A method for carbon sequestration and emission reduction regulation of paddy field soil based on ferrihydrite-organic matter complex

Abstract

The present application belongs to the technical field of soil environment regulation, and particularly relates to a paddy soil carbon fixation and emission reduction regulation method based on a ferrihydrite-organic matter complex. The ferrihydrite-organic matter complex is prepared through coprecipitation, the mineral combination degree of organic carbon in soil is improved, and the biological availability thereof is reduced, so as to alleviate the process of organic carbon decomposition and greenhouse gas emission mediated by microorganisms. Under the condition of redox fluctuation, the complex can maintain good structural stability, slow down the process of iron reduction and dissolution and the process of organic carbon release accompanying the same. In addition, the ferrihydrite as an electron acceptor can participate in the electron competition process under anaerobic conditions, so as to inhibit the production of methane to a certain extent. Therefore, the ferrihydrite-organic matter complex can realize the synergistic regulation of organic carbon stabilization and greenhouse gas emission reduction in paddy soil.

Description

Technical Field

[0001] This invention belongs to the field of soil environment regulation technology, specifically relating to a method for regulating carbon sequestration and emission reduction in paddy field soil based on a water-iron-ore-organic matter complex. Background Technology

[0002] Against the backdrop of global climate change, agricultural greenhouse gas emission reduction and soil carbon sequestration have become important directions for the green and low-carbon development of agriculture. Paddy soil, as a typical constructed wetland ecosystem, is constantly undergoing alternating periods of waterlogging reduction and drainage oxidation. Its soil carbon transformation processes are active, possessing both carbon sequestration potential and being a significant source of greenhouse gases such as methane. Under waterlogging reduction conditions, microbial anaerobic metabolism is active, easily producing large amounts of methane gas. Under drainage conditions, some organic carbon easily undergoes mineralization and decomposition, releasing carbon dioxide. Therefore, paddy soils generally face the challenge of synergistically optimizing organic carbon sequestration and greenhouse gas emissions. How to improve soil organic carbon stability under alternating redox conditions while simultaneously suppressing greenhouse gas release is a pressing technical problem to be solved in the field of low-carbon regulation of paddy fields.

[0003] Existing paddy field soil carbon sequestration and emission reduction technologies mainly include intermittent irrigation, straw return to the field, biochar application, and microbial inoculant regulation. While these technologies can reduce carbon dioxide and methane emissions and enhance soil carbon sequestration under certain conditions, they still have the following shortcomings: First, the regulation effect is unstable. For example, while intermittent irrigation can reduce methane emissions, it may increase carbon dioxide or nitrous oxide emissions under oxidizing conditions. Second, exogenous carbon inputs are easily converted into active substrates. For example, straw return to the field, while increasing soil carbon input, may provide microorganisms with easily decomposable carbon sources in the short term, thus promoting greenhouse gas release. Third, material adaptability and economy are problematic. For example, biochar performance is greatly affected by raw materials, preparation conditions, and soil environmental conditions, resulting in high costs and uncertain long-term effects. Fourth, microbial regulation methods are greatly affected by the environment, exhibiting problems such as insufficient colonization stability and complex regulation mechanisms. Therefore, there is an urgent need to develop a new carbon sequestration and emission reduction regulation method that combines high stability, low cost, and eco-friendliness.

[0004] Iron oxides are important active mineral components in paddy field soils. Among them, ferrous hydroxide, a short-range ordered iron oxide, is characterized by its large specific surface area, high surface reactivity, and strong binding capacity with organic matter, playing a crucial role in the stabilization of organic carbon. However, under the alternating redox conditions in paddy fields, free ferrous hydroxide and low-molecular-weight organic matter may still undergo mineral transformation, reductive dissolution, or rapid mineralization of organic carbon, making it difficult to maintain a stable carbon sequestration and emission reduction effect in the long term. Summary of the Invention

[0005] The purpose of this invention is to address existing problems by providing a method for regulating carbon sequestration and emission reduction in paddy field soil based on a hydroiron-organic matter complex.

[0006] This invention is achieved through the following technical solution: A method for regulating carbon sequestration and emission reduction in paddy field soil based on a ferrous ore-organic matter complex includes the following steps: Step 1: Preparation of ferrohydrate-organic complex: (1) Take iron source and organic matter according to the carbon-iron molar ratio of 1.2-1.4; (2) Mix the iron source solution with the organic matter solution and stir. Add the alkaline solution dropwise under stirring to adjust the pH to 7.0-7.5 to form a coprecipitation complex. (3) The obtained composite was aged, centrifuged, and washed until the conductivity was less than 50 μS / cm, and then freeze-dried to obtain ferrohydrate-organic composite powder. Step 2: Apply the ferrohydrate-organic complex obtained in Step 1 to the paddy soil. The amount of ferrohydrate applied is 6% of the soil mass, and the amount of organic carbon applied is controlled at 25 mg of organic carbon equivalent per 10 g of dry soil.

[0007] Further, the organic matter mentioned in step (1) is selected from at least one of humic acid (HA), fulvic acid (FA) and polygalacturonic acid (PGA).

[0008] Furthermore, the iron source mentioned in step (2) is Fe(NO3)3·9H2O, and the prepared solution is Fe(Ⅲ) solution; The concentration of the Fe(Ⅲ) solution is 0.1 mol / L.

[0009] Furthermore, the stirring speed in step (2) shall not be less than 500 rpm.

[0010] Further, the alkaline solution mentioned in step (2) is a 1 mol / L NaOH solution.

[0011] Furthermore, step (2) also includes X-ray diffraction analysis of the ferrohydrate-organic complex to detect whether there is a broad peak near 2θ=35°, in order to confirm the amorphous structure of the ferrohydrate.

[0012] Furthermore, the aging conditions described in step (3) are: aging at room temperature with stirring in the dark for 24 hours; Centrifugation conditions: 5000 rpm for 10 min; The freeze-drying conditions were -50℃ for 24 h.

[0013] Furthermore, the paddy field soil mentioned in step two is paddy field soil that has been flooded for a long time, has obvious redox cycles, and has a high greenhouse gas emission intensity.

[0014] Furthermore, the ferrous ore-organic matter complex is applied to paddy soil once or multiple times, with additional application during the critical growth period of rice, depending on the soil's physicochemical properties and carbon emission flux.

[0015] Furthermore, the method is used to achieve at least one of the following effects: (1) Increase the content of iron oxide-bound organic carbon in the soil; (2) Reduce the content of dissolved organic carbon in the soil; (3) Suppress the emission of methane and / or carbon dioxide; (4) Inhibit the abundance of iron-reducing bacteria Geobacter and / or iron-oxidizing bacteria Gallionella; (5) Inhibit the activity of carbohydrate hydrolases and / or carbon oxidases.

[0016] Furthermore, the ferrohydrate-organic complex maintains structural stability under alternating anaerobic and aerobic conditions, thus slowing down the reduction and dissolution of iron and the accompanying release of organic carbon.

[0017] Furthermore, the method utilizes ferrohydrate as an electron acceptor to participate in the electron competition process under anaerobic conditions, thereby inhibiting methane formation.

[0018] The present invention has the following advantages over the prior art: 1. This invention prepares a ferruginous hydrate-organic matter complex through co-precipitation, which improves the mineral binding degree of organic carbon in the soil and reduces its bioavailability, thereby mitigating microbial-mediated organic carbon decomposition and greenhouse gas release processes. Under fluctuating redox conditions, the complex maintains good structural stability, slowing down the iron reduction and dissolution process and its accompanying organic carbon release. Furthermore, ferruginous hydrate, as an electron acceptor, can participate in electron competition processes under anaerobic conditions, thus inhibiting methane formation to some extent. Therefore, the ferruginous hydrate-organic matter complex can achieve synergistic regulation of organic carbon stabilization and greenhouse gas emission reduction in paddy field soils.

[0019] 2. This invention achieves effective regulation of soil environmental processes by preparing and applying a ferrous ore-organic matter coprecipitation complex, which has the advantages of significant regulation effect, strong process stability and high environmental friendliness.

[0020] (1) This invention improves the mineral protection of organic carbon in soil and reduces its bioavailability through the synergistic effect of minerals and organic matter, thereby slowing down the rapid mineralization of organic carbon and the greenhouse gas release process caused by it, and has a good synergistic regulation effect of carbon sequestration and emission reduction.

[0021] (2) The composite described in this invention has good structural stability under alternating redox conditions, which can slow down the reduction and dissolution of iron minerals and the accompanying release of organic carbon to a certain extent, which is beneficial to improving the stability and sustainability of the regulation process.

[0022] (3) This invention does not introduce toxic and harmful chemical reagents. The ferrous ore and organic matter used are widely available and have good environmental compatibility. It is not easy to cause secondary pollution and is suitable for long-term regulation of carbon sequestration and emission reduction in paddy field ecosystems.

[0023] It should be noted that different types of organic matter in this invention can form complexes with ferrous ore that have differentiated regulatory characteristics. Among them, the complexes formed with humic acid as the organic matter component show better effects in regulating carbon sequestration and emission reduction in paddy field soil. Attached Figure Description

[0024] Figure 1 XRD patterns of ferrohydrate and different types of ferrohydrate-organic complexes; Figure 2 This represents the change in dissolved Fe(II) concentration under alternating anaerobic and aerobic conditions. Figure 3 The changes in the content of dissolved organic carbon (a) and iron oxide-bound organic carbon (b) in soil under anaerobic-aerobic alternating conditions; Figure 4 The changes in CH4(a) and CO2(b) emissions under anaerobic-aerobic alternation; Figure 5 This represents the copy number of the 16S rRNA gene of soil microorganisms. Figure 6 To and Gallionella (a) Geobacter (b) Copy number of relevant functional microbial genes; Figure 7 Changes in soil carbon hydrolysis / oxidation enzyme activity CBH(a) β -GC(b), POD(c), PPO(d); Figure 8 This is a technology roadmap. Detailed Implementation

[0025] To further explain the present invention, the following specific embodiments are described.

[0026] Example 1: Structural characteristics of the ferrohydrite-organic complex 1.1 Synthesis and characterization of ferrohydrate-organic complex: A coprecipitation method was used to synthesize a ferrohydrite-organic complex. A 0.1 mol / L Fe(III) solution (using Fe(NO3)3·9H2O as the iron source) and an organic carbon solution containing the corresponding organic substances (FA, PGA, HA) were prepared. The feed amounts were calculated based on the organic carbon content to ensure an initial C / Fe molar ratio of 1.4. The Fe(III) solution and organic solution were thoroughly mixed and continuously stirred at ≥500 rpm using a magnetic stirrer to promote the complexation of Fe(III) with the carboxyl and hydroxyl groups in the organic matter. Subsequently, a 1 mol / L NaOH solution was added dropwise under vigorous stirring. The pH of the reaction system was monitored and adjusted to 7.0–7.5 in real time using a pH meter to prevent localized over-alkali precipitation that could lead to the separate precipitation of Fe(OH)3, ensuring that the iron hydrolysis products and organic matter formed a homogeneous complex through coprecipitation. The mixture was aged at room temperature in the dark for 24 h with stirring to promote the growth of ferrohydrate crystal nuclei and the embedding of organic matter. The precipitate was then collected by centrifugation at 5000 rpm for 10 min and repeatedly washed with ultrapure water until the conductivity was below 50 μS / cm to remove free ions and unbound organic matter. The washed sample was freeze-dried at -50℃ for 24 h to obtain the composite powder. The C / Fe molar ratio was finally verified by measuring the total iron and total organic carbon content. Strict control of the pH range (7.0-7.5), stirring speed, and type of organic matter was required during the experiment to enhance the stability of the composite. 2-line ferrohydrate was synthesized using Fe(NO3)3 solution using the same method. X-ray diffraction (XRD) analysis of the synthesized iron-organic composite confirmed the amorphous structure of the ferrohydrate by detecting a broad peak near 2θ = 35°.

[0027] 1.2 Test soil: Paddy soil (corresponding to the Chinese soil system classification of hydragric anthrosols), formed from long-term flooded rice cultivation, was collected. Roots and gravel were removed, and the soil was ground and sieved through 2 mm and 0.149 mm (100 mesh) sieves, respectively, for later use. The pH of the tested soil was 6.76, and the organic carbon content was 36.08 g·kg⁻¹. -1 The total iron (FeT) content is 21.44 g·kg. -1 .

[0028] 1.3 Experimental treatments and culture conditions: A stable iron reduction system was constructed through a pre-culture experiment. The specific method was as follows: 10 g of air-dried soil sample was accurately weighed into a sterile vial, and 20 mL of ultrapure water was added at a soil-to-water ratio of 1:2 (w / v). The vial was sealed and incubated at 25°C in the dark for 14 days. The Fe(II) concentration in the liquid phase was monitored periodically during the incubation period until equilibrium was reached. After pre-culture, the soil moisture content was measured, and ultrapure water was added at a soil-to-water ratio of 1:2 (w / v), along with the corresponding ferrohydrate-organic matter complex. Ferrohydrate was added at 6% (w / w) of the soil mass, and the total organic carbon was uniformly controlled at 25 mg (based on 10 g of dry soil), as shown in Table 1. Eight treatment groups (n = 3) were set up: (1) paddy soil + ultrapure water (DI); (2) paddy soil + ferrous sulfate (Fh); (3) paddy soil + fulvic acid (FA); (4) paddy soil + polygalacturonic acid (PGA); (5) paddy soil + humic acid (HA); (6) paddy soil + ferrous sulfate-fulvic acid complex (Fh-FA); (7) paddy soil + ferrous sulfate-polygalacturonic acid complex (Fh-PGA); and (8) paddy soil + ferrous sulfate-humic acid complex (Fh-HA). The treatment systems were cultured under alternating anaerobic and aerobic conditions: anaerobic conditions were established by introducing high-purity nitrogen gas to replace headspace gas from 0 to 30 days; aerobic conditions were established by introducing filtered air to replace headspace gas from 30 to 40 days.

[0029] Table 1 Experimental Design ; like Figure 1 As shown, the XRD patterns of ferrihydrite (Fh) alone and different ferrihydrite-organic complexes (Fh-HA, Fh-PGA, Fh-FA) all exhibit two broad diffraction peaks near 2θ of 30-40° and 60-70°. This indicates that the co-precipitation of organic matter with different molecular weights did not alter the amorphous, weakly crystalline structure of the ferrihydrite matrix, and the complexes maintained extremely high reactivity.

[0030] Example 2: The Influence of the Water-Iron Ore-Organic Complex on the Dynamic Coupling of Iron and Carbon The experimental method was the same as in Example 1. During the experiment, samples were collected systematically at 0, 3, 6, 10, 15, 20, 25, 30 (anaerobic stage) and 35, 40 days (aerobic stage) to measure relevant indicators.

[0031] Figure 2 These are curves showing the change in dissolved Fe(II) concentration with culture time under different treatments. Figure 2It was observed that during the anaerobic stage (0-30 days), the Fe(II) concentration gradually increased in all treatments, indicating a significant reduction and dissolution phenomenon. Treatments 6 (soil + Fh-HA) and 2 (soil + Fh) showed the fastest Fe(II) formation rate and the highest Fe(II) concentration. Among the different complexes, the trend was Fh-FA > Fh-PGA > Fh-HA, and the overall Fe(II) concentration in the complex treatments was lower than that in the treatments with only the addition of ferrous sulfate. Upon transitioning to the aerobic stage (after 30 days), Fe(II) was rapidly oxidized, and the Fe(II) concentration in the solution decreased significantly.

[0032] Figure 3 This demonstrates the dynamic changes in dissolved organic carbon (DOC) and iron-bound organic carbon (Fe-OC) during cultivation. Figure 3 As shown in Figure a, treatments that only added organic matter (such as treatment 3 soil + FA) showed a significant increase in DOC content due to large-scale dissolution and release of DOC in the early stages of anaerobic digestion; while the combined treatments maintained low DOC content with gradual changes. Figure 3 As shown in b, during the anaerobic stage, the Fe-OC content in each treatment showed a decreasing trend as iron was reduced and dissolved; however, after transitioning to the aerobic stage (30-40 days), the Fe-OC content rapidly rebounded. Among them, the Fe-OC content of treatment 8 (soil + Fh-HA) was consistently significantly higher than that of other treatments throughout the entire alternating culture cycle. P < 0.05), approaching 14 g / kg at the end of the aerobic period. These results indicate that the ferrohydrate-organic matter complex has a strong ability to fix organic carbon in the soil, and the Fh-HA complex exhibits the best resistance to dissolution and long-term carbon fixation potential.

[0033] Figure 4 This represents the changes in CH4 and CO2 emissions during cultivation. For example... Figure 4 As shown in Figure a, during the anaerobic culture stage, treatment 3 (soil + FA) with only organic matter added exhibited a higher peak CH4 emission. In contrast, treatments with added ferrous sulfate and its complex (treatments 2, 6, 7, and 8) showed significantly reduced CH4 emissions. Figure 4 As shown in b, compared to other complexes, treatment 6 (soil + Fh-FA) exhibited the highest CO2 emissions, indicating a rapid degradation of organic carbon; while treatment 8 (soil + Fh-HA) showed relatively moderate CO2 emissions. This suggests that the iron-organic complex can effectively suppress methanogenesis through a competitive mechanism involving the terminal electron acceptor, and that the Fh-HA and Fh-PGA complexes can effectively balance greenhouse gas emission reduction with organic carbon sequestration.

[0034] Example 3: Effects of the ferrohydrate-organic matter complex on iron-cycling microorganisms and carbon-related enzymes The experimental method was the same as in Example 1. During the experiment, samples were systematically collected at days 0, 3, 20, and 30 (anaerobic stage) and days 35 and 40 (aerobic stage) to determine relevant indicators. Real-time quantitative PCR technology was used to analyze soil samples for... Gallionella , Geobacter Absolute quantitative analysis of 16S rRNA gene copy numbers in functional microorganisms and soil microorganisms was performed. The Bio-Rad CFX96 real-time quantitative PCR system was used, and amplification reactions were performed in 96-well plates. Each sample was replicated three times to ensure data reliability. The total reaction volume was 25 μL, containing SYBR Green premixed reagent, specific primers, and template DNA. Specific component ratios and thermal cycling parameters are detailed in Tables 2 and 3. Calibration curves were constructed using serially diluted plasmid standards containing specific clonal fragments of the target gene. The concentrations of these plasmids were accurately quantified using a Qubit 2.0 fluorometer, and the gene copy number concentration gradient was calculated based on the plasmid molecular weight and Avogadro's constant. Fluorescence signals were monitored in real-time during amplification, and the absolute copy number of the target gene in the sample was calculated by the linear relationship between the threshold cycle number (Ct value) and the standard curve.

[0035] Table 2 qPCR amplification system ; Table 3 16S rRNA, Gallionella and Geobacter Detailed information on gene primer pairs and qPCR thermal cycling parameters ; like Figure 5 , 6 As shown, different complex treatments significantly altered the abundance of iron-cycling-related microorganisms. During the anaerobic phase (3–20 days), iron-reducing bacteria… Geobacter Gene copy number increased significantly; however, during the aerobic period (30-40 days), iron-oxidizing bacteria... Gallionella The abundance of microorganisms increased significantly. Treatment 6 (soil + Fh-FA) showed higher total microbial abundance and functional gene abundance, while treatments 8 (soil + Fh-HA) and 7 (soil + Fh-PGA) showed relatively lower microbial abundance. Table 4 shows the correlation analysis results between iron cycling functional genes and soil carbon hydrolysis / oxidase activity. Table 4 shows that iron-reducing bacteria... Geobacter With carbohydrate hydrolases ( β -GC, CBH) showed a highly significant positive correlation ( p < 0.01); while iron-oxidizing bacteria Gallionella It showed a significant positive correlation with carbon oxidase (POD, PPO). p < 0.05). (Combined) Figure 7Changes in soil enzyme activity indicate that the ferrohydrate-organic matter complex inhibited the secretion of carbon-related enzymes to varying degrees. These results demonstrate, from a biological perspective, that this ferrohydrate-organic matter complex can effectively block the mineralization and loss of organic carbon by regulating the composition and enzyme activity of iron-related microbial communities, thus achieving the dual functions of soil sequestration and emission reduction.

[0036] Table 4. Correlation analysis between iron cycling functional genes and soil carbon hydrolysis / oxidation enzyme activity ; * indicates a significant correlation at the 0.05 level (two-tailed). **Indicates a significant correlation at the 0.01 level (two-tailed). The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for regulating carbon sequestration and emission reduction in paddy field soil based on a water-iron ore-organic matter complex, characterized in that, Includes the following steps: Step 1: Preparation of ferrohydrate-organic complex: (1) Take iron source and organic matter according to the carbon-iron molar ratio of 1.2-1.4; (2) Mix the iron source solution with the organic matter solution and stir. Add the alkaline solution dropwise under stirring to adjust the pH to 7.0-7.5 to form a coprecipitation complex. (3) The obtained composite was aged, centrifuged, and washed until the conductivity was less than 50 μS / cm, and then freeze-dried to obtain ferrohydrate-organic composite powder. Step 2: Apply the ferrohydrate-organic complex obtained in Step 1 to the paddy soil. The amount of ferrohydrate applied is 6% of the soil mass, and the amount of organic carbon applied is controlled at 25 mg of organic carbon equivalent per 10 g of dry soil.

2. The method for regulating carbon sequestration and emission reduction in paddy field soil based on a water-iron ore-organic matter complex according to claim 1, characterized in that, The organic matter mentioned in step (1) is selected from at least one of humic acid, fulvic acid and polygalacturonic acid.

3. The method for regulating carbon sequestration and emission reduction in paddy field soil based on a water-iron ore-organic matter complex according to claim 1, characterized in that, The iron source mentioned in step (2) is Fe(NO3)3·9H2O, and the prepared solution is Fe(Ⅲ) solution; The concentration of the Fe(Ⅲ) solution is 0.1 mol / L.

4. The method for regulating carbon sequestration and emission reduction in paddy field soil based on a water-iron ore-organic matter complex according to claim 1, characterized in that, The stirring speed in step (2) shall not be less than 500 rpm.

5. The method for regulating carbon sequestration and emission reduction in paddy field soil based on a water-iron ore-organic matter complex according to claim 1, characterized in that, The alkaline solution mentioned in step (2) is a 1 mol / L NaOH solution.

6. The method for regulating carbon sequestration and emission reduction in paddy field soil based on a water-iron ore-organic matter complex according to claim 1, characterized in that, Step (2) also includes X-ray diffraction analysis of the ferrohydrate-organic complex to detect whether there is a broad peak near 2θ=35°, in order to confirm the amorphous structure of the ferrohydrate.

7. The method for regulating carbon sequestration and emission reduction in paddy field soil based on a water-iron ore-organic matter complex according to claim 1, characterized in that, The aging conditions described in step (3) are: aging at room temperature with stirring in the dark for 24 hours; Centrifugation conditions: 5000 rpm for 10 min; The freeze-drying conditions were -50℃ for 24 h.

8. The method for regulating carbon sequestration and emission reduction in paddy field soil based on a water-iron ore-organic matter complex according to claim 1, characterized in that, The paddy soil mentioned in step two is paddy soil that has been flooded for a long time, has obvious redox cycles, and has a high greenhouse gas emission intensity.

9. The method for regulating carbon sequestration and emission reduction in paddy field soil based on a water-iron ore-organic matter complex according to claim 1, characterized in that, The ferrohydrate-organic complex is applied to paddy soil once or multiple times, with additional application during the critical growth stages of rice, depending on the soil's physicochemical properties and carbon emission flux.

10. The method for regulating carbon sequestration and emission reduction in paddy field soil based on a water-iron ore-organic matter complex according to claim 1, characterized in that, The method is used to achieve at least one of the following effects: (1) Increase the content of iron oxide-bound organic carbon in the soil; (2) Reduce the content of dissolved organic carbon in the soil; (3) Suppress the emission of methane and / or carbon dioxide; (4) Inhibit the abundance of iron-reducing bacteria Geobacter and / or iron-oxidizing bacteria Gallionella; (5) Inhibit the activity of carbohydrate hydrolases and / or carbon oxidases.

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