Are crops significant sources of Volatile Organic Compounds? A bi-compartmented chamber setup for investigating VOC emissions from aboveground and belowground

Plants are the primary worldwide source of Volatile Organic Compounds (VOCs) on Earth and therefore play a significant role in the atmosphere's gas composition. While emissions from foliage have been well documented for decades, emissions from soils with living roots and associated microorganisms are not well understood. In particular, field studies show a wide and inconsistent range of soil-derived VOC net fluxes, possibly due to the variable abiotic and biotic conditions during measurements. In order to figure out the main drivers of soil VOC exchanges, studies under controlled experimental conditions are necessary. 

We developed a new experimental dynamic chamber, allowing to monitor the gas emissions from 1-9 plants simultaneously, both in the aboveground and belowground compartments. Aside from providing controlled experimental conditions, this setup allows to compare the net VOC, CO₂, and H₂O flux dynamics from soil to that from the aboveground plant organs at a high time resolution.

Using a Proton Transfer Reaction – Time Of Fly – Mass Spectrometer, constitutive emissions from soil-grown rapeseed and tomato plants were recorded over 24h. Methanol was the main VOC emitted by both species, followed by, at one order of magnitude lower, methanethiol and monoterpenes for rapeseed and tomato plants, respectively. Although root-derived VOC emissions were generally much lower than shoot-derived ones, belowground DMDS emissions from rapeseed plants were twice higher than those from aboveground.  Interestingly, a negative correlation was observed between several root-derived VOC and root/soil respiration, suggesting a shift in the carbon allocation to specific metabolic pathways in roots or root-associated microorganisms. This experimental approach opens new perspectives for understanding the specific contributions of VOC emissions from soils and roots in agricultural ecosystems, and how these emissions may be linked to plant carbon budget.