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Trees as conduits for connecting belowground microbial processes to aboveground CH4 emissions at the Terrestrial-Aquatic Interface

Active Dates 8/15/2021-8/14/2024
Program Area Environmental Systems Science
Project Description
Trees as conduits for connecting belowground microbial processes to aboveground methane emissions at the

Terrestrial Aquatic Interface

PI:  S.R. Saleska (U. Arizona), J. van Haren (Co-I, U. Arizona); L. Meredith (Co-I, U. Arizona)

D. Ricciuto (Co-I,  Oak Ridge National Laboratory)

P.B. de Camargo (Co-I, University of São Paulo, Brazil)

Raimundo Cosme Oliveira, Co-I, Embrapa, Santarem, Brazil

Jose Mauro S. Moura, Rodrigo da Silva, Raphael Tapajos, Amanda Mortati, Co-Is, Federal University of West-Para, Santarem, Brazil

The seasonally inundated floodplain forests of the Amazon River and its whitewater tributaries, known as várzea forests, are an iconic example of ecosystems at the terrestrial-aquatic interface.  Though the Amazon basin várzea forests have been estimated to emit more methane from the stems of its trees than is emitted from all Arctic wetlands combined, integrated study of the controls, budget and seasonal dynamics of methane cycling in these great forests is lacking.  This project would establish the first continuous whole-ecosystem measurements of methane emissions, via eddy covariance methods,  from a seasonal floodplain forest in the Amazon.  The goal is to improve process-based understanding and modeling of the biogeochemistry of this important greenhouse gas in this globally critical habitat.   

Two puzzles challenge our understanding of Amazonian CH4 cycling: First, the mismatch between top-down estimates (from satellites or aircraft) which show a large source of CH4 to the atmosphere , and bottom-up estimates (from individual sites and soil flux measurements) which show a much smaller source or even a sink. The second puzzle is the gap in our understanding of the controls on methane emission and seasonal dynamics from this forested Terrestrial-Aquatic Interface.

This project will test the hypothesis that both of these puzzles are manifestations of a common mechanism, not well represented in models: the transport of significant amounts of microbially produced CH4 from lower soil depths via tree roots and stems to the atmosphere.

Recent work shows  that this mechanism likely supports large CH4 emissions from Amazonian floodplain forests, and that accounting for this could reconcile top-down and bottom-up estimates. Our proposed work would provide the first whole-forest flux measurements to test whether that mechanism can work.  Our general hypothesis is based on the idea that a similar mechanism (of tree stems as methane conduits) also operates in drier upland forests, whose soils are commonly observed to be CH4 sinks. We will compare CH4 dynamics in a floodplain forest (várzea) of the Amazon river (anchored by the ecosystem scale CH4 eddy flux measurements, brought by this project) to fluxes at a reference AmeriFlux tower at a long-studied upland forest (the km67 site in the Tapajos National Forest). We will quantify individual flux components (from soil/water surfaces and from diverse tree stems, and combine these with novel methane isotope spectroscopic measurements to infer how net fluxes partition between methanogenic production and methanotrophic methane oxidation, coupling this to information on microbial diversity and abundance. This project would thus simultaneously bring a critical but neglected new ecotype (the great iconic várzea forests of the Amazon river) into the growing global database of ecosystem scale measurements of CH4, and also make key important contributions to development and improvement of ecosystem model performance in representing methane dynamics.  We will use DOE’s ELM model to represent tropical Amazon dynamics, and as a basis to more confidently project future CH4 dynamics under climate change, such as expected increases in Amazonian drought frequency. Methane dynamics in these models have been mostly developed and tested in the Arctic or northern temperate systems (e.g. as part of DOE’s SPRUCE experiment), so the opportunity to challenge model simulations in tropical systems is an especially high-value one.
Award Recipient(s)
  • University of Arizona Tucson (PI: Saleska, Scott)