Elevated atmospheric carbon dioxide increases carbon retention in soil

ARGONNE, Ill. (Dec. 20, 2005) — Researchers from the U.S. Department of Energy's Argonne National Laboratory – with collaborators from Oak Ridge National Laboratory, Kansas State University and Texas A&M University– have shown that soils in temperate ecosystems might play a larger role in helping to offset rising atmospheric carbon dioxide (CO₂ ) concentrations than earlier studies had suggested. Results of the new study are published in the current issue of Global Change Biology.

Higher CO₂ concentrations often stimulate plant growth. A subsequent increase in the amount of decaying plant material might then lead to an accumulation of carbon in soil. Yet nearly all field experiments to date have failed to demonstrate changes in soil carbon against the large and variable background of existing soil organic matter.

In this new study, funded by DOE's Office of Science, scientists overcame that issue using a statistical technique called meta-analysis. This analysis of earlier published experiments showed that elevated CO₂ concentrations – ranging from double pre-industrial levels to double current levels – increased carbon in soil surface layers by an average of 5.6 percent across diverse temperate ecosystems. If a response of this magnitude occurred globally for all temperate systems in a CO₂ -enriched world, the authors calculated that increased soil carbon storage might remove 8 to 13 billion metric tons of carbon from the atmosphere over a period of about 10 years.

The researchers also measured comparable increases in soil carbon after Tennessee deciduous forest and Kansas grassland were exposed to elevated CO₂ concentrations for only five to eight years. “At both of our experimental sites, elevated CO₂ apparently caused large enough increases in root litter inputs for us to see measurable accumulation of soil carbon within a relatively short time period,” said Argonne's Julie Jastrow, the study's lead author.

The importance of species variations in both root mass and root life span to the development of soil organic matter was shown in an earlier Argonne study published in November 2003 in Science.

“The Kansas grassland has more abundant fine roots than the Tennessee forest,” said Argonne 's Roser Matamala, a coauthor of the current study and lead author of the Science article. "But the fine roots in this forest have shorter lives and are replaced much faster. After accounting for this difference, we estimated the overall effect of CO₂ enrichment on root-derived inputs to soil organic matter was similar at both sites.”

The current study also reported that more than half of the accumulated soil carbon at the Tennessee and Kansas experimental sites was associated with soil minerals in stable aggregates, which can protect organic matter from rapid decomposition and increase its residence time in the soil.

“This is a key finding,” said Jastrow. “It means all of the added carbon will not be cycled right back to the atmosphere; some of it could stay in the soil for awhile. Even more importantly, it suggests that the ability of these soils to sequester carbon in more stable forms is not yet saturated.”

One difference between the two sites was the distribution of the added soil carbon. Increases in the amount of carbon were found to a depth of 30 cm in the grassland soil but were limited to the top 5 cm of the forest soil.

“The differences in where carbon accumulated are not surprising,” said Michael Miller, another Argonne coauthor. “They followed the normal pattern of soil organic matter development in each ecosystem. In some earlier studies, changes near the surface might have been masked because samples were taken from the top 10-20 cm in one increment.”

The study's experimental results demonstrate that even soils with large amounts of organic matter – such as those at the never-cultivated Kansas grassland site – may be capable of storing additional carbon. The scientists' meta-analysis, which integrated results from a wide range of soil types and environmental conditions, suggests that some increase in temperate soil carbon storage could occur in response to rising atmospheric CO₂. Although the authors caution that this response is not large enough to offset CO₂ emissions from human activities and will not be sustained indefinitely, they believe it is worthy of consideration in modeling efforts to predict the effects of rising CO₂ on global climate change.

Other coauthors for the study were Richard Norby of Oak Ridge National Laboratory, Charles Rice and Clenton Owensby of Kansas State University, and Thomas Boutton of Texas A&M University.

Argonne National Laboratory brings the world's brightest scientists and engineers together to find exciting and creative new solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America 's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.

For more information, please contact Steve McGregor (630/252-5580 or media@anl.gov) at Argonne.