The decaying of animals and plants primarily occurs in the lithosphere and biosphere.
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Lithosphere: This includes the soil and sediments, where the organic matter from decaying plants and animals breaks down through processes like decomposition and mineralization.
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Biosphere: This is the global sum of all ecosystems, encompassing all living organisms including plants and animals. The decaying process is a critical part of nutrient cycling in the biosphere.
While there may be some interactions with the atmosphere (through gas exchange) and the hydrosphere (in the case of organisms decaying in water), the main processes of decay happen in the lithosphere and biosphere.