Methods for determining the potassium permanganate oxidation time during soil active organic carbon determination, and methods for determining soil active organic carbon content.

Abstract

This invention relates to a method for determining the oxidation time of potassium permanganate during the determination of active organic carbon in soil, and a method for determining the content of active organic carbon in soil, belonging to the field of soil chemistry. The method of this invention includes the following steps: determining the content of active organic carbon in soil samples under different oxidation times when potassium permanganate solution is mixed with soil samples for oxidation reaction; establishing a functional relationship y = f(x) between active organic carbon content and oxidation time through data fitting; and taking the x value corresponding to the minimum second derivative of the function f(x) as the potassium permanganate oxidation time. This method can determine the potassium permanganate oxidation time during the determination of active organic carbon in soils of different regions and with different organic carbon content levels, avoiding situations where some samples have incomplete oxidation of active organic carbon or where some samples have inactive organic carbon oxidized, thereby reducing test errors and improving the accuracy of test results.

Description

Technical Field

[0001] This invention relates to a method for determining the oxidation time of potassium permanganate during the determination of soil active organic carbon and a method for determining the content of soil active organic carbon, belonging to the field of soil chemistry. Background Technology

[0002] Active organic carbon, also known as readily oxidizable organic carbon, refers to the active portion of soil organic matter. It has high bioavailability, is easily decomposed and mineralized by soil microorganisms, and can directly supply nutrients to plants. It plays a crucial role in maintaining soil fertility, improving soil activity, and maintaining soil carbon balance. However, soil active organic carbon is not a simple compound; its composition is complex, including dissolved organic carbon, microbial biomass carbon, and light organic carbon, all of which can characterize soil material cycling and evaluate soil quality.

[0003] Because of the complex composition of active organic carbon, it is currently impossible to determine its individual components. The determination of active organic carbon generally employs the potassium permanganate oxidation method. It is generally believed that the portion of organic carbon that is easily oxidized by potassium permanganate is active organic carbon. Alkaline potassium permanganate reacts with the most easily oxidized soil carbon, converting Mn(VII) to Mn(II). The content of active organic carbon can be calculated by measuring the absorbance at 550 nm. This experiment mainly involves two important parameters: the concentration of potassium permanganate and the oxidation reaction time. Many domestic publications, such as Xu Minggang et al.'s "Changes in Active Organic Matter and Carbon Pool Management Index of Red Soil under Long-Term Different Fertilization" (Acta Pedologica Sinica, 2006, (9): 723-729), disclose the use of a higher potassium permanganate concentration (0.333M) and a reaction time of 1 hour. At this concentration, potassium permanganate is difficult to dissolve, making solution preparation difficult and usable. Furthermore, the 1-hour reaction time of this method allows potassium permanganate to react with most of the carbon in the soil, thus failing to accurately characterize the content of active organic carbon. Foreign research literature has adopted lower potassium permanganate concentrations and shorter reaction times. For example, Weil et al. (Estimating active carbon for soil quality assessment: Asimplied method for laboratory and field use. American Journal of Alternative Agriculture, 2003, 18(1):3-17) reported that a potassium permanganate concentration of 0.02M could accurately determine active organic carbon, suggesting a shaking reaction time of 2 minutes. However, this reaction time still has problems. First, the time is too short; in experimental operations, when the number of samples in a batch is large, the time is often difficult to control accurately, causing experimental errors. Second, for most soils, this reaction time is during a period of high oxidation reaction rate, and the active organic carbon has not been completely oxidized, making it impossible to accurately determine the content of active organic carbon.

[0004] When determining soil samples with different organic carbon content levels, simply using a fixed time cannot accurately reflect the active organic carbon content of the soil samples. In actual testing, the optimal oxidation reaction time should be determined based on the actual organic carbon content level of soil samples from different regions or batches to ensure the accuracy of the test. However, currently, there is no good method for determining the optimal time for potassium permanganate oxidation reaction in this test, which affects the accurate determination of active organic carbon in soil. Summary of the Invention

[0005] The purpose of this invention is to provide a method for determining the potassium permanganate oxidation time in the determination of soil active organic carbon, which can provide a targeted potassium permanganate oxidation time for the soil to be tested in a specific region, thereby improving the accuracy of soil active organic carbon content determination.

[0006] Another objective of this invention is to provide a method for determining the content of active organic carbon in soil.

[0007] To achieve the above objectives, the technical solution adopted by the method for determining the potassium permanganate oxidation time in the soil active organic carbon determination process of the present invention is as follows:

[0008] A method for determining the oxidation time of potassium permanganate in the determination of soil active organic carbon includes the following steps: determining the active organic carbon content of soil samples under different oxidation times when potassium permanganate solution is mixed with soil samples for oxidation reaction; establishing a functional relationship y = f(x) between soil active organic carbon content and oxidation time through data fitting, where y is the soil active organic carbon content and x is the oxidation time; then finding the x value corresponding to the minimum value of the second derivative of the function y = f(x), and taking the obtained x value as the potassium permanganate oxidation time.

[0009] The method for determining the potassium permanganate oxidation time in the soil active organic carbon determination process of the present invention can determine the potassium permanganate oxidation time in the soil active organic carbon determination process for soils with different regions and different organic carbon content levels. This avoids the situation where some samples have incomplete oxidation of active organic carbon or some samples have inactive organic carbon oxidation, thereby reducing test errors and improving the accuracy of test results. It can provide technical support for the accurate determination of soil active organic carbon content.

[0010] In the method for determining the potassium permanganate oxidation time during the determination of soil active organic carbon in this invention, the time corresponding to the minimum value of the second derivative of the function y = f(x), i.e., the time point with the minimum reaction acceleration, is taken as the optimal time for the potassium permanganate oxidation reaction. At this time point, the active organic carbon is basically oxidized and begins to transition to the oxidation of inactive organic carbon.

[0011] It is understood that the soil sample is the soil to be tested during the determination of active organic carbon in the soil or soil from the same area as the soil to be tested.

[0012] Furthermore, when mixing the potassium permanganate solution with the soil sample, the volume of potassium permanganate solution used for every 2.5g of soil sample is 18-23mL, for example, 20mL. The concentration of the potassium permanganate solution is 0.015-0.025mol / L, for example, 0.02mol / L, and the pH is 7.1-7.4, for example, pH 7.2.

[0013] To improve detection efficiency and reduce costs while ensuring that the determined oxidation time is specific to the target organism, further, n different oxidation times for potassium permanganate on soil samples are set, arranged in ascending order of oxidation time as: a1, a2, a3, a4, ..., a n Where: 0 < a1 ≤ 2min, a n ≥120min; a i =(1.5~2.5)a i-1 Let i be an integer and 2 ≤ i ≤ n, a i This represents the i-th time interval arranged from shortest to longest oxidation time. For example, the different oxidation times of potassium permanganate on soil, arranged from shortest to longest oxidation time, are 2 min, 5 min, 10 min, 20 min, 30 min, 60 min, and 120 min.

[0014] Furthermore, the functional relationship y = f(x) is:

[0015]

[0016] In the formula, a, b, and c are the parameters to be solved. Using this function for data fitting can effectively simulate the oxidation process of active organic carbon by potassium permanganate, yielding the optimal oxidation time.

[0017] Furthermore, the potassium permanganate solution is obtained by dissolving a water-soluble calcium salt and potassium permanganate in water and then adjusting the pH; the water-soluble calcium salt is calcium chloride; the concentration of calcium chloride in the potassium permanganate solution is 0.08–0.12 mol / L, for example, 0.1 mol / L. Sodium hydroxide is used as the pH adjuster.

[0018] Further, a method for determining the content of active organic carbon in soil samples after oxidation with potassium permanganate for different durations includes the following steps: multiple soil samples are mixed with potassium permanganate solution and oxidized for different times under shaking conditions; solid-liquid separation is performed; the absorbance value of the liquid obtained after solid-liquid separation or a diluted solution of the obtained liquid is measured at 550 nm; and the content of active organic carbon in each soil sample is calculated based on a standard curve; the standard curve is plotted with the concentration of potassium permanganate in the solution on the x-axis and the absorbance value on the y-axis. Further, a method including the following steps is used to plot the standard curve: the absorbance value of potassium permanganate standard solutions of different concentrations at a wavelength of 550 nm is measured to plot the standard curve. The potassium permanganate standard solution is obtained by diluting the potassium permanganate solution.

[0019] It is understandable that the potassium permanganate solution used to prepare the potassium permanganate standard solution is the same as the potassium permanganate solution used to mix with the soil sample.

[0020] To prevent liquid splashing during the oxidation reaction under shaking conditions, the oxidation process, in which the soil sample was mixed with potassium permanganate solution, was carried out under sealed conditions. The sealed conditions were then opened during the solid-liquid separation process.

[0021] Furthermore, the solid-liquid separation involves allowing the system to settle and precipitate after the oxidation reaction has proceeded for a specified time; the settling time is 5–15 minutes, for example, 10 minutes. A potassium permanganate solution prepared using calcium chloride can be directly allowed to settle after shaking, which is simpler to operate compared to other solid-liquid separation methods such as centrifugation. Of course, other commonly used methods such as centrifugation can also be used for solid-liquid separation.

[0022] The technical solution adopted in the method for determining the content of active organic carbon in soil of the present invention is as follows:

[0023] A method for determining the content of active organic carbon in soil includes the following steps: mixing the soil to be tested with a potassium permanganate solution for oxidation reaction, separating the solid and liquid, measuring the absorbance value of the liquid obtained from the solid-liquid separation or the diluted liquid at 550 nm, and calculating the content of active organic carbon in the soil to be tested based on a standard curve; the oxidation reaction time is determined by the method for determining the potassium permanganate oxidation time in the soil active organic carbon determination process described above, and the soil sample is the soil to be tested or soil from the same area as the soil to be tested; the standard curve is plotted with the concentration of potassium permanganate in the solution on the x-axis and the absorbance value on the y-axis.

[0024] The method for determining the content of active organic carbon in soil according to the present invention allows for the determination of the optimal reaction time by taking representative soil samples before conducting large-scale testing of soil samples in a certain area, using the method for determining the oxidation time of potassium permanganate during the determination of active organic carbon in soil according to the present invention. This method can improve the accuracy of detecting the content of active organic carbon in the soil to be tested and provides technical support for the accurate determination of the content of active organic carbon in soil.

[0025] It should be noted that the potassium permanganate solution mixed with the soil to be tested is the same as the potassium permanganate solution mixed with the soil sample mentioned above. When mixing the potassium permanganate solution with the soil to be tested, the volume of potassium permanganate solution used for every 2.5g of soil to be tested is the same as the volume of potassium permanganate solution used for every 2.5g of soil sample when mixing the potassium permanganate solution with the soil sample mentioned above.

[0026] Furthermore, the oxidation reaction between the soil sample and the potassium permanganate solution was carried out under shaking conditions. The solid-liquid separation after the soil sample and the potassium permanganate solution were mixed and oxidized was called static precipitation. The static precipitation time was the same as the time for solid-liquid separation by static precipitation after adding potassium permanganate solution to multiple soil samples and oxidizing them under shaking conditions for different times. Attached Figure Description

[0027] Figure 1 The graph shows the fitted function, its first derivative, and its second derivative in Example 1. Figure 1 (a) is a graph of the fitted function. Figure 1 (b) is a graph of the first derivative of the fitted function. Figure 1 (c) is a graph of the second derivative of the fitted function;

[0028] Figure 2 The graph shows the fitting function, the first derivative of the fitting function, and the second derivative of the fitting function in Example 2. Figure 2 (a) is a graph of the fitted function. Figure 2 (b) is a graph of the first derivative of the fitted function. Figure 2 (c) is a graph of the second derivative of the fitted function;

[0029] Figure 3 The graph shows the fitting function, its first derivative, and its second derivative in Example 3. Figure 3 (a) is a graph of the fitted function. Figure 3 (b) is a graph of the first derivative of the fitted function. Figure 3 (c) is a graph of the second derivative of the fitted function. Detailed Implementation

[0030] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0031] Examples 1-3 below are examples of methods for determining the oxidation time of potassium permanganate during the determination of soil active organic carbon, and Examples 4-6 are examples of methods for determining the content of soil active organic carbon.

[0032] Example 1

[0033] The method for determining the potassium permanganate oxidation time during the determination of soil active organic carbon in this embodiment includes the following steps:

[0034] 1) Preparation of the standard curve:

[0035] To prepare a 0.02 mol / L potassium permanganate solution, KMnO4 was added to a 0.1 mol / L CaCl2 aqueous solution. During the preparation process, the pH was adjusted to 7.2 with 0.1 mol / L NaOH solution before making up the volume (using the 0.1 mol / L CaCl2 aqueous solution). Then, different volumes of 0.02 mol / L potassium permanganate solution and water were used to prepare a series of KMnO4 standard solutions of different concentrations, as shown in Table 1 below. A KMnO4 standard curve was then prepared.

[0036] Table 1. Standard Curve Construction

[0037]

[0038] The absorbance of each standard solution diluted 10-fold at 550 nm was determined: Using a 50 mL centrifuge tube, 49.5 mL of water was added first, followed by 0.5 mL of the standard solution. After thorough mixing, the absorbance was measured at 550 nm using a spectrophotometer. A standard curve was plotted based on the concentrations of the diluted standard solutions and the measured absorbance values: y = ax + b, where y is the potassium permanganate concentration and x is the absorbance.

[0039] (2) Sample testing:

[0040] Take 21 50mL plastic bottles and put 2.5g of soil sample from a certain area in Northeast China into each plastic bottle (the amount of soil sample in each plastic bottle is the same).

[0041] Then, add 20 mL of the 0.02 mol / L potassium permanganate solution prepared in step 1) to the plastic bottle, tighten the cap, and quickly place it on a shaker. Shake at 180 rpm. Set the shaking time to 2, 5, 10, 20, 30, 60, and 120 min, with three replicates for each time period, and time accurately. After each shaking time, remove the three plastic bottles, open the caps, and let them stand on the test bench for 10 minutes. After the standing time, take 0.5 mL of the supernatant into a 50 mL centrifuge tube (pre-added with 49.5 mL of water), mix thoroughly, and measure the absorbance at 550 nm using a spectrophotometer. Calculate the average value of each replicate group, and use the standard curve drawn in step 1) to calculate the soil active organic carbon content at different shaking times.

[0042] (3) Determination of the optimal reaction time:

[0043] Using the soil active organic carbon content obtained from soil samples at different oscillation times, with oscillation time as the independent variable and soil organic carbon content as the dependent variable, the following fitting function f(x) was used for data fitting. The parameters in the fitting function f(x) were a = 0.00112, b = 0.00228, and c = 0.03553, and the goodness of fit R0 was [value missing]. 2 =0.9853.

[0044]

[0045] Taking the second derivative of the fitted function, we obtain the following function:

[0046]

[0047] The time corresponding to the minimum second derivative is 57.1 minutes. Therefore, the optimal potassium permanganate oxidation time in the determination of active organic carbon in soil in Northeast China is 57.1 min.

[0048] The curves of the fitting function, the first derivative of the fitting function, and the second derivative of the fitting function in this embodiment are as follows: Figure 1 As shown.

[0049] Example 2

[0050] The method for determining the potassium permanganate oxidation time in the determination of active organic carbon in soil in this embodiment is for determining the potassium permanganate oxidation time in the determination of active organic carbon in soil in Henan Province. The only difference from the determination method in Example 1 is that the soil sample from Northeast China is replaced with soil from Henan Province.

[0051] The parameters of the function f(x) are: a = 0.00180, b = 0.00420, c = 0.03357; the goodness of fit R0 is 0.03357. 2 = 0.9777. The minimum value of the second derivative of function f(x) corresponds to a time of 46.1 minutes, which is the optimal oxidation reaction time. The curves of the fitting function, the first derivative of the fitting function, and the second derivative of the fitting function in this embodiment are as follows: Figure 2 As shown.

[0052] Example 3

[0053] The method for determining the potassium permanganate oxidation time in the determination of soil active organic carbon in this embodiment is for determining the potassium permanganate oxidation time in the determination of soil active organic carbon in Guizhou region. The only difference from the determination method in Example 1 is that the soil sample from Northeast China is replaced with the soil sample from Guizhou region.

[0054] The parameters of the function f(x) are: a = 0.00073, b = 0.00147, c = 0.04374; the goodness of fit R0 is 0.00073. 2 =0.9884. The optimal oxidation reaction time is 64.5 minutes, when the second derivative of function f(x) reaches its minimum. The curves of the fitting function, the first derivative of the fitting function, and the second derivative of the fitting function in this embodiment are shown below. Figure 3 As shown.

[0055] Example 4

[0056] The method for determining the active organic carbon content in soil in this embodiment includes the following steps:

[0057] Five soil samples from Northeast China were tested using the optimal oxidation reaction time (57.1 minutes) determined in Example 1. The specific testing steps are as follows: 2.5 g of the soil sample from Northeast China was weighed and placed in a 50 mL plastic bottle. Then, 20 mL of 0.02 mol / L potassium permanganate solution was added, the cap was tightened, and the bottle was quickly placed on a shaker and shaken at 180 rpm for 57.1 min. After shaking, the plastic bottle was removed, the cap was opened, and the bottle was left to stand for 10 minutes on the test bench. After standing, 0.5 mL of the supernatant was transferred to a 50 mL centrifuge tube (pre-mixed with 49.5 mL of water), thoroughly mixed, and the absorbance was measured at 550 nm using a spectrophotometer. The active organic carbon content of the soil was calculated using the standard curve plotted in Example 1. The active organic carbon content of each soil sample is shown in Table 2.

[0058] Meanwhile, a comparison was made using the method of Weil (2003) (2 minutes), and the results are shown in Table 2. It can be seen that, using the same 0.02 mol / L potassium permanganate solution, the active organic carbon content of five different soil samples from Northeast China, measured using the method determined in Example 1, was significantly higher than that measured using the literature method. The literature method, due to its shorter reaction time, resulted in incomplete reaction during the experiment, leading to significantly lower measured results.

[0059] Table 2 Comparison of two methods for soil analysis in Northeast China

[0060]

[0061] Example 5

[0062] The method for determining the active organic carbon content in soil in this embodiment includes the following steps:

[0063] Five soil samples from Henan Province were tested using the optimal oxidation reaction time (46.1 minutes) determined in Example 2. The specific testing steps are as follows: 2.5 g of the soil sample from Henan Province was weighed and placed in a 50 mL plastic bottle. Then, 20 mL of 0.02 mol / L potassium permanganate solution was added, the cap was tightened, and the bottle was quickly placed on a shaker and shaken at 180 rpm for 46.1 min. After shaking, the plastic bottle was removed, the cap was opened, and the bottle was left to stand on the test bench for 10 minutes. After standing, 0.5 mL of the supernatant was transferred to a 50 mL centrifuge tube (pre-mixed with 49.5 mL of water), thoroughly mixed, and the absorbance was measured at 550 nm using a spectrophotometer. The active organic carbon content of the soil was calculated using the standard curve plotted in Example 2.

[0064] Meanwhile, a comparison was made using the method (2 minutes) described by Weil (2003), and the results are shown in Table 3. It can be seen that, using the same 0.02 mol / L potassium permanganate solution, the active organic carbon content of soil samples from Henan Province was significantly lower than that from soils in Northeast China. Comparing the two methods, the active organic carbon content of five different soil samples from Henan Province measured using the method determined in Example 2 was also significantly higher than that measured using the method described in the literature.

[0065] Table 3 Comparison of two methods for soil analysis in Henan Province

[0066]

[0067] Example 6

[0068] The method for determining the active organic carbon content in soil in this embodiment includes the following steps:

[0069] Five soil samples from Guizhou Province were tested using the optimal oxidation reaction time (64.5 minutes) determined in Example 3. The specific testing steps are as follows: 2.5 g of the soil sample from Guizhou Province was weighed and placed in a 50 mL plastic bottle. Then, 20 mL of 0.02 mol / L potassium permanganate solution was added, the cap was tightened, and the bottle was quickly placed on a shaker and shaken at 180 rpm for 64.5 min. After shaking, the plastic bottle was removed, the cap was opened, and the bottle was left to stand on the test bench for 10 minutes. After standing, 0.5 mL of the supernatant was transferred to a 50 mL centrifuge tube (pre-mixed with 49.5 mL of water), thoroughly mixed, and the absorbance was measured at 550 nm using a spectrophotometer. The active organic carbon content of the soil was calculated using the standard curve plotted in Example 3.

[0070] Simultaneously, the method described by Weil (2003) (2 minutes) was used for comparison, and the results are shown in Table 4. It can be seen that, using the same 0.02 mol / L potassium permanganate solution, the active organic carbon content of soil samples from Guizhou was all above 1000 mg / kg. The average value of the five samples was slightly higher than that of the five soil samples from Northeast China. This may be because continuous soil conservation work in the area has increased the soil carbon pool content. The active organic carbon content of the five different soil samples from Guizhou, measured using the method determined in Example 3, was significantly higher than that measured by the literature method.

[0071] Table 4 Comparison of two methods for soil analysis in Guizhou region

[0072]

Claims

1. A method for determining the duration of potassium permanganate oxidation in a process for determining soil active organic carbon, characterized in that: The method comprises the following steps: The active organic carbon content of the soil sample is determined when the potassium permanganate solution is mixed with the soil sample to perform the oxidation reaction for different oxidation durations, and a function relationship y=f(x) between the active organic carbon content of the soil and the oxidation duration is established through data fitting, wherein y is the active organic carbon content of the soil, and x is the oxidation duration; then the x value corresponding to the minimum value of the second derivative of the function y=f(x) is obtained, and the obtained x value is taken as the potassium permanganate oxidation duration; the function relationship y=f(x) is: ； In the formula, a, b and c are parameters to be solved.

2. The method for determining the duration of potassium permanganate oxidation in a process for determining soil active organic carbon according to claim 1, characterized in that: When the potassium permanganate solution is mixed with the soil sample, the volume of the potassium permanganate solution corresponding to 2.5 g of the soil sample is 18-23 mL; the concentration of the potassium permanganate solution is 0.015-0.025 mol / L, and the pH is 7.1-7.

4.

3. The method for determining the duration of potassium permanganate oxidation in a process for determining soil active organic carbon according to claim 1, characterized in that: n potassium permanganate oxidation time lengths are set for the soil samples, and the oxidation time lengths are arranged from short to long as follows: a1, a2, a3, a4,..., an n , wherein: 0 < a1≤ 2 min, a n ≥ 120 min; a i =(1.5~2.5)a i-1 , i is an integer and 2≤i≤n, a i is the ith time length arranged from short to long.

4. The method for determining the duration of potassium permanganate oxidation in the process of measuring soil active organic carbon according to any one of claims 1-3, characterized in that: The potassium permanganate solution is obtained by dissolving a water-soluble calcium salt and potassium permanganate in water and adjusting the pH; the water-soluble calcium salt is calcium chloride; and the concentration of the calcium chloride in the potassium permanganate solution is 0.08-0.12 mol / L.

5. The method for determining the duration of potassium permanganate oxidation in a process for determining soil active organic carbon according to claim 4, characterized in that: The method for determining the active organic carbon content of the soil when the potassium permanganate is oxidized for different durations comprises the following steps: a plurality of soil samples are mixed with potassium permanganate solution and then oxidized for different durations under oscillation, solid-liquid separation is performed, the absorbance value of the obtained liquid or a diluted solution of the obtained liquid at 550 nm is determined, and the active organic carbon content in each soil sample is calculated according to a standard curve; the standard curve takes the concentration of potassium permanganate in the solution as the abscissa and the absorbance value as the ordinate.

6. The method for determining the duration of potassium permanganate oxidation in a process for determining soil labile organic carbon according to claim 5, characterized in that: The process of mixing the soil sample with the potassium permanganate solution and then performing the oxidation reaction is carried out in a sealed condition.

7. The method for determining the duration of potassium permanganate oxidation in the process of determining soil active organic carbon according to claim 5, characterized in that: The solid-liquid separation is static precipitation of the system after the oxidation reaction for a corresponding duration; the static precipitation time is 5-15 min.

8. A method for determining the content of soil labile organic carbon, characterized in that: The method comprises the following steps: The soil sample to be measured is mixed with the potassium permanganate solution to perform the oxidation reaction, solid-liquid separation is performed, the absorbance value of the obtained liquid or a diluted solution of the obtained liquid at 550 nm is determined, and the active organic carbon content in the soil to be measured is calculated according to a standard curve; The oxidation duration is determined by the method for determining the potassium permanganate oxidation duration in the process of measuring the active organic carbon of the soil according to any one of claims 1-7, and the soil sample is the soil to be measured or the soil in the same region as the soil sample to be measured; The standard curve takes the concentration of potassium permanganate in the solution as the abscissa and the absorbance value as the ordinate.

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