Biogeochemical Cycles — Drills · Grade 10 Science

Chapter: Ecosystems & Conservation Decisions

What these drills covers

By the end of Biogeochemical Cycles you should be able to:

6 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: biogeochemical cycle, carbon cycle, nitrogen fixation, transpiration, photosynthesis, cellular respiration, nitrification, denitrification.

Problem 1

Carbon moves from the atmosphere into living things mainly through
  1. respiration
  2. photosynthesis
  3. combustion
  4. decomposition
Answer and explanation

photosynthesis

Photosynthesis fixes atmospheric carbon dioxide into organic molecules; the other three processes return carbon to the atmosphere.

Problem 2

Nitrogen fixation converts
  1. nitrates into nitrogen gas
  2. atmospheric nitrogen gas into forms organisms can use
  3. ammonia into protein
  4. nitrogen into carbon
Answer and explanation

atmospheric nitrogen gas into forms organisms can use

Nitrogen gas is unreactive, and fixation by certain bacteria, by lightning, or industrially converts it into usable compounds.

Problem 3

A forest reservoir holds 800 t of carbon, gains 45 t in a year and loses 30 t. What is the net change in tonnes?
Answer and explanation

15 t

45 − 30 = 15 t gained, so the reservoir is growing. Net change is the difference between the fluxes in and out.

Problem 4

Of 60 t of carbon leaving a soil reservoir annually, 48 t leaves by decomposition. What percentage is that?
Answer and explanation

80 %

48 / 60 × 100 = 80 percent, identifying decomposition as the dominant outward flux.

Problem 5

Explain why nitrogen is often a limiting nutrient for plant growth even though the atmosphere is mostly nitrogen.
Answer and explanation

Atmospheric nitrogen exists as N₂, in which the two atoms are held by a strong triple bond that makes the molecule unreactive, and plants cannot use it in that form. Plants require nitrogen as nitrate or ammonium, which must be produced by nitrogen-fixing bacteria, by lightning, or industrially. The supply of usable nitrogen therefore depends on the rate of those conversions rather than on atmospheric abundance, which is why nitrogen limits growth in many soils.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 6

Describe two processes that return carbon to the atmosphere and one that removes it for long periods.
Answer and explanation

Respiration by all organisms releases carbon dioxide, and decomposition of dead material releases it as decomposers respire; combustion of wood or fossil fuels also returns it. Long-term removal occurs when carbon is buried as organic matter or incorporated into carbonate rock and sediments, where it can remain for geological spans, and such reservoirs hold carbon out of circulation until they are exposed, eroded, or burned.

Response criteria

  • State a biologically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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