Resolving the unresolved: new insights into soil chemical reactions using microdialysis / / by Olivier Humphrey, Elliott Hamilton and Matthew Ogley

Olivier Humphrey is an environmental chemist with a particular focus on assessing short-term interactions of micronutrients and potentially toxic elements in soils and plants and their effects on ecosystem and human health.

Elliott Hamilton is an environmental chemist within the Inorganic Geochemistry Facility, specialising in the development and application of analytical techniques to measure and track the mobility of potentially harmful elements and their species in environmental and biological systems.

Matthew Ogley, a placement student from the University of Surrey, is currently working in the Inorganic Geochemistry facility to support the research efforts being made on microdialysis, as well as undertaking routine analyses in the laboratories.

Chemical reactions in soils are difficult to monitor due to the speed with which they start and finish, but measuring them is necessary to increase our understanding of how nutrients and toxic compounds are transferred into plants.

There are a number of techniques capable of collecting and measuring dissolved compounds in water within soil, but the complex nature of soil demands methods that can collect samples faster and at smaller scales without disturbing the natural chemistry of the soil. Microdialysis - originally developed for use in neuroscience and pharmacokinetics - is a new technique that uses small probes to sample compounds dissolved in soil solution (Fig. 1), without the need to dig up soil and potentially alter its chemical properties. BGS’ Inorganic Geochemistry Facility is at the forefront of developing microdialysis as a soil sampling technique, with the ability to assess rapid chemical reactions with unparalleled temporal and spatial resolution without significantly disturbing in-situ physicochemical soil properties.

 

Figure 1: microdialysis equipment a) syringe pump b) microdialysis in soil microcosm, sample collection vials d) ICP-QQQ for analysis and e) microdialysis probe. The small size of the probe enables high spatial resolution of sampling as you can place multiple probes in very close proximity

We first began developing the application of microdialysis during Olivier Humphrey’s PhD (completed in 2019) at the Centre for Environmental Geochemistry (BGS / University of Nottingham), where he was investigating short-term iodine soil interactions following a simulated natural/anthropogenic iodine addition event (full paper). This marked the first occasion of continuous sampling of soil solution to investigate the short-term behaviour of an essential micronutrient, the results of which could inform future food fortification studies aimed at alleviating hidden hunger (aligned with the United Nations Sustainable Development Goal 2: ‘Zero Hunger’). The use of microdialysis to investigate soil-elemental speciation dynamics has been further developed by Elliott Hamilton during his PhD (completed in 2020), which will be summarised in a future blog.

 

Figure 2: Microdialysis sampling for on-line and off-line analysis

We have since been awarded a Royal Society of Chemistry grant to continue this research, and have developed a project aimed at establishing harmonised methods for both microdialysis sampling in soils and subsequent data interpretation. Despite the potential advantages of microdialysis, considerable effort is required to develop and progress the analytical chemistry and theoretical frameworks to apply the technology to soil solution multi-elemental analysis. By providing accurate and precise data on the bioavailability of beneficial and harmful elements in soil, substantial progress can be made within soil science, botany and agricultural sciences. A better understanding of what is beneath our feet can only help us in tackling real world problems (incorporating the SDGs); microdialysis truly has the potential to pave the way in finding the solutions to widespread food security and nutrition issues, as well as aiding the promotion of sustainable agriculture and good health and well-being.

We aim to establish microdialysis as a tool capable of assessing fine-scale, rapid soil chemistry interactions to better inform existing geochemical models that influence phytoremedial and crop biofortification strategies globally, and look forward to updating you in the future as this exciting technique is developed further.