What was done
The authors studied the effects of elevated CO2 (ambient, +175, +350 ppm) and nitrogen fertilization (unfertilized, +100, +200 kg N ha^-1 yr^-1, provided as ammonium sulfate) on C and N accumulations in the biomass of ponderosa pines (Pinus ponderosa Laws, grown from seed) and the soils that supported them in a six-year open-top chamber experiment conducted near Placerville, California, USA.

What was learned
"This study, like several others," according to Johnson et al., "showed that growth response to elevated CO2 more than offset declines in tissue N concentrations, necessitating increased N uptake by trees," which led them to ask: "How did the trees manage to obtain this 'extra' N in an N-limited environment?"

In the fertilized treatments, the four researchers say the extra N could readily have been supplied by the added fertilizer; but in the unfertilized treatments they opine that a substantial amount of the N uptake "probably came from the soil," as both wet and dry deposition were not great enough to have supplied all of the extra N and "no symbiotic N fixer was present in the study plots." Additionally citing a number of other investigators' results as supplying circumstantial evidence for what they finally concluded, they thus suggested that "the additional N needed to respond to elevated CO2 came from the soil and was facilitated by greater root exploration under elevated CO2."

What it means
Acknowledging that they "cannot provide an accurate prediction from the results of this study," Johnson et al. nevertheless state that they "see no evidence that either growth or additional N uptake at the +350 ppm CO2 level are being inhibited by PNL [progressive nitrogen limitation] as of year 6 in this study." Hence, since so many other researchers are finding the same thing (Norby and Iversen, 2006; Finzi et al., 2006; Hungate et al., 2006; Luo et al., 2006), there is no reason not to believe that earth's plant life will be able to continually benefit from the ongoing rise in the air's CO2 content, especially since it rises by such a small increment each year compared to the large increases employed in nearly all CO2 enrichment experiments, where compelling evidence for the PNL hypothesis has yet to be found.

References
Finzi, A.C., Moore, D.J.P., DeLucia, E.H., Lichter, J., Hofmockel, K.S., Jackson, R.B., Kim, H.-S., Matamala, R., McCarthy, H.R., Oren, R., Pippen, J.S. and Schlesinger, W.H. 2006. Progressive nitrogen limitation of ecosystem processes under elevated CO2 in a warm-temperate forest. Ecology 87: 15-25.

Hungate, B.A., Johnson, D.W., Dijkstra, P., Hymus, G., Stiling, P., Megonigal, J.P., Pagel, A.L., Moan, J.L., Day, F., Li, J., Hinkle, C.R. and Drake, B.G. 2006. Nitrogen cycling during seven years of atmospheric CO2 enrichment in a scrub oak woodland. Ecology 87: 26-40.

Luo, Y., Hui, D. and Zhang, D. 2006. Elevated CO2 stimulates net accumulations of carbon and nitrogen in land ecosystems: A meta-analysis. Ecology 87: 53-63.

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.