medium difficulty7 min

Nutrient Cycling

Nutrient cycling is the repeated movement of essential elements between living organisms and the physical environment through biological, geological and chemical processes.

In one line: Energy flows away; nutrients come around again.

Concept Practice Coming Soon

Understand it

Unlike energy, nutrients are recycled in ecosystems.
Elements such as carbon, phosphorus, nitrogen and sulphur move between organisms, soil, water and atmosphere.
A reservoir pool is the main storage site of a nutrient, while an exchange pool is the part actively exchanged with organisms.
Gaseous cycles usually have the atmosphere or hydrosphere as the major reservoir, while sedimentary cycles have rocks or sediments as the reservoir.
Carbon cycle involves photosynthesis, feeding, respiration, decomposition and combustion.
Phosphorus cycle mainly moves through rocks, soil, water, plants, animals and decomposers and lacks a major atmospheric phase.
Exams often ask students to compare carbon and phosphorus cycles.

Example

Carbon dioxide from the atmosphere is fixed by green plants during photosynthesis, transferred to animals through food, returned by respiration and decomposition, and also added by burning organic fuels.

Rule to remember

Key comparison rule: gaseous cycles have atmospheric or hydrospheric reservoirs, while sedimentary cycles have rock or sediment reservoirs.

Carbon cycle is mainly gaseous; phosphorus cycle is mainly sedimentary.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

1For a carbon cycle diagram, show atmospheric carbon dioxide entering plants through photosynthesis, moving to animals through feeding, and returning through respiration, decomposition and combustion.
2This shows both biological transfer and environmental return.

Method you can reuse

1Define nutrient cycling.
2Name the nutrient or cycle asked.
3Identify the reservoir pool.
4Describe entry into producers.
5Show transfer through food chain.
6Explain return by respiration, excretion, decomposition, weathering or sedimentation as applicable.
7Add one comparison if carbon and phosphorus are both mentioned.

Sketch it in your answer

Use carbon and phosphorus cycle diagrams. Carbon labels include atmospheric carbon dioxide, photosynthesis, feeding, respiration, decomposition and combustion. Phosphorus labels include rocks, weathering, soil phosphate, plants, animals, decomposers, sediments and aquatic bodies.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

How is nutrient cycling different from energy flow?

Think first, then show answer

Nutrient cycling involves repeated movement of matter between organisms and the environment, so nutrients are reused. Energy flow is one-way because energy is lost as heat at each trophic transfer.

Common wrong answer

Writing that nutrients are lost as heat like energy.

Why it loses marks: Matter is recycled through ecosystem compartments.

Don't mix this up

Students often write that phosphorus has a major gaseous phase like carbon. Phosphorus cycling is mainly sedimentary, with rocks as an important reservoir.

Exam tip

In nutrient cycle answers, identify the reservoir first, then explain how the nutrient enters organisms and returns to the environment.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Asked as labelled carbon or phosphorus cycle diagrams, comparison between gaseous and sedimentary cycles, reservoir-pool questions, and short answers distinguishing nutrient cycling from energy flow.

Answer plan

Define nutrient cycling. State that nutrients are recycled between biotic and abiotic components. Identify the reservoir and exchange pool. Describe the main steps of the named cycle with a labelled diagram. Add one specific example such as carbon or phosphorus.

1
Enough for 1 mark
Nutrient cycling is the repeated movement of essential elements between living organisms and the physical environment through biological, geological and chemical processes.
2
Add for 2 marks
Example: Carbon dioxide from the atmosphere is fixed by green plants during photosynthesis, transferred to animals through food, returned by respiration and decomposition, and also added by burning organic fuels.
3
Add for 3 marks
Unlike energy, nutrients are recycled in ecosystems.

Practice

Concept practice is coming soon

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