What is a biogeochemical cycle. What is role of reservoir in a biogeochemical cycle. Give an example of a sedimentary cycle with reservoir located in earth's crust. 

The movement of nutrients through various elements of ecosystem is called nutrient cycling or biogeochemical cycle. Nutrient cycling ensures that nutrients are never lost from the ecosystem. They keep on getting replenished in the reservoirs in ecosystem. 
Role of Reservoir: Whenever there is an imbalance between efflux and Influx of a particular nutrient, there can be deficit of that nutrient. The reservoir counters this deficit by supplying the required nutrient. 
Phosphorus Cycle: The reservoir of Phosphorus is located in the earth's crust. Phosphorus cycle is one of the examples of sedimentary cycle. Rocks are the natural reservoirs of Phosphorus. It is present in rocks in the form of phosphates. During weathering of rocks, minute quantities of phosphates get dissolved in soil solution. 
Phosphates in soil solution are absorbed by roots of plants. The primary and secondary consumers get phosphates from plants. 
When dead organisms are decomposed by phosphates-solubilising bacteria, Phosphorus is released back to the environment.