Originally Posted by danielt
In the saltwater aquarium you don't have plant roots. Denitrification occurs in the live rock and the sand bed if it's not stirred and deep enough to have anaerobic areas. Some of the bacteria in the biofilm are optional denitrifiers in the absence of oxygen.
This also applies to freshwater aquariums. What happens is the plants can alter the substrate to extract nutrients from it with their roots.
As long as the substrate has nutrients in it, it will help plants and roots will grow. If the sand is inert and has a low granularity it will promote growth of anaerobic microorganisms and reactions resulting in all kinds of nasties to form in there. Gases like methane and hydrogen sulfide which are highly toxic will result from these anaerobic reactions.
Hydrogen sulfide will probably form. If not released, life will likely form to break it down. Pockets like this form in sw beds when disturbed but are shown to vanish with time. The goal is to only disturb very small sections at a time (snails and worms) .
A fine sand will be used. Maybe even mineralized dirt without a cap. Since the flow will be minimal and there won't be any large fauna to stir up the bed it may work. Such a bed would be better suited for microorganisms and would also release micronutrients into the wc.
Edit - water column alkilinity may be an issue