Method for measuring content of iron bound carbon isotope in soil

Abstract

The invention discloses a high-precision determination method for determining iron-bound organic carbon delta 13C in soil, and belongs to the technical field of determination of the content of carbon isotope in soil. A liquid extracting solution is dropped into a tin boat and freeze-dried, an extracting solution of the iron-bound organic carbon is converted into a solid form, and the content of the iron-bound organic carbon delta 13C in the soil is determined. The problem that 13C abundance of a liquid sample is difficult to directly measure is solved, the isotope characterization method of the metal mineral binding state organic carbon is provided for the first time, the problems that mass spectrum signals of the liquid sample are unstable and form mixed measurement errors are large are avoided, accurate analysis (RSD is smaller than or equal to 1%) of form-divided delta 13C is achieved, and a key technical support is provided for research of a soil carbon circulation mechanism.

Description

Technical Field

[0001] The present invention relates to a method for determining the carbon isotope content in soil, in particular to a method for determining the iron-bound carbon isotope content in soil, and belongs to the technical field of determining the carbon isotope content in soil. Background Art

[0002] Iron-bound organic carbon (Fe-OC) is a key component of soil carbon pool. 13 The C value can reveal the carbon source and stability mechanism, but there is currently no dedicated determination method for this indicator. Existing technology is difficult to directly determine the δ 13 C: Iron ion complexation causes mass spectrometry signal fluctuations, and the high salt matrix corrodes the instrument ion source. The diversity of Fe-OC forms (free / amorphous / complexed) is ignored, and the average value of the measurement masks the differences in different forms. 13 C difference (the embodiment shows that the mixed measurement error is >4‰), which restricts the study of soil iron-carbon coupled cycle.

[0003] 13 Technical challenges of C isotope determination;

[0004] The high concentration of iron ions (>1000 mg / L) in the iron extract complexed with organic carbon, resulting in fluctuations in the EA-IRMS signal.

[0005] When liquid is directly sampled, salt and iron ions will vaporize at high temperature and corrode the ion source of the mass spectrometer, shortening the life of the equipment. 13 C signal is weak, resulting in 13 C abundance measurements are inaccurate, especially in complex liquid environments, and it is difficult to obtain high-precision data using traditional isotope determination methods.

[0006] Due to the differences in chemical behavior of free / amorphous / complexed Fe-OC, the average value of the measured values ​​covers the different forms δ 13 C Features

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

[0008] The complexity of sample preparation;

[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, oven) trigger isotope fractionation:

[0011] When the nitrogen blowing temperature is greater than 40℃, the loss of volatile organic carbon will cause δ 13C is positive by 1.2 to 1.8‰; heating in an oven (60°C) causes the iron-carbon complex to rearrange, changing 13 C / 12 C ratio.

[0012] The chemical properties of free (Fed), amorphous (Feo), and complexed (Fep) iron-bound carbon are quite different. Conventional pretreatment methods may not be able to effectively separate and purify iron-bound organic carbon and its isotopic characteristics, resulting in inaccurate measurement results. 13 Therefore, a method for determining the content of iron-bound carbon isotopes in soil was designed to solve the above problem. Summary of the Invention

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

[0014] The purpose of the present invention can be achieved by adopting the following technical solutions:

[0015] A method for determining the iron-bound carbon isotope content in soil comprises the following steps:

[0016] The steps include:

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

[0018] Step 2: solidifying the liquid of the control extract and the iron-bound extract;

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

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

[0021] Among them, δ 13 C f and δ 13 C CK The iron-bound extract and the control extract were 13 C value;

[0022] C f and C CK are the soluble organic carbon contents of the two, respectively.

[0023] Preferably, in step 1, the extraction of free iron oxide-bound carbon adopts a sodium dithionite, sodium citrate, and sodium bicarbonate method, which specifically includes the following steps:

[0024] Weigh 0.3-0.5g of soil sample, add 0.3mol / L sodium citrate solution and 1mol / L sodium bicarbonate solution, heat to 80℃ in a water bath, and then add sodium dithionite;

[0025] Stir and maintain at 80°C for 10-20 minutes, cool and centrifuge, and adjust the volume of the extract to 250 mL.

[0026] Preferably, the extraction of amorphous iron oxide-bound carbon in step 1 adopts the ammonium oxalate method, which specifically comprises the following steps:

[0027] Weigh 0.5-1.0g of soil sample and add 0.2mol / L ammonium oxalate solution at a soil-liquid ratio of 1:40-1:60;

[0028] Shake in the dark for 1.5-2.5 hours, centrifuge and filter the supernatant for testing.

[0029] Preferably, the extraction of the complexed iron-bound carbon in step 1 adopts a sodium pyrophosphate method, which specifically comprises the following steps:

[0030] Weigh 1.0-2.0g of soil sample and add 0.3mol / L sodium pyrophosphate solution at a soil-liquid ratio of 1:15-1:25;

[0031] After shaking for 2-3 hours, let it stand overnight and centrifuge to make up the volume of the supernatant to 100 mL.

[0032] Preferably, the freeze-drying conditions in step 2 are a pre-freezing temperature of -30°C to -50°C, a vacuum degree ≤10Pa, and a drying time of 12-24h.

[0033] Preferably, the isotope determination in step three adopts an element analyzer-isotope ratio mass spectrometer combined system, the oxidation furnace temperature is 900-1000°C, and the reduction furnace temperature is 600-700°C.

[0034] Preferably, the control extract in step 4 is a reagent blank solution with the same concentration as the iron-bound extract, and does not contain a soil sample.

[0035] Preferably, the method further comprises a quality control step: setting a reagent blank for each batch of samples, the relative deviation of parallel samples is ≤2%, and the recovery rate of the standard substance is 95%-105%.

[0036] Beneficial technical effects of the present invention:

[0037] The three extraction methods of the present invention do not interfere with each other and can respectively obtain pure extracts of Fed, Feo and Fep, laying the foundation for the determination of iron-bound organic carbon isotopes, eliminating cross contamination, and solving the technical bottleneck of morphological mixed measurement using traditional methods.

[0038] Deficiencies of existing technology: Iron-bound organic carbon mostly exists in liquid extracts. Traditional isotope determination requires converting the liquid into gas (such as CO2), but iron ions easily form complexes with carbon, resulting in unstable mass spectrometry signals, and liquid sampling easily contaminates instrument pipelines.

[0039] Advantages of the present invention: The liquid extract is converted into solid salts through freeze-drying technology (pre-freezing at -40°C, vacuum degree ≤10Pa), so that the iron-bound organic carbon is attached to the tin boat in solid form, avoiding volatilization loss and mass spectrometry interference caused by liquid injection;

[0040] Solid samples can be directly converted into CO2 through high-temperature oxidation (950°C) in an elemental analyzer (EA), avoiding corrosion of the mass spectrometer ion source by iron ions and improving injection repeatability (RSD≤1%).

[0041] The technical limitations of isotope determination of liquid samples were solved, and the 1 3 The determination of C abundance becomes possible, broadening the application scenarios of isotope technology in the analysis of soil liquid components.

[0042] For the first time, the isotopic analysis of iron-bound carbon was achieved, providing key technical support for revealing the iron-carbon coupling cycle.

[0043] By measuring the δ13C values ​​of different forms of iron-bound carbon, the contribution ratio of plant-derived (C3 / C4 plant-derived) and microbial-derived organic carbon can be distinguished, and the mechanism of action of iron oxides in carbon sink stabilization can be elucidated.

[0044] Application scenario innovation: serving ecological environment management in farmland soil, through δ 13 The change of C value can be used to evaluate the effect of fertilization measures on the iron-bound carbon pool. In wetland restoration, monitoring the isotopic composition of complexed iron-bound carbon can indicate the carbon pool response under pollution stress. In climate change research, the δ 13 The C fractionation characteristics can provide a quantitative basis for the sensitivity of soil carbon pools to warming.

[0045] The technological paradigm innovation freeze-drying-solid-state sampling technology has broken through the technical barriers to the determination of liquid trace organic carbon isotopes. Its standardized process (pre-freezing temperature, vacuum degree, 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 The present invention is a flow chart of a preferred embodiment of a method for determining the iron-bound carbon isotope content in soil. DETAILED DESCRIPTION

[0047] In order to make the technical solution of the present invention more clear and specific to those skilled in the art, the present invention is further described in detail below with reference to embodiments and drawings, but the embodiments of the present invention are not limited thereto.

[0048] Extraction of three types of iron-bound organic carbon:

[0049] Free iron oxide (Fed) analysis: Free iron oxide (Fed) was determined using the sodium dithionite-sodium citrate-sodium bicarbonate (DCB) method:

[0050] First, weigh 0.3 g of soil sample and place it in a 50 mL centrifuge tube. Add 20 mL of 0.3 mol / L sodium citrate and 2.5 mL of 1 mol / L sodium bicarbonate successively. Heat in a water bath to 80 °C, then add 0.50 g of sodium dithionite. Stir continuously and maintain at 80 °C for 15 min.

[0051] After cooling, centrifuge and transfer the extract into a 250mL volumetric flask, repeat once, and finally wash the centrifuge tube twice with 1mol / LNaCl, pour the supernatant into the same volumetric flask, and dilute to 250ml for testing.

[0052] Simultaneously prepare a test tube and add NaCl solution with the same concentration as the extract to extract the DOC content as a CK control.

[0053] 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, 25 mL of 0.2 mol / L ammonium oxalate solution was added at a soil-liquid ratio of 1:50, and then the sample was placed in a double-layer cloth bag, sealed and shielded from light, shaken for 2 h, and then centrifuged at 4000 r / min. After centrifugation, the clear liquid was poured into a conical flask and stoppered for testing.

[0054] Simultaneously prepare a test tube and add NaCl solution with the same concentration as the extract to extract the DOC content as a CK control.

[0055] Analysis of complexed iron (Fep): Complexed iron oxide and aluminum (Fep and Alp) in the soil were extracted using sodium pyrophosphate solution: 2.0 g of soil sample was weighed into a conical flask, and 40 mL of freshly prepared 0.3 mol / L sodium pyrophosphate solution was added at a soil-liquid ratio of 1:20. The solution was shaken for 2 h and then left overnight.

[0056] Then centrifuge for ten minutes, transfer the supernatant into another dry conical flask, dilute to the fixed volume and then test.

[0057] Simultaneously prepare a test tube and add NaCl solution with the same concentration as the extract to extract the DOC content as a CK control.

[0058] The DOC contents of the three solutions and their control CK treatment were determined using a TOC meter.

[0059] Sample morphology processing method:

[0060] The liquid extract is dropped into a tin boat, and the iron-bound organic carbon extract is converted into a solid form through freeze-drying technology to facilitate isotope determination.

[0061] During the freeze-drying process, iron-bound organic carbon exists in the form of precipitated salts, avoiding the limitations and interference of isotope determination of liquid samples.

[0062] Isotope determination method:

[0063] The interference of water-soluble organic carbon (CK control) was corrected using the formula to accurately calculate the iron-bound organic carbon. 13 C abundance:

[0064]

[0065] Where: δ 13 Cf and δ 13 CCK refers to the δ of iron-bound and CK control extracts, respectively. 13 C value (‰);

[0066] Cf and C CK are the contents of soluble organic carbon in the extracts of iron-bound and CK control (mg·kg －1 ).

[0067] Application value:

[0068] This method realizes the isotopic analysis of the morphology of iron-bound carbon for the first time, providing a quantitative tool for the assessment of carbon sink stability and a new path for the study of carbon bound to other metal minerals.

[0069] By measuring the different forms of iron-bound organic carbon 13 C abundance can be used to compare the carbon sequestration capacity of different types of soil iron oxides.

[0070] Reveal the role of iron oxides in soil organic carbon cycle, especially the influence on the enrichment characteristics of new carbon and old carbon, and further use δ 13 C fingerprint tracing of organic carbon sources (C3 / C4 plant contribution ratio).

[0071] Quantify the impact of fertilization measures on the soil mineral-bound carbon pool (for example, a 1.2‰ decrease in the δ13C value of amorphous iron in rice fields indicates a 15% increase in new carbon sequestration rate) to guide the optimization of farming systems.

[0072] The specific embodiment can supplement δ 13 Demonstration of the relationship between C value and carbon age;

[0073] Isotopes are divided into radioactive isotopes and stable isotopes. 12 C and 13 There are two types of C, with natural abundances of 98.89% and 1.11% respectively. Isotopic abundance refers to the ratio of the number of atoms of a particular isotope to the total number of atoms of the element in a mixture of isotopes of an element. The ratio of the abundance of heavy isotopes to light isotopes of the same element is often expressed as the isotope ratio (R), for example 13 C / 12 C. The abundance of light isotopes in nature is much greater than that of heavy isotopes, so the isotope ratio is very small. In actual work, for ease of use, the isotope ratio (δ value) is often used to express the relative thousandth difference between the R values ​​of two isotopes in a sample relative to the corresponding R value of a certain standard. The expression is: δ(‰) = (R sample / R standard-1) × 1000‰. In the formula, the R standard varies depending on the standard, and the resulting δ value is also different. The standard material for carbon isotopes is Pee Dee Belemnite (PDB) from the Pee Dee Formation of the Cretaceous System in South Carolina, USA, and its R value is 1.124×10 -4 , which is now exhausted. In practice, artificially prepared V-PDB standards are often used, with an R value of 1.11797×10 -2 .

[0074] Different photochemical pathways (C3, C4 and CAM) have different degrees of C fractionation due to photosynthetic carboxylases (RuBPase, PEPase), resulting in δ 13 There are differences in carbon. For example, there is a 13‰ to 15‰ difference between C3 (-22‰ to -32‰) and C4 (-9‰ to -17‰) plants. Due to the differences in carbon isotope composition of plants, the ground humus and rhizosphere sediments produced are different. In the process of microbial decomposition of plant debris, they tend to choose lighter 12 C, in turn, heavy carbon in the soil matrix ( 13 C) become rich.

[0075] 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 sampling volume, minimal damage, and high accuracy, has become an indispensable tool in soil organic carbon research. Stable carbon isotope technology can be used to quantitatively study the source, turnover, migration, stability, and decomposition of soil organic matter.

[0076] Increase the application field and availability, such as 13 The C value is used to evaluate the stability of soil carbon sequestration. In frozen soil, forest, farmland and other scenes, the δ 13 C differences can directly indicate the impact of anthropogenic management / climate warming on carbon pool stability.

[0077] Because each plant 13 The C range is different. For example, the difference between C3 plants (-22‰ to -32‰) and C4 plants (-9‰ to -17‰) is 13‰ to 15‰.

[0078] Example 1: Determination of iron-bound carbon isotopes in paddy soil;

[0079] Sample preparation: Soil from 0-20 cm of paddy fields in the Taihu Lake Basin was extracted using three methods:

[0080] Free state: 0.4 g soil sample + 20 mL 0.3 mol / L sodium citrate + 2.5 mL 1 mol / L sodium bicarbonate, add 0.5 g sodium dithionite in an 80°C water bath and maintain for 15 min;

[0081] Amorphous form: 0.8 g soil sample + 40 mL 0.2 mol / L ammonium oxalate, shield from light and shake for 2 h;

[0082] Complex state: 1.5g soil sample + 30mL0.3mol / L sodium pyrophosphate, shake for 2.5h and then let it stand overnight.

[0083] Measurement results: free iron bound to carbon δ 13 The C value is -25.3±0.1‰ (indicating that it is mainly derived from C3 plants), the amorphous form is -28.7±0.1‰ (fresh carbon accounts for 60%), and the complexed state is -22.1±0.1‰ (significant microbial modification characteristics).

[0084] Example 2: Evaluation of restoration effects in red soil erosion areas;

[0085] Application scenario: Comparing the soil of eroded bare land and vegetation restoration area, it was found that the δ 13 The C value was 2.3‰ more negative than that of bare land, and its content increased by 3 times, which confirmed that vegetation restoration promoted soil carbon sequestration by enhancing the carbon sequestration of amorphous iron (based on δ 13 Calculations of the carbon pool renewal rate for C show a 12% increase in annual turnover).

[0086] Example 3: Quality Control Verification;

[0087] GBW07460 (soil standard material) was measured, and the free iron-bound carbon δ 13 The deviation between the measured C value and the standard value is ≤0.2‰, the relative deviation of the parallel samples is 1.2%, and the recovery rate of the standard substance is 98.3%, which meets the precision requirements of soil environmental monitoring.

[0088] The above is only a further embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes based on the technical solutions and concepts of the present invention within the scope disclosed by the present invention, which fall within the scope of protection of the present invention.

Claims

1. A method for determining the iron-bound carbon isotope content in soil, characterized by: The steps include: Step 1: Extract different forms of iron from the soil; Step 2: solidifying the liquid of the control extract and the iron-bound extract; Step 3: Determine the isotopic composition of the control extract and the iron-bound extract; Step 4: Calculate the isotopic signature of the iron-bound carbon species based on the carbon content and isotopic abundance differences between the control extract and the iron-bound extract.

2. The method for determining the iron-bound carbon isotope content in soil according to claim 1, wherein: Calculate the iron-bound carbon 13 The C isotope ratio is calculated using the formula: Among them, δ 13 C f and δ 13 C CK The iron-bound extract and the control extract were 13 C value; C f and C CK are the soluble organic carbon contents of the two, respectively.

3. The method for determining the iron-bound carbon isotope content in soil according to claim 1, wherein: In step 1, the extraction of free iron oxide-bound carbon is carried out using a 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 to 80℃ in a water bath, and then add sodium dithionite; Stir and maintain at 80°C for 10-20 minutes, cool and centrifuge, and adjust the volume of the extract to 250 mL.

4. The method for determining the iron-bound carbon isotope content in soil according to claim 1, wherein: The extraction of amorphous iron oxide-bound carbon in step 1 is carried out 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-liquid ratio of 1:40-1:60; Shake in the dark for 1.5-2.5 hours, centrifuge and filter the supernatant for testing.

5. The method for determining the iron-bound carbon isotope content in soil according to claim 1, wherein: The extraction of the complexed iron-bound carbon in step 1 is carried out using a 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-liquid ratio of 1:15-1:25; After shaking for 2-3 hours, let it stand overnight and centrifuge to make up the volume of the supernatant to 100 mL.

6. The method for determining the iron-bound carbon isotope content in soil according to claim 1, wherein: The freeze-drying conditions in step 2 are: pre-freezing temperature -30°C to -50°C, vacuum degree ≤10Pa, and drying time 12-24h.

7. The method for determining the iron-bound carbon isotope content in soil according to claim 1, wherein: In step 3, isotope determination is performed using an element analyzer-isotope ratio mass spectrometer system, with the oxidation furnace temperature at 900-1000°C and the reduction furnace temperature at 600-700°C.

8. The method for determining the iron-bound carbon isotope content in soil according to claim 1, wherein: The control extract in step 4 is a reagent blank solution with the same concentration as the iron-bound extract, and does not contain a soil sample.

9. The method for determining the iron-bound carbon isotope content in soil according to claim 1, wherein: Quality control steps are also included: reagent blank is set for each batch of samples, the relative deviation of parallel samples is ≤2%, and the recovery rate of standard substances is 95%-105%.

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