I apologize for the long delay in responding to your inquiry.
This is a unique and really great science fair project idea. An example from real life is always a good topic and here are suggestions for developing the experimental protocol:
Your daughter should try to use plants that are well established in pots and have a quantitative method for measuring the amount of nitrate applied to each plot and leached from the pot. Since there is no procedure written for this, I recommend doing a small pilot scale experiment with one pot and see if it is possible to get reproducible results on measuring leached nitrates.
There are test kits for nitrate available at aquarium stores, but these might be expensive to use for the quantity of testing that needs to be done. Here is a description of two nitrate testing methods:http://water.epa.gov/type/rsl/monitoring/vms57.cfm
And here is a power point that shows pictures of different methods.http://www.usu.edu/cpl/Nitrate%20analys ... ometti.pdf
If your daughter can identify a suitable quantitative method, then she should be able to do this experiment.
What is your daughter thinking about doing for an experimental design? How is she going to reproduce the report of the nitrate being converted to ammonia under low oxygen conditions? It sounds like bacteria in the soil may have been responsible for the conversion, so she should try to think of a way to add a suitable source of microorganisms to one pot. What will be use for controls for the experiment?
Anaerobic conditions are not usually good for the plants, so she should include an evaluation of the health of the plants and make a note of any yellowing leaves or wilting
I recommend using just one type of plant, and include at least two pots of each plant, if possible.
Please post again in this topic if you have any other questions.