Peatlands: a global warming threat?

Peatlands: A global warming threat?

For millennia, peatlands have captured and stored carbon, preventing its atmospheric dissipation as carbon dioxide, a potent greenhouse gas. But a new study indicates some of these soggy carbon-storage depots may be changing from carbon "sinks" to significant carbon dioxide emitters--an unexpected and potentially alarming transformation.

Methane-producing bacteria, or methanogens, are among the plant-decay microbes that ordinarily thrive at peaty sites. But where sulfate levels are high, aggressive sulfate-reducing bacteria (SRBs) tend to move in, starving methanogens out. At least that's the conventional wisdom. But the situation can play itself out quite differently, according to a year-long study by Joseph B. Yavitt of Cornell University in Ithaca, N.Y., and R. Kelman Wieder of Villanova (Pa.) University.

SRBs and methanogens coexist at West Virginia's Big Run Bog, they found. Previously, "microbiologists would have laughed at this assertion and called it impossible," says Yavitt. Moreover, while the bog's sulfate levels are only about one-thousandth those in marine sediments, its SRBs produce unexpectedly copious amounts of carbon dioxide -- comparable to rates for oceanic SRBs, he says. Finally, Yavitt says his preliminary study of the bog's carbon budget--what goes in and comes out--quite unexpectedly showed that "more carbon is being lost as greenhouse gases than is stored by plants."

Methanogens produce two greenhouse gases--methane and carbon dioxide. Yavitt and Wieder found Big Run Bog's SRBs 50 percent more efficient than methanogens at producing carbon-based greenhouse gases. And by coexisting in this ecosystem, Yavitt says, these two microbe types more than double the bog's greenhouse-gas output over expected levels.

These data suggest sulfate deposition by acid rain at presently pristine peatlands will lure SRBs to more wetlands, converting them from carbon sequesterers to potent carbon dioxide sources, Yavitt says. And with peatlands estimated to hold some 15 to 20 percent of land-stored carbon, their emissions might significantly exacerbate climate change, he notes. Finally, Yavitt points out that carbon dioxide production at peatlands can be expected to climb even more if climate warming contributes to their drying and colonization by oxygen-using microbes, which decompose carbon-based plant debris more efficiently than do anaerobes such as methanogens and SRBs.