Research On Inorganic Nitrogen Detection Method Based On Soil Dielectric Measurement

Nitrogen is one of the important elements needed for crops,which is an important indicator of soil fertility.Real-time and rapid determination of inorganic nitrogen in the soil is of great social significance to improve nitrogen fertilizer use efficiency,reduce production cost and reduce agricultural environmental pollution.A great deal of research shows that the dielectric properties can reflect the ion parameters of soil.It can not only retrieve the volume water content of the soil,but also retrieve the ion concentration and species of soil through the complex dielectric constant.At present,the research on soil dielectric for inorganic nitrogen content is mostly carried out at a single frequency,and the polarization behavior in wide-band water nitrogen coupling is less.Besides,the research on soil dielectric under the coupling of different soil water content and soil specific inorganic nitrogen ions is insufficient.Aiming at the problem that nitrogen in farmland cannot be measured in real time,the study on the dielectric characteristics of soil nitrogen can not only analyze the polarization behavior under the coupling condition of soil,water and nitrogen,but also provide a new idea for solving the problem of soil nitrogen measurement.Based on bilinear theory,the Anritsu-MS2028B vector network analyzer and customized probe were used in the process.This paper takes Qujing laterite in Yunnan Province and Yangling loess in Shaanxi Province as the research objects,the spectrum of the dielectric constant（complex dielectric constant and apparent dielectric constant）of laterite under 4 volume water contents（0.10-0.25 cm³/cm³）and 5 ammonium nitrogen contents（0.1-0.5 mol/L）and the spectrum of dielectric constant of loess under 5 volume water contents（0.10-0.30 cm³/cm³）and 6 nitrate nitrogen levels（0-0.30 mol/L）were measured at 10MHz～6 GHz.The dielectric constant spectrum of Lou soil under low level nitrate nitrogen concentration（1-10 g/L）was measured.The relationship between soil inorganic nitrogen（NH₄⁺、NO₃^-）concentration and soil dielectric constant under the influence of soil water content and frequency was analyzed.Based on the experimental conditions and research contents of this paper,the main conclusions are as follows:（1）The dielectric measurement system based on bilinear theory has good applicability for measuring the complex dielectric constant of media.The S₁₁ characteristics measured on the 14 mm end open circuit custom probe can be reliably translated into the complex dielectric constant of the tested medium by using the Anritsu-MS2028B vector network analyzer and computer processing in MATLAB.（2）The soil dielectric constant is affected by the compound influence of soil water content,ammonium nitrogen concentration and frequency.In the measurement frequency band,the influence of diverse ammonium nitrogen content on the real part of the complex permittivity is concentrated in the frequency band of 10MHz～100MHz,and the impact on the imaginary part of the complex permittivity is concentrated in the frequency band of 100MHz～1GHz.Besides,the effect of nitrate concentration change on the imaginary part of the complex permittivity is more obvious.The imaginary part model of dielectric detection of NH₄⁺ions in laterite was established at 101.05 MHz.The model determination coefficient R²=0.937established by regression analysis shows that the empirical formula has good accuracy.（3）In the measurement frequency band,the volume water content and soil nitrate nitrogen（NO₃^-）concentration have significant effects on the soil dielectric constant.It is found that the real part of the complex dielectric constant is more sensitive to the change in soil water content.The real part of the complex dielectric constant is used for the third-order fitting of soil water content at different frequency points,and the correlation coefficient R² is more than0.98.Secondly,the dielectric performance of the real part,imaginary part and apparent dielectric constant of the complex dielectric constant under broadband are compared.The imaginary part of the complex dielectric constant is used to describe the different nitrate nitrogen content in the soil,which has good stability and discrimination.The correlation between soil nitrate nitrogen content and the imaginary part of complex dielectric constant was analyzed at the typical frequency point of 300.9 MHz.R² is between 0.94～0.98.（4）The dielectric detection model of nitrate nitrogen at low nitrate nitrogen ion concentration level was established.Low nitrate nitrogen concentration causes little change in the real part of the complex dielectric constant at each moisture content,which can be concentrated below 200 MHz.At 100MHz～700MHz,the imaginary part of complex dielectric constant is distinguished obviously.With the increase of soil water content,the effect of distinguishing the imaginary part of the complex dielectric constant of soil nitrate nitrogen concentration is better.Compared with the fitting effect of linear,quadratic and cubic equations,it is found that the detection model of soil low nitrate nitrogen level established by cubic equation has better results.300.9 MHz is the best frequency point for soil nitrate nitrogen detection,and the determination coefficient R² is 0.935.