Electrocatalytic nitrate reduction to ammonia offers an environmentally sustainable approach for nitrogen fixation. P-block metals, especially indium, have emerged as prominent materials in electrocatalysis. The integration of indium with carbon supports represents a key advancement in catalyst design. Drawing from prior experimental synthesis and the density functional theory calculations, a strong interaction is proposed between indium atomic chains and carbon edges, driven by p-p coupling that significantly redistributes the electronic structures of indium. Nitrate reduction proceeds efficiently on these indium atomic chains, overcoming an energy uphill of only 0.09 eV through an electron donation and back-donation mechanism that activates the N─O bonds. Furthermore, the continuous band-like electronic cloud surrounding indium atomic chain along zigzag edges can enable proton transport, facilitating the hydrogenation of reaction intermediates. This promotes tandem reactions while suppressing competing hydrogen evolution. These insights advance the understanding of electrocatalytic nitrate reduction and pave the way for designing novel p-block metal-based catalysts.