Decade-long research reveals new understanding of the impact of climate change on Arctic soil carbon stores

Using one of the Arctic’s oldest ecosystem experiments, a team of researchers led by Colorado State University has developed a better understanding of the interaction between plants, microbes and soil nutrients – results that offer new insights into how critical carbon deposition may be. released by the thawing of Arctic permafrost.

Estimates suggest that Arctic soils contain nearly twice the amount of carbon currently in the atmosphere. As climate change has caused parts of Earth’s northernmost polar regions to thaw, scientists have long worried about significant amounts of carbon being released as greenhouse gases, a process fueled by microbes .

Much of the effort to study and model this scenario has focused specifically on how rising global temperatures would disrupt the carbon currently locked in Arctic soils. But warming also has other impacts on the region, including changing plant productivity, the overall composition of vegetation in the landscape, and the balance of nutrients in the soil. These changes in plant composition will also affect how carbon is recycled from the soil back into the atmosphere, according to a study published this week in the journal Climate change.

The work was led by Megan Machmuller, a research scientist in the CSU Department of Soil and Crop Sciences.

“Our work has focused on identifying the mechanisms responsible for controlling the fate of carbon in the Arctic,” Machmuller said. “We know that temperature plays an important role, but there are also changes in ecosystems that accompany climate change in this region.”

In particular, Machmuller said, the region is experiencing a kind of “shrub-ification,” an increase in the abundance and growth of shrubs. And what Machmuller and his co-authors found is that, over long periods of time, these shrubs can help retain more carbon in the soil.

“There’s been a lot of focus on the direct effects of warming on soil carbon,” said co-author Laurel Lynch, an assistant professor at the University of Idaho, “but what we’re seeing with this work, is that it is more complex. We must consider this ecosystem as a community as a whole with many interacting parts and competing mechanisms.

A surprising discovery

For this new work, Machmuller and his team tested soil samples from a 35-year ecosystem experiment in the Arctic. In 1981, scientists began adding nutrients to test plots at the Long-Term Arctic Ecological Research Site in northern Alaska, located near Toolik Lake at the foot of the Brooks Mountain Range. The initial idea was to understand how Arctic vegetation would respond to additional nutrients over time, but the experiment also allowed scientists to examine the long-term impact of changes in the soil on carbon storage.

After 20 years, scientists discovered that there had been a significant loss of soil carbon when nutrients were added compared to control plots, an important discovery that shaped a broad scientific understanding of how soil Arctic could respond to climate change. These experiments continued, and Machmuller and his team retested the plots after 35 years of continuous nutrient application.

However, instead of a continued loss of carbon, they found that the trend had reversed. After 35 years, the amount of carbon stored in the test plots had either recovered or exceeded the amount present in neighboring control plots.

“We were really surprised by these results and became curious about the underlying mechanism,” Machmuller said.

Machmuller and his team conducted advanced isotope tracing experiments in the laboratory to learn more about how carbon moved through the system. What they found was that when nutrients were first added, they stimulated microbial decomposition – a natural process that involves microbes moving through soil organic matter, resulting in the release of carbon dioxide.

But this changed over time, as nutrients were continually added to the test plots. “The shrubs conditioned the soil in a way that altered microbial metabolism, slowing decomposition rates and allowing soil carbon stocks to replenish,” Lynch said. “We didn’t expect this.”

“This offers a potential biological mechanism that could explain why we observed a net carbon loss in the first 20 years, but not after 35 years,” Machmuller said.

The importance of looking long term

According to Machmuller, these results demonstrate that how the Arctic might respond to climate change is more complicated than previously thought. “It’s a complex puzzle,” she said, “and this study highlighted for us the importance of using long-term studies to advance our understanding of ecosystem processes.”

Gus Shaver, a research scientist who helped establish Toolik Lake’s first experimental plots in 1981 and co-authored the study, also stressed the importance of doing this type of work over longer periods of time. periods.

“We showed that long-term experiments offer frequent surprises when we follow the trajectory of their responses over time,” Shaver said. “What you discover in the first few years of an experience is often not what you learn from year 10, 15, or 35.”

Lynch noted that as this ecosystem evolves, other factors must be considered beyond just carbon. Although an increase in shrub abundance may prevent more soil carbon from transferring into the atmosphere, other impacts are not as beneficial, she said.

“When you have a plant species that massively outcompetes the rest of the community, that has major implications for the ecosystem,” Lynch said. For example, she said, “the habitat and food sources of many Arctic animals depend on diverse plant communities, and the loss of this diversity can ripple across the entire ecosystem.” .

Lauren Gifford, associate director of CSU’s Soil Carbon Solutions Center, who was not involved in the study, said the work highlights the need for more robust and detailed modeling to better anticipate the impact of climate change on carbon stored in the Arctic.

“This is a remarkable 35-year study of one of Earth’s most vulnerable ecosystems,” Gifford said. “Even with extensive long-term studies, the impacts of climate change often remain uncertain. Interventions aimed at adapting to and mitigating climate change may lead to similar, contradictory results, or produce unintended consequences.”

For his part, Machmuller hopes this work will encourage future research on this topic. “Arctic carbon research has long been a hot topic because of the critical role it plays in regulating our global climate,” she said. “But we still don’t know exactly what the future carbon footprint will look like.”