Smithsonian Environmental Research Center

Scientists agree that carbon dioxide (CO₂) levels in the atmosphere have risen dramatically with global industrialization. Some estimates suggest that CO2 has risen up to 35 percent since the mid-1800s and will continue to rise to almost double preindustrial levels by the end of this century. No one is certain how the Earth will respond to this change, but a new study by Patrick Megonigal at the Smithsonian Environmental Research Center sheds light on the role soils may play.

The general assumption among scientists has been that soils either will not respond much at all, or will serve as a buffer by storing some of the excess carbon taken up by plants. Soil organic matter represents a very large carbon pool that under most conditions is a fairly stable storage reservoir for carbon in the environment. Megonigal’s findings question that assumption, suggesting that the soil may release carbon as atmospheric CO₂ rises. According to Megonigal, increased atmospheric CO₂ may have a “priming effect,” stimulating microbes in the soil to decompose soil organic matter and release CO₂ in the process.

Megonigal and his colleagues, Amy Wolf and Bert Drake, exposed marsh plants grown in pots in a greenhouse to both normal and elevated CO ₂. They then measured the CO₂ released from their pots. Using soil and plants composed of different stable carbon isotopes, they were able to differentiate between CO₂ arising from the microbe respiration and CO₂ arising from plant respiration. They found that soil microbe respiration increased by 157 percent in the pots exposed to elevated CO₂.

“Adding a little labile CO₂ could stimulate the decomposition of old recalcitrant carbon in the soil,” Megonigal said, adding that this type of priming effect could play a big role in influencing what happens as atmospheric CO₂ continues to rise.

There has been a lot of discussion focused on whether or not plants will manage to take up some of the increased CO₂ in the atmosphere. Although it remains unclear how much of a buffering effect plants will have against the impacts of rising atmospheric CO₂, Megonigal’s findings add a level of complexity to the issue. In this new light, soils may serve to amplify the impacts of human activity by increasing the rate at which atmospheric CO₂ rises.

Megonigal will present his work in a talk at the four-day Estuarine Research Federation conference in Norfolk, VA., on Wednesday, Oct. 19.

During the meetings, Megonigal may be reached at megonigalp@si.edu.

For information about attending the meetings, see: http://erf.org/meetings.html