Fungi and other soil microbiota in Hugelkultur

I've been giving hugelkultur a certain amount of thought, in the context of making it work, especially in the light of my recent reading on soils. I know a lot of people have had trouble making hugelkultur work, while others swear by it.

My own trials involving burying woody trimmings from my hedge (not hugelkultur) have not been a success.

I've been wondering if one of the factors involved may be the microbiota in the pile, specifically the balance between fungi and bacteria.

Okay, some basics for people who don't know about this. In general, annual plants like, and produce root exudates to encourage, a soil dominated by bacteria.

Woody perennials like, and produce root exudates to encourage, a soil dominated by fungi. You'll probably find almost as many bacteria in a forest soil as you will in a soil growing annual plants, but you'll find many times the mass of fungi in the former than in the latter.

So, hugelkulture involves a woody base, usually with more rapidly decomposing nitrogen-rich additions (which will be decomposed by a range of microorganisms, maybe including secondary decomposer fungi, but these would probably not be necessary) and soil. In other words, the substrate is a mix between additions that encourage fungi and additions that encourage bacteria.

With few exceptions, bacteria can't break down lignin in wood. In general, the only things able, certainly effectively in the short to medium term, to tackle the woody base will be some invertebrate soil fauna (but most of the suitable ones won't be able to get at it since you buried it), primary decomposer fungi, followed by secondary decomposer fungi, followed by, maybe, exudates from mycorrhizal fungi which might become established on your trees, after a while.

So, is it worth inoculating some of your logs with at least suitable primary decomposer fungi before building the hugelkultur bed? Alternatively do you just accept that the wood is going to stay there (hopefully, theoretically, wicking water), sequestering carbon, and break down very slowly under the actions of the few bacteria that can decompose the stuff, and treat the bed as something that creates lots of microclimates in your ecosystem?

Is it possible one of the reasons some people have had trouble with this technique is the existing balance of soil microbiota in the pile lacking suitable fungal decomposers?

Neil Layton wrote:Alternatively do you just accept that the wood is going to stay there (hopefully, theoretically, wicking water), sequestering carbon, and break down very slowly under the actions of the few bacteria that can decompose the stuff, and treat the bed as something that creates lots of microclimates in your ecosystem?

This is my attitude toward buried wood. I don't expect the wood itself to feed the plants I'm growing on top of it. I'm fine if the wood just sits there. It seems to be helping by holding moisture.

Hi Neil
From all that I have read, it seems that fungi are the only organisms that can break down lignin. I have read scattered statements that bacteria have been shown to, but, then again, the majority of studies I have read state that this is purely a fungal process.

Thus, I would say that the wood sitting these is being decayed by fungi. As such, I would (and do) suggest inoculating buried wood with preferred mushroom-forming white rot fungi if you wish to speed up the decay process and potentially get some mushrooms in return (if these logs are close to the soil horizon).

Is it possible one of the reasons some people have had trouble with this technique is the existing balance of soil microbiota in the pile lacking suitable fungal decomposers?
Hard to say. Fungi initiate a cascade of succession in soil and avobeground ecologies. Without these keystone species, various other dynamics may not be possible, thereby inhibiting the support of the plants and other organisms.

Peter