by Kristen Minogue

Tidal marshes have long been lauded as carbon sinks for their ability to pull CO₂ from the atmosphere and bury it in the soil, what scientists have taken to calling “blue carbon.” But wetlands are also notorious methane emitters. Now ecologists suspect that only a select few wetland types can reliably act as sinks, and that number may shrink as sea levels rise.

Scientists estimate wetlands are responsible for anywhere from 15 to 45 percent of all methane emissions – a wide range that makes predicting their role in climate change difficult. However, that role could prove critical in the years to come. Methane (CH₄) is a far more potent greenhouse gas than carbon dioxide. Over the course of a century, a single gram of methane is roughly 25 times more powerful than a gram of CO₂.

It all comes down to what’s in the soil, according to SERC ecologist Pat Megonigal. The various microbes that battle for dominance beneath the surface can determine whether a wetland ultimately becomes a greenhouse gas sink or a greenhouse gas source. Some microbes in tidal marshes – the sulfate-reducers – get their energy by respiring or “breathing” sulfur oxides in sea water, a relatively harmless process as far as the atmosphere is concerned. Others – the methanogens – break down CO₂ and re-emit it as methane. If the sulfate-reducers win, the wetland remains a sink. If the methanogens win, it turns into a source.

Megonigal and his colleagues Hanna Poffenbarger and Brian Needelman of the University of Maryland discovered that the salinity of the water flooding marsh soils may tip the scale one way or the other. Sea water is spiked with high concentrations of sulfate (SO₄), enabling the sulfate-reducing microbes to outcompete the methanogens. But in freshwater marshes, methanogens have more of an edge. After looking at data from 31 different marshes, some from previous studies and some from unpublished measurements in SERC’s own tidal marsh, Megonigal and his team found what could be the critical salinity for a marsh to go from a greenhouse gas source to a sink: 18 parts per thousand, or about half as salty as sea water.

While there were some less salty marshes that could act as sinks, the data varied too widely to make reliable predictions. Only when salinity reached 18 parts per thousand could the researchers be 95 percent confident the marsh would benefit climate – meaning the methane the marsh emitted was less powerful than the CO₂ it absorbed. Unfortunately, says Megonigal, this number rules out most marshes in Chesapeake Bay.

But there’s another catch. As sea level rises, the marshes could start to emit even more methane. That’s because if the water table is too high, the soil becomes anaerobic, smothering out the oxygen that could help break down the methane.

Oxygen enables a third kind of microbe, methane-oxidizing bacteria called methanotrophs, to stop methane before it escapes into the atmosphere. The methanotrophs undo the work of the methane-releasing methanogens by oxidizing methane into less harmful CO₂. The most effective wetlands can oxidize up to 100 percent of the methane in the soil. But without enough oxygen, their work becomes impossible.

While working on the SERC marsh with Megonigal’s lab this summer, interns Shannon Hagerty and Lillian Aoki found that a rising water table could make marshes emit more methane from their soils. They simulated sea level rise by filling cylindrical columns with dirt and plants from the marsh and placing them at various elevations in the water, ranging from 35 cm higher to 10 cm lower than current sea level. Then they measured which cylinders emitted the most methane.

Hagerty’s preliminary findings suggested the cylinders submerged the most – the ones mimicking conditions if sea level rose 10 cm – emitted roughly nine to 12 times as much methane as the higher, drier cylinders.

After looking at her preliminary findings, Aoki discovered something equally troubling with the plants in the marsh. The invasive Phragmites australis, a reed from Europe that has begun to take over the marsh in recent years, emits more methane than two of the marsh’s most common native plants. And when CO₂ levels rise, another likely scenario as climate change progresses, even the native plants emit more methane.

Combined, these influences suggest that creating or restoring the saltiest wetlands has promise as a way of absorbing greenhouse gases, provided they continue to sit well above sea level. But as the oceans continue to rise, if the saltier marshes can’t keep up, those marshes would also be the first to drown and disappear.