What was done
The authors quantified carbon (C) and nitrogen (N) inputs to the soil from fine-root mortality under ambient (380 ppm) and elevated (560 ppm) atmospheric CO2 concentrations at several soil depths in a deciduous sweetgum (Liquidambar styraciflua) forest in eastern North America (the Oak Ridge National Laboratory FACE facility) by combining allometric relationships derived from samples of individual roots with a long-term minirhizotron dataset, which enabled them to estimate root biomass and N content from root length and diameter, after which they used these relationships to calculate annual and cumulative root biomass and N input as a function of soil depth over the entire span of the study, which to date has covered the nine-year period running from 1998 to 2006.

What was learned
Iversen et al. say that the approximate 47% increase in atmospheric CO2 concentration "nearly doubled the production of root biomass, and, contrary to what we expected ... it appeared that the largest increases in root production under elevated CO2 were deeper in the soil." In addition, and also "contrary to expectations," they report that "root N did not decline under elevated CO2."

What it means
Also contrary to the early expectations of many scientists -- see Nitrogen (Progressive Limitation Hypothesis) in our Subject Index -- the three researchers state that "limited soil N availability has not constrained forest production or stand N uptake in response to elevated CO2 thus far in any of the forested FACE experiments (Norby et al., 2005; Finzi et al., 2007; Iversen and Norby, 2008)." In fact, they say that increased soil exploration by fine roots has "facilitated greater N acquisition under elevated CO2 in forested ecosystems (Norby et al., 2004; Norby and Iversen, 2006; Finzi et al., 2007; Pritchard et al., 2008), and root proliferation could further stimulate soil N availability throughout the soil profile by supplying a 'fresh' source of organic matter and energy to the microbial community (cf. Fontaine et al., 2007)." The findings of the current study thus provide additional support for the emerging positive view of the ability of atmospheric CO2 enrichment to induce a long-term stimulation of growth and carbon sequestration by earth's forests.

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.

Fontaine, S., Barot, S., Barre, P., Bdioui, N., Mary, B. and Rumpel, C. 2007. Stability of organic carbon in deep soil layers controlled by fresh carbon supply. Nature 450: 277-280.

Iversen, C.M. and Norby R.J. 2008. Nitrogen limitation in a sweetgum plantation: implications for carbon allocation and storage. Canadian Journal of Forest Research 38: 1021-1032.

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.

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.

Norby, R.J., Ledford, J., Reilly, C.D., Miller, N.E. and O'Neill, E.G. 2004. Fine-root production dominates response of a deciduous forest to atmospheric CO2 enrichment. Proceedings of the National Academy of Sciences, USA 101: 9689-9693.

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. 2008. 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.