What was done
In another of Ecology's Special-Feature papers that address the progressive nitrogen limitation (PNL) hypothesis, Luo et al. (2006) conducted a meta-analysis of C and N processes in plants and soils in response to atmospheric CO2 enrichment based on experimentally-derived data found in 104 scientific publications.

What was learned
The researchers' analyses revealed that in response to atmospheric CO2 enrichment, the "C and N contents in all [our italics] the plant and soil pools significantly increase[d], leading to more net C and N accumulations in ecosystems at elevated than ambient CO2." Specifically, they found that the mean CO2-induced increases in C pools of shoot, root, whole plant, litter and soil were 22.4%, 31.6%, 23.0%, 20.6% and 5.6%, respectively, while the corresponding CO2-induced increases in N pools were 4.6%, 10.0%, 10.2%, 25.4% and 11.2%. In addition, they report that "N accumulations in ecosystems have long been documented in association with C accumulations during both primary and secondary successions (Crocker and Major, 1955; Binkley et al., 2000; Vitousek, 2004)."

What it means
Luo et al. conclude that "the net C and N accumulations revealed in this study," which were produced by atmospheric CO2 enrichment, "together with [the results of] studies of C and N dynamics during succession over hundreds to millions of years, suggest that ecosystems may have intrinsic capabilities to stimulate N accumulation by C input," which latter phenomenon is typically increased by atmospheric CO2 enrichment. They further conclude that "the net N accumulation likely supports long-term C sequestration in response to rising atmospheric CO2 concentration." Finally, to underscore this point, they say that the "concomitant increases in C and N contents in plant and soil pools at elevated CO2 as shown in this study point toward a long-term trend of terrestrial C sequestration in response to rising atmospheric CO2 concentration," which is clearly just the opposite of what is predicted by the PNL hypothesis.

References
Binkley, D., Son, Y. and Valentine, D.W. 2000. Do forests receive occult inputs of nitrogen? Ecosystems 3: 321-331.

Crocker, R.L. and Major, J. 1955. Soil development in relation to vegetation and surface age at Glacier Bay, Alaska. Journal of Ecology 43: 427-448.

Vitousek, P.M. 2004. Nutrient Cycling and Limitation: Hawaii as a Model System. Princeton University Press, Princeton, New Jersey, USA.