A study examining the effects of higher temperatures on soil shows that warming alone does not increase levels of carbon dioxide emitted from the soil. Instead, higher temperatures combined with more added carbon – and more nutrients like nitrogen and phosphorus – led to higher carbon dioxide levels released from the soil.

The findings provide another piece of the puzzle reflecting the role nature plays in the delicate balancing act between carbon storage in soil and carbon dioxide emissions into the atmosphere.

Much of the carbon dioxide emissions from soil come from microbes, tiny organisms like bacteria, fungi, viruses and others, that live in soil and “breathe out” carbon dioxide – just like people.

“When things warm up, there is more plant photosynthesis, more ‘food’ for microbes to metabolize on, more activity for microbes,” said Debjani Sihi, an assistant professor with joint appointments in NC State’s Department of Plant and Microbial Biology and Department of Crop and Soil Sciences and corresponding author of a paper describing the research.

“The question here is whether warming was enough to cause more carbon dioxide release from soil. The findings show that if you don't have the carbon and nutrients in easily available forms that soil microbes need to grow and thrive, then heating alone will not increase the loss of carbon.”

Sihi added that adding heat and nutrients alone also did not increase carbon dioxide emissions from the studied soil, which came from a long-term field-warming experimental site in the southeastern United States. Soil carbon in an easily available form was required for carbon dioxide levels from soil to increase.

Until recently, warming studies have mostly been conducted in cold (e.g., Arctic, boreal or temperate) climates, Sihi said, as researchers attempt to understand the effects in places where a little bit of warming might lead to large changes.

This study, in contrast, examined unfertile soil from a subtropical climate – Athens, Georgia, home to one of the longest-running soil-warming facilities on the planet.

“This study occurs in former cotton fields converted to forest land, not in native forest land,” Sihi said. “Cotton is an exhaustive crop, so the soil doesn’t contain many nutrients or carbon; the soil is not fertile or healthy.”

The researchers gathered soil from the field site and brought it to a lab to undergo heating – up to 2.5 degrees Celsius. They also examined a number of complex pathways in the soil carbon cycle, the process by which carbon is either stored in or expelled from the soil.

Soil holds many different forms of organic matter, from plant material to living and dead microbes, all of which play a part in the carbon cycle. Microbes are constantly searching for food to survive and grow. The researchers tracked how much carbon is stored in these different pools.

“Microbes are breathing and they are getting their energy from carbon. And then they're also fulfilling their demand of nutrients from the same food that they're getting,” Sihi said. “Like humans who need a balanced diet – an energy source, proteins, fiber – you can think about a similar parallel with microbes. They use some of the carbon to build biomass. And they will invest some energy to build enzymes that they need to break down complex organic matter into carbon and nutrients in forms that are easy for them to ingest. The remainder will just be expelled, because that's part of their metabolism.

“Nature emits carbon, but it also absorbs carbon. If you know how much CO2 comes from the natural system, then you can identify targets for different other industries or economic sectors to reduce carbon emissions.”

Sihi said that ongoing collaborative work is also examining a range of ecosystems, including two field warming experiments from the tropics – Puerto Rico and Panama – to understand how warming influences soil carbon loss.

“It appears in this case that warming alone may not stimulate microbial activities because these microbes actually don’t have a lot of resources to thrive in,” Sihi said. “In other words, depleted microbial resources constrain warming effects.”

The paper appears in Biogeochemistry. Yaxi Du, a former graduate student of Sihi’s, is the first author. Jacqueline Mohan and Paul Frankson from the University of Georgia co-authored the paper and maintained the long-term field-warming experiment used in the study. Greta Franke and Zhilin Chen are undergraduate researchers who assisted in Sihi’s lab.

Funding for the research was provided by the U.S. Department of Energy’s Environmental System Science Program awards DE-SC0024410 and DE-SC0025314.

- kulikowski -