Decades-long Harvard forest experiment finds warming soils may release “stable” carbon

A 37-year soil warming experiment in Massachusetts demonstrates that prolonged warming can destabilize forms of soil carbon previously considered resistant to decomposition. This process may create an additional feedback loop that accelerates climate change.

Since the late 1980s, researchers at Harvard Forest have heated underground soil plots by 5°C (9°F) above ambient temperatures, establishing the world’s longest-running soil warming experiment. The study indicates that decades of elevated temperatures have altered microbial activity, enabling microbes to decompose persistent organic matter and release additional carbon dioxide into the atmosphere.

“Soil holds more carbon globally than the atmosphere and all plant life combined,” the researchers stated. They noted that the gradual breakdown of these carbon reserves became evident only after decades of continuous warming. These findings challenge longstanding assumptions that certain forms of soil organic matter remain effectively sequestered for centuries.

Jerry Melillo, a distinguished scientist at the Marine Biological Laboratory and co-author of the study, explained that microbes play a central role in recycling nutrients and decomposing organic matter in forest ecosystems. As warming alters microbial communities, the loss of soil carbon may accelerate. Researchers further noted that incorporating these long-term soil responses into climate models could improve projections of future warming.

The study, titled “Three decades of continuous warming in temperate forests destabilizes persistent forms of soil organic matter,” was published on April 7, 2026, in Science of The Total Environment.

Sources: Science of The Total Environment study abstract; Harvard Forest