ALBUQUERQUE—Because forests can lock away carbon in their woody trunks, planting vast swaths of trees on barren land could provide a means for countries to mitigate their carbon emissions. But a comprehensive new analysis warns that pine plantations can rapidly deplete the soil of its nutrients—and carbon—thereby reducing the benefits.

"It's a tough balancing act," says ecologist Sean Berthrong of Duke University. "Plantations are a usable but imperfect tool for carbon sequestration."

Since 2005 140 million hectares of land have been converted to forest globally, and an average of two million new hectares of forests are added each year. Such afforested areas account for only 4 percent of Earth's forests, but they already supply 35 percent of the world's wood products, according to the United Nations's Food and Agriculture Organization.

Berthrong's team analyzed data from 153 tree plantations—mostly eucalyptus and pine—to understand how they influenced soil nutrients and carbon. On average, new forests reduced soil carbon by 6.7 percent and nitrogen by 15 percent. Pine plantations, which are the most common, are the worst, causing a 15 percent reduction in soil carbon and a 20 percent reduction in nitrogen. Tree plantations can also cause other problems in soils, such as making them more acidic.

To minimize carbon and nutrient loss, Berthrong recommends foresters leave woody debris on the site after harvest as well as reduce plowing, which increases erosion and decomposition of buried organic matter. He also says that any program that provides carbon credits for afforestation projects take soil carbon into account.

Berthrong presented the research here at the Ecological Society of America Meeting, and it will be published in the journal Ecological Applications.