Well, there's one reference for Elodea suggesting that range of nutrients for each form of N.
http://keur.knaw.nl/21259/1/21259.pdf
Some issues are present with the study for folks making th assumption.
1. The NH4 was not maintained at 2ppm during the entire course of the test.
2. PO4 and other nutrients also whre not maintained at the higher levels.
3. CO2 was not added
4. Higher light was not added.
5. NO3 needs to be higher than 5ppm and kept high, like the NH4 at 2ppm for the entire test.
6. The test was only for 96 hours............
7. Plants can and will adapt some to difference species of N. This adaptation needs done prior to the test. This is particularly true for NO3, much more so than H4, thus this could skew the data/results.
I agree, given a choice, many species will use NH4, but it must be converted into amanio acids rapidly. This requires lower levels of NH4 and also the carbon skeletons for the amino acids.
So if you are low on CO2..............or add too much NH4, it will inhibit the plant. NO3 is relatively non toxic on the otherhand but the plant needs time to repond to higher levels of NO3 ppm in the water to gear up their enzymes. They can easily store large amounts of NO3 in their central vacuole, they cannot store NH4/NO2.
Algae are similar............
As far algae blooms, the NH4 + higher light (and CO2 perhaps)need much less to gerimate a spore. The issue with vegetative growth is a different question.
I do not think in practical terms folks see much difference betwee KNO3 dosing and Say NH4Cl dosing etc.
Fish waste add plenty of NH4 as it is and we see little difference there vs no fish or lightly stocked tanks.
I think small amouts of NH4 are fine if you are experienced and have lower light, good CO2. Should not do much harm as long s you do not dose that much, say no moee than 0.2-0.4ppm per day. Max uptake migh be about 0.8pm for higher light.
It's a bit like playing with fire and you do not get much, if any gain from this. I just use fish waste and KNO3 and I get awesome results, I've tried to improve it via NH4, but found little evidence it offers any significant difference.
If you mess with CO2 and add NH4 with higherlight, it's a good recipe for algae blooms.
But is dependent on light and CO2 as well, whereas NO3, not nearly as much if at all.
It's also very toxic and folks have a lot of sloppy habits and end up tossing in way too much thinking more is better. Then they will blame me
Regards,
Tom Barr