Can we consider the nitrate contain in tank is one type of fertilizer.? If YES then how the plant absorb it ?and how much nitrate ppm can one plant absorb During 24 hrs? Any information any where about it?.
The nitrates (NO3) are a source of nitrogen (N) for plants. Not the best one, but the next best. Ammonia (NH3) or better yet-Ammonium (NH4+) is even better. It is said that plants will need to convert nitrates to ammonia to be absorbed or that ammonia is absorbed through the leaves and nitrates through the roots.
How much a day? I'd say 4 ppm (based on fishless cycle). Fast growing plants will use more. Slow growing plants-less. Dying plants will add nitrates to your tank and will not take any.
Question is: how about the Carbon? Plants need a lot more Carbon than Nitrogen. There is a proportion between N-P-K and all three are relative to how much Carbon is available. For aerial plants, that's not a problem. We complain about too much Carbon (CO2) in the air. Aquatic plants- different story.
The plants will grow according to the minimum supplied element. The way i see it is: ever baked a cake? It needs 1 pound of flour, 2 eggs, milk, and a teaspoon of butter. If I have 1 pound of butter and a teaspoon of flour, I can't even make one cupcake.
Nitrate is actually sold as an aquarium fertiliser for heavily planted tanks- I've bought it and used it myself when the plants were using them up faster than my fish could produce them. I added it directly to the water column. However the ppm used in 24 hours would certainly vary by plant species and not be the same across the board.