Background
The productivity of earth's temperate forests is often limited by the availability of soil nitrogen (Vitousek and Howarth, 1991).  Especially is this so in the southeastern United States, where the growth of pine-hardwood forests often removes so much nitrogen from the soils in which they grow that they induce what Finzi and Schlesinger (2003) describe as "a state of acute nutrient deficiency that can only be reversed with fertilization."  It would seem only natural, therefore, to presume (as they originally hypothesized in the early stages of the Duke Forest FACE study), that "the increase in carbon fluxes to the microbial community under elevated CO2 would increase the rate of N immobilization over mineralization," which would ultimately lead to a decline in -- and perhaps the total negation of -- the significant CO2-induced stimulation of forest net primary production that developed over the first two years of the experiment (DeLucia et al., 1999; Hamilton et al., 2002).

What was done
To test this hypothesis, Finzi and Schlesinger measured and analyzed the pool sizes and fluxes of inorganic and organic N in the forest floor and top 30 cm of mineral soil during the first five years of differential atmospheric CO2 treatment in the Duke Forest FACE study, wherein half of the experimental plots were fumigated to maintain them at a mean CO2 concentration 200 ppm above ambient.

What was learned
The authors note that the presence of the extra CO2 "significantly increased the input of C and N to the forest floor and the mineral soil."  Nevertheless, they report "there was no statistically significant change in the cycling rate of N derived from soil organic matter under elevated CO2."  Indeed, they say that "neither the rate of net N mineralization nor gross ¹⁵NH4⁺ dynamics were significantly altered by elevated CO2."  In addition, they note "there was no statistically significant difference in the concentration or net flux of organic and inorganic N in the forest floor and top 30-cm of mineral soil after 5 years of CO2 fumigation," adding that "microbial biomass was not a larger sink for N."

What it means
Based on these results from the first five years of the Duke Forest FACE study, Finzi and Schlesinger have rejected their original hypothesis that elevated levels of atmospheric CO2 would significantly increase the rate of N immobilization by the microbial community.  Nevertheless, they continue to believe that "elevated CO2 will only increase the productivity of this forest during the initial stages of stand development, with N limitation constraining additional C sequestration under elevated CO2 well before this stand reaches its equilibrium biomass."

Will the two researchers ultimately be proven to be correct?  Or will the real world surprise them once again?  We will have to wait and see, trusting that their diligence and attention to details of observation and analysis will someday reveal the truth of the matter ... whatever it may be ... whenever it is clearly evident.

References
DeLucia, E.H., Hamilton, J.G., Naidu, S.L., Thomas, R.B., Andrews, J.A., Finzi, A., Lavine, M., Matamala, R., Mohan, J.E., Hendrey, G.R. and Schlesinger, W.H.  1999.  Net primary production of a forest ecosystem with experimental CO2 enrichment.  Science 284: 1177-1179.

Hamilton, J.G., DeLucia, E.H., George, K., Naidu, S.L., Finzi, A.C. and Schlesinger, W.H.  2002.  Forest carbon balance under elevated CO2.  Oecologia 131: 250-260.

Vitousek, P.M. and Howarth, R.W.  1991.  Nitrogen limitation on land and in the sea: how can it occur?  Biogeochemistry 13: 87-115.