Restored wetlands – do they emit or absorb greenhouse gases?

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Yesterday was World Wetlands Day, so we had the opportunity to remind ourselves of the important function wetlands play in the environment. We could also hear and read about the need to restore drained areas to their natural state in order to achieve climate neutrality faster. But will restoring wetlands make them immediately start absorbing greenhouse gases?

Restoration of wetland ecosystems

It has been widely accepted by environmentalists that restoring habitats to their original state can increase carbon storage, improve water quality and strengthen wildlife populations. In recent decades, wetlands have gained much recognition as key ecosystems that perform important environmental functions. As a result, various initiatives have been taken to bring them back, by eliminating barriers, reducing pollution and removing invasive species.

However, recent studies show that the effect of wetland restoration is not always what we would expect. It’s all about greenhouse gas emissions. The findings of biologists from Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley indicate that a wetland exposed to a small influx of seawater releases larger amounts of methane into the environment than normal. The phenomenon affects such areas located at the mouths of rivers to the sea. This suggests that the dynamics governing greenhouse gas storage and emissions in natural landscapes are more complex and unpredictable than previously assumed.

Salt water and greenhouse gases

Global warming is causing sea levels to rise, causing ecosystems to change – usually negatively. However, scientists see some positive aspect to this situation. The increased influx of seawater into tidal wetlands in estuaries makes them less favorable for methane-producing microorganisms. As a result, wetlands should generate less of this greenhouse gas. Logical, right?

Unfortunately, the situation is not so clear-cut. Berkeley Lab biologists analyzed 11 wetland zones for microbiology, chemistry and geology. Studies have shown that an area exposed to even a small influx of seawater releases methane in amounts that exceed those observed in freshwater sites.

We checked how many methanogens, methane-producing organisms, were present in the soils at these sites, and it did not correlate well with the observed amount of methane. And even looking at the number of methanotrophs, organisms that eat methane, in combination with methanogens, it doesn’t seem to be fully explained – says Susannah Tringe, author of the study and director of the Environmental Genomics & Systems Biology Division, Lawrence Berkeley National Laboratory.

Wetlands and their complex relationships

Susannah Tringe and her team took soil samples from 11 wetland sites to study the DNA of the microorganisms they found, such as bacteria, viruses and fungi. They analyzed the genes associated with the relevant vital functions of the microorganisms. In this way, they identified genes involved in nitrogen metabolism and genes of bacteria that use sulfate in respiration. They then considered how the information obtained, juxtaposed with chemical factors in soil and water, might affect the observed methane emissions.

At most sites, the amount of methane emitted decreased as salt water inflow into the river increased. However, one area showed something quite different. A grassy pasture that in 2010. restored to its original wetland habitat, emitted an elevated amount of methane despite the moderate influx of salt water.

The sulfates in seawater were expected to trigger competition between the bacteria that use them for energy and the methanogens that take upCO2. However, it turned out that there are significant influences from yet other groups of bacteria, such as nitrogen cyclers, making it difficult to explain these relationships in simple terms. Methane emissions cannot be explained solely by the amount of available sulfate or the number of methanogens. A study by Duke University researchers also confirms this. They took soil samples from coastal freshwater wetlands and treated them with artificial seawater, with and without sulfate. In both scenarios, methane production increased.

Wetlands – to restore or not to restore?

Since restored wetlands can emit more greenhouse gases, is it worth investing in them in times of climate crisis? It turns out that it is still a profitable investment, but rather a long-term one. Dennis D. Baldocchi, co-author of the above study, has conducted modeling work indicating that despite current greenhouse gas emissions to the atmosphere, the ecosystem of restored wetlands will stabilize and turn into a net carbon sink within 100 to 150 years. However, this tests the patience of investors who wanted to restore these areas for rapid carbon sequestration.