Why Do Coastal Wetland Methane Emissions Increase with Warming?

Four years of data show that coastal wetland methane emissions greatly increase with warming, driven by both biogeochemical and plant trait mechanisms.

Schematic of mechanisms driving enhanced methane (CH4) emissions in response to warming. SCAM and SPPA are the dominant plant species. (Left) Processes under ambient conditions. (Right) Altered processes, driven by direct and indirect warming effects.

[Reprinted under a Creative Commons Attribution 4.0 International License (CC BY 4.0) from Noyce, G.L., et al. “Biogeochemical and Plant Trait Mechanisms Drive Enhanced Methane Emissions in Response to Whole-Ecosystem Warming.” Biogeosciences 18(8), 2449–2463 (2021). DOI: 10.5194/bg-18-2449-2021]

The Science

SMARTX is a whole-ecosystem warming experiment that was established in a Chesapeake Bay tidal marsh in 2016 to understand the ecosystem-scale effects of warming on carbon gain, via plant inputs into the soil, and carbon loss, mainly via methane (CH4) emissions. The research team measured monthly CH4 emissions for four years and found that 5°C of warming more than doubled CH4 emissions compared to ambient conditions. This effect was driven by direct and indirect warming effects, but it also was dependent on plant traits and growth patterns.

The Impact

Methane fluxes are a metric of broader shifts in wetland biogeochemical cycling and carbon preservation. While previous studies have predicted that CH4 emissions will increase in a warming climate, there has been minimal work to determine the underlying mechanisms, as in this study. Without knowing these mechanisms, it is difficult to develop prognostic models to forecast wetland CH4 emissions and to scale from site-based studies to larger areas.

Summary

Climate warming perturbs ecosystem carbon cycling, causing both positive and negative feedbacks on greenhouse gas emissions. Terrestrial ecosystem responses to warming are typically mediated by complex plant-soil interactions. While CH4 emissions from coastal wetlands offset a portion of the carbon sequestered into these ecosystems annually, there is still minimal knowledge about how warming will alter coastal wetland CH4 emissions, even though these feedbacks have the potential to shift coastal wetlands from being a net sink of carbon to a net source.

In SMARTX, heating treatments run year-round along a gradient from ambient to +5.1°C above ambient and warming spans from above the plant canopy to 1.5 m in soil depth. The researchers measured CH4 emissions monthly for four years and coupled these flux measurements with analysis of porewater biogeochemistry and vegetation biomass and composition. Using these data, the team propose four mechanisms that increase CH4 emissions under warming conditions: (1) rates of CH4 production increase more than rates of CH4 oxidation, (2) overall substrate availability increases, (3) sulfate (SO4) reducers become SO4 limited and no longer outcompete methanogens, and (4) plant traits alter substrate and oxygen supply.

Principal Investigator

Genevieve Noyce (noyceg@si.edu) and Pat Megonigal (megonigalp@si.edu)
Smithsonian Environmental Research Center

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 work was supported by the Office of Biological and Environmental Research (DE-SC0014413 and DE-SC0019110) within the U.S. Department of Energy Office of Science, the National Science Foundation Long-Term Research in Environmental Biology Program (DEB-0950080, DEB-1457100, and DEB-1557009), and the Smithsonian Institution.

References

Noyce, G.L., et al. "Biogeochemical and Plant Trait Mechanisms Drive Enhanced Methane Emissions in Response to Whole-Ecosystem Warming." Biogeosciences 18 (8), 2449–2463  (2021). https://doi.org/10.5194/bg-18-2449-2021.