Do you have any more recent pics for your Hugel beds? Be great to see and hear how they've performed since you put them in.
How long ago did you build them? Also, where's the topsoil from?
If it is fungi that break down the wood in hugelkulture, then the process won’t include using lots of nitrogen. It is the bacterial break down of organic material that uses so much nitrogen. This may mean the problem of your logs absorbing nitrogen does not exist as it does with mulches, with logs buried and damp it is possible that they will be rotted by funguses that don’t not need nitrogen.
What worries me about using wood, is not that, the nitrogen used to rot the wood will leave little nitrogen for plants. It is having heard, years ago that the fungi that parasites on dead wood might spread to live wood. If different fungi parasite dead wood than live wood this does not matter, if you have a parasitic fungus on your wood in your hugelkulture maybe it does.
I read a gardening book but I can’t remember which, that talks about using urine, if you have it, to provide the nitrogen, bacteria digesting organic material need. It says that a straw bale will turn to sweet earth in three months if you pee on it. Another book says that you can keep a family in cabbages with the same resource if there are two men or two unprudish females in the family.
What do the panel feel the minimum depth of soil is required to cover a hugel "core" when planting trees?