Droughts are occurring with increased frequency and duration in tropical rainforests due to climate change, having a significant impact on soil C dynamics. The role of microbes as drivers of changing C flow, particularly in relation to volatile organic compound (VOC) cycling, remains largely unknown. Here, we aimed to characterize microbial responses to drought using an integrative, multiple ‘omics approach, and hypothesized that microbial communities will adapt by altering their C allocation strategies. Specifically, during pre-drought, primary metabolic pathways will be more active with microbes using C towards growth, whereas during drought, microbes will divert C to secondary metabolite (including VOC) production in response to stress. To test this, we conducted an ecosystem-wide 66-day drought experiment in the tropical rainforest biome at Biosphere 2, a glass- and steel-enclosed facility near Tucson, AZ. To track carbon allocation by microbes, we injected C1 or C2 position-specific 13C-pyruvate solution into a 25 cm2 region within a soil flux chamber collar (n=6 locations) and measured C isotope ratios of VOC and CO2 emissions. Soil was collected at 0, 6, and 48 hours after pyruvate addition to examine responses in soil metatranscriptomics, metagenomics, and metabolomics (1H nuclear magnetic resonance [NMR] and Fourier-transform ion cyclotron resonance [FTICR]). Our results indicated that 13CO2 (primarily emitted from C1-13C-pyruvate) fluxes decreased during drought, indicating diminished microbial activity. 13C-VOCs (primarily emitted from C2-13C-pyruvate) fluxes also differed between pre-drought and drought. Furthermore, drought-induced increases in activity of VOC-producing metabolic pathways, including acetate and acetone biosynthesis, were evident, as inferred from volatilome, metabolome, and metatranscriptome data. Overall, these results indicate that integration of multiple ‘omics datasets reveal specific impacts of drought on microbial activity affecting carbon flow in the tropical rainforest soil.

The Biosphere 2 (B2) is a large-scale research facility owned by the University of Arizona that comprises seven enclosed biomes. Among these is the Tropical Rainforest (TRF), where environmental factors such as temperature and rainfall are managed in order to simulate a natural ecosystem. Controlling the elements that define a biotic community provides the opportunity to use B2 as a model to study the effect of climate change on carbon cycling and GHG emissions. Large amounts of carbon are stored in tropical soils, proceeding form organic compounds and metabolites that are released into the soil by plant roots. These exudates along with other plant and animal residues and living microbial biomass constitute soil organic matter. Since C released from soils contributes to a positive feedback loop of changing climate, this study seeks to characterize organic metabolites from root, rhizosphere, and bulk soil in the TRF of B2. Water extractable organic matter (OM) from collected root, rhizosphere, and bulk soil samples were cleaned up using Solid Phase Extraction prior to analysis with Fourier Transform Ion Cyclotron Resonance Mass Spectrometry and subsequently, bioenergetic potentials (nominal oxidation state of carbon (NOSC) and Gibbs Free Energy of OC oxidation (ΔG°Cox) half reactions) of the detected compounds were calculated. We observed major differences in OM composition throughout the sampled roots, rhizosphere and bulk soil, with root and rhizosphere displaying similar chemical composition when compared to bulk soil. High amounts of lipids and proteins in the bulk soil were indicative of microbial biomass residues, suggesting that OM is utilized and transformed by microbial communities. Lignins, tannins, and condensed hydrocarbons revealed plant inputs into surface soil. After comparing estimated values of NOSC and ΔG°Cox, we concluded that OM in roots have the lowest bioenergetic potentials and are first picked up by microorganisms to undergo transformations. Moving from rhizosphere to bulk soil, the bioenergetic potential of the OM increases, indicating the refractory nature of the OM in the bulk soil. This data provides further insight to understand belowground biogeochemical cycles in soil and C cycling under extreme weather scenarios.