Alia Al-Haj, with the Smithsonian Environmental Research Center, is testing how sea level rise and warming affect the microbes that dwell in tidal marsh sediments. Photo by Christian Elliott

USA - A 'Carbon Bomb' Lurks In America's Marshlands

Turf wars between microbes dictate how much carbon salt marshes store and how much methane they pump into the air.

Alia Al-Haj strides down the half-kilometer boardwalk, a wagon full of power tools and PVC pipes in tow. Off to each side, a vast marsh stretches into the distance, the wet ground punctuated by tall sedges waving in the cool spring breeze. Al-Haj closes her eyes and takes in a deep breath of the salty, sulfury aroma.

“It’s really important to me being able to smell the marsh,” she says. “I know it may not always be there. That smell is something I think about a lot, how much I’ll miss it.”

For the past 35 years, scientists with the Smithsonian Environmental Research Center, including Al-Haj, have been coming here to the Global Change Research Wetland in coastal Maryland to conduct extensive long-term research. The scientists have turned the tidal marsh off the Chesapeake Bay into a giant experimental laboratory designed to simulate how marshes might respond to climate conditions decades into the future.

Tidal salt marshes are coastal guardians. They filter runoff, protect the shore against storm surges, and shelter wildlife. On a grander scale, they are incredibly effective buffers against climate change. Tidal marshes sequester carbon at a rate 10 times higher than even mature tropical rainforests.

Land development has already stripped the world of half its salt marshes. Along the US East Coast there’s an ongoing push to restore these rare, service-rich ecosystems. But with sea level rise, severe storms, and warming temperatures, scientists are concerned salt marshes could wash away entirely. About 1,500 square kilometers has disappeared in just the past 20 years, according to new NASA research. Globally, that loss of carbon storage is equivalent to the annual emissions of 3.5 million vehicles.

Short of their outright disappearance, there’s a chance the marshes could unpredictably flip from carbon sinks to net carbon emitters, causing a measurable shift in the carbon balance of the world. That’s where Al-Haj’s experiment comes in.

At the end of the boardwalk, Al-Haj unloads the wagon. Here and at a nearby freshwater marsh she’s building “marsh organs”: a series of sedges planted in white PVC piping that she submerges in the marsh at different elevations in an attempt to simulate future scenarios of sea level rise. Sitting in the marsh water, they look like very lost pipe organs. To simulate warming, the pipes have heating elements that maintain a constant temperature 6 °C above the ambient air. Nearby, other odd infrastructure covers the marsh, such as the octagonal plastic igloos pumped full of carbon dioxide to test the effects of elevated emissions.

In tidal marshes, sedges suck carbon dioxide out of the atmosphere and preserve it underground in the form of dead root material. But when microbes decompose the material under low oxygen conditions, they can emit methane and other greenhouse gases. Exactly which gases get produced depends on a number of factors.

The funky, salty smell permeating the air is from hydrogen sulfide gas, Al-Haj explains. That the odorous gas is so prevalent is a good sign. It means that in the submerged sediment beneath the boardwalk, microbes that consume sulfate—a component of seawater—are outcompeting other microbes that produce methane.

“It’s like a schoolyard battle, or a war for resources,” Al-Haj says. “We don’t think about it, we can’t see it, but we can measure the amount of methane and carbon dioxide coming out, so we can get at the outcomes of those battles.”

For now, in this marsh, the sulfate-consumers are winning. But if the methane producers take over, the outcome of this microscopic battle has the potential to change the makeup of the atmosphere.

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