Background
The authors write that "temperate forests are assumed to be organic carbon (OC) sinks, either because of biomass increases upon elevated CO2 in the atmosphere and large nitrogen deposition, or due to their age structure." Yet they say that "respective changes in soil OC and total nitrogen (TN) storage have rarely been proven," and that "it is still unresolved where the carbon goes," citing Ballantyne et al. (2012) and Levin (2012).

What was done
In an attempt to resolve some of these issues, Schrumpf et al. "analyzed OC, TN and bulk densities of 100 soil cores sampled along a regular grid in an old-growth deciduous forest at the Hainich National Park, Germany, in 2004 and again in 2009."

What was learned
The three German researchers found that "concentrations of OC and TN increased significantly from 2004 to 2009, mostly in the upper 0-20 cm of the mineral soil, with average annual accumulation rates of 65 ± 29 g OC/m²/yr and 7.8 ± 2 g N/m²/yr." And they say that the soil OC increases "are estimated to be ~10% of aboveground C gains," and that both the "soil OC and TN gains at the Hainich site seem to be due to increasing litter input, likely because the forest is accumulating biomass."

What it means
Schrumpf et al. conclude that their work "supports earlier studies indicating that the old-growth forest at the Hainich National Park is still accumulating OC in soil." And they say that "it adds to the growing number of studies showing that soils of temperate forests are currently C sinks."

References
Ballantyne, A.P., Alden C.B., Miller, J.B., Tans, P.P. and White, J.W.C. 2012. Increase in observed net carbon dioxide uptake by land and oceans during the past 50 years. Nature 488: 70-72.

Levin, I. 2012. The balance of the carbon budget. Nature 288: 35-36.