[John K]

Assuming the substrate is moist, even a week in transit is nothing for the bacteria. Theoretically, it's no different than disassembling a tank, moving it across the room and reassembling it. Since nitrate is being produced, it sure seems like a cycle is happening. I would suspect that decaying plant matter, dead snails, etc could be the ammonia source.

But of course, it's important to make sure it can handle whatever the intended fish load is. Adding ammonia is a good way to do this.