Is the dirt capped with anything, or is the dirt the only substrate in there?
Also, are you still using the same filter and media you had on your previous tank, or are you using new/different filter/media?
Best guess I have right now is to get a bucket or something that will hold water, fill it with decholrinated water, and move your fish/critters to that, with a filter ideally. Then I guess muck about with the new tank. see if the soil keeps leaching ammonia, or if it was just an initial burst from added fertilizers that will quickly taper off.
If that's not an option, I guess just keep on top of the water changes, do whatever you can to keep the ammonia/nitrite as low as possible until the biological filter picks up. Maybe try throwing in some floaters, or some houseplant cuttings (riparium style), since plants with access to the atmosphere will grow quicker (and take up more ammonia) than most submerged types.