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Differential Effects of Elevated CO2 on Old and New Soil Carbon Pools
Cardon, Z.G., Hungate, B.A., Cambardella, C.A., Chapin, F.S., Field, C.B., Holland, E.A. and Mooney, H.A.  2001.  Soil Biology & Biochemistry 33: 365-373.

What was done
The authors erected open-top chambers on two Mediterranean grassland communities in California, USA, after which they fumigated them for two years with air containing either ambient or twice-ambient atmospheric CO2 concentrations.  In addition, plants were grown with either low or high soil nutrient availability.  The main thrust of this research was to use isotopic labeling to study the effects of elevated atmospheric CO2 and soil nutrient availability on the decomposition of old and new organic carbon in the two grassland soils.

What was learned
When soil nutrient availability was high, elevated CO2 reduced the decomposition of older soil organic carbon by approximately 30% throughout the study.  Thus, the turnover time and stabilization of this soil carbon pool was increased by elevated CO2 exposure.  However, the movement of newly-fixed carbon into the older stabilized pools was decreased with atmospheric CO2 enrichment, due to its preferential utilization by soil microbes.  Thus, soil microbes switched from using older to newer soil organic carbon under CO2-enriched conditions.

What it means
As the atmosphere's CO2 content continues to rise, carbon sequestration in the soils of Mediterranean grasslands will likely increase for two different reasons.  First, it should rise as a consequence of the greater retention times conferred upon the carbon in older soil organic carbon pools, which represent the largest reservoir of terrestrial carbon on earth.  Second, even though soil microbes exhibit a preference for newer carbon under CO2-enriched conditions, it should rise because of the great increase in the amount of carbon going into newer soil carbon pools due to CO2-enhanced root exudation, root turnover and other types of litter production.

Reviewed 10 July 2002