Don't plants take up gases through pores in their leaves?
I've read my suggestion of letting detritus collect in the substrate from many planted tank sources. Now I'm not talking about a monster tank with a few plants (this tank will already have high nitrates), but a heavily planted tank with CO2 and lots of light.
You're correct that the ammonia will be absorbed first, but when there's none left, the nitrates will be absorbed too.