What is a major reservoir for ammonia?

The major reservoir for Ammonia is the natural soil, where it is present in abundance.

Explanation:

In soil, ammonia is found in quite abundance, where it is changed into nitrate through a process commonly knows as Nitrification. In the nitrification process, first, nitrite is formed which is followed by its conversion into nitrate by a bacteria known as Nitrobacteria. Finally, this nitrate is consumed by the plants, which are then eaten up by the animals.