Reverse microdialysis: A window into root exudation hotspots

Plant roots release a variety of low-molecular weight compounds, such as sugars, amino acids or organic acids into the soil, impacting microbial activities and physico-chemical soil processes in their surroundings. These compounds are a source of easily available Carbon (C) and energy for soil microbes, potentially accelerating microbial decomposition of soil organic matter in the immediate vicinity of roots. However, knowledge about processes in root exudation hotspots remains limited due to experimental difficulties in investigating such hotspots in soil. Microdialysis, a passive sampling technique based on diffusion, has been successfully used to collect soil solutes at small spatial scales. Reverse microdialysis, also termed retrodialysis, can be used to introduce solutes into the soil, mimicking passive root exudation. However, little is known about the dynamics of substances released by passive diffusion into intact soil, a crucial prerequisite for applying reverse microdialysis to study root exudation hotspots in undisturbed soils. Here, we used reverse microdialysis to investigate the spatial and temporal dynamics of thirteen different organic compounds passively introduced into two different intact soils. Diffusion of compounds into soils was substantially lower than into water, and was not – as in water – determined by molecular size. Interestingly, butyrate, oxalate and propionate showed the highest diffusive fluxes into soil combined with the lowest rate of back retrieval after input, indicating that they were quickly removed from the soil solution by biotic or abiotic processes. In contrast, glucose and fructose unexpectedly accumulated around the membrane after input without removal. Furthermore, diffusive fluxes of compounds into soils showed a fluctuating temporal pattern, which may be explained by an observed 2-h delay of microbial respiration of added 13C-labelled compounds. During the course of 12 days, approximately one third of 13C-labelled compounds introduced into soil was respired while 8% ended up in microbial biomass. Our results demonstrate that introducing compounds into intact soil triggers complex biotic and abiotic responses at the time scale of hours. Reverse microdialysis proved to be an excellent tool to investigate such responses as well as the dynamics and metabolic consequences of passively released compounds into intact soil, and – in combination with 13C labelled substrate and respiration measurements - to shed light