Monitoring macronutrient dynamics in soil and water with droplet microfluidic sensors

Abstract

Inorganic nitrogen soil is a key indicator of soil health and its analysis still mainly relies on time and labour-intensive grab sampling; with sample degradation being a major source of error in these studies it is imperative that sensors are developed that can monitor this area; with the advent of different microsampling techniques such as microdialysis and ultrafiltration this is becoming possible.
In this work, a droplet microfluidic analyser using microdialysis and ultrafiltration was developed and field-tested to monitor nitrate in soil. This colorimetric sensor was calibrated using both solutions and spiked soil samples (0.8 to 20 mM NO3-) of varying moisture contents (30 to 100% of maximum water holding capacity), showing good linearity across the ranges tested. From this, the sensor was used to detect changes in soil amended with glucose, from which significant nitrate consumption was monitored, showing its potential in recording changing nitrate levels. 4 measurements per day were taken over a period of 12 days with low reagent consumption per measurement (0.198 mL), showing the ability for this sensor to be used in field deployment.
Analysers were then deployed at both Highfield Campus (Southampton, UK) and the Writtle Forest (Essex, UK), with a deployment length of two weeks and a year respectively, using only 96 µL of reagent per measurement whilst producing high quality data through two sampling methods, with small error (2-3% for ultrafiltration, 10% for microdialysis) whilst only requiring monthly maintenance. Further from this, a combined analyser was developed to monitor nitrate, nitrite, and ammonium simultaneously, showing potential to monitor inorganic nitrogen in multiple forms from a single source in-situ. Variations of the analyser were made to either run intermittently or continuously, allowing for many areas of potential deployment ranging from agricultural to bacterial analysis. This analyser was deployed to analyse total inorganic nitrogen in soil (measuring NH4+ and NO3-) and then in conjunction with a bioreactor (measuring NH4+, NO2- and NO3-), showing changes in inorganic N makeup throughout both deployments.
With further development and technology maturation, this multinutrient analyser can be revolutionary in monitoring inorganic N change forms allowing for the development of automated feedback loops for more efficient growth periods.

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Identifiers

ORCID for James Lunn: orcid.org/0000-0003-1961-3336

ORCID for Adrian Nightingale: orcid.org/0000-0003-2445-4827

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