Research on Soil Inorganic Nitrogen Detection Technology Based on Dielectric Response

Efficient monitoring of soil inorganic nitrogen is crucial for precision agriculture fertilization and ecological environmental protection. Traditional detection methods are complex and challenging for real-time in situ measurements. This study proposes an innovative approach based on dielectric response characteristics, enabling non-destructive and rapid detection by analyzing soil polarization behavior in an electromagnetic field. Using a vector network analyzer (E5071-C), we systematically measured the complex dielectric spectra of red soil and yellow clay loam across a wide frequency range from 10 MHz to 4.5 GHz. Coupled with water–nitrogen interaction experiments (volume water content: 0.05–0.25 cm3/cm3; nitrogen concentration: 0–0.2 mol/L), we established a high-frequency–low-frequency collaborative detection model. The study found that at the 3.8 GHz high-frequency band, the interface polarization weakening effect allows for the precise measurement of soil water content (R2 = 0.82; RMSE = 0.030 cm3/cm3). In the 100–200 MHz low-frequency band, based on ion migration dynamics, we successfully identified characteristic sensitive frequency bands for NH4+ (136–159 MHz) and NO3− (97–129 MHz). Notably, at 127 MHz, the water–nitrogen coupling model predicted inorganic nitrogen content with a determination coefficient of 0.721. This method effectively overcomes the water interference issue inherent in traditional single-frequency dielectric methods through a dual-frequency decoupling mechanism. The findings lay a theoretical foundation for developing in situ sensors for farmland. Real-time monitoring can significantly improve nitrogen fertilizer utilization efficiency and reduce environmental pollution, offering substantial application value for advancing precision agriculture and sustainable development.