What charge does nitrogen have?

Elemental nitrogen has no charge.

In ammonia (NH3) and ammonium ions (NH4+) it has a oxidation value of -3 (and actually only a partial negative charge as part from a polar covalent, non-ionic bond).

In Nitrate (NO3-) its oxidation value is +5, in nitrite +3 (but only a partial positve charge in both)