Timber harvesting may release significant amounts of carbon into the atmosphere, according to a new study. This challenges a longstanding belief that using wood for energy is a green alternative to fossil fuels.

Analyses of carbon emissions often ignore the carbon stored in deep soils, the study authors say. But after reviewing multiple recent research papers about decreases in soil carbon levels, the authors concluded that intensive forest management practices can cause large amounts of underground carbon to flow into the atmosphere.

“Our paper suggests the carbon in the mineral soil may change more rapidly, and result in increases in atmospheric carbon dioxide, as a result of disturbances such as logging,” says Andrew Friedland, a professor at Darthmouth College and a co-author of the study. “Our paper suggests that increased reliance on wood may have the unintended effect of increasing the transfer of carbon from the mineral soil to the atmosphere.”

Policymakers should reevaluate calls to boost the use of trees for biofuel, the researchers recommend. Forest biomass currently comprises about 75 percent of global biofuel production.

The study appears in the journal Global Change Biology-Bioenergy.