2.3 Nitrogen Cycle
Nitrogen makes up about 78% of the air, yet plants and animals cannot absorb it directly. The nitrogen cycle continuously recycles nitrogen between living organisms and the environment through processes that add and extract nitrate ions.
Learning Objectives
- Define the nitrogen cycle and state its role in an ecosystem.
- Identify the chemical form of nitrogen that plants absorb through their roots.
- Explain the processes that add nitrate ions to the soil.
- Explain the processes that extract nitrate ions from the soil.
- Explain the importance of the nitrogen cycle to living organisms and agriculture.
The Complete Pathway of the Nitrogen Cycle
Nitrogen moves between the atmosphere, living organisms and soil through addition and extraction processes.
Short Explanation
Definition
The nitrogen cycle is a natural cycle that recycles nitrogen between plants and animals, and the atmosphere, soil and water.
Nitrogen gas makes up approximately 78% of the Earth's atmosphere, but plants cannot absorb it directly from the air. Plants can only absorb nitrogen in the form of dissolved nitrate ions through their roots.
Two groups of processes
The nitrogen cycle is divided into two primary categories: processes that add nitrate ions to the soil, and processes that extract nitrate ions from the soil.
Processes that add nitrate ions to the soil
1. Lightning and volcanic eruptions
High temperatures oxidise atmospheric nitrogen into nitrogen dioxide. Rain dissolves nitrogen dioxide to form nitric acid, which reacts with soil minerals to form nitrate ions.
2. Nitrogen fixation
Nitrogen-fixing bacteria living freely in soil or water, or symbiotically in legume root nodules, convert nitrogen gas into nitrate ions.
3. Nitrification
Decomposing bacteria and fungi break down proteins into ammonium compounds. Nitrifying bacteria then convert ammonium compounds into nitrite ions, and finally into nitrate ions.
Processes that extract nitrate ions from the soil
1. Absorption by plant roots
Plant roots absorb nitrate ions to synthesise plant proteins. When animals eat plants, the proteins are transferred and reassembled into animal proteins.
2. Denitrification
Denitrifying bacteria break down nitrate ions and convert them back into nitrogen gas, which returns to the atmosphere.
3. Nitrate leaching
Soluble nitrate ions dissolve in rainwater and are washed into rivers or seep into underground water tables, reducing nitrate concentration in the topsoil.
Try to Answer First
Answer in your mind, then press “Check Answer”.
In what chemical form do plants absorb nitrogen from the soil through their roots?
Name the two sequential intermediate products formed when decomposing bacteria and nitrifying bacteria convert dead organic matter into nitrate ions.
Why does excessive, continuous rainfall lead to a reduction of nitrate content in agricultural soil?
Common Mistakes
- Confusing nitrogen-fixing bacteria with nitrifying bacteria. Nitrogen-fixing bacteria convert free nitrogen gas into nitrate ions; nitrifying bacteria convert ammonium compounds into nitrite ions, then into nitrate ions.
- Confusing nitrite ions with nitrate ions. Ammonium is converted first into nitrite ions, which are then converted into nitrate ions that plants absorb.
- Believing that plants absorb nitrogen gas through the stomata of their leaves. Plants cannot use nitrogen gas directly; they absorb nitrogen solely as dissolved nitrate ions through root hair cells.
- Forgetting that nitrate leaching is an extraction process, not an addition process. Leaching carries dissolved nitrates away from the soil, reducing soil fertility.
Concept Misunderstandings
Denitrification is a beneficial process for crop farming because it cleans the soil.
Denitrification is essential for replenishing atmospheric nitrogen, but for farmers it removes valuable nitrate ions from the soil, thereby reducing soil fertility.
Legume plants fix atmospheric nitrogen on their own without external aid.
Legumes themselves do not fix nitrogen; it is the symbiotic nitrogen-fixing bacteria living inside the root nodules of the legumes that perform the conversion.
Rain always increases soil nitrates because of lightning.
Rain accompanied by lightning adds nitrates to the soil, but heavy, prolonged rain also causes nitrate leaching, which washes existing nitrates out of topsoil into waterways.
Summary
- Nitrogen gas makes up 78% of the atmosphere, but cannot be utilised directly by plants.
- Plants absorb nitrogen exclusively as nitrate ions through their roots.
- Three processes add nitrate ions to soil: lightning and volcanic eruptions, nitrogen fixation, and nitrification.
- Three processes extract nitrate ions from soil: absorption by plant roots, denitrification, and nitrate leaching.
- The nitrogen cycle maintains atmospheric nitrogen at 78%, sustains soil fertility and crop yields, supplies proteins for growth and repair, and reduces environmental pollution through natural decay and composting.
Short Activity
A. Objective Quiz
1 Which of the following bacteria is responsible for converting nitrate ions in the soil back into nitrogen gas that returns to the atmosphere?
2 During a thunderstorm, what chemical compound is formed when nitrogen dioxide dissolves in rainwater before reacting with soil minerals?
B. Fill in the Blanks
3 Nitrifying bacteria convert ammonium compounds into ions before converting them into nitrate ions.
4 The natural process in which dissolved nitrate ions are washed out of the soil into rivers or underground water is called nitrate .
C. Drag and Drop: Match the Process to Its Description
Drag each process or component to the correct description. On a phone, tap an answer first, then tap a matching box. Tap the Answer Choices area to return a selected item.
C. Match the Process to Its Description
Write the correct process or component next to each description.
| Description | Process / Component |
|---|---|
| 1. Converts nitrogen in the air into nitrate ions in root nodules or soil | |
| 2. Converts animal and plant protein in dead remains into ammonium compounds | |
| 3. Converts nitrate ions in the soil into nitrogen gas that returns to the air | |
| 4. Uptake of dissolved nitrate ions by plant roots to synthesise plant proteins |
Answer choices: Nitrogen-fixing bacteria • Decomposing bacteria and fungi • Denitrification • Nitrate ion absorption
