deep sand bed or not ?

Yes, this is the most common scenario. Gases typically diffuse into the sand bed faster than organics, so, usually there are more gases than organics. The most common exception is if the sandbed becomes stagnant and the bacterial population dies off. This dead bacteria then provides a source of OC and shifts the gas to carbon ratio, in favor of OC.

There are other possible exceptions though. I have certainly seen the black hydrogen/metal compounds form in tanks with seemingly lively sandbeds and lots of circulation. I don't think it is nearly as easy to keep a sandbed from going stagnant as the tank. For the tank, we can just add powerheads and let the water flow. For sandbeds the surface flow helps to a point, until you get deeper. Then we start having to rely more and more on infauna. So we have less control IMO.

I do like DSBs and they have their place. I think if our experienced and understand the dangers, the dangers can be managed. As I said, the risk is small. However, for many I think even the small risks outweigh the benefits. Lately I've been running 2-3" sandbeds and have a lot of diverse fauna and have not had nitrate issues in any of my recent tanks. So, while the risk is small, I'm not sure the benefit is either.

For a DSB, I like the idea of a display style refugium, with a DSB. This way if you ever need to move things you can disconnect it from the main display. Just in case... It's certainly a personal preference though.

 

The benefit is denitrification and a more complete mini eco sytems full of micro fauna that feeds the inhabitants.

When I took down my tank the 3-4" sand bed was fine after 3.5 years. Just fishy smelling. I employed the methods above. Rinsed it in tank water and sold it as live sand. Lots of snails and worms.