UPSC Prelims 2021 GS Paper I · Q27 of 98 Environment & Ecology medium

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 ✓ UPSC's answer
  4. (d) Sulphur cycle

Why the answer is (c)

• The phosphorus cycle is a sedimentary cycle with no significant gaseous phase; phosphate enters ecosystems mainly when phosphate-bearing rocks (apatite) weather and release it into soil and water — option (c).

• Carbon (a), nitrogen (b) and sulphur (d) all have major atmospheric reservoirs (CO2, N2, SO2/H2S), so weathering is not their main entry route.

• Because rock weathering is slow, phosphorus is often the limiting nutrient in lakes and oceans, and excess phosphate from fertilisers causes eutrophication.

• Hence option (c).

Why the other options are wrong

(a) Carbon cycle
Carbon enters mainly via atmospheric CO2 and photosynthesis.
(b) Nitrogen cycle
Nitrogen enters mainly via fixation from atmospheric N2.
(d) Sulphur cycle
Sulphur has a significant atmospheric phase.

Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2021, held on 10 October 2021. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.

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