What was done
Spring wheat (Triticum aestivum L.) was grown for two seasons at atmospheric CO2 concentrations of 370 and 570 ppm in FACE plots located in Arizona, USA.  At each CO2 concentration, one-half of each experimental plot also received high soil nitrogen supplies, while the other half received low soil nitrogen supplies.  The authors of this paper report on the use of stable-carbon isotopic tracers to detect the entry of new carbon into soil organic carbon pools associated with this experimental protocol.

What was learned
In this particular study, elevated CO2 did not significantly impact the addition of new carbon to soil organic carbon pools, regardless of soil nitrogen treatment.  However, by itself, elevated nitrogen supply did do so, increasing new carbon additions by 6% and 5% in the 0-15 and 15-30 cm soil horizons, respectively.

What it means
Although elevated CO2 did not directly impact new additions to soil organic carbon pools in this study, it is likely that future increases in the air's CO2 content will do so indirectly, by stimulating the productivity of nitrogen-fixing plants in various natural and agro-ecosystems.  The authors' experiment, for example, demonstrates that high nitrogen availability increased carbon inputs to soil organic carbon pools; and as elevated CO2 is known to stimulate biological nitrogen-fixation, this phenomenon should ultimately increase soil nitrogen availability and, therefore, indirectly increase soil carbon sequestration on a long term basis.