What was done
In the second of Ecology's Special-Feature papers that address the progressive nitrogen limitation (PNL) hypothesis, Finzi et al. (2006) tested the concept "using data on the pools and fluxes of C and N in tree biomass, microbes and soils" that were obtained from the first six years of the Duke Forest FACE study of (primarily) loblolly pine (Pinus taeda) trees (with a smattering of broadleaf understory species) growing in the Piedmont region of North Carolina (USA), which study has been the subject of several Journal Reviews on our website (see FACE Experiments (Trees - Pine) in our Subject Index). This location is ideal for testing the PNL hypothesis, because the growth of pine-hardwood forests in the southeastern United States 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," which operation, however, is not employed in their long-term CO2 enrichment study.

What was learned
As in the case of the sweetgum forest study described by Norby and Iversen (2006), Finzi et al. likewise found that "during the first six years of this experiment there was no reduction in the average stimulation of net primary production by elevated CO2," even though "significantly more N was immobilized in tree biomass and in the O [soil] horizon under elevated CO2." Also, and "in contrast to the PNL hypothesis," as they describe it, "microbial-N immobilization did not increase under elevated CO2, and although the rate of net N mineralization declined through time, the decline was not significantly more rapid under elevated CO2." In addition, the twelve researchers report that "mass balance calculations demonstrated a large accrual of ecosystem N capital," and they say that the rate of extra N accrual was "much greater than the estimated rate of N input via atmospheric deposition or heterotrophic N fixation," noting further that "there are no plant species capable of symbiotic N fixation in this ecosystem." In other words, by some unknown means the loblolly pine trees obtained the extra N they needed to stave off the negative effects predicted by the PNL hypothesis, possibly, in the words of Finzi et al., by roots "actively taking up N and redistributing N from deeper in the soil profile."

What it means
Against all apparent odds, the Duke Forest loblolly pine plantation has shown no indication of progressive nitrogen limitation of the growth stimulation provided by the aerial fertilization effect of atmospheric CO2 enrichment over the first six years of its existence.

References
Finzi, A.C. and Schlesinger, W.H. 2003. Soil-nitrogen cycling in a pine forest exposed to 5 years of elevated carbon dioxide. Ecosystems 6: 444-456.

Norby, R.J. and Iversen, C.M. 2006. Nitrogen uptake, distribution, turnover, and efficiency of use in a CO2-enriched sweetgum forest. Ecology 87: 5-14.