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A Journey from Roots to Bulk Soil: Organic Matter Characterization in the Biosphere 2 Tropical Rainforest
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  • Rachel Cruz Pérez,
  • Roya AminiTabrizi,
  • Teverrick Chee,
  • Hans Gieschen,
  • Laura Meredith,
  • Jane Fudyma,
  • Malak Tfaily
Rachel Cruz Pérez
University of Puerto Rico Rio Piedras Campus

Corresponding Author:rachel.cruz@upr.edu

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Roya AminiTabrizi
University of Arizona
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Teverrick Chee
Arizona State University
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Hans Gieschen
University of Arizona
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Laura Meredith
University of Arizona
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Jane Fudyma
University of Arizona
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Malak Tfaily
University of Arizona
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Abstract

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.