Exudation
Plant roots and soil microbes release a great diversity of small chemical compounds into soil through a process called exudation. These compounds can function as signalling compounds, constitute an energy source for the food web, and interact with soil minerals to mobilize carbon and nutrients.
Exudation occurs in the Climate and carbon regulation function model. Exudate compounds are rich in carbon; hence, root exudation delivers an important supply of labile carbon to the soil fuelling microbial activity and growth. Microbial exudates and other products are an important source of mineral-associated organic matter. In the Nutrient cycling model, root exudation is considered under the process of root foraging because of its role in mobilizing nutrients from unavailable soil resources. In the Disease and pest management model, exudation is also indirectly present in the processes of antibiosis, resistance and defence and plant metabolism enhancement because exudates would function as the signalling and defence compounds regulating these processes.
Measuring exudation by plant roots or soil microbes in the field is labour-intensive and technically challenging[1]. Exudates can be collected through sampling directly from the root after excavating or extracting soil samples, yet in these latter samples it is difficult to trace the source of the exudate (plant vs. microbe vs. decomposing organic matter). Therefore, exudation is often measured under more controlled conditions in the greenhouse or lab. Collected exudates can be assessed for total organic carbon content as well as for specific compounds including sugars, organic acids, amino acids and phenolic compounds using liquid or gas chromatography.
[1] Oburger E, Jones DL. 2018. Sampling root exudates – Mission impossible? Rhizosphere 6: 116–133.