Determination method for inorganic carbon delta13C in karst soil

Abstract

The invention provides a method for determining inorganic carbon delta 13C in karst soil, and belongs to the technical field of inorganic carbon determination, the method comprises the following steps: step 1, sample reaction under a closed condition, including experimental material preparation, closed device installation, air tightness inspection and soil sample treatment; step 2, gas collection and determination: performing exhaust and gas extraction treatment on a closed device, and determining delta13C-CO2; and step 3, carrying out inorganic carbon delta 13C calculation on the extracted gas by utilizing a keelling plot model. The device provided by the invention is simple and convenient in arrangement, low in cost and simple in detection process, and the limitation of real-time online measurement of inorganic carbon delta 13C by virtue of a vacuum system and expensive equipment in the past is reduced. The Keeling Plot model is utilized to eliminate the influence of other CO2 sources with complex components, and stable and accurate inorganic carbon delta 13C determination is realized; by using the keeling plot model, the overall trend of the data can be well captured, and abnormal values are ignored, so that the defect of high distribution heterogeneity of the Karst inorganic carbon can be overcome.

Description

Technical Field

[0001] The present invention relates to the technical field of inorganic carbon determination, in particular to a method for determining inorganic carbon δ 13 Method for determination of C. Background Art

[0002] Karst soils develop from carbonate rocks, and the lime soils formed are rich in inorganic carbon, mainly in the form of carbonates. In the past, CO2 release from karst soils was traditionally defined as the release of organic carbon mineralization, while inorganic carbon release was ignored. In fact, the contribution of inorganic carbon to soil CO2 emissions is higher than expected, even exceeding that of organic carbon. Therefore, distinguishing the ratio of organic carbon mineralization and inorganic carbon release in karst soils is a key step in exploring karst carbon sinks. 13 The difference in soil organic carbon δ 13 The determination method of soil inorganic carbon δ 13 There are relatively limited methods for determining C. Usually, soil reacts with concentrated phosphoric acid in a vacuum system to generate trace amounts of CO2, which is then connected to an isotope mass spectrometer (IRMS) for real-time online determination. Due to the high cost, complex operation, and expensive equipment of this method, it is necessary to develop a simpler inorganic carbon δ 13 C determination method to meet the actual application needs. Summary of the invention

[0003] The present invention aims to provide a kind of karst soil inorganic carbon delta 13 C determination method to solve the existing karst soil inorganic carbon δ 13 The determination method of C is complex and costly. 13 C Technical problem of large calculation error.

[0004] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0005] A kind of inorganic carbon delta in karst soil 13 A method for determining C, the method comprising the following steps:

[0006] Step 1: Sample reaction under closed conditions, including preparation of experimental materials, installation of closed device, checking of airtightness and soil sample processing;

[0007] Step 2: Gas collection and measurement, including exhaust and extraction of the closed device, δ 13 C-CO2 determination;

[0008] Step 3: Use the keeling plot model to analyze the inorganic carbon delta of the extracted gas. 13C calculation, in which the collected gas can be preserved and the gas extraction measurement is affected by time and number of times.

[0009] Furthermore, the specific process of the experimental preparation in step 1 is as follows:

[0010] (1) Prepare a 200 ml syringe to test the tightness and extract the gas from the Mason jar;

[0011] (2) Prepare 750 ml glass sealed Mason jars that are resistant to acidity and can withstand a fixed high temperature under corresponding conditions;

[0012] (3) Prepare hot melt adhesive and sealing film to strengthen the seal and ensure the sealing performance;

[0013] (4) Prepare phosphoric acid with a concentration of 85%. The inorganic carbon content of karst grassland soil is higher than the set value. Concentrated phosphoric acid reacts with carbonates in the soil to generate CO2. Phosphoric acid does not affect the structure of soil organic matter. When 85% phosphoric acid is used, the generated CO2 is relatively pure and does not contain impurities. Impurities will not affect δ 13 Determination of C.

[0014] Furthermore, the specific process of installing the sealed device in step 1 is to modify the experimental device according to the needs of the experiment. The modified experimental device includes a Mason jar and a nitrogen bottle. The Mason jar cap needs to be processed. Use a puncher to punch two holes with a diameter of 6 mm on the cap, fix them at both ends of the holes with a partition straight-through connector, tighten them, insert four PU air pipes into the connector holes respectively, and point one long and one short PU air pipes into the bottle, and seal and reinforce the interface with hot melt adhesive, wait for it to cool to room temperature and then check the air tightness.

[0015] Furthermore, the specific process of checking the air tightness in step 1 is as follows: check the air tightness of the Mason jar, first ensure that the syringe interface is well sealed, connect the 200ml syringe joint to the two-way valve, and wrap it with a sealing film, then connect a two-way valve of the Mason jar, open the two two-way valves at the same time, and use a syringe to draw air to check the sealing of the Mason jar. If the syringe rebounds strongly, it proves that the air tightness is good.

[0016] Furthermore, the specific process of soil sample treatment in step 1 is: weigh 5 g of 2 mm air-dried soil into a Mason bottle, use a pipette to draw 20 ml of 85% concentrated phosphoric acid and add it to the Mason bottle, cover the bottle cap tightly, check that the two-way valve is in the closed state, and wrap the sealing film to seal the bottle cap knob.

[0017] Furthermore, the specific process of exhaust in step 2 is: connect the air delivery hose of the nitrogen bottle to one end of the short tube of the Mason jar, open all the two-way valves, fill nitrogen from the short tube, and exhaust air from the long tube, observe the flow controller, and adjust the gas flow rate at 11L / min, maintain the flow rate for 3 minutes, and ensure that the air in the bottle is exhausted. After the exhaust is completed, reduce the flow rate, first close the long tube two-way valve, then close the short tube two-way valve, and finally unscrew the knob at the connection between the air delivery hose and the short tube of the Mason jar, and the exhaust is completed.

[0018] Furthermore, the specific process of extracting gas in step 2 is: put the Mason jar into a 70°C oven to react for 3 hours. After the reaction is completed, take out the Mason jar, connect a 200ml syringe to the long tube, open the two-way valve, extract 150ml of gas, close the two-way valve of the syringe, and seal it with a sealing film. The gas sample is measured using a Picarro instrument.

[0019] Further, in step 3, the inorganic carbon δ 13 The specific process of C calculation is:

[0020] The Picarro instrument measures the CO2 concentration and carbon isotope abundance of the sample. 13 C. According to the Keeling Plot formula, the CO2 in the Mason jar is divided into two sources. The soil source is the CO2 released by soil inorganic carbon and other sources. The other sources are the source of CO2 produced by soil organic carbon mineralization and mixed with nitrogen. The carbon in the Mason jar can be divided into:

[0021] C m =C n +C s (1)

[0022] Among them, C m , C n and C s They represent the actual CO2 concentration in the Mason jar, the CO2 concentration produced by other sources, and the CO2 concentration released by the soil;

[0023] According to the two-source mixed mass balance model, we can get:

[0024] δ 13 C m ×C m =δ 13 C n ×C n +δ 13 C s ×C s (2)

[0025] where δ 13 C m ,δ 13 Cn and δ 13 C s Represents the actual δ in the Mason jar 13 C-CO2, δ from other sources 13 C-CO2 and δ released by soil 13 C-CO2;

[0026] Considering the CO2 concentration and δ 13 C-CO2 does not change within a fixed time, and the CO2 concentration of the soil source is much higher than the CO2 concentration produced by other sources. Through formulas (1) and (2), the Keeling Plot curve can be obtained:

[0027] δ 13 C m =C n ×(δ 13 C n -δ 13 C s ) / C m +δ 13 C s (3)

[0028] Keeling Plot curve is δ 13 C m With 1 / C m A linear curve is drawn, where the intercept of the curve is the isotope value of the soil source, and the stable carbon isotope value δ of the soil source can be obtained. 13 C s -CO2, which is inorganic carbon delta 13 C, because CO2 concentration and δ 13 C is stable but unmeasurable. The Keeling Plot model method can avoid the influence of interference factors on the measurement accuracy. n ×(δ 13 C n -δ 13 C s ) as a whole parameter, so there is no need to n and δ 13 C n Performing separate measurements not only simplifies the experimental process, but also improves δ 13 C accuracy. Moreover, the spatial heterogeneity of inorganic carbon content in karst limestone soil is high, and δ 13 C s The Keeling Plot model was used to capture the overall trend of the data and ignore the outliers, thus obtaining a more accurate soil inorganic carbon δ 13 C.

[0029] The present invention has the following beneficial effects due to the adoption of the above technical solution:

[0030] The present invention has a simple device, low cost, simple detection process, and reduces the vacuum system and expensive equipment for real-time online determination of inorganic carbon delta 13 The Keeling Plot model can eliminate the influence of other complex CO2 sources and achieve stable and accurate inorganic carbon δ 13 C determination; the use of the keeling plot model can well capture the overall trend of the data while ignoring outliers, and can overcome the weakness of high heterogeneity of karst inorganic carbon distribution. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The inorganic carbon δ 13 Schematic diagram of the closed Mason jar apparatus and nitrogen filling for C determination;

[0032] Figure 2 The inorganic carbon δ 13 Schematic diagram of gas extraction for C determination;

[0033] Figure 3 The Keeling Plot model method is used in Example 1 of the present invention to calculate the soil inorganic carbon δ 13 Schematic diagram of C.

[0034] In the attached drawings, 1-Mason jar, 2-nitrogen bottle, 3-bottle cap, 4-partition straight-through connector, 5-short PU air tube, 6-long PU air tube, 7-two-way valve, 8-flow meter, 9-syringe injector, 10-air supply hose, 11-air supply tube. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and preferred embodiments. However, it should be noted that many details listed in the specification are only for the purpose of enabling the reader to have a thorough understanding of one or more aspects of the present invention, and these aspects of the present invention can be implemented even without these specific details.

[0036] A kind of karst soil inorganic carbon delta 13 A method for determining C, the method comprising the following steps:

[0037] Step 1: Sample reaction under closed conditions

[0038] Step 1.1: Experimental preparation

[0039] (1) 200ml syringe: used to test the sealing and extract the gas in the Mason jar. It is easy to operate and convenient for subsequent sample testing.

[0040] (2) 750ml sealed glass Mason jar: It has good sealing performance, is resistant to acid, and can withstand a certain high temperature under appropriate conditions, which is conducive to the experiment.

[0041] (3) Hot melt adhesive and sealing film: used to strengthen the seal and ensure the sealing performance.

[0042] (4) Phosphoric acid with a concentration of 85%: Karst grassland soil has a high inorganic carbon content. Concentrated phosphoric acid can effectively react with carbonates in the soil to produce CO2. Compared with strong acids such as hydrochloric acid, phosphoric acid has a lower reactivity and usually does not significantly affect the structure of soil organic matter. Therefore, it is ideal to use phosphoric acid. When using concentrated phosphoric acid, the generated CO2 is relatively pure and does not contain other impurities. This is very important for isotope analysis because any impurities may affect δ 13 Determination of C.

[0043] Step 1.2: Installation of the enclosure

[0044] Modify the experimental device according to the needs of the experiment. The Mason jar cap needs to be processed. Use a punch to punch two holes with a diameter of 6 mm on the bottle cap. Use a partition straight-through connector to fix the two ends of the hole and tighten it. Insert the four PU air tubes into the connector holes respectively, and point one long and one short PU air tube into the bottle. Use hot melt glue to seal and reinforce the interface. Wait for it to cool to room temperature and then check the airtightness. The airtight device is as shown in the attached Figure 1 shown.

[0045] The sealing device includes a Mason bottle 1 and a nitrogen bottle 2, and the Mason bottle 1 is connected to the nitrogen bottle 2. The upper end of the nitrogen bottle 2 is connected to an air supply pipe 11, and a flow meter 8 is arranged on the air supply pipe 11. One end of the air supply pipe 11 is connected to an air supply hose 10, and the air supply hose 10 is connected to one end of the short tube of the Mason bottle 1. The air supply hose 10 of the nitrogen bottle 2 is connected to the PU air pipe at the upper end of the short PU air pipe 5 of the Mason bottle 1, so that nitrogen comes out from the air supply hose 10 of the nitrogen bottle 2, passes through the PU air pipe at the upper end of the short PU air pipe 5, and then enters the interior of the Mason bottle 1 from the short PU air pipe 5. The upper end of the Mason bottle 1 is provided with a bottle cap 3, and the bottle cap 3 is provided with two holes, which are fixed to the upper and lower ends of the hole through a partition straight joint 4 and tightened, and the upper and lower ends of the partition straight joint 4 are respectively provided with PU air pipes, and the PU air pipes are inserted into the joint holes. A PU air tube is arranged at the upper end of a partition straight-through joint 4, and a short PU air tube 5 is arranged at the lower end; a PU air tube is arranged at the upper end of another partition straight-through joint 4, and a long PU air tube 6 is arranged at the lower end. Two-way valves 7 are arranged on the two PU air tubes at the upper end of the partition straight-through joint 4, and one long and one short PU air tubes are arranged at the lower end of the partition straight-through joint 4, namely, the short PU air tube 5 and the long PU air tube 6; and the interface is sealed and reinforced by hot melt adhesive, and the air tightness of the Mason bottle 1 is checked after cooling to room temperature. The Mason bottle 1 can adopt a 750ml glass sealed jar Mason bottle 1, which has good sealing performance, acid resistance, and the glass bottle can withstand a certain high temperature under appropriate conditions, which is conducive to the experiment. The 200ml syringe injector 9 is used to detect the sealing performance of the Mason bottle 1 and extract the gas in the Mason bottle 1, which is convenient and simple to operate, and is also convenient for subsequent sample testing. Hot melt adhesive and sealing film are used to strengthen the seal and ensure the sealing performance.

[0046] Step 1.3: Check for airtightness

[0047] Check the air tightness of the Mason jar. First, ensure that the syringe interface is well sealed. Connect the 200ml syringe joint to the two-way valve and secure it with a sealing film. Then connect a two-way valve of the Mason jar, open both two-way valves at the same time, and use a syringe to draw air to check the air tightness of the Mason jar. If the syringe rebounds strongly, it proves that the air tightness is good.

[0048] The specific operation of checking the air tightness of the Mason jar 1 is as follows: first, ensure that the syringe interface of the syringe 9 is well sealed, connect the syringe joint of the 200ml syringe 9 to the two-way valve 7, and fix it with a sealing film, then connect a two-way valve 7 of the Mason jar 1, open the two two-way valves 7 at the same time, and use a syringe to draw air to check the sealing of the Mason jar 1. If the syringe rebounds strongly, the air tightness of the Mason jar 1 is good.

[0049] Step 1.4: Soil sample processing

[0050] Weigh 5 g of 2 mm thick air-dried soil into a Mason bottle, use a pipette to draw 20 ml of 85% concentrated phosphoric acid into the Mason bottle, close the bottle cap tightly, check that the two-way valve is in the closed state, and wrap the sealing film to seal the bottle cap knob.

[0051] Due to the high inorganic carbon content in karst grassland soil, the use of 85% concentrated phosphoric acid can effectively react with carbonates in the soil to generate CO2. Compared with strong acids such as hydrochloric acid, phosphoric acid has a lower reactivity and usually does not significantly affect the structure of soil organic matter. Moreover, the generated CO2 is relatively pure and does not contain other impurities, which is very important for isotope analysis because any impurities may affect δ 13 Determination of C.

[0052] Step 2: Exhaust and Pump Down

[0053] Step 2.1: Exhaust

[0054] Connect the gas supply hose of the nitrogen bottle to one end of the short tube of the Mason jar, open all the two-way valves, fill nitrogen from the short tube, and exhaust air from the long tube. Observe the flow controller and adjust the gas flow rate to 11L / min, maintain the flow rate for 3 minutes, and ensure that the air in the bottle is emptied. After the exhaust is completed, reduce the flow rate, first close the long tube two-way valve, then close the short tube two-way valve, and finally unscrew the knob at the connection between the gas supply hose and the short tube of the Mason jar. The exhaust is completed, as shown in the attached Figure 1 shown.

[0055] Step 2.2: Pumping

[0056] Place the Mason jar in a 70°C oven for 3 hours. After the reaction is complete, take out the Mason jar, connect a 200ml syringe to the long tube, open the two-way valve, draw 150ml of gas, close the two-way valve of the syringe, and seal it with a sealing film. Figure 2 The gas samples were measured using a Picarro instrument.

[0057] Step 3: Inorganic Carbon Delta 13 C calculation

[0058] The Picarro instrument measures the CO2 concentration and carbon isotope abundance (δ 13 C). According to the Keeling Plot formula, the CO2 in the Mason jar is divided into two sources: soil source (CO2 released from soil inorganic carbon) and other sources (CO2 produced by soil organic carbon mineralization and mixed in nitrogen, etc.). Therefore, this part of carbon can be divided into:

[0059] C m =C n +C s (1)

[0060] Among them, Cm , C n and C s They represent the actual CO2 concentration in the Mason jar, the CO2 concentration produced by other sources, and the CO2 concentration released by the soil.

[0061] According to the two-source mixed mass balance model, we can get:

[0062] δ 13 C m ×C m =δ 13 C n ×C n +δ 13 C s ×C s (2)

[0063] where δ 13 C m ,δ 13 C n and δ 13 C s Represents the actual δ in the Mason jar 13 C-CO2, δ from other sources 13 C-CO2 and δ released by soil 13 C-CO2.

[0064] Considering the CO2 concentration and δ 13 C-CO2 does not change within a certain period of time, and the CO2 concentration from the soil source is much higher than the CO2 concentration from other sources. Using formulas (1) and (2), the Keeling Plot curve can be obtained:

[0065] δ 13 C m =C n ×(δ 13 C n -δ 13 C s ) / C m +δ 13 C s (3)

[0066] Keeling Plot curve is δ 13 C m With 1 / C m A linear curve is formed, where the intercept of the curve is the isotope value of the soil source. This method can be used to obtain the stable carbon isotope value of the soil source δ 13 C s -CO2, which is inorganic carbon delta 13 C. Because the CO2 concentration and δ 13C is stable but unmeasurable. The Keeling Plot model method can avoid the influence of interference factors on the measurement accuracy. n ×(δ 13 C n -δ 13 C s ) as a whole parameter, so there is no need to n and δ 13 C n Performing separate measurements not only simplifies the experimental process, but also improves δ 13 C accuracy. Moreover, the spatial heterogeneity of inorganic carbon content in karst limestone soil is high, and δ 13 C s The Keeling Plot model was used to capture the overall trend of the data and ignore the outliers, thus obtaining a more accurate soil inorganic carbon δ 13 C.

[0067] Examples:

[0068] This case takes the soil of karst grassland ecosystem as an example and applies this experimental method to determine the inorganic carbon delta in the 0-10cm and 10-20cm soil layers. 13 C, calculated using the Keeling Plot model method 13 C. Draw 1 / C using Origin 2021 software m With δ 13 C m The two-dimensional scatter plots of different soil layers are linearly fitted to the data to obtain the fitting equation y = ax + b (the intercept b is the required δ 13 C s -CO2 value) and the model's determination coefficient R 2 , as attached Figure 3 As shown. 2 The closer it is to 1, the better the equation fits. 2 >0.95,δ 13 C s -CO2 is 7‰~9‰, indicating that the application of this measurement method and the use of KeelingPlot model calculation can obtain relatively accurate data.

[0069] In the past, the determination of soil inorganic carbon δ 13The process of CO2 is highly dependent on equipment, especially the gas isotope mass spectrometer and related gas purification, sample processing and data processing systems. The equipment needs to ensure extremely high accuracy and sensitivity to detect very small isotope differences, so the data accuracy depends to a large extent on the quality, stability and operating technology of the detection equipment. The simple device used in this case to collect and measure CO2, combined with the Keeling Plot model, can eliminate the influence of other sources. The CO2 content of other sources is low and the composition is complex, making direct measurement difficult and error-prone. In addition, the previous inorganic carbon δ 13 The C data was calculated by taking the average value of three replicates. The spatial heterogeneity of inorganic carbon content in karst limestone soil is high, so the parameter C m The range of variation is large ( Figure 3 ), the Keeling Plot model can capture the overall trend of the data well and ignore the outliers, thus obtaining a more accurate soil inorganic carbon δ 13 In conclusion, compared with the previous studies on inorganic carbon δ 13 The method for determining C is simpler, has lower cost, is easy to maintain, and has higher data accuracy.

[0070] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A karst soil inorganic carbon delta 13 The method for determining C is characterized by: The method comprises the following steps: Step 1: Sample reaction under closed conditions, including preparation of experimental materials, installation of closed device, checking of airtightness and soil sample processing; Step 2: Gas collection and measurement, including exhaust and extraction of the closed device, δ 13 C-CO2 determination; Step 3: Use the keeling plot model to analyze the inorganic carbon delta of the extracted gas. 13 C calculation, in which the collected gas can be preserved and the gas extraction measurement is affected by time and number of times.

2. The karst soil inorganic carbon δ 13 The method for determining C is characterized by: The specific process of experimental preparation in step 1 is as follows: (1) Prepare a 200 ml syringe to test the tightness and extract the gas from the Mason jar; (2) Prepare 750 ml glass sealed Mason jars that are resistant to acidity and can withstand a fixed high temperature under corresponding conditions; (3) Prepare hot melt adhesive and sealing film to strengthen the seal and ensure the sealing performance; (4) Prepare 85% phosphoric acid. The inorganic carbon content of karst grassland soil is higher than the set value. Concentrated phosphoric acid reacts with carbonates in the soil to generate CO2. Phosphoric acid does not affect the structure of soil organic matter. When 85% phosphoric acid is used, the generated CO2 is relatively pure and does not contain impurities. Impurities will not affect δ 13 Determination of C.

3. The karst soil inorganic carbon δ 13 The method for determining C is characterized by: The specific process of installing the closed device in step 1 is to modify the experimental device according to the needs of the experiment. The modified experimental device includes a Mason jar and a nitrogen bottle. The Mason jar cap needs to be processed. Use a puncher to punch two holes with a diameter of 6 mm on the cap, fix them at both ends of the holes with a partition straight-through connector, tighten them, insert four PU air tubes into the connector holes respectively, and point one long and one short PU air tubes into the bottle, and seal and reinforce the interface with hot melt adhesive, wait for it to cool to room temperature and then check the air tightness.

4. The karst soil inorganic carbon δ 13 The method for determining C is characterized by: The specific process of checking the air tightness in step 1 is as follows: Check the air tightness of the Mason jar, first ensure that the syringe interface is well sealed, connect the 200ml syringe joint to the two-way valve, and wrap it with sealing film, then connect a two-way valve of the Mason jar, open the two two-way valves at the same time, and use a syringe to draw air to check the sealing of the Mason jar. If the syringe rebounds strongly, it proves that the air tightness is good.

5. The karst soil inorganic carbon δ according to claim 4 13 The method for determining C is characterized by: The specific process of soil sample treatment in step 1 is as follows: weigh 5 g of 2 mm thick air-dried soil into a Mason bottle, use a pipette to draw 20 ml of 85% concentrated phosphoric acid and add it to the Mason bottle, cover the bottle cap tightly, check that the two-way valve is in the closed state, and wrap the sealing film to seal the bottle cap knob.

6. The karst soil inorganic carbon δ 13 The method for determining C is characterized by: The specific process of exhaust in step 2 is: connect the gas supply hose of the nitrogen bottle to one end of the short tube of the Mason jar, open all the two-way valves, fill nitrogen from the short tube, and exhaust the air from the long tube. Observe the flow controller and adjust the gas flow rate at 11L / min, maintain the flow rate for 3 minutes, and ensure that the air in the bottle is exhausted. After the exhaust is completed, reduce the flow rate, close the long tube two-way valve first, then close the short tube two-way valve, and finally unscrew the knob at the connection between the gas supply hose and the short tube of the Mason jar to complete the exhaust.

7. The karst soil inorganic carbon δ according to claim 4 13 The method for determining C is characterized by: The specific process of gas extraction in step 2 is: put the Mason jar into a 70°C oven to react for 3 hours. After the reaction is completed, take out the Mason jar, connect a 200ml syringe to the long tube, open the two-way valve, extract 150ml of gas, close the two-way valve of the syringe, and seal it with a sealing film. The gas sample is measured using a Picarro instrument.

8. The karst soil inorganic carbon δ according to claim 4 13 The method for determining C is characterized by: Inorganic carbon δ 13 The specific process of C calculation is: The Picarro instrument measures the CO2 concentration and carbon isotope abundance of the sample. 13 C. According to the Keeling Plot formula, the CO2 in the Mason jar is divided into two sources. The soil source is the CO2 released by soil inorganic carbon and other sources. The other sources are the source of CO2 produced by soil organic carbon mineralization and mixed with nitrogen. The carbon in the Mason jar can be divided into: C m =C n +C s (1) Among them, C m , C n and C s They represent the actual CO2 concentration in the Mason jar, the CO2 concentration produced by other sources, and the CO2 concentration released by the soil; According to the two-source mixed mass balance model, we can get: d 13 C m ×C m =d 13 C n ×C n +d 13 C s ×C s (2) where δ 13 C m , δ 13 C n and δ 13 C s Represents the actual δ in the Mason jar 13 C-CO2, δ from other sources 13 C-CO2 and δ released by soil 13 C-CO2; Considering the CO2 concentration and δ 13 C-CO2 does not change within a certain period of time, and the CO2 concentration of the soil source is much higher than the CO2 concentration produced by other sources. Through formulas (1) and (2), the Keeling Plot curve can be obtained: d 13 C m =C n ×(d 13 C n -d 13 C s ) / C m +d 13 C s (3) Keeling Plot curve is δ 13 C m With 1 / C m A linear curve is drawn, where the intercept of the curve is the isotope value of the soil source, and the stable carbon isotope value δ of the soil source can be obtained. 13 C s -CO2, which is inorganic carbon delta 13 C, because CO2 concentration and δ 13 C is stable but unmeasurable. The Keeling Plot model method can avoid the influence of interference factors on the measurement accuracy. n ×(δ 13 C n -δ 13 C s ) as a whole parameter, so there is no need to n and δ 13 C n Performing separate measurements not only simplifies the experimental process, but also improves δ 13 C accuracy. Moreover, the spatial heterogeneity of inorganic carbon content in karst limestone soil is high, and δ 13 C s The Keeling Plot model was used to capture the overall trend of the data and ignore the outliers, thus obtaining a more accurate soil inorganic carbon δ 13 C.

Images