What's the effect of freezing and thawing on soil microorganisms?

Freezing can kill off many soil microbes up to a certain depth, though this is not necessarily bad because some of these microbes may be pathogenic like fusarium.  Other microbes are capable of going dormant in freezing temps.

For germination:  seeds of wild plants generally require stress and water to soften the seed coat so the seed may germinate.  Those left under snow will freeze, but once they thaw, they will germinate readily as long as the soil temperature is high enough.

Certain trees and shrubs (mostly fruit trees) require a stretch of time with freezing temperatures or they do not produce fruit (or harvest is severely limited).  Their reproduction cycle need cold to function properly.  So, global warming can adversely affect these crops.