• Nitric acid oxidation dramatically enhances the microwave absorption of carbonized melamine foam, achieving a minimum reflection loss of −21.51 dB at 13.20 GHz.
• The surface treatment yields a broad effective absorption bandwidth of 7.04 GHz, covering a wide frequency range useful for electromagnetic interference shielding.
• The improved performance arises from strengthened dielectric loss, better impedance matching, and increased polarization losses induced by oxidized surface functional groups.
• The work presents a lightweight, low-density surface-modification strategy for designing high-performance carbon-based microwave absorbers for aviation, civilian, and military applications.