Preparation method of iron-modified biochar and its application in rice-wheat rotation planting

Abstract

The present invention relates to the field of agricultural technology, and more particularly to a method for preparing iron-modified biochar and its application in rice-wheat rotation planting. The method for preparing iron-modified biochar of the present invention comprises the following steps: rice straw is crushed and put into a ferric sulfate solution, and impregnated for 24-48 hours; the impregnated rice straw is dried at 60-80°C; the dried rice straw is pyrolyzed at 500-600°C for 1-3 hours to obtain iron-modified biochar. The present invention, by applying iron-modified biochar in a rice-wheat rotation system, not only effectively reduces greenhouse gas emissions in the rice-wheat rotation system, but also increases crop yield, and also avoids soil heavy metal pollution, thereby achieving a win-win situation in the economic, environmental and social benefits of agricultural production.

Description

Technical Field

[0001] The present invention relates to the field of agricultural technology, and in particular to a method for preparing iron-modified biochar and its application in rice-wheat rotation planting. Background Art

[0002] Greenhouse gas (GHG) emissions have become a global concern. Agriculture, as a major source of GHG emissions, particularly methane (CH4) and nitrous oxide (N2O), has a significant impact on global climate change. In my country, rice-wheat rotation is a widely used farming system. However, traditional rice-wheat rotation systems are often associated with high GHG emissions, particularly CH4 during the rice planting phase and N2O during the wheat growing phase.

[0003] To address this challenge, researchers have begun exploring the use of biochar to reduce GHG emissions in rice-wheat rotation systems. Biochar is a carbon-rich solid material with a highly porous structure and a large specific surface area. It can improve the physical and chemical properties of the soil, thereby reducing methane production and reducing nitrous oxide emissions by improving nitrogen use efficiency. However, the emission reduction effect of ordinary biochar is uncertain, and its ability to adsorb nitrogen and regulate microbial communities is limited, which limits its effectiveness in practical applications. This urgently requires us to explore new methods to improve its emission reduction efficiency. Modification of biochar is considered to be a very promising regulatory strategy and is receiving increasing attention. Summary of the Invention

[0004] The present invention provides a preparation method of iron-modified biochar and its application in rice-wheat rotation planting, so as to solve the problem of greenhouse gas emissions in the rice-wheat rotation system, while overcoming the uncertainty of the emission reduction effect of ordinary biochar and avoiding the risk of soil heavy metal pollution caused by modification.

[0005] According to a first aspect of the present invention, the present invention provides a method for preparing iron-modified biochar, comprising the following steps: crushing rice straw and putting it into a ferric sulfate solution for soaking for 24-48 hours; drying the soaked rice straw at 60-80°C; and pyrolyzing the dried rice straw at 500-600°C for 1-3 hours to obtain iron-modified biochar.

[0006] Experiments have shown that iron-modified biochar, produced by modifying rice straw with iron sulfate, effectively reduces greenhouse gas emissions and improves soil quality in rice-wheat rotations. By controlling parameters such as impregnation time, drying temperature, and pyrolysis time, iron-modified biochar that meets the requirements can be consistently produced, providing a feasible preparation process for the practical application of this technical solution.

[0007] Furthermore, the rice straw and Fe3+ The mass ratio is 1: (20-30); the concentration of the ferric sulfate solution is 0.4-0.6 mol L -1 The preparation conditions of iron-modified biochar were further optimized by limiting the 3+ The mass ratio of iron sulfate and the concentration range of ferric sulfate solution can better control the modification effect of biochar, making it perform better in greenhouse gas emission reduction and soil improvement.

[0008] Furthermore, the pH value of the iron-modified biochar is 5-6, and the total carbon content is 420.37-500.35 g kg -1 , with a total nitrogen content of 0.98-1.16 g kg -1 , with a specific surface area of ​​96.78-111.06 m 2 g -1 , pore volume of 0.24-0.26 cm 3 g -1 The pore size is 8.68-9.04 nm. These physical and chemical properties of iron-modified biochar enable it to better exert its adsorption, improve soil structure, and regulate microbial communities in the soil, thereby more effectively reducing greenhouse gas emissions while ensuring the safety of the soil environment.

[0009] Furthermore, the iron-modified biochar contains 70-75% C by mass, 20-25% O by mass, 2.5-3% N by mass, 2-2.5% Fe by mass, 0.08-0.1% Cd by mass, and 0.5-0.7% As by mass. This further clarifies the mass percentages of each element in the iron-modified biochar, ensuring that while it can achieve greenhouse gas reduction effects, it does not pollute the soil environment due to excessive levels of heavy metals (such as Cd and As), thus ensuring the environmental safety of this technical solution.

[0010] According to the second aspect of the present invention, the present invention also provides the application of the above-mentioned iron-modified biochar preparation method in rice-wheat rotation planting, and the application method is: after the rice is harvested, the collected rice straw is used to prepare iron-modified biochar using the iron-modified biochar preparation method; before wheat is sown, the obtained iron-modified biochar is applied to the soil at one time and mixed evenly; and fertilizer is added during the rice-wheat rotation.

[0011] The rice-wheat rotation planting method of the present invention applies iron-modified biochar in the rice-wheat rotation and utilizes its modified properties to effectively reduce the emission of greenhouse gases (CH4 and N2O) in the rice-wheat rotation system without causing exogenous heavy metal pollution to the soil. At the same time, it improves soil fertility and crop yield, thereby achieving green and sustainable agricultural development.

[0012] Furthermore, the application amount of the iron-modified biochar is 8-12 t hm -2 By clarifying the application range of iron-modified biochar, we can ensure that it can achieve the best greenhouse gas emission reduction and soil improvement effects in the rice-wheat rotation system, while avoiding negative impacts on the soil environment and crop growth due to inappropriate application.

[0013] Furthermore, the amount of fertilizer applied is 240 kg hm2 of pure nitrogen. -2 、P2O5146 kg hm -2 and K2O 52 kghm -2 By limiting the amount of fertilizer applied within a reasonable range, it is ensured that while applying iron-modified biochar, sufficient nutrients can be provided for the rice-wheat rotation, ensuring the normal growth and yield of crops, and achieving a balance between greenhouse gas emission reduction and crop production.

[0014] Furthermore, during the application process, 40% of the nitrogen and all of the phosphorus and potassium are applied to the soil as a compound fertilizer before wheat sowing or rice transplanting as a base fertilizer, and the remaining 60% of the nitrogen is applied to the soil as a topdressing fertilizer in the form of urea at a ratio of 40%:20%. By properly allocating the ratio of base fertilizer to topdressing, the nutrient needs of crops at different growth stages can be better met, improving fertilizer utilization efficiency. Combined with the application of iron-modified biochar, this can further optimize greenhouse gas emissions reduction and crop yield in the rice-wheat rotation system.

[0015] Furthermore, the compound fertilizer includes 15% by weight of N, 15% by weight of P2O5 and 15% by weight of K2O.

[0016] Furthermore, the rice variety is Yongyou 7850 or Yongyou 12; the wheat variety is Yangmai 34 or Zhenmai 18.

[0017] Furthermore, rice is planted in one hole with two seedlings, and one pot with two holes; wheat is planted in one pot with 20-40 plants.

[0018] Furthermore, the application method also includes field management measures: during wheat planting, the land is leveled, the soil is loosened, and insects and weeds are removed; in terms of weed control, one or more pesticides including mesosulfuron, dimethoate, and bispyribac are used before sowing or after seedling emergence to carry out closed or post-emergence control of broadleaf weeds and grass weeds; in terms of pest control, wheat fields should be inspected regularly to pay attention to the occurrence of aphids, leaf rust, and sheath blight, and pesticides should be applied in a timely manner when the insect population base or lesions are found; at the same time, the field should be kept clean, weeds and stumps should be removed, and the source of pests and diseases should be reduced;

[0019] During the rice planting process, pests and weeds are removed; 7-10 days after transplanting, during the tillering stage and the heading stage, one or more pesticides including pymetrozine, chlorflucythrin and tricyclazole are used to control rice planthoppers, leaf rollers and rice blast; for weed control, one or two herbicides including bensulfuron-methyl or isoproturon-methyl are sprayed during the peak weed season; in terms of water management, water is flooded in advance before transplanting, and a 3-8 cm water layer is maintained from the early stage of transplanting to the late tillering stage. The field is drained and dried for 3-4 days at the end of tillering, and the field is dried one week before the yellow ripening stage for harvesting.

[0020] By rationally optimizing field management measures, it is possible to ensure that the application effects of iron-modified biochar and fertilizers are fully utilized during the rice-wheat rotation process, while ensuring the crop growth environment and soil quality, and achieving synergistic optimization of greenhouse gas emission reduction, soil improvement and crop production.

[0021] Beneficial effects of the present invention:

[0022] The rice-wheat rotation planting method provided by the present invention not only effectively reduces greenhouse gas emissions in the rice-wheat rotation system and increases crop yields through the application of iron-modified biochar, but also avoids heavy metal pollution in the soil, achieving a win-win situation in the economic, environmental and social benefits of agricultural production. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the technical solutions in the present invention or the prior art, a brief introduction is given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0024] Figure 1 It is a scanning electron microscope image of the iron-modified biochar Fe-BC provided in Example 1 of the present invention and the biochar BC of Comparative Example 2.

[0025] Figure 2 It is a Fourier spectrum diagram of the iron-modified biochar Fe-BC provided in Example 1 of the present invention and the biochar BC of Comparative Example 2.

[0026] Figure 3 This is a dynamic diagram of greenhouse gas (CH4, N2O) emissions from two years of rice-wheat rotation provided by Example 1, Comparative Example 1 and Comparative Example 2 of the present invention.

[0027] Figure 4 This is a two-year rice-wheat rotation CH4 cumulative emission diagram provided by Example 1, Comparative Example 1 and Comparative Example 2 of the present invention.

[0028] Figure 5This is a two-year rice-wheat rotation N2O cumulative emission graph provided by Example 1, Comparative Example 1 and Comparative Example 2 of the present invention.

[0029] Figure 6 It is a two-year rice-wheat rotation yield graph provided by Example 1, Comparative Example 1 and Comparative Example 2 of the present invention.

[0030] Figure 7 This is a two-year global warming potential map of rice-wheat rotation provided by Example 1, Comparative Example 1 and Comparative Example 2 of the present invention.

[0031] Figure 8 This is a two-year rice-wheat rotation greenhouse gas emission intensity diagram provided by Example 1, Comparative Example 1 and Comparative Example 2 of the present invention.

[0032] Figure 9 This is a comparison chart of heavy metal content in soil after the test provided by Example 1, Comparative Example 1 and Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0033] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Obviously, the embodiments described are only some of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0034] Example 1

[0035] This embodiment provides a rice-wheat rotation planting method (referred to as the Fe-BC treatment) using the rice variety Yongyou 7850 and the wheat variety Yangmai 34. Three treatments were performed, each with three replicates. The method specifically includes the following steps:

[0036] After rice harvest, rice straw was collected to prepare iron-modified biochar Fe-BC. The preparation method of iron-modified biochar Fe-BC was as follows: 15 kg of straw was washed and set aside; 0.5 mol L -1 ferric sulfate solution to control the relationship between straw and Fe 3+ The iron-modified biochar was prepared by dissolving 2.2 kg of iron sulfate powder in 11 L of water. Crushed rice straw was added and immersed for 24 hours with occasional stirring. The crushed straw was filtered out and dried at 70°C to constant weight. The dried rice straw was pyrolyzed at 550°C for 2 hours to obtain iron-modified biochar.

[0037] At the end of November 2022, before wheat sowing, 10 t hm of iron-modified biochar was applied once. -2 The application amount was applied to the soil and mixed well, and no biochar was added subsequently.

[0038] Fertilizer was added during the rice-wheat rotation, with the application rate of pure nitrogen 240 kg hm -2 、P2O5146 kg hm -2 and K2O52 kg hm -2 . 40% of the N and all the P and K are applied to the soil as base fertilizer in the form of compound fertilizer before wheat sowing or rice transplanting, and the remaining N is applied to the soil as topdressing in the form of urea at a ratio of 40%:20% (two topdressings, including tillering fertilizer and ear fertilizer for rice; jointing fertilizer and ear fertilizer for wheat).

[0039] Field management includes: Wheat management measures, compound fertilizer (N 96 kg hm -2 、P2O5146kg hm -2 、K2O 52 kg hm -2 ), combined with deep plowing 10cm, and fine raking to level the soil. After sowing, apply nitrogen fertilizer 96kg hm2 at the greening stage. -2 At the end of jointing, apply nitrogen fertilizer 48 kg hm -2 , to promote tillering and grain filling. For weed control, pre-sowing or post-emergence weed control, agents such as mesosulfuron, dimethoate, and florasulam can be used to control broadleaf and grass weeds. The recommended dosage should be diluted with water and applied evenly by spraying. For pest and disease control, wheat fields should be regularly inspected to monitor the occurrence of aphids, leaf rust, and sheath blight. When a population or lesions are detected, pesticides should be applied promptly. At the same time, fields should be kept clean, and weeds and stumps removed to reduce the source of pests and diseases.

[0040] In terms of rice management, compound fertilizer (N 96 kg hm -2 、P2O5146 kg hm -2 、K2O 52 kg hm -2 ), deep plowing and leveling the land. Pymetrozine, cyhalothrin, and tricyclazole were used 7–10 days after transplanting, during the tillering stage, and during the booting stage to control rice planthoppers, leaf rollers, and rice blast. For weed control, herbicides such as bensulfuron-methyl or isoproturon-methyl were sprayed during the peak weed season. Regarding water management, a 3–8 cm water layer was maintained from the initial transplanting stage to the late tillering stage. The field was drained and air-dried for 3–4 days at the end of tillering, and then dried one week before the yellow ripening stage to facilitate harvesting. Combined with inter-row tillage and shallow loosening (3–5 cm) to remove weeds and conserve moisture, 96 kg hm-3 of urea was applied during the tillering and booting stages. -2 and 48 kg hm -2 Through stubble cleaning, rational crop rotation and scientific management, the yield and quality of wheat and rice are fully guaranteed.

[0041] Comparative Example 1

[0042] This comparative example provides a rice-wheat rotation planting method (referred to as CF treatment), which is different from Example 1 in that: during the rice-wheat rotation, only fertilizer is added, and no iron-modified biochar is added.

[0043] Comparative Example 2

[0044] This comparative example provides a rice-wheat rotation planting method (abbreviated as BC treatment), which is different from Example 1 in that biochar BC is used instead of iron-modified biochar.

[0045] The carbonization step of biochar (BC) is to pyrolyze it at 500-600℃ for 2 hours to obtain ordinary biochar (BC). Before application, pass it through a 100-mesh sieve to ensure that it is thoroughly mixed with the soil.

[0046] Comparative Example 3

[0047] This comparative example provides a rice-wheat rotation planting method (abbreviated as nZVI-BC treatment), which is different from Example 1 in that nano-zero-valent iron biochar (nZVI-BC) is used instead of iron-modified biochar.

[0048] Nano zero-valent iron biochar (nZVI-BC) was prepared by dissolving 5.56 g of biochar (BC) in 100 ml of deionized water, adding 2.78 g of nano-ferroferric oxide, and stirring the mixture in a nitrogen atmosphere for 60 min. The mixture was then washed three times with ethanol and deionized water respectively. The resulting solid was then transferred to an oven to remove moisture. The dried composite material was named nano biochar (nZVI-BC).

[0049] The obtained nano-biochar (nZVI-BC) was applied to the soil, and a potted experiment was carried out with reference to the Fe-BC treatment group. The results showed that methane emissions were reduced by an average of about 16% compared with the CF treatment group, and nitrous oxide emissions were reduced by an average of about 11%.

[0050] The scanning electron microscope images of the iron-modified biochar Fe-BC of Example 1 and the biochar BC of Comparative Example 2 are as follows: Figure 1 As shown, it can be seen from the SEM images of the two biochars at different magnifications that the surface of BC presents rough, irregular, fragmented and porous particles, while Fe-BC shows a relatively rough granular structure with fine particles gathered on the surface.

[0051] The Fourier transform infrared (FTIR) spectra of the iron-modified biochar Fe-BC of Example 1 and the biochar BC of Comparative Example 2 are as follows: Figure 2 As shown in the figure, it can be seen that the infrared spectra of BC and Fe-BC reveal the changes in their functional groups. First, at 802 cm -1The absorption peak at 1106 cm is related to the plane vibration of the C-H bond on the aromatic carbon. -1 The CO stretching vibration absorption peak of Fe-BC is significantly enhanced compared with BC, while the absorption peak at 1388 cm -1 The absorption peak at 1610 cm corresponds to the weakening of CN vibration. -1 The C=C absorption peak intensity of Fe-BC is weaker than that of BC. -1 At 3432 cm -1 The absorption peak at reflects the stretching vibration of the OH bond, which is stronger in BC than in Fe-BC.

[0052] The basic physicochemical properties of the iron-modified biochar Fe-BC of Example 1 and the biochar BC of Comparative Example 2 are shown in Table 1 below.

[0053] Table 1

[0054]

[0055] Table 1 shows the differences in the main physical and chemical properties of BC and Fe-BC. After the Fe-BC was impregnated with Fe2(SO4)3 solution, its pH dropped from 10 of BC to 5.73, and the total carbon (TC) and total nitrogen (TN) contents decreased by 20.3 g kg -1 and 2.44 gkg -1 BET analysis showed that the specific surface area and pore volume of Fe-BC decreased, but the pore diameter increased slightly. XPS analysis results showed that the carbon (C) content in Fe-BC increased to 71.24%, while the contents of oxygen (O) and nitrogen (N) decreased. At the same time, the iron (Fe) content in Fe-BC increased significantly to 2.36%, while the contents of heavy metal elements such as cadmium (Cd) and arsenic (As) changed little, increasing by only 0.01% and 0.1%, respectively.

[0056] Indicator Measurement: Greenhouse gas emissions from rice-wheat rotations are monitored using a combination of a static darkroom and gas chromatography. Gas samples are collected from sealed fields using a static darkroom. Gas sampling is conducted weekly, with more frequent sampling after fertilization. After collection, the samples are analyzed using an Agilent 7890A gas chromatograph to determine greenhouse gas emission fluxes, and cumulative emissions are calculated.

[0057] The emission fluxes of N2O and CH4 were calculated using the following formulas:

[0058] F=ρ×V / A×dc / dt×273 / (273+T)×P / 1013

[0059] F represents CH4 (mg m -2 h -1 ) or N2O (μg m -2 h -1 ) emission flux, ρ represents CH4 (0.53 g L -1 ) or N2O (1.25 g L -1 ) density, V represents the volume of the sampling box (m 3 ), A represents the bottom frame coverage area (m 2 ), dc / dt represents the gas emission rate, T is the temperature in the sampling box (℃), and P is the local atmospheric pressure. Finally, the N2O (g pot -1 ) and CH4 (g pot -1 ) Cumulative emissions.

[0060] Comprehensively consider N2O and CH4 emissions to calculate the greenhouse effect (GWP, g CO 2-eq pot -1 ) and greenhouse gas intensity (GHGI, g CO 2-eq g -1 ):

[0061] GWP = 273×N2O×44 / 28+27.2×CH4×16 / 12

[0062] GHGI = GWP / grain yield

[0063] The numbers 273 and 27.2 represent the warming potentials of N2O and CH4, respectively.

[0064] Pot test results

[0065] 1. Dynamics of greenhouse gas (CH4, N2O) emissions from two-year rice-wheat rotation

[0066] like Figure 3 As shown in Figure 2, greenhouse gas emissions from a two-year rice-wheat rotation following the application of different biochars revealed higher CH4 emissions during the rice season and lower CH4 emissions during the wheat season. CH4 emissions were higher during flooding during the rice season, but were nearly absent during the field baking and wet irrigation periods. Soil N2O emissions peaked within 7 to 10 days after fertilization during the wheat season, with a significant peak also observed during the field baking period during the rice season.

[0067] 2. Cumulative CH4 emissions from two-year rice-wheat rotation

[0068] like Figure 4As shown in the figure, cumulative CH4 emissions over the two years were calculated. In 2023, the CF treatment had the highest CH4 emissions, while those from the BC treatment decreased significantly. CH4 emissions from the Fe-BC treatment decreased further, while those from wheat increased slightly, but remained relatively low overall. In 2024, CH4 emissions from the CF treatment decreased compared to 2023, but remained higher than those from the other treatments. CH4 emissions from the BC treatment decreased significantly. The Fe-BC treatment had the lowest CH4 emissions.

[0069] 3. Cumulative N2O emissions from two-year rice-wheat rotation

[0070] like Figure 5 As shown, in 2023, N2O emissions were higher during the rice season under the CF treatment and relatively lower during the wheat season. N2O emissions decreased slightly during the rice season under the BC treatment. N2O emissions from the Fe-BC treatment were similar to those from the BC treatment. N2O emissions from the wheat season under the CF treatment in 2024 increased compared to those in 2023. N2O emissions from the BC treatment decreased compared to those from the CF treatment. The Fe-BC treatment was similar to the BC treatment, with lower N2O emissions during both the rice and wheat seasons than the CF treatment.

[0071] 4. Yield of two-year rice-wheat rotation

[0072] like Figure 6 As shown, in 2023, the CF treatment had the highest rice yield, while wheat yield was relatively low. After biochar application, the BC treatment had a slightly lower rice yield than the CF treatment. The Fe-BC treatment had a slightly higher rice yield than both the CF and BC treatments, while its wheat yield was similar to those of the CF and BC treatments. In 2024, the BC treatment had a significantly higher rice yield than the CF treatment, while wheat yield increased slightly. The Fe-BC treatment had the highest yields for both rice and wheat.

[0073] 5. Global Warming Potential (GWP) of a Two-Year Rice-Wheat Rotation

[0074] like Figure 7 As shown in Figure 3, after calculating the global warming potential of rice-wheat rotation over two years, the CF treatment in 2023 had the highest GWP.

[0075] In the BC treatment, biochar application significantly reduced the GWP of rice, while the GWP of wheat remained largely unchanged. The Fe-BC treatment also showed a GWP reduction compared to the CF treatment, but the GWP remained close to that of the BC treatment. In 2024, the BC treatment reduced the GWP of rice, while the GWP of wheat remained similar to that of 2023. Biochar continued to reduce GWP in the second year. The Fe-BC treatment showed the greatest GWP reduction, with both rice and wheat having lower GWPs than the other treatments.

[0076] 6. Greenhouse Gas Intensity (GHGI) of a Two-Year Rice-Wheat Rotation

[0077] like Figure 8As shown, the GHGI for the CF treatment was the highest in 2023. The GHGI for the BC treatment decreased significantly. The GHGI for the Fe-BC treatment was lower than that for the CF treatment and comparable to that for the BC treatment. In 2024, the GHGI for the CF treatment remained the highest, while the GHGI for the BC treatment decreased significantly, particularly for rice, indicating that biochar maintained its strong emission reduction effect in the second year. The GHGI for the Fe-BC treatment further decreased to the lowest level, with both rice and wheat showing lower GHGIs than the other treatments, highlighting the more significant emission reduction advantage of Fe-BC in the second year.

[0078] 7. Heavy metal content in soil at the end of the experiment

[0079] like Figure 9 As shown in the data, the contents of four major heavy metals (lead, arsenic, chromium, and cadmium) in the soil under BC and Fe-BC treatments were not significantly different from those under CF treatment, indicating that the application of BC and Fe-BC did not significantly increase the content of heavy metals in the soil and did not bring additional heavy metal pollution risks.

[0080] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. An application of iron-modified biochar in rice-wheat rotation planting, characterized in that: The application method is as follows: after rice harvest, the collected rice straw is crushed and then put into the iron sulfate solution for immersion for 24-48 hours; the immersed rice straw is dried at 60-80°C; the dried rice straw is pyrolyzed at 500-600°C for 1-3 hours to obtain iron-modified biochar; the rice straw and Fe 3+ The mass ratio is 1: (20-30); the concentration of the ferric sulfate solution is 0.4-0.6 mol L -1 ; The pH value of the iron-modified biochar was 5-6, and the total carbon content was 420.37-500.35 g kg -1 , with a total nitrogen content of 0.98-1.16 g kg -1 , with a specific surface area of ​​96.78-111.06 m 2 g -1 , pore volume of 0.24-0.26 cm 3 g -1 , pore size is 8.68-9.04 nm; In the iron-modified biochar, the mass percentage of C is 70-75%, the mass percentage of O is 20-25%, the mass percentage of N is 2.5-3%, the mass percentage of Fe is 2-2.5%, the mass percentage of Cd is 0.08-0.1%, and the mass percentage of As is 0.5-0.7%; Before wheat sowing, the obtained iron-modified biochar is applied to the soil once and mixed evenly; fertilizer is added during the rice-wheat rotation; the application rate of the iron-modified biochar is 8-12 t / hm 2 .

2. The use of the iron-modified biochar in rice-wheat rotation according to claim 1, characterized in that: The fertilizer application rate is 240 kg hm2 of pure nitrogen. -2 、P2O5 146 kg hm -2 and K2O 52 kg hm -2 .

3. The use of the iron-modified biochar in rice-wheat rotation planting according to claim 2, characterized in that: During the application process of the fertilizer, 40% of the N and all of the P and K are applied to the soil in the form of compound fertilizer as base fertilizer before wheat sowing or rice transplanting, and the remaining 60% of the N is applied to the soil in the form of urea at a ratio of 40%:20% as topdressing.

4. The use of the iron-modified biochar in rice-wheat rotation according to claim 1, characterized in that: The rice variety is Yongyou 7850 or Yongyou 12; the wheat variety is Yangmai 34 or Zhenmai 18; the rice is planted in a hole with two seedlings, or two holes in a pot; the wheat is planted in a pot with 20-40 plants.

5. The use of the iron-modified biochar in rice-wheat rotation planting according to claim 1, characterized in that: The application method also includes field management measures: during wheat planting, the land is leveled, the soil is loosened, and insecticides and weeds are removed; in terms of weed control, one or more pesticides including mesosulfuron, dimethoate, and bispyribac are used before sowing or after seedling emergence to carry out closed or post-emergence control of broadleaf weeds and grass weeds; in terms of pest and disease control, wheat fields are regularly inspected to pay attention to the occurrence of aphids, leaf rust, and sheath blight, and pesticides are applied in a timely manner when the insect population base or lesions are found; at the same time, the field is kept clean, weeds and stumps are removed, and the source of pests and diseases is reduced; During the rice planting process, pests and weeds are removed; 7-10 days after transplanting, during the tillering stage and the heading stage, one or more pesticides including pymetrozine, chlorflucythrin and tricyclazole are used to control rice planthoppers, leaf rollers and rice blast; for weed control, one or two herbicides including bensulfuron-methyl or isoproturon-methyl are sprayed during the peak weed season; in terms of water management, water is flooded in advance before transplanting, and a 3-8 cm water layer is maintained from the early stage of transplanting to the late tillering stage. The field is drained and dried for 3-4 days at the end of tillering, and the field is dried one week before the yellow ripening stage for harvesting.

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