Concept:Decaying organic matter adds nutrients and improves soil, but the increased microbial activity it causes may sometimes be harmful.Explanation:Bacterial and fungal growth is enhanced by decaying organic matter. This can be a disadvantage because it may increase disease-causing organisms in the soil.Releasing minerals into the soil is an advantage. Improving the physical condition and water-holding capacity of the soil are also advantages.So the only disadvantage among the given options is enhanced bacterial and fungal growth.Answer:A. Bacterial and fungal growth is enhanced.