Microdialysis in soil environments: Current practice and future perspectives

Microdialysis is emerging as a sensitive tool in environmental sciences, allowing for in situ sampling of solutes with minimal disturbance to soil environments. This perspective presents the theoretical foundations and practical applications of the technique with a focus on its use in soil, microbial and plant sciences. Using small probes (usually 0.5 mm diameter) fitted with permeable membranes, soil solutes are sampled, so that solute flux (and potentially concentration) can be calculated at root-relevant scales. However, physical and biological characteristics of soil systems impose challenges to sampling efficiencies with microdialysis, particularly where solute concentrations are low. Experimental parameters such as perfusate flow rates, probe design and sampling times can also influence the findings, and reduce the comparability between studies. We explore how equipment setup and experimental conditions can be optimised for soil environments, and how standardisation and calibration techniques may improve cross-study comparability. We show that the technique's strength lies in the ability to integrate many soil factors into a biologically relevant measure of solute availability. Microdialysis has so far provided new insight regarding the bioavailability of soil nitrogen (N) and phosphorus (P), where fluxes have been related to mot uptake rates and gaseous fluxes from soil. The technique has also been used as a mot simulator, mimicking exudation of organic acids to mobilise soil P, and to measure the potential contribution of transpiration-induced mass flow on N availability at the root surface. With further development, and paired with sensitive analytical methods and equipment, microdialysis has much potential to explore fragile and dynamic soil processes in biologically-active zones such as the rhizosphere, and could contribute to solving challenges relating to under- and oversupply of nutrients to plants. We conclude that, with further advancements and critical evaluation, microdialysis could become an important instrument in the soil analysis toolkit.