In a low organic carbon soil 13 Methods to enhance C-NMR signals
By combining HF solution and separation techniques, the problem of carbon concentration in low organic carbon soils was solved, high-quality NMR spectra were obtained, the signal-to-noise ratio was improved, the scanning time was shortened, and the cost was reduced.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- INST OF SOIL SCI CHINESE ACAD OF SCI
- Filing Date
- 2023-04-20
- Publication Date
- 2026-07-07
AI Technical Summary
Existing technologies are unable to effectively concentrate carbon concentration in soils with low organic carbon, resulting in low NMR spectrum quality, poor signal-to-noise ratio, long scanning time, and high cost.
Mineral residues and organic matter are separated by HF solution combined with centrifugation, low-temperature drying or freeze-drying. Mineral residues are further removed by tilting-separation-collection and reciprocating-separation-collection operations to achieve high concentration of organic carbon and enhance NMR signal.
It increased the organic carbon concentration, improved the signal-to-noise ratio, enhanced the quality of NMR spectra, shortened the scanning time, and reduced experimental costs.
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Abstract
Description
Technical Field
[0001] This invention belongs to the field of chemical detection technology, specifically relating to a method for detecting substances in low organic carbon soil. 13 Methods for enhancing C nuclear magnetic resonance signals. Background Technology
[0002] Soil organic matter is the core of soil fertility, a key indicator of soil health, and a significant source of greenhouse gases. Studying the chemical structure of soil organic matter allows us to understand carbon fixation and the stability of organic matter at the molecular level. This is crucial for a deeper understanding of the biogeochemical cycle of organic matter and its response to global climate change, as well as for achieving an efficient and sustainable green agricultural economy.
[0003] solid state 13 Nuclear magnetic resonance (NMR) technology has unique advantages in determining the chemical structure of soil organic matter, capturing its true state. Considering the complexity of soil samples, paramagnetic substances need to be removed to reduce signal interference before NMR measurements; minerals in the soil also need to be reduced to increase the carbon content. Typically, the carbon concentration of samples for NMR is at least 3%, with 10% or higher being optimal for NMR measurements, yielding high-quality spectra with a high signal-to-noise ratio. However, the organic carbon content in Chinese farmland soils, except for some highly fertile black soils, rarely exceeds 10%. Therefore, appropriate pretreatment of soil samples is necessary to concentrate the carbon content. Commonly used chemical reagents include hydrofluoric acid, citric acid, stannous oxide, and sodium bisulfite.
[0004] Currently, the recommended method is the one described by Skjemstad et al. in their article "The removal of magnetic-materials from surface soils - a solid-state 13C CP / MAS NMR study" (Australian Journal of Soil Research. 1994, 32(6): 1215-1229). This method is characterized by using a 2% hydrofluoric acid (HF) solution and continuous extraction (5 shakes for 1 hour, 3 shakes for 16 hours, and 1 shake for 64 hours), which yields relatively good results. 13C NMR spectra. However, the soil organic carbon content used in this method is generally high (2.8%-13.5%), far exceeding the average organic carbon content of 1.43% in Chinese farmland soils. For farmland soils with low organic carbon content, soils treated with similar methods still contain a considerable amount of minerals, resulting in limited organic carbon concentration and low enrichment of carbon content, thus significantly increasing the sample scanning time (exceeding 10 hours); the NMR spectrum quality is often relatively coarse, with low NMR signal resolution and peak broadening, making it difficult to distinguish and quantify carbon chemical structures. The extended NMR time also increases experimental costs.
[0005] Another common method to improve NMR spectra is to increase the concentration of HF. In their article "The effect of 10% HF treatment on the resolution of CPMAS 13C NMR spectra and on the quality of organic matter in Ferralsols" (Geoderma, 2003, 116(3): 373-392), the authors recommended using 10% HF for sample treatment. This method yields higher quality NMR spectra when the soil sample carbon concentration is high. However, when the sample carbon concentration is low (0.47%), even after eight cycles of 10% HF treatment, the carbon concentration can only be concentrated to 1.47%, resulting in a broad spectrum with overlapping peaks.
[0006] According to the "Soil Basic Nutrient Data Set for Soil Testing and Fertilizer Recommendation" (2005-2014) and data from the Second National Soil Survey, the average organic carbon content of topsoil in my country is only 1.43% (Yang Fan, Xu Yang, Cui Yong, Meng Yuanduo, Dong Yan, Li Rong, Ma Yibing, 2017. Changes in Organic Matter Content of Topsoil in Chinese Farmland over the Past 30 Years. Acta Pedologica Sinica 54, 1047-1056.). In North China, there are numerous alluvial soils, brown soils, and alluvial-brown soils, which have high pH values and are alkaline, with particularly low organic carbon content, averaging only 0.82% (Li Jinquan, Li Zhaolei, Jiang Guofu, Cheng Hao, Fang Changming, 2016. Current Status and Controlling Factors of Organic Carbon in Topsoil of Chinese Farmland. Fudan Journal (Natural Science Edition) 55, 247-256+266.). In addition, my country has as much as 1.17 billion mu of marginal land, including various types of land such as soda saline-alkali soil in Northeast China, coastal saline-alkali soil, inland saline-alkali soil in Northwest China, and red soil in the mountains and hills of Southern China. Among them, 850 million mu can be used as undeveloped reserve resources for arable land (Cao Xiaofeng, Sun Bo, Chen Huabang, Zhou Jianmin, Song Xianwei, Liu Xiaojing, Deng Xiangdong, Li Xiujun, Zhao Yuguo, Zhang Jiabao, Li Jiayang, 2021. Pathways and research progress for increasing productivity and improving ecological benefits of marginal land in my country. Bulletin of the Chinese Academy of Sciences 36, 336-348.). Marginal land generally suffers from low soil organic carbon content. How to accurately obtain information on this type of soil with low organic carbon content is a key issue. 13 Using existing methods, it is obviously very difficult to obtain C NMR spectra. Increasing the concentration of HF or increasing the number of extractions will also face the following problems: (1) the carbon recovery rate will decrease, further aggravating the loss of soluble components (carbohydrates and amino acids, etc.), thereby leading to the distortion of the soil organic carbon chemical structure; (2) minerals cannot be removed more effectively, and the carbon enrichment degree is difficult to continue to improve, thereby preventing the NMR spectra from being improved at the root. Summary of the Invention
[0007] For soils with low organic carbon content, existing methods cannot effectively concentrate carbon concentration, and even with increased processing time, it is still difficult to obtain high-quality NMR spectra. The technical problem this invention aims to solve is to provide a method for obtaining high-quality NMR spectra in low organic carbon soils. 13 A method for enhancing C nuclear magnetic resonance signals can achieve high concentration of organic carbon, thereby increasing the intensity of NMR signals, increasing the signal-to-noise ratio, improving spectral quality, and shortening the processing time.
[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:
[0009] In a low organic carbon soil 13The method for enhancing the NMR signal involves using HF solution to remove paramagnetic substances and some minerals from soil samples, and washing away excess acid with water. Mineral residues and organic matter are separated into layers through centrifugation combined with low-temperature drying or freeze-drying. Finally, mineral residues are further removed through a "tilting-separation-collection" and "reciprocating-separation-collection" process, achieving a high concentration of organic carbon. This increases the NMR signal intensity, improves the signal-to-noise ratio, enhances spectral quality, and shortens processing time.
[0010] The low organic carbon soil 13 The method for enhancing C nuclear magnetic resonance signals is used in soils with low organic carbon content (less than 1%), with soil sample size of 8–10 g.
[0011] The low organic carbon soil 13 To enhance the C-NMR signal, excess acid in the sample is washed away with deionized water: add 40 ml of deionized water, shake at 200 rpm for 10 min, then centrifuge at 4000 rpm (2810 g) for 10 min, and discard the centrifuged liquid; repeat the above operation twice. Centrifugation before low-temperature drying or freeze-drying is an important step, utilizing the differences in physical properties between mineral residues and organic matter in the sample to achieve preliminary stratification, facilitating further separation.
[0012] The low organic carbon soil 13 The method for enhancing C-NMR signals involves preparing large weighing paper after centrifugation. The centrifuge tube is tilted 25°-35° with the opening facing down, and slowly moved in the same direction. The residue and organic matter samples move downwards, appearing as a regular planar distribution on the weighing paper, completing the first step of coarse separation between the mineral residue and organic matter samples. Simultaneously, a thin layer of adsorbed white minerals will appear at the bottom of the tube; this should be discarded. The white sample, mainly composed of mineral residue, and the darker-colored organic matter sample are collected separately.
[0013] The low organic carbon soil 13 The method for enhancing C-NMR signals involves the first step of a "tilt-separation-collection" operation: Transfer the coarsely separated mineral residue or organic matter sample to weighing paper A, tilting it at a 25°-35° angle. The sample falls onto weighing paper B, which is placed flat on the experimental table. Simultaneously, some white sample in a planar distribution will be adsorbed onto weighing paper A, which can be discarded. Collect the remaining sample on weighing paper B, completing one "tilt-separation-collection" operation. Repeat this "tilt-separation-collection" operation 3-4 times for the remaining sample on weighing paper B.
[0014] The second step, "reciprocating-separating-collecting" operation, involves the following: The remaining sample still contains residue and organic matter. Press the weighing paper with your index and middle fingers and make regular reciprocating movements. The mineral residue and organic matter will appear as strips on the weighing paper. Use a weighing spoon to separate the white residue and collect the remaining sample, completing one "reciprocating-separating-collecting" operation. Repeat the above "reciprocating-separating-collecting" operation 5 to 6 times on the remaining sample.
[0015] The organic matter sample was ground and passed through a 100-mesh sieve to obtain the NMR sample.
[0016] The low organic carbon soil 13 For methods to enhance C-NMR signals, for calcareous soils, it is necessary to first remove calcium from the soil with hydrochloric acid (HCl); for acidic soils, this step is not required.
[0017] The low organic carbon soil 13 The method for enhancing the C-NMR signal involves removing calcium from the soil using hydrochloric acid (HCl). The specific procedure is as follows: the soil sample is passed through a 2 mm sieve, and 50 ml of 0.1 M HCl is added to a 100 ml plastic centrifuge tube. The sample is shaken at 200 rpm for 0.5 h, then centrifuged at 4000 rpm (2810 g) for 10 min. The centrifuged liquid is discarded. The above method is repeated by adding HCl and shaking 1 to 2 times, and the centrifuge tube is observed until no bubbles are generated.
[0018] The low organic carbon soil 13 The method for enhancing C nuclear magnetic resonance signals involves the following steps for HF solution treatment: Add 70 mL of 2% HF and shake at 220 rpm and 25°C for 1 hour; after shaking, centrifuge at 4000 rpm (2810 g) for 20 minutes and discard the centrifuged liquid; repeat the above operation 4 times, then add 2% HF solution again, and set the shaking conditions to 3 shakes for 16 hours and 1 shake for 64 hours in sequence; the main purpose of this step is to remove paramagnetic substances and minerals from the soil sample.
[0019] The low organic carbon soil 13 To enhance the nuclear magnetic resonance signal, the sample in the centrifuge tube is dried at 40–60°C. During the operation, the centrifuge tube must be kept vertical and not tilted or laid flat. Do not knock the centrifuge tube during the freeze-drying process to avoid disrupting the layering pattern of the sample. After drying, the centrifuge tube should also be handled gently and not shaken.
[0020] After low-temperature drying or direct freeze-drying, soil samples in centrifuge tubes will exhibit two layers; due to gravity separation, the bottom layer consists of denser minerals (white), while the surface layer contains concentrated organic matter (darker). The key point of this invention is to separate the mineral residue at the bottom and retain the organic matter sample on the surface, rather than directly grinding and sieving the entire dried or freeze-dried sample together for NMR determination.
[0021] The low organic carbon soil 13 Methods for enhancing C-NMR signals in soil samples 13 C NMR measurements: Detected on a Bruker Avance 400 nuclear magnetic resonance spectrometer using a solid-state dual resonance probe and a 4mm rotor; 13 The detection resonant frequency of C is 100MHz; the magic angle spin frequency is 14kHz, the cycle delay time is 0.35s, and the 90°... 13 The C pulse width is 4μs.
[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:
[0023] (1) In this invention, the amount of soil sample used in the test should not be too small. It should be increased from 2-5g in the conventional method to 8-10g, so as to facilitate the subsequent sample separation and concentration.
[0024] (2) Centrifugation before low-temperature drying or freeze-drying is an important step. It utilizes the differences in physical properties of mineral residues and organic matter in the sample to achieve preliminary stratification, which facilitates further separation.
[0025] (3) The “tilt-separation-collection” and “reciprocating-separation-collection” operations after low-temperature drying or freeze-drying are key, which can significantly increase the carbon concentration in the sample and thus obtain high-quality NMR spectra. Attached Figure Description
[0026] Figure 1 This is a process flow diagram of the present invention;
[0027] Figure 2 This is an image of the sample collected after the first step of coarse separation in Example 1;
[0028] Figure 3 An image showing the appearance of the white residue sample on weighing paper A used in the first step of fine separation;
[0029] Figure 4 The image shows the appearance of the sample on the weighing paper after the second reciprocating operation of fine separation.
[0030] Figure 5 The image shows the appearance of the sample collected after separation by a weighing spoon in the second step of fine separation.
[0031] Figure 6This is an image of the sample used in the NMR instrument.
[0032] Figure 7 Soil samples measured using the method of this invention and conventional methods 13 C NMR spectrum. Detailed Implementation
[0033] The present invention will be further described below with reference to specific embodiments.
[0034] Reagents required for this invention:
[0035] Hydrochloric acid (0.1 mol·L) -1 HCl solution: Measure commercially available concentrated hydrochloric acid (mass fraction 37%, density 1.179 g / cm³). -3 Dissolve 8.37 ml in 1 L of deionized water to prepare a 0.1 mol·L⁻¹ solution. -1 HCl solution.
[0036] Hydrofluoric acid (2% HF) solution: Dissolve 50 ml of commercially available hydrofluoric acid (40% by mass) in 1 L of deionized water to prepare a 2% HF solution.
[0037] Required equipment:
[0038] Horizontal constant temperature shaking incubator (HZ-9310KB, Taicang Hualida Experimental Equipment Co., Ltd.), centrifuge (TD25-WS, Changsha Xiangzhi Centrifuge Instrument Co., Ltd.), electric constant temperature drying oven (DGG-9140B, Shanghai Senxin Experimental Instrument Co., Ltd.), electronic balance (ME204, Mettler Toledo Instruments (Shanghai) Co., Ltd.), elemental analyzer (Elementar Vario EL III, Elementar GmbH, Germany), nuclear magnetic resonance spectrometer (Bruker Avance 400, Bruker GmbH, Germany).
[0039] Example 1
[0040] The tested soil was typical alluvial soil from the North my country Plain, planted with wheat and corn. The soil was alkaline with a pH of 8.65 and low organic carbon content, averaging 0.48%. The following describes the process for highly concentrating the organic carbon in this soil; the process flow diagram is shown below. Figure 1 As shown, the specific steps are as follows:
[0041] (1) Add 50 ml of 0.1 mol·L⁻¹ soil to 8 g of soil (passed through a 2 mm sieve). -1 Add HCl to a 100ml plastic centrifuge tube, shake at 200rpm for 0.5h, centrifuge at 4000rpm (2810g) for 10min, and discard the centrifuged liquid; repeat the above process of adding HCl and shaking twice.
[0042] (2) Add 70 ml of 2% HF and shake at 220 rpm and 25 ℃ for 1 h. After shaking, centrifuge at 4000 rpm (2810 g) for 20 min and discard the centrifuged liquid. Repeat the above operation 4 times. Then add 2% HF solution and set the shaking conditions to 16 h (3 times) and 64 h (1 time) respectively. The main purpose of this step is to remove paramagnetic substances and minerals from the soil sample.
[0043] (3) Wash away excess acid in the sample with deionized water: Add 40 ml of deionized water, shake at 200 rpm for 10 min, then centrifuge at 4000 rpm (2810 g) for 10 min and discard the centrifuged liquid: Add 40 ml of deionized water and repeat the above operation twice.
[0044] (4) Dry the sample in the centrifuge tube at a low temperature (60℃), keeping the centrifuge tube vertical during the operation; after drying, handle the centrifuge tube gently without shaking; calculate the weight m of the sample at this time by weighing. 常规 (Represents the sample weight obtained by conventional methods);
[0045] (5) Prepare large-sized weighing paper. Tilt the centrifuge tube at approximately 30° with the opening facing downwards. Move it slowly in the same direction. The residue and organic matter samples will both move downwards, appearing in a regular planar distribution on the weighing paper. This completes the first step of coarse separation of mineral residue and organic matter samples. Figure 2 As shown; simultaneously, a thin layer of adsorbed white minerals is present at the bottom of the tube, which is collected separately (in actual operation, this part of the white minerals is not needed and can be discarded directly; here, in order to compare the effects of conventional methods and the method of this invention, it is necessary to collect it separately for subsequent measurements and other operations); collect the white sample mainly composed of mineral residues and the blackish organic sample separately, such as Figure 2 As shown, the results are presented after organic matter and slag are collected. The sample in the lower left corner of the image is mainly black organic matter, while the samples in the middle and upper right corners of the image are mainly white slag.
[0046] (6) Next, fine separation is carried out in two steps: First, the "tilt-separate-collect" operation is performed to separate the samples mainly composed of organic matter (hereinafter referred to as "organic matter samples") or the samples mainly composed of mineral residues (hereinafter referred to as "residue samples"). The organic matter samples or residue samples are transferred to weighing paper A, which is tilted at about 30°, allowing the samples to fall onto weighing paper B placed flat on the experimental table under the action of gravity. At the same time, some samples will be adsorbed on weighing paper A, which will be distributed in a white surface (e.g., Figure 3 As shown), this part of the residue is collected at the mineral residue; collect the remaining sample on weighing paper B, and repeat the above "tilt-separate-collect" operation 3 times on the remaining sample on weighing paper B.
[0047] The second step, "reciprocating-separating-collecting," involves the following: The remaining sample still contains residue and organic matter. Using your index and middle fingers, press the weighing paper and make regular reciprocating movements. Due to the different densities of the residue and organic matter, they will appear as banded patterns on the weighing paper (e.g., ...). Figure 4 (As shown); use a weighing spoon to separate the white residue, collect the remaining sample, and the collection effect is as follows. Figure 5 As shown. By Figure 5 As can be seen, the left side of the figure is the organic matter sample, and the right side is the white residue sample. It can be seen that there is still white residue in the organic matter sample on the left, and there is also a small amount of black organic matter in the white residue on the right. Therefore, it is necessary to repeat the separation-collection operation steps. The remaining sample is subjected to the above "reciprocating-separation-collection" operation 5 times.
[0048] The organic matter sample was ground and passed through a 100-mesh sieve to obtain the NMR sample (e.g., Figure 6 (As shown), and at the same time, the separated mineral residue samples were collected and summarized.
[0049] The target organic matter sample and mineral residue sample were weighed separately, and the carbon and nitrogen contents were determined using an elemental analyzer. The organic carbon (C) and nitrogen (N) contents in the sample obtained by conventional methods (i.e., the sample washed and dried at low temperature) were calculated using the following formulas:
[0050] C 常规 =(C 残渣 ×m 残渣 +C 有机质 ×m 有机质 ) / m 常规 ;
[0051] N 常规 =(N 残渣 ×m 残渣 +N 有机质 ×m 有机质 ) / m 常规 ;
[0052] In the formula, the subscript "general" indicates that the sample is dried at low temperature after washing with water using conventional methods.
[0053] The enrichment factor is used to assess the degree of carbon concentration. The enrichment factor is the ratio of the organic carbon content after sample treatment to the content before treatment.
[0054] Table 1 Comparison of organic carbon content and enrichment coefficient of samples treated by conventional methods and the method of this invention.
[0055]
[0056] The organic carbon content and enrichment coefficient of the samples were obtained after three parallel experiments and conventional treatment. As shown in Table 1, the organic carbon enrichment coefficient of the sample treated with conventional methods was only 2.91. With the present invention, the organic carbon content was increased from 1.38% to 4.23%, the enrichment coefficient was increased to 8.89, and the carbon content increased by 205.5%.
[0057] The formula for calculating carbon recovery rate is as follows:
[0058] %C recovery rate = (C 处理后 ×m 处理后 ) / (C 处理前 ×m 处理前 )×100;
[0059] The carbon recovery rate using conventional methods is 65.8%, while the carbon recovery rate of this invention is 58.5%; this recovery rate is significantly higher than... Low organic carbon samples (<30%) treated with 10% HF.
[0060] The formula for determining whether the organic matter composition of a sample changes after treatment is as follows: (The formula is not provided in the original text.)
[0061] R = (C 处理前 / N 处理前 ) / (C 处理后 / N 处理后 );
[0062] Calculations showed that the average R value after treatment using the method of this invention was 1.24, while the R value of the conventional method was 1.20. Compared with the conventional method, the organic matter composition after treatment by this invention did not change significantly.
[0063] (7) Sample 13 C NMR measurements were performed on a Bruker Avance 400 NMR spectrometer using a solid-state dual-resonance probe and a 4mm rotor. 13 The detection resonant frequency of C is 100MHz. The magic angle spin frequency is 14kHz, the cycle delay time is 0.35s, and the 90°... 13 The C-pulse width is 4 μs. The sample was scanned 16384 times.
[0064] Processed using TopSpin 3.2 software. 13 C NMR spectra, test results of this invention and conventional methods are as follows: Figure 7 As shown. By Figure 7 It can be seen that the NMR signal has high resolution, high signal-to-noise ratio, and high spectrum quality, indicating that the method is effective; conventional methods require far more scans than this invention, yet still cannot obtain a spectrum of the same quality.
Claims
1. In a low organic carbon soil 13 A method for enhancing C-NMR signals, characterized in that... HF solution was used to remove paramagnetic substances and some mineral residues from soil samples, and excess acid was washed away with water. Mineral residue samples and organic matter samples were separated into layers by centrifugation combined with low-temperature drying or freeze-drying. Finally, mineral residues were further removed using weighing paper through a "tilting-separation-collection" and "reciprocating-separation-collection" process. The organic matter samples were collected and processed. 13 C NMR analysis showed that the organic carbon content of the low-organic-carbon soil was less than 1%. After centrifugation, prepare large weighing paper. Tilt the centrifuge tube at 25°~35° with the opening facing down and move it slowly in the same direction. The residue and organic matter will move downwards and appear in a regular planar distribution on the weighing paper, completing the first step of coarse separation of mineral residue and organic matter samples. At the same time, there will be a thin layer of adsorbed white minerals at the bottom of the tube, which should be discarded directly. Collect the white sample, which is mainly composed of mineral residue, and the blackish organic matter sample separately. Step 1, "Tilting-Separation-Collection": Transfer the coarsely separated mineral residue or organic matter sample to weighing paper A. Tilt weighing paper A at 25°-35°, allowing the sample to fall onto weighing paper B, which is placed flat on the experimental table. At the same time, some white sample in a planar distribution will be adsorbed onto weighing paper A, which can be discarded directly. Collect the remaining sample on weighing paper B, completing one "Tilting-Separation-Collection" operation. Repeat the "Tilting-Separation-Collection" operation 3-4 times for the remaining sample on weighing paper B. The second step, "reciprocating-separating-collecting" operation: Press the weighing paper with your index and middle fingers and make a regular reciprocating motion. The mineral residue sample and the organic matter sample will appear in strips on the weighing paper. Use a weighing spoon to separate the white residue part and collect the remaining sample to complete one "reciprocating-separating-collecting" operation. Repeat the above "reciprocating-separating-collecting" operation 5 to 6 times for the remaining sample. The organic matter sample was ground and passed through a 100-mesh sieve to obtain the NMR sample.
2. In the low organic carbon soil according to claim 1 13 A method for enhancing C-NMR signals, characterized in that... The amount of soil sample used is 8-10 g.
3. In the low organic carbon soil according to claim 1 13 A method for enhancing C-NMR signals, characterized in that... Wash away excess acid from the sample with deionized water: add 40 ml of deionized water, shake at 200 rpm for 10 min, then centrifuge at 4000 rpm for 10 min and discard the centrifuged liquid; repeat the above operation twice.
4. In the low organic carbon soil according to claim 1 13 A method for enhancing C-NMR signals, characterized in that... For calcareous soils, hydrochloric acid must be used to remove calcium from the soil first; this step is not necessary for acidic soils.
5. In the low organic carbon soil according to claim 4 13 A method for enhancing C-NMR signals, characterized in that... To remove calcium from soil using hydrochloric acid, the specific procedure is as follows: pass the soil sample through a 2 mm sieve, add 50 ml of 0.1 M HCl to a 100 ml plastic centrifuge tube, shake at 200 rpm for 0.5 h, centrifuge at 4000 rpm for 10 min, and discard the centrifuged liquid; repeat the above process of adding HCl and shaking 1-2 times, and observe the centrifuge tube until no more bubbles are generated.
6. In the low organic carbon soil according to claim 1 13 A method for enhancing C-NMR signals, characterized in that... The specific procedure for HF solution treatment is as follows: add 70 ml of 2% HF and shake at 220 rpm and 25°C for 1 h; after shaking, centrifuge at 4000 rpm for 20 min and discard the centrifuged liquid; repeat the above operation 4 times, and then continue to add 2% HF solution, setting the shaking conditions to 3 shakes for 16 h and 1 shake for 64 h respectively.
7. In the low organic carbon soil according to claim 1 13 A method for enhancing C-NMR signals, characterized in that... The temperature for low-temperature drying is 40~60°C.
8. In the low organic carbon soil according to claim 1 13 A method for enhancing C-NMR signals, characterized in that... Soil samples 13 C NMR measurements: Detected on a Bruker Avance 400 nuclear magnetic resonance spectrometer using a solid-state dual resonance probe and a 4mm rotor; 13 The detection resonant frequency of C is 100 MHz; the magic angle spin frequency is 14 kHz; the cycle delay time is 0.35 s; and the 90°... 13 The C pulse width is 4 μs.