Horticultural scientists and research has not supported the use of hugelkultur, in part because the layering of coarse material under finer material (soil) is opposite of what would be found in natural systems, and it can create a perched water table that may stress or damage roots in the soil above the coarse material. The microbes decomposing wood also require a nitrogen source, and they will take it from the soil surrounding the wood as they work, potentially robbing nearby roots of that important nutrient. Organic matter in the soil does provide the benefits of moisture and nutrient retention, but as particles already broken-down to the point where they can be mixed evenly with the soil, not as still-composting coarser material.

Given those caveats, we do not recommend that you bury fallen leaves and instead reserve them for use on the surface as a mulch, which not only is where they should be decomposing, but the resource will also be valuable food and shelter for a variety of wildlife until the leaves have decayed completely. (In fact, many of those same decayer organisms may not be able to reach the leaves if they are buried.)

If you have a raised bed or other above-grade method of planting that you would like to use, we recommend amending the soil with compost and topsoil instead, as they are more uniform materials that will promote better root growth. While it's true they can subsist (sink and compress a bit) over time, the same would happen with buried woody material and leaves (except to a greater extent in that case). Any time layers of soil differ in composition and coarseness, the water draining through it will be affected, and certain layers may stay too wet while others stay too dry, stressing or damaging roots moving from one layer to another. It also might create an uneven distribution of nutrients, as different materials can bind nutrients to different degrees. Generally, the more uniform the root zone, the better.

Miri

Thank you for this thorough answer. I will have to rethink how I'm going to fill this bed now. :-)
Take care. 

You're welcome.