What was done
Wheat and soybean plants were grown in open-top field chambers exposed to CO2 concentrations of 350 and 500 ppm with or without 35 ppb ozone at two soil moisture levels from 1994 to 1996.  By means of chemical and physical fractionation and spectroscopic analyses, the authors assessed the impacts of these treatments on qualitative and quantitative changes in soil organic matter in these wheat-soybean agroecosystems.

What was learned
Soil particulate organic carbon content was increased under atmospheric CO2 enrichment, as was the potential for the sequestration of a greater proportion of unstable carbon.  Also increased were the amounts of dissolved carbon and humic and fulvic acids present in the soil solution.  Elevated ozone levels tended to produce the opposite effects; but they were usually overpowered by the effects of elevated CO2.

What it means
In the words of the authors, "one of the main benefits arising from the greater supply of organic residues to soils under [atmospheric] CO2 enrichment is an improvement of soil structure."  And better soil structure makes for better plant growth and bigger crop yields.  So, as the air's CO2 content continues to rise, that is what we should see, in addition to a slowing of the rate of rise of the atmosphere's CO2 concentration, as greater amounts of carbon are stored in the world's soils.