http://links.jstor.org/sici?sici=002...3E2.0.CO%3B2-X

This is a good paper, unfortunately we can only see about 1 page of it.
It does detail out that higher growth rates = no stress in terms of plants and that less growth = stress.

That is what slows plant growth down.
From that point, folks can experiment and reduce nutrient levels down and adjust them to see their impacts on various species.

This study shows that these plants can adapt to a range of 10X concentrations using Hoagland's solution, quite a bit.

However, simply because a ppm level exists in a natural system, we do not know if that system is limiting or infertile and if and to what degree the plants are stressed.

So what reference can/do we use?
Like these folks and other plant scientists: a maxmimal non limiting level of nutrients.

From there, we can reduce the various parts down one by one and note the effects on various species of plants. Some are more strongly impacted by lower nutrients than others.

Micrantheum umbrosum is a typical higher NO3 plant while an Anubias could probably live for years with hardly any.

Regards,
Tom Barr

Another paper raises the question, is it the plants, or the surface area and bacteria that grow on them that are rmeoving the NH4?

http://links.jstor.org/sici?sici=002...3E2.0.CO%3B2-O

Interesting idea and perhaps some truth to it for us?
Certainly could be.

Regards,
Tom Barr