UPSC Prelims 2016 CSAT Paper II · Q67 of 79 Comprehension medium

Passage

Passage-4 An increase in human-made carbon dioxide in the atmosphere could initiate a chain reaction between plants and microorganisms that would unsettle one of the largest carbon reservoirs on the planet – soil. In a study, it was found that the soil, which contains twice the amount of carbon present in all plants and Earth's atmosphere combined, could become increasingly volatile as people add more carbon dioxide to the atmosphere. This is largely because of increased plant growth. Although a greenhouse gas and a pollutant, carbon dioxide also supports plant growth. As trees and other vegetation flourish in a carbon dioxide-rich future, their roots could stimulate microbial activity in soil that may in turn accelerate the decomposition of soil carbon and its release into the atmosphere as carbon dioxide.

Which among the following is the most logical corollary to the above passage ?

  1. (a) Carbon dioxide is essential for the survival of microorganisms and plants
  2. (b) Humans are solely responsible for the release of carbon dioxide into the atmosphere
  3. (c) Microorganisms and soil carbon are mainly responsible for the increased plant growth
  4. (d) Increasing green cover could trigger the release of carbon trapped in soil ✓ UPSC's answer

Why the answer is (d)

• The passage states that increased atmospheric CO2 supports plant growth, leading to flourishing vegetation.

• It explains that the roots of these plants stimulate microbial activity in the soil.

• This heightened microbial activity accelerates the decomposition of soil carbon.

• Consequently, the decomposed soil carbon is released back into the atmosphere as CO2.

• Therefore, the logical corollary is that increasing green cover (plant growth) triggers the release of carbon trapped in the soil.

Why the other options are wrong

(a) Carbon dioxide is essential for the survival of microorganisms and plants
The passage does not state that CO2 is essential for the survival of microorganisms, only that it supports plant growth which indirectly affects microbes.
(b) Humans are solely responsible for the release of carbon dioxide into the atmosphere
The passage attributes the increase to human-made CO2 but does not claim humans are solely responsible for all CO2 release, as natural processes like soil decomposition also release it.
(c) Microorganisms and soil carbon are mainly responsible for the increased plant growth
The passage identifies increased atmospheric CO2 as the driver of plant growth, not microorganisms and soil carbon.

Asked in the CSAT Paper II of the UPSC Civil Services Preliminary Examination 2016, held on 7 August 2016. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.

Reading the answer is not the same as getting it right under a clock. Practise this question with UPSC's negative marking, and anything you miss goes into an error notebook until you get it right twice.

Practise this paper free