Saved by fungi?
In forecasting the effect of carbon dioxide emissions on Earth's climate, scientists may be misjudging the response to global warming from some of the planet's tiniest terrestrial inhabitants.
As a result, researchers could be overestimating the average warming the planet might experience.
That is the implication of a growing body of studies focusing on the release back into the atmosphere of carbon in soil, a process known as soil respiration. Respiration from plants and soils sits opposite plant absorption of CO2 on the terrestrial carbon-cycle teeter-totter.
The latest piece in the respiration puzzle comes from a team at the University of Oklahoma in Norman, where scientists spent a year artificially warming prairie test plots at a fixed level above the ambient air and comparing the results with unheated plots.
All plots released carbon dioxide at higher rates in the summer than in the winter, as expected.
But over the course of the year, the rate at which CO2 was released through roots and the activity of fungi and tiny soil organisms was no greater in the warmed plots than in the unheated soils.
This runs counter to a longstanding notion - embraced in textbooks as well as climate models - that the hotter it gets, the faster soils pump out CO2. For every 10 degrees C that temperatures rise, CO2 emissions from soils were thought to double.
By contrast, the team concluded that the soils' organisms quickly adjust to higher temperatures. Like Northeastern "snowbirds" moving to Florida and facing warmer temperatures year-round, they "just get used to it."
Combined with additional data suggesting that the prairie grasses in the heated plots thrived under the additional warmth, the study also suggests that the world's grasslands may soak up more CO2 than they emit, according to Linda Wallace, a plant ecologist at the University of Oklahoma and a member of the team, which is reporting its results in the today's edition of the journal Nature.
"This is good news on a global basis," Dr. Wallace says. "Grasslands cover a such a huge proportion of the terrestrial portion of globe. We'd been kind of discounting them, saying that as things warm up, they would become carbon sources. But in reality, these grasslands could become carbon sinks."
The concept of acclimatization is important in understanding how terrestrial ecosystems could respond to global change, researchers say.
Long-term experiments already have shown that, faced with increased concentrations of CO2, trees go through a relatively short "growth spurt," and then their growth rate slows as they become acclimatized to higher CO2 levels, notes Lindsey Rustad, a soil ecologist with the US Forest Service office in Durham, N.H.
Thus, relying on them as "sinks" for the CO2 that humans are pumping into the atmosphere may be a short-term approach at best for trying to meet emissions targets set out in the 1997 Kyoto Protocols.
Down at the root level, soil respiration pumps up to 14 times as much CO2 into the atmosphere each year as human activity currently does.
Thus, with human-induced warming, soils might be expected to increase their CO2 output as temperatures rise, adding atmospheric insult to injury. Climate modelers use a representation of that rate in their computer simulations of global warming.
This relationship was based primarily on the physiology of the various organisms themselves, Wallace notes, as opposed to detailed measurement of changes in the respiration rate driven by different temperature regimes.
Other studies in different ecosystems have documented an initial spike in soil CO2 emissions when the soil is heated.
But those emissions quickly fall as the organisms use up other resources.
These studies, combined with the Oklahoma team's results, lead Dr. Rustad to conclude:
"The biggest take-home message is the idea that we really have to reexamine what sort of inputs we have in our models and how we derive those."