The ratio of methanogens to methanotrophs and water-level dynamics drive methane transfer velocity in a temperate kettle-hole peat bog

• Main Authors: C. Rey-Sanchez, G. Bohrer, J. Slater, Y.-F. Li, R. Grau-Andrés, Y. Hao, V. I. Rich, G. M. Davies
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-08-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/16/3207/2019/bg-16-3207-2019.pdf

<p>Peatlands are a large source of methane (<span class="inline-formula">CH₄</span>) to the atmosphere, yet the uncertainty around the estimates of <span class="inline-formula">CH₄</span> flux from peatlands is large. To better understand the spatial heterogeneity in temperate peatland <span class="inline-formula">CH₄</span> emissions and their response to physical and biological drivers, we studied <span class="inline-formula">CH₄</span> dynamics throughout the growing seasons of 2017 and 2018 in Flatiron Lake Bog, a kettle-hole peat bog in Ohio. The site is composed of six different hydro-biological zones: an open water zone, four concentric vegetation zones surrounding the open water, and a restored zone connected to the main bog by a narrow channel. At each of these locations, we monitored water level (WL), <span class="inline-formula">CH₄</span> pore-water concentration at different peat depths, <span class="inline-formula">CH₄</span> fluxes from the ground and from representative plant species using chambers, and microbial community composition with a focus here on known methanogens and methanotrophs. Integrated <span class="inline-formula">CH₄</span> emissions for the growing season were estimated as <span class="inline-formula">315.4±166</span>&thinsp;<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">mg</mi><mspace width="0.125em" linebreak="nobreak"/><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="78pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="ef20001b32af2721b2dc95616d9bbffd"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-3207-2019-ie00001.svg" width="78pt" height="16pt" src="bg-16-3207-2019-ie00001.png"/></svg:svg></span></span> in 2017 and <span class="inline-formula">362.3±687</span>&thinsp;<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">mg</mi><mspace width="0.125em" linebreak="nobreak"/><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="78pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="76b89e953aa6d4e973a65484dea0a34b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-3207-2019-ie00002.svg" width="78pt" height="16pt" src="bg-16-3207-2019-ie00002.png"/></svg:svg></span></span> in 2018. Median <span class="inline-formula">CH₄</span> emission was highest in the open water, then it decreased and became more variable through the concentric vegetation zones as the WL dropped, with extreme emission hotspots observed in the tamarack mixed woodlands (Tamarack) and low emissions in the restored zone (18.8–30.3&thinsp;<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">mg</mi><mspace linebreak="nobreak" width="0.125em"/><msub><mi mathvariant="normal">CH</mi><mn mathvariant="normal">4</mn></msub><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">d</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="78pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="22bd542194aabf3918bdd58dbd499748"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-16-3207-2019-ie00003.svg" width="78pt" height="16pt" src="bg-16-3207-2019-ie00003.png"/></svg:svg></span></span>). Generally, <span class="inline-formula">CH₄</span> flux from above-ground vegetation was negligible compared to ground flux (<span class="inline-formula">&lt;0.4</span>&thinsp;%), although blueberry plants were a small <span class="inline-formula">CH₄</span> sink. Pore-water <span class="inline-formula">CH₄</span> concentrations varied significantly among zones, with the highest values in the Tamarack zone, close to saturation, and the lowest values in the restored zone. While the <span class="inline-formula">CH₄</span> fluxes and pore-water concentrations were not correlated with methanogen relative abundance, the ratio of methanogens to methanotrophs in the upper portion of the peat was significantly correlated to <span class="inline-formula">CH₄</span> transfer velocity (the <span class="inline-formula">CH₄</span> flux divided by the difference in <span class="inline-formula">CH₄</span> pore-water concentration between the top of the peat profile and the concentration in equilibrium with the atmosphere). Since ebullition and plant-mediated transport were not important sources of <span class="inline-formula">CH₄</span> and the peat structure and porosity were similar across the different zones in the bog, we conclude that the differences in <span class="inline-formula">CH₄</span> transfer velocities, and thus the flux, are driven by the ratio of methanogen to methanotroph relative abundance close to the surface. This study illustrates the importance of the interactions between water-level and microbial composition to better understand <span class="inline-formula">CH₄</span> fluxes from bogs and wetlands in general.</p>