An efficient static sampling method for in situ measurement of rhizosphere volatile organic compounds in plant-soil...

Volatile organic compounds (VOCs) play a crucial role in mediating the interactions of plants with their environment. Research has predominantly addressed aboveground VOCs, whereas belowground emissions-especially in undisturbed plant-soil systems-remain largely unexplored. Our study aimed to develop a method for the in situ trapping of rhizosphere VOCs using a static sampling approach. Apple (Malus domestica Borkh.) plantlets were grown in soil-filled, perforated rhizoboxes. The holes were integrated to insert silicone-based polymeric phases (Sorb-Stars^®) for the collection of VOCs. Sampling was repeated four times at one-week intervals. As VOCs found in the rhizosphere may derive from multiple origins, additional trapping setups were employed to obtain emissions specifically from roots (hydroponic-grown plants), shoots (aboveground VOC collection), and soil (soil-only rhizoboxes).

Using thermal desorption followed by gas chromatography/ mass spectrometry (GC/MS), a total of twelve VOCs were detected in the rhizosphere samples. Seven of these were successfully identified: 2-furanmethanol, longifolene, 2-(2-hydroxypropoxy)-1-propanol, α-dihydroterpineol, α-terpineol, 3(2H)-benzofuranone, and butylated hydroxytoluene. All of these have been documented in previous studies as VOCs emitted in biological systems. The other five VOCs could not be annotated. Six of the detected VOCs could be classified as the "core volatilome", being present in all trapping setups, while the others showed more setup-specific occurrence. Notably, 3(2H)-benzofuranone was the only VOC detected exclusively in the rhizosphere samples, highlighting the necessity of plant-soil interactions for its production. The setup and sampling time at which the highest relative abundance was observed varied considerably between compounds, emphasizing the dynamic nature of belowground VOC emissions. Therefore, an optimal sampling time could not be identified.

We demonstrated that the proposed method offers a suitable approach to non-invasively capture VOCs in the rhizosphere of plant-soil systems. It was further shown that additional setups, focusing on isolated below- and aboveground components, may help determine the VOC emitters.