RAS question
In the phosphorus cycle, the main reservoir of phosphorus is:
Correct answer: (C) Rocks and sediments.
In the phosphorus cycle, the main reservoir of phosphorus is rocks and sediments, especially phosphate rocks and marine sediments.
Explanation
The phosphorus cycle is distinctive among biogeochemical cycles because it has no gaseous phase. That is why the atmosphere is not its main store. The principal reservoir is phosphate rocks and marine sediments. Weathering releases phosphorus from these rocks into soil and water, where plants take it up and move it into food chains. It then returns through decomposition after organisms die or release waste. This slow rock-to-soil-to-biota pathway also explains why phosphorus is treated as a limiting nutrient in freshwater ecosystems: it is essential, but its supply depends on release from rocks and sediments rather than rapid atmospheric circulation.
Why the other options are wrong
- (A) Atmosphere is wrong because phosphorus has no significant atmospheric phase, which is the key contrast with cycles such as carbon and nitrogen.
- (B) Living organisms contain phosphorus and move it through food chains, but they are not the main reservoir of the element.
- (D) Oceans receive phosphorus and marine sediments store it, but the reservoir is not oceans only; phosphate rocks and sediments are the main store.
Concept
This tests biogeochemical cycles, especially the distinction between gaseous and sedimentary nutrient cycles. It recurs in RAS Environment because reservoir location, weathering, food-chain entry and limiting nutrients are standard ecology hooks.