Because higher CO2 concentrations usually stimulate plant growth, a parallel accumulation of soil carbon derived from decaying roots and other plant material might also be expected. Yet nearly all elevated CO2 experiments to date have failed to detect changes in soil carbon against the large and variable background of existing soil organic matter, leading to predictions that increased deposits of dead plant material will decompose rapidly with little accumulation of carbon in soil.
In a study funded by the U.S. Department of Energy's Office of Science, researchers from Argonne and Oak Ridge National Laboratories and Kansas State and Texas A&M Universities evaluated the collective results of earlier studies by using a statistical procedure called meta-analysis. They found that on average elevated CO2 increased soil carbon by 5.6 percent over a two to nine year period. They also measured comparable increases in soil carbon for Tennessee deciduous forest and Kansas grassland after five to eight years of experimental exposure to elevated CO2.
"We were able to see measurable carbon accumulation within a relatively short time period because the vegetation at both of our sites responded to elevated CO2 with large increases in root growth," said Argonne's Julie Jastrow, the study's lead author. "We also found over half of the accumulated carbon was associated with soil minerals in stable aggregates, which can protect organic matter from rapid decomposition. This suggests that some of the carbon added to these soils could stick around for awhile."