Increased deforestation and agricultural production in the world’s tropical regions may be releasing previously untracked carbon dioxide into the atmosphere, according to a new study published in Nature Geoscience.

Researchers investigated 19 sites in the Democratic Republic of Congo for dissolved organic carbon that had been drained into streams and rivers. As thousand-year-old soil is upturned and disrupted, they found that older dissolved organics from areas that had seen higher rates of deforestation were richer in energy and more chemically diverse.

“We estimate that while deforestation reduces the overall flux of dissolved organic carbon by approximately 56%, it does not significantly change the yield of biolabile dissolved organic carbon,” wrote the authors. “Ultimately, the exposure of deeper soil horizons through deforestation and agricultural expansion releases old, previously stable, and biolabile soil organic carbon into the modern carbon cycle via the aquatic pathway.”

Areas that have been heavily deforested were more likely to have leached organic carbon older and more biodegradable than organic carbon put off. Older, more unstable organic carbon released when the soil is upturned or disrupted. Microscopic organisms that consume released CO₂ then pump it back into the atmosphere, potentially worsening impacts of the greenhouse effect.

"In many ways, this is similar to what happened in the Mississippi River Basin 100 years ago, and in the Amazon more recently," said study author Rob Spencer in a statement. "The Congo is now facing conversion of pristine lands for agriculture. We want to know what that could mean for the carbon cycle."

Altogether, the researchers write that their findings indicate there could well be unaccounted sources of carbon leaches into the atmospheres, particularly in the face of increased land conversion.

"At this point, it's hard to know the magnitude of this flux and thus the relative importance of this process compared to other anthropogenic sources of CO₂, but it is likely to grow with additional deforestation and land-use conversion," said Travis Drake, the study's lead author. "We hope this paper stimulates more research into the relative importance of this process."

The authors note that the process means formerly trapped carbon sequestered in Earth’s soils could re-enter the carbon cycle, highlighting the need to identify the effects of deforestation and land conversion while also accounting for other methods of carbon release.

"This research focuses on the Congo because the tropics are really at the forefront of agriculture-driven land-use conversion," said Spencer.

"Ultimately, it depends on the preservation of forests that maintain and store carbon in soils over longer timescales," Drake added. "When land-use conversion does occur, better practices such as terracing, use of buffer strips and application of organic residues could ameliorate some of the observed organic carbon leaching."