JAMB / WAEC Biology: Nutrient Cycling Explained (Part 1) — Nitrogen, Carbon & Water Cycles

Nutrient cycling is the continuous movement of essential elements and compounds, such as nitrogen, carbon and water, between living organisms and the non-living environment. Unlike energy, which flows through an ecosystem and is eventually lost as heat, nutrients are used over and over again.

This is Part 1 of our Nutrient Cycling series, covering the nitrogen cycle, the carbon cycle and the water cycle. These cycles are regular features in JAMB and WAEC Biology, usually tested through questions on the stages of each cycle and the organisms involved. This lesson explains all three clearly.

Quick takeaways

• Nutrient cycling is the repeated movement of elements like nitrogen, carbon and water between living things and the environment, so that they are never permanently used up.
• In the nitrogen cycle, bacteria convert atmospheric nitrogen into forms plants can absorb, and other bacteria return nitrogen to the air.
• In the carbon cycle, photosynthesis removes carbon dioxide from the air, while respiration, decay and burning return it.
• In the water cycle, water moves between the oceans, the air and the land through evaporation, transpiration, condensation and precipitation.
• Exam questions often ask you to name the stages in order, identify the organisms involved (such as Rhizobium), or state the human activities that disturb a cycle.

Timestamps

• 0:00 Introduction
• 0:51 Carbon Cycle — Definition
• 0:59 Carbon Cycle — Diagram and Process
• 3:58 The Greenhouse Effect
• 4:17 Global Warming
• 5:33 Melting Ice Glaciers
• 5:50 Water Cycle — Definition
• 6:02 Water Cycle — Diagram and Process
• 7:07 Infiltration
• 7:28 Percolation

What is nutrient cycling?

Nutrient cycling, also called biogeochemical cycling, is the continuous movement of essential substances between the living parts of an ecosystem (plants, animals, microorganisms) and the non-living parts (air, soil, water). It matters because the Earth has a fixed supply of these materials.

Decomposers and other processes keep returning them to a usable form, so life can continue generation after generation.

What is the nitrogen cycle?

Nitrogen makes up about 78% of the air, but most plants and animals cannot use it in its gaseous form. The nitrogen cycle shows how nitrogen is converted into usable forms and then returned to the atmosphere. The main stages are:

1. Nitrogen fixation — gaseous nitrogen is converted into nitrogen compounds. This is done by nitrogen-fixing bacteria such as Rhizobium (living in the root nodules of leguminous plants like beans), by free-living bacteria in the soil, and by lightning.
2. Nitrification — nitrifying bacteria convert ammonium compounds in the soil into nitrites and then into nitrates, which plants can absorb.
3. Assimilation — plants absorb nitrates through their roots and use them to build proteins. Animals obtain nitrogen by eating plants or other animals.
4. Ammonification (decay) — when plants and animals die, or when animals excrete waste, decomposers break down the proteins and release ammonia and ammonium compounds back into the soil.
5. Denitrification — denitrifying bacteria convert nitrates in the soil back into gaseous nitrogen, which returns to the atmosphere.

What is the carbon cycle?

Carbon is the basic building block of all organic compounds, such as carbohydrates, fats and proteins. The carbon cycle describes how carbon moves between the atmosphere, living organisms and the Earth. The main processes are:

• Photosynthesis — green plants absorb carbon dioxide from the air and use it to make food, locking carbon into organic compounds.
• Feeding — animals obtain carbon by eating plants or other animals.
• Respiration — plants, animals and microorganisms release carbon dioxide back into the air as they break down food to release energy.
• Decomposition — when organisms die, decomposers break down their remains and release carbon dioxide through respiration.
• Combustion — burning wood, coal, oil and gas releases stored carbon back into the air as carbon dioxide.
• Fossil fuel formation — in some conditions, dead organic matter does not fully decay and over millions of years forms coal, oil and natural gas, which store carbon.

What is the water cycle?

The water cycle describes the continuous movement of water between the oceans, the atmosphere and the land. The main stages are:

1. Evaporation — heat from the sun turns water from oceans, rivers and lakes into water vapour.
2. Transpiration — plants lose water vapour through their leaves, adding moisture to the air.
3. Condensation — as water vapour rises and cools, it condenses into tiny droplets that form clouds.
4. Precipitation — water falls back to the Earth as rain, hail or snow.
5. Runoff and infiltration — some water flows over the land into streams, rivers and the sea (runoff), while some soaks into the soil and becomes groundwater (infiltration), from where plants absorb it or it eventually returns to water bodies.

How do human activities affect these cycles?

Several human activities can disturb nutrient cycles:

• Burning fossil fuels and clearing forests increase the carbon dioxide in the air, since fewer trees are left to absorb it.
• Overuse of nitrogen fertilizers can wash into rivers and lakes, polluting water and disturbing aquatic life.
• Deforestation reduces transpiration and can reduce local rainfall, disturbing the water cycle.

These points link directly to the pollution and conservation topics in our Human and His Environment lesson.

Common mistakes students make with nutrient cycles

• Mixing up nitrification (ammonium to nitrates) with denitrification (nitrates back to nitrogen gas)
• Forgetting that nitrogen fixation requires specific bacteria such as Rhizobium, and that plants cannot use gaseous nitrogen directly
• Confusing photosynthesis (removes carbon dioxide from the air) with respiration (releases carbon dioxide)
• Leaving out transpiration from the water cycle and mentioning only evaporation

Frequently asked questions

Q: What is the role of Rhizobium in the nitrogen cycle? Rhizobium is a nitrogen-fixing bacterium that lives in the root nodules of leguminous plants and converts atmospheric nitrogen into compounds the plant can use, in a mutually beneficial relationship.

Q: What is the difference between nitrification and denitrification? Nitrification is the conversion of ammonium compounds into nitrites and then nitrates, which plants can absorb, while denitrification is the conversion of nitrates back into gaseous nitrogen, returning it to the atmosphere.

Q: How is carbon dioxide returned to the atmosphere in the carbon cycle? Carbon dioxide is returned through respiration by living organisms, decomposition of dead organic matter, and combustion of wood and fossil fuels.

Q: What is the difference between evaporation and transpiration? Evaporation is the conversion of water from surfaces like oceans, rivers and soil into water vapour, while transpiration is the loss of water vapour from the leaves of plants.

Want to see how energy moves through the same ecosystems? Read our Energy Flow lesson → or revisit respiration → and how human activities affect the environment →

This is one topic from our full WAEC & JAMB Biology course — see everything covered →