Some good thinking here on this thread.. The way I see it, there are 3 important forces that might act on the buried tape:
1) Subjection to higher and more constant humidity (moisture and temperature insulation) than in a surface environment;
2) Strongest magnetic force on the tape becomes the earth's magnetic field, which is more or less monodirectional compared to manmade sources of magnetic force;
3) A different rate of oxidation (given that the signal on the tape itself is physically rust, this is probably the most important factor).
All 'effects' are a form of signal degradation, so the most important criteria here are:
1) Does the burying of the tape result in an audible level of signal degradation (given that the surface of most of the tape is coiled and not directly exposed to the elements)?
2) Is any resulting audible degradation considered subjectively desirable?
All of these quantitative considerations can be measured and assessed. However, the remote possibility exists that there is an unknown exponent in this practice of burying the tape. It does seem to bring to a head the philosopher's question: 'If a tree falls in the forest and nobody is around to hear it, does it make a sound?'
We might well ask: 'If Lee Perry made a riddim in the studio when there was nobody else around, decided to bury it in his garden among the weeds at night, and forgot all about it the next day, does that riddim exist?'
In a real sense, the riddim may exist despite that he forgot it (maybe it's still there?), as the electrical patterns in his brain when he made this riddim will have made up a part of what we know as 'Lee Perry'. For example, photographs of a man mixing dubs at the Black Ark studio may exist today, but that man does not, although we call a certain entity 'Lee Perry'. This is because not a single atom in the entity we call Lee Perry today was around in 1970s Jamaica. He has completely recycled, as have we all after 12 years or so, all the atoms in his body. We have simply given a name to a set of brain 'programs', and this software is the accumulation of experience (experience that may or may not include the production of a riddim he has forgotten about and which nobody else knows about).
Quantum mechanics makes this situation even more curious. This field of science predicts to a very high degree of accuracy the position of sub-atomic units of material energy, using wave functions. The implications are, among other things, that a subatomic unit may exist in more places than one. And when we look at the what a dub tape actually is, it is a wave-function written on rust. By this standard, it's arguably a self-referential material entity and the dub can be said to 'exist' whether it's above or below the surface of the earth.