Great thread guys...Tyler and Robert, your thoughts have brought to mind what I read in Noss' Redwood Ecology text:
The rate of decomposition is inversely proportional to the retention of carbon and nitrogen in the soil.
I interpreted this to indicate that the extremely slow (upwards of 2x the life of the tree) but efficient decomposition of redwood is a big part of why the coastal redwood forests have the highest biomass on Earth by a large margin. This biomass over 50% more than the closest other terrestrial ecosystem (in Hoh Rainforest of Olympic NP), and 5-10x that of a tropical rainforests where decomposition is rapid. Alongside the slow decomposition of the redwood (which is really great for growing redwood and things that redwoods benefit from but not much else), is a plethora of other, faster decomposing biomass. This must create edge effects abound, and may be why the soil life in these ancient forests is the most diverse known (again, my memory of Noss), with 15,000 soil invertebrate animal species/m2 in some measurements.
My hypothesis is that the fact that available Nitrogen is the limiting factor for maximal biomass in these forests (again, Noss) has selected for this slow decomposition dynamic in the redwood ecosystem, as the immense biomass of these forests is how they keep enough water in the system to remain catastrophic fire-proof despite dry summers. The lushest parts of the redwood coast have no history of catastrophic fire. The ecosystems that got past a tipping point of biomass to environmentally self-regulate through their own fog and rain creation and temperature moderation became fire proof. Those ecosystems had to have trees with tannins potent enough to slow decomposition to the point where it became extremely Nitrogen efficient and rich in nitrogen fixating plants and fungus. Tadaa... Redwood forests!
In regard to the question of sequestration it seems like hugelkulture is analogous to a safe, high-return short term investment that can form the foundation for a longer term investment strategy. The carbon forms the anchor/sponge for all the other components of humus to latch on to, and in turn grows plants and fungus that sequester more carbon and nitrogen and produce more organic matter for more hugel.