Researchers at UC Irvine, Colorado State University and the Yale School of Forestry and Environmental Studies have found that as global temperatures increase, microbes in soil become less efficient over time in converting carbon in soil into carbon dioxide, which is a key contributor to climate warming.

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 for growth. As warmer temperatures persist, 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, lead author of the study and assistant professor of ecology and evolutionary biology at UC Irvine.

"Microbes function like humans-they take in carbon-based fuel and breathe out carbon dioxide. They are the engines that drive carbon cycling in soils. 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."

The study, "Soil-Carbon Response to Warming Dependent on Microbial Physiology," contradicts the results of older models that assume microbes will continue to spew ever-increasing amounts of carbon dioxide into the atmosphere as the climate continues to warm.

The new simulations suggest that 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 increases in enzyme activity-emissions could intensify.

"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."

Mark Bradford, assistant professor of terrestrial ecosystem ecology at Yale, said 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," he said. "More research in this area will help reduce uncertainties in climate prediction."

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