A method for enhancing gypsum amendment of alkaline soils by overcoming the sulfate common ion effect through in situ layered double hydroxide mineralization

Abstract

This invention discloses a method for improving alkaline soils by overcoming the sulfate common ion effect through in-situ layered bimetallic hydroxide mineralization. The method comprises the following steps: S1, applying gypsum to the alkaline soil, causing the gypsum to dissolve and release calcium ions to replace exchangeable sodium ions on soil colloids, while sulfate ions accumulate in the soil solution; S2, after step S1, applying a synergist including magnesium nitrate and aluminum nitrate to the alkaline soil, causing magnesium and aluminum ions to undergo an in-situ co-precipitation reaction with the accumulated sulfate ions in the soil solution, generating Mg-Al-SO4 type layered bimetallic hydroxides, thereby consuming the accumulated sulfate ions, breaking the inhibition of gypsum dissolution by the sulfate common ion effect, and promoting the continuous dissolution and release of calcium ions from gypsum. This method, without increasing the amount of gypsum used, can overcome the limitation of the common ion effect and increase the calcium content of gypsum. 2+ Release efficiency.

Description

Technical Field

[0001] This invention belongs to the field of soil improvement technology, specifically relating to a method for enhancing the effect of gypsum in improving alkaline soil by overcoming the sulfate common ion effect through in-situ layered bimetallic hydroxide (LDHs) mineralization. Background Technology

[0002] Sodic soil is one of the most serious forms of land degradation globally, characterized by excessively high salicylic acidity (ESP), alkaline pH, deteriorated soil structure, and reduced permeability. It is estimated that there are over 1 billion hectares of saline-alkali land worldwide, and this area continues to expand. China is a major country with saline-alkali land, possessing approximately 3.7 × 10⁻⁶ hectares. 7 The area of ​​saline-alkali land resources, covering several hectares, is mainly distributed in the Songnen Plain of Northeast China, the inland areas of Northwest China, and coastal regions. Excessive Na+ in alkalized soils... + The high content of certain substances leads to the dispersion of soil colloids, the destruction of aggregates, and a sharp decline in permeability, severely restricting agricultural production and ecological environment security.

[0003] Gypsum (CaSO4·2H2O) treatment is currently the most widely used chemical remediation strategy for alkaline soils. Its core mechanism involves the release of Ca2+ through the dissolution of gypsum. 2+ Replace exchangeable Na on soil colloids + Meanwhile, Ca 2+ With basic anions (HCO3-) and CO3 2 The reaction produces insoluble calcite (CaCO3), thereby lowering soil pH and ESP. However, gypsum remediation faces a fundamental thermodynamic bottleneck in practical applications—the sulfate common-ion effect. The gypsum dissolution reaction is a reversible equilibrium process: CaSO4·2H2O Ca 2+ + SO4 2- + 2H₂O. As the modified reaction proceeds, the calcite precipitation consumes Ca. 2+ It reacts with basic anions, but leaves behind the byproduct SO4. 2 It accumulates continuously in the soil solution. According to Le Chatelier's principle, SO42- 2- The increase in concentration shifts the chemical equilibrium to the left, inhibiting further dissolution of gypsum and leading to an increase in Ca2+ concentration. 2+ Release efficiency is significantly reduced.

[0004] To overcome this bottleneck, traditional methods involve excessive application of gypsum or heavy irrigation leaching. However, excessive gypsum application leads to degradation of soil cation exchange capacity (CEC) and secondary salinization, while heavy irrigation is unsustainable in water-scarce regions. Therefore, the question arises: how to effectively consume or fix accumulated SO4 without increasing gypsum application? 2- Breaking the limitations of the common ion effect is the core challenge in optimizing gypsum technology for improving alkaline soil.

[0005] In recent years, layered bimetallic hydroxide (LDH) mineralization technology has been introduced into the field of alkali soil improvement. The general formula for LDHs is [M... 2+ 1-x M 3+ x (OH)2] x+ (An-) x / n ·yH2O is a type of anionic clay with adjustable lamellar cations and exchangeable interlayer anions, possessing the ability to fix specific anions through structural reconstruction or coprecipitation. In the prior art, Sun et al. (2022) disclosed a method for improving soda saline-alkali soil by in-situ formation of a CaFe-LDH ultrastable mineralization structure using CaSO4+Fe2(SO4)3+attapulgite (Chemosphere, 300, 134543). This method utilizes Ca… 2+ and Fe 3+ In alkaline soil environment with OH - and CO3 2- Co-precipitation forms CaFe-LDH, which fixes OH- in the soil. - and CO3 2- This lowers the pH. However, this method has the following drawbacks: First, its mineral agent uses a Ca / Fe sulfate system, and the dissolution of Fe2(SO4)3 introduces additional SO4 into the soil. 2- Instead of alleviating the common ion effect, it exacerbates it; secondly, its technical goal is to fix OH groups. - / CO3 2 Lowering the pH did not involve consuming the SO4 accumulated during the gypsum modification process. 2- To overcome the common ion effect; third, the formation of CaFe-LDH consumes Ca. 2 + The calcium released by the gypsum 2+ The secondary fixation in the LDH lattice weakens the Ca... 2+ To Na + The replacement efficiency.

[0006] In addition, Chai et al. (2025) disclosed the effects of vermiculite on in-situ hyperstable mineralization for improving alkaline soils (Environmental Technology & Innovation, 38, 104156), Diao et al. (2025) disclosed a method for synergistic improvement of saline-alkali soils using Ca-bentonite and in-situ LDH hyperstable mineralization (Science of the Total Environment, 971, 179084), and Fei et al. (2026) disclosed a sulfate-attapulgite composite amendment (Soil and Tillage Research, 256, 106872). All of these existing technologies use a Ca-Fe sulfate system as the mineralizing agent, focusing on fixing OH- through LDHs. - / CO3 2- Lowering the pH did not involve addressing SO4 during the gypsum modification process. 2- The Mg / Al nitrate system was not used as a synergist to overcome the cumulative effect of common ions.

[0007] Therefore, there is an urgent need in the existing technology for a method that can effectively consume the SO4 accumulated during the process of gypsum remediation of alkaline soil. 2- Breaking the common ion effect of sulfate and simultaneously avoiding Ca 2+ Innovative technical solutions for secondary fixation. Summary of the Invention

[0008] To address the aforementioned shortcomings of existing technologies, the present invention aims to provide a method for enhancing gypsum-based alkaline soil improvement by overcoming the sulfate common ion effect through in-situ layered bimetallic hydroxide mineralization. This method avoids the degradation of soil cation exchange capacity (CEC) and secondary salinization caused by excessive gypsum application, as well as the waste of water resources caused by excessive irrigation and leaching. Furthermore, it breaks the limitation of the common ion effect and increases the Ca content of gypsum. 2+ Release efficiency.

[0009] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for improving alkaline soil, comprising the following steps: S1. Applying gypsum to alkaline soil causes the gypsum to dissolve and release calcium ions to replace exchangeable sodium ions on soil colloids, while sulfate ions accumulate in the soil solution. S2. After step S1 is completed, a synergist including magnesium nitrate and aluminum nitrate is applied to the alkalized soil, so that the magnesium ions and aluminum ions in the synergist react with the sulfate ions accumulated in the soil solution in situ to form Mg-Al-SO4 type layered bimetallic hydroxide, thereby consuming the accumulated sulfate ions, breaking the inhibition of gypsum dissolution by the sulfate common ion effect, and promoting the continuous dissolution of gypsum to release calcium ions; wherein, the molar ratio of magnesium nitrate to aluminum nitrate in the synergist is (2~3):1.

[0010] In some embodiments of the present invention, the alkalinity (ESP) of the alkaline soil is greater than 15% and the soil pH is greater than 8.5.

[0011] In some embodiments of the present invention, the amount of gypsum applied is 0.5% to 5% of the mass of the alkalized soil.

[0012] In some embodiments of the present invention, the amount of the synergist applied is 0.3% to 2.0% of the mass of the alkalized soil.

[0013] In some embodiments of the present invention, the synergist is applied 1 to 30 days after applying gypsum in step S1; preferably, after applying gypsum in step S1, water is added to the soil to saturation, and the soil is pre-cultured at 20 to 30°C for 5 to 10 days to fully dissolve the gypsum and allow sulfate ions to accumulate to dynamic equilibrium.

[0014] In some embodiments of the present invention, the magnesium nitrate is magnesium nitrate hexahydrate Mg(NO3)2·6H2O, and the aluminum nitrate is aluminum nitrate nonahydrate Al(NO3)3·9H2O.

[0015] In a preferred embodiment of the present invention, the molar ratio of magnesium nitrate to aluminum nitrate in the synergist is 3:1.

[0016] In some embodiments of the present invention, the chemical formula of the Mg-Al-SO4 type layered bimetallic hydroxide generated by the in-situ co-precipitation reaction is Mg 12 Al4(OH) 24 The Gibbs free energy of the (SO4)2 formation reaction is -48.17 eV.

[0017] In some embodiments of the present invention, the method further includes step S3: after the synergist is applied in step S2 and reacts fully for ≥3 days, irrigation and leaching are performed to remove the displaced sodium ions and soluble salts.

[0018] In a second aspect, the present invention provides the use of synergistic compositions comprising magnesium nitrates and aluminum nitrates in any of the following: A1) Gypsum can be used to improve alkaline soil or as a gypsum synergist; furthermore, the improvement of alkaline soil is manifested in reducing pH value, reducing alkalinity, and reducing SO4. 2- Concentration, increasing water-soluble Ca 2+ Concentration, increase water-soluble Mg 2+ Concentration, reducing water-soluble Na + Concentration, increase NO3 - One or more of the concentrations; A2) Promote crop growth; further, the crop is rice, and the promotion of growth is manifested in one or more of increasing plant height and increasing aboveground dry weight; In the synergistic agent composition, the molar ratio of magnesium nitrate to aluminum nitrate is (2-3):1; The application involves applying the synergistic agent composition to alkalized soil after gypsum application, thereby consuming the sulfate ions accumulated in the soil through the in-situ formation of Mg-Al-SO4 type layered bimetallic hydroxides, thus overcoming the sulfate common ion effect and enhancing the calcium ion release efficiency of gypsum.

[0019] In a preferred embodiment of the present invention, the molar ratio of magnesium nitrate to aluminum nitrate is 3:1.

[0020] The present invention has the following beneficial effects: (1) The common ion effect of sulfate is effectively overcome for the first time. This invention proposes and verifies for the first time the method of consuming accumulated SO4 through in-situ mineralization of LDHs. 2- A technical strategy was developed to overcome the common ion effect in gypsum-modified alkaline soils. Experimental results showed that G+S treatment reduced soil SO4 levels. 2 The concentration was reduced by 25.2% compared to gypsum treatment alone, and the water-soluble Ca... 2+ Increased by 51.8%, Ca 2+ / Na + The molar ratio increased by about 800%, confirming the effective overcoming of the common ion effect.

[0021] (2) The improvement effect is significantly better than existing technologies. The G+S treatment reduced the pH of alkaline soil by 2.0 units and the ESP by 41.8%, which is 0.24 pH units and 44.5% higher than the gypsum-only treatment, respectively. Exchangeable Na + It was reduced by 65.9% compared to the control group, and further reduced by 46.6% compared to the gypsum treatment alone.

[0022] (3) Avoiding Ca 2+ Secondary fixation to achieve SO4 2-Directional solidification. Density functional theory (DFT) calculations show that the formation energy of Mg-Al LDHs (ΔG = -48.17 eV) is significantly lower than that of Ca-Al LDHs (ΔG = -40.63 eV), and its thermodynamic formation advantage can effectively avoid Ca... 2+ It is fixed a second time, thus ensuring Ca 2+ Soil Na + The substitution efficiency is high. This characteristic is a unique technical advantage that existing Ca-Fe LDHs systems do not possess—the phase formation process of Ca-Fe LDHs itself requires a large amount of Ca... 2+ The aforementioned regulatory effect cannot be achieved. Furthermore, the affinity adsorption order of LDHs for anions is: CO3... 2- SO4 2- >OH >Cl NO3 - CO3 in alkalized soil improved with gypsum 2- The content decreases significantly, effectively eliminating interference from competing anions and benefiting SO42-. 2- Selective curing.

[0023] (4) Ion matrix nutrient transformation. Synergists transform the soil ion composition from HCO3- to HCO3--. CO3 2- The toxic type, mainly NO3, has been transformed into a type of NO3. Mg 2+ Ca 2+ Primarily nutritional. NO3 - The concentration increased by 21596.7% compared to the control, water-soluble Mg 2+ Compared to gypsum treatment alone, it increased by 318.0%, providing essential nutrients such as nitrogen and magnesium for crop growth.

[0024] (5) Significantly promoted crop growth. The aboveground dry weight of rice seedlings increased by 72.6% and the plant height increased by 21.8% compared with the gypsum treatment alone. The survival rate was 161.2% to 176.9% higher than the control.

[0025] (6) Simple operation and low cost. This invention only requires an additional application of synergist after the application of gypsum, without the need for special equipment, and Mg / Al nitrate is a common chemical raw material, so the cost is controllable and it is suitable for large-scale promotion. Attached Figure Description

[0026] Figure 1The changes in key salinity parameters of alkalized soil under different treatments are shown. Among them, (a) represents soil pH, (b) represents soil alkalinity (ESP), and (c) represents soil electrical conductivity (EC). CK is the control, G is the gypsum treatment alone, and G+S is the gypsum + synergist treatment. The bar chart represents the mean ± standard deviation (n=5), and different lowercase letters indicate significant differences between treatments (p<0.05, LSD test).

[0027] Figure 2 The reconstruction of soil water-soluble ionic matrix under different treatments is shown; where (af) is the concentration of major water-soluble ions, (g) is the mass percentage of major water-soluble ions; CK is the control, G is gypsum treatment alone, and G+S is gypsum + synergist treatment.

[0028] Figure 3 The dynamic changes in soil exchangeable cations under different treatments are shown; CK is the control, G is gypsum treatment alone, and G+S is gypsum treatment plus synergist.

[0029] Figure 4 The images provide mineralogy evidence for the formation of LDHs in situ. Among them, (a) is the full XRD spectrum of the soil, (b) is the magnified X-ray diffraction (XRD) spectrum in the range of 2θ = 10° to 50°, highlighting the characteristic peaks of gypsum and LDHs, (c) is the Fourier transform infrared spectrum (FTIR), and (df) are scanning electron microscope (SEM) images processed by CK, G and G+S, respectively.

[0030] Figure 5 The effects of different treatments on mitigation of phytotoxicity and growth promotion in rice seedlings are shown. Among them, (a) represents the survival rate, (b) represents the plant height, (c) represents the aboveground dry weight, and (d) represents a representative seedling photograph.

[0031] Figure 6 Analysis of the main soil drivers for rice seedling growth; (a) is a visualization of the Mantel test, and (bd) are the relative importance of soil variables to survival rate, plant height and aboveground dry weight quantified by random forest analysis.

[0032] Figure 7 Synergistic agents with different Mg / Al molar ratios on soil pH improvement and SO42- reduction 2- Removal and Ca 2+ The effects of release were compared, and different lowercase letters indicated significant differences between treatments (p<0.05, LSD test). Detailed Implementation

[0033] As described in existing research and background art, a key but often overlooked limiting factor in the practical application of gypsum-based soil improvement is the common ion effect induced by reaction byproducts. As the improvement process continues, calcium carbonate precipitation consumes calcium ions and basic anions, leading to the formation of a common ion effect caused by the counterion sulfate (SO42-). 2- Sulfate ions are free and continuously accumulate in the soil solution. According to Le Chatelier's principle, the continuous accumulation of sulfate ions will reverse the gypsum dissolution equilibrium, thereby strongly inhibiting further dissolution of gypsum.

[0034] To overcome this thermodynamic bottleneck, practical applications can only rely on excessive application of gypsum or large-scale irrigation to leach accumulated sodium sulfate. However, excessive gypsum application can damage the soil's cation exchange capacity (CEC) and trigger secondary salinization; while large-scale irrigation lacks hydrological sustainability in water-scarce areas. Therefore, the core challenge in optimizing gypsum-based improvement of saline-alkali soils is not simply increasing the amount of gypsum applied, but rather consuming or fixing the accumulated sulfate ions in situ at the reaction interface, thereby breaking the constraint of the common ion effect.

[0035] LDH is a collective term for hydrotalcite (HT) and hydrotalcite-like compounds (HTLCs). A series of supramolecular materials assembled by intercalation of these compounds are called hydrotalcite-like intercalated materials (LDHs). A typical example is magnesium aluminum hydrotalcite (Mg6Al2(OH)). 16 CO3·4H2O, its lamellar structure is similar to that of brucite (Mg(OH)2), but it is separated by trivalent metal ions (such as Al). 3+ Introducing positive charges into the plates. The solubility product constant (Ksp) of LDHs is typically >10. -30 It is a very stable mineral product. In view of this, this invention, by adding materials capable of forming LDHs after improving saline-alkali soil with desulfurized gypsum, can generate anions in the soil to form SO42-. 2- The main component is hydrotalcite, which helps reduce the large amount of SO4 introduced during the application of desulfurized gypsum to improve saline-alkali soil. 2- This reduces the inhibitory effect of the common ion effect on gypsum dissolution, breaks the gypsum precipitation-dissolution balance, and achieves multiple effects such as promoting gypsum dissolution, synergistic improvement of soil physicochemical properties, and promoting crop growth.

[0036] In a first aspect, the present invention provides a method for improving alkaline soil, comprising the following steps: S1. Applying gypsum to alkaline soil causes the gypsum to dissolve and release calcium ions to replace exchangeable sodium ions on soil colloids, while sulfate ions accumulate in the soil solution. S2. After step S1 is completed, a synergist including magnesium nitrate and aluminum nitrate is applied to the alkalized soil, so that the magnesium ions and aluminum ions in the synergist react with the sulfate ions accumulated in the soil solution in situ to form Mg-Al-SO4 type layered bimetallic hydroxide, thereby consuming the accumulated sulfate ions, breaking the inhibition of gypsum dissolution by the sulfate common ion effect, and promoting the continuous dissolution of gypsum to release calcium ions; wherein, the molar ratio of magnesium nitrate to aluminum nitrate in the synergist is (2~3):1.

[0037] Based on the above technical solution, unlike the existing technology that uses Ca... 2+ and Fe 3+ In alkaline soil environment with OH and CO3 2- Co-precipitation forms CaFe-LDH to fix OH in the soil. and CO3 2- The present invention addresses the technical problem of effectively consuming or fixing accumulated SO4 without increasing the amount of gypsum used, by incorporating the concept of lowering pH. 2- This invention overcomes the limitations of the common ion effect and optimizes gypsum-based soil remediation technology. The inventors have taken a novel approach: after applying gypsum to alkaline soil, they apply magnesium and aluminum nitrates. This causes magnesium and aluminum ions to co-precipitate in situ with accumulated sulfate ions in the soil solution, generating Mg-Al-SO4 type layered bimetallic hydroxides. This breaks the inhibition of gypsum dissolution by the common ion effect of sulfate, promoting the continuous dissolution and release of calcium ions. This invention is the first to propose and verify the use of in-situ LDHs mineralization to consume accumulated SO4. 2- A technical strategy to break the common ion effect in gypsum-modified alkaline soil.

[0038] According to embodiments of the present invention, the target soil for improvement is alkalized soil, which refers to a soil type in which soil colloids adsorb a large amount of exchangeable sodium and exhibit a strongly alkaline reaction. Further, in this invention, the alkalinity (ESP) of the alkalized soil is greater than 15%, and the soil pH is greater than 8.5. Compared to using gypsum alone for improvement, the method of the present invention can significantly reduce the pH and ESP of alkalized soil. As an example, this invention selected alkalized soil from the Songnen Plain (45°14'N, 123°14'E) in Jilin Province, China, for experiments. The sampling depth was 0–20 cm of topsoil, with an ESP of 62.9% and a pH of 9.66.

[0039] According to embodiments of the present invention, the amount of gypsum applied is 0.5% to 5% of the mass of the alkaline soil, including but not limited to 2%. The main component of the gypsum is calcium sulfate dihydrate (CaSO4·2H2O). Exemplarily, the purity of the gypsum is ≥99.0%. It is understood that the purity of the gypsum does not necessarily have to be ≥99.0%, and industrial-grade products can also be used. Depending on the source, the gypsum can be selected from one or more of natural gypsum, desulfurized gypsum, and phosphogypsum. The amount of gypsum applied needs to be comprehensively determined based on the degree of soil alkalization, crop type, and improvement goals. This invention, by breaking the common ion effect in gypsum improvement of alkaline soil, can promote the continuous dissolution and release of calcium ions from gypsum, significantly increasing the soil's calcium content. 2+ / Na + This promotes the stability of soil aggregate structure and reduces the risk of clay particle dispersion and pore blockage, while Ca 2+ With basic anions (HCO3-) and CO3 2- The reaction produces insoluble calcite (CaCO3). Furthermore, the Mg dissolved by the synergist... 2+ It will participate in replacing exchangeable Na on soil colloids. + This further reduces soil ESP and can prevent excessive application of desulfurized gypsum.

[0040] According to an embodiment of the present invention, the application amount of the synergist (i.e., the total application amount of magnesium nitrate and aluminum nitrate) is 0.3% to 2.0% of the mass of the alkalized soil, including but not limited to 0.9%. The application amount of the synergist is determined to enable the accumulated sulfate ions in the soil to undergo an in-situ co-precipitation reaction and be fully converted into Mg-Al-SO4 type layered bimetallic hydroxides. Insufficient application amount cannot break the common ion effect in gypsum-modified alkalized soil, while excessive application amount may lead to a significant increase in soil salinity. The high concentration environment formed in the short term may still exert osmotic stress on seedlings, thereby inhibiting crop growth.

[0041] According to an embodiment of the present invention, the synergist is applied from day 1 to day 30 after applying gypsum in step S1; preferably, after applying gypsum in step S1, water is added to the soil to saturation, and the soil is pre-cultured at 20 to 30°C for 5 to 10 days to fully dissolve the gypsum and allow sulfate ions to accumulate to dynamic equilibrium, such as static pre-culture at room temperature for 7 days.

[0042] In this application, the inventors selected magnesium nitrate and aluminum nitrate as divalent and trivalent metal salts for the formation of LDHs based on the following considerations: 1) The purpose of this invention is to consume accumulated SO4 through in-situ LDHs mineralization. 2- This aims to overcome the common ion effect in gypsum-modified alkaline soils, while the existing Ca-Fe LDHs self-phase formation process requires a large amount of Ca.2+ 1) The above-mentioned regulation effect cannot be achieved; 2) The formation energy of Mg-Al LDHs (ΔG = -48.17 eV) is significantly lower than that of Ca-AlLDHs (ΔG = -40.63 eV), and its thermodynamic formation advantage can effectively avoid Ca 2+ It is fixed a second time, thus ensuring Ca 2+ Soil Na + The replacement efficiency. 3) The affinity adsorption order of LDHs for anions is: CO32- 2- SO4 2- >OH >Cl NO3 - CO3 in alkalized soil improved with gypsum 2- The content decreases significantly, effectively eliminating interference from competing anions and benefiting SO42-. 2- Selective solidification. In addition, NO3 in the soil... and water-soluble Mg 2+ Compared to gypsum treatment alone, this method also provides essential nutrients such as nitrogen and magnesium for crop growth. For example, the magnesium nitrate is magnesium nitrate hexahydrate Mg(NO3)2·6H2O, and the aluminum nitrate is aluminum nitrate nonahydrate Al(NO3)3·9H2O.

[0043] According to an embodiment of the present invention, the molar ratio of magnesium nitrate to aluminum nitrate in the synergist is 3:1. The inventors have discovered that when the divalent and trivalent metal salts are composed of Mg... 2+ Al 3+ Furthermore, when the molar ratio is 3:1, LDHs exhibit the best mineralization effect, improving soil pH and reducing SO4 levels. 2- Removal and Ca 2+ The release ratio is significantly higher than 2:1, which may be related to the fact that the chemical formula of the generated Mg-Al-SO4 type layered bimetallic hydroxide is Mg 12 Al4(OH) 24 Related to (SO4)2.

[0044] According to an embodiment of the present invention, the method further includes step S3: irrigation and leaching are performed ≥3 days after the application of the synergist in step S2 to remove the displaced sodium ions and soluble salts.

[0045] In a second aspect, the present invention provides the use of synergistic compositions comprising magnesium nitrates and aluminum nitrates in any of the following: A1) Gypsum is used to improve alkaline soil or as a gypsum synergist; further, the improvement of alkaline soil is manifested in reducing pH value, reducing ESP, and reducing SO4. 2- Concentration, increasing water-soluble Ca 2+ Concentration, increase water-soluble Mg2+ Concentration, reducing water-soluble Na + Concentration, increase NO3 - One or more of the concentrations; A2) Promotes crop growth; further, the crop is rice (such as rice with moderate salt tolerance). Oryza sativa L The growth promotion of the variety "Songliao No. 5" is manifested in one or more of the following: increased plant height and increased aboveground dry weight. In the synergistic agent composition, the molar ratio of magnesium nitrate to aluminum nitrate is (2-3):1; The application involves applying the synergistic agent composition to alkalized soil after gypsum application, thereby consuming the sulfate ions accumulated in the soil through the in-situ formation of Mg-Al-SO4 type layered bimetallic hydroxides, thus overcoming the sulfate common ion effect and enhancing the calcium ion release efficiency of gypsum.

[0046] More preferably, the molar ratio of magnesium nitrate to aluminum nitrate is 3:1.

[0047] The present invention will now be described in further detail with reference to specific embodiments. The given embodiments are merely illustrative of the invention and not intended to limit its scope. The embodiments provided below can serve as a guide for further improvements by those skilled in the art and do not constitute a limitation on the invention in any way.

[0048] Unless otherwise specified, the methods used in the following embodiments are conventional methods, performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Unless otherwise specified, the materials and reagents used in the following embodiments are commercially available.

[0049] Example 1 1. Experimental Materials Alkaline soils were collected from the Songnen Plain, Jilin Province, China (45°14'N, 123°14'E), with sampling depths ranging from 0 to 20 cm in the topsoil. The soil was air-dried, ground, and sieved through a 2 mm nylon sieve before use. The initial physicochemical properties of the soil were as follows: pH 9.66, electrical conductivity 1403.6 μS / cm, and water-soluble potassium... + The concentration was 43.3 mg / kg, and the water-soluble Na... + The concentration was 425.3 mg / kg, and the water-soluble Ca... 2+ The concentration was 22.6 mg / kg, and the water-soluble Mg... 2+ The concentration was 50.2 mg / kg, and the water-soluble NO3 was... - The concentration is 7.8 mg / kg, water-soluble SO4 2- The concentration was 94.6 mg / kg, and the water-soluble CO3 content was...2- The concentration was 217.4 mg / kg, and the water-soluble HCO3- - The concentration was 662.9 mg / kg, and the alkalinity (ESP) was 62.9%. This soil is a typical strongly alkaline soil.

[0050] Chemical reagents: Gypsum (CaSO4·2H2O, purity ≥99.0%), magnesium nitrate hexahydrate (Mg(NO3)2·6H2O, purity ≥99.0%), and aluminum nitrate nonahydrate (Al(NO3)3·9H2O, purity ≥99.0%), all purchased from Shanghai Titan Technology Co., Ltd. Deionized water was used in the experiments.

[0051] 2. Experimental Design Three treatments were set up: (1) CK (control): 200 g of alkaline soil without any amendment; (2) G (gypsum alone): 2% (w / w) gypsum was added to the soil; (3) G+S (gypsum + synergist): 2% (w / w) gypsum was added to the soil first, followed by 0.9% (w / w) synergist. Each treatment was replicated 5 times in a completely randomized design.

[0052] The experimental container was a transparent polyethylene container (top dimensions 7 cm × 3 cm, bottom dimensions 4.5 cm × 4.5 cm, height 6.8 cm).

[0053] 3. Sequential application procedure Step 1 (Gypsum Application and Pre-cultivation): Add 60 mL of deionized water (corresponding to the soil saturation moisture content) to all treatments, stir thoroughly, and then pre-cultivate statically at room temperature for 7 days to allow the gypsum to fully dissolve and SO42- to be released. 2- It accumulates in the soil solution until a dynamic reaction equilibrium is reached.

[0054] Step 2 (Synergistic Agent Application): Dissolve 1.2 g Mg(NO3)2·6H2O and 0.6 g Al(NO3)3·9H2O (molar ratio 3:1) in 15 mL of deionized water and add it evenly to the G+S treatment. Simultaneously, add 15 mL of deionized water to both the CK and G treatments to maintain consistent moisture levels. Continue static incubation at 20–25°C for 3 days. The total incubation period is 10 days. During incubation, deionized water is added periodically by weighing to maintain saturated moisture content.

[0055] 4. Determination of soil physicochemical properties After 3 days of cultivation, 80 ml of deionized water was added to each treatment to maintain a soil surface water level of approximately 3 cm, and the mixture was stirred thoroughly. After standing for 2 days, the top layer of water was slowly poured out to simulate the actual paddy field slurry drainage process. Once drainage was complete, rice seeds (variety "Zhongkefa 5") that had been soaked in deionized water for 1 day were evenly sown on the soil surface of each treatment, ensuring close contact between the seeds and the soil. Ten rice seeds were sown in each treatment. The top layer water level was then maintained at approximately 3 cm until the end of the experiment. The main test parameters for the leachate included pH, EC, and K. + Na + Ca 2+ Mg 2+ SO4 2- NO3 - .

[0056] After cultivation (day 10), soil samples were collected for physicochemical property determination. Soil extraction was performed at a 1:1 (w / v) soil-to-water ratio. pH and conductivity were determined using potentiometry; water-soluble anions (SO42-) were measured. 2- and NO3 - The determination was performed using ion chromatography; alkaline anions (CO3) were measured. 2- and HCO3 - The determination was performed using a dual-indicator neutralization titration method; water-soluble cations (K...) + Na + Ca 2+ Mg 2+ The concentrations of exchangeable cations (CEC) were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES); exchangeable cations were determined by ICP-AES after extraction with 1.0 M ammonium acetate (pH=7.0); CEC was defined as the sum of exchangeable cations; ESP = EX - Na + / CEC × 100%.

[0057] 5. Mineralogical Characterization X-ray diffraction (XRD): A MiniFlex600 (Japan) diffractometer was used, with a step size of 0.02°, a scanning range of 2θ = 3° to 80°, and a scanning speed of 10° / min.

[0058] Scanning electron microscopy (SEM): Soil microstructure was observed using a Sigma 360 (ZEISS, Germany).

[0059] Fourier transform infrared spectroscopy (FT-IR): IS5 (Thermo Fisher) spectrometer, KBr pellet method, wavenumber range 4000–400 cm⁻¹ -1 .

[0060] 6. DFT theoretical calculation Dmol using Materials Studio software 3 The module performs first-principles calculations. Exchange-correlation interactions are described using a Perdew-Burke-Ernzerhof functional under the generalized gradient approximation. Core electrons are treated with an effective nuclear potential, and Kohn-Sham orbitals are expanded using a dual-numerical polarization basis set. The Brillouin zone is sampled using a 2×2×1 k-point grid, with a global orbital cutoff radius of 3.7 Å. Geometric optimization employs the BFGS algorithm, with a convergence criterion of energy 2.0×10⁻⁶. -5 Hartree, maximum force 0.004 Hartree / Å, maximum displacement 0.005 Å. Calculate Mg respectively. 12 Al4(OH) 24 (SO4)2 and Ca 12 Al4(OH) 24 Gibbs free energy for the formation of (SO4)2.

[0061] 7. Experimental Results (1) Improvement of soil salinity and alkalinity parameters: such as Figure 1 As shown, the G+S treatment reduced soil pH by 2.0 units and ESP by 41.8% compared to the CK, both significantly better than the G treatment (reducing pH by 0.24 units and ESP by 44.5%, respectively). The electrical conductivity of the G+S treatment increased by 491.7% compared to the CK and by 40.6% compared to the G treatment, but the ionic composition changed from toxic to nutrient-rich.

[0062] (2) Reconstruction of water-soluble ionic matrix: such as Figure 2 As shown, G+S processing reduces SO4 2- The concentration was reduced by 25.2% compared to treatment G, and the water-soluble Ca... 2+ Increased by 51.8%, water-soluble Mg 2+ Increased by 318.0%, water-soluble Na + NO3 decreased by 23.1% - The concentration increased by 21596.7% compared to the control (CK). 2+ / Na + The molar ratio of CK increased by approximately 800%. The ionic composition changed from HCO3 in CK. - CO3 2- NO3 that shifts from dominant to G+S - / Mg 2+ / Ca 2+ Dominant type.

[0063] (3) Dynamics of exchangeable cations: such as Figure 3 As shown, G+S treatment makes EX-Na + It decreased by 65.9% compared to the control (CK) and further decreased by 46.6% compared to the treatment with glucose (G); EX-Mg2+ Increased by 288.3% compared to CK; EX-Ca 2+ There was no significant difference in Na content between the G and G+S treatments. + The emissions of G+S are significantly higher than those of G and SO4. 2- The emission of G+S was significantly lower than that of G, confirming that SO4 emissions were lower. 2- The original position is fixed.

[0064] (4) Mineralogical evidence: such as Figure 4 As shown, in the XRD pattern, the G+S treatment exhibits characteristic LDH diffraction peaks at 2θ = 11.4°, 23.4°, 35.0°, and 47.7°, corresponding to the (003), (006), (012), and (018) crystal planes, respectively (PDF#35-0964). In the FT-IR pattern, the G+S treatment shows peaks at 1384 cm⁻¹. -1 NO stretching vibration peaks (interlayer NO3) were observed at this location. - ), 1197 cm -1 The SO symmetric vibration peak is enhanced at 1449 cm⁻¹. -1 The C=O vibration peak is weakened. Typical hexagonal plate-like LDH crystals were observed in the G+S processed SEM image.

[0065] (5) DFT calculation results: Mg 12 Al4(OH) 24 The Gibbs free energy for the formation of (SO4)2 is ΔG = -48.17 eV, Ca 12 Al4(OH) 24 The ΔG of (SO4)2 is -40.63 eV, and the difference between the two is 7.54 eV, confirming that the formation of Mg-Al LDHs is thermodynamically significantly preferential to Ca-Al LDHs.

[0066] Example 2 1. Experimental Methods After the 10-day soil culture in Example 1, biological verification of rice seedlings was conducted. The rice variety was "Songliao 5" (…). Oryza sativa L This variety is widely planted in the Songnen Plain and exhibits moderate salt and alkali tolerance. After surface disinfection and soaking in deionized water for 24 hours to break dormancy, 10 seeds were evenly sown in each treatment, maintaining a water layer of approximately 3 cm to simulate paddy field conditions, and cultured for 12 days. Survival rate, plant height, and aboveground dry weight (dried at 85°C to constant weight) were measured.

[0067] 2. Experimental Results like Figures 5-6 As shown: (1) Survival rate: The survival rates of G and G+S treatments were 161.2% and 176.9% higher than those of CK, respectively, but there was no significant difference between the two.

[0068] (2) Plant height: The plant height of rice in the G+S treatment increased significantly by 21.8% compared with the G treatment and by 183.7% compared with the CK treatment.

[0069] (3) Aboveground dry weight: The aboveground dry weight of rice seedlings treated with G+S increased significantly by 72.6% compared with that treated with G.

[0070] (4) Random forest analysis showed that soil HCO3 - It is the main controlling factor for survival rate, pH is the primary determinant of plant height, and ESP is the most important limiting factor for aboveground dry weight. NO3 - and Ca 2+ It was identified as a significant positive contributor to the aboveground dry weight.

[0071] The above results confirm that G+S treatment lowers pH and ESP, and increases Ca2+. 2+ and NO3 - The availability of the soil effectively alleviated the stress of alkaline soil on rice seedlings and significantly promoted biomass accumulation.

[0072] Example 3: Comparison of synergistic effects with different Mg / Al molar ratios 1. Experimental Design. To investigate the effect of the Mg / Al molar ratio in the synergist on the in-situ LDHs mineralization effect, the following treatments were set up: Seven days after the application of 2% (w / w) gypsum in Example 1, synergists with Mg / Al molar ratios of 2:1 and 3:1 (total application amount 0.9% w / w) were applied, respectively. After 10 days of cultivation, soil pH, ESP, and water-soluble Ca were measured. 2+ and SO4 2- Concentration. The rest is the same as in Example 1.

[0073] 2. Experimental Results: like Figure 7 As shown: When the divalent and trivalent metal salts are composed of Mg 2+ Al 3+ Furthermore, when the molar ratio is 3:1, LDHs exhibit the best mineralization effect, improving soil pH and reducing SO4 levels. 2- Removal and Ca 2+ The release rates were significantly higher than other treatments. The pH decreased by 0.11 units compared to the Mg2Al treatment, and the water-soluble Ca... 2+ Significantly increased by 13.2%, water-soluble SO4 2- A significant reduction of 8.3%. This indicates that the LDHs mineralization effect at a molar ratio of 3:1 is significantly better than that at 2:1, specifically manifested in improved soil pH and reduced SO4 levels. 2- Removal and Ca 2+ The release rates were all significantly higher than 2:1.

[0074] The present invention has been described in detail above. Those skilled in the art will recognize that the invention can be practiced in a wide range of ways with equivalent parameters, concentrations, and conditions without departing from its spirit and scope. While specific embodiments have been provided, it should be understood that further modifications can be made to the invention. In summary, according to the principles of the invention, this application is intended to include any changes, uses, or improvements to the invention, including modifications made using conventional techniques known in the art that depart from the scope disclosed herein.

Claims

1. A method for improving alkaline soil, characterized in that, Includes the following steps: S1. Applying gypsum to alkaline soil causes the gypsum to dissolve and release calcium ions to replace exchangeable sodium ions on soil colloids, while sulfate ions accumulate in the soil solution. S2. After step S1 is completed, a synergist including magnesium nitrate and aluminum nitrate is applied to the alkalized soil, so that the magnesium ions and aluminum ions in the synergist react with the sulfate ions accumulated in the soil solution in situ to form Mg-Al-SO4 type layered bimetallic hydroxide, thereby consuming the accumulated sulfate ions, breaking the inhibition of gypsum dissolution by the sulfate common ion effect, and promoting the continuous dissolution of gypsum to release calcium ions; wherein, the molar ratio of magnesium nitrate to aluminum nitrate in the synergist is (2~3):

1.

2. The method for improving alkaline soil according to claim 1, characterized in that, The alkalinity (ESP) of the alkaline soil is greater than 15%, and the soil pH is greater than 8.

5.

3. The method for improving alkaline soil according to any one of claims 1-2, characterized in that, The amount of gypsum applied is 0.5% to 5% of the mass of the alkaline soil.

4. The method for improving alkaline soil according to any one of claims 1-3, characterized in that, The application rate of the synergist is 0.3% to 2.0% of the mass of the alkalized soil; And / or, the synergist is applied 1 to 30 days after applying gypsum in step S1; preferably, after applying gypsum in step S1, water is added to the soil to saturation, and the soil is pre-cultured at 20 to 30°C for 5 to 10 days to fully dissolve the gypsum and allow sulfate ions to accumulate to dynamic equilibrium.

5. The method for improving alkaline soil according to any one of claims 1-4, characterized in that, The magnesium nitrate is magnesium nitrate hexahydrate Mg(NO3)2·6H2O, and the aluminum nitrate is aluminum nitrate nonahydrate Al(NO3)3·9H2O.

6. The method for improving alkaline soil according to any one of claims 1-5, characterized in that, The molar ratio of magnesium nitrate to aluminum nitrate in the synergist is 3:

1.

7. The method for improving alkaline soil according to any one of claims 1-6, characterized in that, The chemical formula of the Mg-Al-SO4 type layered bimetallic hydroxide generated by the in-situ coprecipitation reaction is Mg 12 Al4(OH) 24 The Gibbs free energy of the (SO4)2 formation reaction is -48.17 eV.

8. The method for improving alkaline soil according to any one of claims 1-7, characterized in that, It also includes step S3: after the synergist is applied in step S2 and the reaction is complete for ≥3 days, irrigation and leaching are carried out to remove the replaced sodium ions and soluble salts.

9. The use of synergistic compositions comprising magnesium nitrates and aluminum nitrates in any of the following: A1) Gypsum can be used to improve alkaline soil or as a gypsum synergist; furthermore, the improvement of alkaline soil is manifested in reducing pH value, reducing alkalinity, and reducing SO4. 2- Concentration, increasing water-soluble Ca 2+ Concentration, increase water-soluble Mg 2+ Concentration, reducing water-soluble Na + Concentration, increase NO3 - One or more of the concentrations; A2) Promote crop growth; further, the crop is rice, and the promotion of growth is manifested in one or more of increasing plant height and increasing aboveground dry weight; in, The molar ratio of magnesium nitrate to aluminum nitrate in the synergist composition is (2-3):1; The application involves applying the synergistic agent composition to alkalized soil after gypsum application, thereby consuming the sulfate ions accumulated in the soil through the in-situ formation of Mg-Al-SO4 type layered bimetallic hydroxides, thus overcoming the sulfate common ion effect and enhancing the calcium ion release efficiency of gypsum.

10. The application according to claim 9, characterized in that, The molar ratio of magnesium nitrate to aluminum nitrate is 3:1.

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