Nitrogen is the only nutrient I think that will not last for many years. Bacteria oxidize it to NH4=>NO2=> NO3 if the plants do not get it. So after awhile, not much is left.
But with a slow rate of non CO2 plant tank growth, and with fish waste, the N needs are typically met.
You can add a little KNO3 once every 1-2 weeks, say 2-4ppm worth of NO3, will not hurt.
I measured a few sediments over time and they all pretty much drop to near zero for NO3/NH4. NO3 does not bind to CEC, which is why many places have NO3 in their ground water.