Mosses reduce methane emissions in wetlands

17.05.2016

News from Climate Science: Once a month, climate researchers report on their latest findings in the newspaper "Hamburger Abendblatt". Christian Knoblauch is currently investigating the amount of methane released from the soil in the arctic tundra, and the role that plants play in this.

Wetlands, like these in the arctic Tundra, are the main natural source of the greenhouse gas methane.
Dr. Christian Knoblauch has shown that where mosses grow, methane can be converted almost 100 percent.
Mosses live in symbiosis with methane-converting bacteria. Here oxygen bubbles can be seen rising to the surface.
Vascular plants such as grasses act like chimneys, pumping methane directly into the atmosphere.

Small lakes and pools surrounded by lush greenery – it’s summer in the arctic tundra. On the unassuming island of Samoylov in northeast Siberia, water accumulates above the permanently frozen soil. Here grasses, mosses and shrubs now flourish. In the wetlands large quantities of carbon are converted and can escape into the atmosphere in the form of the climate-relevant gases methane and carbon dioxide.

Microorganisms are the culprit: They break down the organic material found in plant remains, for example, to produce methane – a process that occurs without oxygen in the water-saturated soil. As soon as oxygen becomes available, other bacteria convert the methane to carbon dioxide. This is important because carbon dioxide has a much better greenhouse-gas balance than methane: One ton of methane has roughly the same impact on the atmosphere as 30 tons of carbon dioxide.

My colleagues and I were able to show that not just microorganisms but also plants have a major influence on the formation and release of methane. The amount of methane that is produced, converted into carbon dioxide and released into the atmosphere depends on the types of plants that are present. On the basis of numerous measurements taken in Siberia, we were able to distinguish between two areas in the shallow pools.

We found significantly higher methane flows at the edges of the pools than in the middle. The reason: At the edges there are grasses, and mosses grow beneath the water’s surface. Further toward the center, there are only mosses. Each of these plants regulates the formation, breakdown and release of methane differently.

Mosses produce oxygen under the water, which bacteria then use to transform methane into carbon dioxide – as a result of which up to 99 percent of the methane is converted. Grasses, on the other hand, act like chimneys, belching the methane from the soil directly into the atmosphere. The methane is transported through the grasses’ tissues, which also supply oxygen to the roots in the water-saturated soil. In areas where there is grass, only much smaller amounts of methane can be converted, because the gas isn’t subjected to the combined effects of mosses and bacteria.

Previously, the water level in the soil was considered to mark the border between these two zones, in which metabolic processes occur with or without oxygen, respectively. The fact that mosses produce oxygen under water was not taken into account. However, this process is important when it comes to calculating the overall methane balance.
 
The wetlands in northeast Siberia are sensitive to changes in the climate, and the amount of methane released depends on the temperature, the water level and the plants. Precisely what will happen as the permafrost in the arctic tundra continues to thaw is not yet clear, but our findings will help develop more accurate predictions of how much methane will be released from the thawing permafrost. Future studies will have to attach more significance to the role of different plants.


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