# Nitrogen and nutrient cycling

> IB Biology SL · IB Biology SL 2025 Syllabus
> Source: https://www.owlsprep.com/study/ib-biology-sl-u3-nitrogen-and-nutrient-cycling/

This module covers core principles of nutrient recycling, the key steps and microbial roles in the nitrogen cycle, and human impacts on natural nutrient cycling, a frequently tested topic in IB Biology SL ecology.

**Prerequisites:** [Ecosystem structure and energy flow](https://www.owlsprep.com/study/ib-biology-sl-u3-ecosystem-energy-flow/)

## Learning objectives

- Describe the key differences between energy flow and nutrient cycling in ecosystems
- Outline the roles of microorganisms in each step of the nitrogen cycle
- Explain the impact of human activity on natural nutrient cycling
- Interpret exam questions about mutualistic relationships in the nitrogen cycle

## Principles of Nutrient Recycling

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).

**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.

**Worked example:** Explain why energy flow is described as linear, while nutrient cycling is described as cyclical.

1. Step 1: Energy enters ecosystems as sunlight, which is converted to chemical energy by producers, then transferred between trophic levels.
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. Step 3: When organisms die, decomposers break down their organic tissue, releasing inorganic nutrients back into the soil.
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.

## Key Processes of the Nitrogen Cycle

Atmosphere is 78% nitrogen gas (N₂), but almost all organisms cannot use N₂ 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**: N₂ → ammonia (NH₃) done by nitrogen-fixing bacteria
- **Nitrification**: Ammonia → nitrites → 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 → N₂ released back to atmosphere done by anaerobic bacteria

**Worked example:** Explain why the relationship between Rhizobium bacteria and leguminous plants is mutualistic.

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

## Role of Microorganisms in Nitrogen Cycling

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.

> **info**
>
> Nitrogen-fixing bacteria can be free-living in soil (e.g. *Azotobacter*) or symbiotic in plant root nodules (e.g. *Rhizobium*).

**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. Step 1: Decomposers release ammonia from dead organic matter in soil, but plants cannot easily use large amounts of ammonia. They need nitrates.
2. Step 2: Nitrifying bacteria first convert ammonia to nitrites (genus *Nitrosomonas*), then convert nitrites to nitrates (genus *Nitrobacter*).
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.

## Human Impacts on Nutrient Cycling

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.

**Worked example:** Outline the steps leading to eutrophication after fertilizer runoff enters a lake.

1. Step 1: Excess nitrogen and phosphorus from runoff increases nutrient concentrations in the lake water.
2. Step 2: High nutrient levels trigger rapid uncontrolled growth of algae, called an algal bloom.
3. Step 3: The algal bloom blocks sunlight from submerged plants, which die because they cannot photosynthesize.
4. Step 4: When the algae die, they are broken down by aerobic decomposer bacteria.
5. Step 5: Decomposition uses up almost all of the dissolved oxygen in the lake water, creating hypoxic (low oxygen) conditions.
6. Step 6: Most aquatic organisms like fish cannot survive low oxygen, leading to mass death and a 'dead zone' with very low biodiversity.

## Common pitfalls

- **Wrong:** Claiming energy is recycled along with nutrients in ecosystems
  - Why it fails: Students frequently mix up the two core ecology concepts, as both move through trophic levels
  - Correct: Always state energy flows one-way and is lost as heat, while nutrients are continuously cycled
- **Wrong:** Confusing denitrification with ammonification (decomposition)
  - Why it fails: The similar naming of nitrogen cycle steps leads to common mix-ups
  - Correct: Denitrification returns N₂ to the atmosphere, ammonification releases ammonia from dead organic matter
- **Wrong:** Stating that plants absorb nitrogen directly from the air
  - Why it fails: Students forget atmospheric nitrogen is inert and only bacteria can fix it
  - Correct: Plants only absorb nitrates or ammonium from soil through their roots
- **Wrong:** Only describing the benefit to the plant when explaining Rhizobium mutualism
  - Why it fails: IB exam questions require you to show you understand mutualism benefits both organisms
  - Correct: Always state Rhizobium gets carbohydrates and protection, the plant gets fixed nitrogen
- **Wrong:** Claiming algae use up all the oxygen during eutrophication
  - Why it fails: Students misattribute the cause of oxygen depletion in eutrophic water bodies
  - Correct: Oxygen is depleted by aerobic bacterial decomposition of dead algae and plants after the bloom dies

## Cheatsheet

| Process | Carried out by | Product |
| --- | --- | --- |
| Nitrogen fixation | N-fixing bacteria (Rhizobium, Azotobacter) | Ammonia (NH₃) |
| Nitrification | Nitrifying bacteria | Nitrites → Nitrates (NO₃⁻) |
| Assimilation | Plants/animals | Organic nitrogen (amino acids, DNA) |
| Ammonification | Saprotrophic decomposers | Ammonia |
| Denitrification | Anaerobic denitrifying bacteria | Atmospheric N₂ |

## 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.

- [Conservation of Biodiversity](https://www.owlsprep.com/study/ib-biology-sl-u3-conservation-of-biodiversity/)
- [Theme D: Continuity and Change](https://www.owlsprep.com/study/ib-biology-sl-u4-overview/)
- [DNA replication and transcription](https://www.owlsprep.com/study/ib-biology-sl-u4-dna-replication-and-transcription/)

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