Exploring Isotopically-Resolved Subsurface and Root Volatile Organic Compound Signatures at Biosphere 2

Volatile organic compounds (VOCs) provide insight into nutrient processing and ecosystem signaling during external forcing, including in plant root systems and the rhizosphere. Primary and secondary metabolites exuded by plant roots have been studied under laboratory conditions, but patterns of root exudation in natural communities is limited. Recent advances in chemical ionization mass spectrometry have enabled high sensitivity, fast (1 Hz or better), and selective VOC measurements. Further, coupling Proton Transfer Reaction Mass Spectrometry (PTRMS) with fast, online gas chromatography enables quantitative separation of structural isomers. Time-resolved structural concentrations are critical for apportioning VOC emissions to specific biological processes. Here we present initial results exploring nutrient processing during the 2019 B2 WALD large-scale simulated ecosystem measurement campaign at a tropical rainforest inside Biosphere 2 (B2). A Vocus GC-PTRMS was coupled to a root chamber sampling system during 13C spiking of ambient CO2 into the entire ecosystem, as well as individual plant feeding of 13C pyruvate, which is a key metabolic intermediate. Isotopic signatures of the measured VOCs reveal the degree to which the labelled carbon sources entered subsurface VOC production pathways and how those processes were altered by drought and wetting cycles. Coupled with a gas chromatograph, the GC-Vocus also provided insights into the isomeric composition of measured VOCs. In previous laboratory investigations we observed dynamic and species-specific responses in soil VOC concentrations in response to soil wetting of soils collected from B2. In this presentation, we connected the Vocus to subsurface, in situ diffusive probes at B2 to allow for direct quantification of soil VOCs. The results are considered in the context of isotopic N2O and CH4 measurements from the same probes. These results may unveil potential interactions between VOCs' production and plant-soil-microbes increasing the understanding of VOC's cycling in natural environment.