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Global Warming May Not Raise Soil Carbon
A study by the University of Hawaii appears to disprove the theory that global warming increases the release of carbon from mineral soils into the atmosphere.
"We found no correlation between increased mean annual temperature and the carbon decomposition rates in forest mineral soils," says Christian Giardina, a soil scientist at the University of Hawaii. "If this is true, then global warming may not increase carbon loss from mineral soils."
Based on short-term laboratory incubation experiments and on-site litter decomposition studies, climate scientists assumed that carbon's decay rate in mineral soil increased with temperature. However, data from longer-term forest soil heating experiments and latitudinal comparisons of soil carbon decomposition rates have challenged this belief.
"We found no differences in rates of carbon loss from mineral soils across either incubation temperature for long-term lab experiments or mean annual temperature for the in-field studies," says Giardina. "Rates of carbon loss for mineral soils up to 30 centimeters were actually faster at cool temperate sites than at warm tropical sites, exactly opposite of what climate change models predict."
By compiling and analyzing soil carbon decomposition data from 82 sites in five continents, the study showed that decomposition rates were remarkably constant across the globe, suggesting there is no relationship between carbon decomposition rates in mineral soil and temperature.
However, Bill Parton, senior research scientist at Colorado State University's Natural Resources Ecology Laboratory, finds the University of Hawaii study's results problematic.
"There is evidence that suggests temperature does have an effect on soil carbon," he says.
Parton questions the study's findings because the University of Hawaii researchers didn't examine the relationship between mineral soil carbon and temperature long enough.
"Year-long experiments are not adequate to say anything definitive about the effect of temperature on the slow pools of soil carbon," he says. "The comparison of real world soil carbon data and scientific models explain the variability in the slow pools of carbon in different geographic regions."
Despite Parton's objections, Giardina contends his study contains several important implications.
Given that mineral soils contain twice the carbon of the atmosphere, climate models suggest that global warming will spur forests to release more carbon dioxide through respiration than they take in through photosynthesis, thereby transforming them from carbon sinks into sources.
"Our data suggest that the effects of global warming on carbon decomposition rates may be greatly overestimated," he says.
Giardina contends global warming is unlikely to spur the release of carbon from mineral soils because temperature does not limit the activity of the soil microorganisms responsible for its decomposition. Rather, microbial activity may be regulated by the amount of labile (or easily decomposed) carbon available for microbes to use.
"Temperature elevation only increases the release of carbon from mineral soils when labile carbon is abundant. Typically it's very low," he says.
These findings are good news for foresters, Giardina says.
"If warming does not increase carbon decomposition in mineral soils, but elevated CO2 and increased atmospheric nitrogen deposition stimulate photosynthesis, then the importance of forests as sinks for atmospheric carbon will increase with time. Therefore, forestry may have a major opportunity to positively impact global warming."
For information, contact Christian Giardina, University of Hawaii Branch Station, 461 West Lanikuala Street, Hilo, HI 96720; (808) 934-9512; fax (808) 974-4110; email: firstname.lastname@example.org.
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Title: RMRS Public
Affairs: News Clippings: Global Warming May Not Raise Soil Carbon
Publish Date: April 3, 2001
Last Update: April 3, 2001
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