A method for determining ammonia nitrogen content in saline-alkali soils of estuarine wetlands using a continuous flow analyzer

Abstract

This method for determining ammonia nitrogen content in saline-alkali soil of estuarine wetlands using a continuous flow analyzer belongs to the field of soil chemical analysis and environmental monitoring technology. The method first collects saline-alkali soil samples from estuarine wetlands and extracts them with KCl solution. A modified buffer solution containing disodium hydrogen phosphate, potassium sodium tartrate, and sodium hydroxide, an ammonium sulfate standard solution, a sodium dichloroisocyanurate colorimetric solution, and a sodium salicylate-sodium nitroprusside colorimetric solution are prepared. The instrument's injection tube is replaced with a high-concentration injection tube, and the standard solution and the soil sample solution are placed in the sampler of the continuous flow analyzer for analysis. A standard curve is fitted based on the test results, and the concentration of ammonia nitrogen in the soil sample solution is calculated and output based on the standard curve. The soil ammonia nitrogen content is calculated based on the concentration of ammonia nitrogen in the soil sample solution, combined with the mass of the soil sample before extraction and the volume of KCl solution used. This invention eliminates the interference of the sample matrix on the colorimetric reaction.

Description

Technical Field

[0001] This invention belongs to the field of soil chemical analysis and environmental monitoring technology, and relates to a method for determining the ammonia nitrogen content in saline-alkali soils of estuary wetlands based on a continuous flow analyzer. Specifically, it relates to a method for determining the ammonium nitrogen content in saline-alkali soils such as estuary wetlands using a continuous flow analyzer in conjunction with an improved reagent system. Background Technology

[0002] Estuarine wetlands are a globally important ecosystem type, playing an irreplaceable role in carbon sequestration, biodiversity maintenance, and pollutant interception. Nitrogen is a crucial nutrient element in wetland ecosystems, and soil ammonium nitrogen content is a key indicator for assessing wetland soil fertility, nitrogen cycle efficiency, and vegetation growth. Accurately measuring the ammonium nitrogen content in estuarine wetland soils is of great significance for understanding wetland nitrogen cycle processes, assessing wetland health, and developing ecological restoration strategies.

[0003] Currently, the determination of ammonium nitrogen in soil mainly employs the potassium chloride solution extraction-indophenol blue colorimetric method. With advancements in analytical technology, the Continuous Flow Analyzer (CFA), with its advantages of high automation, fast analysis speed, and good precision, has been widely used in soil and plant nitrogen determination. Its principle for determining ammonia nitrogen is based on the sodium salicylate method: under alkaline conditions, ammonium ions react with sodium salicylate and sodium dichloroisocyanurate in the presence of sodium nitroprusside catalyzed to form a blue-green indophenol complex, which is then determined colorimetrically.

[0004] However, in practical applications, when using the standard sodium salicylate method with a continuous flow analyzer to determine ammonium nitrogen in extracts from saline-alkali soils in estuarine wetlands, abnormal sample response signals often occur, leading to the inability to obtain effective soil ammonia nitrogen data and severely hindering related research. Analysis shows that this problem mainly occurs in soil types with high pH or high salinity. When the soil pH is high or contains a high concentration of salt ions, the high pH and high salinity sample matrix may interfere with the normal progress of the sodium salicylate colorimetric reaction, resulting in abnormal sample signals. Coastal estuarine wetland soils, due to long-term tidal influence, are typically characterized by alkaline pH and high salinity, making them highly susceptible to the aforementioned signal anomalies when using conventional reagent systems. Furthermore, this matrix interference problem is not unique to coastal saline-alkali soils; it can also exist in inland saline-alkali soils, some calcareous soils, and high-salinity soils affected by human activities.

[0005] While existing technologies document salinity interference in continuous flow analysis, most methods compensate using common techniques such as increasing the carrier salinity. This approach is not only cumbersome but also fails to fundamentally eliminate the specific interference of complex soil matrices on the colorimetric reaction. Currently, there are no optimized reagent solutions for extracts from high-pH, high-salinity samples, such as those from saline-alkali soils.

[0006] Therefore, there is an urgent need for a method that can effectively eliminate the interference of high pH and high salinity soil sample extracts on the sodium salicylate method colorimetric reaction, and enable continuous flow analyzers to accurately determine the ammonium nitrogen content of this type of soil. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention proposes a method for determining the ammonia nitrogen content in saline-alkali soils of estuarine wetlands based on a continuous flow analyzer.

[0008] The technical solution of the present invention is as follows: A method for determining ammonia nitrogen content in saline-alkali soils of estuarine wetlands based on a continuous flow analyzer, characterized by comprising the following steps: Step S1: Collect saline-alkali soil samples from the estuary wetland, sieve them, extract them with KCl solution, shake and filter them to obtain the soil test solution; Step S2: Prepare an improved buffer solution containing disodium hydrogen phosphate, potassium sodium tartrate, and sodium hydroxide; prepare a sodium dichloroisocyanurate (DCI) colorimetric solution; prepare a sodium salicylate-sodium nitroprusside colorimetric solution; prepare an ammonium sulfate standard stock solution and perform stepwise dilutions to obtain a series of standard solutions with concentration gradients. Step S3: Place the standard solution and the soil test solution into the injector of the continuous flow analyzer, and replace the low-concentration injection tube with a high-concentration injection tube. Step S4: Using the improved buffer solution, sodium dichloroisocyanurate colorimetric solution and sodium salicylate-sodium nitroprusside colorimetric solution as colorimetric reagents, connect them to the corresponding reagent lines of the continuous flow analyzer to sequentially measure the standard solution and the soil test solution. Step S5: The continuous flow analyzer fits a standard curve based on the response signal value of the standard solution and the corresponding ammonia nitrogen concentration, and calculates and outputs the concentration of ammonia nitrogen in the soil test solution based on the standard curve; Step S6: Calculate the soil ammonia nitrogen content based on the concentration of ammonia nitrogen in the soil test solution, combined with the mass of the soil sample before soaking and the volume of KCl solution used.

[0009] In step S1: the particle size of the sieved soil is less than or equal to 2 mm; the solid-liquid ratio of the sieved soil to the KCl solution is 5 mL to 20 mL of KCl solution per gram of soil; the concentration of the KCl solution is 0.5 mol / L to 2 mol / L; and the shaking time is 1 hour to 2 hours.

[0010] In step S2, the concentrations of each substance in the improved buffer solution are as follows: disodium hydrogen phosphate 30g / L~40g / L, potassium sodium tartrate 40g / L~60g / L, and sodium hydroxide 5g / L~15g / L.

[0011] In step S2, the concentration of the sodium dichloroisocyanurate colorimetric solution is 2 g / L to 4 g / L.

[0012] In step S2, the concentration of sodium salicylate in the sodium salicylate-sodium nitroprusside colorimetric solution is 35 g / L to 45 g / L, and the concentration of sodium nitroprusside is 0.5 g / L to 2.0 g / L.

[0013] In step S3, the low-concentration injection tube is a low-flow peristaltic pump tube, and the high-concentration injection tube is a high-flow peristaltic pump tube.

[0014] In step S4, the soil test solution is first mixed with a modified buffer solution to adjust the pH of the reaction system to 12.6-12.9 before being tested on the instrument. Then, it is mixed with sodium salicylate-sodium nitroprusside colorimetric solution and sodium dichloroisocyanurate colorimetric solution in sequence. Under the catalysis of sodium nitroprusside, a blue-green indophenol complex is generated, and finally, colorimetric determination is performed.

[0015] In step S4, the measurement parameters of the continuous flow analyzer are: detection wavelength of 660 nm; optical path of the cuvette of 10 mm or 30 mm; and reaction temperature of 37℃ to 50℃.

[0016] In step S5, the continuous flow analyzer fits a standard curve using univariate linear regression based on the linear relationship between the ammonia nitrogen concentration in the standard solution and its corresponding response signal value, and then outputs the response signal value of the soil test solution as an ammonia nitrogen concentration value based on the standard curve.

[0017] In step S6, the formula for calculating soil ammonia nitrogen content is: in, This represents the mass fraction of ammonia nitrogen in the soil. c represents the concentration of ammonia nitrogen in the soil test solution calculated by the continuous flow analyzer. V is the volume of KCl solution, mL; m is the mass of the soil sample before soaking, g.

[0018] The beneficial effects of this invention are as follows: (1) Effectively eliminates interference from high pH and high salinity soil matrix and solves the technical problem of abnormal sample response signal: This invention is designed for the characteristics of high pH and high salinity of saline-alkali soil. By replacing the conventional sodium citrate buffer system with a modified buffer system of disodium hydrogen phosphate-potassium sodium tartrate-sodium hydroxide, and removing sodium hydroxide from the sodium dichloroisocyanurate (DCI) solution, and replacing the injection tube with a high-concentration injection tube, the reaction environment of the colorimetric reaction is significantly improved, and the sample response signal is restored to normal, changing from negative or abnormally low values ​​to stable positive values, thus realizing the accurate determination of ammonium nitrogen in saline-alkali soil.

[0019] (2) Simple operation, no additional pretreatment steps required: This invention only requires targeted adjustments to the reagent formulation and injection tube of the continuous flow analyzer. There is no need for additional tedious pretreatment such as neutralization, dilution or matrix matching of soil samples. It does not increase the complexity of operation and is suitable for routine analysis of large batches of soil samples.

[0020] (3) Strong universality and wide range of applications: This invention is not only applicable to saline-alkali soils in coastal estuary wetlands, but can also be extended to the determination of ammonium nitrogen in inland saline-alkali soils, calcareous soils and other high-pH, high-salinity soils, providing a reliable analytical means for the study of nitrogen cycle in saline-alkali soils. Attached Figure Description

[0021] Figure 1 This is a schematic flowchart of the method of the present invention; Figure 2 This is a spectrum showing a negative sample response peak when measured using a conventional reagent system; Figure 3 The spectrum shows the sample response peak recovering to a normal positive value when measured using the improved reagent system of this invention. Detailed Implementation

[0022] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0023] The method for determining ammonia nitrogen content in saline-alkali soil of estuarine wetlands based on a continuous flow analyzer, as described in this invention, has the following basic procedure: Figure 1 As shown. In this embodiment, typical saline-alkali soil from estuary wetlands was selected as the sample to be tested. The specific implementation steps are as follows: Step S1: Soil sample collection and pretreatment.

[0024] Samples of saline-alkali soil from estuarine wetlands were collected. The freshly collected saline-alkali soil samples were thoroughly mixed and passed through a 2 mm sieve. The sieved soil was then extracted with a 1 mol / L KCl solution, with a solid-liquid ratio of 10 mL of KCl solution per gram of soil. After shaking for 1 hour, the soil was filtered to obtain the soil test solution.

[0025] Step S2: Prepare the improved reagent system.

[0026] This step involves preparing the buffer solution, colorimetric reagent, and standard solution according to the following formula: (1) Improved buffer solution: containing disodium hydrogen phosphate, potassium sodium tartrate and sodium hydroxide, wherein the concentration of disodium hydrogen phosphate is 35.8 g / L, the concentration of potassium sodium tartrate is 50 g / L and the concentration of sodium hydroxide is 10 g / L; (2) Sodium dichloroisocyanurate (DCI) colorimetric solution: The concentration of sodium dichloroisocyanurate colorimetric solution is 3 g / L.

[0027] (3) Sodium salicylate-sodium nitroprusside colorimetric solution: The concentration of sodium salicylate in the sodium salicylate-sodium nitroprusside colorimetric solution is 40 g / L and the concentration of sodium nitroprusside is 1 g / L.

[0028] (4) Ammonium sulfate standard solution: Prepare ammonium sulfate standard stock solution and dilute it stepwise to obtain a series of standard solutions with different concentration gradients; Step S3: Replace the sample inlet tube of the continuous flow analyzer.

[0029] The sample inlet tubing of the continuous flow analyzer was replaced with a high-flow-rate peristaltic pump tubing for high concentrations instead of the conventional low-flow-rate tubing used for low-concentration samples. This adjustment helps improve the flow characteristics of high-pH, high-salinity samples in the sample introduction system and reduces signal interference from the samples.

[0030] Step S4: Measurement on the instrument.

[0031] The series of ammonium sulfate standard solutions with varying concentrations prepared in step S2, or the soil test solution prepared in step S1, are placed in the sampler of a continuous flow analyzer. The modified buffer solution, sodium dichloroisocyanurate (DCI) colorimetric solution, and sodium salicylate-sodium nitroprusside colorimetric solution prepared in step S2 are used as colorimetric reagents and connected to the corresponding reagent lines of the continuous flow analyzer. Before analysis, the soil test solution or standard solution is first mixed with the modified buffer solution to adjust the pH of the reaction system to 12.7. Then, it is sequentially mixed with the sodium salicylate-sodium nitroprusside colorimetric solution and the sodium dichloroisocyanurate (DCI) colorimetric solution. Under the catalysis of sodium nitroprusside, a blue-green indophenol complex is generated, and finally, colorimetric determination is performed. Following the above steps, the standard solutions and soil test solutions are sequentially analyzed using the analyzer.

[0032] like Figure 2 As shown, when using a conventional reagent system, the response peaks at each concentration point of the first six standard curves in the figure are normal, but the response peaks of the subsequently injected soil test solution all drop below the baseline and show negative values, making it impossible to obtain valid data.

[0033] like Figure 3As shown, after using the improved reagent system prepared in step S2 of this invention and replacing the high-concentration, high-flow-rate peristaltic pump tube in step S3, the sample response peak in the same batch of soil samples recovered to a stable positive value, with regular peak shape and normal signal.

[0034] Step S5: Standard curve fitting.

[0035] The continuous flow analyzer uses a univariate linear regression to fit a standard curve based on the linear relationship between the ammonia nitrogen concentration in the standard solution and its corresponding response signal value. Based on the standard curve, the response signal value of the soil test solution is converted into an ammonia nitrogen concentration value and output.

[0036] Step S6: Calculate the result.

[0037] Based on the concentration of ammonia nitrogen in the soil test solution, combined with the mass of the soil sample before soaking and the volume of KCl solution used, the content of ammonium nitrogen in the soil is calculated according to the following formula: in, This represents the mass fraction of ammonium nitrogen in the soil. c represents the concentration of ammonia nitrogen in the soil test solution calculated by the continuous flow analyzer. V is the volume of KCl solution, mL; m is the mass of the soil sample before soaking, g.

[0038] In this embodiment, the method of the present invention was used to measure saline-alkali soil samples from estuarine wetlands. All sample response peaks were normal positive values, and the linear correlation coefficient R of the standard curve was [value missing]. 2 The spiked recovery rate was >0.999, and the spiked recovery rate was between 95% and 105%, indicating that the method of the present invention is accurate and reliable and can effectively eliminate matrix interference in saline-alkali soil.

[0039] To verify the accuracy of the results obtained in this invention, a method comparison experiment was conducted using spectrophotometry as a reference method in this embodiment. KCl extracts from the same batch of soil samples were used to determine ammonia nitrogen using both the method of this invention (continuous flow analyzer) and the indophenol blue colorimetric method (spectrophotometer). The spectrophotometric determination conditions were as follows: sodium salicylate as the colorimetric reagent, sodium dichloroisocyanurate as the chlorine source, and sodium nitroprusside as the catalyst. The absorbance was measured at 660 nm using a 10 mm cuvette. The operation method followed the method for determining soil ammonium nitrogen in "Methods for Soil Agricultural Chemical Analysis" (edited by Lu Rukun, China Agricultural Science and Technology Press, 1999).

[0040] The same saline-alkali soil sample was measured using the two methods described above, and the values ​​measured by the continuous flow analyzer were compared with those measured by the spectrophotometer. The results are shown in Table 1.

[0041] Table 1: Comparison of measurement results between the method of the present invention and the spectrophotometric method

[0042] Linear regression analysis was performed on the two sets of measurement results, and the regression equation was: y = 0.99x + 0.05; where x is the measurement value obtained by the method of this invention, y is the measurement value obtained by spectrophotometry, and the correlation coefficient R is 0.99x + 0.05. 2 =0.993, indicating that the results of the method of the present invention are highly consistent with those of the spectrophotometric method, further proving that the method of the present invention can accurately determine the ammonium nitrogen content in high saline-alkali soil.

Claims

1. A method for determining ammonia nitrogen content in saline-alkali soils of estuarine wetlands based on a continuous flow analyzer, characterized in that, Includes the following steps: Step S1: Collect saline-alkali soil samples from the estuary wetland, sieve them, extract them with KCl solution, shake and filter them to obtain the soil test solution; Step S2: Prepare an improved buffer solution containing disodium hydrogen phosphate, potassium sodium tartrate, and sodium hydroxide; prepare a sodium dichloroisocyanurate colorimetric solution; prepare a sodium salicylate-sodium nitroprusside colorimetric solution; prepare an ammonium sulfate standard stock solution and perform stepwise dilutions to obtain a series of standard solutions with varying concentration gradients. Step S3: Place the standard solution and the soil test solution into the injector of the continuous flow analyzer, and replace the low-concentration injection tube with a high-concentration injection tube. Step S4: Using the improved buffer solution, sodium dichloroisocyanurate colorimetric solution and sodium salicylate-sodium nitroprusside colorimetric solution as colorimetric reagents, connect them to the corresponding reagent lines of the continuous flow analyzer to sequentially measure the standard solution and the soil test solution. Step S5: The continuous flow analyzer fits a standard curve based on the response signal value of the standard solution and the corresponding ammonia nitrogen concentration, and calculates and outputs the concentration of ammonia nitrogen in the soil test solution based on the standard curve; Step S6: Calculate the soil ammonia nitrogen content based on the concentration of ammonia nitrogen in the soil test solution, combined with the mass of the soil sample before soaking and the volume of KCl solution used.

2. The method for determining ammonia nitrogen content in saline-alkali soil of estuarine wetlands based on a continuous flow analyzer according to claim 1, characterized in that, In step S1: the particle size of the sieved soil is less than or equal to 2 mm; the solid-liquid ratio of the sieved soil to the KCl solution is 5 mL to 20 mL of KCl solution per gram of soil; the concentration of the KCl solution is 0.5 mol / L to 2 mol / L; and the shaking time is 1 hour to 2 hours.

3. The method for determining ammonia nitrogen content in saline-alkali soil of estuarine wetlands based on a continuous flow analyzer according to claim 1, characterized in that, In step S2, the concentrations of each substance in the improved buffer solution are as follows: disodium hydrogen phosphate 30g / L~40g / L, potassium sodium tartrate 40g / L~60g / L, and sodium hydroxide 5g / L~15g / L.

4. The method for determining ammonia nitrogen content in saline-alkali soil of estuarine wetlands based on a continuous flow analyzer according to claim 1, characterized in that, In step S2, the concentration of the sodium dichloroisocyanurate colorimetric solution is 2 g / L to 4 g / L.

5. The method for determining ammonia nitrogen content in saline-alkali soil of estuarine wetlands based on a continuous flow analyzer according to claim 1, characterized in that, In step S2, the concentration of sodium salicylate in the sodium salicylate-sodium nitroprusside colorimetric solution is 35 g / L to 45 g / L, and the concentration of sodium nitroprusside is 0.5 g / L to 2.0 g / L.

6. The method for determining ammonia nitrogen content in saline-alkali soil of estuarine wetlands based on a continuous flow analyzer according to claim 1, characterized in that, In step S3, the low-concentration injection tube is a low-flow peristaltic pump tube, and the high-concentration injection tube is a high-flow peristaltic pump tube.

7. The method for determining ammonia nitrogen content in saline-alkali soil of estuarine wetlands based on a continuous flow analyzer according to claim 1, characterized in that, In step S4, when performing the test on the instrument, the soil test solution or standard solution is first mixed with the modified buffer solution to adjust the pH of the reaction system to 12.6~12.

9. Then, it is mixed with sodium salicylate-sodium nitroprusside colorimetric solution and sodium dichloroisocyanurate colorimetric solution in sequence. Under the catalysis of sodium nitroprusside, a blue-green indophenol complex is generated, and finally, the colorimetric determination is performed.

8. The method for determining ammonia nitrogen content in saline-alkali soil of estuarine wetlands based on a continuous flow analyzer according to claim 1, characterized in that, In step S4, the measurement parameters of the continuous flow analyzer are: detection wavelength of 660 nm; optical path of the cuvette of 10 mm or 30 mm; and reaction temperature of 37℃ to 50℃.

9. The method for determining ammonia nitrogen content in saline-alkali soil of estuarine wetlands based on a continuous flow analyzer according to claim 1, characterized in that, In step S5, the continuous flow analyzer fits a standard curve using univariate linear regression based on the linear relationship between the ammonia nitrogen concentration in the standard solution and its corresponding response signal value, and then outputs the response signal value of the soil test solution as an ammonia nitrogen concentration value based on the standard curve.

10. The method for determining ammonia nitrogen content in saline-alkali soil of estuarine wetlands based on a continuous flow analyzer according to claim 1, characterized in that, In step S6, the formula for calculating soil ammonia nitrogen content is: in, denoted as ρ, where c is the mass fraction of ammonia nitrogen in the soil; c is the concentration of ammonia nitrogen in the soil test solution calculated by the continuous flow analyzer; V is the volume of the KCl solution; and m is the mass of the soil sample before soaking.

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