The electrochemical nitrate reduction reaction (NitRR) to ammonia (NH₃) offers a sustainable route for simultaneous wastewater treatment and green NH₃ synthesis, yet it demands highly active and selective catalysts. Single-atom Fe-N-C catalysts show promise but often suffer from aggregation and limited mass transport. Herein, we constructed a composite electrocatalyst by confining Fe single atoms within an alkali-modified biochar support (Fe-N-C + OHBC). The OHBC framework features a hierarchical porous structure that enhances the mass transfer, increases the electrochemically active surface area, and creates a confined microenvironment around the Fe–N_x sites. This synergistic integration significantly boosts the NitRR performance. The Fe-N-C + OHBC catalyst achieves a remarkable NH₃ yield rate of 11.46 mg·h⁻¹·cm^–2 at −0.71 V vs. reversible hydrogen electrode (RHE) and a high Faradaic efficiency of 93.03% at −0.45 V vs. RHE, substantially outperforming its individual components. Mechanistic studies reveal that the Fe single atoms are the primary active centers, while OHBC facilitates proton-coupled electron transfer and enriches local reactant concentration. Furthermore, the catalyst demonstrates excellent stability over 80 h and maintains high performance under various pH levels and in the presence of common interfering ions, showcasing its potential for practical nitrate remediation and decentralized NH₃ production.