WOODS HOLE, Mass. — What will happen to the 1.8 billion tons of carbon stored in the frozen peat of Arctic tundra, as the world warms? Will carbon dioxide be belched into the atmosphere from the northern polar regions, dramatically exacerbating climate change?
An unexpected new insight into this major concern, based on an Alaskan soil experiment conducted since 1981 by Gaius Shaver, senior scientist emeritus at the Marine Biological Laboratory (MBL) and colleagues, is published this month in Nature Climate Change. The tundra vegetation, it indicates, is transitioning to shrubs and actually becoming more efficient at storing carbon.
In the 35-year experiment, Shaver, and colleagues continually added nitrogen, which stimulates plant growth, to test plots of Arctic soil to see how the plant community –mostly grasses and sedges –would be affected compared to control plots. They noted, over time, a gradual conversion in the fertilized plots to low-lying, deciduous shrubs, such as birch and alder.
Eventually, they asked how carbon storage in the system is being impacted by changes to its soil and vegetation. That’s where major twists and turns in the data started coming from the experiment, which is based at the Arctic Long-Term Ecological Research site at Toolik Lake, Alaska.
After 20 years of the experiment, Shaver and colleagues measured total soil carbon and found a significant carbon loss from the fertilized plots as compared to the control plots. This important finding, reported in Nature in 2004, shaped broad scientific understanding of how the Arctic might respond to climate change.
Yet 15 years later, when scientists measured soil carbon again, they unexpectedly discovered the trend has reversed. As reported this month, the amount of carbon stored in the fertilized test plots had either recovered or exceeded the amount in the nearby control plots.
What was going on? “We were really surprised by these results and became curious about the underlying mechanism,” said Megan Machmuller, a research scientist at Colorado State University and first author on the report.
Machmuller and her team ran advanced isotope tracing experiments in the lab to learn more about how carbon was moving through the system. What they found was that when the nutrients (nitrogen) were first added, they stimulated microbial decomposition — a natural process in which microbes break down organic matter in the soil, resulting in the release of carbon dioxide.
But that changed over time, as nutrients were continuously added to the test plots and they converted from a mixed plant cover to a “shrubbier” one.
“Shrubs conditioned the soil in a way that shifted microbial metabolism, slowing rates of decomposition and allowing soil carbon stocks to rebuild,” said Laurel Lynch of University of Idaho, a co-author on the paper “We didn’t expect that.”
Shrub cover is expanding across the Arctic, Shaver said, as indicated by numerous other studies. Therefore, the results of the present study are a harbinger of what may come.
“This study on our fertilized plots shows that the shrubs produce a different kind of litter for microbes to decompose, which led to increased retention of carbon in the system,” Shaver said. As the Arctic becomes shrubbier, “the soils beneath shrubs in unfertilized tundra are starting to look like the soils beneath shrubs in our fertilized tundra. … The basic implication here is that increased shrubbiness will lead to increased retention of carbon in these ecosystems.”
The importance of long-term study of ecosystem change can’t be overemphasized, said Shaver, who noted that what they observed after the first year of their experiment in 1981 had already changed by year five.
“By following long-term trajectories of ecosystem response, you’ll learn a lot of things that you wouldn’t have seen if you just collected data once or twice over a short period of time,” he said.
“These kinds of changes in the balance between carbon production and microbial decomposition are being seen in ecosystems around the world, from temperate forests to Arctic tundra,” Shaver said. “It’s another reason why you need to follow these experiments over the long term.”
Paper Citation:
Megan B. Machmuller et al. (2024) Arctic soil carbon trajectories shaped by plant–microbe interactions. Nature Climate Change, DOI: 0.1038/s41558-024-02147-3
Research Briefing:
Megan B. Machmuller & Laurel M. Lynch. Plant–microbe interactions explain the surprising recovery of Arctic soil carbon stocks. Nature Climate Change, 03 October 2024.
News & Views Commentary:
Wild, Birgit (2024) Dynamic nutrient effects on soil carbon. Nature Climate Change, DOI: 10.1038/s41558-024-02120-0.
Read the press announcement from Colorado State University.
—###—
The Marine Biological Laboratory (MBL) is dedicated to scientific discovery – exploring fundamental biology, understanding marine biodiversity and the environment, and informing the human condition through research and education. Founded in Woods Hole, Massachusetts in 1888, the MBL is a private, nonprofit institution and an affiliate of the University of Chicago.