What was done
The authors explored the fate of carbon (C) and nitrogen (N) in the soil of a sweetgum tree (Liquidambar styraciflua L.) plantation in Oak Ridge, Tennessee (USA) at the conclusion of a nearly 12-year free-air CO2 enrichment (FACE) study.

What was learned
The four researchers report that net primary productivity increased in response to atmospheric CO2 enrichment during their long-term study "even though production in the sweetgum stand was limited by soil N availability," and they say that "the majority of the additional C fixed under elevated CO2 was allocated belowground to the production of fine roots," with the largest increases in root biomass production and mortality being "at relatively deep soil depths (i.e., below 30 cm)," with the end result that "soil C and N contents were greater throughout the soil profile under elevated CO2 at the conclusion of the experiment." With respect to the former of these elements, for example, they state that "soil C content was ~19% greater throughout the soil profile after more than 11 years of CO2 enrichment."

What it means
Iversen et al. conclude that "the greater residence time of C in deeper soil indicates that inputs from deep roots under elevated CO2 may increase the potential for long-term storage of C and N in forested ecosystems," adding that "this finding suggests greater C accrual in elevated CO2 compared with ambient CO2 during the experiment, consistent with the conclusion of a meta-analysis that indicated increased ecosystem C storage under elevated CO2 (Luo et al., 2006)," all of which observations presage, in their words, "the potential of future forests to store C and mitigate some portion of rising atmospheric CO2," as they state in the concluding sentence of their report.

Reference
Luo, Y., Hui, D. and Zhang, D. 2006. Elevated CO2 stimulates net accumulations of carbon and nitrogen in land ecosystems: a meta-analysis. Ecology 87: 53-63.