With Earth’s northernmost polar regions thawing, climatologists have long been raising questions and concerns about what will happen to the significant caches of carbon stored in the cold soils of the Arctic.
The University of Toledo’s Dr. Michael Weintraub, a soil ecologist and professor in the Department of Environmental Sciences, recently collaborated on new research exploring this question at one of the region’s longest-running ecosystem experiments. It offers insights into the process by which organic matter within the soil is decomposed and released into the atmosphere, mostly in the form of the greenhouse gas carbon dioxide.
Dr. Michael Weintraub, a soil ecologist and professor in the Department of Environmental Sciences, and Emma Campbell, a full-time laboratory and field technician. Weintraub recently collaborated on new research exploring how rising temperatures may impact caches of carbon stored in Arctic soils.
Led by Dr. Megan Machmuller at Colorado State University and published in the peer-reviewed journal Nature Climate Change, the new research specifically digs into the role of vegetation and how a shift toward shrubs may affect the way carbon is cycled.
“Soils contain two to three times more carbon than is currently in the atmosphere, mostly in the form of decomposing organic matter,” Weintraub said. “When soils warm, organic matter is decomposed more quickly into carbon dioxide. So, warming by fossil fuel emissions of greenhouse gases can stimulate faster decomposition in soils, causing more soil carbon to get released as carbon dioxide, which in turn may further accelerate warming. Because Arctic soils contain large accumulations of organic matter due to low temperatures, their potential for increased decomposition and carbon dioxide release with warming is an important area of research in climate science.”
“But it’s very difficult to predict the effects of climate change on an ecosystem,” he continued. “Often we can model the effects on one part of an ecosystem, like soil carbon, but it becomes much more complex when we consider how this one component interacts with countless others that are simultaneously responding to rising temperatures.”
Therein lies the value of a long-term study like the National Science Foundation’s Arctic Long-Term Ecological Research site in northern Alaska, he said. Established in 1981, it allows researchers to track effects over decades.
Scientists have long been adding nutrients to test plots on the site to study the ecological response over time. Greater nutrient availability with increased decomposition was predicted to be an effect of warming. The site allows scientists to examine how long-term changes to the soil can impact carbon storage, which proved critical to the new research described in Nature Climate Change.
Much of the research into Arctic carbon storage so far has focused specifically on how rising global temperatures will disrupt the carbon currently locked in the permafrost. But warming impacts the region in other ways, too, including changing plant growth, the distribution of vegetation across the landscape and the availability of nutrients in the soil.
These changes in plant composition also will affect the way carbon is cycled from the soil into the atmosphere, according to the researchers. For example, slower decomposition with a shift from fast-decomposing grasses to slow-decomposing woody shrubs can decrease soil carbon dioxide emissions and increase soil carbon storage.
“Our work focused on pinpointing the mechanisms that are responsible for controlling the fate of carbon in the Arctic,” said Machmuller, a research scientist in Colorado State University’s Soil and Crop Sciences Department. “We know temperature plays a large role, but there are also ecosystem shifts that are co-occurring with climate change in this region.”
In particular, Machmuller said, the region is experiencing a kind of “shrub-ification” — an increase in shrub abundance and growth. Machmuller and her co-authors found that over long periods those shrubs may contribute to keeping more carbon in the ground.
Scientists had recorded a significant loss of soil carbon in nutrient-addition plots at the research site after 20 years, an important finding that shaped broad scientific understanding of how the Arctic might respond to increased nutrient availability with faster decomposition due to climate change. But after 35 years of continuous nutrient application, they found the trend had reversed and the amount of carbon stored in these plots had either recovered or exceeded the amount in the nearby control plots.
The research team ran experiments in the lab to learn more about how carbon was moving through the system using advanced tools like isotope tracing. What they found was that when the nutrients were first added, they stimulated the decomposer microbes that churn through organic matter in the soil and release carbon dioxide in the process.
But that changed over time, as nutrients were added to the test plots every year.
“Shrubs conditioned the soil in a way that shifted microbial metabolism, slowing rates of decomposition and allowing soil carbon stocks to rebuild,” said Dr. Laurel Lynch, another co-author and collaborator who is an assistant professor at the University of Idaho. “We didn’t expect that.”
For additional information about the project, read the Colorado State University announcement.
