An Electrochemical Approach to in-Situ Continuous Soil Nitrate Monitoring

Abstract

Soil nitrogen is the most limiting nutrient affecting plant growth. However, due to poor soil management due to lack of soil health information, high-N fertilizer use increased exponentially. The UN has labeled nitrogen pollution one of the most important pollution issues facing humanity[1]. Not only does it affect life on land and underwater, it is also 300 times more potent at heating the climate than carbon dioxide while remaining active for 100 years in the atmosphere. Currently, soil nitrogen assessment techniques require destructive sampling where a soil core is collected then shipped to a lab to measure nitrogen content. Not only does this technique have high costs, it only measures nitrogen levels at that specific time point. In this work, we utilized commercially available screen printed electrodes (SPE) with a nitrate ionophore for low cost real-time continuous tracking of soil nitrate levels. The ionophore coating utilizes tetradodecylammonium nitrate (TDDA) as the selective reagent as it’s highly specific to nitrate ions. Previous work done using TDDA used open circuit potential (OCP) to measure nitrate concentration. Soil water content, pH, environmental noise, and other ions impact OCP thus making it unreliable for long term continuous measurements. This work utilizes electrochemical impedance spectroscopy (EIS) instead as it is more robust against the previously mentioned parameters. By opting to use a higher frequency of 1 kHz to extract nitrate concentration, we avoid environmental noise that are dominant at lower frequencies. The sensor output was calibrated in three different soil textures from 2 – 512 ppm presented in Fig. 1(a) for Clay soil. Then the sensor accuracy was validated to the gold standard cadmium reduction method presented in Fig. 1(b) showing non-significant differences with one sample only having a *p-value of 0.049. Lastly, the sensor stability was measured for 7 days continuously in soil with a coefficient of variation of 17% which is under the acceptable standard 20%.Figure 1 (a) Calibrated dose response for Clay soil with 50% water content. (b) vaidation of proposed sensor against gold standard in various soil types. (c) longitudinal study showing stablity and consistency of sensor performance in-situ Figure 1