Some climate change researchers have placed high hopes in forest and grassland soils and their ability to act as carbon “sinks.” These sinks store excess atmospheric carbon and thus partially offset the effect of increasing amounts of carbon dioxide in the atmosphere. Unfortunately, a recent study by Washington State University environmental scientist Richard Gill and his colleagues indicate the sink may be reaching capacity.

Although carbon dioxide has been increasing in the atmosphere for the last 10,000 years, the increase has been especially rapid in the last 150 years because of the industrial revolution and the conversion of land to agricultural uses. The rate of increase has been slowed somewhat because some of the carbon has been stored in organic matter in the soil, moderating increases in atmospheric carbon dioxide. However, says Gill, the storage capacity is dependent on how both plants and soils respond to the changes caused by the rising carbon dioxide. Increased carbon dioxide tends to make plants more productive, but their productivity begins to create problems in the soil.

Gill’s research shows that there is a limit to how much carbon can be stored in soils and that soil capacity may be at an important threshold. In the Texas tallgrass prairie soil he and his colleagues studied, they found that at high carbon dioxide concentrations, soils were not able to continue to store excess carbon.

“It appears,” says Gill, “that soil carbon storage is very sensitive to nitrogen availability-which changed dramatically with rising carbon dioxide. If the ability of soils to continue to absorb carbon dioxide is limited, we may soon begin to see rapid increases of greenhouse gasses in the atmosphere, leading to potentially swift changes in global climate.”

Gill and his colleagues reported their results in the May 16, 2002, issue of Nature. Gill’s research is funded by a United States Department of Agriculture National Research Initiative Program grant.