What was done
The authors reviewed the scientific literature to determine how soil carbon sequestration rates change with changes in land-use and soil management.  In particular, they reviewed studies that measured changes in soil organic carbon sequestration rates when abandoned agricultural fields were succeeded by either woody plants/forests (48 data points) or grasses (30 data points).

What was learned
From analyzing the results of these several studies, the authors determined that soil organic carbon accumulation rates increase from temperate to subtropical regions, most likely due to greater organic matter inputs, which increase with temperature and moisture along this gradient.  Typical rates of soil organic carbon accumulation were similar for re-established forests and grasslands (about 33.5 g of carbon per square meter per year) and can be enhanced by increasing soil organic matter inputs, placing organic matter inputs deeper into the soil profile, and by reducing tillage, which physically breaks up soil aggregates and exposes organo-mineral complexes to decomposers.

When applying this soil organic carbon accumulation rate to the entire land surface of the United States that has been reclaimed from croplands and converted into forests and grasslands over the past 50 years, the authors calculated a soil organic carbon accumulation rate of 0.05 Pg of carbon per year (1 Pg = 10¹⁵ g), which is but a small, yet significant, fraction of the 1-2 Pg of carbon per year rate of storage that has been inferred to be occurring for terrestrial ecosystems in the Northern Hemisphere.  Thus, if the Northern Hemispheric soil organic carbon accumulation rate of 1-2 Pg of carbon per year is correct, nearly all of that carbon is being stored within plant biomass and surface litter, and not within soils.

What it means
Abandoned agricultural fields that have been reforested or converted into pasturelands will likely possess vegetation that will exhibit increasing growth rates as the air's CO2 concentration continues to rise.  With additional biomass accumulating in such vegetation as time progresses, it is likely that some of its fixed carbon will work its way into the organic matter component of soils, where it can be stored for longer periods of time.  However, the present review paper suggests that nearly all of the Northern Hemisphere's current carbon sink exists within vegetative biomass, rather than soils.  Nevertheless, substantial gains in soil organic carbon can still be achieved by converting croplands into forests or grasslands; and, eventually, a goodly portion of the ongoing increase in aboveground carbon storage will make its way into the ground, greatly augmenting what is there now.