Advertisement - Click to support our sponsors.

Study disputes
global warming tenet

A UH-led team finds that
temperature doesn't increase the
decomposition of organic carbon
in forest mineral soils

By Helen Altonn
Star-Bulletin

A widely held belief that global warming will increase the release of carbon dioxide from the soil into the atmosphere has been challenged by a Hawaii research team.

Christian Giardina of the University of Hawaii College of Tropical Agriculture and Human Resources led a study showing temperature doesn't influence the decomposition of organic carbon in forest mineral soils.

Findings by Giardina and his colleague, Michael Ryan of the U.S. Forest Service's Rocky Mountain Research Station in Fort Collins, Colo., appeared in the April 20 issue of the British journal Nature.

The two are among researchers tracking where all the carbon goes in a eucalyptus forest loaned by C. Brewer & Co. in Pepeekeo, on the Big Island.

Their findings are based on soil carbon decomposition data from 82 sites on five continents.

Decomposition rates are relatively constant across the planet, indicating no connection with mean annual temperature, the researchers reported.

Their paper is one of three highlighted by the journal, which notes that forests are a major source of carbon cycling, with about 60 billion tons cycled annually between the world's vegetation and the atmosphere.

It has been assumed that global warming will increase respiration from plants and decomposition of plant material, resulting in vegetation putting more carbon dioxide into the atmosphere.

Giardina and Ryan found, however, that temperature isn't likely to release more carbon from tropical and temperate forests, which contain about 40 percent of the planet's soil carbon.

"The findings are pretty controversial, I'd say, because they contradict, basically, all the common wisdom regarding what global warming is going to do to the large stock of carbon in soil," Giardina said in an interview.

Mineral soils hold about two times more carbon than the entire atmosphere and release about five times more carbon dioxide into the air annually than is released by fossil-fuel burning, he said.

No pattern found

"So, folks obviously are concerned with global warming if release of carbon dioxide (a greenhouse gas) to the atmosphere increases."

Giardina said a feedback would result, with increased activity of microorganisms, release of more carbon from mineral soils and further warming of the planet.

But, when they looked at decomposition rates, he said, "There wasn't a pattern of decompensating with temperature."

He said global warming may increase carbon release from soils in high latitudes because soils are frozen or waterlogged in boreal and arctic regions for much of the year.

If global warming thaws more soil and lowers the soil water table, findings indicate soil carbon decompensation rates will be more rapid than ecosystem models show.

Giardina said research hasn't been done to get a good look at how carbon responds to changing temperature at a specific place.

"People have assumed there is large variation in decomposition rates. ... If the assumption is that soil decomposes faster in Africa or Central America than Wisconsin, it must mean decomposition also goes up with global warming."

In the Pepeekeo project, the researchers plan to bury heated cables at various places in the soil and warm it artificially to see what happens at a single site.

Giardina, research scientist managing the project, said the main focus is to try to understand why tree growth slows with age.

Where does the carbon go?

They're tracking where all the carbon in the forest goes.

"We're into the carbon cycle of a forest," he said, "so we're going to use all our carbon budget skills that we learned here ... We need that carbon budget to quantify how much trees are aging."

Unlike people, Giardina pointed out, growth of trees doesn't stop with age. "The older they are, the bigger and taller they are."

The project is similar to studies trying to understand human aging, he said.

Besides compiling detailed estimates of soil carbon produced by photosynthesis in the leaves and wood, the researchers are studying tree physiology, nutrition, how growth patterns differ between closely and widely spaced trees and how fertilization affects patterns of growth over long periods of time.