Nitrogen cycle diagram showing key processes and components in an ecosystem.

Let’s solve this step by step.

We are looking at the nitrogen cycle diagram and need to match each process (like Denitrification, Mineralisation, etc.) with the correct lettered arrow or box in the diagram. The word bank gives us:

- Nitrogen Fixation
- Denitrification
- Mineralisation
- Nitrification

And we have 4 blanks labeled A, B, C, D — so we need to assign one term to each blank based on what’s happening in the diagram.

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First, let’s recall what each term means in simple terms:

1. Nitrogen Fixation – This is when nitrogen gas from the air (N₂) gets turned into a form plants can use (like ammonia). It’s done by special bacteria (often in soil or root nodules of legumes). In diagrams, it usually shows arrows going FROM the atmosphere INTO the soil/plants via bacteria.

2. Mineralisation – This is when dead stuff (dead plants, animals, poop) breaks down and releases ammonium (NH₄⁺) into the soil. So look for decomposition → ammonium.

3. Nitrification – This is when ammonium (NH₄⁺) gets turned into nitrites (NO₂⁻) and then nitrates (NO₃⁻) by bacteria. Plants love nitrates! So look for arrows going from ammonium → nitrate.

4. Denitrification – This is when nitrates get turned back into nitrogen gas (N₂) and released into the air. Done by certain bacteria in low-oxygen conditions. Look for arrows going FROM soil/water BACK to the atmosphere as N₂ gas.

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Now let’s look at the diagram clues (even though we’re not describing the image, we’ll reason based on standard nitrogen cycle layouts):

Typical flow:

Atmosphere (N₂) → [Nitrogen Fixation] → Ammonia/Ammonium → [Nitrification] → Nitrates → taken up by plants → eaten by animals → waste/death → [Mineralisation] → back to ammonium → some goes to [Denitrification] → back to N₂ gas in atmosphere.

Also, lightning can fix nitrogen too — that’s another path from atmosphere to soil.

In most school diagrams:

- Arrow from atmosphere to soil via “lightning” or “bacteria” = Nitrogen Fixation
- Arrow from organic matter (dead stuff) to ammonium = Mineralisation
- Arrow from ammonium to nitrate = Nitrification
- Arrow from nitrate back to atmosphere as N₂ = Denitrification

Assuming the letters A, B, C, D correspond to these processes in order (as commonly taught), here’s how they likely map:

Let’s suppose:

→ A: Process turning atmospheric N₂ into usable form → Nitrogen Fixation

→ B: Process breaking down dead matter into ammonium → Mineralisation

→ C: Process turning ammonium into nitrate → Nitrification

→ D: Process turning nitrate back into N₂ gas → Denitrification

This matches the standard teaching sequence.

Double-checking logic:

- Nitrogen must enter the system first → fixation (A)
- Then recycling of nutrients from dead things → mineralisation (B)
- Then conversion to plant-friendly form → nitrification (C)
- Then return to atmosphere → denitrification (D)

Yes, that makes sense.

Final Answer:
A: Nitrogen Fixation
B: Mineralisation
C: Nitrification
D: Denitrification