Validation of a portable electrochemical sensor for real-time soil nitrate monitoring in cotton rhizosphere

Abstract

Soil nitrate (NO₃) exhibits rapid spatial and temporal variability that conventional point-sampling methods cannot effectively capture, limiting timely nutrient management and plant stress detection. This study aimed to validate a low-cost, three-dimensional-printed electrochemical NO₃ sensor for continuous in-soil monitoring and to evaluate whether sensor-derived NO₃ dynamics reflect plant physiological responses. For this purpose, cotton (Gossypium hirsutum L.) plants were grown in pots under greenhouse conditions. The sensor was evaluated in two sequential phases: an initial calibration phase to characterize sensor response across a broad NO₃ concentration range, followed by a validation phase in the cotton plant rhizosphere. In the calibration phase, the sensor exhibited a sensitivity of −73.99 mV/dec and a limit of detection of 0.12 ppm. In the validation phase, cotton plants were grown under three NO₃ treatment levels: no NO₃ (0 ppm), low NO₃ (150 ppm), and high NO₃ (250 ppm), with three replicate pots per treatment. Sensor NO₃ measurements were validated against commercial grade Ward Laboratory data obtained from the same soil samples. Our sensor accurately captured fertilizer-induced NO₃ spikes and depletion trends and showed strong agreement with laboratory measurements (R² = 0.89, root mean square error = 14.77 ppm). F-score, coefficient of variation, and Tukey's honestly significant difference post hoc analyses confirmed a clear distinction between fertilized and unfertilized soil following NO₃ application. Furthermore, cotton plants’ physiological parameters, such as leaf area index (LAI), chlorophyll index, and stomatal conductance, were measured and correlated with soil NO₃ concentrations. LAI and chlorophyll index increased with NO₃ availability, indicating enhanced canopy development and leaf nitrogen status. In contrast, stomatal conductance decreased with increasing NO₃, reflecting physiological stress under excessive or sudden NO₃ supply. These results demonstrate that the sensor provides chemically accurate and biologically meaningful NO₃ measurements, supporting its potential for precision nutrient management and future field-scale agricultural deployment.