What was done
In the words of the authors, "free air CO2 and O3 enrichment (FACE) technology was used at the Aspen FACE project in Rhinelander, Wisconsin [USA] to understand how elevated atmospheric CO2 and O3 interact to alter pCO2 and DIC [dissolved inorganic carbon] concentrations in the soil."  The experimental setting consisted of three blocks of four treatments - control, elevated CO2, elevated O3, and elevated CO2 + O3 - where ambient CO2 was 360 ppm, elevated CO2 was 542 ppm, ambient O3 was 33 ppb, and elevated O3 was 49 ppb, and where half of each FACE ring was planted with trembling aspen, a quarter with a 1:1 mix of trembling aspen and paper birch, and a quarter with a mix of trembling aspen and sugar maple.  The CO2 and O3 treatments were applied over the 2002 growing season, which ran from 28 May to 11 October, during which period bi-weekly samples of the soil air and solution were retrieved from depths of 15, 30 and 125 cm for various chemical analyses in the laboratory.

What was learned
Karberg et al. report that "measured concentrations of soil CO2 and calculated concentrations of DIC increased over the growing season by 14 and 22%, respectively, under elevated atmospheric CO2 and were unaffected by elevated tropospheric O3."  In addition, they say "the increased concentration of DIC altered inorganic carbonate chemistry by increasing system total alkalinity by 210%, likely due to enhanced chemical weathering [of primary minerals]," and they note that a mixing model they employed "showed that new atmospheric CO2 accounted for approximately 90% of the C leaving the system as DIC."

What it means
The Michigan scientists state that the CO2-induced increase in soil solution DIC, which ultimately makes its way to rivers that reach oceans, "represents a potential long-lived sequestration reservoir in deep ocean sediments," noting further that it suggests that "aggrading forest ecosystems may be used to capture and sequester atmospheric CO2 through inorganic processes," which have the potential to transfer it to the bottoms of faraway seas.  See also, in this regard, our reviews of the studies of Raymond and Cole (2003) and Wang and Cai (2004).  And, of course, it is important to note that this phenomenon can be significantly enhanced by increases in the air's CO2 concentration.

References
Raymond, P.A. and Cole, J.J.  2003.  Increase in the export of alkalinity from North America's largest river.  Science 301: 88-91.

Wang, Z.A. and Cai, W.-J.  2004.  Carbon dioxide degassing and inorganic carbon export from a marsh-dominated estuary (the Duplin River): A marsh CO2 pump.  Limnology and Oceanography 49: 341-354.