[dice]

Actinovate has a minimum soil temperature of around 53F or
something close to that. Down 10', the soil is always at that
temperature, but the top foot gets quite a bit colder than that
in northern winters, so I do not know if it would survive until
spring on the roots of cold-tolerant winter cover crops. Maybe.

Other beneficial microbes will, though. Many rhizobia, including
nitrogen-fixing rhizobia that infect root nodules in cold-tolerant
legumes (vetches, winter dormant alfalfa, fava beans, many
clovers, etc), and free nitrogen fixing bacteria (azospirillum,
azotobacter, etc) that merely inhabit root zones (without
stimulating nodulation in legumes) can apparently take colder
temperatures without being eradicated from the soil (although
they are not very active at cold temperatures).

So it seems to me, too, that keeping a diversity of live roots
in the typical rhizosphere of your crop plants through winter
and summer keeps populations of beneficial microbes high.
That paper described azospirillum populations for example
as 100 times higher in a root zone (rhizosphere) than in bare
soil without live roots. It lives mostly on stuff exuded from
the roots of plants. I bet that is true of many other beneficial
microbes as well. The roots are where they find their food
in greatest abundance.

I had read long ago that plants needed nitrogen in nitrate form
to use it, but the nitrogen fixed by nitrogen-fixing rhizobia
in the nodules on the roots of legumes is ammonia compounds.
I wonder whether the legumes use it directly or convert it to nitrates
internally before use. High levels of ammonia in tomato plants has
been implicated by some studies as a contributing factor in BER.

There are some archaea from the sea (part of Biozome) that
use ammonia compounds as an energy source and produce
nitrates as a metabolic byproduct. There may be bacteria or
other microbes in the soil that do this as well.