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Rising seas release more methane from wetlands than expected

In a startling revelation, experts have found that rising sea levels are leading to higher-than-anticipated methane emissions from certain wetland areas. This finding challenges the previously held belief that increasing seawater in tidal wetlands would suppress methane production.

The study, conducted by researchers from the Berkeley Lab and UC Berkeley, involved an extensive examination of 11 different wetland zones. 

Unexpected results

Contrary to expectations, a low-salinity Bay Area estuary ecosystem was found to emit significantly high levels of methane. This was especially surprising given that freshwater sites were emitting far less methane.

The research team analyzed DNA from organisms in soil samples using high-throughput sequencing, identifying genes involved in various metabolic processes. 

Complicated dynamics

The results indicate that the factors governing how much greenhouse gas is stored or emitted in natural landscapes are more complex and difficult to predict than we thought. 

“We looked at how many methanogens, the organisms that make methane, are present in soils at these sites and it wasn’t really well correlated with the amount of methane observed. And even if you look at the amount of methanotrophs, organisms that eat methane, in combination with methanogens, that doesn’t seem to fully explain it,” said study senior author Susannah Tringe. 

Surprising methane emissions 

One particular site, restored in 2010 from pastureland to its original wetland state, showed high methane emissions despite moderate seawater levels. This finding challenges the assumption that more sulfate from seawater would lead to less methane production, as sulfate-using bacteria were thought to outcompete methanogens.

“Ultimately, we found that there were significant influences from other bacterial groups like the ones that break down carbon and even organisms that are better known as nitrogen cyclers, and we couldn’t readily explain the methane emissions by something as simple as, for example, how much sulfate is available or how many methanogens are there,” explained Tringe.

Restored wetlands

Wetlands are increasingly recognized as critical ecosystems that boost carbon storage, improve water quality, and support wildlife. In recent years, there have been widespread efforts to restore wetlands. 

Modeling work by study co-author Dennis D. Baldocchi suggests that while the restored wetland site currently adds greenhouse gases to the atmosphere, it might become a net carbon sink within 100 to 150 years. This timeline is significant for stakeholders who aim for immediate carbon sequestration through ecosystem restoration.

“We want to know if these systems will act as long-term carbon sinks,” said Baldocchi. “And these microbiological investigations can help refine our models and predictions.” 

Study implications 

Tringe noted that similar findings of increased methane production with increased salinity have been observed in other studies, including experiments by Duke University and North Carolina State University. Collectively, the research highlights that the effects of seawater intrusion and ecosystem restoration are more complicated than previously thought.

“There was this expectation that sulfate would be the most important thing. And in those studies, not only did salt water stimulate methane production, which again is kind of counter to the dogma that sulfate is important, it happened whether you had sulfate there or not; in fact the sulfate didn’t have a big effect on the methane emissions,” said Tringe. 

“So I think these experimental manipulations are reconfirming the story that there’s more nuanced effects of seawater intrusion than just a sulfate addition, and also more nuanced factors behind ecosystem restoration.”

The study is published in the journal mSystems

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