Continuous in-situ monitoring of nitrate concentration in soils – a key for groundwater protection from nitrate pollution

Abstract. Lack of real-time information on nutrient availability in cultivated soils inherently leads to excess application of fertilizers in agriculture. As a result, nitrate, which is a soluble, stable and mobile component of fertilizers, leaches below the root zone through the unsaturated zone and eventually pollutes the groundwater and other related water resources. Rising nitrate concentration in aquifers is recognized as a worldwide environmental problem that contributes to water scarcity. Accordingly, developing technologies for continuous in-situ measurement of nitrate concentration in the soils are essential for optimizing fertilizer application and preventing water resource pollution by nitrate. Here we present a conceptual approach for a monitoring system that enables in-situ and continuous measurement of nitrate concentration in soil. The monitoring system is based on absorbance spectroscopy techniques for direct determination of nitrate concentration in soil porewater without pretreatment, such as filtration, dilution, or reagent supplementation. A new analytical procedure was developed to improve measurement accuracy while eliminating the typical measurement interference caused by soil dissolved organic carbon. The analytical procedure was tested at four field sites over 2 years and proved to be an effective tool for nitrate analysis in untreated soil. A soil nitrate-monitoring apparatus, combining specially designed optical flow cells with soil porewater-sampling units, enabled for the first time, real-time continuous measurement of nitrate concentration in the soil. The system provided outstanding and explicit data revealing the complexities of the temporal variations in soil nitrate concentrations in response to irrigation cycles and fertilizer-application pattern. Such real-time measurements of soil nitrate levels are crucial for optimizing fertilizer application to increase agricultural yield while reducing the potential threat of groundwater contamination by down-leaching of nitrate from the soil.