Curious about planting advice

??? My crypt has rhizome, and I bury them all the time---never had a problem.

This suggests that a lack of O2 is the problem...not a whole lot of info though.

http://aquabotanic.blogspot.com/2009/05/cryptocoryne-rhizome-planted-tank.html

It is not a process that happens overnight. It could be gradual over a period of months. It is anaerobic conditions in the substrate that causes the rhizome or bulb to rot in the same way that roots turn black and rot.

Cryptocoryne rhizomes are thick and woody and not as sensitive as rhizomes that are more fleshy like Anubias, although an anaerobic substrate could trigger the common Cryp melt- a condition where all the leaves melt down to the rhizome.

Rhizomes on ferns such as Microsorum or Bolbitis when buried in the substrate will start to loose leaves as the rhizome gets anaerobic. In its native habitat these plants grow in and out of the water attached to rocks or wood often in fast moving streams that are highly oxygenated. Bolbitis in particular for this reason does well in an aquarium positioned in the path of the filter outflow.

Tubers such as Aponogetons, Crinums, and Tiger lotus can easily rot in the substrate, sometimes very quickly. Often people bury only enough of the tuber to keep it anchored, or simply allow the tuber to sit on top of the gravel. In this case the roots from the tuber will still easily find their way into the substrate.

Fungus growing on the rhizome or tuber can also be the cause, but this is much more rare, and aquatic fungus thrives in anaerobic conditions where dissolved organic compounds are high. You can save a plant by cutting off the portion that is soft or rotting.