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Biology

Class 12 · Science & Environment

Ch 12 · Ecosystem

Real UPSC Prelims questions from 2016–2026, linked to this book. Try each one first, then open the answer. 2026 answers follow UPSC's provisional answer key.

Prelims 2021 · Q27

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?

  1. (a)Carbon cycle
  2. (b)Nitrogen cycle
  3. (c)Phosphorus cycle
  4. (d)Sulphur cycle
Show answer

UPSC's answer: (c)

NCERT answers it: Class 12 Biology, Ch 12 (Ecosystem); Class 9 Exploration, Ch 13 (Earth as a System: Energy, Matter, and Life).

The one fact NCERT doesn't give: Phosphorus has no stable gaseous form in the air, unlike carbon, nitrogen and sulphur, so it enters the cycle mainly from rocks as they weather.

Class 12 Biology, ch 12: the carbon cycle's reservoir is the air or water, but the phosphorus cycle's reservoir is the Earth's crust. Class 9 Exploration, ch 13: nitrogen's largest reservoir is the air. Class 10 Contemporary India II, ch 6: sulphur dioxide is one of the gases in polluted air. With the fact, only phosphorus has to come from rocks. Answer (c).

Hindi version: UPSC's official question paper (2021), question 27 ↗

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