First, the reason aquatic plants use nitrate in tanks with fish is very simple. You cannot add ammonia to a tank with fish in it. So the only ammonia available is what is created in the tank. And while this is a constant process, it is also ay a very low rate. In an established tank there will never eb an ammonia reading because the bacteria and plants use it as fast as it is creatd 24/7. Next, if a tank does not procuce enough ammonia/ammonium to support the plant load, we have to add ferts. Since we cannot add ammonia, that leaves nitrate.
Lets start with some science here. In water ammonia (NH3) turns mostly into ammonium. The lower the pH and the cooler the water, the more of the Total Ammonia (TA), NH3 + NH4, in a tank will be in the form of ammonium. At about 6.0 pH it is virtually all ammonium. However, the science also shows us that the bacteria are able to adapt to this. But they process ammonium much (NH4) less efficiently than than ammonia (NH3).
Next, there is no such thing as a planted tank with 0 nitrifying bacteria in it. On the other hand, there are plenty of tanks filled with bacteria and no plants at all.
So, it is not possible to have only plants doing the ammonia processing as 10 tanks mistakenly has stated. Not only do I have many bookmarks on the scientific studies relating to FW aquariums and all connected with them. But I also have over 25 papers on aquaitic plant feeding.
Next, if you have any bacteria in a tank, you also have some small amount of denitrifying bacteria in the same biofilms as the nitrifyers. If you have the proper substrate or massive bio-media, you will have more of the denitrifiers. But unless one has the latter situation, the amount of denitrification is minimal. In my tanks with massive Hamburg Mattenfilters I never have nitrates and none of these tanks has live plants. Because there are no lights on my pleco breeding and grow tanks, There is almost no algae either. All there tanks get is minimal ambient light most of the time. There is some daylight that comes into the room and I do use lights when I clean the tanks.
Consider this as a start:
Jampeetong, Arunothai & Brix, Hans. (2009). Nitrogen nutrition of Salvinia natans: Effects of inorganic nitrogen form on growth, morphology, nitrate reductase activity and uptake kinetics of ammonium and nitrate. Aquatic Botany. 90. 67-73. 10.1016/j.aquabot.2008.06.005.
Abstract
In this study we assessed the growth, morphological responses, and N uptake kinetics of Salvinia natans when supplied with nitrogen as NO3−, NH4+, or both at equimolar concentrations (500 μM). Plants supplied with only NO3− had lower growth rates (0.17 ± 0.01 g g−1 d−1), shorter roots, smaller leaves with less chlorophyll than plants supplied with NH4+ alone or in combination with NO3− (RGR = 0.28 ± 0.01 g g−1 d−1). Ammonium was the preferred form of N taken up. The maximal rate of NH4+ uptake (Vmax) was 6–14 times higher than the maximal uptake rate of NO3− and the minimum concentration for uptake (Cmin) was lower for NH4+ than for NO3−. Plants supplied with NO3− had elevated nitrate reductase activity (NRA) particularly in the roots showing that NO3− was primarily reduced in the roots, but NRA levels were generally low (<4 μmol NO2− g−1 DW h−1). Under natural growth conditions NH4+ is probably the main N source for S. natans, but plants probably also exploit NO3− when NH4+ concentrations are low. This is suggested based on the observation that the plants maintain high NRA in the roots at relatively high NH4+ levels in the water, even though the uptake capacity for NO3− is reduced under these conditions.
from
ammonium and nitrate
or
Stephen G. Nelson, Barry D. Smith, Bruce R. Best,
Kinetics of nitrate and ammonium uptake by the tropical freshwater macrophyte Pistia stratiotes L.,
Aquaculture, Volume 24, 1981, Pages 11-19,
ISSN 0044-8486,
Abstract
In this study we assessed the growth, morphological responses, and N uptake kinetics of Salvinia natans when supplied with nitrogen as NO3−, NH4+, or both at equimolar concentrations (500 μM). Plants supplied with only NO3− had lower growth rates (0.17 ± 0.01 g g−1 d−1), shorter roots, smaller leaves with less chlorophyll than plants supplied with NH4+ alone or in combination with NO3− (RGR = 0.28 ± 0.01 g g−1 d−1). Ammonium was the preferred form of N taken up. The maximal rate of NH4+ uptake (Vmax) was 6–14 times higher than the maximal uptake rate of NO3− and the minimum concentration for uptake (Cmin) was lower for NH4+ than for NO3−. Plants supplied with NO3− had elevated nitrate reductase activity (NRA) particularly in the roots showing that NO3− was primarily reduced in the roots, but NRA levels were generally low (<4 μmol NO2− g−1 DW h−1). Under natural growth conditions NH4+ is probably the main N source for S. natans, but plants probably also exploit NO3− when NH4+ concentrations are low. This is suggested based on the observation that the plants maintain high NRA in the roots at relatively high NH4+ levels in the water, even though the uptake capacity for NO3− is reduced under these conditions.
I can keep on posting studies if folks want, but the facts do not change.
edited to change the word nitrogen o nitrate in the first sentence and typos.