How burying wood reduces carbon dioxide emissions

To combat climate change, scientists have been developing technologies to reduce the amount of carbon dioxide in the atmosphere. These include pulling CO₂ out of the air and storing it underground. Ning Zeng, an atmospheric scientist at the University of Maryland, is proposing another carbon-removal strategy: digging trenches and then filling them with logs before filling them back in.

He presented the research last week at the American Geophysical Union conference in San Francisco.

The idea is simple. It buries unused logs from trees in managed forests a few meters underground. Then, they are covered in clay to create an oxygen-poor environment that would stop or reduce the decay and thus release of CO₂ from the wood. “The logs can be preserved for thousands of years or longer,” Zeng told C&EN.

In a pilot study that began in 2007 on Maryland’s Eastern Shore, his team buried 100 cross sections of loblolly pine at different depths and noted that it took a few years for the wood to degrade at the surface compared to several decades at the 80 cm depth. In a follow-up 2013 project in Montreal, the researchers entombed 35 tons of unwanted logs in a 4-m-deep trench. About nine years later, the team observed nearly no wood decay below 1.5 m depths. Extrapolating from the results, the researchers estimated that 97% of this wood willremain undegraded for more than 100 years.

Now, a company called Carbon Lockdown that Zeng started in 2019 is burying 5,000 tons CO₂ -equivalent of wood from nearby forestry operations. This project is located on a farm near the Potomac River in southern Maryland and is currently listed on a carbon offsets registry. Earlier this year, a Swedish investment group called Kinnevik bought the first 1,000 tons of this sequestered carbon.

Going forward, the researchers expect to scale up the operation and hope others will replicate their approach. But Elaine Pegoraro, a soil and climate scientist at the Lawrence Berkeley National Laboratory who was not involved in the work, worries about taking away wood that would otherwise decompose and contribute to soil carbon. But she also feels that, realistically, such wood burial may reduce only a small proportion of anthropogenic CO₂ emissions. Convincing foresters to not sell wood that’s in high demand for producing wood chips or furniture may be another challenge. “But [for now] we’re focusing on using unmerchantable waste wood,” Zeng said.