Non-destructive soil nitrate detection via electrochemical reduction

A number of methods are available for the analysis of nitrate in soil, each with its own advantages and limitations. One of the main limitations of existing methods is that they involve destructive soil sampling with subsequent extraction. Therefore, there is a demand for a rapid, sensitive, and accurate procedure for analysis of nitrate levels in the soil. Here, we developed and tested a method for quantitative analysis of nitrate in soils based on electrochemical reduction of the nitrate ion to gaseous nitrogen (N) species and subsequent real-time and online quantification of the emitted N gases. For this purpose, we subjected slurries from different soils to electrolysis with different electrode materials under a range of conditions. The N gases developing in the slurry during electrolysis were continuously purged out of the solution into the headspace of the electrolysis cell by a stream of dry nitrogen gas and directed to an infrared laser absorption analyzer for online analysis.

We found that the emission of N₂O, one of the products of the electrolysis of nitrate in the soil slurry, was the most suitable indicator of the nitrate concentration in the sample because it is easy to measure with high sensitivity. To test the linearity of the method, the soil samples were amended with different amounts of nitrate, resulting in nitrate contents of the soils ranging from 20 to 180 mg NO₃^--N kg^-1. Preliminary results showed a linear correlation between nitrate concentrations and N₂O production. However, it became evident that the variability in soil structure and pH significantly impacted the electrochemical reduction pathways and efficiency. To address these limitations, a phosphate buffer was introduced to stabilize the soil pH. This adjustment minimized pH fluctuations, thereby reducing their influence on N₂O production. This newly developed method offers advantages such as fast analysis time and the ability to measure nitrate directly in situ.