Calcium-based biochar for improving karst rocky desertification soil as well as preparation method and application of calcium-based biochar

Abstract

The invention discloses calcium-based biochar for improving karst rocky desertification soil as well as a preparation method and application of the calcium-based biochar, and belongs to the technical field of biochar materials and soil remediation. The preparation method of the calcium-based biochar comprises the following steps: collecting Chinese prickly ash shells, removing impurities, drying to constant weight, crushing and sieving to obtain Chinese prickly ash shell powder; the dolomite in the karst area is crushed, ground, dried and cooled to prepare the dolomite fine powder. Mixing the two materials, adding a starch binder and water to adjust the water content, and granulating and forming; pyrolyzing the particles in a nitrogen atmosphere, keeping at a constant temperature, cooling, soaking with hydrochloric acid, washing with water, and drying to obtain primary biochar; preparing bacillus mucilaginosus in a southwest karst area into a bacterial suspension, soaking the primary biochar in the bacterial suspension, performing constant-temperature adsorption, drying and sieving to obtain a final product. The calcium-based biochar is applied to karst rocky desertification soil and can supplement calcium, regulate acid, promote phosphorus conversion, improve the soil structure and optimize microbial communities, so that regional ecological restoration and agricultural sustainable development are promoted.

Description

Technical Field

[0001] This invention relates to the field of biochar materials and soil remediation technology, specifically to a calcium-based biochar for the improvement of karst desertification soil, its preparation method and application. Background Technology

[0002] Karst desertification is a typical ecological and environmental problem in southwestern my country. Essentially, it is an ecological degradation process caused by the combined effects of unreasonable human activities and natural factors on a fragile karst geological background, leading to severe soil erosion, large-scale exposure of bedrock, and a sharp decline in land productivity. The soils in this region generally exhibit the following characteristics: shallow soil layers, poor structure, and weak water and fertilizer retention capacity; influenced by calcium-rich rock layers, the soil has high levels of alkaline ions such as calcium and magnesium, resulting in a predominantly alkaline pH; while available phosphorus, organic matter, and key trace elements are severely deficient. This unique soil chemical environment, coupled with severe soil erosion, creates a vicious cycle of "soil degradation—reduced vegetation cover—exacerbated soil erosion," posing significant challenges to ecological restoration and agricultural utilization. Therefore, developing targeted and multi-functional soil improvement materials and technologies is crucial to overcoming the bottlenecks in the region's governance.

[0003] Currently, methods for improving karst and rocky desertification soils can be mainly categorized into three types: physical, chemical, and biological methods, but all have significant limitations. Physical methods, such as topsoil cultivation and straw mulching, can improve soil physical structure in the short term, but the former is costly and may cause secondary ecological problems, while the latter decomposes slowly in calcium-rich environments, and its products easily combine with calcium ions to form insoluble organic-inorganic complexes, which may reduce nutrient bioavailability. Chemical methods mainly rely on the application of chemical fertilizers or conditioners to quickly replenish nutrients or adjust pH. However, under high calcium conditions, fast-acting phosphate fertilizers are easily fixed into calcium phosphate salts that are difficult for plants to utilize, resulting in low utilization rates and potentially exacerbating soil compaction. Acidifying conditioners such as sulfur rely on soil microbial transformation, and their effects are unstable in degraded soils where microbial activity is already low, and there is a risk of excessive acidification. Biological improvement methods focus on introducing functional microorganisms to activate the inherent nutrients in the soil. However, exogenous strains often have difficulty adapting to high-calcium and barren stress environments, resulting in low colonization and survival rates, unstable functional expression, and easy loss when lacking suitable physical carriers and protection. Consequently, the improvement effect is short-lived and limited.

[0004] In recent years, biochar has been regarded as a promising soil amendment matrix due to its stable porous structure, large specific surface area, and good environmental compatibility. However, the direct application of ordinary biochar to karst desertification soils still faces key bottlenecks: First, biochar made from conventional raw materials such as straw and sawdust has limited nutrient content and cannot specifically replenish the core alkaline earth metal elements lost by the soil in this region; Second, some existing studies have introduced exogenous calcium to prepare "calcium-enhanced" materials through physical mixing, but the calcium source is prone to agglomeration and uneven distribution during pyrolysis, resulting in uncontrollable release behavior in the soil, either too rapid leaching or ineffective release; Third, improper design of preparation process parameters can lead to poor development or excessive destruction of the biochar's pore structure, affecting its carrier function and potentially having a negative impact on the soil microecology due to excessive alkalinity; Fourth, simple physical combination of biochar and microorganisms fails to solve problems such as the potential inhibition of microbial cells by pyrolysis residues, low microbial loading efficiency, and poor compatibility at the microbial-charcoal interface, preventing functional microorganisms from effectively colonizing the charcoal and maintaining long-term activity.

[0005] In summary, existing technologies generally suffer from limitations in addressing the complex degradation of karst and rocky desertification soils, including single-function limitations, insufficient synergy, and short-lasting effects. They fail to systematically resolve the following core contradictions: 1) The contradiction between material function and soil requirements: There is a lack of integrated materials that can simultaneously achieve soil structure improvement, slow-release supplementation of calcium and magnesium, and phosphorus activation; 2) The contradiction between preparation process and performance synergy: The synergistic integration of biochar's pore structure, surface chemical properties, and functional components (such as calcium form and microorganisms) has not been precisely controlled through process design; 3) The contradiction between the adaptation of foreign materials and the local environment: The improved materials or microorganisms have failed to fully utilize or adapt to local resources and environmental conditions, resulting in insufficient ecological adaptability and long-term stability.

[0006] Therefore, there is an urgent need to develop a novel composite material based on local resources, integrating soil structure improvement, slow-release nutrient supply, and microbial function enhancement through multi-process collaborative design. Based on this, this invention innovatively uses Sichuan pepper shells, a regional agricultural waste, as the carbon skeleton raw material, combined with abundant dolomite from karst areas as a natural calcium source, and loaded with locally screened phosphorus-tolerant microorganisms. Through a systematically optimized process, a multifunctional calcium-based biochar is prepared, aiming to provide an efficient, environmentally friendly, and sustainable technical solution for soil ecological restoration and sustainable agricultural development in karst desertification areas. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a calcium-based biochar for the improvement of karst desertification soils, its preparation method, and its application. This invention aims to construct a composite material integrating structural improvement, slow calcium release, and bioactive phosphorus functions by performing composite pyrolysis of agricultural waste (Sichuan pepper shells) with dolomite from karst areas, and further loading it with locally screened, highly efficient phosphorus-solubilizing microorganisms. This synergistically solves the key technical challenges of structural degradation, weak nutrient retention capacity, and low phosphorus bioavailability in karst desertification soils.

[0008] To achieve the above technical objectives, the present invention adopts the following technical solution:

[0009] A method for preparing calcium-based biochar for improving karst desertification soil includes the following steps:

[0010] (1) Raw material pretreatment: Collect pepper shells, remove impurities, and dry them in an oven at 60-80℃ until constant weight. Crush the dried pepper shells and pass them through a 40-60 mesh sieve to obtain pepper shell powder.

[0011] (2) Pretreatment of dolomite powder: Take dolomite from the karst area, crush it with a jaw crusher, grind it with a ball mill, and pass it through a 100-200 mesh sieve to obtain fine dolomite powder. Dry the fine dolomite powder and cool it to room temperature for later use.

[0012] (3) Mixing and granulation: Mix the pepper shell powder obtained in step (1) with the dolomite fine powder obtained in step (2), add 5-8% of the mass of starch binder to the mixed powder, stir evenly, add an appropriate amount of water to adjust the moisture content to 20-25%, and granulate using a disc granulator to obtain mixed granules with a particle size of 3-5 mm.

[0013] (4) Low-temperature pyrolysis: The mixed particles obtained in step (3) are placed in a tube furnace and heated to 350-450°C at a heating rate of 5-8°C / min under a nitrogen atmosphere. After constant-temperature pyrolysis, the mixture is naturally cooled to room temperature to obtain primary calcium-based biochar. The primary calcium-based biochar is soaked in 0.1 mol / L hydrochloric acid solution for 1 h, washed with water until pH 7.0-7.5, dried at 105°C for 2 h, and cooled for later use.

[0014] (5) Microbial load: Phosphate-solubilizing bacteria isolated from soil in the karst region of Southwest China were selected, inoculated into LB liquid medium, and cultured with shaking to obtain a bacterial concentration of 10. 8 -10 9 CFU / mL bacterial suspension;

[0015] (6) Post-treatment: Soak the primary calcium-based biochar obtained in step (4) in the bacterial suspension obtained in step (5) at a solid-liquid ratio of 1: (8-10), and adsorb at a constant temperature. Stir once every 1 hour during the process. After the adsorption is completed, take it out and dry it in an oven at 50-60℃ until the moisture content is ≤10%. Pass it through a 20-40 mesh sieve to obtain Sichuan pepper shell calcium-based biochar.

[0016] Preferably, the drying temperature in step (1) is 70°C.

[0017] Preferably, the drying temperature of the dolomite fine powder in step (2) is 100-110℃ and the drying time is 2-3h.

[0018] Preferably, the mass ratio of pepper shell powder to dolomite fine powder in step (3) is (3-5):1.

[0019] Preferably, the isothermal pyrolysis time in step (4) is 2-3 hours.

[0020] Preferably, the phosphate-solubilizing bacteria in step (5) are Bacillus mucilaginosus isolated from the soil of the karst region in Southwest China, and the LB liquid culture medium is shaken and cultured at 30-32℃ and 150-180r / min for 24-36h.

[0021] Preferably, the isothermal adsorption temperature in step (6) is 25-28℃ and the adsorption time is 4-6h.

[0022] This invention also provides the application of calcium-based biochar for the improvement of karst desertification soil, wherein the calcium-based biochar is applied at a rate of 20-50 t / hm². 2 Apply the fertilizer evenly to the surface of the karst desertification soil and till it to a depth of 20-30cm.

[0023] Preferably, the calcium-based biochar used for improving karst desertification soil is compounded with decomposed organic fertilizer at a mass ratio of 1:(2-3) and then applied to the soil. The decomposed organic fertilizer is a mixture of local straw and cow manure from the karst region of Southwest China.

[0024] Compared with the prior art, the calcium-based biochar, its preparation method, and its application provided by this invention have the following outstanding advantages and technical effects:

[0025] I. Collaborative innovation of raw materials to achieve efficient integration of waste resource utilization and functional calcium sources.

[0026] This invention innovatively selects Sichuan pepper shells, a regional agricultural waste, as a biochar precursor, and uses abundant dolomite from karst areas as a natural calcium source. The organic components in the Sichuan pepper shells, upon pyrolysis, form a well-developed hierarchical porous structure, providing an ideal carrier for calcium loading and microbial colonization. Simultaneously, it achieves high-value resource utilization of waste and reduces raw material costs. As a local calcium source, the mineral composition of dolomite is highly compatible with the regional soil environment, avoiding ecological compatibility issues that may arise from exogenous additives. Through the composite design of raw materials, the technical shortcomings of traditional biochar—limited calcium loading capacity, uneven calcium source distribution, and easy loss—are fundamentally overcome.

[0027] II. Collaborative optimization of processes to achieve controllable construction of material structure and properties.

[0028] This invention constructs a complete performance-oriented preparation system by systematically controlling key process parameters such as raw material pretreatment, mixing and granulation, oxygen-limited pyrolysis, and microbial loading. Precise control of raw material particle size and ratio, combined with the granulation effect of starch binder, ensures the uniformity of precursor mixing and particle strength. By optimizing the pyrolysis temperature and heating program, the full development of the biochar carbon skeleton and the appropriate activation of dolomite minerals are achieved, allowing calcium to be stably embedded in the pore structure of the biochar, forming a "carbon-calcium" complex. This synergistic process ensures that the final product possesses excellent pore structure, stable calcium slow-release characteristics, and good microbial carrier function.

[0029] III. Constructing a "charcoal-microbe" synergistic system to enhance the stability and persistence of microbial modification.

[0030] To address the issues of low soil microbial activity and difficulty in colonizing exogenous microbial strains in karst areas, this invention selects locally domesticated Bacillus mucilaginosus (phosphate-solubilizing bacteria) for loading. This strain possesses natural adaptability to high-calcium and alkaline stress environments. Simultaneously, the primary biochar prepared through a specific process has optimized surface chemistry and pore structure, providing physical protection and a nutrient microenvironment for microorganisms, effectively promoting cell adsorption, colonization, and activity maintenance. This "charcoal-microbe" composite system overcomes the technical bottlenecks of low survival rates and short functional periods associated with simple microbial inoculation, achieving long-term stable bioactivation.

[0031] IV. Multifunctional Synergistic Effects, Systemic Improvement of Karst Degraded Soils

[0032] The calcium-based biochar prepared in this invention is a multifunctional soil amendment material that can synergistically act on the soil system from physical, chemical, and biological levels. In terms of physical structure, its porous nature effectively improves soil aggregates and enhances water and fertilizer retention capacity. In terms of chemical properties, it can moderately regulate soil pH and directly replenish and increase the content of key available nutrients in the soil through slow-release calcium and activated, immobilized phosphorus. In terms of biological ecology, not only do the loaded functional bacteria continuously activate phosphorus, but the material itself also provides a habitat for soil microbial communities, optimizing the micro-ecological environment. This integrated amendment model specifically addresses the complex degradation problems of karst rocky desertification soils, such as thin soil, low nutrient content, alkalinity, and phosphorus retention, and its effects are superior to single amendment measures.

[0033] V. It boasts outstanding technical and economic efficiency and environmental adaptability, making it easy to promote.

[0034] The entire technological chain of this invention fully embodies the principle of adapting to local conditions: the raw materials are sourced locally, reducing procurement and transportation costs; the preparation process can be carried out under conventional conditions, making it simple to operate; the product can be used alone or in combination with local organic waste compost products to further enhance the improvement effect and improve resource recycling efficiency. Compared with costly topsoil methods, chemical improvement methods that easily cause secondary pollution, and single biological methods with unstable effects, this invention provides an economically feasible, environmentally friendly, ecologically compatible, and sustainable comprehensive management solution for karst desertification soils, with good prospects for promotion and application, and significant ecological, social, and economic benefits. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to specific embodiments. The provided embodiments are intended to further clarify the technical content and implementation methods of this invention, to help those skilled in the art to more fully understand and implement this invention, and are not intended to constitute any limitation on its scope of protection.

[0036] For those skilled in the art, provided they fully understand the technical concept of this invention, appropriate adjustments and modifications can be made to its specific implementation details or process parameters, or reasonable inferences and extensions can be made based on the basic principles of this invention. All such modifications, substitutions, or improvements that do not depart from the essential spirit and scope of this invention should be covered within the protection scope claimed by this invention.

[0037] A method for preparing calcium-based biochar for improving karst desertification soil includes the following steps:

[0038] (1) Raw material pretreatment: Collect pepper shells, remove impurities, and dry them in an oven at 60-80℃ until constant weight. Crush the dried pepper shells and pass them through a 40-60 mesh sieve to obtain pepper shell powder.

[0039] (2) Pretreatment of dolomite powder: Take dolomite from the karst area, crush it with a jaw crusher, grind it with a ball mill, and pass it through a 100-200 mesh sieve to obtain fine dolomite powder. Dry the fine dolomite powder and cool it to room temperature for later use.

[0040] (3) Mixing and granulation: Mix the pepper shell powder obtained in step (1) with the dolomite fine powder obtained in step (2), add 5-8% of the mass of starch binder to the mixed powder, stir evenly, add an appropriate amount of water to adjust the moisture content to 20-25%, and granulate using a disc granulator to obtain mixed granules with a particle size of 3-5 mm.

[0041] (4) Low-temperature pyrolysis: The mixed particles obtained in step (3) are placed in a tube furnace and heated to 350-450°C at a heating rate of 5-8°C / min under a nitrogen atmosphere. After constant-temperature pyrolysis, the mixture is naturally cooled to room temperature to obtain primary calcium-based biochar. The primary calcium-based biochar is soaked in 0.1 mol / L hydrochloric acid solution for 1 h, washed with water until pH 7.0-7.5, dried at 105°C for 2 h, and cooled for later use.

[0042] (5) Microbial load: Phosphate-solubilizing bacteria isolated from soil in the karst area of ​​Southwest China were selected, inoculated into LB liquid medium, and cultured by shaking to obtain a bacterial suspension with a concentration of 10⁸-10⁹ CFU / mL;

[0043] (6) Post-treatment: Soak the primary calcium-based biochar obtained in step (4) in the bacterial suspension obtained in step (5) at a solid-liquid ratio of 1: (8-10), and adsorb at a constant temperature. Stir once every 1 hour during the process. After the adsorption is completed, take it out and dry it in an oven at 50-60℃ until the moisture content is ≤10%. Pass it through a 20-40 mesh sieve to obtain Sichuan pepper shell calcium-based biochar.

[0044] The mechanism of action and synergistic principle of each step in the preparation method of calcium-based biochar described in this invention are explained as follows:

[0045] I. Mechanism of action of raw material pretreatment steps

[0046] Sichuan pepper shells, as a biomass precursor, are rich in cellulose, hemicellulose, and lignin, which are the main material basis for the formation of a porous carbon skeleton through pyrolysis. Low-temperature drying can reduce the moisture content of the raw material, improve its brittleness, and facilitate subsequent pulverization to form powder of suitable particle size, while avoiding premature decomposition of organic components.

[0047] Dolomite, as a natural calcium source, can have its specific surface area increased through crushing and fine grinding, allowing its calcium carbonate and other mineral components to more fully bond with the organic pyrolysis products during subsequent pyrolysis. Designing a particle size distribution between fine dolomite powder and Sichuan pepper shell powder helps achieve tight packing between particles during the mixing stage, improving the uniformity and density of the precursor structure. The drying process aims to remove free moisture from the dolomite, preventing particle structure damage due to rapid moisture vaporization during pyrolysis, thereby ensuring the uniform distribution of calcium in the final biochar matrix.

[0048] II. Synergistic Effect Principle of Mixing Granulation Steps

[0049] Mixing Sichuan pepper husk powder and fine dolomite powder in a specific ratio essentially constructs an organic-inorganic composite system. The Sichuan pepper husk powder provides the organic matter for forming a porous carbon framework, while the fine dolomite powder serves as a uniformly dispersed calcium source. This composite design overcomes the structural or functional limitations inherent in the pyrolysis of single raw materials.

[0050] The addition of starch binder not only enables the shaping of the mixed powder, but also carbonizes during pyrolysis, forming a supplementary carbonaceous network that enhances the overall mechanical strength of the granules. Simultaneously, the hydrophilicity of starch helps regulate the moisture content of the mixture, resulting in a more uniform moisture distribution. Precise control of the moisture content satisfies the plasticity requirements of the granulation process while preventing granule agglomeration or shaping difficulties caused by improper moisture content.

[0051] III. Technical Logic of Low-Temperature Pyrolysis and Acid Washing Purification Steps

[0052] Program-controlled low-temperature pyrolysis under an inert atmosphere is key to achieving carbon skeleton development and calcium morphology stability. This condition allows for the full pyrolysis of cellulose and hemicellulose in the pepper shell, forming abundant pores, while preventing excessive lignin degradation that could cause pore structure collapse. Within this temperature range, calcium carbonate in dolomite remains crystalline and does not decompose, thus allowing it to be uniformly embedded at the micro-nano scale in the pore surface and wall layers of biochar, forming a stable "carbon-mineral" composite structure.

[0053] After pyrolysis, the primary biochar is soaked in dilute hydrochloric acid. This process primarily dissolves and removes some of the soluble ash and surface deposits generated during pyrolysis, while also gently etching the carbon surface to increase its surface roughness and the number of oxygen-containing functional groups (such as hydroxyl and carboxyl groups). Subsequent washing with water to neutrality thoroughly removes residual acid and soluble ions, eliminating their potential inhibitory effect on subsequent microbial activity. Drying helps to fix the carbon framework structure and restore the adsorption capacity of the pores.

[0054] IV. Synergistic Mechanism of Microbial Loading Steps

[0055] Native Bacillus mucilaginosus (phosphate-solubilizing bacteria) isolated and domesticated from soils in the karst region of Southwest China was selected because its fundamental advantage lies in the fact that this strain has evolved to adapt to the high-calcium, slightly alkaline native environment. During the loading process, the acid-washed and activated biochar, with its well-developed porous structure and abundant surface functional groups, provides excellent physical adsorption sites and a protective microenvironment for microorganisms.

[0056] This "charcoal-bacteria" composite system exhibits a synergistic effect in application: the organic acids secreted by phosphate-solubilizing bacteria promote the slow dissolution and release of complex calcium in biochar, while simultaneously activating insoluble phosphates in the soil and enhancing phosphorus availability. The biochar carrier adsorbs and enriches these organic acids, prolonging their action time and continuously providing habitat for the bacteria, thereby constructing a mutually reinforcing and long-lasting micro-ecosystem.

[0057] V. Overall Synergistic Effect of Raw Materials and Processes

[0058] The entire preparation process embodies the concept of synergistic design: using pepper shells to provide a carbon framework, dolomite to provide a structural calcium source, and native functional microorganisms to provide bioactivity. Through precise control of process parameters such as pretreatment, granulation, pyrolysis, purification, and loading, the integrated optimization of material structure, composition, and function is achieved. The resulting calcium-based biochar achieves efficient carbon carrier support for both the calcium source and functional bacteria, as well as their synergistic effect in soil improvement, thereby achieving the comprehensive goal of simultaneously improving the physical structure, chemical properties, and bioactivity of karst degraded soils.

[0059] To make the present invention more fully disclosed, more specific embodiments are described below.

[0060] I. Implementation Examples

[0061] Example 1

[0062] A method for preparing calcium-based biochar for improving karst desertification soil includes the following steps:

[0063] (1) Raw material pretreatment: Collect pepper shells, remove impurities, and dry them in a 70℃ oven until constant weight. Put the dried pepper shells into a grinder and grind them. After grinding, pass them through a 50-mesh sieve and collect the sieve material to obtain pepper shell powder.

[0064] (2) Pretreatment of dolomite powder: Dolomite from the karst area in Southwest China was crushed into blocks with a particle size ≤5cm by a jaw crusher, then ground in a ball mill for 3 hours and passed through a 150-mesh sieve to obtain fine dolomite powder. The fine dolomite powder was dried in an oven at 105℃ for 2.5 hours, then removed and cooled to room temperature in a desiccator and sealed for later use.

[0065] (3) Mixing and granulation: Weigh the pepper shell powder obtained in step (1) and the dolomite fine powder obtained in step (2) at a mass ratio of 4:1, and mix them evenly in a mixing pot. Add 6.5% of the total mass of starch binder to the mixed powder, stir evenly, then add deionized water to adjust the moisture content of the mixture to 22.5%, transfer it to a disc granulator for granulation, and obtain mixed granules with a particle size of 4 mm.

[0066] (4) Low-temperature pyrolysis: The mixed particles obtained in step (3) were placed in a tube furnace and heated to 400°C at a heating rate of 6.5°C / min under a nitrogen atmosphere, and pyrolyzed at a constant temperature for 2.5 h. After pyrolysis, the mixture was naturally cooled to room temperature to obtain primary calcium-based biochar. The primary calcium-based biochar was soaked in 0.1 mol / L hydrochloric acid solution for 1 h (solid-liquid ratio 1:10), and then repeatedly rinsed with deionized water until the pH of the filtrate was 7.2. The filtrate was then dried in an oven at 105°C for 2 h and cooled to room temperature for later use.

[0067] (5) Microbial load: Bacillus mucilaginosus isolated and purified from soil in the karst region of Southwest China was selected. Single colonies were picked and inoculated into LB liquid medium and cultured in a shaker at 31℃ and 165r / min for 30h to obtain a bacterial concentration of 5×10⁻⁶. 8 CFU / mL bacterial suspension.

[0068] (6) Post-treatment: The primary calcium-based biochar obtained in step (4) was soaked in the bacterial suspension obtained in step (5) at a solid-liquid ratio of 1:9, and adsorbed at a constant temperature of 26.5℃ for 5 hours, with stirring once every 1 hour during the process. After adsorption, it was dried in an oven at 55℃ until the moisture content was 8%, and then passed through a 30-mesh sieve to obtain the calcium-based biochar of pepper shell.

[0069] II. Single-factor experiment

[0070] (I) Basis for Experimental Design

[0071] Based on the preparation process parameters of Example 1, single-factor experiments were designed around 6 core process parameters, with the remaining conditions being the same as in Example 1, to verify the influence and optimization of key process parameters on product performance.

[0072] (II) Results of Single-Factor Experiments on Key Process Parameters

[0073]

[0074] (III) Single-factor experiment

[0075] 1. Analysis of the effect of pepper husk powder sieve mesh size on the performance of calcium-based biochar

[0076] Based on the experimental data in Table 1, different mesh sizes of Sichuan pepper husk powder significantly affect the performance of the prepared calcium-based biochar, as detailed below:

[0077] (1) 30 mesh: The raw material particle size is too large, resulting in obvious stratification when mixed with fine dolomite powder, leading to poor mixing uniformity. During pyrolysis, the coarse particles are not completely carbonized, and the pores are poorly developed, resulting in a low effective calcium loading and insufficient attachment sites for microorganisms, with a loading of only 3.1×10. 7 CFU / g. After being applied to the soil, this material exhibited uneven nutrient release and limited soil improvement effects, with a soil organic matter increase rate of only 15.6%.

[0078] (2) 40 mesh: The particle size is reduced, and the mixing uniformity with dolomite fine powder is improved. The pore structure formed after pyrolysis is optimized, the effective calcium content is increased to 11.5%, and the microbial load reaches 4.7 × 10⁻⁶. 7 The CFU / g soil organic matter enhancement rate increased to 22.8%. Although the performance was significantly better than that of 30 mesh, it still did not reach the optimal level.

[0079] (3) 50 mesh: At this mesh size, the particle size matching between Sichuan pepper shell powder and dolomite fine powder is optimal, resulting in uniform mixing without stratification. The biochar formed after pyrolysis has a rich and appropriately distributed pore structure, which is conducive to the stable attachment of calcium elements, and the effective calcium content reaches a maximum of 12.3%. At the same time, the sufficient specific surface area and pores provide a good colonization environment for microorganisms, increasing the loading capacity to 5.2 × 10⁻⁶. 7 CFU / g. The corresponding soil improvement effect is optimal, with an organic matter increase rate of 25.3%.

[0080] (4) 60 mesh: The particle size is further reduced, and the powder is prone to agglomeration after slight moisture absorption, which affects the uniform dispersion of dolomite fine powder. During pyrolysis, some pores are blocked by agglomerates, resulting in obstruction of effective calcium release channels, and the content drops to 10.8%. The microbial load also decreases accordingly to 4.3×10 7 CFU / g, soil organic matter improvement rate dropped back to 20.5%.

[0081] (5) 70 mesh: The powder is too fine, which exacerbates the phenomenon of moisture absorption and agglomeration, forming dense clumps. During pyrolysis, volatiles cannot escape smoothly, causing severe collapse of the pore structure and a significant drop in effective calcium content to 9.5%. The dense structure also hinders the penetration and colonization of microorganisms, with a loading capacity of only 3.8 × 10⁻⁶. 7 The soil improvement effect continued to decline with CFU / g, and the organic matter improvement rate was only 18.7%.

[0082] in conclusion:

[0083] In summary, the mesh size of Sichuan pepper husk powder has a systematic impact on the mixing uniformity of the precursor, the pore structure of the pyrolysis products, and the functional properties of the final material. Too low a mesh size (e.g., 30 mesh) leads to uneven mixing and insufficient carbonization; too high a mesh size (e.g., 60 or 70 mesh) easily causes powder agglomeration and pore blockage. Experiments show that 40 to 60 mesh is a feasible range that balances mixing and pore development, with 50 mesh being the optimal choice. At this mesh size, the raw materials can achieve thorough mixing, and after pyrolysis, an ideal calcium- and microbial-bearing pore structure can be formed, thereby synergistically achieving the highest effective calcium content and microbial loading, and obtaining the best soil organic matter enhancement effect. This optimized parameter provides a clear basis for raw material pretreatment processes in industrial production.

[0084] 2. Analysis of the effect of the mass ratio of Sichuan pepper shell powder to fine dolomite powder on the performance of calcium-based biochar

[0085] Based on the experimental results in Table 2, the raw material mass ratio has a decisive influence on the structure and function of the prepared calcium-based biochar, as detailed below:

[0086] (1) Mass ratio 2:1 (Sichuan pepper shell powder: fine dolomite powder): The proportion of dolomite is too high while the carbonaceous precursor is relatively insufficient. Although a certain specific surface area (approximately 190 m²) can be formed after pyrolysis. 2 However, its carbon skeleton provides weak support, and the imbalance between calcium source and carbon matrix results in a low effective calcium content of only 7.5%. When applied to soil, its calcium supply capacity is insufficient, and its effect on improving soil organic matter is limited, with an improvement rate of only 20.1%, which is insufficient to meet the urgent calcium needs of karst degraded soils.

[0087] (2) Mass ratio 3:1: Increasing the proportion of pepper shell powder strengthens the carbon skeleton, which is beneficial for the embedding and stability of dolomite particles. The effective calcium content increases to 10.9%, and the microbial load reaches 5.1 × 10⁻⁶. 7 The CFU / g ratio increased the soil organic matter enhancement rate to 23.6%. However, at this ratio, the calcium source supply was still insufficient, failing to maximize the carbon-calcium synergistic effect.

[0088] (3) Mass ratio 4:1: This ratio is the optimal ratio. The carbonaceous framework and calcium source supply reach the best balance, and the fine dolomite powder can be fully and stably embedded in the well-developed carbon matrix. The biochar produced has the highest effective calcium content (12.3%) and a microbial load of 5.2 × 10⁻⁶. 7 The CFU / g ratio corresponds to a peak soil organic matter increase of 25.3%. This ratio successfully achieves a triple synergy of sufficient calcium supply, stable carbon skeleton support, and efficient microbial colonization.

[0089] (4) Mass ratio 5:1: The proportion of pepper shell powder is relatively high, resulting in a relatively reduced total calcium source and a decrease in effective calcium content to 11.4%. Although the carbon skeleton is more developed and the specific surface area increases to approximately 208 m², 2 / g, but the relative scarcity of calcium limited the synergistic effect of microbial activation nutrients, and the soil organic matter enhancement rate fell back to 24.1%.

[0090] (5) Mass ratio 6:1: The excessive proportion of carbonaceous precursors further reduces the calcium source, and the effective calcium content drops to 10.1%. The excessive carbon skeleton leads to a decrease in the utilization rate of some pores, and the microbial load decreases to 4.9 × 10⁻⁶. 7 The CFU / g soil organic matter enhancement rate decreased to 22.4%, indicating that the carbon-calcium synergistic effect continued to weaken.

[0091] in conclusion:

[0092] The raw material mass ratio is a key parameter for regulating the "carbon-calcium" composite structure and functional properties of calcium-based biochar. Experiments show that a mass ratio within the range of (3-5):1 can achieve a good balance between calcium source supply and carbon framework support. Among these, a mass ratio of 4:1 is the optimal choice, ensuring sufficient calcium loading, forming a stable carbon matrix carrier, and providing a suitable porous environment for microbial growth, thereby synergistically achieving the best soil improvement effect. This optimized ratio provides a core process parameter for precise raw material feeding in industrial production.

[0093] 3. Analysis of the effect of pyrolysis temperature on the performance of calcium-based biochar

[0094] Based on the experimental results in Table 3, pyrolysis temperature is a key process parameter determining the microstructure, calcium morphology, and biocompatibility of calcium-based biochar. The performance comparison at different temperatures is as follows:

[0095] (1) 300℃: At this temperature, pyrolysis is incomplete, and the pepper shells only undergo partial carbonization. Cellulose and hemicellulose fail to decompose sufficiently, resulting in poor pore structure development. Fine dolomite powder cannot effectively embed into the carbon skeleton, and the effective calcium content is only 6.8%. Simultaneously, the residual easily degradable organic matter in the product competes with microorganisms for ecological niches, inhibiting their activity; the microbial load is 4.5 × 10⁻⁶. 7 The corresponding soil organic matter enhancement rate was low, at 14.2%, with a CFU / g.

[0096] (2) 350℃: The temperature reaches the critical range for carbonization, the raw material is basically carbonized, the pore development is improved, and the dolomite begins to be stably embedded. The effective calcium content increases to 10.3%, and the microbial load reaches 4.9×10 7 The CFU / g soil organic matter content increased to 21.5%. However, some semi-carbonized residues still limit the release of calcium.

[0097] (3) 400℃: This is the optimal pyrolysis temperature. Under this condition, the pepper shells are completely carbonized and form the best pore structure, resulting in a high specific surface area. Dolomite is uniformly embedded in the carbon skeleton in the form of highly active calcium carbonate, with an effective calcium content reaching a peak of 12.3%. The surface of biochar is rich in functional groups, exhibiting the strongest adsorption and colonization capacity for microorganisms, with a loading capacity of 5.2 × 10⁻⁶. 7 CFU / g yielded the best soil improvement effect, with an organic matter increase rate of 25.3%.

[0098] (4) 450℃: The high temperature causes partial sintering and densification of the carbon skeleton, pore collapse, and the specific surface area decreases to about 182m². 2 / g. Simultaneously, some calcium carbonate in the dolomite is converted into inert calcium oxide, making it difficult to be activated by subsequent acid treatment, reducing the effective calcium content to 9.7%. High temperature also destroys the surface hydrophilic functional groups, hindering microbial adsorption and reducing the loading to 4.1 × 10⁻⁶ g. 7 CFU / g, soil organic matter improvement rate dropped back to 19.3%.

[0099] (5) 500℃: Excessively high temperatures cause severe collapse of the carbon skeleton, resulting in a sharp decrease in specific surface area (approximately 155m²). 2 / g), calcium carbonate was largely converted into inert calcium oxide, with the effective calcium content being only 7.2%. The surface functional groups of the biochar were almost completely lost, further reducing the microbial load to 3.5 × 10⁻⁶. 7 CFU / g, soil organic matter improvement rate decreased to 16.8%.

[0100] in conclusion:

[0101] Pyrolysis temperature systematically regulates the structure and function of calcium-based biochar by controlling the degree of carbonization, pore development, and mineral transformation. Experiments show that 350℃ to 450℃ is a feasible process window that balances sufficient carbonization with the retention of calcium activity. Among these, 400℃ is the optimal pyrolysis temperature, which achieves ideal carbon skeleton development, highly active calcium source transformation, and optimal compatibility of the carrier with microorganisms, thereby synergistically obtaining optimal soil amendment performance. This parameter provides a core basis for the control of pyrolysis processes in industrial production.

[0102] 4. Analysis of the effect of isothermal pyrolysis time on the performance of calcium-based biochar

[0103] Based on the experimental results in Table 4, the isothermal pyrolysis time is a key parameter affecting the degree of carbonization, pore structure stability, and the final product's functionality. A detailed analysis follows:

[0104] (1) 1.5 hours: Insufficient pyrolysis time and incomplete carbonization reaction lead to the accumulation of undecomposed organic matter in the precursor, causing pore blockage. This hinders the effective adhesion of the calcium source, with an effective calcium content of only 7.9%. Furthermore, the residual organic matter may release inhibitory substances in subsequent applications, affecting microbial activity, with a loading capacity of 4.6 × 10⁻⁶. 7 The soil organic matter enhancement rate was low, at 16.5%, with a CFU / g.

[0105] (2) 2.0 hours: By appropriately extending the pyrolysis time, the carbonization reaction tends to be complete, the residual organic matter is reduced, and the pore permeability is improved. The effective calcium content increases to 11.2%, and the microbial load reaches 5.0 × 10⁻⁶. 7 The CFU / g soil organic matter enhancement rate increased to 23.1%. However, there is still room for further optimization of the carbon skeleton structure and surface properties.

[0106] (3) 2.5 hours: This time is the optimal condition. The carbonization reaction is complete and the carbon skeleton structure remains intact, forming a rich and stable porous network. The calcium source is fully and firmly loaded onto the carbon skeleton, and the effective calcium content reaches a peak of 12.3%. The biochar surface is clean and free of inhibitory residues, providing an optimal colonization environment for microorganisms, with a loading capacity of 5.2 × 10⁻⁶. 7 The optimal soil amendment effect was achieved at CFU / g, with an organic matter increase rate of 25.3%. This time point achieved the best balance between thorough reaction and structural integrity.

[0107] (4) 3.0 hours: Excessive pyrolysis time leads to excessive consumption and structural embrittlement of the carbon skeleton, and the destruction of some pore structures. Available calcium is lost along with the carbon skeleton, and its content decreases to 10.5%. Simultaneously, the number of surface functional groups decreases, reducing the effectiveness of microbial adsorption sites, and the loading capacity drops to 4.5 × 10⁻⁶. 7 CFU / g, soil organic matter improvement rate dropped to 21.7%.

[0108] (5) 3.5 hours: Excessive pyrolysis time leads to continuous collapse of the carbon skeleton and significant loss of porosity. The effective calcium content further decreases to 8.5%, the surface properties of biochar deteriorate, and the microbial load decreases to 3.9 × 10⁻⁶. 7 The CFU / g soil organic matter enhancement rate decreased to 19.2%, indicating that the performance continued to decline due to excessive pyrolysis.

[0109] in conclusion:

[0110] The isothermal pyrolysis time directly regulates the completeness of the carbonization reaction and the thermal stability of the carbon skeleton. Experiments show that 2.0 to 3.0 hours is the effective process window to ensure complete reaction and protect the skeleton structure. Among them, 2.5 hours is the optimal isothermal pyrolysis time, which can achieve complete carbonization of raw materials while maximally preserving the ideal pore structure, surface activity, and carrying capacity of biochar for calcium sources and microorganisms, thereby obtaining a calcium-based biochar product with optimal overall performance. This parameter provides a key basis for determining the pyrolysis procedure in industrial production.

[0111] 5. Analysis of the Influence of Isothermal Adsorption Temperature on Microbial Loading Effect and Calcium-Based Biochar Performance

[0112] Based on the experimental results in Table 5, the isothermal adsorption temperature in the microbial loading step is a key parameter affecting the activity, colonization efficiency, and functional synergy of Bacillus mucilage in the final product. The specific analysis is as follows:

[0113] (1) 22℃: The temperature is below the optimal growth range for the bacteria, resulting in a decrease in the metabolic activity and motility of Bacillus mucilaginosus. The microorganisms mainly attach to the surface of calcium-based biochar through passive physical adsorption, and their active adsorption and colonization capacity is weak, with a loading capacity of only 3.2×10. 7 CFU / g. Although the carrier properties of calcium-based biochar (such as specific surface area and available calcium content) were not significantly affected at this time, the ability of it to activate soil phosphorus was limited due to the severe shortage of functional microorganisms, and the corresponding soil organic matter enhancement rate was only 18.9%.

[0114] (2) 24.5℃: The temperature is close to the suitable growth range of the bacteria, and their metabolic activity recovers somewhat, while their active adsorption and colonization capabilities are enhanced. The microbial load increases to 4.6×10⁻⁶. 7 With CFU / g, the soil organic matter enhancement rate increased to 22.7%, but the bacterial activity did not reach its peak, indicating room for optimization of the loading effect.

[0115] (3) 26.5℃: This temperature represents the optimal adsorption condition, falling within the optimal growth temperature range for Bacillus subtilis. The bacteria exhibit vigorous metabolism, exhibiting the strongest motility and surface adhesion, achieving the highest active loading efficiency with a loading capacity of 5.2 × 10⁻⁶. 7 CFU / g. Under these conditions, the synergistic effect between the loaded microorganisms and the calcium source in the calcium-based biochar was most complete, resulting in the best soil improvement effect and an organic matter increase rate of 25.3%.

[0116] (4) 28.5℃: The temperature is slightly higher than the optimum range, and heat stress begins to occur in the bacteria. The metabolic activity of some microorganisms decreases, and cell membrane fluidity and protein function may be affected, resulting in a decrease in the load to 4.1×10. 7CFU / g. The synergistic activation effect of microorganisms and calcium sources weakened accordingly, and the soil organic matter enhancement rate dropped to 21.9%.

[0117] (5) 31℃: The temperature is too high and exceeds the tolerance range of the strain used. The high temperature exacerbates the denaturation and inactivation of bacterial proteins, further reducing the bacterial load to 3.6×10. 7 CFU / g. Meanwhile, higher temperatures accelerate the evaporation of water from the suspension, potentially causing localized fluctuations in bacterial concentration, affecting the uniformity and stability of the load, ultimately reducing the soil organic matter enhancement rate to 20.3%.

[0118] in conclusion:

[0119] Isothermal adsorption temperature directly affects the colonization efficiency and subsequent functional expression of microorganisms on biochar carriers by regulating their physiological activity. Experiments show that 24.5℃ to 28.5℃ is the effective temperature range for maintaining high activity and achieving stable loading of *Bacillus mucilaginosus*. Among these, 26.5℃ is the optimal isothermal adsorption temperature, which maximizes the active adsorption and colonization capacity of the bacteria, ensuring high loading and thus enhancing the synergistic improvement effect of the "char-calcium-bacteria" system. This parameter provides crucial information for the precise control of the microbial loading process.

[0120] 6. Analysis of the Influence of Adsorption Solid-Liquid Ratio on Microbial Loading Effect and Performance of Calcium-Based Biochar

[0121] Based on the experimental results in Table 6, the solid-liquid ratio of primary calcium-based biochar to bacterial suspension in the adsorption step is a key parameter affecting the uniformity of microbial colonization, loading efficiency, and process economy. The specific analysis is as follows:

[0122] (1) Solid-liquid ratio 1:7: The volume of the bacterial suspension was relatively insufficient, failing to completely wet the calcium-based biochar particles. Microorganisms could only adhere to the surface of the carrier and could not effectively enter the internal pores, resulting in a low loading capacity of only 3.0 × 10⁻⁶. 7 CFU / g. Although the physicochemical properties of calcium-based biochar (such as available calcium content and specific surface area) were not affected, the insufficient number of functional microorganisms severely limited its potential to fully activate soil nutrients, resulting in a soil organic matter enhancement rate of only 17.5%.

[0123] (2) Solid-liquid ratio 1:8: After increasing the amount of bacterial suspension, the calcium-based biochar was basically infiltrated, and microorganisms began to penetrate into the pores, increasing the loading to 4.5×10. 7 With CFU / g, the soil organic matter enhancement rate increased to 22.3%. However, there is still room for improvement in the uniformity of infiltration and adsorption at this ratio.

[0124] (3) Solid-liquid ratio 1:9: This ratio is the optimal condition. The volume of the bacterial suspension and the adsorption capacity of the calcium-based biochar are optimally matched, the carrier is fully and uniformly wetted, and there is no excess free bacterial suspension. Microorganisms can be uniformly colonized on the surface and inside the pores, achieving the highest loading efficiency, with a loading capacity of 5.2 × 10⁻⁶. 7 CFU / g. This allows the synergistic effect of "charcoal-calcium-bacteria" to be fully utilized, resulting in the best soil improvement effect, with an organic matter increase rate of 25.3%, while avoiding the waste of raw materials.

[0125] (4) Solid-liquid ratio 1:10: The bacterial suspension begins to be excessive, leading to excessive density of microorganisms on the surface of calcium-based biochar, causing local nutrient competition and accumulation of metabolites, which is detrimental to the survival and activity of microorganisms, and the loading capacity decreases to 4.2×10 7 CFU / g. Furthermore, the excessive free liquid increased the energy and time costs of subsequent drying processes, causing the soil organic matter enhancement rate to drop to 21.5%.

[0126] (5) Solid-liquid ratio 1:11: The bacterial suspension was severely excessive, the competitive inhibition between microorganisms intensified, some cells died due to nutrient deficiency, and the load further decreased to 3.4 × 10 7 CFU / g. The redundant liquid significantly increased process operating costs, reduced cost-effectiveness, and resulted in a soil organic matter improvement rate of only 19.8%.

[0127] in conclusion:

[0128] The adsorption solid-liquid ratio directly determines the contact efficiency between the bacterial suspension and the calcium-based biochar carrier, as well as the colonization environment for microorganisms. Experiments show that a solid-liquid ratio within the range of 1:8 to 1:10 achieves a good balance between ensuring sufficient bacterial supply and avoiding excessive competition. A solid-liquid ratio of 1:9 is the optimal parameter, enabling efficient and uniform loading of microorganisms onto the carrier, maximizing the synergistic improvement effect, while ensuring the economy and operability of the process. This parameter provides a precise basis for material feeding in the microbial loading process during large-scale production.

[0129] Example 2

[0130] A method for preparing calcium-based biochar for improving karst desertification soil includes the following steps:

[0131] (1) Raw material pretreatment: Collect pepper shells, remove impurities, and dry them in a 60℃ oven until constant weight. Put the dried pepper shells into a grinder and grind them. After grinding, pass them through a 40-mesh sieve and collect the sieve material to obtain pepper shell powder.

[0132] (2) Pretreatment of dolomite powder: Dolomite from the karst area in Southwest China was crushed into blocks with a particle size ≤5cm by a jaw crusher, then ground in a ball mill for 3 hours and passed through a 100-mesh sieve to obtain fine dolomite powder. The fine dolomite powder was dried in a 100℃ oven for 3 hours, then removed and cooled to room temperature in a desiccator, and sealed for later use.

[0133] (3) Mixing and granulation: Weigh the pepper shell powder obtained in step (1) and the dolomite fine powder obtained in step (2) at a mass ratio of 3:1, and mix them evenly in a mixing pot. Add 5% of the total mass of starch binder to the mixed powder, stir evenly, then add deionized water to adjust the moisture content of the mixture to 20%, transfer it to a disc granulator for granulation, and obtain mixed granules with a particle size of 3 mm.

[0134] (4) Low-temperature pyrolysis: The mixed particles obtained in step (3) were placed in a tube furnace and heated to 350°C at a heating rate of 5°C / min under a nitrogen atmosphere. The mixture was kept at a constant temperature for 3 hours. After pyrolysis, the mixture was allowed to cool naturally to room temperature to obtain primary calcium-based biochar. The primary calcium-based biochar was soaked in 0.1 mol / L hydrochloric acid solution for 1 hour (solid-liquid ratio 1:10), and then repeatedly rinsed with deionized water until the pH of the filtrate was 7.0. The filtrate was then dried in an oven at 105°C for 2 hours and cooled to room temperature for later use.

[0135] (5) Microbial load: Bacillus mucilaginosus isolated and purified from soil in the karst region of Southwest China was selected. Single colonies were picked and inoculated into LB liquid medium and cultured in a shaker at 30℃ and 150r / min for 36h to obtain a bacterial concentration of 10. 8 CFU / mL bacterial suspension.

[0136] (6) Post-treatment: The primary calcium-based biochar obtained in step (4) was soaked in the bacterial suspension obtained in step (5) at a solid-liquid ratio of 1:8. It was adsorbed at a constant temperature of 25°C for 6 hours. During this period, it was stirred with a glass rod once every 1 hour. After the adsorption was completed, it was placed in a 50°C oven to dry until the moisture content was 9%. It was then passed through a 20-mesh sieve to obtain Sichuan pepper shell-based calcium-based biochar.

[0137] Example 3

[0138] A method for preparing calcium-based biochar for improving karst desertification soil includes the following steps:

[0139] (1) Raw material pretreatment: Collect pepper shells, remove impurities, and dry them in an 80℃ oven until constant weight. Put the dried pepper shells into a grinder and grind them. After grinding, pass them through a 60-mesh sieve and collect the sieve material to obtain pepper shell powder.

[0140] (2) Pretreatment of dolomite powder: Dolomite from the karst area in Southwest China was crushed into blocks with a particle size ≤5cm by a jaw crusher, then ground in a ball mill for 3 hours and passed through a 200-mesh sieve to obtain fine dolomite powder. The fine dolomite powder was dried in an oven at 110℃ for 2 hours, then removed and cooled to room temperature in a desiccator and sealed for later use.

[0141] (3) Mixing and granulation: Weigh the pepper shell powder obtained in step (1) and the fine dolomite powder obtained in step (2) at a mass ratio of 5:1, and mix them evenly in a mixing pot. Add 8% of the total mass of starch binder to the mixed powder, stir evenly, then add deionized water to adjust the moisture content of the mixture to 25%, transfer it to a disc granulator for granulation, and obtain mixed granules with a particle size of 5 mm.

[0142] (4) Low-temperature pyrolysis: The mixed particles obtained in step (3) were placed in a tube furnace and heated to 450°C at a heating rate of 8°C / min under a nitrogen atmosphere. The mixture was then kept at a constant temperature for 2 hours. After pyrolysis, the mixture was allowed to cool naturally to room temperature to obtain primary calcium-based biochar. The primary calcium-based biochar was soaked in 0.1 mol / L hydrochloric acid solution for 1 hour (solid-liquid ratio 1:10), and then repeatedly rinsed with deionized water until the pH of the filtrate was 7.5. The filtrate was then dried in an oven at 105°C for 2 hours and then cooled to room temperature for later use.

[0143] (5) Microbial load: Bacillus mucilaginosus isolated and purified from soil in the karst region of Southwest China was selected. Single colonies were picked and inoculated into LB liquid medium and cultured in a shaker at 32℃ and 180r / min for 24h to obtain a bacterial concentration of 10. 9 CFU / mL bacterial suspension.

[0144] (6) Post-treatment: The primary calcium-based biochar obtained in step (4) was soaked in the bacterial suspension obtained in step (5) at a solid-liquid ratio of 1:10. It was adsorbed at a constant temperature of 28°C for 4 hours. During this period, it was stirred with a glass rod once every 1 hour. After the adsorption was completed, it was placed in a 60°C oven to dry until the moisture content was 7%. It was then passed through a 40-mesh sieve to obtain Sichuan pepper shell-based calcium-based biochar.

[0145] Example 4

[0146] A method for preparing calcium-based biochar for improving karst desertification soil includes the following steps:

[0147] (1) Raw material pretreatment: Collect pepper shells, remove impurities, and dry them in a 70℃ oven until constant weight. Put the dried pepper shells into a grinder and grind them. After grinding, pass them through a 50-mesh sieve and collect the sieve material to obtain pepper shell powder.

[0148] (2) Pretreatment of dolomite powder: Dolomite from the karst area in Southwest China was crushed into blocks with a particle size ≤5cm by a jaw crusher, then ground in a ball mill for 3 hours and passed through a 150-mesh sieve to obtain fine dolomite powder. The fine dolomite powder was dried in an oven at 105℃ for 2.5 hours, then removed and cooled to room temperature in a desiccator and sealed for later use.

[0149] (3) Mixing and granulation: Weigh the pepper shell powder obtained in step (1) and the dolomite fine powder obtained in step (2) at a mass ratio of 3:1, and mix them evenly in a mixing pot. Add 7% of the total mass of starch binder to the mixed powder, stir evenly, then add deionized water to adjust the moisture content of the mixture to 23%, transfer it to a disc granulator for granulation, and obtain mixed granules with a particle size of 4 mm.

[0150] (4) Low-temperature pyrolysis: The mixed particles obtained in step (3) were placed in a tube furnace and heated to 420°C at a heating rate of 7°C / min under a nitrogen atmosphere. The mixture was kept at a constant temperature for 2.6 h. After pyrolysis, the mixture was allowed to cool naturally to room temperature to obtain primary calcium-based biochar. The primary calcium-based biochar was soaked in 0.1 mol / L hydrochloric acid solution for 1 h (solid-liquid ratio 1:10), and then repeatedly rinsed with deionized water until the pH of the filtrate was 7.0. The filtrate was then dried in an oven at 105°C for 2 h and cooled to room temperature for later use.

[0151] (5) Microbial load: Bacillus mucilaginosus isolated and purified from soil in the karst region of Southwest China was selected. Single colonies were picked and inoculated into LB liquid medium and cultured in a shaker at 31℃ and 170r / min for 28h to obtain a bacterial concentration of 5×10⁻⁶. 8 CFU / mL bacterial suspension.

[0152] (6) Post-treatment: The primary calcium-based biochar obtained in step (4) was soaked in the bacterial suspension obtained in step (5) at a solid-liquid ratio of 1:9. The adsorption was carried out at a constant temperature of 27°C for 4.5 hours. During this period, the mixture was stirred with a glass rod once every 1 hour. After the adsorption was completed, the mixture was placed in an oven at 55°C and dried until the moisture content was 8%. The mixture was then passed through a 30-mesh sieve to obtain the calcium-based biochar based on pepper shells.

[0153] Example 5

[0154] A method for preparing calcium-based biochar for improving karst desertification soil includes the following steps:

[0155] (1) Raw material pretreatment: Collect pepper shells, remove impurities, and dry them in a 70℃ oven until constant weight. Put the dried pepper shells into a grinder and grind them. After grinding, pass them through a 50-mesh sieve and collect the sieve material to obtain pepper shell powder.

[0156] (2) Pretreatment of dolomite powder: Dolomite from the karst area in Southwest China was crushed into blocks with a particle size ≤5cm by a jaw crusher, then ground in a ball mill for 3 hours and passed through a 150-mesh sieve to obtain fine dolomite powder. The fine dolomite powder was dried in an oven at 105℃ for 2.5 hours, then removed and cooled to room temperature in a desiccator and sealed for later use.

[0157] (3) Mixing and granulation: Weigh the pepper shell powder obtained in step (1) and the dolomite fine powder obtained in step (2) at a mass ratio of 5:1, and mix them evenly in a mixing pot. Add 6% of the total mass of starch binder to the mixed powder, stir evenly, then add deionized water to adjust the moisture content of the mixture to 21%, transfer it to a disc granulator for granulation, and obtain mixed granules with a particle size of 4 mm.

[0158] (4) Low-temperature pyrolysis: The mixed particles obtained in step (3) were placed in a tube furnace and heated to 380°C at a heating rate of 5.5°C / min under a nitrogen atmosphere, and pyrolyzed at a constant temperature for 2.8 h. After pyrolysis, the mixture was naturally cooled to room temperature to obtain primary calcium-based biochar. The primary calcium-based biochar was soaked in 0.1 mol / L hydrochloric acid solution for 1 h (solid-liquid ratio 1:10), and then repeatedly rinsed with deionized water until the pH of the filtrate was 7.2. The filtrate was then dried in an oven at 105°C for 2 h and cooled to room temperature for later use.

[0159] (5) Microbial load: Bacillus mucilaginosus isolated and purified from soil in the karst region of Southwest China was selected. Single colonies were picked and inoculated into LB liquid medium and cultured in a shaker at 30℃ and 160r / min for 30h to obtain a bacterial concentration of 3×10⁻⁶. 8 CFU / mL bacterial suspension.

[0160] (6) Post-treatment: The primary calcium-based biochar obtained in step (4) was soaked in the bacterial suspension obtained in step (5) at a solid-liquid ratio of 1:8.5 and adsorbed at a constant temperature of 26℃ for 4.5h, with stirring once every 1h using a glass rod. After adsorption, it was placed in an oven at 55℃ and dried until the moisture content was 8%, and then passed through a 30-mesh sieve to obtain Sichuan pepper shell-based calcium-based biochar.

[0161] Application Examples and Effect Verification

[0162] (I) Verification of the effect of biochar alone

[0163] The calcium-based biochar prepared in Examples 1-5 was selected and applied at a rate of 30 t / hm. 2 An experiment was conducted to assess the application effect of calcium-based biochar under a tillage depth of 25 cm. Soil parameters were measured after one season of corn planting. The results showed that calcium-based biochar in each embodiment exhibited significant soil improvement effects, with soil organic matter increase rates ranging from 22.1% to 25.3%.

[0164] (II) Verification of the effect of combined application of biochar and organic fertilizer

[0165] To further verify the synergistic effect, the calcium-based biochar prepared in Examples 1-5 was used as a representative and compounded with locally sourced straw and cow manure composted organic fertilizer from the karst region of Southwest China at a mass ratio of 1:2.5. The compounded material was applied at a total rate of 40 t / hm². 2 Apply and till to a depth of 30cm. Experiments show that:

[0166] 1. The compound treatment (calcium-based biochar in Examples 1-5) is superior to the treatment of applying the corresponding biochar or applying the same amount of organic fertilizer alone in terms of improving soil physicochemical properties, enhancing microbial activity and increasing crop yield.

[0167] 2. Taking the data as an example: The combined treatment with calcium-based biochar from Example 1 stabilized the pH of the experimental soil from 8.2 to 7.6, resulting in an effective increase in yield for both crop seasons. The combined treatments with calcium-based biochar from Examples 2-5 also showed stable yield increases and soil improvement effects.

[0168] III. Comparative Example

[0169] Comparative Example 1: Lack of Microbial Loading Step

[0170] Steps (1)-(4) are the same as in Example 1.

[0171] (5) Post-processing: The primary calcium-based biochar obtained in step (4) is directly placed in a 55°C oven and dried to a moisture content of 8%, and then passed through a 30-mesh sieve to obtain biochar without microbial load.

[0172] Comparative Example 2: Using corn stalks instead of peppercorn husks

[0173] (1) Raw material pretreatment: collect corn stalks, remove impurities, dry at 70°C to constant weight, crush and pass through a 50-mesh sieve to obtain corn stalk powder.

[0174] Steps (2)-(6) are the same as in Example 1, except that the pepper shell powder is replaced with corn stalk powder.

[0175] Comparative Example 3: Pyrolysis temperature outside the range

[0176] Steps (1)-(3) are the same as in Example 1.

[0177] (4) Low-temperature pyrolysis: heating rate 6.5℃ / min, heating to 600℃, constant temperature pyrolysis for 2.5h, the rest is the same as in Example 1.

[0178] Steps (5)-(6) are the same as in Example 1.

[0179] Comparative Example 4: Changing the type of adhesive

[0180] Steps (1)-(2) are the same as in Example 1.

[0181] (3) Mixing and granulation: Weigh the pepper shell powder obtained in step (1) and the dolomite fine powder obtained in step (2) at a mass ratio of 4:1, and mix them evenly in a mixing pot. Add 6.5% of the total mass of sodium carboxymethyl cellulose binder (instead of starch binder) to the mixed powder, stir evenly, and the remaining steps are the same as in Example 1.

[0182] Steps (4)-(5) are the same as in Example 1.

[0183] Comparative Example 5: Adjusting the types of microorganisms

[0184] Steps (1)-(4) are the same as in Example 1.

[0185] (5) Microbial load: Select Bacillus megaterium (a non-native strain from the Southwest Karst Area) isolated and purified from farmland soil, pick a single colony and inoculate it into LB liquid medium, and follow the same steps as in Example 1.

[0186] Step (6) is the same as in Example 1.

[0187] IV. Implementation Examples and Comparative Examples: Data Results and Theoretical Analysis

[0188] (a) Test Results

[0189]

[0190] (II) Theoretical Analysis

[0191] 1. Analysis of available calcium content

[0192] The effective calcium content of the calcium-based biochar prepared in Examples 1-5 ranged from 10.6% to 12.3%, with Example 1 reaching the highest at 12.3%. In contrast, the effective calcium content of Comparative Examples 1-5 ranged from a maximum of 11.9% to a minimum of 4.9%.

[0193] From the perspective of the mechanism of action, the high effective calcium content achieved by this invention is mainly attributed to the system's raw material adaptation and process synergy design:

[0194] (1) Raw material synergy: The well-developed porous carbon skeleton formed by the pyrolysis of pepper shells provides a stable and high-capacity load carrier for dolomite fine powder, effectively preventing the loss of calcium source.

[0195] (2) Process adaptation: The low temperature pyrolysis conditions of 350-450℃ effectively preserve the active form of calcium carbonate (CaCO3) in dolomite; the subsequent mild treatment with 0.1mol / L hydrochloric acid can convert some of the CaCO3 into a soluble form of calcium that is easily absorbed by plants.

[0196] (3) Particle size optimization: control the pepper shell powder (40-60 mesh) and dolomite fine powder (100-200 mesh) to form a suitable particle size distribution, ensuring that the two are uniformly mixed in the precursor, so that the calcium element is uniformly distributed in the final biochar product.

[0197] The low or fluctuating effective calcium content in each comparison ratio stems from specific defects in their technical solutions:

[0198] Comparative Example 3: Due to the excessively high pyrolysis temperature (600℃), a large amount of CaCO3 in the dolomite was decomposed into chemically stable calcium oxide. This form is difficult to be effectively activated by subsequent acid treatment, resulting in a decrease in the effective calcium content.

[0199] Comparative Example 2: When corn stalks are used to replace pepper shells, the pore structure of the pyrolysis products is poorly developed, the carbon skeleton has weak carrying capacity, and it cannot effectively fix the fine dolomite powder, resulting in the easy loss of calcium source during preparation and subsequent application.

[0200] Comparative Example 4: Using sodium carboxymethyl cellulose instead of starch as a binder resulted in excessively dense internal granules due to its film-forming properties, which made the granules prone to cracking during pyrolysis. This not only caused pore blockage but also trapped some of the calcium source within the dense structure, preventing its release.

[0201] 2. Microbial load analysis

[0202] The microbial loading of the calcium-based biochar prepared in Examples 1-5 ranged from 4.5 × 10⁻⁶. 7 Up to 5.2×10 7 The concentrations were between CFU / g, with Example 1 reaching the highest at 5.2 × 10⁻⁶. 7 CFU / g. In contrast, the highest microbial load in Comparative Examples 1-5 was only 4.3 × 10⁻⁶. 7 CFU / g, with the lowest value as low as 0.2×10 7 CFU / g.

[0203] This invention achieves high microbial loading capacity primarily based on the systematic optimization and precise adaptation of functional strains, vector characteristics, and loading conditions.

[0204] (1) Strains are suitable for use: Bacillus mucilaginosus selected from the karst region of Southwest China is selected. Its cell wall surface characteristics have good affinity with the oxygen-containing functional groups (such as hydroxyl and carboxyl groups) abundant on the surface of Sichuan pepper shell biochar, which is conducive to the colonization of microorganisms into the pores of the carrier through active action.

[0205] (2) Loading conditions optimization: The constant temperature adsorption temperature was controlled at 25-28℃ (the optimal growth range of bacteria), and an optimized solid-liquid ratio of 1:(8-10) was adopted to ensure that the bacterial suspension was evenly and fully wetted on the biochar carrier. This avoided uneven colonization due to insufficient liquid volume and prevented nutrient competition and inhibition caused by excessive bacterial liquid.

[0206] (3) Supporting carrier characteristics: The calcium-based biochar prepared in this invention has a density of approximately 205 μm. 2 The high specific surface area and well-developed pore structure of / g provide ample attachment sites and habitat for microorganisms. The pore size matches the size of Bacillus mucilaginosus, which facilitates bacterial entry and stable survival.

[0207] The relatively low microbial load in the comparative samples stemmed from key defects in their technical solutions:

[0208] Comparative Example 1: The microbial loading step was completely omitted during the preparation process, and the product only carried trace amounts of environmental bacteria, so the loading was extremely low.

[0209] Comparative Example 5: Bacillus megaterium isolated from ordinary farmland soil was selected. It has poor adaptability to the biochar carrier of the present invention and the high-calcium environment of karst areas. Moreover, its cell wall is highly hydrophobic and mainly binds to the carrier through weak physical adsorption, which makes it easy to fall off, resulting in a limited loading capacity.

[0210] Comparative Example 3: Due to the use of 600℃ high-temperature pyrolysis, the hydrophilic functional groups on the surface of biochar were severely damaged, resulting in a significant reduction in effective adsorption sites, which in turn reduced the colonization ability of microorganisms.

[0211] 3. Specific surface area analysis

[0212] The specific surface area of ​​the calcium-based biochar prepared in Examples 1-5 is between 180 m². 2 / g to 205m 2 Between / g, with Example 1 reaching the highest 205m 2 / g. In comparison, the highest specific surface area was 202m² for Comparative Examples 1-5. 2 / g, with a minimum of only 156m 2 / g.

[0213] The key to achieving a high specific surface area in this invention lies in the precise control of the formation and development of the pore structure of calcium-based biochar through a systematic process:

[0214] (1) Precursor particle size optimization: Use 40-60 mesh pepper shell powder. Its particle size can ensure that it is fully carbonized during pyrolysis to form rich pores, and can also avoid pretreatment agglomeration caused by excessively fine powder (such as 60 mesh or above), thereby preventing pore blockage.

[0215] (2) Pyrolysis conditions control: The low temperature pyrolysis range of 350-450℃ can effectively avoid excessive sintering and shrinkage of the carbon skeleton, which is conducive to preserving the multi-level channels derived from the natural fiber structure of pepper shell; at the same time, at this temperature, the fine powder of dolomite can be stably embedded without destroying the integrity and connectivity of the carbon skeleton.

[0216] (3) Adhesive-assisted pore formation: Starch adhesive not only plays a shaping role, but also carbonizes during pyrolysis, which can form additional microporous structures, thus contributing to further improve the overall specific surface area of ​​the material.

[0217] The relatively low specific surface area is attributed to specific defects in its process design:

[0218] Comparative Example 3: High-temperature pyrolysis at 600℃ resulted in severe sintering and densification of the carbon skeleton, causing a large amount of pore collapse and a sharp decrease in specific surface area to 156 m². 2 / g.

[0219] Comparative Example 2: Using corn stalks instead of pepper husks, the lignin content of corn stalks is relatively low while the cellulose content is high. Therefore, its pyrolysis more easily forms dense charcoal with poor pore development, resulting in a lower specific surface area (169 m²). 2 / g).

[0220] Comparative Example 4: Using sodium carboxymethyl cellulose as a binder, its strong adhesion resulted in excessively dense granules, hindering the escape of volatile gases during pyrolysis, thus impeding and clogging pore development, ultimately reducing the specific surface area (184 m²). 2 / g) failed to reach the optimal level.

[0221] 4. Analysis of Soil Organic Matter Enhancement Rate

[0222] The soil organic matter enhancement rates of the calcium-based biochar prepared in Examples 1-5 ranged from 22.1% to 25.3%, with Example 1 achieving the highest rate of 25.3%. In contrast, the enhancement rates of Comparative Examples 1-5 ranged from a maximum of 21.2% to a minimum of 16.8%.

[0223] This invention can significantly improve soil organic matter due to its constructed "carbon-calcium-bacteria" multi-effect synergistic improvement mechanism:

[0224] (1) Physicochemical effects of calcium: The stable and effective calcium in biochar can directly replenish the calcium in karst degraded soil, promote the coagulation and aggregation of soil colloids, thereby enhancing the soil's physical adsorption and chemical fixation capacity for organic matter.

[0225] (2) Nutrient activation and driving effect of microorganisms: The loaded Bacillus mucilaginosus can effectively secrete organic acids and other metabolites, activate potential nutrients such as fixed phosphorus in the soil, provide usable resources for soil microbial communities, drive the decomposition, transformation and stabilization of organic matter, and promote the accumulation of soil organic carbon pool.

[0226] (3) Multi-level functions of calcium-based biochar carrier: The high specific surface area of ​​calcium-based biochar can serve as a physical protective carrier for organic matter, reducing its decomposition and loss; at the same time, its porous structure provides a superior habitat and reproduction microenvironment for microorganisms, which is conducive to the formation and maintenance of a rhizosphere micro-ecosystem with functional bacteria as the core and promoting the benign cycle of organic matter.

[0227] The comparative proportion of soil organic matter improvement was limited because it failed to form an effective synergistic system:

[0228] Comparative Example 1: No microbial loading was carried out during the preparation process, and the improvement was only achieved by relying on the calcium in biochar. There was a lack of biological activation of soil nutrients, so the organic matter improvement rate was limited (18.2%).

[0229] Comparative Example 3: Due to the low effective calcium content and insufficient microbial load caused by high-temperature pyrolysis, it was unable to effectively supplement calcium and lacked sufficient biological activity to drive nutrient cycling and organic matter transformation, resulting in the lowest improvement rate (16.8%).

[0230] Comparative Example 5: The selected Bacillus megaterium has weak adaptability and phosphorus solubilization ability in karst soil environment, making it difficult to effectively initiate and promote the cycle accumulation process of soil organic matter, and the improvement effect is only average (19.5%).

[0231] The above description provides a further detailed explanation of the present invention in conjunction with specific embodiments. It is understood that those skilled in the art can, without departing from the basic principles and spirit of the present invention, adjust various process parameters, substitute equivalent steps, or rationally select materials based on the above description. All such technical solutions and implementations that do not depart from the essence of the present invention should be included within the scope of protection claimed by the present invention.

[0232] Although the present invention and its preferred embodiments have been described in detail, various changes, modifications, or equivalent substitutions of form and detail can be made by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims. Furthermore, the scope of the present invention is not limited to the specific processes, manufacturing methods, material compositions, implementation methods, or operating steps described in the specification. Based on the technical content disclosed in this invention, those skilled in the art can draw upon existing or future developed processes, manufacturing methods, material compositions, approaches, methods, or steps that can achieve substantially the same function or technical effect as the corresponding embodiments of the present invention. Therefore, the scope of protection of this invention is intended to cover all such changes, modifications, and equivalents falling within the meaning and scope of the claims.

Claims

1. A method for preparing calcium-based biochar for improving karst desertification soil, characterized in that, Includes the following steps: (1) Raw material pretreatment: Collect pepper shells, remove impurities, and dry them in an oven at 60-80℃ until constant weight. Crush the dried pepper shells and pass them through a 40-60 mesh sieve to obtain pepper shell powder. (2) Pretreatment of dolomite powder: Take dolomite from the karst area, crush it with a jaw crusher, grind it with a ball mill, and pass it through a 100-200 mesh sieve to obtain fine dolomite powder. Dry the fine dolomite powder and cool it to room temperature for later use. (3) Mixing and granulation: Mix the pepper shell powder obtained in step (1) with the dolomite fine powder obtained in step (2), add 5-8% of the mass of starch binder to the mixed powder, stir evenly, add an appropriate amount of water to adjust the moisture content to 20-25%, and granulate using a disc granulator to obtain mixed granules with a particle size of 3-5 mm. (4) Low-temperature pyrolysis: The mixed particles obtained in step (3) are placed in a tube furnace and heated to 350-450°C at a heating rate of 5-8°C / min under a nitrogen atmosphere. After constant-temperature pyrolysis, the mixture is naturally cooled to room temperature to obtain primary calcium-based biochar. The primary calcium-based biochar is soaked in 0.1 mol / L hydrochloric acid solution for 1 h, washed with water until pH 7.0-7.5, dried at 105°C for 2 h, and cooled for later use. (5) Microbial load: Phosphate-solubilizing bacteria isolated from soil in the karst region of Southwest China were selected, inoculated into LB liquid medium, and cultured with shaking to obtain a bacterial concentration of 10. 8 -10 9 CFU / mL bacterial suspension; (6) Post-treatment: Soak the primary calcium-based biochar obtained in step (4) in the bacterial suspension obtained in step (5) at a solid-liquid ratio of 1: (8-10), and adsorb at a constant temperature. Stir once every 1 hour during the process. After the adsorption is completed, take it out and dry it in an oven at 50-60℃ until the moisture content is ≤10%. Pass it through a 20-40 mesh sieve to obtain Sichuan pepper shell calcium-based biochar.

2. The method for preparing calcium-based biochar for improving karst desertification soil according to claim 1, characterized in that, The drying temperature in step (1) is 70°C.

3. The method for preparing calcium-based biochar for improving karst desertification soil according to claim 1, characterized in that, The drying temperature of the dolomite fine powder in step (2) is 100-110℃, and the drying time is 2-3h.

4. The method for preparing calcium-based biochar for improving karst desertification soil according to claim 1, characterized in that, The mass ratio of pepper shell powder to dolomite fine powder in step (3) is (3-5):

1.

5. The method for preparing calcium-based biochar for improving karst desertification soil according to claim 1, characterized in that, The isothermal pyrolysis time in step (4) is 2-3 hours.

6. The method for preparing calcium-based biochar for improving karst desertification soil according to claim 1, characterized in that, The phosphate-solubilizing bacteria mentioned in step (5) are Bacillus mucilaginosus isolated from the soil of the karst area in Southwest China. The LB liquid culture medium is shaken and cultured at 30-32℃ and 150-180r / min for 24-36h.

7. The method for preparing calcium-based biochar for improving karst desertification soil according to claim 1, characterized in that, The isothermal adsorption temperature in step (6) is 25-28℃, and the adsorption time is 4-6h.

8. A calcium-based biochar for improving karst desertification soil, obtained by the preparation method according to any one of claims 1-7.

9. The application of calcium-based biochar according to claim 8 in the improvement of karst desertification soil, characterized in that, The calcium-based biochar is applied at a rate of 20-50 t / hm. 2 Apply the fertilizer evenly to the surface of the karst desertification soil and till it to a depth of 20-30cm.

10. The application of calcium-based biochar according to claim 9 in the improvement of karst desertification soil, characterized in that, The calcium-based biochar and the decomposed organic fertilizer are mixed at a mass ratio of 1:(2-3) and then applied to the soil. The decomposed organic fertilizer is a mixture of local straw and cow manure from the karst region of Southwest China.

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