Maarg Manthan · Topic 5.1

Concept of Biogeochemical Cycle: Parts and Types

Environment & Ecology › Nutrient Cycling (Biogeochemical Cycles) · Topic 5.1

A biogeochemical cycle is the pathway by which a chemical element or compound moves between the living (bio) and non-living (geo) parts of the Earth, through chemical, physical and biological processes. Matter is finite and is used again and again, unlike energy, which flows one way and is lost as heat. Each cycle has two parts: a reservoir pool, a large, slow-moving store such as the atmosphere, rocks or oceans, and an exchange or cycling pool, a smaller, fast-moving store between organisms and their immediate surroundings. Cycles are grouped into gaseous cycles, with the atmosphere or ocean as the main reservoir (carbon, nitrogen, oxygen, water), and sedimentary cycles, with the Earth's crust as the reservoir (phosphorus, sulphur, calcium). Human activity now disturbs most cycles.

6 min read · UPSC Prelims · Environment & Ecology · Nutrient Cycling (Biogeochemical Cycles)

At a Glance

Meaning Movement of elements through life and earth
Two pools Reservoir and exchange
Gaseous cycles Carbon, nitrogen, oxygen, water
Sedimentary cycles Phosphorus, sulphur, calcium
Energy versus matter Energy flows, matter cycles
Exam link UPSC Prelims, Environment & Ecology: Nutrient Cycling (Biogeochemical Cycles)

Where Does It Fit?

This is the first topic of the macro on nutrient cycling. It sets out the common vocabulary used in every cycle that follows: carbon, water, nitrogen, oxygen, sulphur and phosphorus. Previous topic: Human-Modified Ecosystems. Next topic: Carbon Cycle: Steps, Long-Term and Short-Term Cycles, Carbon as a Climate Buffer.

What Are the Parts and Types of a Biogeochemical Cycle?

Feature	Details
Meaning	Circulation of nutrients and elements between biotic and abiotic components. Also called nutrient cycles or mineral cycles
Reservoir (storage) pool	A large, slow, mostly non-biological store, such as the atmosphere for nitrogen, rocks for phosphorus, and oceans for carbon. Elements stay here for a long time
Exchange (cycling) pool	A small, active pool where an element moves rapidly between organisms and their immediate environment, such as nutrients in soil water and plant tissues
Gaseous cycle	Main reservoir is the atmosphere or hydrosphere. Cycles are fast and tend to be self-correcting. Examples: carbon, nitrogen, oxygen, hydrological
Sedimentary cycle	Main reservoir is the lithosphere. Cycles are slow, and elements may be locked in rocks for millions of years. Examples: phosphorus, sulphur, calcium, potassium
Residence time	Average time an atom stays in a pool. Short in the atmosphere for water vapour (about nine days), long for carbon in rocks
Flux	The rate of movement of an element from one pool to another

Why Do Cycles Matter and How Are They Disturbed?

Life support: all essential elements, such as carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur, are recycled so that life continues with a finite supply
Link with energy: energy flows one way, from the sun to heat, but drives the movement of matter through photosynthesis and decomposition
Role of decomposers: bacteria and fungi release nutrients from dead matter and return them to the exchange pool
Biological, geological and chemical processes: photosynthesis, respiration, weathering, volcanism, sedimentation and precipitation move elements
Human disturbance: burning fossil fuels, fertiliser use, deforestation, mining and urbanisation speed up some fluxes
Consequences: climate change, eutrophication, acid rain, ozone depletion and soil degradation
Gaseous cycles are more stable: because the atmosphere buffers disturbances, though human emissions now exceed this capacity
Sedimentary cycles are less perfect: because some material is lost to deep sediments for geological time

What Else Should You Remember?

Nutrient cycle: another name emphasising mineral elements needed by plants
Macronutrients: needed in large amounts: carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, calcium, potassium and magnesium
Micronutrients: needed in traces, such as iron, zinc, copper, boron and molybdenum
Perfect cycle: an idealised cycle with little loss, as in gaseous cycles with a large atmospheric pool
Imperfect cycle: a cycle where some material is lost to sediments, as in phosphorus
Atmosphere: reservoir for nitrogen (about 78 percent), oxygen (about 21 percent) and carbon dioxide (a small fraction)
Lithosphere: reservoir for phosphorus, sulphur and most carbon in carbonate rocks
Hydrosphere: reservoir for water and dissolved carbon
Biosphere: living matter, a small but active pool
Weathering: breaks rocks and releases minerals
Decomposition: converts organic matter to inorganic nutrients
Mineralisation: release of inorganic nutrients from organic matter
Immobilisation: uptake of nutrients into microbial and plant tissues
Leaching: loss of nutrients dissolved in water moving downwards
Run-off: loss of nutrients carried over the surface to rivers and lakes
Sedimentation: settling of material in oceans and lakes, forming rocks over time
Nutrient retention in forests: tropical forests hold most nutrients in biomass, while temperate forests hold much in soil and litter
Hubbard Brook study: a forest experiment in New Hampshire showed that cutting trees greatly raised nutrient loss in streams
Link to ecosystem: see the macro on Ecosystem Structure and Function for energy flow
Link to pollution: see Air, Marine, Noise and Other Pollution macro for excess nutrients and eutrophication
Link to climate: see Climate Change Basics and Causes macro for carbon and nitrogen gases
Caution: some books class the water cycle separately as a physical cycle, though it is also included as a gaseous cycle
Biological pump: movement of carbon from surface seawater to the deep ocean by sinking organisms
Anthropogenic fluxes: human-caused flows now rival natural flows for nitrogen and sulphur
Planetary boundaries: a framework that includes nitrogen and phosphorus flows as limits that should not be crossed
Exam hint: remember the grouping: carbon, nitrogen, oxygen and water as gaseous, and phosphorus and sulphur as sedimentary

Why Does It Matter for the Exam?

Pools: reservoir and exchange
Types: gaseous and sedimentary
Examples: carbon and nitrogen versus phosphorus
Current affairs link: planetary boundaries and nitrogen management
Common errors: saying energy cycles, or calling phosphorus a gaseous cycle

As an analytical point, biogeochemical cycles link all the spheres of the Earth, and human activity has speeded up flows of carbon, nitrogen and phosphorus. Understanding cycles is the base for climate, pollution and soil management policy.

Why Is This Topic Important?

Pools Reservoir and exchange
Types Gaseous and sedimentary
Disturbance Humans

What Came Next?

The next topic covers the carbon cycle, with its long-term and short-term loops and its role as a climate buffer. Next topic: Carbon Cycle: Steps, Long-Term and Short-Term Cycles, Carbon as a Climate Buffer.

Key People and Terms

Reservoir pool Large slow store
Exchange pool Small fast store
Residence time Time an atom stays in a pool
Flux Rate of transfer
Mineralisation Release of inorganic nutrients
Leaching Loss by downward water

Exam Corner

Points to Remember

Matter cycles; energy flows.
Gaseous: C, N, O, water.
Sedimentary: P, S, Ca.
Reservoir is large and slow.
Decomposers release nutrients.

Do Not Confuse With

Reservoir and exchange pool Slow large store versus fast small store
Gaseous and sedimentary cycle Atmosphere versus crust reservoir
Energy flow and nutrient cycle One-way versus circular

Memory Hook

Matter goes round, energy goes through; gases in air, sediments in rock.

Mains Angle

Questions ask for pools and types. These points help.

Concept Pools
Types Gaseous, sedimentary
Human impact Disturbed fluxes

A Question You May Face

An original practice question, not a past paper question.

“What is a biogeochemical cycle? Distinguish gaseous and sedimentary cycles with examples, and explain how human activity disturbs them.”

How to Answer

Define Cycle of elements.
Pools Reservoir, exchange.
Types Gaseous, sedimentary.
Impact Human.

GS Relevance

Prelims: definitions, pools, gaseous versus sedimentary cycles, examples. GS Paper 3: ecology, climate and human impact on cycles.

Frequently Asked Questions

What is a biogeochemical cycle?

A biogeochemical cycle is the path an element or compound follows as it moves through living organisms, the atmosphere, water and rocks. It explains how a finite supply of matter, such as carbon or nitrogen, is reused repeatedly in the biosphere, driven by solar energy and the activity of decomposers.

What is the difference between reservoir pool and exchange pool?

The reservoir pool is the large, slow-moving store of an element, usually non-living, such as rocks, the atmosphere or oceans. The exchange pool is the smaller, fast-moving portion that cycles quickly between organisms and their immediate surroundings, such as nutrients in soil water and plant tissues.

What are gaseous and sedimentary cycles?

In gaseous cycles, such as carbon, nitrogen, oxygen and water, the main reservoir is the atmosphere or ocean and the cycle is fast. In sedimentary cycles, such as phosphorus and sulphur, the main reservoir is the Earth's crust and the cycle is slow, with elements released by weathering.

Why is energy flow different from nutrient cycling?

Energy enters the ecosystem from the sun, is passed along food chains and is finally lost as heat, so it flows in one direction and needs a constant supply. Nutrients, in contrast, are finite and are used again and again, moving in a circle between organisms and the environment.

How do humans disturb biogeochemical cycles?

Humans disturb cycles by burning fossil fuels and clearing forests, which raises carbon dioxide, using nitrogen and phosphorus fertilisers, which pollute waters, and mining and burning sulphur-rich fuels, which cause acid rain. These changes cause climate change, eutrophication and soil degradation.

PYQ Practice — Statement Analysis

1 Carbon and nitrogen cycles are gaseous cycles.

True

The atmosphere is the reservoir.

2 Phosphorus has a major atmospheric reservoir.

False

It is a sedimentary cycle.

3 Energy flows through ecosystems in one direction.