* Oxidation State of Nitrogen: In ammonium ion, nitrogen has an oxidation state of -3. This is because hydrogen has an oxidation state of +1, and the overall charge of the ion is +1.

* Tendency to Lose Electrons: Nitrogen in ammonium ion can be oxidized to a higher oxidation state, meaning it can lose electrons. This loss of electrons is the characteristic of a reducing agent.

Example:

In the following reaction, ammonium ion acts as a reducing agent:

```

NH₄⁺ + 2O₂ → NO₃⁻ + 2H⁺ + H₂O

```

Here, nitrogen in ammonium ion (-3 oxidation state) is oxidized to nitrate (NO₃⁻) with a +5 oxidation state. This is because ammonium ion loses electrons, which are gained by oxygen in the reaction.

Key Takeaway:

A reducing agent is a substance that donates electrons and gets oxidized in the process. Since ammonium ion can lose electrons and get oxidized, it acts as a reducing agent.