[tmz]

I wouldn't expect the sand to do much denitrification, personally. Diffusion is irrelevant here: only bulk flow can move enough nutrients into the sand to make even a 1" bed useful.

I agree with Jonathan 's assessment; most of the denitrificatin in a sand bed occurs in the the first half inch or less because the bacteria performing this function need nutrients( organic carbon,dissolved nitrogen and phosphorus) .

The bacteria performing dentirification are facultative;they use free oxygen and nitrate for oxygen when free oxygen is limited. They are heterotrophs that need , organic carbon, phosphate and nitrogen. A deep bed without some force to move nutrients down into it does very little. Diffusion a molecular force is too weak and slow.

The old deep buckett threads just don't account for the nature of the bacteria expected to do the nitrate removal. These bacteria use the oxygen in an area assimilating some nitrogen and phosphate as they grow and then switch to nitrate for oxygen when free oxygen is exhausted. At that stage the nitrate is reduced to N,some of which binds with other N forming N2 gas which bubbles out of the tank further reducing nitrate.These bacteria create low oxygen areas and use nitrate in the upper layers of the bed, live rock porous media or even in their own mulm but need to be in an area where the nitrogen phosphate and organic carbon are available to them.

Th black stains noted in the above posts are probably sulfides, residue from sulfate reducing bactrerial activity. When there is some organic carbon present in an areas where both free oxygen and nitrate have been exhausted by the denitrifying bacteria, sulfate reducing bacteria move in to consume the organic carbon using sulfate( SO4 for oxygen. This process produces sulfides and toxic hydrogen sulfide gas as by products.

Sand is useful for critters that need it and can be aesthetically pleasing when it's kept clean.Some enjoy observing various sand critters and reactions in the sand too. It also provides a lot of surface area for bacteria to colonize but it's is not needed at any significant depth for nitrification and is mostly sterile down deep unles nutreints are moving down ther via a viable population of sand criiters ,channeling or some other force. Anoxic areas ( no oxygen and no nitrate) with organic material in them are alos a conern vis a vie sulfate reducing bacteria.