biogeochemical-cycles MCQs for UPSC Prelims

25 practice questions on biogeochemical-cycles from the Earth as a System: Energy, Matter, and Life section of the UPSC Prelims syllabus. 25 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

7 Easy 13 Medium 5 Hard

Sample questions

Q1
easy
Which process puts water vapour into the air from plants?
  1. A Run off
  2. B Condensation
  3. C Transpiration
  4. D Precipitation
Show answer and explanation

Correct answer: C - Transpiration

Plants release water vapour into the air by transpiration, which feeds the water cycle alongside evaporation from rivers, lakes and oceans. Precipitation brings water down as rain, hail or snow. Condensation turns vapour into cloud droplets. Run off is water flowing over the ground instead of soaking into it.
Q2
hard
Nitrifying bacteria act in two steps. Which sequence of substances do they produce?
  1. A Nitrogen gas to nitrate, and then nitrate to ammonia
  2. B Nitrate to nitrite, and then nitrite to ammonia
  3. C Ammonia to nitrate, and then nitrate to nitrite
  4. D Ammonia to nitrite, and then nitrite to nitrate
Show answer and explanation

Correct answer: D - Ammonia to nitrite, and then nitrite to nitrate

Nitrosomonas converts ammonia into nitrite, and Nitrobacter then converts that nitrite into nitrate, and together these two steps make up nitrification. Running the pair in the reverse order reverses the whole process. Turning nitrate back towards ammonia is not what nitrifying bacteria do. Converting nitrogen gas straight into a usable compound is fixation, the work of quite different bacteria.
Q3
easy
Water vapour rising from the oceans cools high in the air and turns back into tiny droplets that build clouds. What is this change called?
  1. A Infiltration
  2. B Condensation
  3. C Evaporation
  4. D Precipitation
Show answer and explanation

Correct answer: B - Condensation

Vapour turning back into liquid droplets is condensation, and it is how clouds form. Evaporation is the opposite change, liquid water becoming vapour. Precipitation is the fall of rain, hail or snow from those clouds. Infiltration is water seeping down through soil and rock to become groundwater.
Q4
medium
In the fast part of the carbon cycle, which runs over days to years, by which routes does carbon return to the atmosphere?
  1. A Through photosynthesis by plants and through the burial of dead matter
  2. B Through respiration by living things and through decomposition when they die
  3. C Through carbon being taken up into the shells of marine organisms
  4. D Through the slow burial of plants and animals to form rock over millions of years
Show answer and explanation

Correct answer: B - Through respiration by living things and through decomposition when they die

In the fast cycle plants take in carbon dioxide, and it comes back to the air when living things respire and when decomposers break down their dead bodies. Photosynthesis moves carbon the other way, out of the air, and burial holds carbon rather than releasing it. Burial that forms fossil fuel is the slow cycle, taking millions of years. Carbon locked into shells is being stored, not returned.
Q5
medium
What is the slow part of the carbon cycle?
  1. A Plants converting carbon dioxide into glucose over one growing season
  2. B Carbon dioxide dissolving into sea water within a few hours
  3. C Animals eating plants and breathing out carbon dioxide within a day
  4. D Dead plants and animals being buried and turned into coal, oil and gas over millions of years
Show answer and explanation

Correct answer: D - Dead plants and animals being buried and turned into coal, oil and gas over millions of years

The slow cycle is the burial of dead organisms and their conversion into fossil fuels such as coal, oil and gas, a process taking millions of years. Photosynthesis over a season and an animal breathing within a day belong to the fast cycle of days to years. Carbon dioxide passing between the air and the sea happens continuously and quickly, so it is not the slow route either.
Q6
medium
Imagine a forest in which every decomposer suddenly disappeared. What would happen over the following years?
  1. A Dead matter would pile up, and the nutrients locked inside it would stop returning to the soil and the air
  2. B Dead matter would vanish faster, because plants would absorb it whole
  3. C Nitrogen fixation would stop at once, since it depends on decomposers
  4. D Photosynthesis would speed up, because there would be less competition for light
Show answer and explanation

Correct answer: A - Dead matter would pile up, and the nutrients locked inside it would stop returning to the soil and the air

Decomposers are the route by which carbon returns to the air and nitrogen compounds return to the soil, so without them dead bodies and waste would accumulate with their nutrients still locked inside. Plants absorb dissolved nutrients through their roots and cannot take in whole dead bodies. Fixation is done by bacteria such as Rhizobium and Azotobacter working on nitrogen gas, so it would continue. Piled up dead matter gives plants no extra light.
Q7
medium
Denitrifying bacteria act on one substance and produce another. Which pair is right?
  1. A They convert nitrates into nitrogen gas
  2. B They convert nitrogen gas into nitrates
  3. C They convert nitrite into nitrate
  4. D They convert ammonia into nitrite
Show answer and explanation

Correct answer: A - They convert nitrates into nitrogen gas

Denitrifying bacteria such as Pseudomonas turn some of the nitrates in the soil back into nitrogen gas, which escapes to the atmosphere. Turning nitrogen gas into usable compounds is fixation and runs the other way. Converting ammonia to nitrite and nitrite to nitrate are the two nitrification steps, carried out by quite different bacteria.
Q8
medium
Which process is the main one that restores oxygen to the atmosphere?
  1. A Photosynthesis by plants
  2. B Combustion of fuels
  3. C Respiration by animals
  4. D Formation of metal oxides in the Earth's crust
Show answer and explanation

Correct answer: A - Photosynthesis by plants

Plants use sunlight, water and carbon dioxide to make glucose and release oxygen, so photosynthesis is what puts oxygen back into the air. Respiration consumes oxygen and gives out carbon dioxide. Burning fuel also uses up oxygen. Oxygen locked into metal oxides in the crust has been taken out of circulation rather than returned to the air.

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