Agreed with Belay, Amrani and Lokhandwala. All the methods are good one and may be followed.
There are certain things which are more important while storage of AM fungi.
1. The moisture content of the inoculum while storage as such in deep freeze.
2. Shelf life of your inoculum.
The most critical point is the percentage recovery after the storage. Because, -20C storage in glycerol or mineral oil or in sterile water are not sometimes recoverable. Hence, 4C as soil may be recommended, but with optimum moisture and shelf life should be assayed for each inoculum.
I haven't ever gone through -20 oC preservation of AM fungi because I am a learner of this field and currently working on culture and taxonomy so all the inoculum I have, is in the form of running culture. So, It is hard to say, that "deep freeze could kill AMF".
Our colleague (working on mycelial endopytes, i.e., Non-AMF) have experienced that, only few fungi could be revived after preservation in glycerol/mineral oil/sterile water at -20 oC in comparison to that stored at 4 oC (personal observation).
While deep freezing of living cells following stages may occur:
1. Extracellular ice formation: cooling below freezing point may cause water to come out from the cell consequently extracellular ice formation, that may crush the cell membrane. This may also lead to the dehydration of the cell, and hence, cell damage.
2. Intracellular ice formation: formation intracellular ice often fatal to the living cells.
That is why, I have suggested to store at 4 oC.
After all a scientific evaluation could be made by comparing all the methods of preservation for optimization of storage of AMF inoculum (for commercialization).