Answer: First of all, that's a great idea for an experiment! As you probably know, nitrogen is a very important plant nutrient, and one that is often in short supply for commercial and garden crops. (That's why many fertilizers contain significant amounts of nitrogen.)
You may know that legumes (including mung beans) form a symbiotic relationship with soil bacteria that allows them to "fix" their own nitrogen from atmospheric nitrogen. Most other plants can't do that--they need to get their nitrogen from the soil. So it might be interesting to see how mung beans fare when grown with and without supplemental nitrogen--or how they differ when grown in sterile medium vs. a medium that has been inoculated with the special bacteria.
I'm probably getting too far ahead here, so let me get back to your original question. Radishes grow very quickly, so they might be the best choice. I've heard that some students have had trouble getting them to form good "radishes"--that is, the large root--but you can always compare top growth. Mustard would also be a good choice, because it grows quickly. There is a company that sells what are called "Wisconsin Fast Plants"--these are specially-bred brassica seeds (related to mustard) that complete their lifecycles in a short time and are especially good for experiments like yours. Ask your teacher if she/he is familiar with this plant. If not, I can direct you to a source if you are interested.
The mung beans might work. Most people are familar with mung bean sprouts, rather than the mature plant. The sprouts grow in just a few days--but I don't think you'd get very good results comparing just the sprouts, and the plants may take longer to grow to any reasonable size for comparison--I'm not sure.
If I were you, I'd go with radishes, or check out those Wisconsin Fast Plants. And good luck!!!!
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