Hugelkultur meets biogas?

Does anyone know if Hugelkultur bed decomposition is aerobic or anaerobic?  If anaerobic, it would be interesting to put tubes into the beds to siphon off the biogas.  This assumes soil on the outside of the bed could contain the biogas.  But in theory, small plot of land (half-acre or so) with large Hugelkultur beds could produce lots of food on the top while at the same time producing biogas underneath.  Wood decomposes relatively slowly, but could slowly and steadily produce biogas for years with no foreseen downside to the plants above.  Any thoughts?

So basically the same scenario as like a landfill, minus the harmful inputs?

Yes, to give credit, I was inspired by my recent tour of the Monterey waste management facility where they currently generate 5 MW per day from biogas.  http://www.mrwmd.org/. ; If it works, the beauty of it would be that all of the hugelkultur benefits of water retention,micro-climates, etc would still be intact, but you just created a relatively easy energy stream to heat or power your house.

I like it. A clay liner (on top) and some corrugated drain tile would be a pretty simple thing to put together.

To clarify, I was going for more Jean Pain meets hugelkultur than landfill meets hugelkultur.  http://journeytoforever.org/biofuel_library/methane_pain.html. ; Jean used much bigger piles, so I don't know if it would work, but if someone is building a large hugelkultur bed it may be worth the experiment to throw a perferated pipe into the pile and see what happens.

There's usually a mix of aerobic and anaerobic processes in healthy soil, but the anaerobic processes in a biogas digester are much more extreme than that.

I think harvesting the biogas means isolating the decomposing material pretty completely from anything aerated: the parts currently generating harvestable biogas won't be available to tree roots.

IIRC, Jean Pain used a sealed tank in the center of the aerobic pile to generate his biogas.