Answer: As long as the material is permeable by air and water it should not cause direct harm to the soil. You may find however that as the surface layer of mulch rots down and forms a nice layer of humus on top of the fabric that the weed seeds germinate like crazy there. The fabric also limits the ability of the organic matter to reach the soil where it would benefit it. Over time you may want to pull back the fabric periodically and allow the rotted mulch to replenish the soil below.
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