What species do you lack?
What species are in excess?

Tell me that and I can tell you what amendments to add.

The Soil Food Web feeds on organic matter in the soil, so to increase those wee critters and keep them active you7 need to add organic matter, whether from compost or other forms of vegetative waste. What you use depends on what is most readily available to you.
Canada, like the United States, is a big place with many different soils and climate types, so something like Saskatchewan, or the Yukon Territories would be of more help then simply Canada.

Here is a link that might be useful: Soil Biology Primer

lazygardens,
I don't know what I lack or have in excess. All I'd like to do is keep them healthy and in good diversity *without* adding compost for the reasons already stated.

I don't know what I lack or have in excess.

So, for all you know it may be perfectly balanced and teemingly abundant (for its soil type). Resist the urge to do "stuff" unless you know why you are doing it, and what the results of the application will be. The old adage, "If it ain't broke, don't try to fix it" applies to gardening.

Watch your garden for a year or two. Look at what your plants are doing - are they reasonably vigorous? Do you have spots that drain differently than others?

Have a real soil test done by a good lab ... fix any glaring deficiencies.

Instead of trying to compost in bins, use "trench composting" with what materials you have.

Wheat straw or wood chips as a mulch might be beneficial, and cheap.

Is tilling in deciduous leaves an option? They feed fungi, but also bacteria - not to mention earthworms. This is my modus operandi, except that I spade them in rather than till. In the past (in sandy soil) I have also used mixtures of wood chips, manure and leaves.

Thanks for the suggestions. I have lots of trees, so I expect lots of leaves. Mostly poplar - are there any allelopathic effects possible?

none (adding random characters to avoid rejection).

Slimy, at the risk of having tomatoes thrown at me for mentioning biodynamics (throw Sun Golds, please), I'll tell you that I really like the Pfeiffer BD field and garden spray for increasing soil bacteria. I've read the research and feel it's of value, and one packet lasts for years (I got it from the Josephine Porter institute). The research I've read on Pfeiffer is impressive, and have read studies that showed higher levels of soil life in BD soils when compared to organic soils. I'm sure it can all be disputed, whatever, but I like using it -- and it does not have to be stirred like the other BD stuff.