A Journey from Roots to Bulk Soil: Organic Matter Characterization in
the Biosphere 2 Tropical Rainforest
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.