Soil nitrogen leaching capacity monitoring method based on frequency domain reflection method

Abstract

The invention discloses a soil nitrogen leaching capacity monitoring method based on a frequency domain reflectometry, and relates to the technical field of soil water environment monitoring, in particular to a nitrogen leaching online monitoring method based on the frequency domain reflectometry (FDR), the Darcy law and a Van Genuchten (VG) model. And a monitoring method with high precision and sensitivity, multi-parameter synchronous detection and low cost is provided for the soil nitrogen leaching capacity. Comprising the following steps: 1, collecting and pretreating a soil sample; 2, placing the treated soil sample in a soil experiment cylinder; 3, inserting the multi-parameter sensors into the soil experiment cylinder in a layered manner; 4, 1000 ml of water is injected, and data acquisition is carried out; and 5, realizing soil nitrogen leaching capacity monitoring in combination with a Darcy law and a VG model.

Description

Technical Field

[0001] The present invention relates to the technical field of smart agriculture and soil environment monitoring, and specifically to a soil nitrogen leaching capacity monitoring method based on frequency domain reflectometry. Background Art

[0002] Soil nitrogen, a key nutrient for crop growth, leaches from it not only impacts crop yield and quality but also causes environmental problems such as groundwater pollution. Accurately monitoring soil nitrogen leaching is crucial for optimal fertilization and ecological and environmental protection. Currently, traditional soil nitrogen leaching monitoring methods primarily include field sampling combined with laboratory analysis, and fixed-point monitoring using tensiometers and soil solution samplers. While field sampling and analysis can yield accurate data, they suffer from issues such as poor timeliness, high labor intensity, and difficulty achieving continuous monitoring. Furthermore, equipment such as tensiometers and soil solution samplers is limited by complex installation, susceptibility to environmental interference, and high monitoring costs. Furthermore, they cannot reflect the dynamic changes in soil nitrogen in real time. With the increasing demand for precision agricultural management, there is an urgent need for efficient, real-time, and non-destructive soil nitrogen leaching monitoring technology. Frequency domain reflectometry (FDR), based on the propagation characteristics of electromagnetic waves in soil, can rapidly determine the dielectric constant of soil, and subsequently infer parameters such as soil moisture content and salinity. In recent years, this technology has achieved significant results in soil moisture monitoring, but its application to soil nitrogen leaching monitoring remains a technical gap. Therefore, adopting a soil nitrogen leaching monitoring method based on frequency domain reflectometry has important practical significance for promoting the efficient use of agricultural resources and the sustainable development of the ecological environment. Summary of the Invention

[0003] The present invention aims to provide a soil nitrogen leaching monitoring method based on frequency domain reflectometry, enabling real-time and accurate monitoring of soil nitrogen leaching information. By monitoring the dynamic changes of nitrogen leaching in real time, this method provides reliable data support for precision agricultural fertilization and environmental protection.

[0004] To achieve the above object, the present invention provides the following technical solution: a method for monitoring soil nitrogen leaching capacity based on frequency domain reflectometry, comprising the following steps: Step 1: Obtain soil samples and sieve them to remove impurities; Step 2: Place the sieved soil in the soil test tube and compact it during the placement process; Step 3: Insert the calibrated multi-parameter sensors vertically into the equally spaced holes pre-opened on the side wall of the soil test cylinder according to the preset measurement depth. When inserting, the sensor probes must be aligned with the center axis of the holes. Step 4: Accurately measure 1000ml of water and inject it into the test tube at a constant flow rate through the porous disperser at the top of the test tube. Ensure that the water level rises evenly during the injection process to prevent water erosion and soil structure disturbance. Immediately after water injection, start the data acquisition system and set the sampling frequency (collect data every 10 seconds). Monitor until the data is stable. Step 5: Input the soil particle size percentage into the RETC software to obtain the VG model parameters and calculate the unsaturated hydraulic conductivity according to the VG model; Step 6: Combine the unsaturated hydraulic conductivity obtained and substitute it into Darcy's law to calculate the water flux; Step 7: Calculate nitrogen migration flux and leaching capacity.

[0005] Preferably, in step 1, the soil sample is crushed, foreign matter is picked out and passed through a 2 mm soil sieve.

[0006] Preferably, the soil test cylinder in step 2 is a plexiglass cylinder with an inner diameter of 20 cm, a height of 100 cm, and holes spaced 20 cm apart, wherein a porous disperser is installed on the top of the test cylinder. During the filling process, a layered compaction method is used to compact the soil particles to ensure that they are fully and tightly arranged.

[0007] Preferably, the multi-parameter sensor in step three is a soil sensor based on frequency domain reflectometry. An internal oscillator circuit in the sensor generates a specific frequency signal. After the signal is transmitted through the probe into the soil, the complex dielectric constant is calculated by detecting the phase difference and amplitude attenuation between the reflected and incident waves. The real and imaginary parts of the signal reflect the moisture content and conductivity, respectively. The sensor pins must be inserted into the test tube once. After installation, the holes and gaps must be sealed with waterproof sealant to prevent external interference and evaporation of soil moisture along the channels.

[0008] Preferably, the data acquisition system in step 4 includes: FDR sensor → RS485 bus (Modbus RTU protocol) → data recording display (OLED display)

[0009] Preferably, the soil particle size percentage in step 5 is measured by a hydrometer method. 100 g of pretreated and 2 mm sieved soil sample is dispersed with sodium hexametaphosphate and then fixed to 1000 ml. After stirring, the suspension density is measured with a hydrometer at different time points, and the percentage of particles of each size is calculated according to the formula. The calculation formula for unsaturated hydraulic conductivity is: Where, is the volumetric water content, is the saturated moisture content, is the residual moisture content, m is the VG model parameter, is the unsaturated hydraulic conductivity, is the saturated hydraulic conductivity.

[0010] Preferably, the formula for calculating the moisture flux in step 6 is: Where q is the moisture flux (positive if downward), is the soil water potential gradient, and “+1” is the gravity term.

[0011] Preferably, the calculation formula for the nitrogen migration flux in step seven is: Where, is the nitrogen flux, and C is the nitrogen mass per unit volume of soil solution. The calculation formula for the leaching capacity is: Where, is the nitrogen lost due to leaching, is the nitrogen flux, L is the target soil depth, z is the depth, t is the measurement time, and are the start and end times respectively.

[0012] Compared with existing technologies, this method offers the following advantages: Using frequency domain reflectometry, it can accurately and in real time measure changes in soil conductivity and other data, indirectly reflecting nitrogen leaching dynamics and overcoming the lag inherent in traditional sampling. This provides real-time data support for precision fertilization, significantly reducing the risk of nitrogen loss and water pollution caused by excessive fertilization, and improving resource utilization efficiency and environmental sustainability. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 The present invention is a flowchart of the steps of a soil nitrogen leaching capacity monitoring method based on frequency domain reflectometry.

[0014] Figure 2 This is a structural schematic diagram of the equipment used in the soil nitrogen leaching capacity monitoring method based on the frequency domain reflectometry of the present invention.

[0015] Figure 3 The figure is a side view of an apparatus used in a method for monitoring soil nitrogen leaching capacity based on frequency domain reflectometry according to the present invention.

[0016] Figure 4 This is a top view of the equipment used in the soil nitrogen leaching capacity monitoring method based on the frequency domain reflectometry of the present invention.

[0017] Figure 5 This is a schematic diagram of the bottom structure of an apparatus used in a method for monitoring soil nitrogen leaching capacity based on frequency domain reflectometry according to the present invention.

[0018] Reference numerals: 1-Porous disperser, 2-OLED display, 3- RS485 bus, 4-Soil test cylinder, 5-FDR sensor, 6-Soil cylinder hole. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0020] See also Figure 1-5 A soil nitrogen leaching capacity monitoring method based on frequency domain reflectometry comprises the following steps: Step 1: Obtain soil samples and sieve them to remove impurities; Step 2: Place the sieved soil in the soil test tube and compact it during the placement process; Step 3: Insert the calibrated multi-parameter sensors vertically into the equally spaced holes pre-opened on the side wall of the soil test cylinder according to the preset measurement depth. When inserting, the sensor probes must be aligned with the center axis of the holes. Step 4: Accurately measure 1000ml of water and inject it into the test tube at a constant flow rate through the porous disperser at the top of the test tube. Ensure that the water level rises evenly during the injection process to prevent water erosion and soil structure disturbance. Immediately after water injection, start the data acquisition system and set the sampling frequency (collect data every 10 seconds). Monitor until the data is stable. Step 5: Input the soil particle size percentage into the RETC software to obtain the VG model parameters and calculate the unsaturated hydraulic conductivity according to the VG model; Step 6: Combine the unsaturated hydraulic conductivity obtained and substitute it into Darcy's law to calculate the water flux; Step 7: Calculate nitrogen migration flux and leaching capacity.

[0021] Furthermore, in step 1, the soil sample is crushed, foreign matter is picked out and passed through a 2 mm soil sieve.

[0022] Furthermore, the soil test tube 4 in step 2 is a plexiglass cylinder with an inner diameter of 20 cm, a height of 100 cm, and openings 6 spaced 20 cm apart. A porous disperser 1 is installed on the top of the test tube 4. During the filling process, a layered compaction method is used to ensure that the soil particles are fully and tightly arranged.

[0023] Furthermore, the multi-parameter sensor 5 in step 3 is a soil sensor 5 based on frequency domain reflectometry. An internal oscillating circuit in the sensor 5 generates a specific frequency signal. After the signal is transmitted through the probe into the soil, the complex dielectric constant is calculated by measuring the phase difference and amplitude attenuation between the reflected and incident waves. The real and imaginary parts of the signal reflect the moisture content and conductivity, respectively. The sensor 5 pins must be inserted into the test tube 4 once. After installation, the gaps in the holes 6 must be sealed with waterproof sealant to prevent external interference and evaporation of soil moisture along the holes 6.

[0024] Furthermore, the data acquisition system in step 4 includes: FDR sensor 5 → RS485 bus 3 (ModbusRTU protocol) → data recording display 2 (OLED display)

[0025] Furthermore, the soil particle size percentage in step 5 is measured using a hydrometer method. 100 g of pretreated and 2 mm sieved soil sample is dispersed with sodium hexametaphosphate and then fixed to 1000 ml. After stirring, the suspension density is measured using a hydrometer at different time points, and the percentage of particles of each size is calculated according to the formula. The calculation formula for unsaturated hydraulic conductivity is: Where, is the volumetric water content, is the saturated moisture content, is the residual moisture content, m is the VG model parameter, is the unsaturated hydraulic conductivity, is the saturated hydraulic conductivity.

[0026] Furthermore, the formula for calculating the moisture flux in step 6 is: Where q is the moisture flux (positive if downward), is the soil water potential gradient, and “+1” is the gravity term.

[0027] Furthermore, the calculation formula for the nitrogen migration flux in step seven is: Where, is the nitrogen flux, and C is the nitrogen mass per unit volume of soil solution. The calculation formula for the leaching capacity is: Where, is the nitrogen lost due to leaching, is the nitrogen flux, L is the target soil layer depth, z is the depth, t is the measurement time, and are the start and end times respectively.

[0028] For those skilled in the art, it is obvious that the soil nitrogen leaching monitoring method based on the frequency domain reflectometry described in the present invention is not limited to the operational details of the above-mentioned exemplary embodiments, and can be implemented in other specific forms without departing from the technical core or basic features of the present invention. Therefore, no matter from which perspective, the embodiments should be regarded as illustrative rather than restrictive. The scope of protection of the present invention is defined by the attached claims, rather than the above description, and is intended to include all technical changes that fall within the meaning and scope of the equivalent elements of the claims into the scope of protection. Any figure mark in the claims should not be regarded as a limitation on the corresponding claim.

Claims

1. A soil nitrogen leaching capacity monitoring method based on frequency domain reflectometry, characterized in that: include: Step 1: Obtain soil samples and sieve them to remove impurities; Step 2: Place the sieved soil in the soil test tube and compact it during the placement process; Step 3: Insert the calibrated multi-parameter sensors vertically into the equally spaced holes pre-opened on the side wall of the soil test cylinder according to the preset measurement depth. When inserting, the sensor probes must be aligned with the center axis of the holes. Step 4: Accurately measure 1000ml of water and inject it into the test tube at a constant flow rate through the porous disperser at the top of the test tube. Ensure that the water level rises evenly during the injection process to prevent water erosion and soil structure disturbance. Immediately after water injection, start the data acquisition system and set the sampling frequency (collect data every 10 seconds). Monitor until the data is stable. Step 5: Input the soil particle size percentage into the RETC software to obtain the Van Genuchten (VG) model parameters and calculate the unsaturated hydraulic conductivity based on the VG model; Step 6: Combine the unsaturated hydraulic conductivity obtained and substitute it into Darcy's law to calculate the water flux; Step 7: Calculate nitrogen migration flux and leaching capacity.

2. The method for monitoring soil nitrogen leaching capacity based on frequency domain reflectometry according to claim 1, characterized in that: In step 1, the soil sample is crushed, foreign matter is picked out, and the soil sample is passed through a 2 mm soil sieve.

3. The method for monitoring soil nitrogen leaching capacity based on frequency domain reflectometry according to claim 1, wherein: The soil test tube in step 2 is a plexiglass cylinder with an inner diameter of 20 cm, a height of 100 cm, and holes spaced 20 cm apart. A porous disperser is installed on the top of the test tube. During the filling process, a layered compaction method is used to ensure that the soil particles are fully and tightly packed.

4. The method for monitoring soil nitrogen leaching capacity based on frequency domain reflectometry according to claim 1, wherein: The multi-parameter sensor in step three is a soil sensor based on frequency domain reflectometry. The sensor's internal oscillator circuit generates a specific frequency signal, which is transmitted through the probe into the soil. The complex dielectric constant is calculated by measuring the phase difference and amplitude attenuation between the reflected and incident waves. Its real and imaginary parts reflect the moisture content and conductivity, respectively. The sensor pins must be inserted into the test tube once. After installation, the holes and gaps must be sealed with waterproof sealant to prevent external interference and evaporation of soil moisture along the channels.

5. The method for monitoring soil nitrogen leaching capacity based on frequency domain reflectometry according to claim 1, wherein: The data acquisition system in step 4 includes: FDR sensor → RS485 bus (Modbus RTU protocol) → data recording display (OLED display).

6. The method for monitoring soil nitrogen leaching capacity based on frequency domain reflectometry according to claim 1, characterized in that: The soil particle size percentage in step 5 is measured using a hydrometer method. 100 g of pre-treated soil sample that has passed through a 2 mm sieve is dispersed with sodium hexametaphosphate and then fixed to 1000 ml. After stirring, the suspension density is measured using a hydrometer at different time points, and the percentage of particles of each size is calculated according to the formula. The calculation formula for unsaturated hydraulic conductivity is: Where, is the volumetric water content, is the saturated moisture content, is the residual moisture content, m is the VG model parameter, is the unsaturated hydraulic conductivity, is the saturated hydraulic conductivity.

7. The method for monitoring soil nitrogen leaching capacity based on frequency domain reflectometry according to claim 1, characterized in that: The formula for calculating the moisture flux in step 6 is: Where q is the moisture flux (positive if downward), is the soil water potential gradient, and "+1" is the gravity term.

8. The method for monitoring soil nitrogen leaching capacity based on frequency domain reflectometry according to claim 1, wherein: The calculation formula for the nitrogen migration flux in step seven is: Where, is the nitrogen flux, and C is the nitrogen mass per unit volume of soil solution. The calculation formula for the leaching capacity is: Where, is the nitrogen lost due to leaching, is the nitrogen flux, L is the target soil layer depth, z is the depth, t is the measurement time, and are the start and end times respectively.

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