What was done
Twelve identical mini-ecosystems -- each consisting of three longleaf pine (Pinus palustris) seedlings, three wiregrass (Aristida stricta) C4 grass plants, two sand post oak (Quercus margaretta) seedlings, one rattlebox (Croatalaria rotundifolia) C3 perennial herbaceous legume, and one butterfly weed (Asclepias tuberose) herbaceous C3 dicotyledonous perennial -- were grown for three years under natural meteorological conditions in an outdoor soil bin at the National Soil Dynamics Laboratory in Auburn, Alabama (United States) within 12 open-top chambers (half of which were maintained for three years at 365 ppm CO2 and half of which were maintained at 720 ppm CO2 for the same period), during which time the "standing crops" of fine-root length, rhizomorph length and number of mycorrhizal root tips were assessed in the upper (0-17 cm) and lower (17-34 cm) halves of the plants' root zones at 4-week intervals via micro-video cameras installed within each of two mini-rhizotron tubes located within each of the 12 plots into which the soil bin was divided.

What was learned
The greatest impacts of the 97% increase in the air's CO2 content were generally observed in the lower halves of the ecosystems' root zones, where the standing crops of fine roots, rhizomorphs and mycorrhizal root tips were increased by 59%, 66% and 64%, respectively, although the mean standing crop of rhizomorphs in the upper halves of the ecosystems' root zones was increased by 114%.

What it means
McCormack et al. say that as the atmosphere's CO2 content continues to rise, "greater biomass production in deeper soils in the coming decades has the potential to contribute to greater carbon storage in forest soils," since "carbon in deeper soil turns over (decomposes) more slowly than litter nearer the soil surface," citing the work of Trumbore (2000) and Schoning and Kogel-Knabner (2006). In addition, they note that "fungal tissues consist largely of chitin, a potentially recalcitrant compound that may build up soil organic matter and persist for long periods of time relative to more labile carbon," citing Langley and Hungate (2003). Thus, they suggest that "regenerating longleaf pine-wiregrass systems may act as a carbon sink as atmospheric CO2 rises in the coming decades through increased biomass production and potentially through directed allocation of carbon to deeper soils," which they note is "consistent with the recent assertion that greater allocation of forest carbon to deeper soil is a general response to atmospheric CO2-enrichment," citing the work of Iversen (2010). And, very importantly, they state that "significant increases in mycorrhizae and rhizomorphs," as they found in their study, "may explain why the magnitude of the increase in forest net primary productivity caused by elevated CO2, in several long-term demonstrably nitrogen-limited FACE experiments, has not decreased after nearly a decade (Finzi et al., 2007)," helping to explain why the progressive nitrogen limitation hypothesis has been shown to be wrong, time after time after time.

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. 2010. Digging deeper: fine-root responses to rising atmospheric CO2 concentration in forested ecosystems. New Phytologist 186: 346-357.

Langley, J.A. and Hungate, B.A. 2003. Mycorrhizal controls on belowground litter quality. Journal of Ecology 84: 2302-2312.

Schoning, I. and Kogel-Knabner, I. 2006. Chemical composition of young and old carbon pools throughout Camisol and Luvisol profiles under forests. Soil Biology and Biochemistry 38: 2411-2424.

Trumbore, S. 2000. Age of soil organic matter and soil respiration: radiocarbon constraints on belowground C dynamics. Ecological Applications 10: 399-411.