What was done
At the Sky Oaks CO2 enrichment site of San Diego State University (California, USA), which is located in chaparral vegetation dominated by chemise (Adenostoma fasciulatum) shrubs, twelve 2-m by 2-m by 2-m closed chambers were constructed so as to contain a central individual Adenostoma shrub and its surrounding herbaceous plants. Beginning in December 1995, these chambers were maintained at six atmospheric CO2 concentrations ranging from 250 to 750 ppm in 100-ppm increments. Then, at various times throughout 1999, measurements of net ecosystem exchange of CO2 were made, while soil samples were collected for analyses of arbuscular mycorrhizal (AM) fungi and sequestered carbon found within both bulk soil and water-stable aggregates.

What was learned
The authors report that "plants and soils within the chambers took up more carbon under CO2 enrichment." Specifically, they say that "chambers exposed to 250 to 550 ppm CO2 released an average of 703 g C m^-2 year^-1," while "chambers in the 650-750-ppm treatments absorbed an average of 160 g C m^-2 year^-1 [our italics]." Likely driven by these dramatic CO2-induced differences in net ecosystem exchange of CO2, it was not surprising, in their words, that "pools of total carbon in bulk soil and in water-stable aggregates increased 1.5- and three-fold, respectively, between the 250- and 650-ppm treatments." In addition, they found that "the abundance of live AM hyphae and spores rose markedly over the same range of CO2."

What it means
Treseder et al. conclude that the augmentation of carbon pools found in their study, "if common in other ecosystems, appears substantial enough to influence sequestration of CO2 originating from fossil fuel burning and deforestation." And, of course, the nature of that "influence" would be to greatly increase soil carbon sequestration.