A nested application method of a composite amendment for soil improvement of Chinese fir forest

Abstract

The application provides a nested application method of a composite amendment for Cunninghamia lanceolata forest soil improvement. The method sets a plurality of sample plots in a Cunninghamia lanceolata plantation, divides a plurality of small sample plots in each sample plot, respectively applies different gradients of quicklime and biochar, and evaluates the synergistic improvement effect of the two on soil physical and chemical properties and microbial ecology through a nested application system.

Description

Technical Field

[0001] This invention belongs to the field of soil improvement technology, specifically relating to a method for nested application of compound soil conditioners for improving the soil of Chinese fir forests. Background Technology

[0002] As a major afforestation tree species in southern China, Chinese fir plays a vital role in maintaining the stability of forest ecosystems and developing the forestry economy. However, due to unreasonable human activities and natural influences, acid rain in southern my country is intensifying, leading to severe soil acidification, nutrient loss, and decreased microbial activity, thus affecting tree growth and ecological functions. Soil is the largest recipient of acid deposition and also the medium upon which plants depend for survival; basic ions in the soil are essential nutrients for plants, easily leached away under the influence of acid rain, resulting in decreased soil fertility. Furthermore, the soils in southern my country are mostly phosphorus-deficient and aluminum-rich acidic soils. The organic acids and phenols secreted by the autotoxicity of Chinese fir, along with the prominent issue of multiple generations of pure stands, further exacerbate soil acidification in southern China, leading to the depletion of soil nutrient reserves, thereby weakening its productivity and ecological functions, and restricting its potential for carbon sequestration.

[0003] Existing studies mostly use single soil conditioners such as quicklime or biochar for soil improvement: on the one hand, lime applied to the soil surface can quickly increase soil pH, but its effect diminishes over time, and the calcium content in the soil... 2+ Excessive concentration of quicklime leads to increased bonding between clay particles, causing soil compaction and affecting soil quality. On the other hand, biochar can increase pH, alleviate aluminum toxicity, improve the microbial environment in acidic soils, and enhance soil carbon stability, but its improvement effect is not as good as that of quicklime. This is because the oxygen-containing functional groups (such as -COOH and -OH) on the surface of biochar are reduced due to the pyrolysis temperature of biochar, resulting in biochar aging and a further decrease in pH. In summary, although existing methods can increase pH to a certain extent, they suffer from problems such as limited improvement effect, poor durability, and limited improvement on soil structure and microbial community (see Non-Patent Literature 1, 2).

[0004] Therefore, there is an urgent need to develop a compound soil conditioner and its application method that can neutralize soil acidity, improve soil structure, enhance nutrient availability and carbon sequestration capacity.

[0005] Non-patent literature:

[0006] Non-patent literature 1: Hu Ren, Long Qiaoling, Wang Limin, et al. Research progress on the effect and mechanism of biochar on acidic soil improvement [J]. Journal of Huazhong Agricultural University, 2025.

[0007] Non-patent literature 2: Liang Junwei. Effects of quicklime application rate on acidic soil and early growth stage of tobacco [D]. Chinese Academy of Agricultural Sciences, 2021. Summary of the Invention

[0008] This invention aims to address the technical problems of existing soil conditioners with single components, such as the application of quicklime or biochar alone, which result in limited improvement effects, short duration of action, and limited improvement on soil structure and microbial communities in the soil improvement of Chinese fir plantations. The inventors have conducted meticulous research on these issues and proposed a nested application method for composite soil conditioners in Chinese fir forests. This method involves combining biochar and quicklime at different application rate gradients to construct a nested field experimental system. The synergistic effects of these two agents on soil acidification improvement, nutrient enhancement, and carbon sequestration capacity were systematically studied, thereby identifying the optimal composite improvement scheme to achieve comprehensive improvement of acidified soil in Chinese fir plantations.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] A method for nested application of compound soil conditioner for improving soil in Chinese fir forests, specifically including the following steps:

[0011] ① Preparation of composite modifier:

[0012] Preparation of biochar: After drying and crushing the residue of moso bamboo or nan bamboo, place it in an oxygen-limited environment and pyrolyze it at 450℃~550℃ for 2~4 hours. After cooling, pass it through a 20-mesh sieve to obtain biochar (BC), which is then sealed and stored for later use.

[0013] Quicklime pretreatment: The quicklime is industrial grade calcium oxide (CaO) with an effective calcium oxide content of ≥90%. The quicklime is crushed and passed through a 20-mesh sieve and then sealed for storage.

[0014] ② Original soil survey

[0015] Before applying the compound soil conditioner, surface soil samples were collected from various plots and small quadrats to determine the soil bulk density, initial pH value, soil bulk density, organic carbon content, total nitrogen content, and exchangeable base ions, as well as other basic physicochemical properties, which were used as data for subsequent effect evaluation.

[0016] ③ Determination of the dosage of the improver

[0017] Based on the soil bulk density measured in step ②, calculate the mass of topsoil per unit area of ​​1 m². Combining the initial soil pH and the preset improvement target, calculate the required steps for each sample plot and subplot. The amount of compound amendment to be applied should be determined, and then applied in a gradient manner.

[0018] ④ Sample plot setup and pretreatment

[0019] Within the Chinese fir plantation, areas with consistent site conditions were selected to establish 12 sample plots, each measuring 10 m × 10 m. Buffer zones of more than 5 m were set between the sample plots to prevent mutual interference, and the application was carried out in a nested manner.

[0020] ⑤ Field application of compound amendments

[0021] First, clear away surface litter in the sample plot and set it aside. Then, apply the treatment in a nested manner in the following order: First, apply the treatment in the following steps... The selected quicklime was evenly spread across the surface of the entire large sample plot (10 m × 10 m), and then mixed into the top 0-20 cm of soil by tilling or shallow hoeing. After the quicklime application was completed, proceed with the steps... The pre-set biochar gradient is used to evenly spread the determined biochar on the surface of the small plots, and then the soil is tilled again to mix it into the 0-20 cm soil layer to ensure that the amendment is in full contact with the soil. After application, the cleaned-up debris is restored to the original surface to reduce the loss of amendment caused by rainwater erosion.

[0022] ⑥ Effect monitoring and sample data collection

[0023] One year after application, soil samples were collected from the inner surface layer (0-20 cm) of each small plot to determine the following soil physicochemical properties and microbial ecological indicators:

[0024] Soil pH, exchangeable base ions, total nitrogen content, soil organic carbon, particulate organic carbon, mineral-bound organic carbon, soil ammonium nitrogen, soil nitrate nitrogen, soil microbial biomass carbon, and soil enzyme activity (such as β-glucosidase, acid phosphatase, N-acetylglucosidase, xylosidase, leucine aminopeptidase, etc.).

[0025] ⑦ Data Processing and Evaluation

[0026] By comparing the differences in soil physicochemical properties and microbial ecological indicators under nested application of compound amendments with different gradients, the improvement effect of each CaO application group was comprehensively evaluated, and the optimal combination of quicklime and biochar was selected.

[0027] in,

[0028] The steps In this context, the application gradient of quicklime is set as follows:

[0029] Quicklime control group CaO: No quicklime was added in the corresponding sample plot;

[0030] Low-volume lime treatment for CaO1: 2-3 t ha in the corresponding sample plots. -1 Spread quicklime evenly;

[0031] High-volume lime treatment of CaO2: 4-6 t ha in the corresponding sample plots. -1 Spread quicklime evenly;

[0032] The biochar application gradient is set as follows:

[0033] Biochar control group BC0: No biochar was added in the corresponding sample plot;

[0034] Low-volume biochar treatment BC1: Based on the corresponding sample plots, at a rate of 3-5 t ha -1 Spread biochar evenly;

[0035] High-volume biochar treatment of BC2: Based on the corresponding sample plots, at a rate of 7-9 t ha -1 Spread biochar evenly;

[0036] The gradients of the above-mentioned lime application rate and biochar application rate were combined to form 9 combinations, with 4 replicates for each combination, for a total of 36 quadrats.

[0037] In the steps In this process, the nested application involves randomly dividing 12 sample plots into 3 groups of 4 plots each, corresponding to different gradients of quicklime application. Within a 10 m × 10 m large sample plot, the application amount is set according to the group. Then, within each 10 m × 10 m sample plot, 3 smaller sample plots of 1 m × 1 m are further divided, and the application amount is set according to the gradient corresponding to the smaller sample plot, corresponding to different gradients of biochar application.

[0038] Regarding the mechanism of action of composite modifiers:

[0039] The nested application method described in this invention achieves comprehensive improvement of acidified soil in Chinese fir plantations by systematically regulating the gradient application rates of quicklime and biochar.

[0040] First, quicklime has a rapid neutralizing effect. After quicklime (CaO) is applied to the soil, it reacts with water to produce calcium hydroxide (Ca(OH)2), which quickly neutralizes H⁺ and Al³⁺ in the soil, significantly increases the soil pH value, and alleviates soil acidification.

[0041] Secondly, biochar has a continuous ameliorative effect. Biochar has a rich porous structure and a high cation exchange capacity (CEC), which can adsorb exchangeable Al³⁺ in the soil and reduce its toxicity. At the same time, the surface of biochar contains a large number of oxygen-containing functional groups, which can complex H⁺, further enhancing the buffering capacity of soil acidity.

[0042] Finally, through the synergistic effect of quicklime and biochar, the application of quicklime increased the soil pH, creating a suitable environment for microbial activity, while promoting the activation of functional groups on the surface of biochar and adsorption of H⁺; the addition of biochar improved the soil structure and enhanced the soil's ability to adsorb and retain nutrients. The synergistic effect of the two significantly improved soil fertility and carbon sequestration capacity.

[0043] Beneficial effects:

[0044] According to the present invention, a nested application method for compound soil conditioner in Chinese fir forests involves setting up several sample plots within the plantation, each plot further divided into smaller quadrats. Different gradients of quicklime and biochar are applied to each quadrat. The nested application system is used to evaluate the synergistic improvement effects of these two agents on soil physicochemical properties and microbial ecology. Furthermore, this nested experimental design systematically evaluates the synergistic effects of different combinations of lime and biochar on soil improvement, providing a scientific basis for optimizing compound soil conditioner programs. In addition, this application method demonstrates significant and lasting improvement effects. The rapid neutralization by quicklime combined with the continuous improvement by biochar not only solves the problem of soil acidification but also enhances soil nutrients, significantly strengthening the soil's carbon sequestration function. Moreover, the improved soil environment is conducive to microbial growth and increased enzyme activity, promoting the restoration and health of the soil ecosystem. Finally, the application method described in this invention is simple to operate, with a reasonable gradient setting, making it easy to promote and apply in Chinese fir plantation areas in southern China. Attached Figure Description

[0045] Figure 1 : Actual plot layout photos (a) Plot after application of quicklime, (b) Plot after application of biochar

[0046] Figure 2 : Detailed Implementation

[0047] A case study on soil improvement in a Chinese fir plantation in Jinzhai County, Anhui Province, was conducted at the Mazongling Forest Farm, located in a subtropical region. A nested application of quicklime and biochar compound soil conditioner was performed. See the actual plot layout in Figure [Figure Number]. The plots shown are after quicklime application and after biochar application. This area is located on the northern slope of the Dabie Mountains, at an altitude of approximately [Altitude] meters, with a southeast-to-northwest orientation. The average annual precipitation over [Number] mm is over [Number] years, the average annual relative humidity is [Amount], the average annual temperature is [Amount], and the frost-free period is [Number] days. The soil texture is loamy and slightly acidic.

[0048] exist Figure 2 The diagram shows the sample plot layout. Specifically, plots 3, 7, 9, and 12 are the CaO0 control group (no quicklime added, blue group CK); plots 2, 5, 6, and 11 are the CaO1 control group (low amount of quicklime added, green group L, application rate 2.5 t ha). -1Sample plots 1, 4, 8, and 10 were designated as the CaO2 group (high-volume quicklime addition group, yellow group denoted as H, application amount 5 t / ha). -1 Furthermore, nested experiments were conducted based on this, where the subplots L and H within each plot represented the addition of low-level and high-level biochar, respectively (the low-level application rate was 4 t ha). -1 The high-volume application rate is 8 t ha -1 ).

[0049] The specific method for nested application of compound modifiers is as follows:

[0050] Preparation of composite modifier:

[0051] Preparation of biochar: The residue of moso bamboo harvesting is air-dried, crushed, and placed in an oxygen-limited environment for pyrolysis at about 500℃ for 4 hours. After cooling, it is passed through a 20-mesh sieve to obtain biochar, which is then sealed and stored for later use.

[0052] Quicklime pretreatment: The quicklime is purchased industrial-grade calcium oxide (CaO) with an effective calcium oxide content of ≥90%. The quicklime is crushed and passed through a 20-mesh sieve, then sealed and stored for later use.

[0053] Soil original survey

[0054] Before applying the compound soil conditioner, soil samples were collected from each plot and within 20 cm of each sample plot. The initial pH value, soil bulk density, organic carbon content, total nitrogen content, and exchangeable basic physicochemical properties were measured. The data were recorded and shown in Table 1 (where the cross data of CaO0 and BC0 is the background data (control group)).

[0055] Plot setup and pretreatment, determination of amendment application rate and application: In August 2024, 12 plots were established in a 20-year-old Chinese fir plantation in this forest farm. Each plot was 10 m × 10 m in size and square in shape. To prevent mutual interference between plots, the distance between each plot was greater than five meters. The 12 plots were randomly divided into 3 groups (see...). Figure 2 The three groups (blue, green, and yellow) each correspond to different gradients of quicklime application rates: no quicklime, low-level quicklime application (2.5 thalassium), and low-level quicklime application (no quicklime, 2.5 thalassium). -1 Add 5 t ha -1In addition, to ensure the accuracy of the experimental measurements, the experiment was repeated for each group of four plots (subsequent experimental data are the average values ​​of the measured results); within each 10 m × 10 m plot, three 1 m × 1 m subplots were further divided, each corresponding to a different gradient of biochar application, forming a nested application method. While the amount of quicklime applied to each large plot was constant, the amount of biochar applied to each subplot was different. In the subplot designation, L represents low biochar addition, and H represents high biochar addition, meaning no biochar was added to any subplot within each plot. Figure 2 (The schematic diagram of the small sample plot is omitted in the text.) The low-volume application rate is 4 t ha. -1 The high-volume application rate is 8 t ha -1 .

[0056] Effect monitoring and sample data collection

[0057] Soil samples from the improved topsoil layer (0-20 cm) were collected in September 2025. A comprehensive quantitative analysis of the soil's physicochemical properties and microbial ecological indicators was conducted, including soil pH, exchangeable basic ions, organic carbon, total nitrogen content, particulate organic carbon (POC), mineral-bound organic carbon (MAOC), ammonium nitrogen, nitrate nitrogen, soil microbial biomass carbon, and soil enzyme activity. Specific data are shown in Table 1.

[0058] Table 1. Baseline data and soil index data collected and applied according to this invention.

[0059]

[0060] Continued table

[0061]

[0062] This study comprehensively evaluated the effects of different gradient nested application strategies of quicklime and biochar composite amendments on improving forest soil acidification and enhancing nutrient availability by measuring a series of soil physicochemical properties. Specific analysis is as follows:

[0063] As shown in Table 1, regarding pH changes, after one year of adding the composite soil conditioner, under BCO conditions, the low-dosage quicklime treatment increased soil pH by 0.780 units, while the high-dosage quicklime treatment increased it by 0.895 units. Without quicklime, the low-dosage biochar treatment increased soil pH by 0.155 units, and the high-dosage biochar treatment increased it by 0.336 units. However, with the combined application of both, the CaO2×BC1 treatment showed the most significant increase in soil pH, increasing it by 1.1 units compared to the no-addition treatment. This indicates that applying biochar or quicklime alone is slower and less effective than the combined application of both. Therefore, the combined application of the composite soil conditioner of this invention is more effective in improving soil pH.

[0064] The nested application of exchangeable basic ions in soil resulted in varying degrees of increase in their content. For example, as shown in Table 1, biochar addition increased the exchangeable potassium ion content in the soil by 6.94-41.51%, with low-volume quicklime addition and high-volume addition increasing by 44.07% and 5.94%, respectively. The highest increase in exchangeable potassium ion content was 63.21% under the nested application of CaO1×BC2. Furthermore, the addition of quicklime and biochar also led to varying degrees of increase in exchangeable calcium ion content in the soil. The increase was greatest with only low-volume quicklime addition, approximately 4.5 times that of the control group. Low-volume biochar addition and high-volume biochar addition increased by 68.94% and 136.62%, respectively. The CaO1×BC2 nested application resulted in the highest level, 3.5 times that of the control group, similar to the changes in the other two cations. Furthermore, as shown in Table 1, the exchangeable magnesium ion content also increased to varying degrees. Adding quicklime significantly increased the exchangeable magnesium ion content in the soil, with high-volume quicklime addition reaching 12 times that of the control group, and low-volume quicklime addition approximately 11 times. The addition of biochar alone only slightly increased the exchangeable magnesium ion content in the soil, with low and high volumes increasing it by 2.07% and 39.93%, respectively. The nested application method also showed a significant increase, approximately 6.5-10.7 times that of the control group. Therefore, the nested application of the composite soil conditioner of this invention significantly increases the exchangeable basic ion content in the soil, indicating that the application method of this invention is effective.

[0065] Regarding soil organic carbon content, adding biochar alone significantly increased the POC content in the soil, with the largest increase reaching 77.86% under high biochar addition. Most nested applications also showed an increase, reaching a maximum of 65.17%. With increasing single biochar addition, the MAOC content in the soil gradually decreased. The results of nested applications were similar to those of single biochar addition, showing varying degrees of decrease compared to the control group, ranging from 11.62% to 42.13%. Under low-lime treatment, the microbial biomass carbon content increased compared to both the no-lime and biochar treatments, ranging from 4% to 53%, with the CaO1×BC1 treatment group showing the most significant effect. Under high-lime treatment, the microbial biomass carbon content decreased. Therefore, the application method of the composite amendment of this invention can improve the soil organic carbon content, and different dosages of the composite amendment can be applied to different plots to achieve the best utilization effect.

[0066] Regarding available nutrients in the soil, the content of ammonium nitrogen decreased continuously with increasing quicklime addition. Under conditions of no quicklime or low quicklime addition, its content also decreased continuously with increasing biochar addition. The most significant decreases in ammonium nitrogen content were observed in the treatments with both low and high quicklime addition and the treatment with only high quicklime addition, decreasing by 50% and 55.26% respectively compared to the control group. In the cases of no quicklime or low quicklime addition, the content of nitrate nitrogen increased with increasing biochar addition. The increases in the treatments with both low and high quicklime addition and the treatment with only high biochar addition were 11.51% and 55.38% respectively compared to the control group. Table 1 shows that the soil conditioner increased soil pH, allowing ammonium nitrogen to be better retained in the soil solution and soil colloids, facilitating plant absorption and utilization. Furthermore, the increased pH promoted nitrification, increasing nitrate nitrogen content, thus resulting in an overall decrease in ammonium nitrogen content. Finally, considering the combined effect of the two, the improvement effect of CaO1×BC2 is the best.

[0067] Regarding soil enzyme activity, carbon cycle-related enzyme activities increased to varying degrees under most treatments. The treatment with simultaneous addition of low-level quicklime and high-level biochar resulted in the largest increase in BG activity, reaching 1.46 times that of the control group, while xylosidase activity was 1.59 times that of the control group. Nitrogen cycle-related enzyme activities showed different trends. N-acetylaminopeptidase activity, under nested application, saw the highest increase, reaching 1.49 times that of the control group, but some treatments showed a decrease of 21.58%. Leucine aminopeptidase activity generally decreased under this experimental mode, with the treatment with simultaneous addition of high-level quicklime and biochar showing the largest decrease of 53.05%. Acid phosphatase activity generally decreased under nested application, with the treatment with simultaneous addition of high-level quicklime and biochar showing the largest decrease, 41.11% lower than the control group. Therefore, the application of the composite soil conditioner of this invention, under nested application, will affect the activity of various soil enzymes. Thus, the dosage of quicklime and biochar in the composite conditioner can be adjusted according to the actual application to achieve the optimal effect required by the soil.

[0068] In summary, the combined application of quicklime and biochar in different ratios has a more significant effect on improving acidified soil, regulating the content of exchangeable basic ions in soil, improving nutrient availability, and adjusting soil enzyme activity. Furthermore, the combined application has a better improvement effect than a single amendment, especially when the improvement effect is best when CaO1×BC1 or CaO1×BC2 is observed in multiple physicochemical properties.

[0069] Therefore, the nested application method of the compound soil conditioner of this invention improves the soil environment to promote microbial growth and enhance enzyme activity, significantly strengthening the soil carbon sequestration function and promoting the restoration and health of the soil ecosystem. The compound soil conditioner can be applied according to local conditions based on experimental data to achieve the best improvement effect. Furthermore, the application method of this invention is simple to operate and, through reasonable gradient settings, is easy to promote and apply in plantation areas of Cunninghamia lanceolata in southern China.

Claims

1. A method for nested application of a compound soil conditioner for improving soil in Chinese fir forests, characterized in that, Includes the following steps: ① Preparation of composite modifier: Preparation of biochar: After drying and crushing the residue of moso bamboo or nan bamboo, place it in an oxygen-limited environment and pyrolyze it at 450℃~550℃ for 2~4 hours. After cooling, pass it through a 20-mesh sieve to obtain biochar, which is then sealed and stored for later use. Quicklime pretreatment: The quicklime is industrial-grade calcium oxide with an effective calcium oxide content of ≥90%. The quicklime is crushed and passed through a 20-mesh sieve, then sealed and stored for later use. ② Original soil survey Before applying the compound soil conditioner, surface soil samples were collected from various plots and small quadrats to determine their soil volume, including initial pH value, soil bulk density, organic carbon content, total nitrogen content, available phosphorus content, total iron content, and metal ions, as data for subsequent effect evaluation. ③ Determination of the dosage of the improver Based on the soil bulk density measured in step ②, calculate the mass of topsoil per unit area of ​​1 m². Combining the initial soil pH and the preset improvement target, calculate the required steps for each sample plot and subplot. The amount of compound amendment to be applied should be determined, and then applied in a gradient manner. ④ Sample plot setup and pretreatment Within the Chinese fir plantation, areas with consistent site conditions were selected to establish 12 sample plots, each measuring 10 m × 10 m. Buffer zones of more than 5 m were set between the sample plots to prevent mutual interference, and the application was carried out in a nested manner. ⑤ Field application of compound amendments First, clear away any surface debris from the sample plot and set it aside. Then, apply the lime in the following order: first, set it according to the quicklime group, and then proceed with the steps... The selected quicklime was evenly spread on the surface of a 10 m × 10 m plot of land, and then mixed into the 0–20 cm soil surface layer by tilling or shallow hoeing. After the quicklime has been applied, follow the steps. The pre-set biochar gradient is used to evenly spread the determined biochar on the surface of the small sample plot, and then the soil is tilled again to mix it into the 0-20 cm soil layer to ensure that the amendment is in full contact with the soil. After application, restore the cleaned debris to its original state to reduce the loss of amendments due to rainwater erosion; ⑥ Effect monitoring and sample data collection Collect samples one year after application. The following soil physicochemical properties and microbial ecological indicators were measured from the surface soil samples within small quadrats: Soil pH, exchangeable basic ions, total nitrogen content, soil organic carbon, particulate organic carbon, mineral-bound organic carbon, soil ammonium nitrogen, soil nitrate nitrogen; soil microbial biomass carbon, soil enzyme activity; Data Processing and Evaluation By comparing the differences in soil physicochemical properties and microbial ecological indicators before and after the nested application of different gradients of lime and biochar, the improvement effect of each application group was comprehensively evaluated, and the best combination of quicklime and biochar ratio was selected.

2. The application method according to claim 1, characterized in that, The steps In this context, the application gradient of quicklime is set as follows: Quicklime control group CaO: No quicklime was added in the corresponding sample plot; Low-volume lime treatment for CaO1: 2-3 t ha in the corresponding sample plots. -1 Spread quicklime evenly; High-volume lime treatment of CaO2: 4-6 t ha in the corresponding sample plots. -1 Spread quicklime evenly; The biochar application gradient is set as follows: Biochar control group BC0: No biochar was added in the corresponding sample plot; Low-volume biochar treatment BC1: Based on the corresponding sample plots, at a rate of 3-5 t ha -1 Spread biochar evenly; High-volume biochar treatment of BC2: Based on the corresponding sample plots, at a rate of 7-9 t ha -1 Spread biochar evenly; The gradients of lime application rate and biochar application rate were combined to form different sample plots.

3. The application method according to claim 1, characterized in that, In the steps In this process, the nested application involves randomly dividing 12 sample plots into 3 groups of 4 plots each, corresponding to different gradients of quicklime application. Within a 10 m × 10 m large sample plot, the application rate is set according to the group. Then, within each 10 m × 10 m sample plot, 3 smaller sample plots of 1 m × 1 m are further divided, and the application rate is set according to the gradient corresponding to the smaller sample plot, corresponding to different gradients of biochar application.

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