Study Guide

Nitrogen and nutrient cycling

IB Biology SLΒ· Theme C: Interactions and Interdependencies, Unit 3Β· 15 min read

1. Principles of Nutrient Recyclingβ˜…β˜…β˜†β˜†β˜†β± 4 min

Unlike energy, which enters ecosystems as sunlight and is eventually lost as heat, nutrients (carbon, nitrogen, phosphorus, water) are continuously reused. They cycle repeatedly between abiotic components (soil, air, water) and biotic components (living organisms).

πŸ“˜ Definition

Nutrient cycling

The continuous movement of essential inorganic elements from the environment to living organisms, and back to the environment, sustaining life on Earth.

Example:

Nitrogen atoms in your DNA may have previously been part of a dinosaur or an ancient plant.

Saprotrophic bacteria and fungi (decomposers) are the key organisms that drive this process, breaking down dead organic matter to release inorganic nutrients back into soil for uptake by new plant growth.

πŸ“ Worked Example

Explain why energy flow is described as linear, while nutrient cycling is described as cyclical.

  1. 1

    Step 1: Energy enters ecosystems as sunlight, which is converted to chemical energy by producers, then transferred between trophic levels.

  2. 2

    Step 2: At every trophic level, energy is lost as heat from cellular respiration, and heat cannot be reused by the ecosystem. Energy never cycles, so flow is one-way (linear).

  3. 3

    Step 3: When organisms die, decomposers break down their organic tissue, releasing inorganic nutrients back into the soil.

  4. 4

    Step 4: These nutrients are then taken up by new plants to build new tissue, so the same atoms are reused indefinitely, making the process cyclical.

2. Key Processes of the Nitrogen Cycleβ˜…β˜…β˜…β˜†β˜†β± 5 min

Atmosphere is 78% nitrogen gas (Nβ‚‚), but almost all organisms cannot use Nβ‚‚ directly. The nitrogen cycle converts nitrogen between different chemical forms to make it available to build biological molecules like amino acids and DNA.

  • Nitrogen fixation: Nβ‚‚ β†’ ammonia (NH₃) done by nitrogen-fixing bacteria

  • Nitrification: Ammonia β†’ nitrites β†’ nitrates (NO₃⁻) done by nitrifying bacteria

  • Assimilation: Plant roots take up nitrates, incorporate into organic molecules, passed up food chain

  • Ammonification: Decomposers break down organic nitrogen into ammonia from dead matter/waste

  • Denitrification: Nitrates β†’ Nβ‚‚ released back to atmosphere done by anaerobic bacteria

πŸ“ Worked Example

Explain why the relationship between Rhizobium bacteria and leguminous plants is mutualistic.

  1. 1

    Step 1: Rhizobium is a nitrogen-fixing bacteria that lives in root nodules of legumes (beans, clover).

  2. 2

    Step 2: Rhizobium converts atmospheric nitrogen into ammonia, which the plant uses to make amino acids and nucleic acids. This is the benefit to the plant.

  3. 3

    Step 3: In return, the plant provides Rhizobium with carbohydrates from photosynthesis, and a protected, oxygen-free environment required for nitrogen fixation. This is the benefit to the bacteria.

  4. 4

    Step 4: Since both organisms benefit from the relationship, it is classified as mutualism.

Exam tip:

You will almost always get a question asking you to name or describe one step of the nitrogen cycle, so memorize all five core processes.

3. Role of Microorganisms in Nitrogen Cyclingβ˜…β˜…β˜…β˜†β˜†β± 4 min

All major chemical transformations in the nitrogen cycle are carried out exclusively by prokaryotic microorganisms. Without these bacteria, almost no life on Earth could access usable nitrogen, making them critical to all ecosystems.

πŸ“ Worked Example

A farmer has soil with very low nitrate concentration. Which group of bacteria should they encourage to grow to increase nitrate levels, and why?

  1. 1

    Step 1: Decomposers release ammonia from dead organic matter in soil, but plants cannot easily use large amounts of ammonia. They need nitrates.

  2. 2

    Step 2: Nitrifying bacteria first convert ammonia to nitrites (genus Nitrosomonas), then convert nitrites to nitrates (genus Nitrobacter).

  3. 3

    Step 3: Nitrates are the form of nitrogen most readily absorbed by plant roots, so encouraging the growth of nitrifying bacteria will increase soil nitrate levels.

4. Human Impacts on Nutrient Cyclingβ˜…β˜…β˜…β˜†β˜†β± 5 min

Human activity has doubled the amount of fixed nitrogen entering global ecosystems, mostly from artificial fertilizer production for agriculture. This excess nitrogen causes widespread environmental harm.

Excess nitrogen and phosphorus from fertilizer is often washed off agricultural land into rivers, lakes, and oceans, a process called runoff. This causes eutrophication, a common exam topic.

πŸ“ Worked Example

Outline the steps leading to eutrophication after fertilizer runoff enters a lake.

  1. 1

    Step 1: Excess nitrogen and phosphorus from runoff increases nutrient concentrations in the lake water.

  2. 2

    Step 2: High nutrient levels trigger rapid uncontrolled growth of algae, called an algal bloom.

  3. 3

    Step 3: The algal bloom blocks sunlight from submerged plants, which die because they cannot photosynthesize.

  4. 4

    Step 4: When the algae die, they are broken down by aerobic decomposer bacteria.

  5. 5

    Step 5: Decomposition uses up almost all of the dissolved oxygen in the lake water, creating hypoxic (low oxygen) conditions.

  6. 6

    Step 6: Most aquatic organisms like fish cannot survive low oxygen, leading to mass death and a 'dead zone' with very low biodiversity.

5. Common Pitfalls

Wrong move:

Claiming energy is recycled along with nutrients in ecosystems

Why:

Students frequently mix up the two core ecology concepts, as both move through trophic levels

Correct move:

Always state energy flows one-way and is lost as heat, while nutrients are continuously cycled

Wrong move:

Confusing denitrification with ammonification (decomposition)

Why:

The similar naming of nitrogen cycle steps leads to common mix-ups

Correct move:

Denitrification returns Nβ‚‚ to the atmosphere, ammonification releases ammonia from dead organic matter

Wrong move:

Stating that plants absorb nitrogen directly from the air

Why:

Students forget atmospheric nitrogen is inert and only bacteria can fix it

Correct move:

Plants only absorb nitrates or ammonium from soil through their roots

Wrong move:

Only describing the benefit to the plant when explaining Rhizobium mutualism

Why:

IB exam questions require you to show you understand mutualism benefits both organisms

Correct move:

Always state Rhizobium gets carbohydrates and protection, the plant gets fixed nitrogen

Wrong move:

Claiming algae use up all the oxygen during eutrophication

Why:

Students misattribute the cause of oxygen depletion in eutrophic water bodies

Correct move:

Oxygen is depleted by aerobic bacterial decomposition of dead algae and plants after the bloom dies

6. Quick Reference Cheatsheet

Process

Carried out by

Product

Nitrogen fixation

N-fixing bacteria (Rhizobium, Azotobacter)

Ammonia (NH₃)

Nitrification

Nitrifying bacteria

Nitrites β†’ Nitrates (NO₃⁻)

Assimilation

Plants/animals

Organic nitrogen (amino acids, DNA)

Ammonification

Saprotrophic decomposers

Ammonia

Denitrification

Anaerobic denitrifying bacteria

Atmospheric Nβ‚‚

7. Frequently Asked

Why can't plants use atmospheric nitrogen directly?

Atmospheric nitrogen (Nβ‚‚) has a very strong triple bond that makes it inert. Only nitrogen-fixing bacteria can break this bond to convert it into a form plants can absorb.

Are nutrients ever lost from ecosystems?

Small amounts can be leached into ocean sediments or lost via erosion, but most nutrients are continuously recycled within the biosphere, unlike energy which is permanently lost as heat.

When this came up on past exams

AI-estimated based on syllabus patterns β€” cross-check with official past papers for accuracy. Use only as revision-focus signals.

  • 2022 Β· 1

    MCQ on nitrogen cycle processes

  • 2023 Β· 2

    Explain eutrophication process

  • 2024 Β· 1

    Role of Rhizobium bacteria

Going deeper

What's Next

Mastering nitrogen and nutrient cycling builds a foundation for understanding all other biogeochemical cycles and human impacts on global ecosystems, a key theme in IB Biology SL ecology. This topic is regularly tested in both Paper 1 multiple choice and Paper 2 extended response questions, so focusing on the roles of different microorganisms and the steps of eutrophication will give you easy marks in the exam. Next, you can build on this knowledge to explore other core biogeochemical cycles and human impacts on global biodiversity.