Development of a Subsurface Sensing Probe for Measurement of Nitrogen Oxide Gases

Highlights A novel 3D printed subsurface soil probe device integrated with NDIR N2O and electrochemical NOx sensors was designed. The performances of these sensors were calibrated using compressed gases and by applying inorganic nitrogen fertilizer to soil cores. The readings from the developed sensors closely matched the gas concentration profile measured by the commercially available Gasmet FTIR instrument for both N2O and NO. Results confirmed that the sensing devices can detect subsurface nitrogen oxide gases, presumably microbially induced, including the changes following the application of an inorganic fertilizer and an irrigation event. Abstract. The goal of this study was to develop a subsurface soil probe device that uses sensors for continuous measurement of nitrogen oxide gases (N 2 O and NO x =NO + NO 2 ) generated microbially within the soil. The polymer-based soil probe device is manufactured by 3D printing and consists of a perforated probe that penetrates the soil and an attached above-ground chamber that stores the sensors and the electronics. For N 2 O, we have developed a non-dispersive infrared sensor (NDIR), and for NO x , an electrochemical sensor. Both sensors can measure gases in the ppb-ppm range. The soil probe device was tested with soil cores. To validate the accuracy of the sensors, we used compressed gas tanks of NO and N 2 O to permeate these gases into the soil core. The compressed gases spread through a soil core profile, and the concentrations of the nitrogen oxide gases that diffused into the in-ground probe chamber were measured by the sensors. The sensor readings closely matched the gas concentration profiles measured by the commercial Gasmet instrument for both N 2 O and NO. Both N 2 O and NO x sensors were able to measure gas diffusing through the soil for sustained periods of time. To demonstrate the agricultural relevance of the device, we designed an experiment using soil cores in which the soil was treated with inorganic fertilizer followed by irrigation and then the subsequent evolution of nitrogen oxides was monitored over a period of days using both sensors. These trials illustrated that the in-ground probe chamber and respective sensing devices can measure subsurface nitrogen oxide gases that are microbially induced, including the changes that occur after the application of an inorganic fertilizer and an irrigation event. Keywords: Electrochemical sensor, Greenhouse gases, Nitrogen oxides, Nitrous oxide, Nondispersive infrared (NDIR) sensor, Potentiometric sensing, Soil microbiology.