Leaves shed in autumn tend to have a very low nitrogen content and are often dry. Their main constituents are cellulose and lignin. Because of this, autumn leaves break down far more slowly than most other compost ingredients with very little bacterial decomposition involved.
Time and process
Fungal decomposition of a heap of leaves in damp temperate climates can take between one and three years to break down into a dark brown fine powdery humic matter. A succession of different fungal species may be involved. A range of micro detritivores are also involved in converting the leaf material into a fine-grained humus, including many isopods, millipedes, earthworms, etc.
In the natural environment, the decomposition of leaves provides a moist growing medium for young plants and protects the ground from drying out during periods of low rainfall. It is a significant component of soil organic matter, particularly in temperate deciduous woodland. The slow rate of decomposition gradually releases plant nutrients bound up in the leaves back into the environment to be re-used by plants. Autumn leaves are often collected in gardens and farms into pits or containers for the resultant leaf mold to be used later.
Oxygen and moisture are essential for leaf decomposition. Leaf mold is not high in nutrient content but is an excellent humic soil conditioner because its structure and moisture retention provide a good growing medium for seedling roots.
Leaves collected from roads and pavements may be contaminated by pollutants which can become more concentrated as the leaves decompose into a smaller volume 
- Compost organisms
- "Compost Chemistry". Cornell University. Retrieved 6 October 2016.
- Jana Voříšková1; Petr Baldrian (11 October 2012). "Fungal community on decomposing leaf litter undergoes rapid successional changes". The ISME Journal. 7 (3): 477–486. doi:10.1038/ismej.2012.116. PMC 3578564. PMID 23051693.
- "Leaf litter in street sweepings: investigation into collection and treatment" (PDF). The Environment Agency. Retrieved 6 October 2016.