Uncovering the Secrets of Carbon in Soil Organic Matter


Soil organic matter (SOM) is a heterogeneous mixture of partially decomposed, plant- and microbial-derived materials that plays an important role in the global carbon cycle. The carbon associated with some components of SOM can persist belowground for centuries to millennia, making these pools crucial carbon reservoirs. However, these long-lived carbon pools and the factors controlling their persistence in soil are not well characterized, because they are extremely challenging to isolate. A team led by Argonne National Laboratory used a novel approach combining sequential physical and chemical fractionations with two naturally occurring carbon isotopic tracers to divide the SOM into pools with average turnover times that ranged from 1 to over 3,000 years. They discovered that the SOM pools associated with soil minerals, which typically have been characterized as having extremely long lifetimes, are actually composed of a mixture of rapidly cycling pools and pools with much longer residence times. Further, the study found that the rapidly cycling pools accounted for a much greater proportion of the total soil carbon than is generally represented in SOM models. These results provide new insight into soil carbon dynamics, since SOM pools with long turnover times were previously thought to be relatively homogenous and practically inert. The findings can inform models used to predict the contributions of soils to the carbon cycle and the responses of SOM to climatic change.


O’Brien, S. L., J. D. Jastrow, L. J. McFarlane, T. P. Guilderson, and M. A. Gonzalez-Meler. 2012. “Decadal Cycling within Long-Lived Carbon Pools Revealed by Dual Isotopic Analysis of Mineral-Associated Soil Organic Matter,” Biogeochemistry, DOI: 10.1007/s10533-011-9673-0.