To the question asked by
@WhistlingBadger in post #1, and referenced/mentioned in subsequent posts...there is a nutrient difference between all terrestrial plants and all aquatic plants that are grown submersed.
To clarify what the above means, terrestrial plants grow on land, with the roots in soil. Hydroponic cultivation, where the roots are in water but not the leaves, is the same thing for purposes of this discussion. These are still terrestrial plants. Terrestrial plants take up
nitrate as their source of nitrogen. They do not, so far as I have ever been able to ascertain, take up ammonia/ammonium or nitrite.
Aquatic plants refers to plants that grow submerged, leaves and roots in water. Floating plants in an aquarium are aquatic plants, not terrestrial, and they take up nutrients through their leaves and roots. The vast majority of aquatic plants take up
ammonia/ammonium as their source of nitrogen. They do not take up nitrates, unless forced to by an absence of ammonia/ammonium in balance with other nutrients and light. There is some evidence that aquatic plants would take up nitrite before nitrate, but studies are few and not all that conclusive.
If the aquarium water is high in nitrates because of high nitrate in the source water, aquatic plants are not going to help. Terrestrial plants with their roots in the aquarium water will take up nitrates. If the nitrate occurs from processes within the aquarium, aquatic plants can help control these by taking up more ammonia/ammonium than the nitrifying bacteria/archaea can, and this means significantly fewer nitrites and thus significantly fewer nitrates down the line.
There is another aspect to this. The majority of aquatic plants take up ammonia/ammonium via their leaves, not their roots. The following is Diana Walstad's explanation on this.
If aquatic plants preferred to get ammonium by root uptake from the substrate rather than leaf uptake from the water, their ability to remove ammonium from the water and protect our aquarium fish would be lessened. Fortunately for hobbyists, aquatic plants seem to prefer leaf uptake of ammonium as opposed to sediment uptake. Thus, in split-chamber experiments with marine eelgrass [12], when ammonium was added to the leaf/stem compartment, root uptake of ammonium was reduced by 77%. However, when ammonium was added to the root compartment, leaf uptake of ammonium was not reduced."
Work with other aquatic plant species support the above findings. The seagrass Amphibolis antarctica takes up ammonium 5 to 38 faster by its leaves than its roots [13]. Myriophyllum spicatum planted in sediment containing adequate ammonium, grew fine without ammonium in the water. However, when investigators added ammonium to the water (0.1 mg/l N), plants took up more N from the water than the sediment [14].
Several aquatic plants (Juncus bulbosus, Sphagnum flexuosum, Agrostis canina, and Drepanocladus fluitans) were found to take up 71 to 82% of the ammonium from the leaves; their roots took up only a minor amount [15].
Hobbyists using fertilizer tablets for aquatic plants should understand the aquatic plant preference for leaf uptake of ammonium (as opposed to root uptake). In aquariums, fish-generated ammonium in the water can fulfill most N needs of plants. Moreover, any nitrogen added to substrates, such as in fertilizer tablets, can have bad and unintended consequences. For example, when I added nitrate-containing fertilizers to a fresh soil substrate, the fish became sick from nitrite toxicity. (Soil bacteria had converted the nitrates to toxic nitrites, which then entered the overlying water.
Pathos ..I do pathos for nitrates
Thanks, that's what I thought. Haven't read Walstad's book for a while; need to again. It is also my understanding that aquatic algae mostly dines on nitrates, which can give them an advantage over vascular plants in certain situations. Is that correct as well? I remember that Rachel O'Leary once had a tank of desirable algae wiped out because she added pothos to the HOB filter.
