A Tale of Oxygen Priming in a Wetland

Scientists at the Smithsonian Environmental Research Center (SERC) and the Virginia Institute of Marine Science recently published a paper that details surprising results from a four-year field study. The gist: soil carbon in wetlands is more sensitive to elevated carbon dioxide (CO₂) than to warming because elevated CO₂ stimulates O₂-breathing microbes. Researchers call it oxygen priming. 

“The hypothesis we confirmed is that elevated CO₂ enhances both plant growth and decomposition,” said Patrick Megonigal, associate director of research and principal investigator (PI) at SERC and a joint appointee in Pacific Northwest National Laboratory’s Biological Sciences Division. “Interestingly, we did not see this in plots that had warming only, which tells us that it is important for climate change experiments to look at how multiple climate change factors interact.” 

Megonigal explains that the idea of oxygen priming was first published in 2007. 

“We’re starting to see papers that support the idea that more wetland plant growth means more oxygen for microbes in wetland soils, but mainly with respect to methane. Showing this for soil carbon in a real wetland is novel,” he said. 

Described as a “brackish high marsh on the western shore of the Chesapeake Bay,” the Smithsonian’s Global Change Research Wetland served as the field site. There, researchers tended to 12 plots, treated with two temperature levels and two CO₂ levels. 

“It’s rare that field experiments manipulate temperature and CO₂ levels together,” said Megonigal. “This experiment is only one of two that intentionally do both in wetlands—the other one being a Department of Energy funded experiment in a completely different system, a peatland.” 

In 2007, Megonigal and others were studying the relationship between elevated CO₂ and soil carbon decomposition in small pots. As they increased CO₂, they saw an increase in soil carbon lost to decomposition. But they weren’t sure why it was happening. 

They conducted a similar experiment in much larger pots, and they obtained similar results. 

“We had something that was repeatable but not conclusive,” Megonigal said. 

Genevieve Noyce, also a principal investigator and senior scientist at SERC, got involved in 2016 to help find the causes of the missing carbon. She submitted a proposal for a Large-Scale Research project with the Environmental Molecular Sciences Laboratory (EMSL), a Department of Energy, Office of Science user facility sponsored by the Biological and Environmental Research program. As part of the project, scientists used EMSL’s instrumentation to understand the processes occurring underground. 

“We couldn’t have got the results we did without using EMSL’s FTICR,” said Noyce. “The data we got from it helped us secure more money to install an automated redox system, which was the final piece we needed to confirm our hypothesis.” 

EMSL is home to one of two 21T, for tesla, magnets in the world. The FTICR—Fourier transform ion cyclotron resonance—is used to identify compounds in complex samples. It’s one of many instruments scientists world-wide can use free of charge through requests for proposals. 

Routinely measuring redox in the field is critical to understanding biogeochemical changes. When their FTICR data revealed carbon transformations that could only have occurred with increased oxygen, scientists installed a system in 2020 that has captured redox data every 30 minutes ever since.  

“Roy Rich designed the warming experiment and is also the collaborator who built the real-time redox system. He set up loggers that collected the data and streamed it to our computers so on any given day I could pull up the information and see how things were going,” said Noyce.  

The team then had a Eureka moment—the plants were adept at transmitting oxygen through their stems and roots, raising the redox potential of the soil.  

“It’s rare that you have an entire story to tell from your original hypothesis,” said Noyce. “This story is both interesting and exciting because every piece of data added to or confirmed our hypothesis. It doesn’t usually work like that.” 

Now that researchers know the marshland plants add more oxygen to the soil, they suspect they’re adding something else, too. And that’ll be the next chapter of their story.