Soil conditioner for severely arsenic-polluted rice, treatment method and application of soil conditioner

Abstract

The invention discloses a severely arsenic-polluted rice soil conditioner, a treatment method and application thereof, and relates to the technical field of environmental protection. The modifier is composed of biochar and potassium nitrate; the mass ratio of the biochar to the potassium nitrate is (1-1.4): (2-2.6). According to the method, arsenic in the anaerobic rice soil is fixed, meanwhile, the nutrient circulation function of the soil is improved, and the method has important significance on arsenic-polluted soil remediation and soil fertility improvement.

Description

Technical Field

[0001] The present invention relates to the field of environmental protection technology, and more specifically to a heavily arsenic-contaminated rice soil conditioner, a treatment method and an application thereof. Background Art

[0002] Arsenic is a metalloid element that is ubiquitous in the natural environment. It is widely distributed in the atmosphere, water, soil and sediments. It can enter the human body through water, atmosphere and food chains. High levels of arsenic accumulation can cause serious harm to the human body and lead to cancer in multiple parts of the body. Therefore, the control of arsenic pollution is imminent.

[0003] At present, the use of arsenic has involved medicine, agriculture, metallurgy, telecommunications technology and other disciplines. As a result, arsenic deposits have been developed on a large scale. At the same time, lead, zinc, copper, gold, tin and rare earth mines are often accompanied by a large amount of arsenic ores, and the development of these deposits will bring a large amount of arsenic into the soil. It is reported that arsenic in flooded soil mainly exists in two inorganic valence forms, AsⅤ and AsⅢ, of which the latter is about 100 times more toxic than the former; compared with other cereal crops, rice has a strong ability to absorb As. Exposure to arsenic will reduce seed germination, increase oxidative damage, hinder photosynthesis, and reduce the grain yield of plants; it is reported that the most commonly used and effective soil arsenic pollution remediation technology is solidification and stabilization technology, and the selection of modifiers in solidification and stabilization technology is very important.

[0004] Biochar has the advantages of large porosity, specific surface area and strong adsorption capacity. As an amendment, biochar is widely used in environmental pollution remediation and soil improvement. However, after actual application, it was found that biochar has low efficiency in reducing the biological effectiveness of arsenic in paddy soil. Summary of the invention

[0005] To solve the above problems, the present invention provides a heavily arsenic-contaminated paddy soil conditioner, a treatment method and an application thereof, which are used to improve the efficiency of biochar in reducing the biological effectiveness of arsenic in paddy soil.

[0006] The present invention is achieved through the following technical solutions:

[0007] A first aspect of the present invention provides a soil conditioner for rice soil that is heavily contaminated with arsenic. The soil conditioner consists of biochar and potassium nitrate.

[0008] The mass ratio of the biochar to potassium nitrate is 1-1.4:2-2.6.

[0009] Preferably, the mass ratio of the biochar to potassium nitrate is 1:2.

[0010] Preferably, the biochar is rice straw biochar.

[0011] A second aspect of the present invention provides a method for treating heavily arsenic-contaminated rice soil, comprising: adding the heavily arsenic-contaminated rice soil conditioner to the heavily arsenic-contaminated rice soil.

[0012] The mass ratio of the heavily arsenic-contaminated rice soil conditioner to the heavily arsenic-contaminated rice soil is 3 to 4:40.

[0013] Preferably, the mass ratio of the heavily arsenic-contaminated rice soil conditioner to the heavily arsenic-contaminated rice soil is 3.6:40.

[0014] The heavily arsenic-contaminated rice soil conditioner is used in improving heavily arsenic-contaminated rice soil.

[0015] The heavily arsenic-contaminated rice soil conditioner is used to reduce soil pH and increase soil redox potential, electrical conductivity, soil organic matter and dissolved organic carbon content or more.

[0016] The heavily arsenic-contaminated rice soil conditioner is used to increase the relative abundance of functional microorganisms that are arsenic-resistant and can oxidize AsⅢ to AsⅤ.

[0017] Preferably, the functional microorganism is of the genera Lysobacter, Bacillus, Luteomonas and Arthrobacter.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] The present invention simultaneously applies biochar and potassium nitrate to contaminated paddy soil, and finds that biochar and potassium nitrate have an obvious synergistic effect, which improves the passivation effect on arsenic. In addition, both biochar and potassium nitrate treatments reduce the proportion of AsⅢ in the effective arsenic, so that AsⅢ in the effective arsenic is completely converted into AsⅤ, reducing the biological effectiveness of arsenic in paddy soil.

[0020] The application of biochar and potassium nitrate in the present invention significantly improves soil redox point Eh, electrical conductivity EC, soil organic matter SOM, dissolved organic carbon DOC and NO3 - -N content; significantly reduced soil pH, HCl-extractable FeII and NH4 + -N content, which helps reduce the effectiveness of arsenic in the soil.

[0021] The synergistic application of biochar and potassium nitrate in the present invention directly or indirectly promotes the proliferation of functional microorganisms with arsenic resistance and oxidation of AsⅢ to AsⅤ, such as Lysobacter, Paenibacillus, Bacillus, Luteimonas and Arthrobacter, thereby promoting the adsorption and fixation of arsenic. While fixing arsenic in anaerobic paddy soil, the nutrient cycle function of the soil is also improved, which is of great significance for the restoration of arsenic-contaminated soil and the improvement of soil fertility. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0023] Figure 1 The graph is a graph showing the change in the available As content in the soil after restoration by biochar, potassium nitrate and biochar-based potassium nitrate in the present invention; wherein CK is a blank control group, BC is a biochar treatment group, N is a potassium nitrate treatment group, and BC-N is a biochar-based potassium nitrate treatment group;

[0024] Figure 1 Among them, A is the AsⅤ content diagram after extraction with NaHCO3, and B is the AsⅢ and AsⅤ ratio diagram after extraction with KH2PO4-K2HPO4;

[0025] Figure 2 The figure is a test result of the physical and chemical properties of the soil after applying biochar, potassium nitrate and biochar-based potassium nitrate in the present invention; wherein CK is a blank control group, BC is a biochar treatment group, N is a potassium nitrate treatment group, and BC-N is a biochar-based potassium nitrate treatment group;

[0026] Figure 2 In the figure, A is the pH value test result diagram, B is the redox potential test result diagram, C is the conductivity test result diagram, D is the FeII concentration test result diagram, E is the soil organic matter test result diagram, and F is the dissolved organic carbon test result diagram;

[0027] Figure 3 The figure is a graph showing the test results of effective nutrients in the soil after applying biochar, potassium nitrate and biochar-based potassium nitrate in the present invention; wherein CK is a blank control group, BC is a biochar treatment group, N is a potassium nitrate treatment group, and BC-N is a biochar-based potassium nitrate treatment group;

[0028] Figure 3In the figure, A is the ammonium nitrogen test result diagram, B is the nitrate nitrogen test result diagram, and C is the available phosphorus test result diagram;

[0029] Figure 4 The representative α-richness index detection result diagram of the 16S rRNA gene in the soil bacteria of the present invention; wherein CK is a blank control group, BC is a biochar treatment group, N is a potassium nitrate treatment group, and BC-N is a biochar-based potassium nitrate treatment group;

[0030] Figure 4 In the figure, A is the Ace index test result diagram, B is the Chao index test result diagram, C is the Shannon index test result diagram, and D is the Simpson test result diagram;

[0031] Figure 5 The figure is a test result of soil bacterial community composition after restoration of the present invention; wherein CK is a blank control group, BC is a biochar treatment group, N is a potassium nitrate treatment group, and BC-N is a biochar-based potassium nitrate treatment group;

[0032] Figure 5 In the figure, A is the community richness ratio at the phylum level, B is the community richness ratio at the genus level, C is the Veen graph, and D is the principal component analysis graph at the OTU level;

[0033] Figure 6 The redundancy analysis RDA diagram of the correlation between soil environmental parameters and microbial community composition at the phylum level of the present invention; wherein, Eh: soil redox point; EC: electrical conductivity; AAS: soil available arsenic; Ammonium: ammonium nitrogen; Nitrate: nitrate nitrogen; SOM: soil organic matter; DOC: dissolved organic carbon; AP: soil available phosphorus;

[0034] Figure 7 The Pearson correlation heat map between the abundance at the phylum level and genus level and environmental factors of the present invention; wherein, Eh: soil redox site; EC: electrical conductivity; AAS: soil available arsenic; Ammonium: ammonium nitrogen; Nitrate: nitrate nitrogen; SOM: soil organic matter; DOC: dissolved organic carbon; AP: soil available phosphorus;

[0035] Figure 7 In the figure, A is the Pearson correlation heat map between phylum-level abundance and environmental factors, and B is the Pearson correlation heat map between genus-level abundance and environmental factors;

[0036] Figure 8 This is a heat map of the functional predictability of the soil bacterial community of the present invention; wherein CK is a blank control group, BC is a biochar treatment group, N is a potassium nitrate treatment group, and BC-N is a biochar-based potassium nitrate treatment group;

[0037] Fig. 9 The Pearson correlation coefficient diagram between different physical and chemical properties of the soil of the present invention, P<0.05; P<0.01; wherein, Eh: soil redox point; EC: electrical conductivity; AAS: soil available arsenic; Ammonium: ammonium nitrogen; Nitrate: nitrate nitrogen; SOM: soil organic matter; DOC: dissolved organic carbon; AP: soil available phosphorus;

[0038] Fig.10 It is the X-ray photoelectron spectrum of the present invention;

[0039] Fig.10 In the figure, A is the N1s spectrum of the blank control group, B is the N1s spectrum of the biochar-based potassium nitrate treatment group, C is the Fe 2p spectrum of the blank control group, and D is the Fe 2p spectrum of the biochar-based potassium nitrate treatment group;

[0040] Fig.11 This is a diagram showing the restoration mechanism of As-contaminated paddy soil by biochar-based potassium nitrate of the present invention. DETAILED DESCRIPTION

[0041] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively below, and preferred embodiments of the present invention are provided. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0043] The biochar used in the present invention is rice straw biochar, which is purchased from Henan Lize Environmental Protection Technology Co., Ltd.; the potassium nitrate used is purchased from Guizhou Fenglan Industrial Development Co., Ltd.

[0044] The soil samples used in the present invention were collected from the rice fields polluted by mining in Hezhang County, Guizhou Province, China. They are severely arsenic-contaminated soils with an arsenic content of ≥200 mg kg -1 The specific physical and chemical properties of soil samples are shown in Table 1.

[0045] Table 1 Physical and chemical properties of the experimental soil

[0046]

[0047] Note: “-” means no such item.

[0048] The present invention provides a statistical analysis and drawing method.

[0049] The present invention uses Excel 2016 to summarize and organize the experimental data, and uses IBM SPSS statistic27 to perform significant difference analysis between different treatment groups, and P<0.05 is considered significant. The tools provided by the cloud platform of Majorbio Bio-Pharm Technology Co. Ltd. are used to process the relevant data of the microbial community in the soil and draw graphs. Canoco5.0 software is used to perform principal component analysis of the microbial community, the English name is Principal component analysis, abbreviated as PCA, and its β-diversity is studied. XPSPEAR41 is used to analyze N1s and Fe 2p in the control group and the biochar and potassium nitrate treatment groups, and imported into Origin2022 for drawing.

[0050] The treatment effect of biochar on heavy metals is related to its physical and chemical properties, the adsorption and complexation of metal ions and non-metal ions, and the precipitation process. The environmental and toxic effects of arsenic are more dependent on the transformation of its chemical forms: arsenic in rice soil mainly exists in the form of AsⅤ and AsⅢ inorganic valence states, of which the latter is about 100 times more toxic than the former, and the two can be transformed into each other through the interaction of abiotic and biotic factors. Both are anions, which repel the negative charge on the surface of biochar. The use of biochar alone cannot transform the highly toxic AsⅢ into AsⅤ, which makes biochar inefficient in reducing the biological effectiveness of arsenic in rice soil.

[0051] The present invention provides a severely arsenic-contaminated rice soil conditioner, a treatment method and an application thereof, wherein the severely arsenic-contaminated rice soil conditioner is composed of biochar and potassium nitrate. Biochar and potassium nitrate have an obvious synergistic effect, which improves the passivation effect on arsenic, and potassium nitrate can reduce the repulsive effect between the negative charge on the surface of biochar and the anionic arsenic compound, thereby improving the biological effectiveness efficiency of arsenic in paddy soil. The present invention also provides a method for treating severely arsenic-contaminated rice soil, wherein the mass ratio of the severely arsenic-contaminated rice soil conditioner to the severely arsenic-contaminated rice soil in the treatment method is 3 to 4:40. The simultaneous application of biochar and potassium nitrate solves the problem of low efficiency of biochar in reducing the biological effectiveness of arsenic in paddy soil.

[0052] In order to better illustrate the present invention, the technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0053] Example 1: Treatment of heavily arsenic-contaminated paddy soil with soil conditioner

[0054] (1) Biochar and potassium nitrate were mixed in a mass ratio of 1:2 to obtain biochar-based potassium nitrate, which was a heavily arsenic-contaminated rice soil conditioner. 40.0 g of heavily arsenic-contaminated rice soil sample was added to a clean plastic bottle, and then the heavily arsenic-contaminated rice soil conditioner was added to the soil sample. The mass ratio of the heavily arsenic-contaminated rice soil conditioner to the heavily arsenic-contaminated rice soil sample was 3:40, which was used as the biochar-based potassium nitrate treatment group BC-N. At the same time, a blank control group was set up without adding materials to compare the treatment effect of the heavily arsenic-contaminated rice soil conditioner on the contaminated paddy soil.

[0055] (2) Three parallel samples were set up in the biochar-based potassium nitrate treatment group, and 40 mL of deionized water was added to each sample to create a flooded anaerobic environment, and then the samples were sealed and cultured together at room temperature for 50 days.

[0056] (3) After 50 days, the soil samples were dried, ground through a 100-grit sieve, and sealed for subsequent experiments. Some soil samples were freeze-dried at -80°C and used for microbial analysis.

[0057] Example 2: Treatment of heavily arsenic-contaminated paddy soil with soil conditioner

[0058] (1) Biochar and potassium nitrate were mixed in a mass ratio of 1:2 to obtain biochar-based potassium nitrate, which was a heavily arsenic-contaminated rice soil conditioner. 40.0 g of heavily arsenic-contaminated rice soil sample was added to a clean plastic bottle, and then the heavily arsenic-contaminated rice soil conditioner was added to the soil sample. The mass ratio of the heavily arsenic-contaminated rice soil conditioner to the heavily arsenic-contaminated rice soil sample was 3.6:40, which was used as the biochar-based potassium nitrate treatment group BC-N. At the same time, a blank control group was set up without adding any materials to compare the treatment effect on the contaminated rice soil.

[0059] (2) Three parallel samples were set up in the biochar-based potassium nitrate treatment group, and 40 mL of deionized water was added to each sample to create a flooded anaerobic environment, and then the samples were sealed and cultured together at room temperature for 50 days.

[0060] (3) After 50 days, the soil samples were dried, ground through a 100-grit sieve, and sealed for subsequent experiments. Some soil samples were freeze-dried at -80°C and used for microbial analysis.

[0061] Example 3: Treatment of heavily arsenic-contaminated paddy soil with soil conditioner

[0062] (1) Biochar and potassium nitrate were mixed in a mass ratio of 1.4:2.6 to obtain biochar-based potassium nitrate, which was a heavily arsenic-contaminated rice soil conditioner. 40.0 g of heavily arsenic-contaminated rice soil sample was added to a clean plastic bottle, and then the heavily arsenic-contaminated rice soil conditioner was added to the soil sample. The mass ratio of the heavily arsenic-contaminated rice soil conditioner to the heavily arsenic-contaminated rice soil sample was 4:40, which was used as the biochar-based potassium nitrate treatment group BC-N. At the same time, a blank control group was set up without adding any materials to evaluate the treatment effect on the contaminated paddy soil.

[0063] (2) Three parallel samples were set up in the biochar-based potassium nitrate treatment group, and 40 mL of deionized water was added to each sample to create a flooded anaerobic environment, and then the samples were sealed and cultured together at room temperature for 50 days.

[0064] (3) After 50 days, the soil samples were dried, ground through a 100-grit sieve, and sealed for subsequent experiments. Some soil samples were freeze-dried at -80°C and used for microbial analysis.

[0065] Comparative Example 1: Biochar treatment of contaminated paddy soil

[0066] (1) 40.0 g of heavily arsenic-contaminated paddy soil sample was added to a clean plastic bottle, and then biochar was added to the soil sample. The mass ratio of biochar to soil sample was 1.2:40, which was used as the biochar treatment group BC. At the same time, a blank control group was set up without adding any materials to compare the treatment effect on the contaminated paddy soil.

[0067] (2) Three parallel samples were set up in the biochar treatment group, and 40 mL of deionized water was added to each sample to create a flooded anaerobic environment, and then the samples were sealed and cultured together at room temperature for 50 days.

[0068] (3) After 50 days, the soil samples were dried, ground through a 100-grit sieve, and sealed for subsequent experiments. Some soil samples were freeze-dried at -80°C and used for microbial analysis.

[0069] Comparative Example 2: Treatment of Contaminated Paddy Soil with Potassium Nitrate

[0070] (1) 40.0 g of heavily arsenic-contaminated paddy soil sample was added to a clean plastic bottle, and potassium nitrate was then added to the soil sample. The mass ratio of potassium nitrate to soil sample was 2.4:40, which was used as potassium nitrate treatment group N. At the same time, a blank control group was set up without adding any materials to compare the treatment effects on the contaminated paddy soil.

[0071] (2) Three parallel samples were set up for potassium nitrate treatment group N, and 40 mL of deionized water was added to each of the 12 samples to create a flooded anaerobic environment, and then the samples were sealed and cultured together at room temperature for 50 days.

[0072] (3) After 50 days, the soil samples were dried, ground through a 100-grit sieve, and sealed for subsequent experiments. Some soil samples were freeze-dried at -80°C and used for microbial analysis.

[0073] Example 4: Analysis of changes in available arsenic content

[0074] In this example, available arsenic in soil was extracted and determined under three different treatment conditions: biochar, potassium nitrate, and biochar-based potassium nitrate.

[0075] According to relevant literature reports, in the comparison of significant correlations between As extracted by different extraction methods, 0.5 mol·L -1 NaHCO3 has the best correlation with soil properties and crop absorption. Weigh 1.0g of soil and add 30ml 0.5mol·L -1 The NaHCO3 extract was shaken for 4 h and then centrifuged and filtered, and the filtrate was made up to 50 ml with deionized water.

[0076] Then, without using a reducing agent, that is, using only potassium borohydride, the trivalent arsenic content was determined by atomic fluorescence. Under the condition of using a reducing agent, potassium borohydride + thiourea + ascorbic acid was used to determine the effective total arsenic, including the trivalent and pentavalent contents, and then the effective pentavalent arsenic content was obtained by subtraction.

[0077] At the same time, KH2PO4-K2HPO4 extraction method was used. Weigh 1g of soil sample and add 5mL 0.1mol·L -1 Potassium phosphate buffer, shake and centrifuge twice, the first shake for 240 minutes and then centrifuge for 10 minutes, pour the supernatant into a polyethylene bottle. Add 10mL of ultrapure water to the remaining sample in the centrifuge tube and continue shaking for 240 minutes and then centrifuge for 10 minutes, mix the supernatant with the solution in the previous step and filter; take 5mL of the filtrate and add 2mL of HNO3 and HClO4 mixed solution to digest at 180℃, cool to room temperature and add 2mL of 6mol·L -1 The HCl solution was continued to be digested, and then the inner wall of the beaker and the inner side of the watch glass were rinsed with ultrapure water. The resulting solution was transferred to a 20 mL test tube and 1 mL of 12 mol·L -1 HCl solution, shake to volume and take 3mL, add 2mL of thiourea + ascorbic acid and HCl mixture, the volume ratio of the two is 1:1, and the volume is adjusted to 15mL. After standing for 40 minutes, the arsenic concentration in the extract is determined. The results are as follows Figure 1 shown.

[0078] Changes in available arsenic content in contaminated soils with different treatments Figure 1As shown in A. It can be seen that the application of biochar-based potassium nitrate has a certain positive effect on improving soil arsenic pollution. Compared with the blank control group, the soil effective arsenic content increased from 3.01±0.03mg·kg after single biochar treatment. -1 Significantly reduced to 2.6±0.08mg·kg -1 , P < 0.05. The arsenic stabilization efficiency of soil treated with single biochar and potassium nitrate was 13.5% and 17.6%, respectively. In contrast, the addition of biochar-based potassium nitrate had a significant effect on the reduction of available As in the soil, with the available As content increasing from 3.01 ± 0.03 mg kg -1 Reduced to 2.24±0.08mg·kg -1 , the stabilization efficiency reached 26.6%, showing a synergistic effect.

[0079] Depend on Figure 1 In Figure B, it can be seen that after applying single biochar, potassium nitrate and biochar-based potassium nitrate, there are differences in the proportion of AsⅢ and AsⅤ in the effective As extracted by K2HPO4-KH2PO4 in different material treatments. The proportions of AsⅢ and AsⅤ in the blank control group CK group were 2.36% and 97.64%, respectively, the proportions of AsⅢ and AsⅤ in the biochar treatment group were 0.30% and 99.70%, respectively, and the proportions of AsⅢ and AsⅤ in the potassium nitrate treatment group were 0.1% and 99.9%. For the biochar-based potassium nitrate treatment, AsⅢ was not detected, and the proportion of AsⅤ in the effective arsenic reached 100%. This shows that the application of biochar-based potassium nitrate can promote the conversion of AsⅢ in the effective arsenic in the soil to AsⅤ, thereby promoting the fixation of arsenic.

[0080] The surface of biochar is usually negatively charged, which helps it form complexes with cations present in a specific solution. Arsenic is negatively charged or electrically neutral within the normal range. Therefore, biochar has limited or no adsorption capacity for arsenic, and may even cause the conversion of stable arsenic in the soil to active arsenic. Therefore, biochar modification is required to further improve its adsorption performance. In this study, the bioavailability of arsenic showed a decreasing trend when biochar was applied to the soil, which may be related to the biochar-mediated transformation of soil iron minerals, thereby forming an arsenic-iron mineral-organic matter ternary complex.

[0081] In fact, in the presence of nitrate, 99.8% of the highly toxic arsenic As aqueous solution will be converted into nitrate NO3 in 2 to 8 days, which is a combination of dissolved As and Fe. - The combination of reduction and oxidation of AsⅢ is an important nitrogen-arsenic coupling process commonly found in paddy fields: NO3 - It itself is converted into N2; at the same time, NO3 -It can also undergo redox reactions with divalent Fe, and the resulting Fe exists in the soil in the form of Fe or other iron mineral phases, which can also promote the adsorption and fixation of As, thereby fixing As in the form of low-toxic and stable AsV. Studies have also shown that nitrates can inhibit the reductive dissolution of iron, thereby inhibiting the release of arsenic under flooded anaerobic conditions.

[0082] Example 5: Analysis of changes in soil physical and chemical properties

[0083] In this example, the physical and chemical properties of soil under three different treatment conditions of biochar, potassium nitrate, and biochar-based potassium nitrate were measured.

[0084] The soil physical and chemical property indexes involved in the present invention include soil pH, redox potential Eh, electrical conductivity EC, dissolved organic carbon DOC, soil organic matter SOM and hydrochloric acid extractable divalent iron content.

[0085] The soil pH and Eh values ​​were measured at a soil-water ratio of 1:2.5, using a PHSJ-4F acidity meter purchased from Shanghai, China, after standing for 30 minutes; the EC was measured at a soil-water ratio of 1:5, using a DDS-11A conductivity meter purchased from Shanghai, China, after shaking for 30 minutes.

[0086] DOC was determined by extracting the soil sample with deionized water at a mass ratio of 1:10, shaking for 60 minutes and then centrifuging and filtering. The filtrate was diluted 1000 times and measured by a vario TOC element analyzer purchased from Germany.

[0087] SOM was determined by potassium dichromate-external heating method, i.e. H2SO4-K2Cr2O7. Weigh 0.15g of soil sample treated with biochar and 0.25g of other soil samples, add 5mL 0.8mol·L -1 After adding 5 mL of K2Cr2O7 standard solution and concentrated sulfuric acid, it was boiled in a water bath at 100°C for 15 min, and then 0.2 mol·L -1 Calculation by titration of FeSO4 solution.

[0088] The iron content was determined by spectrophotometry. Weigh 1.5 g of soil sample and add 20 mL of 0.5 mol·L -1HCl, shake and extract for 4 hours, then centrifuge and filter, take 1mL of filtrate, mix with 8mL 10% sodium acetate and 10mL 0.1% 1,10-phenanthroline, color for 30min, measure at 510nm with UV-visible spectrophotometer, and calculate the FeII content; in addition, take 1mL of supernatant, mix with 8mL 10% sodium acetate, 2mL 10% hydroxylamine hydrochloride and 10mL 0.1% 1,10-phenanthroline, color for 30min, measure at 510nm with UV-visible spectrophotometer, and calculate the total Fe content. The difference between total iron and divalent iron is the value of trivalent iron in hydrochloric acid extraction state.

[0089] Changes in soil physicochemical properties under different treatments Figure 2 As shown in Figure 2, the pH value of the soil increased significantly after the application of biochar, while the addition of nitrogen fertilizer and biochar-based potassium nitrate reduced the soil pH value. Figure 2 The reasons may include the following aspects: First, there are various basic groups -COO-, -O-, -OH, etc. on the surface of biochar, and it itself has alkalinity that can neutralize H in the soil environment. + At the same time, a large amount of exchangeable base ions will be released in the soil solution, resulting in an increase in soil pH. - If NO3 - If it is not absorbed by plants or microorganisms, it will cause soil acidification.

[0090] The Eh of the soil decreased significantly after applying biochar, while the addition of nitrogen fertilizer and biochar-based potassium nitrate increased the soil Eh value. Figure 2 As shown in B in the figure. According to relevant literature, soil is a complex redox system with a variety of organic and inorganic oxidized and reduced substances. Generally, free oxygen and high-valent metal ions in the soil air are oxidants, and organic matter in the soil and its decomposition products under anaerobic conditions and low-valent metals are reducing agents. The Eh of flooded rice fields is generally negative and is in a moderately reduced state. Studies have shown that biochar can change the redox process of flooded soils through complex chemical-microbial coupling effects, especially when the available soil electron donors are insufficient for reduction. It can affect the redox process through different surface functional groups, such as quinone functional groups, as electron mediators, regulating the electron distribution in the anaerobic respiration process of microorganisms. The application of nitrogen fertilizer introduces oxidizing NO3 into the soil. - , thereby increasing the soil redox potential compared with the blank control group.

[0091] The electrical conductivity of the soil decreased significantly after the application of biochar, while the addition of nitrogen fertilizer and biochar-based potassium nitrate increased the electrical conductivity of the soil. Figure 2The reason is as follows: (1) Although the addition of biochar will increase the number of freely movable positive and negative charges in the soil, biochar is almost non-conductive. Therefore, the addition of biochar will change the pore structure of the soil and the charge transfer path, and the dielectric constant of the soil-biochar complex will decrease, causing the system charge transfer process to slow down and the system to become more stable. (2) Within a certain concentration range, the soluble salt content is positively correlated with the conductivity, and the conductivity increases with the increase of salt content; with the input of nitrogen fertilizer, the application of external salt will break the inherent buffering capacity of the soil, and the soluble ions will continue to migrate and move in the soil solution, causing the soil conductivity to be on an upward trend.

[0092] The concentration of hydrochloric acid-extractable FeII in the soil increased significantly after biochar application, while the addition of nitrogen fertilizer and biochar-based potassium nitrate reduced the concentration in the soil. Figure 2 The reason is that FeII generally exists in rice soil in free form or in the form of complexes such as -NOM and -OM, which is easy to react with NO3 - A redox reaction occurs, namely nitrate-dependent oxidation (NDFO), in which microorganisms mediate a coupled process of nitrate reduction and ferrous iron oxidation, ultimately leading to the oxidation of ferrous iron to produce nitrite, nitrous oxide or nitrogen gas.

[0093] The soil SOM value increased significantly after applying biochar and biochar-based potassium nitrate, while the addition of nitrogen fertilizer had no significant effect. Figure 2 As shown in E. According to relevant studies, SOM is directly related to carbon input in the soil, and biochar contains a large amount of carbon with high bioavailability; and pyrolysis converts unstable carbon into a stubborn form that is not available for microbial degradation, an aromatic form, so it stays in the soil for a longer time.

[0094] The soil DOC value did not change significantly after applying biochar, while the addition of nitrogen fertilizer and biochar-based potassium nitrate reduced and significantly increased the soil DOC value, respectively. Figure 2 As shown in Figure F, the results show that the combined application of biochar and fertilizer increases the storage of nutrient elements and allows the soil to fully retain nutrients. Therefore, the continuous interaction between the two will increase the content of inorganic nitrogen and organic carbon in the soil, thereby allowing microorganisms to reproduce and convert a large amount of large-molecule organic carbon into small-molecule organic carbon under anaerobic conditions, thereby increasing the DOC content.

[0095] Example 6: Analysis of changes in soil available nutrients

[0096] In this example, available nutrients in soil were measured under three different treatment conditions: biochar, potassium nitrate, and biochar-based potassium nitrate.

[0097] The soil available nutrient indicators involved in the determination include soil nitrate nitrogen, ammonium nitrogen and available phosphorus content.

[0098] Among them, the determination of nitrate nitrogen adopts ultraviolet spectrophotometry, and the determination of ammonium nitrogen adopts indophenol blue colorimetry. Weigh 2g of soil sample and add 10mL 2mol·L -1 After KCl solution, shake for 60 minutes and then centrifuge and filter. The filtrate can be directly measured by UV spectrophotometer at 220nm and 275nm wavelength for nitrate nitrogen content; take another 2mL of filtrate and add 8mL of 2mol·L -1 KCl solution, 5 mL of phenol solution and 5 mL of sodium hypochlorite solution were mixed evenly and allowed to stand for 60 min, after which 1 mL of masking agent was added, and the ammonium nitrogen was determined colorimetrically at a wavelength of 625 nm.

[0099] Available phosphorus was determined by molybdenum antimony colorimetry. Weigh 1 g of soil sample and add 20 mL of 0.5 mol·L -1 NaHCO3 solution, shake for 30 minutes and then centrifuge and filter. Then take 5mL of the filtrate and add 15mL of molybdenum antimony solution. After color development for 30 minutes, perform colorimetric determination at a wavelength of 700nm. The results are as follows: Figure 3 shown.

[0100] The concentration of available phosphorus in the soil increased significantly after applying biochar, while the addition of nitrogen fertilizer and biochar-based potassium nitrate had no significant effect. Figure 3 As shown in A. This may be because biochar can adhere to and retain macronutrients through its large surface area; another possible mechanism is that the aliphatic and volatile carbon of biochar gradually decomposes and forms more carboxyl functional groups, thereby increasing the interaction between soil minerals and nutrients and maintaining the effective phosphorus content in the soil. In addition, biochar also promotes the growth of phosphate-dissolving bacteria, thereby accelerating the conversion of soil-bound phosphorus into phosphate under the action of phosphatase, phytase, organic acid, etc., thereby increasing the effective phosphorus content in the soil.

[0101] The application of biochar, nitrate and biochar-based potassium nitrate all reduced the ammonium nitrogen content in the soil, among which biochar-based potassium nitrate played a significant role in reducing the content of ammonium nitrogen in the soil. Figure 3 The reason is that (1) biochar has a large specific surface area and sufficient pore space, which is conducive to the absorption of NH4 + -N has excellent adsorption capacity. In addition, due to the oxygen atom giving NH4 +-N coordinated lone electron pair causes the shared electron pair between O and C atoms to move along the O direction, the binding energy of C=O increases, and the exchange sites provided by oxygen-containing surface functional groups can also cause electrostatic interactions, which can also cause ammonium nitrogen to be adsorbed on biochar, reducing its content in the soil. (2) According to research, there is a significant positive correlation between the relative changes in N input and output and the relative changes in fixed ammonium, and the application of external nitrogen fertilizers provides sufficient nitrogen for the soil; secondly, ammonium nitrogen can also be converted into nitrate nitrogen under aerobic conditions through the action of nitrite and nitrifying bacteria.

[0102] There was no significant change in soil nitrate nitrogen content after applying biochar, while the addition of nitrogen fertilizer and biochar-based potassium nitrate increased and significantly increased the nitrate nitrogen content, respectively. Figure 3 As shown in C. Studies have shown that the addition of exogenous carbon stimulates the activity of soil microorganisms, regulates the fixation and modification of mineral nitrogen, and thus converts easily lost inorganic nitrogen into more stable organic nitrogen, thereby improving the soil's nitrogen fixation capacity; in addition, the applied potassium nitrate slowly releases nitrogen nutrients, which also improves the sustainable supply capacity of soil nitrate nitrogen. In addition, compared with single potassium nitrate, biochar-based potassium nitrate input significantly increased the soil nitrate nitrogen content and promoted the reproduction of soil microorganisms such as nitrifying bacteria, thereby promoting the conversion of endogenous organic nitrogen in the soil to inorganic nitrogen.

[0103] Example 7: Analysis of changes in soil microbial communities

[0104] In this example, the microbial community in the soil under three different treatment conditions of biochar, potassium nitrate, and biochar-based potassium nitrate was determined.

[0105] The ELISA kit was purchased from Omega Bio-tek, Norcross, GA, USA. Soil DNA kit was used to extract DNA of bacteria in soil from 0.5 g soil sample. Primer pair 338F of the highly variable region V3-V4 of 16SrRNA gene is as shown in SEQ ID NO.1, which is: 5'-ACTCCTACGGGAGGCAGC AG-3', 806R is as shown in SEQ ID NO.1, which is: 5'-GGACTACHVGGGTWTCTAAT-3' Exosome amplification was performed by continuous denaturation and annealing cycles using ABI GeneAmp 9700PCR thermal cycler purchased from ABI, California, USA. MiSeq sequencing was performed at Majorbio Bio-Pharma Technology Co. Ltd., Shanghai, China. Ace index and Chao index were used for richness analysis of microbial communities, Shannon index, English name Shannon and Simpson index, English name Simpson were used for diversity analysis.

[0106] In Examples 4 to 6, it was speculated that the decrease in arsenic effectiveness and the changes in soil nutrients and physicochemical properties were driven by microbial processes, so the composition of the microbial communities under different treatments was further studied. Figure 4 As shown, Figure 4 A in the figure is the Ace index test result, B is the Chao index test result, C is the Shannon index test result, and D is the Simpson test result: (1) The application of biochar had no significant effect on the richness and diversity of soil bacterial communities; but it had a significant effect on diversity, increasing and decreasing the Shannon and Simpson indices, respectively. The application of biochar can significantly promote the expression of key genes related to antioxidant activity, lignin synthesis, nitrogen transport and energy metabolism, thereby regulating the diversity of microbial communities and promoting nutrient availability in rhizosphere soil. (2) The application of potassium nitrate also had no significant effect on the richness of soil bacterial communities; but it had a significant effect on diversity, decreasing the Shannon index. Studies have shown that the application of nitrogen fertilizers can cause certain ecological impacts, such as soil acidification, which can lead to the destruction of soil structure, thereby causing the extinction of many microbial species, specifically manifested as a decrease in bacterial community diversity. (3) The application of biochar-based potassium nitrate had no significant effect on the richness and diversity of soil bacterial communities.

[0107] Further analysis of the relative abundance of bacteria at the phylum and genus levels, e.g. Figure 5 As shown. Figure 5 As shown in A, in the bacterial community composition at the phylum level, the relative abundance of Proteobacteria and Actinobacteriota increased slightly after the application of biochar, from 21.21% to 22.21%, an increase of 4.71%, and from 10.17% to 11.21%, an increase of 10.23%; while the addition of nitrogen fertilizer increased from 21.21% to 41.69%, an increase of 96.56%, from 11.21% to 26.30%, an increase of 134.61%, and biochar-based potassium nitrate increased from 21.21% to 41.69%, an increase of 96.56%, and from 11.21% to 26.30%, an increase of 134.61%. 1% increased to 32.91%, an increase of 55.16%, from 11.21% to 26.62%, an increase of 137.47%, which significantly increased the content of the two; on the contrary, the relative abundance of Bacteroidota decreased slightly after the application of biochar, from 24.47% to 21.41%, a decrease of 12.51%, while the addition of nitrogen fertilizer and biochar-based potassium nitrate significantly reduced its content, from 24.47% to 0.53%, a decrease of 97.83%, from 24.47% to 0.44%, a decrease of 98.20%. In addition, the relative abundance of Firmicutes increased significantly after the application of biochar-based potassium nitrate, from 15.99% to 22.71%, an increase of 42.03%.

[0108] Microbial arsenite oxidation is catalyzed by either of two distantly related arsenite oxidases, called AIO and ARX. The arx genes encoding ARX and its regulatory proteins were originally defined in the genome of γ-proteobacteria isolated from alkaline soda lakes; the arx gene cluster has been identified in a limited number of bacteria, mainly in γ-proteobacteria. In addition, it has been confirmed that Actinobacteriota also has the ability to alleviate arsenic stress. Therefore, after applying the modifier, the increase in Proteobacteria and Actinobacteriota can fully reduce the effective arsenic content. At the same time, studies have shown that exposure to As2O3 will increase Bacteroidota and reduce Firmicutes, which is consistent with the results obtained in the present invention.

[0109] Depend on Figure 5As shown in B, in the composition of bacterial communities at the genus level, compared with the blank control group, the relative abundance of Bacillus increased slightly after the application of biochar, from 2.25% to 2.71%, an increase of 20.53%, and Lysobacter increased from 0.09% to 0.13%, an increase of 38.49%. The addition of potassium nitrate increased from 2.25% to 8.90%, an increase of 295.95%, and from 0.09% to 23.49%, an increase of 24. 600.97%; and biochar-based potassium nitrate increased from 2.25% to 19.44%, an increase of 764.03%, from 0.09% to 12.59%, an increase of 13144.06%, which significantly increased the content of the two. In addition, the application of the two increased Gaiella, from 1.55% to 1.96%, an increase of 26.45%, from 1.55% to 2.18%, an increase of 63.00%, and Luteimonas, from 1.55% to The relative abundance of Flavisolibacter decreased slightly from 17.61% to 16.71% after biochar application, a decrease of 5.11%, while the addition of nitrogen fertilizer and biochar-based potassium nitrate decreased the relative abundance of Flavisolibacter from 17.61% to 0.048%, a decrease of 99.72%, and the relative abundance of Flavisolibacter decreased from 1.55% to 5.19%, a decrease of 234.84%. The content of Bryobacter decreased from 17.61% to 0.051%, a decrease of 99.71%; the content of Symbiobacteraceae decreased from 0.012% to 0.0081%, a decrease of 33.33%; and the content of Bryobacter decreased from 17.61% to 0.048%, a decrease of 99.72%; and the content of Symbiobacteraceae decreased from 0.012% to 0.0088%, a decrease of 26.67%.

[0110] Bacillus, which belongs to the genus PGPB of plant growth-promoting bacteria, can synthesize indoleacetic acid IAA, siderophore and ACC deaminase, thereby promoting the adsorption and fixation of arsenite and reducing its toxicity; secondly, Bacillus will form a heavy metal detoxification mechanism through EPS to protect other heavy metal-sensitive microorganisms in the community. This symbiotic effect promotes the growth of microbial communities and improves the survival rate of microbial communities under heavy metal stress, thereby greatly increasing the abundance of As resistance genes and activating the systemic resistance and tolerance of plants to heavy metals. In addition, Bacillus also has the ability to oxidize AsIII. In addition, compared with the blank control group, single potassium nitrate and biochar-based potassium nitrate treatments increased the abundance of soil Paenibacillus by 139.4% and 156.0%, and Paenibacillus has the ability to drive nitrate reduction coupled to AsIII.

[0111] Lysobacter has a nearly complete type II secretion system that can secrete cell wall degrading enzymes, so this strain has good arsenic resistance. Flavisolibacter belongs to the Bacteroidota phylum, and similarly, it will decrease with the decrease of As2O3. Gaiella strains are halophilic, which may be the reason why their abundance increased after adding KNO3.

[0112] Studies have revealed network events between symbiotic bacteria and Xanthomonas strains. Two obvious network events were found at the intersection of the network, one of which was between Luteimonas and Symbiobacteraceae, indicating the flow of genetic information between them, that is, exchange and recombination. Therefore, the addition of KNO3 changed the physical and chemical properties of the soil, making the soil environment more suitable for the existence of Luteimonas. The two then carried out a series of exchange and recombination behaviors, which increased the richness of Luteimonas and reduced the richness of Symbiobacteraceae. Bryobacter is a Gram-negative, non-spore-forming rod-shaped cell that appears alone, in pairs, in short chains or in aggregates. Some members have the ability to reduce FeIII and nitrate. Therefore, after the application of KNO3, Bryobacter may participate in the reduction reaction of FeIII and nitrate, resulting in a decrease in its content.

[0113] Depend on Figure 5From the C in the figure, we can see that the application of biochar and biochar-based potassium nitrate increased the number of soil microbial species, while potassium nitrate caused the number of microbial species to decrease instead of increase. The composite treatment group had the largest number of independent OUTs, which was 680. The reasons are as follows: (1) There are many smooth pores in biochar, including micropores and mesopores, and its specific surface area is large, which can provide sufficient habitat and nutrients for microorganisms; at the same time, biochar enhances the C, N, and P components in the soil. These nutrients provide indispensable substrates for enzyme synthesis, and soil enzymes play a vital role in nutrient cycling, energy flow, and material transformation. Therefore, it also provides an excellent living environment for microorganisms, increasing the total amount of soil microorganisms. (2) The application of nitrogen fertilizer will lead to soil acidification problems, reduce the activity of soil urease, sucrase, and catalase, and induce a significant decrease in soil microbial communities.

[0114] Depend on Figure 5 It can be more intuitively seen from D that the application of biochar and potassium nitrate has a significant effect on the composition of soil microbial communities, and the distance between the potassium nitrate treatment group and the blank control group is farther than the distance between the biochar treatment group and the blank control group, so the nitrogen treatment is more significant; there is a certain degree of overlap in the combined treatment of the two.

[0115] Example 8. Correlation analysis between environmental factors and microbial communities

[0116] Depend on Figure 6 It can be seen that the results of redundancy analysis (RDA) show that the R 2 =0.676, p=0.01, R of nitrate 2 =0.9625, p=0.003, FeII R extracted with hydrochloric acid 2 =0.6829, p=0.007, R of DOC 2 =0.5289, p=0.01438 content and Eh value R 2 =0.631, p=0.016 is a key environmental factor that significantly affects soil microbial communities.

[0117] (1) With NH4 + The increase of -N will further stimulate the relative expression of As transporter genes in the soil, thereby increasing the content of available As in the soil and AsIII in pore water. +-N also provides electron donors for the reduction and dissolution of FeIII, resulting in the release of a large amount of AsIII adsorbed on it. (2) Arsenic in the soil will be adsorbed on the surface of minerals through processes such as ligand exchange and chelation. Fe oxides and hydroxides have a large surface area and many available surface binding sites, and have strong arsenic adsorption). Therefore, if the external conditions change and cause these iron minerals to dissolve and reduce, the As adsorbed on them will be released and migrate in the soil. Therefore, the Fe content is significantly positively correlated with the effectiveness of arsenic. (4) DOC will affect the migration and transformation of As in various ways: negatively charged DOC can compete with arsenate for adsorption sites on the surface of soil minerals, thereby promoting the desorption of As; at the same time, it can affect its mobility by forming a ternary complex with As; in addition, DOC can also provide nutrients for various microorganisms, including microorganisms with arsenic methylation function, thereby fully promoting arsenic methylation. (5) The increase in pH increases the negative charge on the surface of soil colloids, including clay, Fe / Mn / Al oxides / hydroxides, etc., which repel anions such as arsenite and arsenate, thereby promoting the desorption of As.

[0118] On the contrary, (1) studies have shown that SOM can inhibit the effectiveness of As through electron shuttling, competitive adsorption and complexation reactions, and under low temperature conditions, organic compounds with high thermodynamic energy will be preferentially degraded by microorganisms, thereby providing sufficient energy to maintain the storage of sediments and hindering the release and migration of AsⅢ. (2) As far as As is concerned, as the Eh value increases, the water solubility and exchangeability of its bioavailable form and the content of its organic binding form will decrease; surveys have shown that as the Eh value decreases, the effectiveness of As increases exponentially. The fitting curve shows that the effectiveness of As at -400mV increases by 3.2 times compared with 200mV. (3) According to studies, nitrates can promote the co-precipitation of arsenic with mineral oxides to fix arsenic, and arsenic oxide As(V) is considered to be immobile.

[0119] Depend on Figure 7 As shown in A, at the phylum level, the effectiveness of arsenic showed a significant positive correlation with Myxococcaceae, Symbiobacteraceae, and Flavisolibacter; and a significant negative correlation with Alcaligenaceae and Bacillus.

[0120] (1) Myxococcaceae showed sequence homology with arsenate reductase and had high activity against arsenate reductase, which would play a role in arsenate reductase and release more As. (2) Symbiobacteraceae is an anaerobic chemical organotrophic bacterium that can use nitrate as an electron acceptor or peptone alone. Studies have found that Symbiobacteraceae occupies an absolute dominant position in the microbial community that catalyzes hydrogenotrophic methane production, promoting the process of arsenic methylation. (3) Among the polluting microorganisms, Flavisolibacter is a major pathogenic fungus that often contaminates rice and produces carcinogenic toxic secondary metabolites. Therefore, it is speculated that it has a toxic effect on certain microorganisms or destroys the soil environment, causing AsⅢ to dissolve and migrate.

[0121] (4) Under As stress, Alcaligenaceae can reduce the toxicity of As to wheat seedlings and reduce the absorption of As by aboveground tissues by improving the efficiency of root energy metabolism, defense and cell wall biosynthesis. (5) In the case of arsenic pollution, plants show symptoms of phosphorus deficiency due to competition between phosphorus ions and arsenic ions. Phosphorus can play an important role in regulating physical reactions and promoting the remediation of heavy metal pollution by increasing energy metabolism, cell division and phospholipid production. Bacillus can act as a phosphate binder and increase the plant availability of phosphate as much as possible to fully utilize the efficacy of phosphorus. In addition, proline is one of the front-line defenses of plants against harsh environmental stress, especially against heavy metals; another important role of proline is metal chelation and signal transduction, so under heavy metal stress, proline can also act as a stabilizer. With the activation of Bacillus in As stress, the severely reduced proline content will return to its original state.

[0122] Depend on Figure 7 From B, we can see that at the genus level, the effectiveness of arsenic shows a significant positive correlation with Acidobacteriota and Bacteroidota, and a significant negative correlation with Actinobacteriota and Proteobacteria.

[0123] According to research, (1) Acidobacteriota is distributed in acidic, neutral and weakly alkaline environments. It can perform FeII oxidation and FeIII reduction reactions, and has the ability to oxidize minerals and synthesize new active substances, so it has a great influence on the effectiveness of arsenic. (2) Bacteroidota contains an ars cluster with arsR, encoding an arsenite response repressor protein, which contains a helix-turn-helix motif and many arsC genes encoding arsenate reductase, so it can promote the reduction of AsⅤ, thereby improving the effectiveness of arsenic.

[0124] (4) The active efflux system of Actinobacteriota plays a role in the removal of arsenic by indica rice: it takes up arsenate through phosphate transporters, converts it into arsenite through arsenate reductase, and then shunts out arsenite III; and Actinobacteriota has a strong arsenic tolerance, so it can be used to reduce the toxic effects of arsenic in the environment. (4) Most genes in the arx gene cluster of Proteobacteria are conserved in more than half of the genome. For enzymes involved in arsenic metabolism, such as AIO arsenite oxidase, the subunits encoded by the gene are used to inhibit the migration and pollution of arsenic.

[0125] Example 9. Microbial community functional abundance

[0126] In this example, the abundance of community functions in soil under three different treatment conditions of biochar, potassium nitrate, and biochar-based potassium nitrate was measured.

[0127] The functional prediction of soil microbial communities can further verify their role in nutrient cycling and immobilization. The heat map of microbial community functions predicted based on the FAPROTAX database is shown in Figure 8 As shown in the figure. After the biochar-based potassium nitrate treatment, the soil acetogenesis, dark hydrogen oxidation, sulfur respiration, methanol oxidation, aromatic compound degradation, nitrate reduction and chemoheterotrophy were better than those of the other three treatment groups. This indicates that microorganisms may reduce the effectiveness of arsenic by exerting the above effects.

[0128] Example 10: Passivation Mechanism of Biochar-based Potassium Nitrate

[0129] Since biochar-based potassium nitrate has a better effect in reducing the bioavailability of arsenic in anaerobic paddy soil, and there is a synergistic effect compared with single potassium nitrate and biochar, the present invention further analyzes the synergistic mechanism of the use of biochar-based potassium nitrate in reducing the effectiveness of arsenic in paddy soil.

[0130] From above Figure 7 The analysis showed that the available arsenic content showed a significant negative correlation with strains of Lysobacter, Bacillus, Luteimonas and Arthrobacter, which may suggest that these microorganisms fix arsenic by driving nitrate reduction coupled with ferrous oxidation / As(III) oxidation.

[0131] The results of Pearson correlation analysis are as follows Fig. 9 Soil EC value, Eh value, DOC, SOM and NO3 --N content was negatively correlated with the content of available As extracted by NaHCO3, P<0.05, indicating that these soil physical and chemical properties are conducive to improving the immobilization of As, which is consistent with the above analysis results.

[0132] In addition, we used X-ray photoelectron spectroscopy, abbreviated as XPS, to study the valence information of N and Fe in the blank control group and the biochar-based potassium nitrate treatment group. Fig.10 Middle A and Fig.10 The N1s spectrum of B in the blank control group shows two fine peaks with binding energies of 399.48 and 401.58 eV, representing NH4 + -N and organic nitrogen; in the biochar-based potassium nitrate treatment group, three fine peaks were shown, with binding energies of 399.36, 402.78 and 406.58 eV, representing NH4 + -N, organic nitrogen and NO3 - -N. And calculated from the different peak areas formed by each valence nitrogen, NH4 + -N content was reduced from 61.14% to 50.93% after biochar-based potassium nitrate treatment; NO3 - The -N content increased from 0% to 21.06%.

[0133] like Fig.10 Middle C and Fig.10 As shown in the Fe 2p spectrum of D in the middle: in the blank control group, there are 5 fine peaks, with binding energies of 710.65 and 724.59 eV representing FeII, 713.00 and 727.00 eV representing FeⅢ, and 719.00 eV representing FeII-FeⅢ mineral phase; after treatment with biochar-based potassium nitrate, the peak of FeII shifted to 710.95 and 724.58 eV, the peak of FeⅢ shifted to 714.38 and 732.58 eV, and the peak of FeII-FeⅢ mineral phase shifted to 718.58 eV. And calculated from the different peak areas formed by each valence state of iron, the content of FeII decreased from 77.43% to 66.21% after treatment with biochar-based potassium nitrate; the content of FeⅢ increased from 18.29% to 23.04%.

[0134] At the same time, the overall analysis Fig.10 , along with NO3 - -N content increased, FeII content decreased and FeⅢ content increased in the soil environment. This further confirmed the oxidation effect of nitrate nitrogen on iron. Combined with the previous report that AsⅢ content was effectively reduced after applying biochar-based potassium nitrate, it can be seen that NO3 - -N not only directly oxidizes AsⅢ to AsⅤ, but also indirectly promotes the adsorption and fixation of AsⅢ on iron minerals through iron-nitrogen conversion.

[0135] In summary, the application of biochar, potassium nitrate, and biochar-based potassium nitrate significantly reduced the content of available arsenic in the soil. The passivation efficiencies of the three were 13.5%, 17.6%, and 26.6%, respectively. The passivation effect of biochar-based potassium nitrate was the highest, and there was an obvious synergistic effect between biochar and potassium nitrate. Both biochar and potassium nitrate treatments reduced the proportion of AsⅢ in available arsenic. All AsⅢ in the available arsenic in the biochar-based potassium nitrate treatment group was converted into AsⅤ. The possible mechanism of the passivation of biochar-based potassium nitrate on arsenic-contaminated paddy soil is as follows: Fig.11 shown.

[0136] NO3 in potassium nitrate - It will directly undergo redox reaction with AsⅢ, oxidizing it to AsⅤ, thereby reducing the toxic effect of arsenic; NO3 - It will also couple with FeII oxidation to oxidize it to FeIII and form iron oxide or hydroxide minerals, which will synergize with biochar to fix AsⅤ and reduce the bioavailability of arsenic in paddy soil.

[0137] The application of biochar-based potassium nitrate significantly increased soil Eh, EC, SOM, DOC and NO3 - -N content; significantly reduced soil pH, HCl-extractable FeII and NH4 + -N content. The environmental conditions of high Eh and low pH have an inhibitory effect on the reductive dissolution of arsenic-bearing iron minerals under flooded anaerobic conditions. The reduction in the content of FeII extracted by HCl also indicates that the reductive dissolution of iron oxides is inhibited, which is beneficial to reducing the effectiveness of arsenic in the soil.

[0138] The application of biochar-based potassium nitrate directly or indirectly promotes the growth of functional microorganisms that are arsenic-resistant and can oxidize As(III) to AsV, such as strains of Lysobacter, Paenibacillus, Bacillus, Luteimonas and Arthrobacter, thereby promoting the adsorption and fixation of arsenic.

[0139] Therefore, biochar-based potassium nitrate is a cheap, easily available, efficient and environmentally friendly high-quality soil conditioner. While fixing arsenic in anaerobic paddy soil, it also improves the soil's nutrient cycling function, which is of great significance for the remediation of arsenic-contaminated soil and the synergistic improvement of soil fertility.

[0140] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0141] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A soil conditioner for rice soil with severe arsenic contamination, characterized in that: The improver consists of biochar and potassium nitrate; The mass ratio of the biochar to potassium nitrate is 1-1.4:2-2.

6.

2. The heavily arsenic-contaminated rice soil conditioner according to claim 1, characterized in that: The mass ratio of the biochar to potassium nitrate is 1:

2.

3. The heavily arsenic-contaminated rice soil conditioner according to claim 1, characterized in that: The biochar is rice straw biochar.

4. A method for treating heavily arsenic-contaminated rice soil, characterized in that: include: Adding the heavily arsenic-contaminated rice soil conditioner according to claim 1 into heavily arsenic-contaminated rice soil; The mass ratio of the heavily arsenic-contaminated rice soil conditioner to the heavily arsenic-contaminated rice soil is 3 to 4:

40.

5. The method for treating heavily arsenic-contaminated paddy soil according to claim 4, characterized in that: The mass ratio of the heavily arsenic-contaminated rice soil conditioner to the heavily arsenic-contaminated rice soil is 3.6:

40.

6. Use of the heavily arsenic-contaminated rice soil conditioner according to claim 1 in improving heavily arsenic-contaminated rice soil.

7. The use according to claim 6, characterized in that The heavily arsenic-contaminated rice soil conditioner is used to reduce soil pH and increase soil redox potential, electrical conductivity, soil organic matter and dissolved organic carbon content or more.

8. The use according to claim 6, characterized in that The heavily arsenic-contaminated rice soil conditioner is used to increase the relative abundance of functional microorganisms that are arsenic-resistant and can oxidize AsⅢ to AsⅤ.

9. The use according to claim 8, characterized in that The functional microorganisms are of the genera Lysobacter, Bacillus, Luteomonas and Arthrobacter.

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