Maybe because decomposition of the matter at the surface is much faster compared to the same matter at depth.
There is much lesser microbial activity plus aeration in terms of available oxygen deep in the soil compared to surface , thereby, microbial decomposition of the buried plant material is relatively slow...
There is much lesser microbial activity plus aeration in terms of available oxygen deep in the soil compared to surface , thereby, microbial decomposition of the buried plant material is relatively slow...
If you add amount of water coming from rain/ dew and responsible for activation of mold and other degradating insects you may have some good explanation.
Plant materials near and on the surface of soil are exposed to light, heat and cold, rain, etc which facilitates the physical breakdown of the materials thereby increasing the exposed surface area for microbial decomposition. Whereas, those buried deeper gets adequate protection from the environmental changes and are far from the active zone of aerobic microbial decomposition.
Rate of decomposition in soil subsurafce will always be slower than the surface where air exchange with atmosphere is so pronounced...
The rate may depend on surface soil microbial ecology which may also be limited to a few, making decomposition slow.
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