Background
The authors write that the progressive nitrogen limitation hypothesis (Luo et al., 2004) suggests that CO2-induced decreases in soil nitrogen availability "could potentially diminish or eliminate greater plant growth in a CO2-enriched atmosphere, thus constraining the long-term storage of anthropogenic CO2 in terrestrial ecosystems," which hypothesis, if proven to be correct, would not bode well for earth's terrestrial biosphere.

What was done
Zak et al. initiated a year-long ecosystem-level ¹⁵N tracer experiment at the Rhinelander, Wisconsin (USA) FACE facility at the seven-year point of a long-term study of aspen (Populus tremuloides) and aspen-birch (P. tremuloides-Betula papyrifera) communities exposed to factorial treatments of CO2 (ambient and elevated to 200 ppm above ambient) and O3 (ambient and elevated to 30-40 ppb above ambient).

What was learned
One year after adding tracer amounts of ¹⁵NH4+ to the forest floor of the young tree stands, the three researchers determined that "both forest communities exposed to elevated CO2 obtained greater amounts of ¹⁵N (29%) and N (40%) from soil, despite no change in soil N availability or plant N-use efficiency," which they attributed to greater belowground root growth and a more thorough exploration of the soil for growth-limiting nitrogen in the CO2-enriched treatment. In contrast, they found that the elevated O3 treatment "decreased the amount of ¹⁵N (-15%) and N (-29%) in both communities." These decreases, however, were significantly smaller than the corresponding CO2-induced increases.

What it means
Zak et al. conclude that "progressive nitrogen limitation is presently not a factor governing plant growth response to elevated CO2 in these young, developing forest communities." In addition, they state that their findings "are consistent with those in young sweet gum (Liquidambar styraciflua) and loblolly pine (Pinus taeda) forests exposed to elevated CO2 (Finzi et al., 2006; Norby and Iversen, 2006)." To date, therefore, most observational data continue to argue against the validity of the progressive nitrogen limitation hypothesis, as may be seen from perusing the several items we have archived under this heading in our Subject Index.

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.

Luo, Y., Su, B., Currie, W.S., Dukes, J.S., Finzi, A., Hartwig, U., Hungate, B., McMurtrie, R.E., Oren, R., Parton, W.J., Pataki, D.E., Shaw, M.R., Zak, D.R. and Field, C.B. 2004. Progressive nitrogen limitation of ecosystem responses to rising atmospheric carbon dioxide. BioScience 54: 731-739.

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.