Can wetland restoration cool the planet?

(American Chemcial Society)
Wetlands are champions at carbon storage, but they also release methane, a greenhouse gas 20 times more potent than CO₂. Scientists are boosting research efforts to determine whether the cooling power of carbon storage outstrips the global warming potential of methane in wetlands. They are finding that the greatest cooling occurs from saltwater marshes.

Jim Nickles/USGS
In the future, farmers could be paid to grow cattails and tules on this “carbon farm” in the Sacramento−San Joaquin River Delta.

This summer, the U.S. Geological Survey (USGS) announced that it was launching a $12.3 million project to capture carbon by growing tules (a species of sedge also known as bulrushes) and cattails in wetlands created on abandoned farmland on islands in California’s Sacramento−San Joaquin River Delta. Two months later, the carbon-storing capacity of wetlands headlined 2 days of workshops at the September 16 meeting of the Association of State Wetland Managers in Portland, Ore.

The USGS project has captured eye-popping amounts of carbon—an average of 3000 grams of carbon per square meter per year (g-C/m²/yr) over the past 5 years. For comparison, reforested agricultural land, eligible for carbon credits under the Kyoto Protocol on climate change, socks away carbon at a rate much less than 100 g-C/m²/yr, says Gail Chmura, a biogeochemist at McGill University (Canada).

Wetlands capture carbon by incorporating CO₂ from the air into new plant growth, explains Roger Fujii, a soil chemist with USGS. When the plant material dies, near-constant water cover keeps oxygen out of the rich mud, slowing decomposition that would otherwise emit CO₂. Undisturbed wetlands are so effective at accreting carbon that their organic peat soils can be 60 feet deep and 7000−10,000 years old, he says.

USGS is now expanding the delta project to see whether it can regain the land elevation lost since farmers drained the delta island marshes 100 years ago, causing the soil to decompose, emit CO₂, and subside, Fujii says. A secondary goal is to find out whether the extraordinary carbon storage capacity of the tule and cattail “farms” could be sold as carbon credits on California’s upcoming CO₂ cap-and-trade market, he says.

Scientists are excited by the prospect of selling wetland carbon credits because the sales could provide money and an additional reason for restoring wetlands, says Scott Bridgham, an ecosystem ecologist at Oregon State University. “But if you don’t know what the methane emissions are, you can’t assume there is a net climate-cooling impact,” he says.

The low oxygen conditions that promote carbon storage also promote release of methane, explains Ramesh Reddy, a biogeochemist at the University of Florida. Microbes prefer using oxygen to produce energy, but if they can’t get oxygen, they can use other electron acceptors such as iron oxides, sulfate, and CO₂. When they use CO₂, they emit methane, he says.

Because the tiny traces of methane gas from microbes are hard to measure, very few data are available on methane releases from wetlands, Bridgham says. He recently coauthored a chapter in The First State of the Carbon Cycle Report, in which he concluded that the climate-warming potential of methane very likely cancels out the climate-cooling potential of CO₂ storage for most North American freshwater wetlands.

Because saltwater is high in sulfate, microbes in saltwater marshes don’t have to use CO₂ as an electron acceptor, and therefore they produce negligible amounts of methane, Chmura says. She estimates that North American salt marshes sequester an average of 210 g-C/m²/yr. These hefty rates, along with an ability to accrete carbon faster as the sea level rises, make saltwater marshes ideal sites for restoration and carbon storage, she says.

Although the USGS project doesn’t have a reliable estimate of methane emissions from the experimental plots, initial measurements suggest that these emissions may not cancel out the climate-cooling potential of the CO₂ storage, says Brian Bergamaschi, a biogeochemist with USGS. He and his colleagues are researching ways to add nutrients to the wetlands and to make water levels fluctuate to maximize carbon storage while minimizing methane emissions.