Technical note: An open-source, low-cost system for continuous monitoring of low nitrate concentrations in soil and open water

Nitrate (NO⁻ ₃ ), mainly leaching with soil porewater, is the primary nonpoint source pollutant of groundwater worldwide. Obtaining real-time information on nitrate levels in soils would allow for gaining a better understanding of the sources and transport dynamics of nitrate through the unsaturated zone. However, conventional nitrate detection techniques (e.g., soil sample analysis) necessitate costly, laboratory-grade equipment for analysis, along with human resources, resulting in a laborious and time-intensive procedure. These drawbacks raise the need to develop cost-effective and automated systems for in situ nitrate measurements in field conditions. This study presents the development of a low-cost, portable, automated system for field measurements of nitrate in soil porewater and open water bodies. The system is based on the spectrophotometric determination of nitrate using a single reagent. The system design and processing software are openly accessible, including a building guide, to allow duplicating or changing the system according to user-specific needs. Three field tests, conducted over 5 weeks, validated the system’s measurement capabilities within the range of 0–10 ppm NO⁻ ₃ –N with a low RMSE of < 0.2 ppm NO⁻ ₃ –N when comparing the results to standard laboratory nitrate analysis. Data derived from such a system allow for tracking of the temporal variation in soil nitrate, thus opening new possibilities for diverse soil and nutrient management studies.