Nitrate　electroreduction　to　ammonia　holds　great　promise　in　sustainable　green　ammonia　synthesis,　yet　faces　a　dearth　of　competent　electrocatalysts　adapted　to　varying　nitrate　concentration,　and　inadequate　ammonia　fixation.　Herein,　we　present　a　high-performance　Ag　single-atom-decorated　Cu2O　nanowire　catalyst　(Ag1@Cu2O)　that　exhibits　concentration-universal　high-rate　nitrate　reduction,　achieving　＞90%　to　near-unity　ammonia　faradaic　efficiency　(FE)　across　nitrate　concentrations　from　0.01　to　0.5　M.　Notably,　at　0.5　M　nitrate　concentration,　it　attains　a　two-ampere-level　current　density　(2.3　A　cm-2)　at　-1　V　vs.　RHE,　resulting　in　a　leading　ammonia　yield　rate　of　184.4　mgNH3　h-1　cm-2.　In　situ　studies　combined　with　theoretical　calculations　elucidate　an　Ag-Cu　inter-site　synergistic　catalytic　mechanism,　in　which　single-atom　Ag　serves　as　an　accelerator　for　active　hydrogen　generation　and　stabilization　on　Cu　sites　to　boost　the　hydrogenation　kinetics　of　N-containing　intermediates,　thus　smoothing　the　energy　barriers　for　ammonia　production　via　the　favorable　*NHO　pathway.　Additionally,　Ag1@Cu2O　demonstrates　near-unity　formate　FE　for　formaldehyde　oxidation,　reaching　a　300　mA　cm-2　current　density　at　a　mere　0.31　V　vs.　RHE.　Motivated　by　this　exceptional　bifunctionality,　we　demonstrate　an　innovative　tandem　electrochemical-chemical　strategy　for　upgrading　ammonia　into　high-value　ammonium　formate　by　coupled　electrolysis　of　nitrate　reduction　and　formaldehyde　oxidation,　followed　by　straightforward　chemical　combination　and　isolation.　In　practice,　membrane　electrode　assembly　(MEA)　electrolysis　at　1.6　V　for　100　h　successfully　outputs　10.7　g　of　high-purity　ammonium　formate.　Furthermore,　the　commonality　of　this　strategy　is　validated　by　application　to　various　nitrate/aldehyde　pairs.　This　work　blazes　a　new　trail　for　scalable,　cost-　and　energy-efficient　green　ammonia　production　and　fixation　from　nitrate　reduction.