medium difficulty5 min

Nitrogen and Oxygen Cycles

Nitrogen and oxygen cycles describe how nitrogen and oxygen move through air, soil, water, and living organisms by biological, chemical, and physical processes.

In one line: Bacteria unlock nitrogen; plants release oxygen.

Practice This Concept

Understand it

Most nitrogen in air cannot be used directly by plants. Nitrogen-fixing bacteria convert it into usable compounds, plants absorb them, animals get nitrogen by feeding, and decomposers return nitrogen compounds to soil. Oxygen is released by photosynthesis and used in respiration, combustion, and decomposition.

Example

Leguminous plants have root nodules with bacteria that help convert atmospheric nitrogen into forms plants can use.

Rule to remember

Rule: photosynthesis releases oxygen; respiration and combustion use oxygen.

Nitrogen fixation converts atmospheric nitrogen into usable nitrogen compounds.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

A pulse crop improves soil nitrogen because bacteria in its root nodules fix atmospheric nitrogen into compounds that plants can use.

Method you can reuse

1For nitrogen, start with atmospheric nitrogen, add fixation, plant absorption, animal feeding, decomposition, and return to soil.
2For oxygen, connect photosynthesis release with respiration use.

Sketch it in your answer

Draw nitrogen cycle with air nitrogen, nitrogen-fixing bacteria, soil nitrates, plants, animals, decomposers, and return to soil. Draw oxygen cycle with photosynthesis and respiration arrows.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why can most plants not use atmospheric nitrogen directly?

Think first, then show answer

Most plants cannot use atmospheric nitrogen directly because nitrogen gas is very stable. It must first be converted into usable nitrogen compounds such as nitrates or ammonium compounds.

Common wrong answer

Plants take nitrogen gas directly through leaves for protein making.

Why it loses marks: Plants mainly absorb usable nitrogen compounds through roots.

Don't mix this up

Students often write that plants directly absorb nitrogen gas from air. Most plants absorb nitrogen mainly as nitrates or ammonium compounds from soil.

Exam tip

For nitrogen cycle, mention nitrogen fixation and absorption by plants. For oxygen cycle, connect photosynthesis with oxygen release and respiration with oxygen use.