In Situ Rapid Detection of Soil Nitrate Nitrogen via Dielectric Spectroscopy Using a Dual-Band Frequency-Splitting Coupled Sensor

Traditional detection methods for soil nitrate nitrogen (NO₃^--N), a critical nutrient for crop growth, suffer from poor timeliness and susceptibility to matrix interference. To address these issues, this study presents a novel dual-band frequency-splitting coupled sensor with a concentric copper ring structure for in situ rapid detection of soil NO₃^--N via dielectric spectroscopy. The key technological innovation lies in using the low-frequency band (1-50 MHz) to isolate water and salinity interferences via stable impedance matching of the copper rings and capturing the characteristic NO₃^--N relaxation signals in the high-frequency band (100-500 MHz) via enhanced electromagnetic coupling. Field trials across five soil types (brown, black, red, saline-alkali, and loess) demonstrated excellent performance, with determination coefficient (<i>R</i>²) values of 0.943-0.987, mean absolute error values of ≤0.75 mg/kg, root-mean-square error values of ≤0.92 mg/kg, and millisecond-level response. Signal drift remained <0.25 mg/kg even under extreme conditions (-5 °C, 90% relative humidity (RH)), with Pearson correlation coefficient values of 0.995-0.999 in typical agricultural scenarios (pre/postfertilization and precipitation). The developed sensor eliminates the need for sampling and pretreatment, reducing the detection time from 3-5 days using traditional methods to milliseconds, and provides high-precision data for dynamic nitrogen management. Moreover, the IoT integration potential of the sensor advances smart agriculture and sustainable development.