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?
Answer & explanation
Answer: (c) Phosphorus cycle
Nutrient cycles are either gaseous (reservoir in the air or water) or sedimentary (reservoir in the Earth's crust). Phosphorus has no significant gaseous form, so its supply to soils and water comes mainly from the slow weathering of phosphate-bearing rocks. Carbon, nitrogen and sulphur all have major atmospheric pools.
- ✓ (c) The phosphorus cycle is sedimentary: rocks are its reservoir, and weathering releases phosphate that plants take up from the soil.
- ✗ (a) Carbon cycles mainly through the atmosphere and oceans as carbon dioxide, entering food chains by photosynthesis.
- ✗ (b) The atmosphere is the largest reservoir of nitrogen; it enters living systems mainly through nitrogen fixation, not rock weathering.
Remember · Gaseous cycles: carbon, nitrogen (sulphur has a large gaseous part). Sedimentary cycle: phosphorus, whose reservoir is the Earth's crust and whose source is rock weathering.
📘 Read it in NCERT: Class 12 Biology, Ch 12 (practise this chapter) · Class 9 Exploration, Ch 13 (practise this chapter)
Sources
- NCERT Class 12 · Biology, Chapter 12 “Atmosphere or hydrosphere is the reservoir for the gaseous type of cycle (carbon), whereas Earth's crust is the reservoir for sedimentary type (phosphorus).”
- NCERT Class 9 · Exploration, Chapter 13 “The largest reservoir of nitrogen is in our atmosphere.”
- University of California Museum of Paleontology, Understanding Global Change: Phosphorus ↗ · reference work “There are no major gaseous forms of phosphorus, so aside from wind distributing small particles of dust containing phosphorus, only very small amounts of phosphorus move through the atmosphere.”
Question and answer: UPSC's official GS Paper I (2021, Series A) — paper ↗ · answer key ↗. Explanation: Minimalist IAS, checked 30 Sept 2026 (how we verify). ·