Lack of oxygen locks up peat’s carbon

When peatlands and permafrost lose moisture, huge amounts of carbon sequestered there can oxidize and return to the atmosphere as carbon dioxide, a greenhouse gas (SN: 12/16/00, p. 396). Now, researchers in Wales, where peat is common, suggest the key that unlocks the carbon is an oxygen-stimulated chemical reaction catalyzed by a single enzyme.

Up to 30 percent of the carbon in the world’s soil is locked into the Northern Hemisphere’s peatlands, and many peatlands have been stockpiling carbon since the last ice age. In these areas, moss and other vegetation grow atop water-saturated, oxygen-poor, and partially decomposed vegetation layers from previous years, says Chris Freeman, a biogeochemist at the University of Wales in Bangor.

The release of carbon dioxide from peat turns out to be a complicated process. Hydrolase enzymes often help break down vegetation and liberate carbon dioxide. Despite their efficiency in many other oxygen-poor environments, these enzymes don’t seem to be active in peat. Other types of wetlands also have slow rates of decomposition, says Freeman. One factor common to these environments, he notes, is the presence of phenolic compounds. In high concentrations, these organic chemicals strongly inhibit the hydrolase enzymes.

Freeman and his colleagues propose in the Jan. 11 Nature that adding oxygen to the peat kindles a cascade of chemical reactions that ultimately produces carbon dioxide. Their experiments show that oxygen stimulates another reaction in which an enzyme in peat, phenol oxidase, helps break down the phenolic compounds. This, in turn, frees hydrolase enzymes to speed decomposition of the vegetation. In the field, the team found that a doubling in phenol oxidase activity led to a comparable increase in carbon dioxide production.

Droughts spurred by global warming could dry and thus aerate the peat, Freeman says. And that could stimulate phenol oxidase reactions in the peatlands to unleash more of the 455 gigatons of carbon-an amount equal to about 60 percent of that in the atmosphere now.