CORVALLIS - A soil scientist at Oregon State University has discovered that adding additional organic matter to Oregon's forest soils may actually increase rather than hinder the release of carbon dioxide into the atmosphere.
Researcher Elizabeth Sulzman's findings fly in the face of what scientists believe about long-term carbon storage by soils and their potential role in ameliorating global climate change.
Sulzman's work - presented at the recent national Ecological Society of America meeting and soon to be published in the international journal Biogeochemistry - shows that the additional organic matter, in the form of conifer needles, may actually prime soil microorganisms to degrade both the new, as well as older, more stable soil carbon stores.
Working at the National Science Foundation-funded H.J. Andrews Long-Term Ecological Research site, located in Oregon's Central Cascade mountain range, Sulzman has shown that exposing forest soils to twice the normal amount of organic matter increased soil carbon releases by 34 percent more than expected. Rather than storing carbon, the additional material fueled a boom of microbial activity that further decomposed soil carbon reserves - ultimately resulting in a net loss of carbon from the soil, returning it to the atmosphere as carbon dioxide.
From roots to decaying wood, there is a tremendous amount of organic material stored beneath the earth's surface. Depending on the forest, 30-50 percent of tree weight can be below ground. Soil microorganisms work to transform all plant material, both above and below ground, into pools of carbon that can remain in the soil for thousands of years.
As soils warm, microbial activity increases. Therefore, soils typically lose more carbon under warmer conditions. "If climate change leads to even warmer temperatures, we could have even greater carbon loss from these soils," said Sulzman, an assistant professor in OSU's Department of Crop and Soil Science. "This would be a double whammy for atmospheric carbon dioxide levels."
It is well-documented that plants are able to use elevated atmospheric carbon dioxide to enhance their growth, a seemingly fortuitous by-product of pollution. But the work of Sulzman and others is finding that above-ground productivity doesn't necessarily translate into long-term storage of carbon below ground, as has long been assumed.
For the past several years, government policies have promoted land management activities designed to store carbon in the soil, so-called carbon sequestration, as a way to mitigate rising atmospheric carbon dioxide levels. While attempting to better understand forest carbon dynamics and the potential for carbon sequestration in these lands, Sulzman's work has given scientists grounds to question this logic.
"It goes against conventional wisdom," said Sulzman's East Coast collaborator, Richard Bowden, an associate professor at Allegheny College in Pennsylvania. Bowden is a researcher at the Harvard Forest Long-Term Ecological Research site in Massachusetts.
"If we add more organic matter, soil should store more carbon," he said. "But if we add organic matter and lose it faster for as yet unknown reasons, we need to think seriously about how well soils can store carbon."
Forest management options, such as leaving slash on the ground or removing it through harvests or burns, subsequently have impacts on carbon storage. "When any management alters the forest floor it has implications for carbon storage, which must be considered," said Sulzman.
"We don't know how forests are really working, and we're asking them to clean up our carbon dioxide pollution," pointed out Bowden, adding that a better understanding of both how forests work as well as their limitations is needed to inform forest policy. "These findings are causing us to rethink our understanding of soil biology at this and other forests where we are conducting these experiments," said Kate Lajtha, a professor in the Department of Botany and Plant Pathology at OSU and a colleague of Sulzman.
Sulzman, a scientist with OSU's Agricultural Experiment Station, is collaborating with scientists all over the world in an attempt to better define below-ground processes that control carbon dynamics. Her work is vital to the debate as it's the only research of its kind being conducted in conifer systems. As such, it will also play a role in future forest management.