I agree with Bangin about plants and cycling, and I second everything Bol wrote (as far as I know plants don't utilize nitrogen if in nitrite form). I would only add that, as per a discussion I had with mrV, apparently in high concentrations ammonium ion may be toxic.
Apparently ammonium is taken up so much more readily because of the savings in energy (nitrates require more energy to take up and metabolise) that plants will deplete carbohydrate reserves and fail to reproduce in the presence of high ammonium-low carbohydrate environments. Also, ammonium in high concentrations may inhibit electron transport in photosynthesis.