Why does frost increase soil fertility?
I found this article on why frost increase soil fertility - specifically nutrient density in the soil.  101 years old!
The Occurrence of Bacteria in Frozen Soil
E. C. Harder  Botanical Gazette  Vol. 61, No. 6 (Jun., 1916), pp. 507-517
https://www.jstor.org/stable/2469064?seq=3#fndtn-page_scan_tab_contents
Relevant text pasted below.  In short: it is bacterial action and nitrogen fixation which contributes to soil fertility.  Freezing increases bacterial population and these metabolic activites.  The theory is that freezing kills bacterial predators on higher trophic levels (protozoans) but the bacteria live and thrive in the absence of predators.

An analogous hypothesis proposed by RUSSEL3 for increases in the number of bacteria after partial sterilization by heat, frost, or other means is that by such partial sterilization the protozoa are killed, thus permitting the unhindered development of bacteria which under normal conditions is held in check by protozoa.
BROWN and SMITH (loc. cit.) in their investigations dealt mainly with the physiological activities of bacteria under conditions of low temperature and frost, although they also made some determinations of the number of bacteria in frozen soil. Their principal conclusions regarding the ammonifying, nitrifying, denitrifying, and nitrogen fixing powers of frozen soils are as follows: (1) that &quot;frozen soils possess a much greater ammonifying power than unfrozen soils&quot;; (2) that &quot;during the fall season, the ammonifying power of the soil increases until the temperature of the soil almost reaches zero, when a decrease occurs, and this is followed by a gradual increase and the ammonifying power of the soil reaches a maximum at the end of the frozen period&quot;; (3) that &quot;the nitrifying power of frozen soils is weak and shows no tendency to increase with extension of the frozen period&quot;; (4) that &quot;frozen soils possess a decided denitrifying power which seems to diminish with the continuance of the frozen period&quot;; (5) that &quot;during the fall season, the denitrifying power of the soil increases until the soil freezes, after which a decrease occurs&quot;; (6) that &quot;frozen soils possess a nitrogen fixing power which increases with the continuance of the frozen period, being independent of moderate changes in the moisture conditions, but restricted by large decreases in moisture&quot;; and (7) that &quot;in the fall, the nitrogen fixing power of the soil increases until the soil becomes frozen, which in almost ceases, after which a smaller nitrogen fixing power is established.&quot;