NH4 is highly toxic and must be rapidly converted into Glutamine endogenously/internally. So they cannot have much around inside........on the other hand, NO3 is very non toxic and they can store vast amounts in the central vacuole.

So you also need NO3- as an anion to balance K+ in there also. the plant can then draw NO3 out as needed rather than depending on the environment for a steady supply of NH4, which often varies widely and is typically very low in planted tanks(with or without a bio section).

Also, while it is true that plants expend some effort to convert NO3=> NH4 to glutamine, it's not that much of an energy loss over all.

Uptake or conversion of CO2, photorespiration etc are much larger issues and draw far more energy in terms of of plant energy budgets than does N conversion.

Plants can do this reduction at their leisure as well.

The other thing to consider: algae prefer NH4 as well and the form of N makes a much larger difference to algae. Why do you think that is?
Carbon......algae have far less demand for CO2 than any plant, so NH4 is really a good form of N to have for them relative to a plant. They are a single cell also, so that small amount of NH4 is worth much more than to a large plant with it's higher CO2 requirements, large storage capacity etc.

Most every plant out there also prefers a ratio of NH4:NO3, not this either or business for optimal growth.

Give the toxicity and algae related issues with NH4, I'd rather have a tad less growth(I've never been able to see any significant differences between NH4 and NO3 dosing, nor have any other folks), minimize algae and reduce the toxic effects of NH4 on critters personally.

That's a very very very small trade off, one I've not seen quantified with algae and other critters present.

If you look at the cited reference from Diana Walstad's book, the figure starts off at 2 ppm of NH4, pretty darn toxic.

Then once the level drops to 0.5 ppm of NH5 or close, the rate slows way down.

At that same time, the rate of NO3 uptake starts. and it never stops even when the NH4 is still present.

So under our typicaly aquarium conditions, that graph supports that NO3, not NH4 is actually preferred in Egeria. One plant also is hardly evidence that all 300 species prefer the same conditions as well.

Note, the experiment was also done in absence of any algae spores etc, sterile conditions............our tanks have algae spores and bacteria growing on all surfaces. When you factor such issues into the real world field test vs a controlled lab setting, the results often are no longer significant.

Poor interpretation of graphs can lead to poor conclusions also.
It depends on where on the graph you are discussing.

At one end of the graph, yes, there appears to be preferences for NH4 vs NO3.
However, at the other end of the graph, this relation is reversed.
And that is the end that applies to us and aquariums.

Most research is this way, you have things change through time, space and concentrations etc. You need to be able to apply the research you read and support the argument that you make.

Like the new GI bill study that was recently lamblasted by a certain prez candidate as reducing military personnel serving by causing many to leave to go to college (16%), but then failed to mention that it would attract 16% in new recruits. So they claimed it would reduce membership in the military and should be defeated. I guess they did not or could not read more than the 1st part of the study? In which case I'm very worried, or that he did, and chose not discuss that part that did not fit in with his agenda. In which case I'm even more scared. It was a rather simple study to read, not too technical.

16%- 16% =0%, or no net loss and the study even said this in the conclusion.
And happier more educated GI's.

Political manipulation of Science has a long history and this is done more often than not. You need to read both sides/parts and acknowledge what aspects you feel are applicable to the real world situation.

Same deal here. At first, it sounds like a reasonable argument.
However, unlike politics and extreme cases, I think that was no bad intent on anyone's part, just over looked the graph. Took me a awhile to see the error myself.

Think about CO2 fixing enzyme Rubisco and how much N it demands.
Simply allowing the plant to be more efficient by providing good stable CO2 levels can radically alter the demands for NH4/N in general, thus leave plenty of energy available for growth.

Likewise, in a non CO2 situation, the rates of growth are much slower, thus N demand are much slower, 10-20X less, so the fish waste etc add plenty in the form of NH4.


Regards,
Tom Barr