A method for determining the content of soil iron-bound carbon isotope

Abstract

The application discloses a high-precision determination method for soil iron-bound organic carbon delta 13 C, belongs to the technical field of determination of soil carbon isotope content, and solves the problem that liquid samples are difficult to be directly determined 13 C abundance by the following steps: the liquid extraction solution is dropped into a tin boat and is subjected to freeze-drying, and the leaching solution of iron-bound organic carbon is converted into a solid form, the first method for isotope characterization of metal mineral-bound organic carbon is proposed, the method avoids the problems of unstable mass spectrum signal of a liquid sample and large morphological mixed measurement error, and accurate analysis (RSD≤1%) of delta 13 C of different morphologies is realized, and key technical support is provided for research on a soil carbon cycle mechanism.

Description

Technical Field

[0001] This invention relates to a method for determining the carbon isotope content in soil, and particularly to a method for determining the iron-bound carbon isotope content in soil, belonging to the technical field of soil carbon isotope content determination. Background Technology

[0002] Iron-bound organic carbon (Fe-OC) is a key component of the soil carbon pool, and its δ... 13 The carbon (C) value can reveal the carbon source and stability mechanism, but there is currently no dedicated method for measuring this indicator. Existing techniques make it difficult to directly measure the δ-value of liquid Fe-OC extracts. 13 C: Iron ion complexation causes fluctuations in the mass spectrometry signal, and the high-salt matrix corrodes the instrument's ion source. The diversity of Fe-OC morphologies (free / amorphous / complexed) is overlooked, and the average value of the statistical measurements masks the different δ-terminal values. 13 The C-variance (the example shows that the mixed test error is >4‰) restricts the study of soil iron-carbon coupling cycle.

[0003] 13 Technical challenges of C isotope determination;

[0004] High concentrations of iron ions (>1000 mg / L) in iron leaching solutions complex with organic carbon, leading to fluctuations in the EA-IRMS signal.

[0005] When liquid samples are directly injected, the high-temperature vaporization of salts and iron ions corrodes the ion source of the mass spectrometer, shortening the equipment's lifespan. The concentration of organic carbon in liquid iron is low, and trace amounts are present. 13 The C signal is weak, leading to 13 C abundance measurements are inaccurate, especially in applications in complex liquid environments, making it difficult to obtain high-precision data using traditional isotope determination methods.

[0006] Due to differences in chemical behavior, the statistical mean values ​​of free / amorphous / complexed Fe-OC mask the δ values ​​of different morphologies. 13 C characteristics

[0007] The contribution of dissolved organic carbon (DOC) from the reagents was not quantified, resulting in a shift in the absolute value of δ13C (error > 1.5‰ when uncorrected);

[0008] The complexity of sample pretreatment;

[0009] Liquid iron samples may contain high concentrations of iron ions, salts, and other impurities, which increases the difficulty of sample pretreatment.

[0010] Existing drying methods (such as nitrogen blowing and oven drying) induce isotope fractionation:

[0011] When the nitrogen blowing temperature is >40℃, the loss of volatile organic carbon leads to δ 13C is positively correlated with 1.2–1.8‰; oven heating (60℃) promotes the rearrangement of iron-carbon complexes, altering... 13 C / 12 C ratio.

[0012] Free (Fed), amorphous (Feo), and complexed (Fep) iron-bound carbon exhibit significant differences in chemical properties. Traditional pretreatment methods may not be able to effectively separate and purify iron-bound organic carbon and its isotopic characteristics, leading to inaccurate measurement results and δ¹⁸O in the mixed solution. 13 The C-value is distorted, so a method for determining the content of iron-bound carbon isotopes in soil is designed to solve the above problems. Summary of the Invention

[0013] The main objective of this invention is to provide a method for determining the content of iron-bound carbon isotopes in soil.

[0014] The objective of this invention can be achieved by adopting the following technical solution:

[0015] A method for determining the content of iron-bound carbon isotopes in soil includes the following steps:

[0016] Includes the following steps:

[0017] Step 1: Extracting iron from the soil;

[0018] Step 2: Solidify the liquids of the comparison leaching solution and the iron-bound leaching solution;

[0019] Step 3: Determine the isotopic composition of the control extract and the iron-bound extract;

[0020] Step 4: Calculate the isotopic differences of iron-bound carbon in this form by using the carbon content difference and isotopic difference between the control extract and the iron-bound extract:

[0021]

[0022] Where, δ 13 C f and δ 13 C CK These are iron-bound extract and control extract, respectively. 13 C value;

[0023] C f and C CK These represent the soluble organic carbon content of the two components.

[0024] Preferably, the extraction of free iron oxide bound carbon in step one employs the sodium dithionite, sodium citrate, and sodium bicarbonate method, specifically including the following steps:

[0025] Weigh 0.3-0.5g of soil sample, add 0.3mol / L sodium citrate solution and 1mol / L sodium bicarbonate solution, heat in a water bath to 80℃, and then add sodium dithionite.

[0026] Stir and maintain at 80°C for 10-20 minutes. After cooling, centrifuge to separate the extract and bring the volume of the extract to 250 mL.

[0027] Preferably, the extraction of amorphous iron oxide bound to carbon in step one is performed using the ammonium oxalate method, which specifically includes the following steps:

[0028] Weigh 0.5-1.0g of soil sample and add 0.2mol / L ammonium oxalate solution at a soil-to-liquid ratio of 1:40-1:60;

[0029] Shake in the dark for 1.5-2.5 hours, centrifuge, and then filter the supernatant for analysis.

[0030] Preferably, the extraction of complexed iron-bound carbon in step one is performed using the sodium pyrophosphate method, which specifically includes the following steps:

[0031] Weigh 1.0-2.0g of soil sample and add 0.3mol / L sodium pyrophosphate solution at a soil-to-liquid ratio of 1:15-1:25;

[0032] Shake for 2-3 hours, let stand overnight, centrifuge, and then bring the supernatant to a final volume of 100 mL.

[0033] Preferably, the freeze-drying conditions in step two are: pre-freezing temperature -30℃ to -50℃, vacuum degree ≤10Pa, and drying time 12-24h.

[0034] Preferably, in step three, the isotope determination is performed using an elemental analyzer-isotope ratio mass spectrometer combined system, with the oxidation furnace temperature at 900-1000℃ and the reduction furnace temperature at 600-700℃.

[0035] Preferably, the control extract in step four is a reagent blank solution of the same concentration as the iron-bound extract, which does not contain soil samples.

[0036] Preferably, it also includes quality control steps: setting a reagent blank for each batch of samples, relative deviation of parallel samples ≤2%, and recovery rate of standard substances 95%-105%.

[0037] Beneficial technical effects of the present invention:

[0038] The three extraction methods of this invention do not interfere with each other and can obtain pure extracts of Fed, Feo, and Fep respectively, laying the foundation for the determination of iron-bound organic carbon isotopes, eliminating cross-contamination, and solving the technical bottleneck of mixed speciation in traditional methods.

[0039] Existing technology drawbacks: Iron-bound organic carbon is mostly found in liquid extracts. Traditional isotope determination requires converting the liquid into a gas (such as CO2). However, iron ions easily form complexes with carbon, leading to unstable mass spectrometry signals. Furthermore, liquid sample introduction can easily contaminate the instrument tubing.

[0040] Advantages of this invention: By using freeze-drying technology (pre-freezing at -40℃, vacuum degree ≤10Pa) to convert liquid extract into solid salts, iron-bound organic carbon is attached to the tin boat in solid form, avoiding volatilization loss and mass spectrometry interference from liquid sample introduction;

[0041] Solid samples can be directly converted into CO2 by high-temperature oxidation (950℃) using an elemental analyzer (EA), avoiding the corrosion of the mass spectrometer ion source by iron ions and improving sample repeatability (RSD≤1%).

[0042] Overcoming the technical limitations of isotope determination in liquid samples, enabling the determination of iron-bound organic carbon by 1 3 The determination of C abundance has become possible, broadening the application scenarios of isotope technology in the analysis of soil liquid components.

[0043] For the first time, we have achieved speciation isotopic analysis of iron-bound carbon, providing key technical support for revealing the iron-carbon coupling cycle.

[0044] By measuring the δ13C values ​​of different forms of iron-bound carbon, the contribution ratio of plant-derived (C3 / C4 plants) and microbial-derived organic carbon can be distinguished, thus elucidating the role of iron oxides in carbon sink stabilization.

[0045] Application scenario innovation: Serving ecological environment management in farmland soil, through δ 13 Changes in C-values ​​can be used to assess the impact of fertilization practices on the iron-bound carbon pool; in wetland remediation, monitoring the isotopic composition of complexed iron-bound carbon can indicate the carbon pool response under pollution stress; in climate change research, the δ¹⁸O value of free iron-bound carbon... 13 C fractionation characteristics can provide quantitative evidence for the sensitivity of soil carbon pools to warming.

[0046] The innovative freeze-drying-solid sample introduction technology has broken through the technical barriers to the determination of trace organic carbon isotopes in liquid. Its standardized procedures (pre-freezing temperature, vacuum degree, and redox furnace temperature parameters) can be extended to the isotope analysis of other soil liquid components (such as aluminum-bound carbon and calcium-bound carbon), expanding the application boundaries of isotope technology in environmental geochemistry. Attached Figure Description

[0047] Figure 1 This is a flowchart of a preferred embodiment of a method for determining the content of iron-bound carbon isotopes in soil according to the present invention. Detailed Implementation

[0048] To enable those skilled in the art to understand the technical solution of the present invention more clearly, the present invention will be further described in detail below with reference to the embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto.

[0049] Extraction of three types of iron-bound organic carbon:

[0050] Analysis of free iron oxide (Fed): Determination of free iron oxide (Fed) using the sodium dithionite-sodium citrate-sodium bicarbonate (DCB) method:

[0051] First, weigh 0.3g of soil sample and put it into a 50mL centrifuge tube. Then, add 20mL of 0.3mol / L sodium citrate and 2.5mL of 1mol / L sodium bicarbonate. Heat the mixture in a water bath to 80℃ and then add 0.50g of sodium dithionite. Stir constantly and maintain the temperature at 80℃ for 15min.

[0052] After cooling, centrifuge to separate the extract. Transfer the extract to a 250 mL volumetric flask and repeat once. Finally, wash the centrifuge tube twice with 1 mol / L NaCl. Pour the supernatant into the same volumetric flask and bring the volume to 250 mL for testing.

[0053] Prepare a test tube and add NaCl solution with the same concentration as the extract to serve as the control (CK) for extracting DOC content.

[0054] Analysis of amorphous iron oxide (FeO): Extraction was performed using the ammonium oxalate method: 0.5 g of soil sample was weighed into a 50 mL centrifuge tube, and 25 mL of 0.2 mol / L ammonium oxalate solution was added at a soil-to-liquid ratio of 1:50. The sample was then placed in a double-layered cloth bag, sealed, and protected from light. After shaking for 2 hours, the sample was centrifuged at 4000 r / min. After centrifugation, the supernatant was poured into an Erlenmeyer flask, stoppered, and ready for analysis.

[0055] Prepare a test tube and add NaCl solution with the same concentration as the extract to serve as the control (CK) for extracting DOC content.

[0056] Analysis of complexed iron (Fep): Complexed iron oxide and aluminum (Fep and Alp) in soil were extracted using sodium pyrophosphate solution: 2.0 g of soil sample was weighed into an Erlenmeyer flask, and 40 mL of freshly prepared 0.3 mol / L sodium pyrophosphate solution was added at a soil-to-solution ratio of 1:20. The mixture was shaken for 2 h and then left to stand overnight.

[0057] Then centrifuge for ten minutes, transfer the supernatant to another dry Erlenmeyer flask, dilute to a fixed volume, and then test.

[0058] Prepare a test tube and add NaCl solution with the same concentration as the extract to serve as the control (CK) for extracting DOC content.

[0059] The DOC content of three solutions and the control CK treatment was determined using a TOC meter.

[0060] Sample morphology processing methods:

[0061] The liquid extract is dropped into a tin boat, and the iron-bound organic carbon extract is converted into a solid form by freeze-drying technology, which facilitates isotope determination.

[0062] During freeze-drying, iron-bound organic carbon exists in the form of precipitated salts, avoiding the limitations and interferences of isotope determination in liquid samples.

[0063] Isotope determination methods:

[0064] The interference from water-soluble organic carbon (CK control) was corrected using a formula to accurately calculate the iron-bound organic carbon. 13 C abundance:

[0065]

[0066] In the formula: δ 13 Cf and δ 13 CCK refers to the δ of iron-bound state and CK control extract, respectively. 13 C value (‰);

[0067] Cf and C CK The content of soluble organic carbon (mg·kg) in iron-bound and CK control extracts were respectively. －1 ).

[0068] Application value:

[0069] This method is the first to achieve isotopic analysis of iron-bound carbon, providing a quantitative tool for assessing carbon sink stability and offering a new pathway for the study of carbon bound to other metal minerals.

[0070] By measuring different forms of iron-bound organic carbon 13 C abundance can be used to compare the carbon sequestration capacity of iron oxides in different types of soil.

[0071] To reveal the role of iron oxides in soil organic carbon cycling, particularly their impact on the enrichment characteristics of new and old carbon, and to further utilize δ¹⁸O₂... 13 C-fingerprint traceability of organic carbon sources (C3 / C4 plant contribution ratio).

[0072] The impact of quantitative fertilization measures on the soil mineral-bound carbon pool (e.g., a 1.2‰ decrease in the δ13C value of amorphous iron in paddy fields indicates a 15% increase in new carbon sequestration rate) can guide the optimization of farming systems.

[0073] Specific implementation methods can be supplemented δ 13 Demonstration of the relationship between C value and carbon age;

[0074] Isotopes are classified into radioactive isotopes and stable isotopes. Stable carbon isotopes are further divided into... 12 C and 13 C has two natural abundances, 98.89% and 1.11%, respectively. Isotopic abundance refers to the ratio of the number of atoms of a specific isotope to the total number of atoms of the element in a mixture of isotopes. The ratio of the abundance of heavy isotopes to light isotopes of the same element is commonly expressed as the isotope ratio (R), for example... 13 C / 12 C. Light isotopes are far more abundant than heavy isotopes in nature, resulting in very small isotope ratios. For ease of use in practical applications, isotope ratios (δ values) are commonly used to represent the relative difference in percentages (‰) between the R values ​​of two isotopes in a sample and their corresponding R values ​​in a standard. The expression is: δ(‰) = (Rsample / Rstandard - 1) × 1000‰. In this formula, Rstandard varies depending on the standard, resulting in different δ values. The carbon isotope standard is PeeDee Belemnite (PDB) from the PeeDee Formation of the Cretaceous in South Carolina, USA, with an R value of 1.124 × 10-1. -4 The reserves are now exhausted. In practice, artificially prepared V-PDB standards are mostly used, with an R-value of 1.11797 × 10⁻⁶. -2 .

[0075] Different photochemical pathways (C3, C4, and CAM) result in varying degrees of C fractionation by photosynthetic carboxylases (RuBP and PEP enzymes), leading to different δ values. 13 There are differences in carbon isotope composition. For example, the difference between C3 (-22‰ to -32‰) and C4 (-9‰ to -17‰) plants is 13‰ to 15‰. Due to the differences in carbon isotope composition, the resulting ground humus and rhizosphere sediments differ. During the decomposition of plant debris by microorganisms, they tend to select lighter particles. 12 C, conversely, heavy carbon in the soil matrix ( 13 C) It becomes richer.

[0076] Therefore, vegetation with different carbon isotope compositions will significantly affect soil organic carbon δ. 13 C. Characteristics. Stable carbon isotope technology, with its advantages of small sample size, minimal disruption, and high accuracy, has become an indispensable research tool in soil organic carbon research. It can be used to quantitatively study the sources, turnover, migration, stability, and decomposition degree of soil organic matter.

[0077] Increase the scope and availability of applications, such as δ 13 The C-value is used to assess soil carbon sequestration stability. In scenarios such as permafrost, forests, and farmland, it represents the δ¹⁸O value of iron-bound carbon. 13 C differences can directly indicate the impact of human management / climate warming on carbon pool stability.

[0078] Because every plant 13 The C ranges are different; for example, the difference between C3 plants (-22‰ to -32‰) and C4 plants (-9‰ to -17‰) is 13‰ to 15‰.

[0079] Example 1: Determination of iron-bound carbon isotopes in paddy field soil;

[0080] Sample processing: Soil samples from paddy fields in the Taihu Lake basin (0-20cm depth) were collected and extracted using three different methods:

[0081] Free state: 0.4g soil sample + 20mL 0.3mol / L sodium citrate + 2.5mL 1mol / L sodium bicarbonate, add 0.5g sodium dithionite in an 80℃ water bath, maintain for 15min;

[0082] Amorphous form: 0.8g soil sample + 40mL 0.2mol / L ammonium oxalate, shaken in the dark for 2 hours;

[0083] Complexed state: 1.5g soil sample + 30mL 0.3mol / L sodium pyrophosphate, shake for 2.5h and let stand overnight.

[0084] Measurement results: Free iron-bound carbon δ 13 The C value is -25.3±0.1‰ (indicating that C3 is the main plant source), the amorphous form is -28.7±0.1‰ (fresh carbon accounts for 60%), and the complexed form is -22.1±0.1‰ (significant microbial modification characteristics).

[0085] Example 2: Evaluation of the remediation effect in red soil erosion areas;

[0086] Application scenario: Comparing soils from eroded bare land and vegetation-restored areas, the δ¹² value of amorphous iron-bound carbon in the restored area was found. 13 The C value was 2.3‰ negative compared to bare land, and its content increased threefold, confirming that vegetation restoration promotes soil carbon sequestration by enhancing the carbon fixation effect of amorphous iron (based on δ). 13 Calculations of the carbon pool renewal rate for C show an annual turnover increase of 12%.

[0087] Example 3: Quality Control Verification;

[0088] The free iron-bound carbon δ was determined by GBW07460 (soil standard material). 13 The deviation of the measured value of C from the standard value is ≤0.2‰, the relative deviation of parallel samples is 1.2%, and the recovery rate of standard substances is 98.3%, which meets the precision requirements of soil environmental monitoring.

[0089] The above description is merely a further embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope disclosed in the present invention, based on the technical solution and concept of the present invention, shall fall within the scope of protection of the present invention.

Claims

1. A method for determining the content of iron-bound carbon isotopes in soil, characterized in that: Includes the following steps: Step 1: Extracting iron in different forms from the soil; Step 2: Solidify the liquids of the comparison leaching solution and the iron-bound leaching solution; The solidification process specifically involves drying the liquid extract to transform it into solid salt residues. Step 3: Determine the isotopic composition of the control extract and the iron-bound extract; Step 4: Based on the difference in carbon content and isotopic abundance between the control extract and the iron-bound extract, calculate the isotopic characteristics of iron-bound carbon.

2. The method for determining the content of iron-bound carbon isotopes in soil according to claim 1, characterized in that: The isotopic characteristics of iron-bound carbon are calculated using the following formula: ; in, and These are iron-bound extract and control extract, respectively. 13 C value; and These represent the soluble organic carbon content of the two components.

3. The method for determining the content of iron-bound carbon isotopes in soil according to claim 1, characterized in that: In step one, the extraction of free iron oxide bound carbon is performed using the sodium dithionite, sodium citrate, and sodium bicarbonate method, which specifically includes the following steps: Weigh 0.3-0.5g of soil sample, add 0.3mol / L sodium citrate solution and 1mol / L sodium bicarbonate solution, heat in a water bath to 80℃, and then add sodium dithionite. Stir and maintain at 80°C for 10-20 minutes. After cooling, centrifuge to separate the extract and bring the volume of the extract to 250 mL.

4. The method for determining the content of iron-bound carbon isotopes in soil according to claim 1, characterized in that: In step one, the extraction of amorphous iron oxide bound to carbon is performed using the ammonium oxalate method, which specifically includes the following steps: Weigh 0.5-1.0g of soil sample and add 0.2mol / L ammonium oxalate solution at a soil-to-liquid ratio of 1:40-1:60; Shake in the dark for 1.5-2.5 hours, centrifuge, and then filter the supernatant for analysis.

5. The method for determining the content of iron-bound carbon isotopes in soil according to claim 1, characterized in that: The extraction of complexed iron-bound carbon in step one employs the sodium pyrophosphate method, which specifically includes the following steps: Weigh 1.0-2.0g of soil sample and add 0.3mol / L sodium pyrophosphate solution at a soil-to-liquid ratio of 1:15-1:25; Shake for 2-3 hours, let stand overnight, centrifuge, and then bring the supernatant to a final volume of 100 mL.

6. The method for determining the content of iron-bound carbon isotopes in soil according to claim 1, characterized in that: In step two, the freeze-drying conditions are: pre-freezing temperature -30℃ to -50℃, vacuum degree ≤10Pa, and drying time 12-24h.

7. The method for determining the content of iron-bound carbon isotopes in soil according to claim 1, characterized in that: Step 3, isotope determination, uses an elemental analyzer-isotope ratio mass spectrometer combined system, with oxidation furnace temperature of 900-1000℃ and reduction furnace temperature of 600-700℃.

8. The method for determining the content of iron-bound carbon isotopes in soil according to claim 1, characterized in that: In step four, the control extract is a reagent blank solution of the same concentration as the iron-bound extract, which does not contain soil samples.

9. The method for determining the content of iron-bound carbon isotopes in soil according to claim 1, characterized in that: It also includes quality control steps: setting reagent blanks for each batch of samples, relative deviation of parallel samples ≤2%, and recovery rate of standard substances 95%-105%.

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