Maine is a sunny state. I would suggest a simple DC solar panel collector, and a computer fan. You can find (cheaply) cooling fans that move 5 cfm that run off straight DC current. Wire directly to the collector. No charger or controller needed. Sun up? Fans on. Sun down, fan off. Cloudy day, slower fan. How many cubic feet is your root cellar? Simple math will tell you how many times you will turn over the airspace.
Do you have vent pipes with a low input and high output to get the air to move?
"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
We have a semi-basement in our house. The floor is/was a thin (1-2 in) concrete with loam below.
The humid and warm summer air was leaving condensation on the floor and the walls. Also the humidity from the loam was drawn into the brick walls (no moisture barrier on the interior walls).
After reading a lot of opinions on that subject and talking to different people, receiving different answers each time, I decided to remove the concrete floor in one room.
The condensing water goes now directly into the loam floor, limiting the humidity to the moisture level in the ground.
We also removed the old paint and plaster and applied earthen plaster. The air quality has improved so far. (There are tree more rooms to convert, so the real effect isn't visible yet.)
If taking out the stone floor is possible, you could replace it with an earthen floor..