Sustainable production method of wheat based on the improvement of alkaline arsenic-cadmium contaminated soil

Abstract

The present invention discloses a sustainable wheat production method based on the improvement of alkaline arsenic-cadmium contaminated soil, belonging to the technical field of soil remediation, and comprising the following steps: before wheat sowing, a iron-based modified biochar composite material is mixed with a functional microorganism microsphere solution and applied to the soil; after soil preparation, wheat is sown; during the jointing stage of wheat, a silicon-selenium nano composite sol is sprayed on the leaf surface; after wheat matures, it is harvested and the iron-based modified biochar in the soil is recovered. The present invention innovatively proposes a synergistic system of iron-based modified biochar and functional microorganisms: the nano zero-valent iron-humic acid complex synchronously fixes As / Cd through a dual mechanism of reduction-adsorption, and the metabolites of functional microorganisms activate the surface of the passivation material to extend the repair cycle; at the same time, a phased strategy is adopted to combine soil passivation and leaf surface barrier to break through the limitation of the alkaline environment on the repair efficiency, and it has both economy and sustainability.

Description

Technical Field

[0001] The present invention belongs to the technical field of soil remediation, and specifically relates to a method for sustainable wheat production based on the improvement of alkaline arsenic-cadmium contaminated soil. Background Art

[0002] The problem of arsenic-cadmium combined pollution in alkaline soil is becoming increasingly serious. Under high pH conditions, arsenic exists stably in the form of As(V), while cadmium is prone to combine with carbonate to form insoluble CdCO3. Traditional passivators (such as lime and phosphate fertilizer) are difficult to simultaneously fix the two heavy metals. In the prior art, the adsorption efficiency of biochar for As is limited by the high pH environment, and single microbial remediation is easily inactivated by the alkaline soil environment. In addition, the foliar barrier technology mostly relies on single components of silicon or selenium and lacks a synergistic inhibition mechanism. Summary of the Invention

[0003] To solve the problem of difficult synchronous remediation of arsenic-cadmium contaminated soil in the prior art, the present invention provides a method for sustainable wheat production based on the improvement of alkaline arsenic-cadmium contaminated soil. This method can complete the remediation of arsenic-cadmium contaminated soil on the premise of ensuring the safe production of wheat, with remarkable remediation effect, and achieves the purpose of remediation while producing.

[0004] The present invention innovatively proposes a synergistic system of iron-based modified biochar and functional microorganisms: the nZVI (nano zero-valent iron)-humic acid complex simultaneously fixes As / Cd through a dual mechanism of reduction-adsorption, and the metabolites of functional microorganisms activate the surface of the passivation material to extend the repair cycle. At the same time, a phased strategy is adopted, combining soil passivation and foliar barrier, to break through the limitation of the alkaline environment on the repair efficiency, and it has both economy and sustainability.

[0005] The method for sustainable wheat production based on the improvement of alkaline arsenic-cadmium contaminated soil provided by the present invention includes the following steps:

[0006] Step 1: Before sowing wheat, mix and apply an iron-based modified biochar composite material and functional microorganism microspheres into the soil;

[0007] The iron-based modified biochar composite material is biochar modified by nano zero-valent iron and humic acid;

[0008] The functional microorganism microsphere solution is prepared from arsenic-reducing bacteria and cadmium-resistant plant growth-promoting bacteria. The arsenic-reducing bacteria ( Shewanella sp. strain PV-4 ) are purchased from Ningbo Mingzhou Biotechnology Co., Ltd., and the cadmium-resistant plant growth-promoting bacteria (Enterobacter ludwigii, Enterobacter ludwigii , strain number S15) are purchased from the China General Microbiological Culture Collection Center, and the preservation number is CGMCC NO.29744.

[0009] Step 2: Sow wheat after land preparation;

[0010] Step 3: At the jointing stage of wheat, spray silicon-selenium nano-composite sol on the leaf surface;

[0011] Step 4: Harvest the wheat after it matures and recover the iron-based modified biochar in the soil.

[0012] Further, in Step 1, a rotary tiller is used to plow the iron-based modified biochar composite material and the functional microbial microspheres to a depth of 20 - 30 cm.

[0013] Further, in Step 2, a low-accumulation wheat variety is selected, and the seeding rate is 12 - 15 kg / mu. When sowing the low-accumulation wheat variety, the base fertilizer is a compound fertilizer with a mass ratio of N:P:K of 16:16:8, and the application rate is 40 kg / mu; 20 kg / mu of urea is topdressed at the tillering stage.

[0014] Further, in Step 1, the preparation method of the iron-based modified biochar composite material includes:

[0015] (a) Pyrolyze rice husks under anoxic conditions at 500 - 600 °C for 2 - 4 hours, and pulverize them to 100 mesh to obtain high-specific surface area biochar;

[0016] (b) Use the chemical reduction method to co-load nano zero-valent iron (nZVI) and humic acid into the pores of the biochar. The mass ratio of nZVI to humic acid is 1:2 - 1:5; and the total mass ratio of nZVI and humic acid to the mass of the biochar is 1:10;

[0017] (c) Calcinate the loaded biochar under nitrogen protection (350 °C, 1 h) to enhance the material stability.

[0018] Further, in Step 1, the preparation method of the functional microbial microspheres includes:

[0019] (A) Mix arsenic-reducing bacteria ( Shewanella sp. strain PV-4 ) and cadmium-resistant plant growth-promoting bacteria (Enterobacter ludwigii, strain number S15) in a volume ratio of 1:1. Before mixing, the bacterial liquid concentrations of arsenic-reducing bacteria and cadmium-resistant plant growth-promoting bacteria are both 1×10 7 -1×10 8 CFU / mL;

[0020] (B) Mix the mixed bacterial solution and sodium alginate solution in a volume ratio of 1:1 to obtain a mixed solution. The concentration of sodium alginate in the sodium alginate solution is 2% w / v. Then, add biochar powder to the mixed solution, and the addition amount of biochar powder is 5% of the mass of the mixed solution. Finally, drop the mixed solution into 5 times the volume of CaCl2 solution for curing. The concentration of CaCl2 in the CaCl2 solution is 2% w / v to obtain a functional microbial microsphere solution.

[0021] Further, in step one, the application amount of the iron-based modified biochar composite is 2-5% of the soil mass; the application amount of the microbial microsphere solution is 1×10 7 -1×10 8 CFU / g soil, that is, the number of colonies in the microbial microsphere solution applied per gram of soil is 1×10 7 -1×10 8 CFU; where:

[0022] When As≤50 mg / kg and Cd≤2 mg / kg in the soil, it is lightly polluted, and the application amount of the iron-based modified biochar is 2-3%;

[0023] When As is 50-100 mg / kg and Cd is 2-5 mg / kg in the soil, it is moderately polluted, and the application amount of the iron-based modified biochar is 3-5%.

[0024] Further, in step three, the preparation method of the silicon-selenium nanocomposite sol is as follows:

[0025] Prepare mesoporous silica nanoparticles (particle size 50-100 nm, pore size 5-10 nm) by the sol-gel method;

[0026] Mix the sodium selenite (Na2SeO3) solution and silica particles in a Se / SiO2 mass ratio of 1:100, ultrasonically disperse for 30 minutes, and obtain a selenium-loaded nanocomposite powder after freeze-drying. Disperse the nanocomposite powder in water to obtain a silicon-selenium nanocomposite sol. When in use, disperse the powder in water according to the ratio, and adjust the selenium concentration to 0.1-0.3 mg / L and the silicon concentration to 50-100 mg / L.

[0027] Further, in step three, the method of foliar spraying the silicon-selenium nanocomposite sol is as follows:

[0028] Use a knapsack electric sprayer (droplet diameter ≤100 μm), spraying pressure 0.3-0.5 MPa, spraying amount 30-50 L / acre, selenium concentration 0.1-0.3 mg / L, and silicon concentration 50-100 mg / L.

[0029] Further, in step four, iron-based modified biochar is recovered by magnetic separation method.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) In the preparation method of the iron-based modified biochar composite material of the present invention, iron-based modified biochar nZVI and humic acid are co-loaded at a mass ratio of 1:2 - 1:5, realizing the synchronous fixation of As / Cd. At this ratio, humic acid can not only buffer the pH of alkaline soil to prevent the oxidation and inactivation of nZVI, but also enhance the fixation of Cd through complexation. The step of calcining at 350°C for 1 hour under nitrogen protection can significantly improve the material stability, avoid the rapid oxidation of nZVI in alkaline soil, and effectively extend the repair cycle.

[0032] (2) In the preparation method of the functional microbial microspheres of the present invention, by introducing 5% biochar powder as a filler into the microspheres, the mechanical strength of the microspheres is enhanced, the degradation of sodium alginate in an alkaline environment is delayed, the slow-release effect of functional microorganisms is ensured, and the synchronous repair of arsenic and cadmium is realized by using their synergistic effect. Arsenic-reducing bacteria reduce As(V) to As(III) and adsorb it on the biochar, and cadmium-resistant bacteria secrete organic acids to dissolve CdCO3, inducing wheat roots to secrete metallothionein to chelate Cd²⁺.

[0033] (3) In the sustainable wheat production method based on the improvement of alkaline arsenic-cadmium contaminated soil of the present invention, a phased repair strategy is adopted: First, soil passivation is carried out before sowing (step one) to reduce the content of available heavy metals in the soil. Second, the elongation stage (step three) when the wheat stem grows rapidly is selected for foliar spraying of silicon-selenium nanosol, rather than the traditional tillering stage. The foliar absorption efficiency of wheat is the highest at the elongation stage, and silicon and selenium synergistically inhibit the transfer of As / Cd to grains, forming a "soil-plant" double barrier.

[0034] (4) In the sustainable wheat production method based on the improvement of alkaline arsenic-cadmium contaminated soil of the present invention, the recycling mechanism is also integrated: Iron-based biochar is recovered by magnetic separation method (recovery rate ≥ 90%). While improving the repair efficiency, the recyclability of materials and the applicability to farmland are taken into account to ensure the technical and economic feasibility. On the one hand, it guarantees the safe production of wheat grains in food agriculture, and on the other hand, it realizes the repair of arsenic-cadmium slightly contaminated farmland soil. Detailed implementation mode

[0035] The technical solution of the present invention will be described in detail below through examples, but the protection scope of the present invention is not limited to the described examples.

[0036] Example 1: Fixation effect of iron-based modified biochar on As / Cd

[0037] Take the heavy metal - polluted farmland (mild pollution) in a certain place of a certain city as the experimental field. The soil pH value is 8.2, the organic matter content is 1.5%, the content of available As (determined by 0.5M NaHCO3 extraction - atomic fluorescence spectrometry) is 45 mg / kg, and the content of available Cd (determined by DTPA extraction - graphite furnace atomic absorption spectrometry) is 1.8 mg / kg.

[0038] Apply the biochar material to the soil and use a rotary tiller to plow it to a depth of 20 - 30 cm. The experimental group prepares an iron - based modified biochar composite material according to nZVI:humic acid = 1:3, and the application rate is 3% of the soil mass (mild pollution gradient). The control groups include unmodified biochar (As fixation rate 35%, Cd fixation rate 40%) and biochar loaded with nZVI alone (As fixation rate 60%, Cd fixation rate 50%). In the experimental group, the available As in the soil decreased by 78% (to 9.9 mg / kg), and Cd decreased by 82% (to 0.32 mg / kg), which is significantly better than the control groups (p < 0.05).

[0039] Among them, the preparation methods of the iron - based modified biochar composite material, unmodified biochar, and biochar loaded with nZVI alone are as follows:

[0040] Specific preparation method of the iron - based modified biochar composite material:

[0041] (a) Pyrolyze 1 kg of dry rice husks under anoxic conditions at 600 °C for 3 hours, and crush them to 100 mesh to obtain high - specific - surface - area biochar;

[0042] (b) Load nZVI and humic acid by chemical reduction method: Dissolve 0.5 g of nano - zero - valent iron (nZVI) and 1.5 g of humic acid (mass ratio 1:3) in 200 mL of deionized water, and ultrasonically disperse for 30 minutes; add 20 g of biochar, stir (500 rpm) for 4 hours, and let it stand for 12 hours;

[0043] Co - load nano - zero - valent iron and humic acid into the pores of biochar. The mass ratio of nano - zero - valent iron to humic acid is 1:3, and the total mass ratio of nZVI and humic acid to the mass of biochar is 1:10.

[0044] (c) Calcinate the loaded biochar under nitrogen protection (350 °C, 1 h) to enhance the material stability.

[0045] 2) Preparation method of the unmodified biochar:

[0046] Pyrolyze dry rice husks in a tube furnace under anoxic conditions at 600 °C for 3 hours, crush them through a 100 - mesh sieve to obtain unmodified biochar.

[0047] 3) Preparation method of nZVI-loaded biochar alone:

[0048] (a) Take 1 kg of dry rice husk, pyrolyze it in a tube furnace under anoxic conditions at 600 °C for 3 hours, crush it and pass through a 100-mesh sieve;

[0049] (b) Dissolve 2.0 g of nano zero-valent iron (nZVI) in 200 mL of deionized water and ultrasonically disperse it for 30 minutes;

[0050] (c) Add 100 g of biochar, stir (500 rpm) for 4 hours, and let it stand for 12 hours;

[0051] (d) The loaded biochar is calcined at 350 °C for 1 hour under nitrogen protection, cooled and passed through a 100-mesh sieve, and stored in a sealed manner.

[0052] Example 2: Sustained-release effect of functional microbial microspheres

[0053] Simulate alkaline soil (pH 8.5) under laboratory conditions, with a temperature of 25 ± 2 °C and a humidity of 60%. Set two treatments. The experimental group is a functional microbial microsphere solution containing 5% biochar, and the strain ratio of arsenic-reducing bacteria to cadmium-resistant plant growth-promoting bacteria is 1:1. The control group is a pure sodium alginate microsphere (without biochar) solution. After 30 days, the bacterial survival rate in the experimental group > 80%, and the bacterial survival rate in the control group is only 30%.

[0054] The arsenic-reducing bacteria ( Shewanella sp. strain PV-4 ) are purchased from Ningbo Mingzhou Biotechnology Co., Ltd., and the cadmium-resistant plant growth-promoting bacteria (Enterobacter ludwigii, Enterobacter ludwigii, strain number S15) are purchased from the China General Microbiological Culture Collection Center, with the preservation number CGMCC NO. 29744.

[0055] The preparation methods of the functional microbial microsphere solution containing 5% biochar and the pure sodium alginate microsphere solution are as follows:

[0056] (1) Preparation method of the functional microbial microsphere solution containing 5% biochar:

[0057] (a) Mix the bacterial solutions of arsenic-reducing bacteria and cadmium-resistant plant growth-promoting bacteria at a volume ratio of 1:1; before mixing, the concentrations of the bacterial solutions of arsenic-reducing bacteria and cadmium-resistant plant growth-promoting bacteria are both 1×10 8 CFU / mL.

[0058] (b) Mix the mixed bacterial solution and the sodium alginate solution in a volume ratio of 1:1 to obtain a mixed solution. The concentration of sodium alginate in the sodium alginate solution is 2% w / v. Then, add biochar powder to the mixed solution, and the addition amount of biochar powder is 5% of the mass of the mixed solution. Finally, drop 5 times the volume of CaCl2 solution to obtain a functional microorganism microsphere solution. The concentration of the CaCl2 solution is 2% w / v, and the diameter of the formed microspheres is 2 - 3 mm.

[0059] (2) Preparation method of pure sodium alginate microsphere (without biochar) solution:

[0060] (a) Mix the bacterial solutions of arsenic-reducing bacteria and cadmium-resistant plant growth-promoting bacteria in a volume ratio of 1:1. Before mixing, the concentrations of the bacterial solutions of arsenic-reducing bacteria and cadmium-resistant plant growth-promoting bacteria are both 1×10 8 CFU / mL.

[0061] (b) Mix the mixed bacterial solution and the sodium alginate solution in a volume ratio of 1:1 to obtain a mixed solution. The concentration of sodium alginate in the sodium alginate solution is 2% w / v; drop it into 5 times the volume of 2% w / v CaCl2 solution for curing to obtain a pure sodium alginate microsphere solution without adding biochar powder.

[0062] Example 3: Synergistic effect of the staged strategy

[0063] I. Test site and soil physical and chemical properties

[0064] Use a cadmium-polluted farmland in a certain city as the test field for field experiments (mild pollution). The soil pH value is 8.3, total arsenic (As) is 85 mg / kg, total cadmium (Cd) is 4.8 mg / kg; available As (content determined by 0.5M NaHCO3 extraction - atomic fluorescence spectrometry) is 38 mg / kg, and available Cd (content determined by DTPA extraction - graphite furnace atomic absorption spectrometry) is 2.5 mg / kg.

[0065] II. Experimental scheme

[0066] Experimental group (staged remediation strategy): Apply iron-based modified biochar and functional microorganism microspheres for foliar spraying; the specific steps are as follows:

[0067] Step 1. Soil passivation and synergy with functional microorganisms: 15 days before wheat sowing, evenly spread the iron-based modified biochar composite material (nZVI: humic acid = 1:3, application amount is 3% of the soil mass) and the functional microorganism microsphere (application amount is 1×10 7 - 1×10 8 CFU / g soil) solution on the ground and use a rotary tiller to plow to a depth of 20 - 30 cm;

[0068] Step 2: Sow wheat after land preparation; The wheat variety is Jimai 22, and the seeding rate is 15 kg / mu. When sowing, the base fertilizer is a compound fertilizer with a mass ratio of N:P:K of 16:16:8, and the application rate is 40 kg / mu; Urea is topdressed at a rate of 20 kg / mu during the tillering stage.

[0069] Step 3: Foliar spraying of silicon-selenium sol: At the jointing stage of wheat, select a sunny and windless day (temperature 15 - 20 °C), and use a knapsack electric sprayer (spraying pressure 0.4 MPa) to spray silicon-selenium nano-composite sol on the leaves (mass ratio of Se / SiO2 is 1:100, selenium concentration is 0.2 mg / L, silicon concentration is 80 mg / L). There is no rainfall within 6 hours after spraying, and both the front and back sides of the leaves are evenly covered.

[0070] Control group 1 (only soil passivation): Only apply iron-based modified biochar, do not add microbial microsphere solution in Step 1, and do not perform the foliar spraying operation in Step 3. The parameters are the same as those of the experimental group.

[0071] Control group 2 (only foliar barrier): Only spray silicon-selenium sol at the jointing stage, do not perform the operation in Step 1, and perform the operations in Step 2 and Step 3. The parameters are the same as those of the experimental group.

[0072] Control group 3 (conventional planting, without any remediation measures): Do not perform the operations in Step 1 and Step 3. The parameters are the same as those of the experimental group.

[0073] Control group 4 (only soil passivation + foliar spraying): Only apply iron-based modified biochar in Step 1, do not add microbial microsphere solution, and perform the operations in Step 2 and Step 3. The parameters are the same as those of the experimental group.

[0074] In the above experimental group and control groups, the preparation method of the iron-based modified biochar is the same as that in Example 1, and the preparation method of the functional microbial microspheres is the same as that in Example 2. Preparation method of silicon-selenium nano-composite sol:

[0075] (a) Prepare mesoporous silica nanoparticles (particle size 50 - 100 nm, pore size 5 - 10 nm) by the sol-gel method;

[0076] Dissolve 1.0 g of CTAB (cetyltrimethylammonium bromide) in a mixed solution of 200 mL of deionized water and 50 mL of absolute ethanol. After stirring and dissolving, add 5 mL of ammonia water (25%); Dropwise add 4 mL of TEOS (tetraethyl orthosilicate, dropping rate 0.5 mL / min), and stir and react at 25 °C for 6 hours; After centrifugation, washing and drying, calcine at 550 °C for 4 hours to obtain mesoporous silica nanoparticles (particle size 80 ± 20 nm, pore size 7 ± 3 nm).

[0077] (b) Mix the sodium selenite (Na2SeO3) solution with silicon dioxide nanoparticles at a Se / SiO2 mass ratio of 1:100, ultrasonically disperse for 30 minutes, and obtain a selenium-loaded nano-composite powder after freeze-drying; disperse the powder in water and adjust the selenium concentration to 0.2 mg / L and the silicon concentration to 80 mg / L.

[0078] III. Data Collection and Analysis

[0079] (1) Soil remediation effect

[0080] The detection indicators include available As in soil (content determined by 0.5M NaHCO3 extraction - atomic fluorescence spectrometry) and available Cd (content determined by DTPA extraction - graphite furnace atomic absorption spectrometry).

[0081] Experimental group: Available As decreased to 8.7 mg / kg (a decrease of 77.1%), and available Cd decreased to 0.45 mg / kg (a decrease of 82.0%).

[0082] Control group 1: Available As was 15.2 mg / kg (a decrease of 60.0%), and Cd was 0.82 mg / kg (a decrease of 67.2%).

[0083] Control group 2: The decreases in As / Cd were both <10%.

[0084] Control group 3: Available As was 38 mg / kg (a decrease of 0%), and available Cd was 2.5 mg / kg (a decrease of 0%).

[0085] Control group 4: Available As was 12.5 mg / kg (a decrease of 67.1%), and available Cd was 0.68 mg / kg (a decrease of 72.8%).

[0086] (2) Wheat growth and heavy metal accumulation:

[0087] Biomass: The grain yield of the experimental group was 482 kg / mu, which was 17.6% higher than that of control group 3 (410 kg / mu); Control group 1: The grain yield was 455 kg / mu; Control group 2: The grain yield was 418 kg / mu; Control group 4: The grain yield was 460 kg / mu.

[0088] The contents of As and Cd in wheat tissues in the experimental group and control groups are shown in Table 1:

[0089] Table 1

[0090]

[0091] Example 4: Recovery and Recycling of Iron-based Biochar

[0092] When planting wheat (Jimai 22) every season, the application rate of the iron-based modified biochar composite is 3% of the soil mass, and the cumulative application rate for 3 consecutive seasons is 9%.

[0093] The iron-based biochar is recycled using a permanent magnet drum magnetic separator (magnetic field intensity 1.0 T, treatment time 40 min), and the recovery rate is 92%. After recovery, pickling regeneration is carried out (0.1M HCl, oscillation rate 150 rpm, time 2 h). The oxidation rate of the iron-based modified biochar composite is <10%. After 3 consecutive seasons of wheat planting, when the recycled material is reapplied, the available As / Cd in the soil is still reduced by more than 70%.

[0094] As described above, although the present invention has been shown and described with reference to specific preferred embodiments, it should not be construed as a limitation of the present invention itself. Various changes can be made to it in form and detail without departing from the spirit and scope of the present invention.

Claims

1. A sustainable wheat production method based on the improvement of alkaline arsenic-cadmium contaminated soil, characterized in that It includes the following steps: Step 1: Before sowing wheat, mix the iron-based modified biochar composite material with the functional microorganism microsphere solution and apply it to the soil; The iron-based modified biochar composite material is biochar modified by nano zero-valent iron and humic acid; The functional microorganism microsphere solution is prepared from arsenic-reducing bacteria and cadmium-resistant plant growth-promoting bacteria; Step 2: Sow wheat after land preparation; Step 3: At the jointing stage of wheat, spray silicon-selenium nano composite sol on the leaf surface; Step 4: Harvest wheat after maturity and recover the iron-based modified biochar composite material in the soil; The preparation method of the functional microorganism microsphere solution is as follows: (A) Mix the bacterial solutions of arsenic-reducing bacteria and cadmium-resistant plant growth-promoting bacteria at a volume ratio of 1:1; before mixing, the concentration of both bacterial solutions is 1×10 7 -1×10 8 CFU / mL; (B) Mix the mixed bacterial solution and sodium alginate solution in a volume ratio of 1:1 to obtain a mixed solution, and then add biochar powder; finally, drop the mixed solution into CaCl2 solution for curing; Among them, the addition amount of biochar powder is 5% of the mass of the mixed solution; the volume ratio of the mixed solution to the CaCl2 solution is 1:5; the concentration of the sodium alginate solution is 2% w / v; the concentration of the CaCl2 solution is 2% w / v.

2. The method according to claim 1, wherein In Step 1, the arsenic-reducing bacterium is Shewanella sp. strain PV-4, The cadmium-resistant plant growth-promoting bacterium is Enterobacter ludwigii S15.

3. The method according to claim 1, wherein In Step 1, the preparation method of the iron-based modified biochar composite material is as follows: (a) Pyrolyze rice husk under anaerobic conditions at 500-600 °C for 2-4 hours, and crush it to 100 meshes to obtain high specific surface area biochar; (b) Co-load nano zero-valent iron and humic acid into the pores of biochar by chemical reduction method: the mass ratio of nano zero-valent iron to humic acid is 1:2 - 1:5, and the mass ratio of the sum of the mass of nano zero-valent iron and humic acid to the mass of biochar is 1:10; (c) Calcinate the loaded biochar under nitrogen protection at 350 °C for 1 h.

4. The method according to claim 1, wherein In Step 1, the application rate of the iron-based modified biochar composite material is 2-5% of the soil mass; the application rate of the microbial microsphere solution is: the number of colonies in the microbial microsphere solution applied per gram of soil is 1×10 7 -1×10 8 CFU.

5. The method according to claim 1, wherein The preparation method of the silicon-selenium nano composite sol is as follows: Prepare mesoporous silica nanoparticles by sol-gel method, the particle size of the silica nanoparticles is 50-100 nm, and the pore size is 5-10 nm; Mix sodium selenite solution and silica particles in a mass ratio of Se to SiO2 of 1:100, disperse them by ultrasonic wave and then freeze-dry to obtain selenium-loaded nano composite powder; disperse the nano composite powder in water to obtain silicon-selenium nano composite sol.

6. The method according to claim 1, characterized in that The spraying amount of the silicon-selenium nano composite sol is 30-50 L / mu; in the silicon-selenium nano composite sol, the selenium concentration is 0.1-0.3 mg / L, and the silicon concentration is 50-100 mg / L.

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