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Warming likely to speed the release of carbon from soils to the atmosphere

Dec. 12, 2016

Writer: Beth Gavrilles, bethgav@uga.edu

Contact: Jacqueline Mohan, jmohan@uga.edu

Rising temperatures are likely to lead to a major increase in the already substantial amount of carbon released into the atmosphere from soil, according to a pair of new papers just published in Proceedings of the National Academy of Sciences and in Nature. Both draw on data collected over roughly 20 years from dozens of soil warming experiments, including one conducted by University of Georgia ecologist Jacqueline Mohan in Whitehall Forest on the university campus. Mohan’s experiment plays an important role in the findings as the southernmost experiment represented.

By analyzing soil warming experiments from across the globe, the papers’ authors found that the impacts of warming vary widely by region, helping to explain a lack of consensus from earlier studies based on single sites. Some of those earlier studies suggested that by enhancing plant growth, warming temperatures could lead to more carbon being stored in soil than released from it. While that may happen in some locations, both new studies concluded that increased releases of carbon, particularly in the Arctic, will more than offset such an effect on a global scale. The findings will help climate scientists make more accurate projections about future climate scenarios.

Mohan, an associate professor in the UGA Odum School of Ecology, said that the studies took different approaches to exploring the impact of warming on carbon cycling in soil.

“In the PNAS paper, we’re looking at how warming affects the rate at which microbes in the soil convert the carbon stored there into carbon dioxide, which is then released into the atmosphere,” she said. “The paper in Nature is concerned with the amount of carbon stored in soils and how we can expect that to change under warming conditions.”

Both papers concluded that the impacts of warming will be particularly powerful in the Arctic.

The study in PNAS was led by Joanna C. Carey and Jianwu Tang of the Marine Biological Laboratory in Woods Hole, Mass., with 41 additional coauthors from 36 institutions, including Odum alumna Megan Machmuller who received her doctorate in 2014 and is now at Colorado State University. It included data from 27 soil warming experiments from North America and Europe. Its main finding was that the warmer the soil, the more carbon is released, up to about 25 degrees Celsius; after that point, soils start to release less carbon. Because far northern latitudes only rarely reach that threshold, the authors project that carbon releases there will accelerate.

The Nature study was led by Yale University’s Thomas Crowther and Mark Bradford, a former Odum professor, with 49 additional coauthors from 44 institutions, including Machmuller and Odum alumna Yolima Carrillo, who received her doctorate in 2007 and is now at Western Sydney University in Australia. The study used data from 49 field experiments from North America, Europe and Asia. It found that carbon release is likely to be greatest in the far North because of the large stocks of carbon that have built up over centuries and been held frozen in the soils there. The authors project that at least 60.6 billion tons of carbon will be released into the atmosphere from warming soils by 2050, or about 17 percent more than from human activities during that time. 

“It’s a very dangerous double whammy,” said Mohan. “The places with the greatest amount of soil carbon are the very places that are warming the most.”

One important region not represented in either study is the tropics. According to Mohan, this is in part because of the logistical challenges posed by installing and running soil warming experiments, which need sustained access to reliable power sources. The lack of data from the tropics lends Mohan’s Whitehall experiment—the southernmost field site represented—added significance.

“Whitehall is important because the soils here in the southern Piedmont are very different from soils in more northern latitudes,” Mohan said. “They are much more similar to soils in the tropics, which account for 20-25 percent of the world’s soils. There are no published soil warming experiments in the tropics that I’m aware of, so the data from Whitehall are really critical. We’re very fortunate to have this research facility in our backyard.”

Exploring the response of tropical and subtropical soils to warming is Mohan’s next research topic. “We need to find out why low-latitude systems show less response to warming,” she said. “We’re proposing to do that with warming experiments in Costa Rica and Puerto Rico, but until those are up and running, Whitehall remains our best window to the lower latitudes and the southern hemisphere.”

Temperature response of soil respiration largely unaltered with experimental warming, by Joanna C. Carey et al., is available online at http://www.pnas.org/content/113/48/13797.full.

Quantifying global soil carbon losses in response to warming, by Thomas Crowther et al. may be found online at http://www.nature.com/nature/journal/v540/n7631/full/nature20150.html.

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