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).

Principal Investigator

Rebecca Neumann
University of Washington, Seattle
rbneum@uw.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

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.

References