In some ways, understanding the effects of global warming comes down to measuring how microbes respond to warming soils. Through their incessant work in breaking down plant and animal matter, microorganisms drive the respiration of gases such as carbon dioxide from the planet’s thin “skin” of soil cover. These emissions account for more than 10 times the carbon dioxide produced by human activity.
It’s well known that as temperature increases, the rate of chemical reactions typically increases along with it. The same is true of the biochemical reactions carried out by microbes, which metabolize organic compounds in the soil and respire carbon dioxide as waste. Hence, higher temperature means more respiration, which means more carbon returned to the atmosphere.
Another, less-understood factor controlling the flow of carbon dioxide from the soil is the efficiency with which microbes use carbon — that is, the ratio of carbon incorporated into their cell structure and the amount respired as waste. Slight changes in microbial efficiency, brought on by changing soil temperatures, could potentially have a significant effect on the total release of soil carbon dioxide.
With this in mind, University of New Hampshire microbial ecologist Serita Frey and colleagues set out to measure the efficiency of microbial communities in soils in response to both short and long-term warming conditions. The study, published in the Jan. 20 edition of Nature Climate Change, contained two main findings.
The first finding was that for more complex, hard-to-break-down carbon compounds, microbial efficiency goes down as temperature increases, meaning that more carbon dioxide is respired as waste. This wasn’t unexpected. Theoretical predictions show that higher operating temperatures in the molecular machinery needed for metabolism reduce overall efficiency in microbes.
But the second result showed that in soils artificially warmed for 18 years, the drop in efficiency from further temperature increases was much lower than in controls. In other words, microbial communities shifted in response to long-term warming, becoming more efficient in their use of carbon. Since current climate models don’t account for this particular efficiency parameter, the study results could mitigate the long-term predicted increases of soil carbon dioxide emissions.
“It all depends on what timescale you’re looking at,” said co-author Johan Six, a professor in the UC Davis Department of Plant Sciences at the time of the study. “We’re finding more and more that there’s quite a bit of difference between what happens in the short-term versus the long-term. It might well be that many systems are a lot more resilient than we think at this point.”
To measure microbial efficiency, the researchers added four easy-to-track synthetic compounds to artificially warmed soils. The compounds roughly approximate the range of complexity found in natural soils, ranging from glucose, a simple sugar, to phenol, a component of the lignin found in wood. Each compound was “tagged” with a heavy isotope of carbon so that researchers could track how much of each compound was incorporated into microbial biomass, and how much was respired as carbon dioxide.
Using this method, the team showed that microbial efficiency is inversely related to temperature in some compounds but not others. Since glucose can be directly assimilated by most soil microbes, its utilization showed no sensitivity to changes in temperature. But phenol, which must be broken down by microbial enzymes into less complex molecules before it can be used, showed a 60 percent drop in efficiency as temperature was increased from five to 25 degrees Celsius.
“I think their key message about incorporating some critical elements of microbial physiology into soil organic models is spot on,” said Josh Schimel, a microbial ecologist at UC Santa Barbara. “Soil microbial communities are enormously complex. So the question is, how do we capture some of the dynamics of those communities that are really important in regulating global carbon cycling?”
In order to study how those dynamics might play out over the longer term — what Six called “the legacy effect” of warming on bacterial communities — the researchers conducted the same measurements on soils taken from plots warmed by underground coils at a research site in Harvard Forest.
For soils that had been continuously warmed five degrees Celsius above ambient temperatures for 18 years, further heating in the range of 10 to 25 degrees showed a much smaller decrease in microbial efficiency compared with the control soils which had not been previously warmed.
One of the remaining challenges is resolving the exact causes underlying this change in microbial efficiency. The study’s authors aren’t certain whether the change is due to physiological adaptations by existing microbes or a wholesale change in the species composition of the microbial community.
“You might imagine that you get a change in organisms living in the soil, maybe different species or maybe changing physiology in some way to deal with the change in resource availability,” Frey said.
OYANG TENG can be reached at firstname.lastname@example.org.