Analyzing radiocarbon dating on a massive database of soil samples from around the world, University of California, Irvine researchers have determined that globally, the average age of the carbon in the ground is about 5,000 years old. In a study published today in Nature Geoscience, a team led by Zheng Shi, UCI postdoctoral scholar in Earth system science, studied shallow- and deep-layer soil from 789 locations worldwide and used machine learning and statistical techniques to fill in gaps where samples were not available. They found the carbon in soil at a depth up to one meter to be quite ancient, and even that at about a foot deep was more than 1,000 years old. The results have implications for climate models that attempt to predict how much carbon from human fossil fuel burning will be cycled out of the atmosphere and into the ground in the coming decades and centuries. “If soil carbon is very young, that means it cycles rapidly, and carbon soaked up by the soil comes back out quickly,” said co-author Steven Allison, UCI professor of ecology & evolutionary biology and Earth system science. “If soil carbon is very old, it cycles slowly, meaning very little makes it from plants into long-lasting carbon forms today, so it will take too long for soils to act like a sponge.” Allison said that before this study, scientists did not have a clear understanding of the age of the world’s soil carbon nor how fast it gets drawn from the atmosphere into subsurface storage. “Our results imply that we cannot depend on soils soaking up carbon at the levels anticipated by current models, so need to take even stronger steps to limit our carbon dioxide emissions,” said co-author James Randerson, Chancellor’s Professor and Ralph J. and Carol M. Cicerone Chair in Earth System Science. Randerson added that the project showed where soil carbon may be vulnerable to human disruption. “Although it may take thousands of years to build up, ancient carbon stored in Arctic permafrost, for example, can be lost rapidly due to changing climate and land use,” he said.