This story first appeared in the SERC quarterly Newsletter winter 2005/2006
Turmoil in the Soil
There's a battle going on underground in marshes and other wetlands that could have serious implications for global climate. The struggle pits microbe against microbe in a competition for the energy resource carbon, and a new study by Patrick Megonigal suggests that it's the plants of these marshes, specifically plant roots, that wield the power to determine the winner.
Below ground microbes come in many different types. Just as humans use oxygen for metabolism, one group of microbes, the iron reducers, uses iron oxides, which are familiar to us as rust. Methanogens, on the other hand, use carbon dioxide and release methane, a significant greenhouse gas. Thus, the amount of methane emitted from the marsh is determined by which species of microbe dominates in the soil. Megonigal's studies showed that in the spring--when plants are most productive and the increased activity in the roots makes iron in the soil available to microbes--the iron reducers prevailed. But, as the summer wore on and plant growth slowed, the iron reducers began losing ground literally to the methanogens. By August, 80 percent of the total underground metabolism in the marsh was by methane producers.
Megonigal suggests that the iron reducers are generally more efficient than the methane producers, and when there's an abundance of iron around, they out compete them. But when there's less iron, the methane producers take over. Though it's still early, this work could have implications for controlling methane if we can figure out how to manipulate plants to support iron reducers over methane producers. One solution may be as simple as introducing a useable form of iron into the soil so that the iron reducers have what they need to win out over their competition.
Sink or Source?
In another recent study, Megonigal questions a long-held assumption that soil may act as one of the earth's natural buffers against rising carbon dioxide. As industrial development and global CO2 levels continue to rise, it has been presumed that soil may serve as a sink for some of the excess carbon. Megonigal's work suggests the opposite, however. Increased atmospheric CO2 may actually have a "'priming effect," stimulating microbes in the soil to decompose old stores of organic matter in the soil and release CO2 in the process. "'Adding a little labile CO2 could stimulate the decomposition of old, recalcitrant carbon in the soil," Megonigal said.
Megonigal and his colleagues exposed marsh plants grown in pots in a greenhouse to both normal and elevated CO2, and found that soil microbe respiration of old soil carbon increased by 157 percent in the pots exposed to elevated CO2. This suggests that rather than serving as a sink for excess carbon, soil just may be a source of excess carbon and soil microbes will amplify CO2 concentrations.
For more information, or to reach Dr. Megonigal, please contact SERC science writer Kristen Minogue.