CN106885780A - A kind of method for determining soil nitrate-N - Google Patents

Abstract

It is specifically a kind of method for determining soil nitrate-N and nitrite nitrogen summation the present invention relates to a kind of method for determining soil nitrate-N.Pedotheque is vibrated into extraction with 4 times of 2mol/L KCl solution of bulking value (g/ml), filtering, gained leaching liquor uses A in 220nm and 275nm absorbances are determined on ultraviolet-visible spectrophotometer₂₂₀–2×A₂₇₅Computational methods try to achieve absorbance, and establishing criteria curve, nitrate in inquiry leaching liquor.The concentration passes through formula：Y=1.021x-0.223 is corrected, and as a result determining ammonium nitrogen with the way of distillation is combined, and is set up complete set, efficient, the accurate way of distillation and is determined the inorganic nitrogen systems of soil available.The inventive method is reproducible, and reappearance is high, simple and easy to apply.

Description

A method for measuring soil nitrate nitrogen

technical field

The invention relates to the determination of nitrate nitrogen in soil, in particular to an improved determination method of soil nitrate nitrogen.

Background technique

Inorganic nitrogen in soil can be divided into water-soluble state, exchange state and fixed state. The determination of ammonium, nitrate and nitrite nitrogen in soil is usually the total amount of water-soluble and exchangeable nitrogen. The exchanged state is the part that can be exchanged and extracted by neutral salt solution (neutral salt solution is potassium chloride or sodium chloride). The neutral salt solution is mixed and shaken with the soil to exchange and leach the ammonium nitrogen adsorbed by the soil, which also includes the water-soluble state. The extract was filtered, and the filtrate was subjected to the determination of various forms of nitrogen. Commonly used methods for the determination of ammonium nitrogen include distillation and indophenol blue colorimetry, but there are many methods for the determination of nitrate nitrogen, including distillation, phenol disulfonic acid method, cadmium reduction method and dual-wavelength ultraviolet spectrophotometry ( Document 1: Lu Rukun, Soil Agricultural Chemical Analysis Methods, Beijing: China Agricultural Science and Technology Press, 2000: 156-165).

What needs to be explained is that nitrite nitrogen is also an exchange nitrogen. During the growth of crops, it is used as an effective nitrogen fertilizer to provide nutrients for crops, even if its content is extremely low, less than 5% of nitrate nitrogen. The values measured in the above methods for measuring soil nitrate nitrogen can all contain nitrite nitrogen. Here we are concerned with methods for determining the sum of nitrate-nitrogen and nitrite-nitrogen in soils.

Commonly used methods for determining nitrate nitrogen have the following disadvantages: the distillation method requires a specific device, consumes a lot of energy, and it is difficult to grind nitrogen-fixing alloys through a 100-mesh sieve; It takes a long time, consumes energy, and interferes with various substances; although the cadmium column reduction method has less interference and high precision, it is cumbersome to operate and cadmium is a highly toxic heavy metal, which is easy to pollute the environment; there is no unified standard for the determination of wavelength in dual-wavelength ultraviolet spectrophotometry (Document 2: Tu Changqing, Wen Xinrong, Determination of soil nitrate nitrogen by dual-wavelength ultraviolet spectrophotometry, Soil Fertilizer, 2006 (1): 50-51).

Contents of the invention

The purpose of the invention is to provide a method for measuring soil nitrate nitrogen with simple operation, no addition of any reagent, good repeatability and high reproducibility.

To achieve the above object, the technical scheme adopted in the present invention is as follows:

A method for measuring soil nitrate nitrogen,

1) Use 2mol/L KCl as a solvent to prepare more than 6 different concentrations of nitrate nitrogen solutions in the range of 0.1-20mg N/L, and use a UV-visible spectrophotometer to measure the concentration of each prepared solution at 220nm and 275nm wavelengths respectively. Solution absorbance A ₂₂₀ and A ₂₇₅ ; use the calculation method of A _220-2 ×A ₂₇₅ to obtain the absorbance, take the nitrate nitrogen concentration as the abscissa, and calculate the absorbance value as the ordinate to make a standard curve;

2) Preparation of soil extract: shake and extract the soil sample with 2 mol/L KCl solution of 4 times the weight volume (g/ml) for more than 1 hour, filter to obtain the extract;

3) Place the extract solution directly in a UV-visible spectrophotometer to measure the absorbance values at 220nm and 270nm, use the calculation method of A ₂₂₀ -2×A ₂₇₅ to obtain the absorbance, and use the obtained absorbance to make the standard in step 1) Query the curve to obtain the concentration of nitrate nitrogen in the extract;

4) The UV spectrophotometry measured value is corrected into the distillation method measured value, the formula is y=1.021x-0.223, wherein x is the soil nitrate nitrogen concentration measured value of the UV spectrophotometric method, and y is the soil nitrate state of the distillation method Measured value of nitrogen concentration.

The specific process of the step 1) preparing the nitrate nitrogen solution is: using 2mol/L KCl as a solvent to prepare nitrate nitrogen solutions with a concentration of more than 6 different concentrations in the range of 0.1-20mg N/L, including 0.1mgN/ Nitrate nitrogen solutions with two extreme concentrations of L and 20mg N/L, and more than 4 different concentrations of nitrate nitrogen solutions from more than 0.1mg N/L to less than 20mg N/L.

The specific process of the step 1) preparing the nitrate nitrogen solution is: using 2mol/L KCl as a solvent to prepare nitrate nitrogen solutions with a concentration of more than 6 different concentrations within the range of 0.1-15mg N/L, including 0.1mgN/ Nitrate nitrogen solutions with two extreme concentrations of L and 15mg N/L, and more than 4 different concentrations of nitrate nitrogen solutions from more than 0.1mg N/L to less than 15mg N/L.

The specific process of the step 1) is as follows: draw 0.05, 0.25, 0.5, 2.5, 5.0, and 7.5 mL of nitrate nitrogen solutions with a concentration of 100 mg N/L in a 50 mL volumetric flask, and add 2 mol/L KCl to the 50 mL mark , shake well, and measure the absorbance at 220nm and 275nm wavelengths with a UV-visible spectrophotometer; use the calculation method of A ₂₂₀ – 2×A ₂₇₅ to obtain the absorbance, take the nitrate nitrogen concentration as the abscissa, and calculate the absorbance value as the ordinate Coordinates to make a standard curve, the linear range of the standard curve is 0.1-15mg N/L, and the standard curve is generally a polynomial equation.

The solute used for preparing the nitrate nitrogen solution is potassium nitrate.

The specific process can be: a method for measuring soil nitrate nitrogen,

1) Make a standard curve: draw 0.05, 0.25, 0.5, 2.5, 5.0, 7.5mL of 100mg N/L nitrate nitrogen solution (prepared with 2mol/L KCl) into a 50mL volumetric flask, add 2mol/L KCl to constant volume, and shake well , use a UV-visible spectrophotometer to measure the absorbance at 220nm and 275nm wavelengths, use the calculation method of A ₂₂₀ – 2×A ₂₇₅ to obtain the absorbance, take the concentration of nitrate nitrogen as the abscissa, and calculate the absorbance value as the ordinate to make a standard curve ;

2) Preparation of soil extract: shake and extract the soil sample with 4 times the weight volume (g/ml) of 2mol/L KCl solution for 1 hour, filter to obtain the extract;

3) Place the extract directly in a UV-Vis spectrophotometer to measure the absorbance at 220nm and 270nm, use the calculation method of A ₂₂₀ -2×A ₂₇₅ to obtain the absorbance, and query the standard curve to calculate the concentration of nitrate nitrogen in the extract ;

4) Correct the value measured by UV spectrophotometry into the value measured by distillation, the formula is y=1.021x-0.223 (R ² =0.9722, n=20), where x is the value measured by UV spectrophotometry, and y is the value of distillation measured value.

Using this method to determine the content of nitrate nitrogen in soil has the following advantages:

1. The method is easy to operate, with few steps, without adding any reagents, and the ultraviolet absorption measurement of the extract is directly carried out, and the human error is small.

2. This method saves trouble. The original distillation method or colorimetric method can measure no more than 100 nitrate nitrogen in one day, but this method can be used to measure 1000 nitrate nitrogen in the same time, and the efficiency is increased by 9 times.

3. The soil nitrate nitrogen concentration measured by this method has an excellent correlation with the value determined by the distillation method (R ² =0.9722, n=20), so the measured value can be corrected and combined with the determination of ammonium nitrogen by the distillation method to establish A complete, efficient and accurate method system for the determination of soil available inorganic nitrogen.

The method of the invention has good repeatability, high reproducibility, and is simple and easy to implement.

Description of drawings

Fig. 1 nitrate nitrogen determination standard curve-1;

Fig. 2 nitrate nitrogen determination standard curve-2;

Figure 3 Comparison of the results of the determination of soil nitrate nitrogen by distillation and ultraviolet spectrophotometry;

Figure 4 validates the correlation between distillation and UV spectrophotometry for the determination of nitrate nitrogen.

detailed description

Example 1

Establishment of Standard Curve for Determination of Nitrate Nitrogen by Ultraviolet Spectrophotometry

Since the concentration of nitrate nitrogen in the soil extract is directly measured without any operation, the concentration of nitrate nitrogen should all fall within the linear range of the standard curve, otherwise the determination result will be inaccurate. After calculation, when the nitrate nitrogen concentration in the soil solution is 15mg N/L, the soil nitrate nitrogen content is about 66kg/mg, which is equivalent to the nitrogen fertilizer application rate of 158kg N/ha (0-20cm soil bulk density is 1.2g/ ^cm3 ), so the maximum concentration of nitrate nitrogen in the standard curve is set to 15m N g/L (if necessary, it can also be set to 20mg N/L), which can fully meet the needs of the experiment.

After determination, both nitrate nitrogen and nitrite nitrogen measured by ultraviolet spectrophotometry have strong absorption peaks at 220nm, so the measurement results of this method include the content of nitrite nitrogen, which is different from the nitrite nitrogen measured by distillation. Matching with the sum of nitrite nitrogen becomes the experimental basis for the mutual verification of this method and the distillation method. In addition, soil nitrite nitrogen is an intermediate process of soil nitrification, and its content is extremely low, which is usually negligible during the determination process. Therefore, the standard curve solution does not add standard nitrite nitrogen, and its absorbance is calculated against nitrate nitrogen.

The specific process is as follows:

1) Preparation of nitrate nitrogen stock solution: Accurately weigh 7.216g of dried potassium nitrate, add 2mol/L KCl to dissolve, and dilute to a 1L volumetric flask. This solution contains 1000mg/L of nitrate nitrogen. Dilute this stock solution by a factor of 10, and constant volume with 2mol/L KCl to obtain a solution with a concentration of 100mg N/L;

2) Make a standard curve: draw 0.05, 0.25, 0.5, 2.5, 5.0, 7.5mL of the above-mentioned nitrate nitrogen solution into a 50mL volumetric flask, add 2mol/L KCl to the mark, shake well, and use a UV-visible spectrophotometer to Absorbance was measured at wavelengths of 220 nm and 275 nm. Use the calculation method of A ₂₂₀ -2×A ₂₇₅ to obtain the absorbance, take the concentration of nitrate nitrogen as the abscissa, and calculate the absorbance value as the ordinate to make a standard curve. The results are shown in Figure 1, the standard curve conforms to the polynomial equation, R ² =0.998 (n=7).

Example 2

Comparison of Ultraviolet Spectrophotometry and Distillation Method for Determination of Nitrate Nitrogen Concentration

The specific implementation process:

1) Preparation of soil extract: soil samples (n=44 soil samples in total) were oscillated and extracted with 4 times the weight volume (g/ml) of 2mol/L KCl solution for 1 hour, filtered to obtain the extract. This extraction sample is divided into two parts, and a part (total n=20 soil samples) is used for determining the correlation of two kinds of methods to measure nitrate nitrogen concentration, and remaining sample (total n=24 soil samples) is used for verifying this correlation accuracy;

2) Make a standard curve: draw 0.05, 0.25, 0.5, 2.5, 5.0, and 7.5 mL of nitrate nitrogen solution with a concentration of 100 mg N/L into a 50 mL volumetric flask, add 2 mol/L KCl to the mark, shake well, and use a UV - Visible spectrophotometer to measure absorbance at wavelengths of 220nm and 275nm. Use the calculation method of A _220-2 ×A ₂₇₅ to obtain the absorbance, take the nitrate nitrogen concentration as the abscissa, and calculate the absorbance value as the ordinate to make a standard curve (Figure 2); the standard curve is generally a polynomial equation;

3) Determination of nitrate nitrogen concentration in the extract by ultraviolet spectrophotometry: the extract in step 1) (total n=20 soil samples) is directly placed in the ultraviolet-visible spectrophotometer to measure the absorbance values at 220nm and 270nm respectively A ₂₂₀ and A ₂₇₅ , use the calculation method of A ₂₂₀ -2×A ₂₇₅ to obtain the absorbance, query the above standard curve to calculate the concentration of nitrate nitrogen in the extract;

4) Determination of nitrate nitrogen concentration in the extract by distillation: take 50mL of the same extract from the above-mentioned determination step 3) (a total of n=20 soil samples) in a retort, and make a blank group in a retort at the same time (with 50mL 2mol/L KCl solution inside) for comparison, add 1g of magnesium oxide respectively, place in the distillation device, immediately pass water vapor into the extraction solution and the blank group for heating and distillation, and receive the distillate at the lower end of the condenser tube When the volume of the distillate in the Erlenmeyer flask reaches 100mL, stop the distillation and remove the Erlenmeyer flask receiving the distillate. The purpose of this step is to remove ammonium nitrogen. Replace the triangular flask receiving the distillate with one containing 15mL of boric acid with a weight concentration of 2% plus one drop (0.05mL) of a mixed indicator (0.066g of methyl red and 0.099g of bromocresol green with a volume concentration of 95% ethanol). 100ml) of the receiving bottle at the lower end of the condenser, continue to add 0.2g nitrogen-fixing alloy (alloy composition: Cu 50%, Al 45%, Zn 5%) to the retort respectively, and continue to feed water vapor into the retort respectively Carry out distillation until the volume of the distillate in the receiving bottle reaches 100mL, stop the distillation and remove the receiving bottle. Titrate with 0.003mol·L ^-1 sulfuric acid standard solution, when the solution turns red from gray, it is the titration end point. Calculate the nitrate nitrogen content according to the amount of sulfuric acid consumed by the sample (document 1: Lu Rukun, Soil Agricultural Chemical Analysis Methods, Beijing: China Agricultural Science and Technology Press, 2000: 156-165);

The measurement results are shown in Figure 3. The concentration of nitrate nitrogen in the randomly selected soil extracts varies greatly, ranging from 0.1-3.7 mg/L, which is highly representative. The correlation coefficient of the two methods is R=0.986, which shows that the correlation between the two methods is good, and the tedious distillation method can be replaced by ultraviolet spectrophotometry for the determination of soil nitrate nitrogen content.

5) Take the remaining 24 soil extracts, according to the above-mentioned 3)-4) steps, use two methods to measure its nitrate nitrogen concentration, and measure the results by ultraviolet spectrophotometry according to the formula y=1.021 x-0.223 correction, the obtained result is compared with the distillation method measurement result, and the result is shown in Figure 4. The results show that the correlation between the two methods is very good, especially for samples with very low concentrations, the measurement results are still very consistent, indicating that the simple operation of ultraviolet spectrophotometry can completely replace the cumbersome distillation method and become an important method for the determination of soil nitrate nitrogen .

Claims (5)

1. it is a kind of determine soil nitrate-N method, it is characterised in that：

1) with 2mol/L KCl as solvent compound concentration be more than 6 in the range of 0.1-20mg N/L not With the nitrate nitrogen solution of concentration, respectively with ultraviolet-visible spectrophotometer at 220nm and 275nm wavelength Determine each and prepare solution absorbance A₂₂₀And A₂₇₅；Use A₂₂₀–2×A₂₇₅Computational methods try to achieve absorbance, With nitrate as abscissa, the absorbance of calculating is ordinate making standard curve；

2) prepared by soil extraction：By pedotheque with 4 times of 2mol/L KCl solution of bulking value (g/ml) Vibration extraction more than 1h, filtering obtains leaching liquor；

3) leaching liquor is placed directly within ultraviolet-visible spectrophotometer and determines 220nm and 270nm absorbances Value, uses A₂₂₀–2×A₂₇₅Computational methods try to achieve absorbance, by the absorbance tried to achieve in step 1) make Standard curve in inquire about obtain leaching liquor in nitrate；

4) correction of ultraviolet spectrophotometry method measured value is turned into way of distillation measured value, formula is Y=1.021x-0.223, wherein x are the soil nitrate-N concentration measurement of ultraviolet spectrophotometry, and y is distillation The soil nitrate-N concentration measurement of method.

2. in accordance with the method for claim 1, it is characterised in that：

The step 1) prepare nitrate nitrogen solution detailed process be：It is solvent compound concentration with 2mol/L KCl It is the nitrate nitrogen solution of more than 6 various concentrations in the range of 0.1-20mg N/L, including 0.1mg The nitrate nitrogen solution of bis- end value concentration of N/L and 20mg N/L, and more than 0.1mg N/L to less than 20 The nitrate nitrogen solution of more than 4 various concentrations of mg N/L.

3. in accordance with the method for claim 1, it is characterised in that：

The step 1) prepare nitrate nitrogen solution detailed process be：It is solvent compound concentration with 2mol/L KCl It is the nitrate nitrogen solution of more than 6 various concentrations in the range of 0.1-15mg N/L, including 0.1mg The nitrate nitrogen solution of bis- end value concentration of N/L and 15mg N/L, and more than 0.1mg N/L to being less than The nitrate nitrogen solution of more than 4 various concentrations of 15mg N/L.

4. according to the method described in claim 1 or 3, it is characterised in that：

The step 1) detailed process be：Respectively draw concentration for 100mg N/L nitrate nitrogen solution 0.05, 0.25th, 0.5,2.5,5.0,7.5mL adds 2mol/L KCl to 50mL to carve in 50mL volumetric flasks Degree line, shake up, with ultraviolet-visible spectrophotometer at 220nm and 275nm wavelength mensuration absorbance； Use A₂₂₀–2×A₂₇₅Computational methods try to achieve absorbance, with nitrate as abscissa, the absorbance of calculating It is worth for ordinate makes standard curve, the standard curve range of linearity is 0.1-15mg N/L, standard curve one As be polynomial equation.

5. according to any described methods of claim 1-4, it is characterised in that：The preparation nitrate nitrogen is molten The solute that liquid is used is potassium nitrate.