I do not think anyone really knows.
You could remove the plants and add KNO3 and then see how long it takes to drop.
But.........you also have an issue with plant roots effecting the sediments and adding O2, this reduces and prevents lowe rRedox levels that are required for denitrification.
Plants also remove NO3 as well as NH4.
So most test will be "artificial" that aquarist can do.
Researchers use the N2O relationship in denitrification or some use labeled N15 stable isotopes of NH4 or NO3.
This way they can track where the % of NO3 goes in a real system or how much is from denitrification.
I think it can vary widely in aquariums. Soem tanks none at all, others, like soil based tanks, quite strong for a few months, then very very klittle there after(all the N is gone in most soils/ADA etc after a few months, 1-2 year range anyway)
The lower redox values provided by such soils are excellent for plants, but weak roots and weak growth will be hampered by lower Redox as well, it's a 2 edged sword.
ADA got around this somewhat using Aqua soil, which are larger clay soil that allows lots of flow through, but each grain is a functional unit and internally has the lower Redox values, this is the best of both worlds.
Over time, this breaks down and the grains decay and fine mulm forms and seals the sediment. But.........by then.......the plant roots are well established and pump plenty of O2 down there.
We measure the total uptake, production and decomposition of NO3 from all metabolic processes.
A filter should be cleaned often, lower flow/current etc=> less flow, less CO2 mixing and also, importantly, less O2!!
Less O2= more fish stress. Clogged filter= overloaded polluted organic matter that sucks out O2 as it decomposes that the fish can use and need.
The same can be said for the sediment if there's no good plant growth or roots.
The sediment will sour after a time from organic matter loading.
Regards,
Tom Barr
Regards,
Tom Barr
