball milling is a water-added wet ball milling process with a ball-to-material mass ratio of (1~10):1, a milling time of 2~4 hours, and a water addition rate controlled at a mass-to-volume ratio of 1:(0.01~0.05) kg / m³ for the mixture. 3 ; The pH of the ball-milled product was adjusted to 3.0 ~ 6.0 using hydrothermal fluid; the evaporation drying temperature was 65 ~ 100 ℃, and the drying time was 0.5 ~ 1.5 h.

4. The preparation method of a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soil according to claim 1, characterized in that, In step (3), the pyrolysis temperature for oxygen-limited pyrolysis is 150-300℃; the pyrolysis time is 3-6h; the oxygen-limited atmosphere is a mixture of oxygen and nitrogen with a volume ratio of 1:(5-10); and the gas flow rate is controlled at 50-150 m³ / h. 3 ·h -1 ; The solution for extracting the pyrogen organic matter from the pyrolysis products by water extraction is as follows: pure water is mixed with the pyrolysis products, stirred, and then filtered. The volume-to-mass ratio of liquid to solid is (2 ~ 5): 1 L / Kg. The stirring speed is controlled at 200 ~ 500 rpm, and the stirring time is controlled at 0.5 ~ 2.0 h. The filtered solution is the pyrogen organic matter solution.

5. The method for preparing a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils according to claim 1, characterized in that, In step (4), the iron-based mineralizer is composed of one or more of the following: ferric chloride, ferric acetate, ferric sulfate, ferric nitrate, polyferric sulfate, and ferric oleate. The organic dispersant is composed of one or more of cationic surfactants, anionic surfactants, or nonionic surfactants; wherein the cationic surfactants include hexadecyltrimethylammonium bromide, the anionic surfactants include ammonium dodecylbenzenesulfonate and sodium dodecyl sulfate, and the nonionic surfactants include one or more of polyvinylpyrrolidone and Triton. The mixed organic acids consist of one or more of acetic acid, citric acid, oxalic acid, and lactic acid; The mass ratio of biochar precursor to iron-based mineralizer is 1:(0.01~0.05); the solid-to-liquid mass-to-volume ratio of biochar precursor, organic dispersant, and mixed organic acid is 1:(0.001~0.005):(0.01~0.05) Kg / L / L. The three-stage gradient hydrothermal system involves placing the material in a high-temperature, high-pressure hydrothermal reactor with a material filling rate of 30-50%. The hydrothermal temperature is set in three gradient ranges: 80-100℃, 150-180℃, and 210-230℃. The hydrothermal time for each stage is 1-3 hours.

6. The method for preparing a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soil according to claim 1, characterized in that, In step (5), the solid-to-liquid mass-to-volume ratio of the mixture of nano-sized magnesium-iron biochar and heat source organic matter solution is 1:(1~5) Kg / L; The compound agent is composed of one or more of the following: wood ash, attapulgite clay, eggshells, furfural residue, zeolite, and biogas residue; The mass ratio of nano-sized magnesium-iron biochar to the compounding agent is controlled at 1:(0.2 ~ 0.4); The pH range of the hydrothermal liquid-adjusted compound granulation product is 3 to 6.

7. A magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils, prepared by the method according to any one of claims 1 to 6.

8. The application of a magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils as described in claim 7, characterized in that, The magnesium-iron biochar-based soil conditioner was applied to saline-alkali soil.

9. The application of the magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils according to claim 8, characterized in that, Follow these steps: Mix magnesium-iron biochar-based soil conditioner with saline-alkali soil at a mass ratio of (1~5):100, and achieve "salt fixation and alkali reduction" remediation of saline-alkali soil through physical remediation methods.

10. The application of the magnesium-iron biochar-based soil conditioner for "salt fixation and alkali reduction" in saline-alkali soils according to claim 8, characterized in that, Saline-alkali soil types include one of the following: carbonate alkaline soil, sulfate alkaline soil, and chloride alkaline soil. The physical repair methods include one or more of leaching and concealed pipe treatment.

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