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U.S. Department of Energy Office of Biological and Environmental Research

PI-Submitted Research Highlights for
Terrestrial Ecosystem Science Program

Plant Root Exudates Increase Methane Emissions Through Direct and Indirect Pathways

Rebecca B. Neumann

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September 19, 2019

Carbon released by plant roots into soil fuels methane production by directly supplying carbon to microbes and by stimulating microbial use of soil organic matter.

The Science
In a plant-growth laboratory experiment conducted with a common wetland sedge (Carex aquatilis) and peat collected from a permafrost-thaw bog, plants were exposed to isotopically labeled carbon dioxide (13CO2) at two time points. Subsequent enrichment of root tissue, rhizosphere soil, and emitted methane (CH4) was used in an isotope mixing model to determine the proportion of plant-derived versus soil-derived carbon supporting methanogenesis. Results showed that carbon exuded by plants was converted to CH4 but also that planted boxes emitted 28 times more soil-derived carbon than was emitted by the unplanted treatments. At the end of the experiment, emissions of excess soil-derived carbon from planted boxes exceeded emissions of plant-derived carbon.

The Impact
In the experiment, an order of magnitude increase in conversion of soil carbon to CH4 was driven by plant growth, which is projected to increase in the boreal region under forecasted climate conditions. The presence of such a large “priming” effect (i.e., the release of carbon by plant roots stimulating a microbial population into breaking down soil organic matter) implies that increased plant productivity potentially could lead to increased conversion of soil carbon to CH4 on climatically relevant scales.

Summary
The largest natural source of CH4 to the atmosphere is wetlands, which produce 20% to 50% of total global emissions. Vascular plants play a key role in regulating wetland CH4 emissions through multiple mechanisms. They often contain aerenchymatous tissues that act as a diffusive pathway for CH4 to travel from the anoxic soil to the atmosphere and for oxygen to diffuse into the soil and enable oxidation of CH4 to CO2. Plants also exude carbon from their roots, stimulating microbial activity and fueling methanogenesis. This study investigated these mechanisms in a laboratory experiment using root boxes containing either C. aquatilis plants, silicone tubes that simulated aerenchymatous gas transfer, or only soil as a control. Methane emissions were over 50 times greater from planted boxes than from control boxes or simulated plants, indicating that the physical transport pathway of aerenchyma was of little importance when not paired with other effects of plant biology. Plants were exposed to 13CO2 at two time points, and the subsequent enrichment of root tissue, rhizosphere soil, and emitted CH4 was used in an isotope mixing model to determine the proportion of plant-derived versus soil-derived carbon supporting methanogenesis. Results showed that carbon exuded by plants was converted to CH4 but also that planted boxes emitted 28 times more soil-derived carbon than was emitted by the other experimental treatments. At the end of the experiment, emissions of excess soil-derived carbon from planted boxes exceeded the emission of plant-derived carbon. This result signifies that plants and carbon exuded by plant roots (i.e., root exudates) altered the soil chemical environment, increased microbial metabolism, and/or changed the microbial community such that microbial utilization of soil carbon was increased (e.g., microbial priming). 

Contacts
BER Program Managers
Daniel Stover
U.S. Department of Energy Office of Science, Office of Biological and Environmental Research
Earth and Environmental Systems Sciences Division (SC-33.1)
Environmental System Science
daniel.stover@science.doe.gov

Jared DeForest, Intergovernmental Personnel Act (IPA) assignment
U.S. Department of Energy Office of Science, Office of Biological and Environmental Research
Earth and Environmental Systems Sciences Division
Environmental System Science

Principal Investigator
Rebecca B. Neumann (University Researcher, Early Career Award)
Associate Professor, Civil & Environmental Engineering
University of Washington, Seattle, WA
rbneum@uw.edu

Funding
This material is based on work supported by the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science, under Award No. DE-SC-0010338. A portion of this research was performed under the Facilities Integrating Collaborations for User Science (FICUS) Program and used resources at the Environmental Molecular Sciences Laboratory (EMSL, grid.436923.9), which is a DOE Office of Science User Facility sponsored by BER and operated under Contract No. DE-AC05-76RL01830. This material is based on work supported by the Office of Science Graduate Student Research (SCGSR) Program of the DOE Office of Science’s Office of Workforce Development for Teachers and Scientists. The SCGSR Program is administered for DOE by the Oak Ridge Institute for Science and Education (ORISE). ORISE is managed by Oak Ridge Associated Universities (ORAU) under Contract No. DE-SC0014664. Students were additionally supported by the University of Washington (UW) College of Engineering Dean’s Fellowship/Ford Motor Company Fellowship, UW Civil & Environmental Engineering Valle Scholarship, UW Mary Gates Scholarship, and Carleton College Kolenkow Reitz Fellowship.

Publication
Waldo, N. B., B. K. Hunt, E. C. Fadely, J. J. Moran, and R. Neumann. “Plant root exudates increase methane emissions through direct and indirect pathways.” Biogeochemistry 145, 213–34 (2019). [DOI:10.1007/s10533-019-00600-6].

University Researcher. Early Career Award.

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