Soil microorganisms absorb carbon dioxide only to convert it into much more potent greenhouse gases, negating almost 17 percent of the Earth's ability to absorb heat-trapping emissions, a new study suggests.

The study, published in this week's issue of Nature, measured the release of methane (CH4) and nitrous oxide (N2O) gases from soils in forests, grasslands, wetlands and agricultural fields, including rice paddies. Although less abundant than carbon dioxide, these two greenhouse gases have about 30 and 400 times the heat-trapping power of CO2, respectively.

"It reminds us that there are many dimensions of soil as a carbon sink," said Bruce Hungate, co-author of the study and a professor of ecosystem ecology at Northern Arizona University. Kees Jan van Groenigen of Trinity College in Ireland and Craig Osenberg of the University of Florida co-authored the paper.

To obtain their energy, soil microbes take in carbon dioxide and emit methane and nitrous oxide. As the atmospheric concentration of carbon is expected to increase in the coming years, the output of CH4 and N2O will greatly accelerate the rate of warming, conclude the authors.

Overall, the methane and nitrous oxide released from methanogens and denitrifiers -- the types of microbes that emit the gases -- canceled out 16.6 percent of the global terrestrial carbon sink, the carbon sequestration area that does not include bodies of water.

Recreating the high-CO2 environment forecast for the next 50 to 100 years, the researchers found that higher concentrations of carbon stimulated overall emissions of N2O by 18.8 percent and wetlands-based emissions of methane by 13.2 percent, compared to current atmospheric levels of CO2. In rice paddies, methane emissions increased by 43.4 percent.

Soiled potential?
The study brings into question the carbon-mitigating qualities of soils, which many have considered a promising option for canceling out carbon emissions and as possible tender for carbon credits.

"Sometimes we operate with blinders on," said Hungate about the rush to center carbon mitigation projects around soil. "We should take a holistic approach, not just focus on carbon."

Carbon sequestration from soils has been touted as an important method for climate change mitigation, and was included in the Clean Development Mechanism methodology for agroforestry, the guidelines for carbon crediting under the Kyoto Protocol, said Steven Shonts of forestry and land-based asset management company CINCS. Shonts authored the recently released report "Soil Carbon Sequestration: An Overlooked Opportunity for REDD+."

Hungate noted that his research was limited by a lack of prior research on global change experiments in tropical ecosystems.

"We need global change experiments in the tropics and emissions [studies] in the global ecosystem," he said. "We have a reason to believe it might be large, but there is no standard to test that hypothesis."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500