In India, agricultural refuse is very simply converted in the field to charcoal particles that can be agglomerated with starch and other ingredients to a good fuel...

An effective way that I can think of is to use it to prepare compost which can be used as natural nutrients in agriculture.

Good concern. I think this huge amount of leaves can be utilized as a feed material of pyrolysis process in order to produce bio-fuel. It's really environment friendly than

burning this leaves for cooking or other heating process.

Pyrolysis is an expensive method which the developing countries can not afford. Regarding it is difficult to degrade leaves by composting. How can we easily degrade these leaves in composting

In cities, lots of biodegradable wastes which contain lots of moisture is collected (i.e. food refuse from cafeterias, hotels, markets etc.). The fallen leaves can be mixed with these leaves very effectively in composting the biodegradable wastes collected. Biodegradable wastes can be collected separately (which will be practiced soon in Colombo Municipality area in Sri Lanka).

Fallen trees could be converted to fuel wood. There is a high demand for fuel wood from industries (for boilers), bakeries and even for home-scale cooking. The city councils could collect the trees (trunks and branches) and process it prior to sales. This can reduce at least some of the demand on forest products.

Fallen leaves can be gather into a pit and cover with ashes or mud or soils whichever is readily available. Keep it for three months. When you open the pit afterwards you can use the deposits as manure for gardening. Burning the fallen leaves is not a good practice. Fallen trees got stink. Collect the fallen trees by cutting it into useable length for family consumption, you may also use it for garden fencing. It also depends on the type of tree that some trees can be used for producing mushroom and planting mushroom seeds into the fallen tree and water it for few months, you will be able to see new mushroom sprouting for family consumption as well as commercial use.

Agricultural wastes can be converted into biomass charcoal briquettes (small fuel pellets) or bio char production through Low cost carbonation process. Fuel pellets can be effectively used for households/small hotels for cooking or industries for boilers heating, biochar could be applied to agricultural crops it reduce soil co2 and increase the carbon content in soil and crop yield also increased. I think this technology is very inexpensive process and easy to produce and environmental friendly.

we can use them for the study of allelopathic effect.

we can use them for the study of allelopathic effect.

Composting is the more sustainable way of use

Fallen leaves shouldn't be seen as a problem. Just heap them below the respective tree

Composting is one of the way which reduces the waste. In rural parts of India, especially coconut tree leaves are used for production of Coconut Broom Sticks and Thatching roof. The problem is not because of the fallen leaves, its because of the grown up cities and towns

We can effectively utilize fallen leaves of plants and trees through biogas production.

Reference:

http://www.omicsonline.org/proceedings/biogas-production-potential-from-fallen-teak-leaves-tectona-grandis-44470.html

Coconut fronds 

The stiff midribs of coconut leaves are used for making brooms in India, Indonesia (sapu lidi), Malaysia, the Maldives, and the Philippines (walis tinting). The green of the leaves (lamina) is stripped away, leaving the veins (wood-like, thin, long strips) which are tied together to form a broom or brush. A long handle made from some other wood may be inserted into the base of the bundle and used as a two-handed broom. The leaves also provide material for baskets that can draw well water and for roofing thatch; they can be woven into mats, cooking skewers, and kindling arrows, as well. Two leaves (especially the younger, yellowish shoots) woven into a tight shell the size of the palm are filled with rice and cooked to make ketupat (Grimwood 1975). Dried coconut leaves can be burned to ash, which can be harvested for lime. In India, the woven coconut leaves are used as pandals (temporary sheds) for marriage functions especially in the states of Kerala, Karnataka, and Tamil Nadu (Wikipedia). At present, coconut fronds are used in composting .

Fallen leaves are undoubtedly  an issue. Forests capture CO2...for a while and  after that are CO2 + CH4 generators. Amazon forest works  in a borderline equilibrium..in few words is not a "dump" for industrial CO2. The question comes how C is accumulated for further planet use.  The problem with fossil fuels (Coal, petroleum) in kind of philosophical terms is not CO,CO2, NOxs...It is basically, that you are heating a molecule unnaturally and altering the atmosphere gas flows (A Pinatubo event is an example) Leaves, afforestation are good in the long run...thousands of years, but the deforestation caused by agriculture worldwide since ~30,000 years ago is passing us the bill. In a several thousands years ago grain crops would have combined with trees we would not be facing the current 'climate change' situation. The Andeans had a principle "When you make war, dont damage the trees" For much more than 5,000 years, there is not a single burnt city...but this is a matter that is not important here!

Leaves are disposable accumulators of energy. They can be friendly and enrich soil, but also can be an environmental enemy since can decay into methane. The permafrost in Canada, Russia etc holds enormous CH4 contents..

Resuming fallen leaves are CO2 gas  storage in sugars and carbohydrates.. If one uses leaves to feed silkworms you have a usable prime fiber. In my experience leaves for gas are very poor performers, for fuel have too much water, for soil enrichment/remediation can become environmentally counterproductive (lots of CH4 on decomposition) Ideal leaves, therefore are the ones that can be integrated  for i.e making silk or feed cattle....Otherwise fallen leaves are a problem...