October 15, 2020

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Getting to the Root of Plant-Mediated Methane Emissions and Oxidation in a Thermokarst Bog

We used multiple methods to identify how plants affect the emissions of methane from a permafrost-thaw bog in Interior Alaska.

Black spruce permafrost forest (dark green areas) and thermokarst bogs (light green areas) where permafrost has thawed, the land has subsided and transformed from a forest into a bog. Location is near Fairbanks, AK, where research was performed.

[Courtesy Rebecca Neumann.]

The Science

Methane is a greenhouse gas with a greater ability to warm the earth than carbon dioxide. Wetlands are the largest natural source of methane to the atmosphere. Working in a wetland that formed due to permafrost thaw, we used multiple methods to identify how wetland plants influence the amount of methane: generated by soil microbes (called production), converted by soil microbes into carbon dioxide (called oxidation), and transported from soil to the atmosphere (called emission). Plants appeared to increase methane production and, to our surprise, decrease methane oxidation. We created a theory for why plants increased methane emissions.

The Impact

To understand future climate change, scientists need to predict the amount of methane released from wetlands. Our work has advanced understanding of plant‐soil interactions that contribute to wetland methane emissions within thawing permafrost landscapes. This understanding will help scientist model and predict wetland methane emissions.

Summary

In a permafrost‐thaw bog in Interior Alaska, we sought to disentangle mechanisms by which vascular vegetation affect methane emissions. Vegetation operated on top of baseline methane emissions, which varied with proximity to the thawing permafrost margin. Emissions from vegetated plots increased over the season, resulting in cumulative seasonal methane emissions that were ~4.5 g m−2 season−1 greater than unvegetated plots. Mass balance calculations signify these greater emissions were due to increased methane production and decreased methane oxidation. Minimal oxidation occurred along the plant‐transport pathway, and oxidation was suppressed outside the plant pathway. Suppression of methane oxidation was stimulated by root exudates fueling competition among microbes for electron acceptors. This contention is supported by the fact that methane oxidation and relative abundance of methanotrophs decreased over the season in the presence of vegetation, but methane oxidation remained steady in unvegetated treatments; oxygen was not detected around plant roots but was detected around silicone tubes mimicking aerenchyma; and oxygen injection experiments suggested that oxygen consumption was faster in the presence of vascular vegetation. Root exudates are known to fuel methane production, and our work provides evidence they also decrease methane oxidation.

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 upon work supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research (Award Number DE‐SC‐ 0010338 to R.B.N.) and the USGS Land Change Science Program. Logistic support was provided by the Bonanza Creek LTER Program, which is jointly funded by NSF (DEB 1026415) and the USDA Forest Service, Pacific Northwest Research Station (PNW01‐ JV112619320‐16). Microbial data came from the Joint Genome Institute (JGI Proposal 1445).

References

Turner, J. C., et al. "Getting to the Root of Plant-Mediated Methane Emissions and Oxidation in a Thermokarst Bog.." Journal of Geophysical Research: Biogeosciences 125 (11), e2020JG005825  (2020). https://doi.org/10.1029/2020JG005825.