A new study in Nature Geoscience suggests that as global temperatures increase, microbes in soil become less efficient over time at converting carbon in soil into carbon dioxide, a key contributor to climate warming.

The results contradict those of earlier studies that assume microbes will continue to spew ever-increasing amounts of carbon dioxide into the atmosphere as the climate continues to warm. If microbial efficiency declines in a warmer world, carbon dioxide emissions will fall back to pre-warming levels, a pattern seen in field experiments. But if microbes manage to adapt to the warmth – for instance, through increased enzyme activity – emissions could intensify.

There is intense debate in the scientific community over whether the loss of soil carbon will contribute to global warming. "The challenge we have in predicting this is that the microbial processes causing this loss are poorly understood," said Mark Bradford, assistant professor of terrestrial ecosystem ecology at Yale. "More research in this area will help reduce uncertainties in climate prediction."

Microbes, in the form of bacteria and fungi, use carbon for energy to breathe, or respire, and to grow in size and in number. A model developed by the researchers shows microbes exhaling carbon dioxide furiously for a short period of time in a warmer environment, leaving less carbon to grow on. As warmer temperatures are maintained, the less efficient use of carbon by the microbes causes them to decrease in number, eventually resulting in less carbon dioxide being emitted into the atmosphere.

"Microbes aren't the destructive agents of global warming that scientists had previously believed," said Steven Allison, assistant professor of ecology&evolutionary biology at UCI and lead author on the study. "Microbes function like humans: They take in carbon-based fuel and breathe out carbon dioxide. They are the engines that drive carbon cycling in soil. In a balanced environment, plants store carbon in the soil and microbes use that carbon to grow. The microbes then produce enzymes that convert soil carbon into atmospheric carbon dioxide."

"When we developed a model based on the actual biology of soil microbes, we found that soil carbon may not be lost to the atmosphere as the climate warms," said Matthew Wallenstein of the Natural Resource Ecology Laboratory at Colorado State University. "Conventional ecosystem models that didn't include enzymes did not make the same predictions."

Citation: Steven D. Allison, Matthew D. Wallenstein, Mark A. Bradford, 'Soil-carbon response to warming dependent on microbial physiology', Nature Geoscience, April 2010; doi:10.1038/ngeo846