Plants can't convert nitrates back to ammonia. There's a 'bio bug' that does ammonification, converting it into ammonia, and water-soluble ammonium salts or so I read quickly from this: http://www.lenntech.com/nitrogen-cycle.htm
... After reading an article on the decomposition of, well, dead people, I would have to guess and say bacs convert most forms of nitrogen faster than plants... But, my logic is not guided by understanding, but deduction. I just know the basics of the nitrogen cycle. I'm not trying to correct you, at all! but learn with you. Probably just an idiot's ramblings.
Anyhow. Perhaps you might be interested in the massive colony of bac on the MattenFilter: http://www.plantedtank.net/forums/lo...-new-post.html
. It's kind of like a sump, inside the tank. No reason you couldn't just have this be in your sump too. You could grow your terrestrial plants on that aquaponically, and submersed, any aquatic rhizome plants can grow on the filter below the water line.
Can we have a close-up of your sump?