Due to leaf litter

It is due to high amount of carbon content. When the C:N ratio is high then decomposition of litter slower down.

For more details, please go through the link given below

https://www.planetnatural.com/composting-101/making/c-n-ratio/

High C:N ratios may be lowered by adding grass clippings or manures. Low C:N ratios may be raised by adding paper, dry leaves or wood chips.

Because of high amount of carbon content.

It may be due to higher carbon content due to protection of organic matter and reduced decomposition rate of organic matter in the lower layer (15-30 cm) than the surface layer.