**The Main Sulphur Reservoir in the Sulphur Cycle**

The sulphur cycle is a biogeochemical cycle that describes the movement of sulphur through various compartments of the Earth's ecosystem. Sulphur is an essential element for all living organisms and plays a crucial role in many biological processes. The main reservoir of sulphur in the sulphur cycle is present in the following:

**1. Soil**
Sulphur is found in soil in various forms, such as organic sulphur compounds and inorganic sulphates. Organic sulphur compounds are derived from the decomposition of organic matter, such as dead plants and animals. Inorganic sulphates are formed through the weathering of rocks containing sulphur minerals. Sulphur in the soil is taken up by plants and incorporated into their tissues through the process of assimilation. It is then passed on to animals and other organisms through the food chain.

**2. Volcanic Eruption**
Volcanic eruptions release large amounts of sulphur dioxide (SO2) into the atmosphere. This sulphur dioxide can react with other compounds in the atmosphere to form sulphate aerosols, which can be transported over long distances by wind and eventually deposited onto land or water bodies. Volcanic eruptions are significant sources of sulphur input into the sulphur cycle and can have both local and global impacts on the sulphur cycle.

**3. Hydrogen Sulphide**
Hydrogen sulphide (H2S) is a gas that is released into the atmosphere from natural sources such as volcanic activity, marshes, and the breakdown of organic matter. It is also produced by certain bacteria during the process of anaerobic decay. Hydrogen sulphide can react with oxygen in the atmosphere to form sulphur dioxide, which then participates in various chemical reactions in the atmosphere and eventually returns to the Earth's surface through deposition.

**4. Water**
Sulphur can also be found in water bodies in the form of sulphates and sulphides. Sulphates are dissolved inorganic compounds that are commonly found in rivers, lakes, and oceans. They are derived from the weathering of rocks containing sulphur minerals. Sulphides, on the other hand, are formed through the decomposition of organic matter in oxygen-deprived environments, such as swamps and wetlands. Sulphur in water can be taken up by aquatic organisms and can also be released back into the atmosphere through various processes, such as evaporation and volcanic activity.

In conclusion, the main sulphur reservoir in the sulphur cycle is present in the soil, where it exists in organic and inorganic forms. However, volcanic eruptions, hydrogen sulphide emissions, and sulphur in water bodies also contribute significantly to the sulphur cycle. These different reservoirs interact with each other through various processes, such as deposition, assimilation, and volatilization, to maintain a balance in the sulphur cycle.