Just building my root cellar now, so i've been doing a bit of reading...'root cellaring' by Bubel seems pretty good. They make a pretty strong argument that proper vents will allow you to control temperature and humidity better. Ideally, they suggest an inlet and exhaust vent. Any scrappy scrounged pipe would work. Cellars larger than six by eight go with 6 inch pipe, otherwise 4 is ok.
The exhaust pipe enters the cellar at the ceiling, the intake pipe comes in at floor level so convection will drive the venting..the two pipes are preferably in opposite corners to encourage airflow across the cellar. Try to space shelving off walls and avoid dead air spaces that get no circulation. Dead air can mean condensation and moulding. If possible have the pipes cut flush where they enter the cellar wall to prevent dead air spaces there.
You'll have to be able to control air throw through the vents by blocking them off with something, perhaps something insulated, to keep things from getting to cold in the cold snaps.
I think i'll give the two vents a try, it doesn't seem like much extra expense or bother.
Intake can be the threshold under the door, but two pipes definitely work better. In MN, you will definitely want a way to close them off to keep from getting too cold. Double airlock doors and a way to heat if it gets close to freezing are important. A candle is probably enough for most people for the few days it is a problem, but a lightbulb is a lot more convenient if you are way north--you can even rig it to a Tstat to turn on automatically when the temp drops.
"You must be the change you want to see in the world." "First they ignore you, then they laugh at you, then they fight you, then you win." --Mahatma Gandhi
"Preach the Gospel always, and if necessary, use words." --Francis of Assisi.
"Family farms work when the whole family works the farm." -- Adam Klaus