Background
The authors write that "elevated CO2 has increased the NPP [net primary production] of tree stands in four large-scale forested free-air CO2 enrichment (FACE) experiments (Norby et al., 2005)," and they say that it has also led to "increased uptake of mineral N [nitrogen] from the soil in three of the four forested FACE experiments, even those that were initially N-limited (Finzi et al., 2007)." Thus, they set about to determine how the CO2-enriched trees may have accomplished this impressive feat.

What was done
Working in the CO2-enriched sweetgum (Liquidambar styraciflua L.) plantation located in Oak Ridge, Tennessee (USA), Iversen et al. used ¹⁵N isotope pool dilution methodology to measure potential gross N cycling rates in laboratory incubations of soil from four depth increments to 60 cm in order to determine whether N is available for root acquisition in deeper soil and to see if it may have altered N cycling rates.

What was learned
When all was said and done, the four researchers concluded that "deeper rooting distributions in response to CO2 enrichment were likely associated with relatively greater N availability in deeper soil rather than altered N cycling rates under elevated CO2," providing evidence that "N mineralization at depth in the soil, combined with increased root exploration of the soil volume under elevated CO2, may be more important than changes in potential gross N cycling rates in sustaining forest responses to rising atmospheric CO2."

What it means
When atmospheric CO2 enrichment provides an opportunity for trees to enhance their growth rates, it also seems to provide a way for them to find the extra nitrogen they need to do so. And, in the words of Iversen et al., "in addition to internal ecosystem N recycling at depth, external sources of N may also lead to greater root proliferation in deep soil," as they note that "root access to inorganic N found in groundwater at >2 m depth in CO2-enriched chambers may have sustained tree production under elevated CO2 in a scrub-oak system in Florida, USA (McKinley et al., 2009)."

References
Finzi, A.C., Norby, R.J., Calfapietra, C., Gallet-Budynek, A., Gielen, B., Holmes, W.E., Hoosbeek, M.R., Iversen, C.M., Jackson, R.B., Kubiske, M.E., Ledford, J., Liberloo, M., Oren, R., Polle, A., Pritchard, S., Zak, D.R., Schlesinger, W.H. and Ceulemans, R. 2007. Increases in nitrogen uptake rather than nitrogen-use efficiency support higher rates of temperate forest productivity under elevated CO2. Proceedings of the National Academy of Sciences, USA 104: 14,014-14,019.

McKinley, D.C., Romero, J.C., Hungate, B.A., Drake, B.G. and Megonigal, J.P. 2009. Does deep soil N availability sustain long-term ecosystem responses to elevated CO2? Global Change Biology 15: 2035-2048.

Norby, R.J., DeLucia, E.H., Gielen, B., Calfapietra, C., Giardina, C.P., King, S.J., Ledford, J., McCarthy, H.R., Moore, D.J.P., Ceulemans, R., De Angelis, P., Finzi, A.C., Karnosky, D.F., Kubiske, M.E., Lukac, M., Pregitzer, K.S., Scarasci-Mugnozza, G.E., Schlesinger, W.H. and Oren, R. 2005. Forest response to elevated CO2 is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences 102: 18,052-18,056.