Current models of global climate change predict warmer temperatures will increase the rate that bacteria and other microbes decompose soil organic matter, a scenario that pumps even more heat-trapping carbon into the atmosphere. But a new study led by a University of Georgia researcher shows that while the rate of decomposition increases for a brief period in response to warmer temperatures, elevated levels of decomposition don't persist.
"There is about two and a half times more carbon in the soil than there is in the atmosphere, and the concern right now is that a lot of that carbon is going to end up in the atmosphere," said lead author Mark Bradford, assistant professor in the UGA Odum School of Ecology. "What our finding suggests is that a positive feedback between warming and a loss of soil carbon to the atmosphere is likely to occur but will be less than currently predicted."
Bradford, whose results appear in the early online edition of the journal Ecology Letters, said the finding helps resolve a long-standing debate about how unseen soil microbes respond to and influence global climate change.
Bradford and his team, which included researchers from the University of New Hampshire, the Marine Biological Laboratory at Woods Hole, Duke University and Colorado State University, found evidence to support both hypotheses and revealed a third, previously unaccounted for explanation: The abundance of soil microbes decreased under warm conditions.
The researchers studied soil microbes at Harvard Forest in Massachusetts, the site of a soil warming experiment that began in 1991. Scientists took soil samples from two plots, one in which buried cables heat the soil to five degrees Celsius above the ambient soil temperature (a condition that is expected to occur around 2100) and a control condition in which cables are buried but not producing heat.
In the first set of experiments, the scientists compared microbial respiration in the two groups and found lower rates of decomposition in the heated plots. This finding supported the idea that respiration decreases after a few years of warming, but didn't explain whether the cause was substrate depletion in the warmer soils or adaptation by the microbes.
In the next set of experiments, they added the simple sugar sucrose to both sets of soils to alleviate any food limitation for the microbes. They found that microbes from both conditions increased their respiration, but that the increase was greater in the unheated control soils than in the heated soils. "That finding told us that substrate depletion played a role," Bradford said, "but it also told us that there were other factors involved."
The researchers then measured microbial biomass and found that there were fewer microbes in the heated soils. To test whether thermal adaptation occurred, they measured respiration while keeping temperature constant. They found that respiration rates were indeed lower in the heated versus the control soils, even when adjusting for microbial biomass.
Study co-author Matthew Wallenstein, a research scientist at Colorado State University, pointed out that the study is among the first to demonstrate that microbes, like many plants and animals, can adapt relatively quickly to changes in climate.