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Don't forget about the root system, a large part of a tree is underground, so naturally buried

I'd call it semi-buried, since it is not isolated from the atmosphere, so in a long term I suspect that almost all its mass gets re-circulated back again – still through microbes, fungi, insects, and perhaps living root systems of surrounding trees. (Last time I tried to get some data about how much carbon remains in the soil after aerobic wood/plants decomposition in agriculture I remember it was pretty much negligible. Forest have way higher limits for maximum soil carbon accumulation, but this limit is reportedly reached after about three centuries of uninterrupted growth.)

Depends on the ecosystem. Anything embedded into bogs or peatland like roots will generally not decay at all, instead building up a very thick layer of captured carbon as peat.

Plants get most of their carbon from CO2 anyway, so in most cases carbon accumulates in the loam (outside of intensive agriculture at least). They produce far more than decomposers have a use for and that's how CO2 accumulates in soil. It only needs to be replenished by the rest of the carbon cycle because of erosion.

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