Q. In case of which one of the following biogeochemical cycles, the weathering of rocks is the main source of release of nutrient to enter the cycle? (UPSC Prelims 2021)
Answer: Phosphorus cycle
Notes: The correct answer is [C] Phosphorus cycle. Biogeochemical cycles are divided into gaseous and sedimentary types based on their primary reservoir.

• Phosphorus Cycle (Correct): Phosphorus is a sedimentary cycle. Unlike nitrogen or carbon, it does not have a significant gaseous phase in the atmosphere. Its primary reservoir is in the Earth's crust. The weathering of phosphate-bearing rocks (apatite) by rain and chemical runoff releases phosphate ions into the soil and water, making them available to plants and the food chain.
• Carbon Cycle (Incorrect): While some carbon is stored in limestone and released through weathering, the "main" source for the active cycle is the atmosphere (as carbon dioxide) and the oceans.
• Nitrogen Cycle (Incorrect): This is a gaseous cycle. Its primary reservoir is the atmosphere, which is 78% nitrogen gas (N_{2}). It enters the biological cycle through nitrogen fixation by bacteria or lightning, not primarily through rock weathering.
• Sulphur Cycle (Incorrect): Sulphur has both gaseous (SO_{2}, H_{2}S) and sedimentary phases. While weathering does release some sulphur from pyrite or gypsum, the atmosphere and volcanic activity are also major components, making the phosphorus cycle the more definitive answer for weathering as the main source.

Phosphorus is a limiting nutrient in many ecosystems because its release through the slow process of geological weathering is much slower than the cycling of atmospheric nutrients.