Researchers have uncovered a mechanism in the Alaskan tundra that doesn't seem to follow the climate change script for soil carbon.
For years, scientists have shown that rising temperatures stimulate microbes that decay plant matter, releasing carbon more quickly into the atmosphere. But when Seeta Sistla, a doctoral student at the University of California, Santa Barbara, came to collect data for a 20-year-old experiment at the U.S. Arctic Long-Term Ecological Research site in northern Alaska, the soil carbon levels at the site were curiously stable.
While climate change is warming the soil and spurring microorganisms to decompose leaves -- which releases carbon into the air -- the growth of more shrubbery in the tundra is soaking up that carbon and redistributing it back into the ground.
"What basically happened is that there was a feedback between the increased plant growth and the increased decomposer activity," said Sistla, who published her findings in a recent paper in Nature.
As microorganism activity goes up, the amount of carbon dioxide that is being released from the soil will increase, and carbon that is stored in the soil will decrease. It takes only a few degrees' temperature increase for microbial activity to double in soils, said Josh Schimel, a professor of environmental studies at UC Santa Barbara and Sistla's academic adviser.
Over the past two decades, researchers have seen changes in the vegetation around the tundra. Tall, deep-rooted trees and shrubs are taking the place of shallow-rooted plants. A survey of the local plant community by soil biologist and co-author John Moore found that dwarf birch shrubs increased by 94 percent, while lichens were almost completely wiped out.
"The question with Arctic warming is whether the increase in plant growth is going to outpace the release of carbon dioxide because of increasing decomposition," Sistla said.
Warming may push carbon more deeply into the soil
The researchers collected samples dating back to 1989 from greenhouses at the Arctic Long-Term Ecological Research site, the longest-running whole system tundra warming experiment.
Overall, there was no change in total soil carbon over 20 years. While the surface layer lost some of its carbon, there was a significant increase in the mineral soils more than 2 feet below the surface. These soils typically don't hold a lot of carbon, but the researchers believe warming has encouraged soil nematodes and mites, which help decompose leaves and other plant matter, to make their way to the deeper soils.
"Deeper soil food webs are looking like surface soil food webs," Schimel said.
This redistribution of soil carbon storage raises questions of whether the balance provided by larger plants will stand in the long term or whether the more active microbes detected in the deeper soils will eventually offset the increased carbon in those deeper soils.
Arctic and boreal ecosystems carry about one-third of total global soil carbon, where plant matter takes a long time to decay in the cold weather. When compared to the body of knowledge on how forests and other "sinks" help prevent the acceleration of climate change, soil carbon dynamics are a relatively new field of study.
A study published in February found that exposing permafrost to sunlight after a collapse could convert soil carbon to carbon dioxide more quickly than previously thought (ClimateWire, Feb. 12).
"Because both processes are often not studied together, the 'net' effects of warming on [carbon] storage in the tundra are poorly known," said Rose Cory, an environmental sciences and engineering assistant professor at the University of North Carolina, Chapel Hill, and co-author of the permafrost collapse study. "This study addresses this knowledge gap by showing that warming increased plant biomass, but also redistributes carbon into the soil resulting in no change in soil [carbon] stocks."
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