A study from the United States suggests that wood may not be so green a biofuel as has previously been claimed and that logging may have greater impact on carbon emissions than previously thought.

Although woody biomass is sometimes touted as a good, sustainable diversification for farm bsuinesses and other landowners, a study led by Dartmouth College has questioned the assumption that using wood for energy is cleaner than using fossil fuels. Dartmouth scientists said logging may in fact release large amounts of carbon stored in deep forest soils.

The Dartmouth team said global atmospheric studies often don't consider carbon in deep (or mineral) soil because it is thought to be stable and unaffected by timber harvesting. But their findings show deep soil can play an important role in carbon emissions in clear-cutting and other intensive forest management practices.

Since making their discovery, the scientists have demanded that calls for an increased reliance on forest biomass are re-evaluated, or at least qualified, and warned that forest carbon analyses are incomplete unless they include deep soil, which stores more than 50 percent of the carbon in forest soils.

Dartmouth Professor and study co-author Andrew Friedland said, "Our paper suggests the carbon in mineral soil may change more rapidly, and result in increases in atmospheric carbon dioxide, as a result of disturbances such as logging. 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. So the intended goal of reducing carbon in the atmosphere may not be met."

Woody biomass, which includes trees grown on plantations, managed natural forests and logging waste, makes up about 75 percent of global biofuel production. Mineral soil carbon responses can vary highly depending on harvesting intensity, surface disturbance and soil type.

"Analysis of forest carbon cycles is central to understanding and mitigating climate change, and understanding forest carbon cycles requires an in-depth analysis of the storage in and fluxes among different forest carbon pools, which include aboveground live and dead biomass, as well as the belowground organic soil horizon, mineral soil horizon and roots," Friedland said.

Friedland's research was carried out in conjunction with researchers at other US institutes, as well as the University of Lund, Sweden.