UPSC Prelims 2021 Q27: Environment & Ecology – Biogeochemical Cycles & Earth Systems
- (a) Carbon cycle
- (b) Nitrogen cycle
- (c) Phosphorus cycle
- (d) Sulphur cycle
Answer: (c) Phosphorus cycle
Explanation
Biogeochemical Cycles: Weathering as Nutrient Source
| Cycle | Primary Source of Nutrient | Weathering-dependent? |
|---|---|---|
| Carbon cycle | Atmosphere (CO2), oceans | NO - gaseous cycle |
| Nitrogen cycle | Atmosphere (78% N2) | NO - gaseous cycle |
| Phosphorus cycle | Weathering of phosphate rocks (apatite) | YES - sedimentary cycle |
| Sulphur cycle | Volcanic emissions, atmosphere, sediments | Partial (mixed cycle) |
The Phosphorus cycle is the classic SEDIMENTARY cycle - it lacks a gaseous phase. Phosphorus enters ecosystems only when phosphate-bearing rocks (apatite) weather and release phosphate ions into soil and water. There is no significant atmospheric reservoir. This makes phosphorus a common limiting nutrient in agriculture and freshwater ecosystems.
Answer: (c) Phosphorus cycle.
Why each option is right or wrong
- (a) Incorrect. Carbon cycle is gaseous (atmospheric CO2 pool), not weathering-driven.
- (b) Incorrect. Nitrogen cycle is gaseous (78% N2 in atmosphere), not weathering-driven.
- (c) CORRECT. Phosphorus is the classic sedimentary cycle - it lacks an atmospheric pool. Phosphorus enters ecosystems only through weathering of phosphate rocks (apatite). Hence phosphorus is often the limiting nutrient in agriculture.
- (d) Incorrect. Sulphur cycle has both gaseous (SO2, H2S) and sedimentary components; weathering is not its main source.
Key point
Phosphorus = sedimentary cycle (no gaseous phase). Enters ecosystems via weathering of phosphate rocks (apatite). Limiting nutrient in agriculture. Carbon, Nitrogen = gaseous cycles. Sulphur = mixed cycle.
Remember
The Phosphorus cycle is a sedimentary cycle where the weathering of rocks (phosphate rocks like apatite) is the main source of nutrient entering the cycle. Phosphorus lacks a gaseous phase, making it the classic limiting nutrient in agriculture. Correct answer: (c).
Common mistakes
- Not distinguishing gaseous cycles (C, N, O) from sedimentary cycles (P, Ca, Fe, S partly). Sedimentary cycles depend on weathering.
- Assuming all nutrients cycle through the atmosphere. Phosphorus does not - hence its scarcity in soils.
- Missing that phosphate rock depletion is a global food security concern (Morocco holds ~70% of world reserves).
Exam tip
Gaseous cycles: C, N, O, S (mostly). Sedimentary cycles: P, Ca, Fe. Phosphorus is the classic sedimentary cycle - always weathering-based. Eutrophication in Indian lakes is often P-driven (from detergents, fertilisers).
Sources
- NCERT Class XII Biology - Chapter 15: Biodiversity and Ecosystems (Nutrient Cycling)
- NCERT Class XI Biology - Ecosystem Nutrient Cycles