Nitrogen-rich Bedrock and Carbon Sequestration

Scientific American:

Nitrogen-rich forest bedrock -- the geologic rock formation located under forest soil -- may aid trees in better sequestering carbon, according to a recent study that offers a new understanding on why some forests store greenhouse gases more efficiently than others.

While geologic rock isn't a carbon sink itself, it plays an important role in helping the soil and trees above absorb CO₂, say the study's authors, who published their findings last week in Nature. But a lack of research on nitrogen has left it largely ignored by climate scientists and policymakers scrambling to identify carbon sinks that mitigate carbon dioxide pollution from large emitters.
A group of University of California, Davis, biogeologists observed two very similar, adjacent forests in Northern California -- South Fork Mountain and the Bear Wallow Diorite Complex.
Both forests had similar soils, vegetation, rainfall and temperature. But the rock below the South Fork Mountain forest was made of nitrogen-rich mica schist, derived from marine sediments from the early Cretaceous Period.
The results were significant: South Fork Mountain held 42 percent more carbon in its trees, as observed in analysis of carbon in leaves and soil, than Bear Wallow.

Without locking the CO2 in a more stable location, I don't see how this information helps much.  We may be able to find places where trees grow better and suck up more carbon, but if it isn't transferred into the soil, won't it be released back into the atmosphere when the trees die? Or is some of the carbon transferred to the soil if the trees decay?  I guess if it is trapped in the soil as organic matter when the trees decay, that might make some sense.