I think that deep rooting is a common feature of most dynamic accumulators, however this isn't always the case. Commonly, the plants are drawing minerals from the subsoil and concentrating them in their bodies. It is possible that plants are drawing in the nutrients they concentrate from the surface as well.
If anyone wants see a grand list of dynamic accumulators and what they accumulate, check out
http://oregonbd.org/Class/accum.htm.
Dave