What was done
Colleen Iversen of the Oak Ridge National Laboratory in Oak Ridge, Tennessee (USA) reviewed the pertinent scientific literature "to examine the potential mechanisms for, and consequences of, deeper rooting distributions under elevated CO2 as they relate to ecosystem carbon and nitrogen cycling," focusing primarily on forests.

What was learned
Iversen writes that "experimental evidence from a diverse set of forested ecosystems indicates that fine roots of trees exposed to elevated CO2 are distributed more deeply in the soil profile relative to trees grown under ambient CO2." As an example, she reports that "in a FACE experiment in a sweetgum (Liquidambar styraciflua) plantation, Iversen et al. (2008) found that, over nine years, there was a 220% stimulation in cumulative carbon inputs from fine roots under elevated CO2 at 45-60 cm soil depth, compared with a 30% stimulation of root carbon inputs at 0-15 cm depth," and she notes that "Pritchard et al. (2008a) found a similar response in a CO2-enriched loblolly pine (Pinus taeda) plantation." In fact, she found that "of those experiments that examined rooting depth responses to elevated CO2, 73% found deeper rooting distributions." In addition, she notes that "increased proliferation at depth in the soil has not been limited to fine roots: increased production of mycorrhizas (Pritchard et al., 2008b) and coarse roots (Liberloo et al., 2006) also occurred deeper in the soil under CO2 enrichment."

What it means
Speaking of what many have called the progressive nitrogen limitation hypothesis, Iversen says that "a disconnect between observed root dynamics and modeled nutrient availability has confounded projections of forest responses to elevated CO2," stating that "while models predict that soil nitrogen availability will limit forest responses to elevated CO2 (Thornton et al., 2007), many of the forested FACE experiments found a sustained increase in nitrogen uptake from the soil in response to CO2 enrichment (Finzi et al., 2007)," after which she goes on to say "there has been much speculation on the source of this 'extra' nitrogen (Johnson, 2006), and a greater cumulative amount of nitrogen available at depth in the soil may be the answer (i.e. a 'bigger box' of nitrogen when deeper soil depths are considered)."

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.

Iversen, C.M., Ledford, J. and Norby, R.J. 2008. CO2 enrichment increases carbon and nitrogen input from fine roots in a deciduous forest. New Phytologist 179: 837-847.

Johnson, D.W. 2006. Progressive N limitation in forests: Review and implications for long-term responses to elevated CO2. Ecology 87: 64-75.

Liberloo, M., Calfapietra, C., Lukac, M., Godbold, D., Luo, Z.-B., Polle, A., Hoosbeek, M.R., Kull, O., Marek, M., Raines, C., Rubino, M., Taylor, G., Scarascia-Mugnozza, G. and Ceulemans, R. 2006. Woody biomass production during the second rotation of a bio-energy Populus plantation increases in a future high CO2 world. Global Change Biology 12: 1094-1106.

Pritchard, S.G., Strand, A.E., McCormack, M.L., Davis, M.A., Finzi, A.C., Jackson, R.B., Matamala, R., Rogers, H.H. and Oren, R. 2008a. Fine root dynamics in a loblolly pine forest are influenced by free-air-CO2-enrichment: a six-year-minirhizotron study. Global Change Biology 14: 588-602.

Pritchard, S.G., Strand, A.E., McCormack, M.L., Davis, M.A. and Oren, R. 2008b. Mycorrhizal and rhizomorph dynamics in a loblolly pine forest during 5 years of free-air-CO2-enrichment. Global Change Biology 14: 1-13.

Thornton, P.E., Lamarque, J.F., Rosenbloom, N.A. and Mahowald, N.M. 2007. Influence of carbon-nitrogen cycle coupling on land model response to CO2 fertilization and climate variability. Global Biogeochemical Cycles 21: 10.1029/2006GB002868.